Activity-dependent peptidergic modulation of the plateau-generating neuron B64 in the feeding network of Aplysia

The Complement of Projection Neurons Activated Determines the Type of Feeding Motor Program in Aplysia

Multiple projection neurons are often activated to initiate behavior. A question that then arises is, what is the unique functional role of each neuron activated? We address this issue in the feeding system of Aplysia. Previous experiments identified a projection neuron [cerebral buccal interneuron 2 (CBI-2)] that can trigger ingestive motor programs but only after it is repeatedly stimulated, i.e., initial programs are poorly defined. As CBI-2 stimulation continues, programs become progressively more ingestive (repetition priming occurs). This priming results, at least in part, from persistent actions of peptide cotransmitters released from CBI-2. We now show that in some preparations repetition priming does not occur. There is no clear seasonal effect; priming and non-priming preparations are encountered throughout the year. CBI-2 is electrically coupled to a second projection neuron, cerebral buccal interneuron 3 (CBI-3). In preparations in which priming does not occur, we show that ingestive activity is generated when CBI-2 and CBI-3 are coactivated. Programs are immediately ingestive, i.e., priming is not necessary, and a persistent state is not induced. Our data suggest that dynamic changes in the configuration of activity can vary and be determined by the complement of projection neurons that trigger activity.

Persistent effects of cyclic adenosine monophosphate are directly responsible for maintaining a neural network state

Network states are often determined by modulators that alter the synaptic and cellular properties of the constituent neurons. Frequently neuromodulators act via second messengers, consequently their effects can persist. This persistence at the cellular/molecular level determines the maintenance of the state at the network level. Here we study a feeding network in Aplysia. In this network, persistent modulation supports the maintenance of an ingestive state, biasing the network to generate ingestive motor programs. Neuropeptides that exert cyclic adenosine monophosphate (cAMP) dependent effects play an important role in inducing the ingestive state. Most commonly, modulatory effects exerted through cAMP signaling are persistent as a consequence of PKA activation. This is not the case in the neurons we study. Instead maintenance of the network state depends on the persistence of cAMP itself. Data strongly suggest that this is a consequence of the direct activation of a cyclic nucleotide gated current.

Peptide Cotransmitters as Dynamic, Intrinsic Modulators of Network Activity

Neurons can contain both neuropeptides and "classic" small molecule transmitters. Much progress has been made in studies designed to determine the functional significance of this arrangement in experiments conducted in invertebrates and in the vertebrate autonomic nervous system. In this review article, we describe some of this research. In particular, we review early studies that related peptide release to physiological firing patterns of neurons. Additionally, we discuss more recent experiments informed by this early work that have sought to determine the functional significance of peptide cotransmission in the situation where peptides are released from neurons that are part of (i.e., are intrinsic to) a behavior generating circuit in the CNS. In this situation, peptide release will presumably be tightly coupled to the manner in which a network is activated. For example, data obtained in early studies suggest that peptide release will be potentiated when behavior is executed rapidly and intervals between periods of neural activity are relatively short. Further, early studies demonstrated that when neural activity is maintained, there are progressive changes (e.g., increases) in the amount of peptide that is released (even in the absence of a change in neural activity). This suggests that intrinsic peptidergic modulators in the CNS are likely to exert effects that are manifested dynamically in an activity-dependent manner. This type of modulation is likely to differ markedly from the modulation that occurs when a peptide hormone is present at a relatively fixed concentration in the blood.

Newly Identified Aplysia SPTR-Gene Family-Derived Peptides: Localization and Function

When individual neurons in a circuit contain multiple neuropeptides, these peptides can target different sets of follower neurons. This endows the circuit with a certain degree of flexibility. Here we identified a novel family of peptides, the Aplysia SPTR-Gene Family-Derived peptides (apSPTR-GF-DPs). We demonstrated apSPTR-GF-DPs, particularly apSPTR-GF-DP2, are expressed in the Aplysia CNS using immunohistochemistry and MALDI-TOF MS. Furthermore, apSPTR-GF-DP2 is present in single projection neurons, e.g., in the cerebral-buccal interneuron-12 (CBI-12). Previous studies have demonstrated that CBI-12 contains two other peptides, FCAP/CP2. In addition, CBI-12 and CP2 promote shortening of the protraction phase of motor programs. Here, we demonstrate that FCAP shortens protraction. Moreover, we show that apSPTR-GF-DP2 also shortens protraction. Surprisingly, apSPTR-GF-DP2 does not increase the excitability of retraction interneuron B64. B64 terminates protraction and is modulated by FCAP/CP2 and CBI-12. Instead, we show that apSPTR-GF-DP2 and CBI-12 increase B20 excitability and B20 activity can shorten protraction. Taken together, these data indicate that different CBI-12 peptides target different sets of pattern-generating interneurons to exert similar modulatory actions. These findings provide the first definitive evidence for SPTR-GF's role in modulation of feeding, and a form of molecular degeneracy by multiple peptide cotransmitters in single identified neurons.

