New insights from small rhythmic circuits

Small rhythmic circuits, such as those found in invertebrates, have provided fundamental insights into how circuit dynamics depend on individual neuronal and synaptic properties. Degenerate circuits are those with different network parameters and similar behavior. New work on degenerate circuits and their modulation illustrates some of the rules that help maintain stable and robust circuit function despite environmental perturbations. Advances in neuropeptide isolation and identification provide enhanced understanding of the neuromodulation of circuits for behavior. The advent of molecular studies of mRNA expression provides new insight into animal-to-animal variability and the homeostatic regulation of excitability in neurons and networks.


Introduction
Many early researchers who wished to understand how circuit dynamics arise from the properties of neurons and their synaptic connections turned to small rhythmic circuits found in invertebrates [1], and this continues today [2**]. While these circuits were initially called 'simple' it became apparent that despite having small numbers of neurons, nothing about them was simple. Indeed, many fundamental principles, now clearly relevant to larger circuits, came first from small, invertebrate circuits. The explosion of new technologies tantalizes us with the hope that the secrets of how larger brain circuits work will reveal themselves. We highlight new insights that are still coming from small circuits of well-identified neurons. Today, as in the past, it is the ability to unambiguously identify neurons, and then establish their connectivity, that is crucial for understanding how a circuit works. A recognizable and well-defined output pattern can be key for interpreting the results of circuit perturbation, so much of what we discuss comes from rhythmically active central pattern generating circuits, with their easily measurable functional outputs.
Space limitations force us to make difficult choices about papers and topics. Notably, we have not treated the large topics of developmental reconfigurations [3*], the use of small circuits in the design of robotic controllers, or much valuable work from Drosophila [4**], C. elegans and other preparations.

Reciprocal inhibition and half-center oscillators
Reciprocal inhibition was one of the earliest circuit elements recognized, both for its role in contrast enhancement in the Limulus retina [5] and for controlling alternating patterns of activity in movement [6]. Here, we focus on work on reciprocal inhibition in rhythmically active invertebrate circuits ( Figure 1) although there is an important literature in the spinal cord of developing and adult vertebrates [7,8].
Reciprocal inhibition is at the core of left-right alternation in many motor systems such as Clione [9], Dendronotus, and Tritonia [10e12] swim circuits and the leech heartbeat system [13e19]. In some systems, the two neurons that participate are copies of the same neuron and can be loosely thought of as 'identical' although no two biological neurons are ever truly identical (Figure 1a). In other instances, more complex circuits have embedded motifs of reciprocal inhibition between neurons of different cell types, and these neurons may make and receive different sets of synaptic inputs (Figure 1b).
Early theoretical work [20] defined two distinct mechanisms that can account for the activity transitions between the two reciprocally inhibitory neurons in a half-center oscillator. In the escape mode the offon transition depends on the inhibited cell depolarizing past its synaptic threshold (Figure 1c). In the 'release' mode the on/off transition depends on the active neuron falling below its synaptic threshold ( Figure 1c). While these two modes can be rigorously distinguished in theoretical work, often transitions show mixed modes of activity [21].
A recent theoretical study is among the first to address the dynamics that can occur in half-center oscillators composed of neurons with different intrinsic properties [22**]. In this study the authors generated a series of model networks with a variety of conductances, characterized their stability, and attempted to find correlation motifs associated with that stability. For example, altering I A and I Cas in opposite directions results in similar effects on circuit stability, and decreasing I H produces losses of rhythmicity. Understanding the relationship between half-center parameters and circuit stability to perturbation is the subject of another recent paper on half-center oscillators [23*]. In this work, the authors used the dynamic clamp to construct half-center oscillators from biological neurons, as has been done previously [24e26] but examined extensively the differential responses of half-centers in release and escape modes to perturbations [23*]. Figure 2 illustrates that the mechanism of oscillation strongly influences the response of the network to perturbation, in this case temperature. Figure 2a compares the responses of dynamic-clamp constructed halfcenters in escape and release modes to a 10 C increase in temperature [23*]. In the left panel, the raw physiological traces show that the temperature change only modestly altered the activity of the circuit in the escape mode, as is seen by the almost invariant cycle frequency illustrated in the spectrogram at the bottom. In contrast, in the right panel, when the circuit is in the release mode, the temperature increased the frequency of the half-center and made its activity more irregular, seen in the spectrogram. Figure 2b contrasts the effects of temperature on two forms of the gastric mill rhythm of the stomatogastric ganglion (STG) [27*,28]. These two forms of the rhythm share the strong reciprocal inhibition between LG and Int1 (see connectivity diagrams) but are activated by stimulation of different descending modulatory neurons. While these two modes of activation are degenerate in the sense that they both activate rhythms characterized by alternation between the DG and LG neurons (Figure 2), they are differentially robust to temperature changes (Figure 2b). At control temperatures they show similar properties (Figure 2b), but the Network based reciprocal inhibition. a) Schematic of a half-center network consisting of two neurons connected by reciprocally inhibitory synapses. Traces in blue and green demonstrate the alternating bursting pattern of activity generated by such networks. b) Half-center oscillators are the building blocks of many CPGs. Top: a simplified circuit diagram of the gastric circuit of Cancer borealis with a half-center oscillator between LG and Int1 neurons forming its core. Intracellular recordings from LG and Int1 showing an alternating bursting pattern of activity. Figure is modified from [106]. Bottom: a circuit diagram of the Tritonia swim CPG, consisting of three types of interneurons: cerebral cell (C2), dorsal swim interneuron (DSI) and ventral swim interneuron (VSI) connected with reciprocal inhibitory and excitatory connections. To the right of the diagram are the intracellular recordings from DSI and VSI showing an alternating bursting pattern of activity. Figure modified from [12]. c) Example traces from half-center oscillators built with dynamic-clamp operating with either escape (top) or release (bottom) mechanism based on differences in the synaptic threshold (V th ) Figure  MCN1 activated rhythm is less robust, and 'crashes' at lower temperatures. There are a number of potential explanations for this: 1) different descending pathways activate circuits operating by different mechanisms [23*] and 2) the strength of the modulatory drive evoked in one pattern of stimulation is significantly higher than the other [29], as we know that activation of a modulatory current can restore oscillatory activity to a release half-center circuit that has lost activity at high temperatures [23*]. Moreover, we know that some neuropeptides can increase the temperature range of stable pyloric rhythm activity [30,31**] and gastric mill activity [32].
Recent work in the leech heartbeat system has focused on the roles of I H and the Na þ /K þ pump on the range of stable alternating half-center patterns of activity [19**]. This work combines computational and experimental data to argue that comodulation of multiple processes is more effective at extending functional operating ranges than modulation of a single current or process.

