Mild traumatic brain injury exacerbates Parkinson's disease induced hemeoxygenase-2 expression and brain pathology: Neuroprotective effects of co-administration of TiO₂ nanowired mesenchymal stem cells and cerebrolysin

Abstract

Mild traumatic brain injury (mTBI) is one of the leading predisposing factors in the development of Parkinson's disease (PD). Mild or moderate TBI induces rapid production of tau protein and alpha synuclein (ASNC) in the cerebrospinal fluid (CSF) and in several brain areas. Enhanced tau-phosphorylation and ASNC alters the molecular machinery of the brain leading to PD pathology. Recent evidences show upregulation of constitutive isoform of hemeoxygenase (HO-2) in PD patients that correlates well with the brain pathology. mTBI alone induces profound upregulation of HO-2 immunoreactivity. Thus, it would be interesting to explore whether mTBI exacerbates PD pathology in relation to tau, ASNC and HO-2 expression. In addition, whether neurotrophic factors and stem cells known to reduce brain pathology in TBI could induce neuroprotection in PD following mTBI. In this review role of mesenchymal stem cells (MSCs) and cerebrolysin (CBL), a well-balanced composition of several neurotrophic factors and active peptide fragments using nanowired delivery in PD following mTBI is discussed based on our own investigation. Our results show that mTBI induces concussion exacerbates PD pathology and nanowired delivery of MSCs and CBL induces superior neuroprotection. This could be due to reduction in tau, ASNC and HO-2 expression in PD following mTBI, not reported earlier. The functional significance of our findings in relation to clinical strategies is discussed.

Introduction

Military personnel are susceptible to Parkinson's disease (PD) due to traumatic mild brain injury (mTBI) induced concussion of varying degree as compared to non-TBI cases (Ascherio and Schwarzschild, 2016; Delic et al., 2020; Gardner et al., 2018; McKee and Robinson, 2014; Taylor et al., 2016; White et al., 2020). Available data suggest that military personnel with mild mTBI exhibited a risk of 54% and with moderate TBI showed 84% risk for developing PD within 3–7 years time (Campdelacreu, 2014; Gardner et al., 2015; Graham et al., 2014; Huang et al., 2018a; LoBue et al., 2019; Nyam et al., 2018; Perry et al., 2016; Taylor et al., 2016; Wong and Hazrati, 2013). These observations suggest that TBI poses a significant risk of developing PD in later life causing additional disability and health problems (Papa et al., 2012; Schiehser et al., 2016). Thus, further studies are extremely warranted to expand our knowledge on factors affecting TBI and PD relationship to develop suitable therapeutic strategies in clinical practice (Schiehser et al., 2016).

Mild TBI induces concussive head injury (CHI) affects military personnel or athletes such as football players each year with long-term progressive neurological symptoms and behavioral disturbances (Fehily and Fitzgerald, 2017; McCrory et al., 2017; VanItallie, 2019; White et al., 2020). CHI is also a growing problem in the elderly with increasing risks for neuropsychiatric abnormalities (Papa et al., 2012; Seabury et al., 2018; Vaishnavi et al., 2009). According to a rough estimate, more than 40 million people are affected by some form of TBI worldwide with neurological consequences every year (Baldwin et al., 2018; Dewan et al., 2018; GBD 2015 Neurological Disorders Collaborator Group, 2017; Iaccarino et al., 2018; Taylor et al., 2017; Vaughn et al., 2019; Winkler et al., 2016). Thus, loss of consciousness for brief periods, psychological disturbances, mood and cognitive alterations and progressive depression episodes are very common in these TBI victims (Choe, 2016; Hon et al., 2019; Jackson and Starling, 2019; Mullally, 2017; Sharp and Jenkins, 2015). Studies show that mild to moderate TBI over a time period of 2–8 years could precipitate in neurodegenerative diseases including PD and Alzheimer's disease (AD) (Gardner et al., 2015; Montenigro et al., 2017; Snyder et al., 2018; Washington et al., 2016). There is a close similarity between TBI, PD and AD with regard to brain pathology (Ganguly et al., 2017; Montagne et al., 2017; Ozkizilcik et al., 2018, Ozkizilcik et al., 2019; Sadlon et al., 2019; Sharma et al., 2016a, Sharma et al., 2018a, Sharma et al., 2018b, Sharma et al., 2019a; Weiner et al., 2014). Leakage of blood-brain barrier (BBB), brain edema, neuronal, glial and axonal damage following TBI, PD and AD are quite common in these victims (Jha et al., 2019; Kaur and Sharma, 2018; Montagne et al., 2017; Sharma et al., 2016b; Sweeney et al., 2018, Sweeney et al., 2019; Zenaro et al., 2017). In addition, deposition of amyloid beta protein (AβP), phosphorylation of tau protein and upregulation of alpha synuclein in the brain and/or CSF occur in several cases of TBI that are hallmark of AD and PD cases (Acosta et al., 2015, Acosta et al., 2019; Bogoslovsky et al., 2017; Castellani and Perry, 2019; Edwards et al., 2017; Goldman et al., 2012; Katsumoto et al., 2019; Mohamed et al., 2019; Okamura et al., 2019; Rubenstein et al., 2017). These observations support the idea of TBI a potential risk factor for both AD and PD.

