Pharmaceutical-mediated neuroimmune modulation in psychiatric/ psychological adverse events

PPAEs during preclinical stages of a new drug ’ s R & D. This review provides a comprehensive summary of the most recent advances in neuroimmune modulation-related mechanisms contributing to the onset of PPAEs and their association with specific pharmaceuticals. Reported data strongly support an association between neuroimmune modulation and the onset of PPAEs. Pharmaceuticals may target specific molecular pathways and pathway elements (e


Introduction
Psychiatric and psychological adverse events (PPAEs) encompassing mood (e.g., depression, schizophrenia, suicide and suicidal ideation) and cognitive changes (e.g., learning and memory impairment, dementia), occur as a consequence of distinct pharmaceutical treatments.Noteworthy, these changes in the central nervous system (CNS) are usually only detected at later stages (e.g., Phase III) of clinical trials or during pharmacovigilance (Phase IV), posing serious concerns to the patient's health and safety (Andronis et al., 2020), especially since the mechanisms underlying the onset of PPAEs remain mostly unclear.
The role of the immune system in the onset of PPAEs has grown major interest in recent years, mostly as a result of the observation that 1) some small non-CNS targeted pharmaceuticals (e.g., ciprofloxacin, a fluoroquinolone that inhibits prokaryotes' DNA gyrase and DNA topoisomerase IV) may trigger PPAEs (Gürbay et al., 2006); and 2) pharmaceuticals that are unable to cross the blood-brain barrier, such as chemotherapeutic agents (e.g., doxorubicin, daunorubicin) or monoclonal antibodies (e.g., natalizumab) may also impair cognitive functions (Cardoso et al., 2020;Stüve, 2008).Interestingly, a recent effort to identify candidate pathways and pathway elements associated with PPAEs based on the statistical analysis of pharmacovigilance reports, evidenced that 66% of the genes identified as being directly linked to PPAEs were also directly or indirectly associated with immune modulation (Andronis et al., 2020).
The mechanisms underlying neuroimmune modulation are complex and involve multiple cell types and signaling pathways.Still, it is of paramount importance to understand these interactions to improve and/ or build new tools able to strengthen the predictability of psychiatric/ psychological adverse events at the preclinical stages of new drug development.
This review aims at summarizing the most recent updates on the role of neuroimmune modulation in the onset of PPAEs, in particular by addressing how candidate pathways and pathway elements targeted by pharmaceuticals associated with a high number of PPAE reports may modulate neuroimmune signaling.Understanding these mechanisms represents an important contribution to highlight potential key immunomodulatory pathways and pathway elements involved in PPAEs, and help improving the safety assessment of new pharmaceuticals.