Additional effects of environmental influences on neurons and circuit mechanisms
The previous section focused on the effects of temperature on half-center driven circuit mechanisms. There is a growing literature on other aspects of the effects of temperature and other environmental influences on small circuits. A recent study documents unexpected blue-light responses of neurons in the crab STG that may allow the animal to be sensitive both to its depth and the time of year [33**]. Stein and Harzsch [34**] provide an excellent review of changes in ocean environments and the effects of these changed environments on the appreciable contribution of marine crustaceans to the earth's biomass. Most notable are the well-known effects of increased sea water temperature and decreased mean ocean pH [34**], with concomitant changes in dissolved O 2 levels. In most cases, the effects of oxygen, temperature, and pH on isolated crustacean circuits have been studied in isolation [35e37], while in the wild, these effects are linked, as pH and oxygen levels vary as a function of ocean temperature [38**]. The obligatory metabolic trade-offs of the biological compensations that occur as animals live close to their temperature limits [38**] highlight the importance of understanding the compensatory mechanisms that neurons and circuits employ to cope with multiple stressors, and the interactions among those multiple stressors. For example, a recent study on the pyloric pacemaker neurons [39*] showed that loss of bursting activity follows different dynamical mechanisms in response to extremes of temperature and pH.
Faria et al. [38**] argue that animals die at extremes of temperature when their metabolic demands become too extreme. The effects of temperature extremes on neuronal and circuit robustness are revealed with in vitro experiments in dissected preparations and continuously exchanged saline [27*,30,37,40,41*]. When the effects of temperature were studied on the pyloric rhythm of crabs, the isolated in vitro and the in vivo rhythms were almost indistinguishable over the temperatures most commonly encountered in the wild, but at higher temperatures, the in vivo and in vitrorecorded rhythms diverged [42]. A recent study, DeMaegd and Stein [41*] studied the effects of temperature on axonal conduction velocity in three identified motor neurons from the crab, C. borealis and showed that temperature has a modest effect on propagation and spike timing in different axons.