Multifaceted Expression of Peptidergic Modulation in the Feeding System of Aplysia

Neuropeptides are present in species throughout the animal kingdom and generally exert actions that are distinct from those of small molecule transmitters. It has, therefore, been of interest to define the unique behavioral role of this class of substances. Progress in this regard has been made in experimentally advantageous invertebrate preparations. We focus on one such system, the feeding circuit in the mollusc Aplysia. We review research conducted over several decades that played an important role in establishing that peptide cotransmitters are released under behaviorally relevant conditions. We describe how this was accomplished. For example, we describe techniques developed to purify novel peptides, localize them to identified neurons, and detect endogenous peptide release. We also describe physiological experiments that demonstrated that peptides are bioactive under behaviorally relevant conditions. The feeding system is like others in that peptides exert effects that are both convergent and divergent. Work in the feeding system clearly illustrates how this creates potential for behavioral flexibility. Finally, we discuss experiments that determined physiological consequences of one of the hallmark features of peptidergic modulation, its persistence. Research in the feeding system demonstrated that this persistence can change network state and play an important role in determining network output.

Intrinsic and Extrinsic Neuromodulation of Olfactory Processing

Neuromodulation is a ubiquitous feature of neural systems, allowing flexible, context specific control over network dynamics. Neuromodulation was first described in invertebrate motor systems and early work established a basic dichotomy for neuromodulation as having either an intrinsic origin (i.e., neurons that participate in network coding) or an extrinsic origin (i.e., neurons from independent networks). In this conceptual dichotomy, intrinsic sources of neuromodulation provide a “memory” by adjusting network dynamics based upon previous and ongoing activation of the network itself, while extrinsic neuromodulators provide the context of ongoing activity of other neural networks. Although this dichotomy has been thoroughly considered in motor systems, it has received far less attention in sensory systems. In this review, we discuss intrinsic and extrinsic modulation in the context of olfactory processing in invertebrate and vertebrate model systems. We begin by discussing presynaptic modulation of olfactory sensory neurons by local interneurons (LNs) as a mechanism for gain control based on ongoing network activation. We then discuss the cell-class specific effects of serotonergic centrifugal neurons on olfactory processing. Finally, we briefly discuss the integration of intrinsic and extrinsic neuromodulation (metamodulation) as an effective mechanism for exerting global control over olfactory network dynamics. The heterogeneous nature of neuromodulation is a recurring theme throughout this review as the effects of both intrinsic and extrinsic modulation are generally non-uniform.

Use of the Aplysia feeding network to study repetition priming of an episodic behavior

Many central pattern generator (CPG)-mediated behaviors are episodic, meaning that they are not continuously ongoing; instead, there are pauses between bouts of activity. This raises an interesting possibility, that the neural networks that mediate these behaviors are not operating under "steady-state" conditions; i.e., there could be dynamic changes in motor activity as it stops and starts. Research in the feeding system of the mollusk Aplysia californica has demonstrated that this can be the case. After a pause, initial food grasping responses are relatively weak. With repetition, however, responses strengthen. In this review we describe experiments that have characterized cellular/molecular mechanisms that produce these changes in motor activity. In particular, we focus on cumulative effects of modulatory neuropeptides. Furthermore, we relate Aplysia research to work in other systems and species, and develop a hypothesis that postulates that changes in response magnitude are a reflection of an efficient feeding strategy.

Functional consequences of neuropeptide and small-molecule co-transmission

Colocalization of small-molecule and neuropeptide transmitters is common throughout the nervous system of all animals. The resulting co-transmission, which provides conjoint ionotropic ('classical') and metabotropic ('modulatory') actions, includes neuropeptide- specific aspects that are qualitatively different from those that result from metabotropic actions of small-molecule transmitter release. Here, we focus on the flexibility afforded to microcircuits by such co-transmission, using examples from various nervous systems. Insights from such studies indicate that co-transmission mediated even by a single neuron can configure microcircuit activity via an array of contributing mechanisms, operating on multiple timescales, to enhance both behavioural flexibility and robustness.