The central nervous system, the immune system and neuroimmune modulation
Neuroimmune modulation may be broadly described as an intricate network of interactions between the CNS, the immune, and the endocrine systems.It encompasses the complex multidirectional communication pathways between these systems, influencing various physiological and pathological conditions, such as cancer, Parkinson's disease, Alzheimer's disease (AD), multiple sclerosis, major depressive disorders and others (Chambers and Schauenstein, 2000;Tiro et al., 2013;Norris and Kipnis, 2018).Here, we specifically focus on the interactions between the CNS and the immune system.Of note, neuroendocrine regulation has already been recently and elegantly reviewed by others (MacDonald et al., 2019).
Specifically, the interplay between the CNS and the immune system comprises both the interactions within the CNS (i.e., between astrocytes, microglia, and oligodendrocytes), and the interaction between the peripheral immune system (namely, the peripheral immune cells), and the CNS.These interactions are physically separated by the blood-brain barrier (BBB), a functional anatomical structure that selectively promotes exchanges between the brain and the rest of the body, controlling the influx and efflux of xenobiotics, biological substances, and cells, to preserve its homeostasis (Xiao et al., 2020).
The BBB is composed of endothelial cells, pericytes, immune cells, astrocytes, and the basement membrane, mostly associated with the formation of impermeable capillaries.Additionally, several proteins create a scaffold in the basement membrane, synergistically interacting to promote a robust barrier against ions and molecules (Sharif et al., 2018).This broad range of intervenients confers unique functionalities to the BBB, which are important to import nutrients, export cellular debris, and for the permeation of immune cells and foreign elements such as pharmaceuticals (Xiao et al., 2020).The internalization of important nutrients is achieved by the solute carrier (SLC) transport family (present in astrocytes that coat the basement membrane, and pericytes), which is responsible for the transport of glucose, lactate, pyruvate, and amino acids into the brain.Astrocytes are further responsible for water and metabolite balance, but also innate immune regulation and electrochemical activity (Sharif et al., 2018;Rodríguez-Arellano et al., 2016).
Astrocytes are important players in neuroimmune modulation, with increasing evidence of their crucial role in CNS homeostasis (Verkhratsky and Nedergaard, 2018).Astrocytes are versatile cells, displaying a plethora of features (as a result of the their high density and diversity of receptors), which allow the binding of immune mediators, ions, or neurotransmitters that render them highly sensitive to most environmental cues (Verkhratsky and Nedergaard, 2018).For example, astrocytes are fundamental for neurotransmitter turnover, reuptake of glutamate and GABA from the synaptic cleft, and their conversion into glutamine (a neurotransmitter precursor).Glutamine is then released back to presynaptic neurons to be reconverted into a neurotransmitter, thus preventing neurotransmitter depletion (Verkhratsky and Nedergaard, 2018).Astrocytes also participate in synaptogenesis, by secreting multiple synaptogenic factors (e.g., cholesterol, integrins, thrombospondins), and synaptic maturation and pruning (by supporting the stability of dendritic spines), and dynamic remodelling, which is essential for memory consolidation (Verkhratsky and Nedergaard, 2018;Eroglu and Barres, 2010;Eroglu et al., 2009).Additionally, astrocytes respond to any kind of insult, triggering a spectrum of changes (i.e., molecular, morphological, and functional) commonly known as astrogliosis, which may vary depending on the severity, intensity, and context of the threat (Sofroniew and Vinters, 2010;Sofroniew, 2014;Hirrlinger and Nimmerjahn, 2022;Halassa et al., 2007).
One of the most relevant examples of neuroimmune modulation by astrocytes is gliotransmission (Hirrlinger and Nimmerjahn, 2022;Halassa et al., 2007).This event was initially described as a tripartite synapse, in which astrocytes located in the proximity of synapses could be triggered by the neuronal electrophysiological signaling, responding with their own Ca 2+ signaling (Araque et al., 1999).However, recent studies have shown that this interaction is far more complex and involves more cells than initially described, as one astrocyte can extend to several synapses, and one synapse can have extensions from different astrocytes (Semyanov and Verkhratsky, 2021;Peng et al., 2023).Semyanov and Verkhratsky have recently presented a new interpretation of the tripartite synapse and gliotransmission (Semyanov and Verkhratsky, 2021).These authors introduced the "active milieu" concept, recognizing that all elements present in the CNS, namely the neurons, microglia, astrocytes, oligodendrocytes, blood vessels, extracellular matrix, are important for the homeostasis and function of the brain as a whole.They also recognized that neurotransmission, gliotransmission, and other synaptic microenvironmental tasks performed by the other elements are not so specific and independent, and should be taken as an integral, complex system.
In particular, astrocytes display branches that express neurotransmitter receptors (e.g., for glutamate and GABA), and that are sensitive to the synaptic transmission events across the synaptic cleft (Fig. 1a).Astrocyte triggering promotes the release of their intracellular Ca 2+ , which can then be transported through cell-to-cell connections into other astrocytes, replicating the Ca 2+ release in the subsequent cells (Fig. 1b) (Rose et al., 2020).This event is described as a Ca 2+ wave effect and promotes morphological, functional, and organizational changes to the astrocyte, which impacts synaptic efficacy, plasticity, and connectivity (Fig. 1c) (Semyanov and Verkhratsky, 2021).This highlights the importance of the endfeet of astrocytes' branches, which are hypothesized to shape into a rosette-like morphology, maximizing the number of receptors available in the membrane, in the vicinity of the synaptic cleft.This impacts the machinery of astrocytic signal transmission since less volume potentially represents a smaller distance between the receptors and the storage of Ca 2+ and Na + (Rose et al., 2020).Also, a smaller volume of astrocytes' endfeet results in better shuttling between neurons and astrocytes of glutamate-glutamine, lactate, and glutathione, which are indispensable for neuronal metabolism.
Microglia are considered the resident effective immune cells of the CNS and are of paramount importance for the protection of the brain against any injury or infection, controlling the inflammatory status of the brain through the maintenance of homeostasis in healthy tissue, and promoting inflammation in compromised tissue (Aloisi, 2001;Davalos et al., 2005).One of the key functions of microglia is to detect and respond to pathogens and injury in the brain.Upon detecting an insult, microglia become activated, releasing cytokines and expressing molecules [e.g., major histocompatibility complex (MHC) class II receptor] that promote a localized immune response (Davalos et al., 2005;Marıń-Teva et al., 2004).In addition, microglia participate in neurogenesis (Wu et al., 2013), modulation of synaptic plasticity (Marıń-Teva et al., 2004), neuronal network maintenance (Verkhratsky and Nedergaard, 2018;Kettenmann et al., 2011) and play a role in synaptic formation and pruning through the elimination of inactive, weak, or excessive synapses (Fig. 2a) (Gomez-Nicola and Perry, 2015).This elimination depends on the microglia triggering receptor expressed on myeloid cells 2 (TREM2), C1q and C3 proteins, and microglia's phagocytic abilities.To-beeliminated neuronal synapses expose phosphatidylserine in the outer surface of the plasma membrane (Fig. 2), triggering phagocytosis by microglia and their subsequent removal from the neuronal network (Fig. 2c) (Scott-Hewitt et al., 2020).Notably, while astrocytes participate in neuronal regulation through direct contact with neurons and synapses, as described in the active milieu model (Semyanov and Verkhratsky, 2021), microglia-associated neuro-and immunemodulatory actions rely on their pro− /anti-inflammatory phenotype and activation, with subsequent expression of cytokines (Peng et al., 2023;Kettenmann et al., 2011).
Oligodendrocytes and oligodendrocyte precursor cells (NG2-glia) interact with both astrocytes and microglia in the regulation of myelination and remyelination events.However, in the grey matter, these cells participate in the neurogliovascular regulation of the BBB.Oligodendrocytes were initially thought to participate solely in neuron myelination (Allen and Lyons, 2018).By wrapping around neuronal axons, NG2-glia differentiate into oligodendrocytes and create myelin sheaths that allow the insulation of the axon, resulting in faster signaling transmission (Peng et al., 2023;Jäkel and Dimou, 2017;Bradl and Lassmann, 2010).However, recent studies have shed light on a plethora of neuroimmune modulatory functions.NG2-glia actively participates in maintaining CNS homeostasis by promoting neuroprotective phenotypes from both astrocytes and microglia, whereas NG2-glia ablation in mice has been shown to aggravate neuroinflammation status and neurodegeneration (Boccazzi et al., 2022;Zhang et al., 2019a).Other studies in rat models, with NG2-glia adult selective ablation trigger, to destroy NG2 cells in adult rats, revealed hippocampal cell death along with microglia activation and increased proinflammatory IL-1β, IL-6, and tumor necrosis factor-alpha (TNF-α) mRNA levels (Nakano et al., 2017).NG2-glia ablation in mice models resulted in aberrant microglia activation, with an increased neuroinflammatory response, whereas astrocytes demonstrated neurotransmitter reuptake impairment from the synaptic cleft (Nakano et al., 2017;Zhang et al., 2019b).Of note, Fig. 1.Simplified scheme of astrocytic gliotransmission.a) the interaction between astrocyte terminals, or endfeet (purple cell), and a synaptic cleft (blue cells: neurons) describes astrocyte predisposition to respond to neurotransmission (e.g., glutamate and GABAblue dots).b) Astrocyte activation promotes a calcium release wave, that travels between cell-to-cell junctions to other astrocytes; c) In turn, these astrocytes affect the neurotransmission of adjacent cells through the release of corresponding neurotransmitter/neurotransmitter precursors (neurons-light blue; neurotransmitter/precursororange dots)-created with BioRender.com.(For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)disruption of glutamatergic neurotransmission, for example, is often associated with depressive-like behavior (Birey et al., 2015).Oligodendrocyte and NG2-glia neuroprotective abilities are thought to rely on their expression of tumor necrosis factor receptor 2 (TNFR2) (Boccazzi et al., 2022).TNFR2 activation has been suggested to decrease neuroinflammation and demyelination since mice selectively lacking TNFR2 expression demonstrated that TNFR2 silencing leads to premature microglia activation and peripheral immune cell infiltration.Ultimately, these lead to increased demyelination, axonal loss, and impaired remyelination (Madsen et al., 2020;Desu et al., 2021).
The knowledge of the role of the peripheral immune system in CNS regulation has substantially evolved over the last decades, having noted its impact on neurodegenerative disorders, such as Alzheimer's and Parkinson's diseases, as previously reviewed (Yang et al., 2020).The authors highlighted that chronic inflammation is often associated with neurodegenerative diseases and PPAEs, such as memory and learning impairment, depression, and cognitive dysfunction (Yang et al., 2020;Chesnokova et al., 2016;Imfeld et al., 2016).However, under physiological conditions, innate immune cells (e.g., leukocytes and neutrophils) have a very low expression in the CNS.To move across the BBB, these cells depend on adhesion processes mediated by several integrins and selectins [e.g., intercellular adhesion molecule 1 (ICAM-1), platelet endothelial cell adhesion molecule 1 (PECAM-1), E-and P-selectins] that secure and facilitate their movement (Sharif et al., 2018).Fig. 3 summarizes the roles of the main immune cells in the CNS (i.e., astrocytes, microglia, oligodendrocytes) in neuroimmune modulation.
The examples of peripheral inflammatory processes influencing the CNS are extensive.For example, lipopolysaccharide (LPS) intraperitoneal injection in mice, followed by pathogenic α-synuclein treatment, resulted in α-synuclein internalization by peripheral monocytes, that move with preferential dissemination towards the brain (Peralta Ramos et al., 2019).On the other hand, monocytes can also act as neuroprotective agents by responding to brain injury and stimulating glutamatergic neuronal transmission in the CNS (Liu et al., 2017).Also, the glial cell line-derived neurotrophic factor (GDNF) expressed by bone marrow macrophages is neuroprotective by preventing dopaminergic neurons' degeneration (Chen et al., 2018).T cell lymphocytes can sense CNS damage and migrate across the BBB, turning from an inactive to an active state upon migration.Then, by either recruiting antigenpresenting cells (APCs) or by interacting with microglia, which may express MHC II, T cells are further activated for a second time in the CNS (Korn and Kallies, 2017).The phenotype acquired by T cells, either Th1 and Th17 (proinflammatory) or Th2 and Treg (anti-inflammatory) depends on multiple environmental factors.Th1 and Th17 cells, which express TNF-α and interferon (IFN)-γ, induce microglia activation and further stimulate neuroinflammation, while Th2 and Treg secrete IL-4 and IL-10, which promote a shift in microglia into an antiinflammatory phenotype (Yang et al., 2020).Also, B cells have been shown to behave as APCs to T cells, and, similarly to other peripheral immune cells (PICs), can act as either pro-inflammatory or antiinflammatory, influencing the progression of neuroinflammation (Yang et al., 2020;Vazquez et al., 2015).