Degenerate mechanisms in small circuits
There is a growing literature that suggests that circuits can have degenerate solutions, that is similar looking behavior with different underlying parameters across individuals [12,43,44,45,46,47**,48,49,50**,51*]. While it is often assumed that genetically identical animals produce similar behavior, this turns out not to be invariably the case. There are numerous studies in worms, flies, fish, and mice, that indicate that genetically identical animals show behavioral diversity similar to that shown in wild-caught animals [52*,53,54,55**, 56]. Moreover, repeated performance of the same task is often associated with variable activity in the network generating this task [57**]. New work in Aplysia suggests a plausible set of synaptic mechanisms that can account for some of this variability [57**].
Although degenerate mechanisms exist and can produce similar motor patterns, because of the differences in their underlying parameters, these solutions are differentially sensitive to extreme perturbations such as those described in the previous section [31**,58,59*]. An example of this is seen in a recent study in Aplysia that illustrates that some changes in task switching can only occur from one of the possible, seemingly degenerate network states [51*]. Moreover, evolutionary studies illustrate that similar motor patterns can result from different connectivity patterns and that seemingly similar looking connectivity can result in differences in behavior [60].
Fahoum and Blitz [67**] studied the effects of modulatory neuron activation on switching of neurons between the fast pyloric and slower gastric mill rhythms of the STG of the crab (Figure 3a). Specifically, the LPG neuron switches its participation from exclusively the pyloric rhythm, to being part of the gastric mill rhythm as modulatory inputs are activated. Nonetheless, hyperpolarization of other gastric mill neurons does not prevent this switching, arguing that it does not depend on specific synaptic inputs from other neurons [67**]. Figure 3b) that builds on earlier work [48], and illustrates that the properties of half-center oscillators are strongly influenced by the presence of a slow negative conductance. Moreover, a five-cell circuit with the same architecture as Gutierrez et al. [48] shows increased stability and switching between fast and slow behaviors that depend on the presence of the slow negative conductance gated by modeling neuromodulatory inputs [68*].

Neuropeptide and amine modulation of small circuits
All circuits are subject to neuromodulation. Studies on small circuits have revealed extraordinary richness in modulatory systems and showed that most modulatory neurons release several cotransmitters, including neuropeptides and small molecules [69,70] (Figure 4). One of the challenges in understanding the organization of neuromodulatory systems is to quantitatively characterize the varieties of motor patterns evoked under different modulatory conditions. A new paper [71**] uses unsupervised dimensionality reduction methods to characterize the dynamics of ordered, disordered and modulated STG rhythms ( Figure 4b).
While comodulation is likely the rule rather than the exception in the regulation of many networks, comodulation systems are often difficult to study rigorously. A new study [72**] quantitatively compares the actions of several peptide neuromodulators on synaptic strength and intrinsic excitability. By looking at single and dual applications of two peptides (CCAP and proctolin) on the same target neurons, the authors establish that the actions of the cotransmitters appear to add linearly on the synaptic strengths, but not so when looking at a voltage-dependent intrinsic current [72**].
Many modulators act on voltage-dependent currents, or themselves have voltage-dependent actions. Consequently, modulators may display a number of statedependent actions [73*], including an interaction between the frequency of the action of the target network and the modulator action ( Figure 4c). Figure 4d shows that the effects of a modulator can depend critically on the mechanisms underlying circuit function.
The effects of modulators on the strength of gap junctions are often overlooked, but gap junction regulation is crucial in the retina and in many body organs [74]. The crustacean cardiac ganglion produces synchronous activity that is necessary for a robust heartbeat. The cardiac ganglion is modulated by many amines and peptides [75], two of which are serotonin and dopamine [76*]. While both serotonin and dopamine are generally excitatory, serotonin can desynchronize bursts but dopamine promotes stable bursting, associated with strengthening of the gap junction coupling [76*].
There are hundreds of crustacean neuropeptides [77**, 78*,79,80**,81**], consisting of approximately 20 neuropeptide families, with multiple isoforms in most of these families. Many of these neuropeptides are biologically active. This richness raises several fascinating questions: a) Are the same isoforms released from all presynaptic release sites? b) How many different isoforms are found in a given presynaptic neuron? c) Do different isoforms show differential stability towards degradation and therefore different time courses of action? d) How different are the dose-response curves of different isoforms of the same peptide?
New advances in Mass Spectroscopic Imaging (MSI) [77**] should bring us closer to resolving the first two of these issues. In MSI, a laser beam is used to generate mass spectrometry profiles at specific tissue localizations, and then these spectra can be analyzed to determine accurately which peptides are where in a tissue [77**,80**]. There are many mass spectrometry methods under development, some of which can be combined with traditional microscopy. However, the resolution and 3D reconstructions for peptides are still not as good as can be done with quality confocal techniques with antibodies [77**]. While the highquality visualizations in 3D now available with conventional immunocytochemistry provide excellent anatomical localizations, peptide antibodies are unlikely to adequately distinguish among all isoforms. Thus, the hope for the future is that MSI localization of peptides in anatomical samples will reach the precision of the best light microscopy except in specific cases [82].  In the feeding system of Aplysia, ingestion and egestion are antagonistic behaviors, and fascinating new work argues that persistent effects of cAMP are important for maintaining a persistent network state [90**,91], as the animal switches between these two behaviors. An intriguing study in the Aplysia feeding system suggests a new mechanism for driving a rhythmic behavior that results from organelle-derived intracellular calcium oscillations [92**].