Activity-dependent increases in [Ca2+]i contribute to digital-analog plasticity at a molluscan synapse

In a type of short-term plasticity that is observed in a number of systems, synaptic transmission is potentiated by depolarizing changes in the membrane potential of the presynaptic neuron before spike initiation. This digital-analog form of plasticity is graded. The more depolarized the neuron, the greater the increase in the efficacy of synaptic transmission. In a number of systems, including the system presently under investigation, this type of modulation is calcium dependent, and its graded nature is presumably a consequence of a direct relationship between the intracellular calcium concentration ([Ca²⁺]_i) and the effect on synaptic transmission. It is therefore of interest to identify factors that determine the magnitude of this type of calcium signal. We studied a synapse in Aplysia and demonstrate that there can be a contribution from currents activated during spiking. When neurons spike, there are localized increases in [Ca²⁺]_i that directly trigger neurotransmitter release. Additionally, spiking can lead to global increases in [Ca²⁺]_i that are reminiscent of those induced by subthreshold depolarization. We demonstrate that these spike-induced increases in [Ca²⁺]_i result from the activation of a current not activated by subthreshold depolarization. Importantly, they decay with a relatively slow time constant. Consequently, with repeated spiking, even at a low frequency, they readily summate to become larger than increases in [Ca²⁺]_i induced by subthreshold depolarization alone. When this occurs, global increases in [Ca²⁺]_i induced by spiking play the predominant role in determining the efficacy of synaptic transmission.NEW & NOTEWORTHY We demonstrate that spiking can induce global increases in the intracellular calcium concentration ([Ca²⁺]_i) that decay with a relatively long time constant. Consequently, summation of the calcium signal occurs even at low firing frequencies. As a result there is significant, persistent potentiation of synaptic transmission.

Specificity of repetition priming: the role of chemical coding

We investigate stimulus specificity of repetition priming in a tractable model system; the feeding network of Aplysia. Previous studies primarily focused on an aspect of behavior that is altered during ingestive priming, radula opening. Priming of radula opening occurs when two modulatory peptides [feeding circuit activating peptide (FCAP) and cerebral peptide-2 (CP-2)] are released from the cholinergic command-like neuron cerebral buccal interneuron 2. Effects of FCAP/CP-2 on radula opening motor neurons are cAMP mediated. The present experiments sought to determine whether FCAP/CP-2 and cAMP are also involved in the priming of radula opening during an incompatible activity, i.e., during egestive motor programs. Egestive priming is induced when motor programs are triggered by afferents with processes in the esophageal nerve. We demonstrate that egestive priming is not FCAP/CP-2 mediated. Instead, it is induced by an unrelated peptide (small cardioactive peptide), which exerts PKC-mediated effects. Our data, therefore, suggest that different feeding motor programs are primed via actions of different sets of intercellular and intracellular substances. We suggest that this accounts for the stimulus specificity that can be characteristic of repetition priming. Different stimuli activate different central pattern generator inputs. These inputs release different modulators, which induce functionally distinct motor programs.

Latent modulation: a basis for non-disruptive promotion of two incompatible behaviors by a single network state

Behavioral states often preferentially enhance specific classes of behavior and suppress incompatible behaviors. In the nervous system, this may involve upregulation of the efficacy of neural modules that mediate responses to one stimulus and suppression of modules that generate antagonistic or incompatible responses to another stimulus. In Aplysia, prestimulation of egestive inputs [esophageal nerve (EN)] facilitates subsequent EN-elicited egestive responses and weakens ingestive responses to ingestive inputs [Cerebral-Buccal Interneuron (CBI-2)]. However, a single state can also promote incompatible behaviors in response to different stimuli. This is the case in Aplysia, where prestimulation of CBI-2 inputs not only enhances subsequent CBI-2-elicited ingestive responses, but also strengthens EN-elicited egestive responses. We used the modularly organized feeding network of Aplysia to characterize the organizational principles that allow a single network state to promote two opposing behaviors, ingestion and egestion, without the two interfering with each other. We found that the CBI-2 prestimulation-induced state upregulates the excitability of neuron B65 which, as a member of the egestive module, increases the strength of egestive responses. Furthermore, we found that this upregulation is likely mediated by the actions of the neuropeptides FCAP (Feeding Circuit Activating Peptide) and CP2 (Cerebral Peptide 2). This increased excitability is mediated by a form of modulation that we refer to as "latent modulation" because it is established during stimulation of CBI-2, which does not activate B65. However, when B65 is recruited into EN-elicited egestive responses, the effects of the latent modulation are expressed as a higher B65 firing rate and a resultant strengthening of the egestive response.