Dysregulation of neuroimmune modulation and psychiatric/ psychologic adverse outcomes
Given the plethora of neuronal functions modulated by glial cells, it becomes clear that the dysregulation of neuroimmune modulation may contribute to the onset of psychiatric/psychological adverse outcomes, such as major depressive disorder (MDD), bipolar disorder, schizophrenia, depression, suicide and suicidal ideation (Jäkel and Dimou, 2017;Miller and Raison, 2016).
Najjar et al. observed a duality in microglia behavior, noting that these cells may either adopt a neuroprotective (e.g., by expressing MHC-II, secreting anti-inflammatory cytokines, and preventing glutamate excitatory neurotoxicity), or a harmful phenotype.The latter is characterized by: 1) lack of MHC-II expression; 2) secretion of proinflammatory cytokines; 3) disruption of the BBB; and 4) release of glutamate into the extracellular environment (Najjar et al., 2013).This effect of pro-inflammatory microglia, usually associated with high levels of pro-inflammatory cytokines (e.g., IL-1β, IL-18, and TNF-α) has been correlated with significantly increased levels of anxiety and memory impairment in mice (Cheon et al., 2021).Similarly, chronic inflammation related to autoimmune diseases, systemic infections, or cancer, can lead to depressive symptomatology as a secondary effect (Dantzer et al., 2008;Raison et al., 2006).The opposite is also true, as MDD and other types of depression are usually accompanied by increased serum levels of pro-inflammatory cytokines, such as IL-6 and IL-1 receptor agonists (Maes et al., 1997).Schlatter et al. (2004) observed that the peripheral blood of dysthymia and MDD patients presented elevated IL-1β, IL-6, and TNFα levels compared to healthy individuals (Schlatter et al., 2004).Lanquillon et al. found that IL-6 was elevated in the whole blood samples of MDD patients unresponsive to amitriptyline antidepressant treatment, while responsive patients presented a decrease in IL-6, after treatment (Lanquillon et al., 2000).Moreover, anti-cytokine treatment has been associated with antidepressant activity (Kappelmann et al., 2018).Indeed, a meta-analysis of clinical trials of chronic inflammatory anti-cytokine treatments, where depressive symptoms were measured as secondary outcomes, established a potential causal role for cytokines in depression, by revealing that, compared to placebo, anti-cytokine treatment, such as anti-TNF-α agents, significantly ameliorated depressive symptoms.
It is not surprising that astrocytes have been reported to be involved in PPAEs (e.g., depression, anxiety, Alzheimer's disease, schizophrenia), considering their ample functional range.For example, transcriptomic and proteomic analysis of GJA1 (connexin 43) in post-mortem tissues from healthy individuals and AD patients revealed a strong correlation between astrocytes lacking GJA1 expression and the cognitive decline associated with AD (Kajiwara et al., 2018).Interestingly, connexins (Cx) 30 and Cx43, mainly expressed by astrocytes, are associated with the formation of tight gap junctions, ion regulation between adjacent cells, and astroglial activation.In contrast, astrocytic Cx dysregulation has been associated with depressive behavior and depression-induced suicide (Peng et al., 2023;Huang et al., 2019).Indeed, studies performed in a mouse model of social defeat stress led to the conclusion that reduced levels of astrocytic Cx30 and Cx43, as well as reduced neuronal activity, were concomitant with depressive-like behavior.In turn, overexpression of Cx30 and Cx43 increased neuronal activity and inhibited the depressive behavior, whereas Cx30 and Cx43 suppression in normal mice replicated the results from the social defeat stress mouse model, further supporting a causal role between astrocytic Cx dysregulation and depressive-like behavior (Huang et al., 2019).
Notably, psychiatric and psychological adverse outcomes are not solely due to microglia and/or astrocyte dysregulation and reactivity.For example, a mouse model of inducible overexpression of mitochondrial catalase in astrocytes revealed that catalase overexpression reduced mitochondrial ROS production both in primary astrocyte cell line and mice brain, affecting brain carbohydrate, lipid, amino acid, and redox metabolic pathways, leading to neuronal dysfunction, behavioral changes, and cognitive impairment (Vicente-Gutierrez et al., 2019).Furthermore, using a double-transgenic mouse that allowed for NG2 preferential ablation, followed by LPS challenging, revealed an exacerbation of neuroinflammation, with the mRNA of pro-inflammatory cytokines IL-1β, IL-6 and TNF-α found overexpressed when compared with non-ablated controls.These NG2-ablated, LPS-challenged animals presented hyperactivated microglia, compared with control animals.This study also indicated that TGF-β2/TGFBR2 derived from NG2 regulated CX3CR1-depended immune activation, demonstrating that NG2-glia cells aid in the maintenance of homeostasis, while their silencing promotes microglia activation, rendering the brain more prone to inflammation (Zhang et al., 2019b).