Homeostasis regulation and ion channel correlations
In long-lived animals, be they crabs or humans, the lifetime of proteins is much shorter than the animal's lifetime. Consequently, the proteins in long-lived neurons must be continuously replaced while the animal maintains its characteristic function. There are strong correlations in mRNA expression of ion channel genes in single identified crustacean neurons [102**]. In a fascinating set of experiments, Santin and Schulz [103**] studied the correlated expression of ion channel genes in single PD neurons from the crab STG. They found that silencing the neurons and removing their synaptic and modulatory inputs produced a loss of some of the characteristic correlations in ion channel expression in these neurons but that these correlations were maintained when the neurons were voltage-clamped to their control voltage waveforms. These results extend and confirm earlier studies [104,105], suggesting that the specific patterns of correlated channel expression arise in an ongoing manner from continuous interactions between activity and programs of gene expression.

Conclusions
Small circuits with identified neurons continue to provide significant advantages for understanding how circuit dynamics arise from the properties of individual neurons. Insights from computation, molecular analyses, and biochemistry are supplementing insights from electrophysiology and behavior. Using these systems, one can hope to achieve the time-honored goals of integrating information from intracellular signaling to circuit function to behavior.

Declaration of interest
None. The authors studied synapse numbers and synaptic partners in isogenic flies raised at two different temperatures. They found that temperature has a profound effect on the numbers of synaptic connections in photoreceptor neurons. These wiring differences give rise to robust behaviors in these flies at their rearing temperature demonstrating that circuits can develop in a manner that is adapted to the environment, even when starting with identical genetic blueprints. 4 * * .

References
Ravbar P, Zhang N, Simpson JH: Behavioral evidence for nested central pattern generator control of Drosophila grooming. Elife 2021, 10:e71508. The authors use behavioral experiments in Drosophila to look at the relationships between multiple movements that are part of behavioral sequences in grooming. This study also looks at the effects of temperature on these actions and shows that "nested" CPGS (those called in sequence in a behavior) are influenced to the same degree by temperature.

5.
Hartline HK, . Neuromodulators influence CPG activity by changing several ionic conductances simultaneously. For example, the modulator myomodulin changes both I H and electrogenic Na + /K + pump currents in the leech heart interneuron half-center oscillator (HCO) despite which the HCO remains stable in its presence. The authors modeled the effect of maintaining a negative correlation between I H and the electrogenic Na + /K + pump, mimicking the effects of myomodulin on the network, and found that the model neurons maintain half-center activity when the two currents change together. The paper highlights the potential value of the multimodal action of neuromodulators. . Onasch S, Gjorgjieva J: Circuit stability to perturbations reveals hidden variability in the balance of intrinsic and synaptic conductances. J Neurosci 2020, 40:3186-3202. This theoretical study explored the stability of a large family of halfcenter oscillator circuits composed of non-identical conductancebased model neurons. The authors changed intrinsic conductances either singly or in combination. They classified the ways in which network outputs varied upon changing conductances singly. The occurrence of particular emergent 'stability states' was found to correlate with modifications of specific conductances across families of degenerate starting states. They also characterized the cumulative impact of changing conductances with high or low correlations.