A discrepancy between clinical data analysis regarding the relationship between TBI and PD in some studies could be due to the magnitude and intensity of TBI and its relationship with PD cases (see (Godbolt et al., 2014; Marras et al., 2014)). In addition, time elapsed after TBI and onset of PD mismatch is also one of the key causes of inconclusive evidences establishing a clear-cut relationship among them. Thus, further studies are needed to explore the inter-relationship between TBI and PD.

So far, there is no suitable drug treatment strategies are available for PD cases effectively. Several drugs to contain PD symptoms or pathophysiology are used in pre-clinical and clinical studies (Armstrong and Okun, 2020; Fox et al., 2018; Tarakad and Jankovic, 2017). However, these drugs have some advantages and disadvantages including serious side effects after their prolonged use. Thus, novel treatment strategies including combination of drugs or nanodelivery of select compounds are needed to enhance superior neuroprotective capability in PD.

This review is focused on novel approaches for treatment strategies using nanomedicine in PD complicated with CHI based on our own investigation. New roles of mesenchymal stem cells (MSCs) in combination with select neurotrophic factors, e.g., cerebrolysin (CBL) following their nanowired delivery seems apparent in inducing superior neuroprotection in PD with CHI. The functional significance of our findings is discussed in the context of current literature in PD.

Traumatic brain injury (TBI) could occur in a variety of populations ranging from military personnel, athletes, boxers, motor vehicle accidents, falls or blunt head trauma (Goldsmith and Plunkett, 2004; Graham et al., 1995; Roozenbeek et al., 2013; Sussman et al., 2018; Tiesman et al., 2011). About 2 million people suffer from TBI from variety of causes annually, which accounts for more than 53 k deaths in the United States of America only (Lo et al., 2020). There could be two main types of TBI that account for either open or perforated skull induced brain injury or closed head injury (CHI) (Kerr, 2013; Lin et al., 2013; Pandey et al., 2018; Schmidt et al., 2005; Sharma et al., 2016a,b; Tabibkhooei et al., 2018; Yamamoto et al., 2018). The degree of CHI could vary from mild to moderate and severe TBI (McKee and Daneshvar, 2015; Pavlovic et al., 2019; Sharma et al., 2016a,b; Zetterberg and Blennow, 2016). Mild CHI could induce concussion that could account for mild to no obvious symptoms or hospital admission. However, even mild or no symptoms persists neural damage, changes in intracranial pressure (ICP), BBB leakage, cerebral edema may continue to develop leading to slowly developing brain pathology (Alla et al., 2012; McKee and Daneshvar, 2015; Servatius et al., 2018).

The chronic consequences of TBI or CHI are often associated with chronic neuroinflammatory or neurodegenerative diseases with cognitive and/or motor problems years after the initial insult (Broussard et al., 2018; Goldstein et al., 2012; Ladak et al., 2019; Schimmel et al., 2017; Witcher et al., 2018; Xiong et al., 2018). The secondary injury processes could lead to oxidative stress, apoptosis, abnormal protein aggregation within neurons or in the extracellular fluid microenvironment, edema and progressive neuronal death (Graham and Sharp, 2019; Johnson et al., 2017; Wilson et al., 2017). Traumatic brain induced chronic secondary injury cascade is thus primarily responsible for the development of several neurodegenerative disorders like AD and/or PD like pathologies (Al-Dahhak et al., 2018; Camacho-Soto et al., 2017; Crane et al., 2016; Daglas and Adlard, 2018; Fiandaca et al., 2018; Jellinger, 2019; Kokiko-Cochran and Godbout, 2018; Ramos-Cejudo et al., 2018).