Molecular targets associated with PPAEs and neuroimmune modulation
Pharmaceuticals can affect the immune status of the CNS in different ways (Fig. 4).The most straightforward involves the permeation of the BBB to these pharmaceuticals and their direct interaction with the immune cells in the CNS, mainly astrocytes and microglia (Fig. 4A and B, respectively).Considering the plethora of receptors expressed by astrocytes (Verkhratsky and Nedergaard, 2018), these cells represent a potentially vulnerable target to several classes of pharmaceuticals.Also, the extensive functions performed by astrocytes (Fig. 3) indicate their importance to the modulation of the molecular mechanisms underlying PPAEs.Likewise, microglia are responsible for the immune defense of the CNS and are sensitive to inflammatory cues that may trigger extensive responses.Furthermore, the association between chronic inflammation in the CNS and the development of neurodegenerative diseases has become a hallmark of this condition, revealing the importance of understanding phenotypic changes and stimuli in microglia.The biological pharmaceuticals, whose properties (e.g., size) prevent BBB permeation, may act by either stimulating the peripheral immune cells, promoting their activation and migration into the CNS, through the BBB (Fig. 4C), or by activating cells in the gut-brain axis, promoting the release of signaling agents that cross the BBB and reach receptors in the brain (Fig. 4D).
Examples of pharmaceuticals triggering PPAEs are fairly common and are not solely linked to CNS-acting pharmaceuticals.For example, ciprofloxacin, a second-generation fluoroquinolone antibiotic, has been reported to induce depression, suicidal ideation, and psychotic episodes (Norra et al., 2003;Ahmed et al., 2011).
One of the main reasons why newly developed pharmaceuticals are found to trigger such PPAEs at late stages of drug development relies on the incomplete understanding of the molecular mechanisms underlying such adverse events.To address this lack of knowledge, Andronis et al. developed a unique workflow, based on the mining of a pharmacovigilance database, followed by evidence-based expert curation of such data, and subsequent signaling pathway functional enrichment and cross-talk analyses.The authors identified 120 genes and common pathway nodes possibly underlying PPAEs.Most importantly, about two-thirds of the identified genes were found to be directly or indirectly associated with immune modulation, supporting the key role played by immune cells in the regulation of CNS function (Andronis et al., 2020).Some of these candidate pathway elements, which, when targeted, may underlie PPAEs and have already been linked to immune modulation, can be grouped as follows:

Immune mediators
Several pharmaceuticals that directly target elements of the immune system, including monoclonal antibodies (e.g., natalizumab, brodalumab) and recombinant cytokines (e.g., interferon family), have been extensively associated with the onset of PPAEs (Lebwohl et al., 2018).Examples of molecular targets of such pharmaceuticals include the recombinant cytokine IFN-α, tyrosine-protein kinase ABL1, and IL-17 receptor.
Treatment with IFN-α, which is used for chronic viral hepatitis, has been associated with neuropsychiatric adverse events such as depression, mania, irritability and agitation (Raison et al., 2005).This drug primarily targets the interferon-α/β receptor (IFNAR), a membrane receptor that binds to the intracellular type I IFN, whose activation, in turn, triggers the Janus kinase/signal transducers and activators of transcription (JAK-STAT), mitogen-activated protein kinase (MAPK), and phosphatidylinositol-3 kinase (PI3K) and serine/threonine kinase family (Akt) pathways (Schreiber and Piehler, 2015).The type I IFNs are usually related to anti-viral immune responses, apoptosis, autophagy, and cell differentiation.The IFNAR has largely been associated with PPAEs such as depression and bipolar disorder (BP) through inflammation-induced brain changes.Additionally, IFNAR polymorphisms have been shown to interfere with IFN response, having been associated with inflammatory-induced changes in the brain, besides being identified in depression and bipolar disease (Salvetat et al., 2022).
Tyrosine-protein kinase ABL1, also known as c-Abl, is a cytoplasmic tyrosine kinase involved in many neuronal processes and development of immune cells (Vargas et al., 2018;Chen et al., 2011;Wu and Berg, 2008;Silberman et al., 2008).This tyrosine kinase can be modulated, for example, by blinatumomab (a bispecific T-cell engager immunotherapeutic used in cancer treatment), which has been involved in a high number of PPAEs.Interestingly, the constitutive expression of activated c-Abl in a transgenic mice model of a neurodegenerative disease (i.e., Alzheimer's), was reported to promote reactive gliosis (Schlatterer et al., 2011).Also, aberrant c-Abl activation by the binding of amyloid beta oligomers (AβO) to tyrosine kinase ephrin receptors A4 (EphA4), has been shown to promote dendritic spine elimination in neuron-to-neuron cell cultures (Vargas et al., 2018).c-Abl participates in NLRP3 inflammasome activation, which in turn induces NF-κB expression.The activation of this transcription factor is then crucial for the further expression of proinflammatory mediators in activated microglia, particularly promoting the expression of TNF-α, IL-1β, IL-18, as well as the production of nitric oxide, and mitochondrial ROS/RNS (Lawana et al., 2017).Moreover, the administration of c-Abl inhibitors to previously LPS-activated mouse primary microglia cells resulted in decreased inflammatory response (Lawana et al., 2017), and attenuated cognitive impairments, evidencing c-Abl as a potential therapeutic target for neurodegenerative pathologies, such as AD, by suppressing inflammation (Vargas et al., 2018;Lawana et al., 2017;Lee and Suk, 2018).
Interleukin-17 is a central pro-inflammatory cytokine that regulates inflammatory response in the brain.Additionally, IL-17 negatively modulates adult neurogenesis and neuronal excitability, while IL-17 deletion has been found to reduce the expression of the proinflammatory cytokines IL-1β, IL-6, TNF-α and IFN-γ (Liu et al., 2014;Zabot et al., 2024), being implicated in neurodevelopmental disorders, including AD and autism spectrum disorder (Mills, 2023).In line with these findings, studies in mice models have associated IL-17 increase with memory deficits, with the potential concomitant triggering of AD, while its neutralization prevented synaptic dysregulation and cognitive impairment (Brigas et al., 2021).The human monoclonal antibody brodalumab, which is used in the treatment of autoinflammatory diseases such as rheumatoid arthritis, psoriasis, and multiple sclerosis (Silfvast-Kaiser et al., 2019), primarily targets IL-17 receptor (IL17R).Notably, case reports from a pharmacovigilance database (Andronis et al., 2020), have associated brodalumab-based therapeutics to a high number of psychiatric adverse effects, including suicidal ideation (Mills, 2023).