*
. Morozova E, Newstein P, Marder E: Reciprocally inhibitory circuits operating with distinct mechanisms are differently robust to perturbation and modulation. Elife 2022, 11:e74363. In this work the authors use the dynamic clamp to construct half-center oscillators from two Gastric Mill (GM) neurons of the crab, Cancer borealis, that are otherwise uncoupled. The dynamic clamp is used to add a modeled I H and modeled synapses, thus allowing the authors to alter those parameters in the half-center at will. They then directly compared the stability of half-centers in the release and escape modes, as a function of sensitivity to parameters, and to changes in temperature. These experiments highlight the fact that a circuit's dynamical mechanism can significantly alter the way it responds to perturbation. An additional challenge is for multiple temperature-robust oscillators to maintain functional synchrony. The authors demonstrate remarkable conservation of coupling between two oscillatory circuits in the C. borealis STG over a range of temperatures. Their findings hint at mechanisms that operate at an additional level of coordination to maintain temperature-invariant integer coupling in this system. . Alonso LM, Marder E: Temperature compensation in a small rhythmic circuit. Elife 2020, 9, e55470. Temperature affects the conductances and kinetics of all membrane currents, but does so in a manner that depends on the details of the protein's structure. Consequently, each membrane conductance depends differently on temperature, and this raises the question of how neuronal circuits can operate over a range of temperatures. In this paper the authors used genetic algorithms to find pyloric rhythms that were robust over a range of temperatures, and show that these neurons can produce smooth transitions between current mechanisms that facilitate their expression of this robustness. 125887. This is an excellent review of the impact of climate change on the neural systems of marine life. The authors collate data tracking changes in multiple ocean water parameters such as temperature, salinity, pH and dissolved oxygen and consider their effects on marine crustaceans, a group with enormous ecological and economic impact. They present the effects of changing each of these parameters independently on nervous system output and organismal development and discuss the importance of studying the collective impact of multifaceted environmental changes. Physiol 2020, 11:312. The thermal tolerance of a species has contributions from phylogeny and its environment. Temperatures close to an animal's thermal limits impact available oxygen and lactate buildup. The authors studied aerobic and anaerobic metabolic processes and enzyme kinetics at the limits of thermal tolerance of 12 different intertidal crab species collected from temperature zones ranging from tropical to sub-Antarctic. They found that tropical and sub-tropical crabs respond differently to acute temperature stress compared with sub-Antarctic, although oxygen consumption and lactate buildup increased with temperature in all species. 39 * . Ratliff J, Franci A, Marder E, O'Leary T: Neuronal oscillator robustness to multiple global perturbations. Biophys J 2021, 120:1454-1468. A fascinating question is whether robustness to one perturbation implies either a greater robustness to other perturbations, or a trade-off between resilience to different perturbations. Here the authors study the effects of pH and temperature on the pacemaker neurons of the stomatogastric ganglion and find that the pacemaker 'crashes' by different paths in response to extremes of temperature and pH.