Clinical studies show that earlier CHI inflicted in military personnel, boxers or athletes are at high risk of developing PD or PD like symptoms several years after the primary insult (Elder et al., 2019; Gardner et al., 2017; Veitch et al., 2013; Weiner et al., 2013, Weiner et al., 2014; White et al., 2020). This is evident from the findings of hallmarks of PD in brain such as progressive degeneration of dopaminergic neurons and upregulation of alpha-synuclein (ASNC) within nerve cells together with inclusion of Lewy bodies in the substantia nigra (SN) (Llorens et al., 2016; Lotankar et al., 2017). ASNC overexpression leads to neuronal damages by disrupting the cell membranes of neurons (Ganjam et al., 2019; Poehler et al., 2014). These observations suggest that ASNC could be the pathological link between TBI and PD (Acosta et al., 2015; Delic et al., 2020; Shahaduzzaman et al., 2013).

Several lines of evidences show that brain or spinal cord injury (SCI) induces upregulation of free radical nitric oxide (NO) probably due to secondary injury induced oxidative stress (Cherian et al., 2004; Conti et al., 2007; Garry et al., 2015; Kimura et al., 2009; Kozlov et al., 2017; Logsdon et al., 2018; Marsala et al., 2007; Sharma et al., 2019b). Up regulation of all the three isoforms of NO synthesizing enzymes nitric oxide synthase (NOS) occurs in TBI, CHI or SCI cases (Dawson and Dawson, 1996). Thus, upregulation of constitutive or neuronal NOS (nNOS), inducible NOS (iNOS) or endothelial NOS (eNOS) are seen following injury to the CNS and in neurodegenerative diseases (Dawson and Dawson, 2018; Sharma, 1998; Sharma and Alm, 2004; Sharma et al., 2006). Drugs modifying NO metabolism or oxidative stress are able to thwart NOS expression in TBI (Sharma, 1998, Sharma, 1999; Sharma and Alm, 2002, Sharma and Alm, 2004).

Upregulation of nNOS and iNOS are also seen in PD brains where substantia nigra, striatum, hippocampus and cerebral cortex showed increased expression of NOS activity as compared to controls (Javier Jimenez-Jimenez et al., 2016; Ozkizilcik et al., 2018, Ozkizilcik et al., 2019; Parlak et al., 2018). Treatment with neuroprotective agents thwarted NOS expression in PD brain (Ozkizilcik et al., 2019; Sharma and Alm, 2004; Tieu et al., 2003). This confirms involvement of NO in the pathophysiology of PD.

Besides NO, another free radical gas carbon monoxide (CO) is also produced within the CNS during injury and in neurodegenerative diseases (Hanafy et al., 2013; Queiroga et al., 2018; Raub and Benignus, 2002; Sharma, 1998). The CO is synthesized by the enzyme hemeoxygenase (HO) that exists in two isoforms namely HO-1 and HO-2 (Boehning and Snyder, 2002; Dawson and Snyder, 1994; Snyder et al., 1998; Verma et al., 1993). The HO-2 is a constitutively expressed enzyme that is upregulated under several noxious insults to the CNS (Boehning et al., 2004; Sharma et al., 1998a; Zakhary et al., 1997). The enzyme HO-1 is inducible and its upregulation is seen following injury to the CNS (Li et al., 2016; Nitti et al., 2018; Qi et al., 2014; Youn et al., 2014). Available evidences suggest that upregulation of HO-1 has endogenous neuroprotective ability (Sharma and Westman, 2003; Sharma et al., 1998a). However, the mechanisms and significance of upregulation of HO-2 expression is not very well known in the CNS (Dore et al., 1999, Dore et al., 2000; Zakhary et al., 1996).