Cholinergic system
The cholinergic system, comprising acetylcholine (nicotinic and muscarinic) receptors (AChRs) and enzymes like acetylcholinesterase (AChE) or butyrylcholinesterase (BChE), is involved in neuroimmune communication, establishing the connection between the peripheral immune system and the CNS.Most importantly, its dysregulation has been associated with inflammatory and autoimmune diseases (Halder and Lal, 2021).For example, AChE, which can be modulated by rivastigmine (a cholinesterase inhibitor used for the treatment of mild to moderate Alzheimer's disease) has been identified as a candidate target underlying the onset of PPAEs (Walczak-Nowicka and Herbet, 2021).AChE plays an important role in cell adhesion and proliferation (Campanha et al., 2014), terminating impulse transmission at cholinergic synapses through rapid hydrolysis of acetylcholine (ACh) (Reale and Costantini, 2021) and participates in the inflammatory response, apoptosis, and oxidative stress.Moreover, it contributes to the catecholaminergic-cholinergic balance in depressive disorders (Walczak-Nowicka and Herbet, 2021), by influencing acetylcholine binding to nicotinic and muscarinic receptors.Notably, these receptors are present in neurons, glial cells, and peripheral immune cells.Furthermore, AChE activity, and consequently, acetylcholine (ACh) levels, have been associated with the immune response and inflammatory levels.Decreased AChE activity has been correlated with higher levels of antiinflammatory cytokines, such as IL-10, and lower levels of ACh.In contrast, higher AChE activity has been associated with increased proinflammatory IL-1β, IL-18, IL-12, and TNFα (Walczak-Nowicka and Herbet, 2021).
The nicotinic family of cholinergic receptors (nAChRs) plays an important role in neurotransmission release, inflammation, cognition, motivation and reward, attention, learning and memory (Sherafat et al., 2021;Iarkov et al., 2021).Nicotinic receptors are composed of combinations of five α and β subunits, from α2 -α10 and β2 -β4 (Sherafat et al., 2021;Letsinger et al., 2022), and the variety of combinations of subunits assembly plays an important part in the downstream binding of ligands, which are differentially expressed across cell populations (Sherafat et al., 2021).For example, α7β2 nAChRs are expressed at a low density in cholinergic neurons and appear to have increased sensitivity to pathologically relevant amyloid β concentrations (Letsinger et al., 2022).Similarly, the cholinergic anti-inflammatory pathway appears to be mediated by α7, mostly in its homopentameric receptor combination (Vilella et al., 2023;Xia et al., 2022).Of note, the activation of nAChRs by nicotine, followed by later withdrawal of this substance, has been associated with anxiogenic behavior, decreased GFAP astrocyte expression, microglial morphological changes, and increased expression of TNF-α, and IL-1β (Sherafat et al., 2021;Adeluyi et al., 2019).This sensitivity may be associated with receptor combinations comprising α4, α6 and β2 and β3 subunits, whereas α5 subunits appear to mediate the adverse effects of nicotine, which may limit its use (Papke and Lindstrom, 2020).Additionally, nicotine modulates neuroimmune response partly through the α7 receptor (Namba et al., 2020).The nAChRs are modulated, for example, by varenicline, a partial agonist to α4β2 nAChR and a total agonist of α7 nAChRs, that is used as a smoke cessation drug with considerable success.However, suicide ideation has also been highly associated with varenicline treatment (Vilella et al., 2023).
The family of muscarinic cholinergic receptors (mAChRs) belongs to the superfamily of G protein-coupled receptors (GPCRs) and are expressed by glia and neurons.The mAChRs are involved in motor, sensory and behavioral functions, and learning and memory (Wess et al., 2007).Most importantly, their modulation has already been associated with PPAEs (Walczak-Nowicka and Herbet, 2021).Such an example was demonstrated in a mouse model with the mAChR M1 null mutation, which presented normal hippocampal processing, but impaired memory, learning abilities, and social functions (Anagnostaras et al., 2003).Additionally, schizophrenic patients with memory impairment have been shown to express a reduced number of mAChR M1 receptors (Scarr and Dean, 2008).Moreover, decreased expression of mAChRs M2 and M3 have been found in the brains of BD and MDD patients (Gibbons et al., 2009).The mAChRs can be modulated, for example, by scopolamine, a non-selective competitive inhibitor of mAChRs, which has been associated with psychotropic effects such as memory impairment, anxiety, and delirium (Lakstygal et al., 2019).Moreover, different studies that used cholinergic agonists and analysed gene transcription, detected mAChRs expression in leukocytes and lymphocytes, suggesting the involvement of these receptors in immune function regulation (Reale and Costantini, 2021;Kawashima and Fujii, 2000).

Serotonergic system
The serotonergic system is involved in the pathophysiology and treatment of depression and has also been implicated in behavioral and cognitive functions (Vaswani et al., 2003).During development, serotonin is fundamental for brain wiring by influencing the synaptic organization.Noteworthy, serotonergic neurons can regrow and repopulate brain regions after an insult.This system is composed of 14 different serotonin receptors (5-HTRs), which are cell type-and region-specific (Vahid-Ansari and Albert, 2021).
The serotonin 5-HT2A receptors, in particular, are expressed extensively throughout the body, and in both neuronal and glial cells within the CNS (Verkhratsky and Nedergaard, 2018;Barnes et al., 2021).The 5-HT2A is associated with memory and learning (Zhang and Stackman, 2015), neuronal excitability (Barnes et al., 2021), immunity, and mood (Shajib and Khan, 2015).In a rat model, upon treating the mothers with LPS to induce inflammation during pregnancy, it was verified that 5-HT2A was upregulated in the offspring.Additionally, 5-HT2A offspring sensitivity was tested by receptor stimulation with the 5-HTR agonist (±)-1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropan hydrochloride (DOI), being noted that the offsprings from LPS-treat female demonstrated more behavioral changes (e.g., locomotor hyperactivity and increased head twitch), and increased c-Fos expression, when compared to offsprings of progenitors not challenged with LPS.This study established a connection between prenatal immune activation and the development of neuropsychiatric symptoms associated with schizophrenia, as well as with the involvement of the 5-HT2A receptor in neuroimmune modulation (Wischhof et al., 2015).
Paroxetine is a selective serotonin reuptake inhibitor (SSRI) used for MDD, anxiety, obsessive-compulsive disorder (OCD), and panic disorder, preventing serotonin reuptake by the presynaptic neuron, thus promoting a longer exposure of the postsynaptic neurons to higher levels of serotonin.Nevertheless, there is increasing evidence associating paroxetine with a high number of PPAEs.Interestingly, the analysis of a human cohort of elderly patients with MDD demonstrated that different 5-HT2A polymorphisms respond differently to paroxetine, and that such polymorphic differences may result in distinct PPAEs, with different severity (Murphy et al., 2003).