*
. DeMaegd ML, Stein W: Temperature-robust activity patterns arise from coordinated axonal Sodium channel properties. PLoS Comput Biol 2020, 16, e1008057. Robust functioning of neuronal output across a wide range of temperatures, despite multiple underlying Q 10 s, has been well-described for the pyloric network of the C. borealis STG. The authors probe the effects of temperature on action potential propagation to test the temperature-robustness of spike timing. They found that temperature has a modest effect on propagation and spike timing in axons that have different physical parameters. From modeling studies they conclude that coordinated changes in sodium channel maximum conductances and activation gate time constants across different temperatures can achieve this result. . Olivares E, Izquierdo EJ, Beer RD: A neuromechanical model of multiple network rhythmic pattern generators for forward locomotion in C. elegans. Front Comput Neurosci 2021, 15, 572339. Pacemakers and stretch receptor feedback are mechanisms that are believed to underlie locomotion control in C. elegans. This computational study explores the possibility of multiple CPGs contributing to locomotion, as an alternative mechanism. They find effective solutions in which gap junctions play a crucial role. 51 * . Wang Y, Weiss KR, Cropper EC: Network degeneracy and the dynamics of task switching in the feeding circuit in Aplysia. J Neurosci 2019, 39:8705-8716. Egestive motor behavior in Aplysia can be elicited through two stimulation paradigms-repetition priming and positive biasing. The authors find that only one method invokes the involvement of an interneuron, B20, and this makes it more difficult to switch to other behaviors (ingestion). Thus, these degenerate circuits can have larger consequences on future behaviors such as task-switching. 52 * . Smith MA, Honegger KS, Turner G, de Bivort B: Idiosyncratic learning performance in flies. Biol Lett 2022, 18, 20210424. This study characterizes individual learning differences amongst isogenic flies. Some flies were better learners than others in many iterations of classical Pavlovian conditioning paradigm, such as to different aversive modalities and different odor cues. Stochastic processes through development can hence lead to significant differences in learning capacities of isogenic individuals. eaba4856. Motor behaviors produced repeatedly vary each time they're performed. The authors address the neuronal underpinnings of variability in motor behavior production within an animal using an Aplysia feeding circuit. They compare two neurons that elicit feeding motor programs, one more variably than the other, and find that a weaker synaptic connection and high synaptic noise drive output variability highlighting circuit-level mechanisms that can underlie variable behaviors. e76579. The ability to classify different network states is a valuable tool for studying network dysfunction. The authors use unsupervised learning techniques to parse a large repository of real-world data from the pyloric network in different conditions to construct maps of different functional regimes and study stereotypies in movements across these states under different conditions. 72 * * . Li X, Bucher D, Nadim F: Distinct co-modulation rules of synapses and voltage-gated currents coordinate interactions of multiple neuromodulators. J Neurosci 2018, 38:8549-8562. Co-modulation is a well-known feature of neural systems. This is a beautiful study of the effects of comodulation on network output. The authors found that comodulation generated simple linear additive effects at the level of synapses but a neuromodulatory current responded in a sub-linear fashion, suggesting the involvement of two opposing intracellular target pathways. The study highlights the complex interactions of modulators that act simultaneously and the difficulties of extrapolating their cumulative effects based on individual analyses. Frequency-dependent action of neuromodulation. eNeuro 2021, 8. ENEURO.0338-21.2021. The effect of a neuromodulatory current on a neuron is dependent on the target neuron's activity. The authors studied this relationship in LP neurons of the C. borealis STG using proctolin to activate a modulatory current, I MI . They found that I MI amplitude and peak time are dependent on pyloric cycle frequency and used voltage ramps with different slopes to uncover two kinetically different currents activated by proctolin. I MI is composed of an additional calcium-permeable fast inactivating current that is activated by positive ramps and is slope-dependent. They further modeled the differential effects of the two I MI components on oscillatory activity. The study demonstrates an important feature of neuromodulator effects, namely their relationship to various features of network activity. e39368. Neuromodulation is a means for networks to achieve flexibility. Underlying degeneracies in intrinsic conductances and circuit configurations can interact with neuromodulation to produce non-uniform effects. The authors examine the effects of two biogenic amines, serotonin and dopamine on the large cells of the C. borealis cardiac ganglion, which are known to have variable maximal conductance values even within an animal. Both modulators impact a K + conductance that is important for maintaining network synchrony, but dopamine has an excitatory effect while serotonin leads to a loss of synchrony. They found that dopamine increases gap junction coupling, potentially increasing synchrony. They demonstrate a novel way for neuromodulators to maintain synchronous output in the face of degeneracy. Mass spectrometric profiling of neuropeptides in Callinectes sapidus during hypoxia stress. ACS Chem Neurosci 2020, 11: 3097-3106. The authors utilize a marine invertebrate, Callinectes sapidus, known to experience and survive a wide range of hypoxia stress to study the impact of changing O 2 environments on neuropeptide families involved in stress responses. They use different mass spectrometry techniques to quantify neuropeptide content in different tissues with various severities of hypoxia and find that each tissue has unique expression profiles under different states of hypoxia. ACS Chem Neurosci 2021, 12:782-798. This study compares the neuropeptides present in the nervous system of the crab, Cancer borealis, in fed and unfed animals. Remarkably, the number of peptides that change in response to feeding is quite large, illustrating that the neuropeptide composition and milieu is not accounted for by a change in only a few feeding related constituents. 81 * * . Hu M, Helfenbein K, Buchberger AR, DeLaney K, Liu Y, Li L:

Neyton J, Trautmann
Exploring the sexual dimorphism of Crustacean neuropeptide expression using Callinectes sapidus as a model organism. J Proteome Res 2021, 20:2739-2750. In this study, the authors document sex differences in neuropeptides in the crab, C. sapidus using mass spectrometry. Obviously, peptides known to play a role in reproduction differ, but also there were a number of sex differences in peptides of other classes. Three members of a peptide family are differentially distributed and elicit differential statedependent responses in a pattern generator-effector system. J Neurophysiol 2018, 119:1767-1781. Neuropeptides come in many isoforms that may exert similar actions through a common receptor. This paper shows that contrary to this belief, members of a peptide family, C-type allatostatins produce different effects on neuronal activity in the cardiac neuromuscular system and are distributed differentially in the American lobster.