There are reasons to believe that HO-2 expression represent the level of cellular stress and/or injury under a variety of CNS insults (Sharma and Alm, 2004; Sharma and Westman, 2003; Sharma et al., 1998b). Thus, TBI, CHI, SCI, hyperthermia induced brain injury, peripheral nerve lesion induced neuropathic pain all could upregulate HO-2 expression in the brain or spinal cord (Alm et al., 2000; Gordh et al., 2000; Sharma and Westman, 2003, Sharma and Westman, 2003; Sharma et al., 2000a, Sharma et al., 2000b, Sharma et al., 2000c). The areas showing HO-2 expression is associated with pathological changes in the neurons, glia cells and axons (Sharma and Westman, 2003; Sharma et al., 1998b). This suggests that HO-2 expression is injurious to the cells and induce CNS pathology. Pretreatment with antioxidant compounds, or neurotrophic factors significantly attenuated cell injury and HO-2 expressions (Alm et al., 2000; Sharma et al., 2000a, Sharma et al., 2000b, Sharma et al., 2000c).

However, expression of HO system in PD brain is still not well known in all its details (Ozkan et al., 2016). High levels of tissue iron are present in the substantia nigra and basal ganglia in PD patients (Genoud et al., 2020; Hare et al., 2012; Thomas et al., 2020). These abnormally high concentrations of iron are responsible for generation of free radicals, reactive oxygen species (ROS) leading to degeneration of dopaminergic neurons (Cuenca et al., 2019; Manoharan et al., 2016; Puspita et al., 2017; Zuo and Motherwell, 2013). Immunohistochemical studies of postmortem PD brain showed marked upregulation of HO-1 immunostaining in the caudate, putamen, globus pallidus and hippocampus as compared to controls (Schipper et al., 1998; Song et al., 2009; Yamamoto et al., 2010). Interestingly HO-1 expression in substantia nigra pars compacta (SNpC) was not much different that the control group. Dopaminergic neurons, perikarya exhibited moderate HO-1 immunoreactivity in SNpC but the difference was not much distinct from the control group. Although, the HO-1 expression in the neuropil of SNpC is much more pronounced in the PD brain from the controls (Schipper et al., 1998; Song et al., 2009). The other areas of the brain including choroid plexus, ependymal and cerebral endothelial cells showing HO-1 immunostaining were not different between PD and control brain. These observations clearly suggest that HO-1 expression is significantly affected in PD brain (Schipper et al., 1998). However, alterations in HO-2 expression in PD are still not well known and require additional investigation.

Neurotrophic factors are endogenous essential secreted proteins for the development, survival and maintenance of the CNS in healthy conditions (Mocchetti and Wrathall, 1995; Sharma et al., 1998a; Skaper, 2018; Tuszynski and Gage, 1994). A lack or impairment of neurotrophic factors secretion or function leads to neurodegeneration (Blesch, 2006; Dragunow et al., 1997). These neurotrophic factors are also impaired in CNS insults such as TBI, stroke or SCI and other neurodegenerative diseases like AD or PD (Bali et al., 2017; Cui, 2006; Du et al., 2018; Ferreira et al., 2018; Goetzl et al., 2019; Korley et al., 2016; Mattson and Scheff, 1994; Otten et al., 1977; Paul and Sullivan, 2019; Schwab and Bartholdi, 1996; Siegel and Chauhan, 2000; Tanila, 2017; Tome et al., 2017). In brain pathology either caused by acute insults or prolong neurodegeneration alterations in the regulation or function of specific neurotrophins and their receptors lead to cell injury or cell death (Chao et al., 1998; Meldolesi, 2018). Exogenous supplement of specific neurotrophic factors could prevent cell death, thwart neurodegeneration process and promote cell growth (Cai et al., 2014; Chen et al., 2006; Houlton et al., 2019). Neurotrophic factors also help in restoring neuronal functions in several brain disorders (Giacobbo et al., 2019; Numakawa et al., 2018; Xu et al., 2019).

Experiments from our laboratory showed significant improvements in cognitive and motor functions in SCI after topical application of several neurotrophic and growth factors (Sharma, 2005a, Sharma, 2005b, Sharma, 2007a, Sharma, 2007b, Sharma, 2010a; Sharma and Johanson, 2007; Sharma and Sharma, 2012a). These include brain derived neurotrophic factors (BDNF), glial-derived neurotrophic factor (GDNF), insulin like growth factor-1, ciliary neurotrophic factor (CNTF) and nerve growth factor (NGF) (Sharma, 2005a, Sharma, 2005b, Sharma, 2007a, Sharma, 2007b). In addition, a select combination of these neurotrophic factors also induce superior neuroprotection in CNS injury. Accordingly, topical application of these neurotrophic factors on the lesion site either alone or in combination resulted in improvement of cellular connections as well as reduction in cell injury (Sharma and Westman, 2003; Sharma et al., 1998a; Winkler et al., 2000a, Winkler et al., 2000b).