Dopaminergic system
The dopaminergic system is involved in a plethora of functions such as pain control, movement, motivation, mood and emotions, cognitive functions and others (Babić Leko et al., 2021).Dopaminergic neuronal degeneration has been associated with cognitive decline, among other pathologies (Wiart, 2014).Noteworthy, dopamine performs several tasks outside the CNS, including lymphocyte inhibition and suppression of phagocytic cells (e.g.neutrophils) (Wenisch et al., 1996).Dopamine is a catecholamine that plays an important role in the catecholaminergiccholinergic balance, associated with depressive disorders, including BD.This balance is pivotal for brain health, since the increase in ACh can potentially decrease dopamine activity, leading to depressive symptomatology, while an increase in dopamine accompanied by the decrease of ACh is observed in mania episodes (Walczak-Nowicka and Herbet, 2021).Dopamine, produced in dopaminergic neurons, binds to five receptors (D1R -D5R).These receptors, similarly, to mAChRs, are GPCRs, and have been associated with PPAEs development and neuroimmune modulation (Andronis et al., 2020;Babić Leko et al., 2021).
Dopamine receptor 2 (D2R), belongs to the receptor subfamily that also includes D3R and D4R, which inhibit adenylate cyclase and modulate Ca 2+ , through voltage-channel inhibition (McKenna et al., 2002).D2R can be modulated by bupropion, an antidepressant used in MDD and a smoking cessation pharmaceutical and plays an important role in attention.Polymorphisms in D2R have been associated with attention-requiring tasks, with a cohort of schizophrenic patients performing worse than a healthy cohort (Nkam et al., 2017).D2R polymorphisms were also related to initiation deficits in psychotic disorders, after the genotyping of psychotic patients (Lencer et al., 2014).Notably, D2R are present in peripheral immune cells, such as lymphocytes, where they appear to regulate proliferation, differentiation and cell viability (McKenna et al., 2002).
The dopamine receptor (D5R), belongs to the receptor subfamily that includes D1R, which stimulates adenylate cyclase and promotes cyclic AMP (cAMP) formation (McKenna et al., 2002).D5R can be modulated by zuclopenthixol, a first-generation antipsychotic pharmaceutical.In a study with a rat model, the exposure of pregnant mothers to nicotine impacted the offspring's development.The adult offspring demonstrated delayed development, and impaired learning, and an increase in D5R expression was noted (Schneider et al., 2011).Notably, this receptor is expressed by peripheral immune cells, including dendritic cells and Tlymphocytes.As such, D5R has been reported to modulate autoimmune encephalomyelitis, in a dual, opposing manner, depending on the type of stimulated T-cell.D5R expression in naïve CD4 + T-cells promoted Th17 cell activation and further pro-inflammatory phenotype development.Similarly, D5R signaling applied to Tregs strengthened the antiinflammatory functions along with the suppression of proinflammatory activity.This D5R duality points to differential cell expression and inflammatory status throughout disease development (Osorio-Barrios et al., 2018).

Glutamatergic system
The glutamatergic system plays a crucial role in the functioning of the CNS and has been implicated in various physiological and pathological processes of mood disorders (Kugaya and Sanacora, 2005;Niswender and Conn, 2010).Glutamate, the major excitatory neurotransmitter in the brain, acts on a diverse array of receptors and is involved in synaptic plasticity, learning, memory, and neural development (Francis, 2003).The glutamatergic system encompasses a complex network of receptors, transporters, and enzymes that collectively regulate glutamate transmission and maintain excitatory balance within the brain.With over a dozen different glutamate receptor subtypes identified, in neuronal and glial cells, the system exhibits remarkable diversity and specificity in its cellular and regional distribution, contributing to its multifaceted roles in both health and disease (Kettenmann et al., 2011;Haroon et al., 2017).
The glutamate ionotropic receptor N-methyl-D-aspartate (NMDA) type GluN2A, encoded by the GRIN2A gene, is essential for the excitatory synaptic transmission in the CNS (Marwick et al., 2015).Polymorphism associated with this receptor appears to confer susceptibility to bipolar disorder (Zarate Jr. et al., 2010), epilepsy (Pierson et al., 2014) and intellectual disability (Marwick et al., 2015).These studies focused on patients genotyping, along with their families (Zarate Jr. et al., 2010;Pierson et al., 2014), and channel voltage analysis in xenopus laevis oocytes of GRIN2A polymorphisms (Marwick et al., 2015).This receptor can be regulated by memantine, a NMDA antagonist that blocks MG 2+ currents and is used for dementia and AD treatment (Marwick et al., 2015), which has also demonstrated antidepressant effects in humans (Müller, 2014).Memantine has neuroimmune modulatory properties by suppressing nerve growth factor IB (Nur77), which promotes mitochondrial impairment, contributing to neurodegeneration in an in vitro PD model.With Nur77 suppression, memantine promotes neuronal survival, preventing neurodegeneration (Wei et al., 2016).