As a follow up, Cerebrolysin (Ever NeuroPharma, Austria) a select combination of neurotrophic factors and active peptide fragments when administered following TBI, CHI, SCI or hyperthermic brain injury (HBI) significant neuroprotection and functional improvement is achieved (Menon et al., 2012; Navrotskaya et al., 2014; Sharma et al., 2011a, Sharma et al., 2011b). Likewise cerebrolysin treatment also reduced PD induced brain pathology and functional impairment (Ozkizilcik et al., 2018, Ozkizilcik et al., 2019). These observations suggest that neurotrophic factors are involved in the pathogenesis of PD and exogenous supplement of neurotrophic factors or cerebrolysin could induce neuroprotection in PD.

It appears that in SCI or TBI neurotrophic factors are able to induce neuroprotection by thwarting the oxidative stress production and upregulation of nNOS and HO-2 expression (Muresanu et al., 2012; Pandey et al., 2012; Sharma and Westman, 2003; Sharma et al., 1998a, Sharma et al., 2009a, Sharma et al., 2015a, Sharma et al., 2018a, Sharma et al., 2018b). This suggests that impairment of neurotrophic factors play important roles in brain pathology induced by injury or chronic neurodegeneration.

Neurotrophic factors alterations also occur in pre-clinical and clinical cases of PD (Huang et al., 2018b; Palasz et al., 2017; Rahmani et al., 2019; Sidorova et al., 2019; Whone et al., 2019; Xu et al., 2018). Decreased neurotrophic factor levels of BDNF, GDNF in substantia nigra in PD is shown that is associated with the pathophysiological outcomes (Anastasia et al., 2011; Huang et al., 2019; Oiwa et al., 2006; Thomsen et al., 2017; Virachit et al., 2019; Wang et al., 2016). BDNF is implicated in the control of central motor system including basal ganglia, cerebellum and brain stem (Gao et al., 2010; He et al., 2013). Deficiency in BDNF in these structures affect neuronal development and survival for normal motor function. In addition, neurotrophic factors are involved in the development and differentiation of cerebellar granule and Purkinje cells as well as survival and maintenance of dopaminergic neurons in the ventral tegmental area and medial substantia nigra pars compacta regions (Blanco-Lezcano et al., 2017; Lindholm et al., 1997; Tsurushima et al., 1993). BDNF through its TrkB receptor signaling inhibits nigrostriatal apoptosis and regulate dopamine D3 receptor function and tyrosine hydroxylase activity (Ding et al., 2011; Scheggi et al., 2020).

BDNF levels and expression are also found decreased in postmortem studies in the SNpC caudate nucleus and putamen in PD brain (Nagatsu and Sawada, 2007; Smith et al., 2019). Decreased levels of BDNF correlate well with the degree of degeneration of dopaminergic neurons in PD. In addition, ASNC overexpression down regulates BDNF transcription and impairs its trafficking in dopaminergic neurons (Sathiya et al., 2013; Yuan et al., 2010).

Apart from BDNF, high upregulation of GDNF occurs in striatum and substantia nigra in postmortem human brain (Hunot et al., 1996). In PD brain severe reduction in GDNF is seen nigra that is 4–6 times greater that decline in BDNF in PD brain. GDNF is about 5–10 times more potent than BDNF in neural survival, differentiation, organization and maintenance (Lu and Hagg, 1997). GDNF receptors are highly expressed in nigral cells but not in striatum indicating its involvement in nigral dopaminergic neurons (Alladi et al., 2010; Sharma et al., 1998b).

Taken together, it appears that BDNF and GDNF are potent neurotrophic factors involved in the cellular mechanisms of PD (Allen et al., 2013; Sun et al., 2005).

British Physician James Parkinson described PD in 1817 as “Shaking Palsy” with signs of motor disease such as postural inability, stiffness, bradykinesia and resting tremor (Kempster et al., 2007; Palacios-Sánchez et al., 2017; Parkinson, 2002). Since then exploration of the cause and therapy of this neurodegenerative disease started world-wide (Cuenca et al., 2019; Fahn, 2008; Obeso et al., 2017; Titova et al., 2017). But so far no great success has been achieved. Thus, in search of new treatment strategies cell therapy appears to be of some importance in neuroregeneration of dopaminergic neurons with symptomatic relief (Kim et al., 2017; Rodríguez-Violante et al., 2018). With regard to cell therapy, mesenchymal stem cells (MSCs) are considered as one of the prominent treatment strategies due to its immunomodulatory and multipotentiality properties (Venkatesh and Sen, 2017; Vilaca-Faria et al., 2019).

Cell therapy using MSCs in PD was first described in 2001 in MPTP induced PD like symptoms in mice (Park et al., 2011). In this experiment MSCs were administered into the striatum and motor behavior using Rota Rod was examined after 30 days. The results showed significant improvement of motor behavior that was correlated with increased tyrosine hydroxylase activity indicating neuroregeneration of dopaminergic cells in MSCs administered mice in PD as compared to controls. Further research showed that MSCs could also enhance autophagic processes of ASNC degradation in PD that was accumulated in neuronal cells in the untreated MPTP induced PD group (Park et al., 2016). These beneficial effects of MSCs therapy in several models of PD were shown by several in vivo and in vitro studies (Bagheri-Mohammadi et al., 2019; Gugliandolo et al., 2017; Mendes Filho et al., 2018).

The success of experimental results in PD on MSCs therapy several clinical trials were initiated from 2009 and onwards (Lee and Park, 2009). In 2010 about 1 million MSCs per kg were transplanted in human PD patients in the sub-lateral ventricular area through stereotaxic surgery. At 10–36 month follow up three out of seven PD patients showed improvement in quality of life with no adverse effects (Venkataramana et al., 2010). After that allogenic MSCs 2 million cells per kg were administered in PD patients in the sub-ventricular zone and followed for 12 months. These patients showed much improvement in United PD Rating Scores (UPDRS) without any serious side effects (Venkatesh and Sen, 2017). Interestingly, patients in early stages of PD showed more improvements with no further progression of the disease (Mendes Filho et al., 2018; Vilaca-Faria et al., 2019).

These studies suggest that MSCs therapy could be the potential neurorestorative strategies in PD in clinics that requires further standardization for routine treatments.

PD is a neurodegenerative disease with a variety of motor and non-motor neurological symptoms due to dopaminergic neurons degeneration in the SNpC and STr (Balestrino and Schapira, 2020; Maiti et al., 2017). Despite its high prevalence and high death rates in population no suitable therapeutic strategies are available so far (Raza et al., 2019).

The current therapies in practice for PD patients include the gold standard treatment with the dopamine precursor l-3,4-dihydroxyphenylalanine (l-DOPA) (Lane, 2019). In addition, dopamine agonist at dopamine D2 receptor Ropinirole, dopamine D2 receptor agonist with partial D1 receptor agonist Bromocriptine, and monoamine oxidase B inhibitors Rasagiline and/or Selegiline are also used as conventional treatment (Clarke and Deane, 2001; Lieberman et al., 1976; Masellis et al., 2016; Riederer and Muller, 2018; Szoko et al., 2018). However, each drug therapy or combination of them has some advantages and disadvantages over the long period of treatment in patients (Borovac, 2016). Continuous use of these drugs has untoward side effects that restrict their use in old and severe PD cases (Mizuno et al., 2018). Thus, exploration of suitable novel therapeutic strategies in treating PD is the need of the hour.

With the advent of nanotechnology for drug delivery in enhancing their penetration into the brain as well as long-term bioavailability with slow metabolism prompted numerous investigation to use nanomedicine in pre-clinical and clinical cases of PD (Broderick and Jacoby, 2017; Ganesan et al., 2015; Hernando et al., 2016; Karthivashan et al., 2020; Leyva-Gomez et al., 2015; Sharma, 2007a, Sharma, 2007b, Sharma, 2007c; Sharma et al., 2016a,b). However, PD treatment is still under investigation in pre-clinical studies using nanomedicine for new and conventional drug delivery (see below).