DNA damage repair mechanisms
As the core unit of genetic information, DNA may experience changes that may cause mutations and originate faulty proteins.To prevent this, DNA damage repair mechanisms are responsible for the removal or replacement of the damaged, or wrongly placed bases, intending to preserve the correct DNA sequence.
Tumor suppressor and transcription factor p53 is involved in a plethora of functions related to genomic integrity maintenance and expression of cell-cycle control, DNA repair and apoptosis-related genes (Ko and Prives, 1996;Almog and Rotter, 1997).Increased levels of p53 were associated with AD and cognitive impairment, in patients' brain tissue, when compared with controls, leading to the conclusion that cognitive decline was potentially promoted by p53-involved neuronal cell death (Cenini et al., 2008).Furthermore, p53 role in neuroimmune modulation includes microglia pro-inflammatory promotion and antiinflammatory response downregulation, as verified in a p53-deficient mouse model, that presented increased expression of the antiinflammatory transcription factor c-Maf (Su et al., 2014).Temozolomide, a chemotherapeutic used to treat glioma, has been associated with p53 modulation (Andronis et al., 2020).This pharmaceutical, when a dose-equivalent treatment was applied in mice, was associated with decreased neurogenesis, with behavioral impact and potential depression development (Egeland et al., 2017).
The ataxia-telangiectasia mutated (ATM) serine/threonine kinase is responsible for the phosphorylation of key proteins that activate DNA checkpoint, promoting cell cycle arrest and either DNA repair or apoptosis.Similarly to p53, ATM may also be modulated by temozolomide (Andronis et al., 2020).In rat-and mouse-extracted hippocampal neurons, ATM was associated with the regulation of GABAergic neuronal neurotransmission, in which reduced levels of ATM produced inhibitory postsynaptic current events, and increased number of GABAergic synapses (Pizzamiglio et al., 2016).This protein potentially impacts neuroimmune modulation since functional ATM absence leads to a childhood neurodegenerative disease characterized by progressive neuronal degeneration, cognitive impairment, cancer susceptibility and immune deficiency.This condition phenotype is also dependent on cytokine profiling, and brain region, whereas LPS-challenge may promote a neuroprotective response, based on TNF-α, or a neurodegenerative one, based on IL-1β.
Other targets of DNA damage repair mechanism and DNA stability have been associated with cognitive impairment (Andronis et al., 2020) such as the above-mentioned c-ABL, and G4 foci and gamma H2A histone family member X (γH2AX), that have been found elevated in lymphocytes of patients with mild cognitive impairment (François et al., 2016).Moreover, single nucleotide polymorphisms (SNP) have been associated with increased susceptibility for mood-related disorders.For example, the SNP rs6023059, in transglutaminase 2 (TGM2), has been associated with estrogen fluctuation and a consequent sensitivity to mood swings and potentially BD (Graae et al., 2012).The polymorphisms rs4753426 and rs794837, of the melatonin MT2 receptor have been shown to affect the potential risk for depressive disorder development (Gałecka et al., 2011).Of note, a systematic review by Kucuker et al. has associated SNPs in DNA repair enzymes with BD and MD (Kucuker et al., 2022).Also, protein kinase Akt1 gene polymorphism rs1130214 and rs3730358, was associated with antidepressant treatment sensitivity in a human cohort analysis (Losenkov et al., 2016).

Epigenetic machinery
Epigenetics represents the study of the mechanisms (e.g., DNA methylation, histone modifications) that modulate gene expression, without tinkering with DNA sequence, in a hereditable, yet reversible manner (Csoka and Szyf, 2009;Kubota et al., 2012).
Histone deacetylases (HDAC) comprise a family of enzymes responsible for the removal of acetyl groups from proteins, not exclusively histones.This change to DNA-binding histones promotes tighter coiling of the DNA, inhibiting gene expression.Andronis and colleagues have indicated that HDAC1, 2, 3, 4, 5, 6, and 8 represent molecular targets of pharmaceuticals associated with PPAEs, and that these HDACs are also linked to immune signaling pathways (Andronis et al., 2020).In line with these findings, HDAC1-dependent deacetylation of histone 3 has been shown to be inhibited in rats exposed to radiotherapy, affecting brain-derived neurotrophic factor (BDNF) promoters and decreasing bdnf mRNA expression.The use of the HDAC inhibitor trichostatin A promoted the rescue of neurogenesis, and alleviated cognitive deficit associated with radiotherapy.This implies that HDAC1 modulates bdnf expression in the CNS, influencing neurogenesis and cognitive functions affected by radiotherapy (Ji et al., 2014).Another study in a rat model for HDAC2 silencing demonstrated that the inhibition of the protein expression improved cognitive deficits (Yuan et al., 2019).Of note, a mouse model for airway inflammation indicated that HDAC2 suppressed IL-17A (Lai et al., 2019)..In a postmortem analysis of subjects with cognitive impairment, AD and controls, HDAC1/ 3 were significantly increased in cognitive impairment and AD (Mahady et al., 2018).Also, in a diabetic rat model and in a human cell line study, HDAC1/ 3 overexpression exacerbated both inflammation and apoptosis (Guo et al., 2019).HDAC4 inhibition in wild-type mice, using siRNA, caused learning and memory impairments (Pardo et al., 2017).HDAC4 participates in the autophagy and vascular inflammatory pathways, by FoxO3a deacetylation (Yang et al., 2018).These studies suggest that cognitive impairments may be modulated with HADC2 and HADC4 inhibition and HDAC1 and HDAC3 overexpression.All these HDACs impact inflammation, while HDAC1 and HDAC3, and HDAC4 also participate in cell death pathways.In a mouse model of LPS-induced inflammation, trichostatin A dampened inflammatory response, where samples presented decreased HDAC2 /5 mRNA together with decreased expression of pro-inflammatory cytokines.This indicated that HDAC2 and HDAC5 potentially participate in the inflammatory response in the CNS, and modulate cognitive functions associated with cytokine expression (Hsing et al., 2015).Similarly, HDAC6 inhibition with the selective inhibitor tubastatin A attenuated LPS-induced neuroinflammation and neuronal loss, which was found to be promoted by p38 MAPK phosphorylation, and TNF-α and IL-6 expression (Song et al., 2019).Inhibition of HDAC8, with the selective inhibitor PCI34051, in a mice model, led to a decrease in the inflammatory molecules -vascular cell adhesion molecule-1 (VCAM-1) and ICAM-1 of angiotensin IIinfused mice (Kee et al., 2019).The examples of pharmaceuticals that modulate HDACs are extensive.For instance, vorinostat and romidepsin (HDAC inhibitors used as anti-cancer pharmaceuticals of T-cell lymphomas), and trichostatin A (an antifungal antibiotic with cell differentiating properties), act on HDACs, interfering with the gene expression status.
DNA topoisomerase IIα, an enzyme important for DNA replication, chromosome condensation, and chromatids separation, has been associated with PPAEs.This enzyme may be modulated, for example, by daunorubicin, an anticancer pharmaceutical used to treat leukaemia and neuroblastoma, associated with psychological symptoms in cancer patients (Zimmermann et al., 2013).A study demonstrated that psychostimulants downregulated elements of the cell cycle and DNA replication machineries, such as DNA topoisomerase Iiα in human fetal astrocytes both from an immortalized cell line and primary culture, compromising cell division (Jackson et al., 2014).Chronic psychostimulant use is associated with cognitive dysfunction and motor impairment, highlighting the potential role of astrocytes, and astrocyte division, as fundamental for cognitive performance (Jackson et al., 2014).