To attenuate the side effects of l-DOPA, the drug was incorporated into chitosan nanoparticles and delivered intranasally resulting into superior neuroprotection in PD models induced by either MPTP or 6-OHDA administrations (Cao et al., 2016; Di Gioia et al., 2015; Sharma et al., 2014). When nano-l-DOPA was administered in rat model of PD motor co-ordination was improved after 1 week of nanodelivery that continued even after discontinuation of nanomedicine for 1 week. When dopamine was delivered through intranasal route to enhance dopamine level in the STr this was achieved by repeated nanodelivery of drug without any side effects (Tang et al., 2019). This indicates that non-invasive nanodelivery of dopamine to brain is possible in PD cases.

Several investigations using Ropinirole either alone or in combination with l-DOPA to reduce its dosage was tested in animal models of PD through nasal delivery (Zhao et al., 2019). Using chitosan or PLGA nanoparticles through intranasal route was found to be long lasting and efficiently delivered to the brain without any side effects (Bi et al., 2016). Nanodelivered drug has also prolonged actions in brain and reduction in tremor as compared to the conventional drug (Sharma and Sharma, 2012; Sharma and Sharma, 2012b, Sharma and Sharma, 2013). When chitosan coated oil in water emulsion containing Ropinirole was delivered through intranasal route in PD model in rats the nanomedicine was able to enhance muscle co-ordination and swimming ability as compared to conventional Ropinirole administration (Azeem et al., 2012; Mustafa et al., 2015). These studies support the idea that nanodelivery of drugs has superior bioavailability, prolonged action and superior effects in lower doses than their conventional treatment in PD.

The dopamine D2 and D1 antagonist drug bromocriptine was approved as an alone treatment or adjuvant to l-DOPA at early stages of PD cases since 1974 (Calne et al., 1974). However, bromocriptine has a short half-life of about 5 h. When bromocriptine was delivered with chitosan nanoparticles using intranasal delivery in a mouse model of PD its bioavailability has increased into the brain with improved penetration and prevented haloperidol induced PD brain pathology as compared to the drug given alone (Md et al., 2013).

Rasagiline is an irreversible and selective inhibitor of MAO type B that blocks dopamine metabolism (Muller and Mohr, 2019). Rasagiline is often used as single therapy or sometimes as adjuvant with l-DOPA (Rascol et al., 2005). Although Rasagiline is neuroprotective in PD there are side effects that include nausea, dizziness and headache thus limiting its application (Leegwater-Kim and Bortan, 2010). Furthermore, its oral bioavailability is less than 40% with a half-life of about 2 h. When Rasagiline is administered with chitosan nanoparticles through intranasal route its bioavailability has increased with prolonged action and superior beneficial effects in PD (Thebault et al., 2004). These effects of nanodelivery of Rasagiline show promising future approaches to treat PD cases in clinics.

Another MAO type B inhibitor Selegiline when combined with chitosan nanoparticles and administered through intranasal route leads sustained release up to 28 h and improved locomotor activity and increased dopamine and catalase activity in the brain of PD model in the rat as compared to the conventional drug (Mishra et al., 2019; Sridhar et al., 2018). These observations clearly demonstrate superior therapeutic values of nanodelivery of compounds in PD (Ozkizilcik et al., 2018, Ozkizilcik et al., 2019).

Apart from the above drugs, urocortin a corticotrophin releasing factor (CRF) like peptide is neuroprotective in PD through activation of CRF1 receptors (Abuirmeileh et al., 2007a, Abuirmeileh et al., 2007b). Treatment with urocortin attenuates loss of dopaminergic nerve fibers in STr in PD models (Abuirmeileh et al., 2007a). When urocortin was associated with nanoparticles and delivered in rat model of PD it significantly reduced apomorphine induced rotational behavior and rescues dopamine neurodegeneration in nigrostriatal region and restored dopamine metabolites in 6-OHDA model of rat PD (Hu et al., 2011).

Taken together these investigations clearly show that nanodelivery of drugs enhanced their bioavailability, concentration and prolonged effects leading to superior beneficial and neuroprotective effects in PD. However, further studies using nanomedicine approach in PD requires to expand on different kinds of nanoformulation, administration routes and various nanomaterials for drug delivery to find out optimal conditions for clinical use. This is a feature that requires additional investigation.