Other molecular targets
There are other pharmaceutical targets that do not fit into any of the above-mentioned categories but that have also been associated with PPAEs.Such an example is the amyloid beta precursor protein (APP), an integral membrane protein that participates in synapse formation and neural plasticity, which can be modulated by daunorubicin.APP is the precursor of amyloid beta peptide (Aβ) (Gallego Villarejo et al., 2022;Frost and Li, 2017), and it is mainly expressed by neurons and astrocytes (Liang et al., 2020).Following a study in an AD mice model and an astrocyte cell line, reactive astrocytes appeared to be autonomously involved in age-dependent APP up-regulation and amyloidogenesis, resulting in Aβ42 production (main amyloid peptide associated with AD).APP may influence immune regulation not only through astrocytes, but also through neurons as the primary inflammatory agents that promote microglia activation and further proinflammatory conditioning from pre-Aβ deposition (Gallego Villarejo et al., 2022;Liang et al., 2020;Orre et al., 2014).
The vitamin D receptor (VDR) is a member of the nuclear receptor family of transcription factors (Eyles, 2021;Qin and Wang, 2019), which can be modulated by alendronate, a pharmaceutical used for osteoporosis prevention and treatment.This receptor is associated with dopaminergic neuronal differentiation and survival (Oo and Burke, 1997), and neuron excitotoxicity prevention (Brewer et al., 2001).Most importantly, it has been associated with the reduction of inflammation (e.g., inhibition of proinflammatory cytokine production, as well as of microglia and astrocyte activation) (Lee et al., 2020;Alessio et al., 2021).In rodents, a lack of VDR has been shown to lead to impaired social behavior and increased anxiety-like traits, denoting its importance (Eyles, 2021;Kalueff et al., 2004).Additionally, a study of a human cohort denoted that VDR polymorphisms may influence depressive symptomatology (Glocke et al., 2013).
The androgen receptor (AR) is localized in the nuclear membrane of cells and acts as a transcription factor that regulates prostate development, upon binding with DNA on androgen response elements.This regulation involves cell cycle progression and proliferation, protein synthesis and cell death (Modi et al., 2016).Dysregulation of AR has been associated with cognitive impairment, hippocampal structural alterations and performance (Raber, 2008), while some AR polymorphisms may introduce autism spectrum disorder susceptibility.
Since autism appears to be highly heritable, a genotyping study of three AR polymorphisms in patients, controls and families was performed, establishing a susceptibility connection between AR, but not a causal one (Henningsson et al., 2009).Notably, AR is expressed by microglia, promoting a neuronal protective effect, in a brain injury stab model (García-Ovejero et al., 2002).Additionally, the pro-inflammatory cytokine TNF-α contains androgen response elements in the promoter region of its gene, therefore, AR can regulate a pro-inflammatory response (McBeth et al., 2015).AR can be modulated by ketoconazole, a CYP3A4 gene transcription inhibitor (Huang et al., 2007) that interferes with adrenal steroid production, reducing anxiety-related behavior in rat models exposed to cat scent (Cohen et al., 2000).
The estrogen receptor (ER) is a G protein-coupled estrogen receptor located in the nuclear membrane of cells that function as transcription factors.This receptor can be found in the brain and has been associated with memory and learning (Carruth and Shahbazi, 2015;Hsu et al., 2018).The ER is highly expressed by microglia and displays a neuroprotective effect against microglia activation and pro-inflammatory cytokines production, preventing dopaminergic neurodegeneration, as verified in a PD mouse model (Guan et al., 2017).The ER can be modulated by raloxifene, a second-generation selective estrogen receptor modulator, indicated in the treatment and prevention of postmenopausal osteoporosis, which has demonstrated cognitive benefits in postmenopausal female patients with schizophrenia (Hsu et al., 2018).

Conclusions and future directions
The onset of pharmaceutical-induced psychiatric and psychological adverse events poses serious concerns regarding patients' health and safety.With the increasing evidence suggesting the impact of neuroimmune modulation on the onset of PPAEs, the need to understand the mechanisms underlying these events has become imperative.
The available literature supports the active involvement of neuroimmune modulation in PPAEs development.Indeed, changes in the function and reactivity of glial cells (i.e., astrocytes, microglia, oligodendrocytes, and oligodendrocyte precursor cells) have been widely associated with neuropsychiatric/psychological disorders, including memory and learning impairment, cognitive dysfunction, anxiety, or depression.Still, neuroimmune modulation is a two-way event, meaning that neurons are also important in the active milieu, and can act as primary inflammatory agents.
Additionally, it is becoming evident that a series of molecular targets of pharmaceuticals that have been extensively associated with a high number of PPAE reports are directly or indirectly involved in immunerelated signaling pathways.Such molecular targets range from the rather "obvious" immune mediators (e.g., cytokine receptors), to targets related to neurotransmission systems, namely the cholinergic (e.g., nicotinic and muscarinic receptors, AChE) and the serotonergic (e.g., 5-HT2A receptor).Of note, in the case of nicotinic receptors, the different possible α and β subunits combinations may translate into distinct signals, with the α7 subunits being the ones most often associated with immune modulation.Moreover, changes in elements of the epigenetic machinery, in particular HDACs, seem to display an extensive impact on mood and cognition, while being associated with neuroimmune modulation.
Considering the implications of these findings, understanding the molecular targets associated with immunomodulation is of paramount importance when developing new pharmaceuticals.By considering the potential impact on neuroimmune modulation, researchers and pharmaceutical companies can work towards preventing the onset of PPAEs.This knowledge can guide the development of safer and more effective medications, minimizing the risks of psychiatric and psychological adverse events and ultimately improving patient outcomes.

Fig. 2 .Fig. 3 .
Fig. 2. Simplified scheme of synaptic pruning (blue cells: neurons; yellow cells: microglia) by microglia.During neuronal network formation, the elimination of inactive, weak, or excessive synapses (neuronal growth conea) is performed by microglia, through the identification of phosphatidylserine in the cellular membrane of to-be-eliminated synapses, by microglial TREM2 complex (b).This signal triggering activates microglia, that proceed to phagocyte the phosphatidylserine-presenting synapses (c).Created with BioRender.com.(For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Fig. 4 .
Fig. 4. Main pathways through which a pharmaceutical may interact with the CNS.A) Small pharmaceuticals (small green triangles) can cross the blood brain barrier (BBB) (dashed orange square) and interact with astrocytes (purple cell, top right, resting state), promoting their activation (high branch density, bottom left) or B) Interact with microglia cells in resting state (yellow cell with high branch density, top), after crossing BBB, promoting their activation (yellow cell with few branches, bottom).In contrast, C) Biological therapies (large dark blue triangles), unable to cross the BBB may interact with peripheral immune cells (dialogue box), promoting their activation and enabling them to pass through the BBB (activated lymphocytes represented in light blue); or D) Act on the gut, influencing the gutbrain axis, and promoting signaling production (signals represented in dark blue) that is sent to the brain.Image created with BioRender.com.(For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)