Neuromodulation of neuronal circuits: back to the future

All nervous systems are subject to neuromodulation. Neuromodulators can be delivered as local hormones, as cotransmitters in projection neurons, and through the general circulation. Because neuromodulators can transform the intrinsic firing properties of circuit neurons and alter effective synaptic strength, neuromodulatory substances reconfigure neuronal circuits, often massively altering their output. Thus, the anatomical connectome provides a minimal structure and the neuromodulatory environment constructs and specifies the functional circuits that give rise to behavior.

What can the spinal cord teach us about learning and memory?

The work of recent decades has shown that the nervous system changes continually throughout life. Activity-dependent central nervous system (CNS) plasticity has many different mechanisms and involves essentially every region, from the cortex to the spinal cord. This new knowledge radically changes the challenge of explaining learning and memory and greatly increases the relevance of the spinal cord. The challenge is now to explain how continual and ubiquitous plasticity accounts for the initial acquisition and subsequent stability of many different learned behaviors. The spinal cord has a key role because it is the final common pathway for all behavior and is a site of substantial plasticity. Furthermore, because it is simple, accessible, distant from the rest of the CNS, and directly connected to behavior, the spinal cord is uniquely suited for identifying sites and mechanisms of plasticity and for determining how they account for behavioral change. Experimental models based on spinal cord reflexes facilitate study of the gradual plasticity that makes possible most rapid learning phenomena. These models reveal principles and generate concepts that are likely to apply to learning and memory throughout the CNS. In addition, they offer new approaches to guiding activity-dependent plasticity so as to restore functions lost to injury or disease.

Feedforward compensation mediated by the central and peripheral actions of a single neuropeptide discovered using representational difference analysis

Compensatory mechanisms are often used to achieve stability by reducing variance, which can be accomplished via negative feedback during homeostatic regulation. In principle, compensation can also be implemented through feedforward mechanisms where a regulator acts to offset the anticipated output variation; however, few such neural mechanisms have been demonstrated. We provide evidence that an Aplysia neuropeptide, identified using an enhanced representational difference analysis procedure, implements feedforward compensation within the feeding network. We named the novel peptide "allatotropin-related peptide" (ATRP) because of its similarity to insect allatotropin. Mass spectrometry confirmed the peptide's identity, and in situ hybridization and immunostaining mapped its distribution in the Aplysia CNS. ATRP is present in the higher-order cerebral-buccal interneuron (CBI) CBI-4, but not in CBI-2. Previous work showed that CBI-4-elicited motor programs have a shorter protraction duration than those elicited by CBI-2. Here we show that ATRP shortens protraction duration of CBI-2-elicited ingestive programs, suggesting a contribution of ATRP to the parametric differences between CBI-4-evoked and CBI-2-evoked programs. Importantly, because Aplysia muscle contractions are a graded function of motoneuronal activity, one consequence of the shortening of protraction is that it can weaken protraction movements. However, this potential weakening is offset by feedforward compensatory actions exerted by ATRP. Centrally, ATRP increases the activity of protraction motoneurons. Moreover, ATRP is present in peripheral varicosities of protraction motoneurons and enhances peripheral motoneuron-elicited protraction muscle contractions. Therefore, feedforward compensatory mechanisms mediated by ATRP make it possible to generate a faster movement with an amplitude that is not greatly reduced, thereby producing stability.

Repetition priming of motoneuronal activity in a small motor network: intercellular and intracellular signaling

The characteristics of central pattern generator (CPG) outputs are subject to extensive modulation. Previous studies of neuromodulation largely focused on immediate actions of neuromodulators, i.e., actions that were exerted at the time when either neuromodulators were present or neuromodulatory inputs to the CPG were active. However, neuromodulatory actions are known to persist when neuromodulators are no longer present. In Aplysia, stimulation of cerebral-buccal interneuron-2 (CBI-2), which activates the feeding CPG, produces a repetition priming of motor programs. This priming is reflected in an increase of firing of motoneurons. As CBI-2 contains two neuromodulatory peptides, FCAP (feeding circuit-activating peptide) and CP2 (cerebral peptide 2), we hypothesized that repetition priming may involve persistent peptidergic neuromodulation. We find that these peptides produce priming-like effects, i.e., they increase the firing of radula-opening (B48) and radula-closing (B8) motoneurons during motor programs. Proekt et al. (2004, 2007) showed that repetition priming of neuron B8 is implemented by modulatory inputs that B8 receives from the CPG. In contrast, our current findings indicate that priming of B48 may be implemented by a direct peptidergic modulation of its intrinsic characteristics via a pathway that activates cAMP. We suggest that the direct versus indirect, i.e., CPG-dependent, repetition priming may be related to the type of input that individual motoneurons receive from the CPG. We suggest that in motoneurons that are driven by concurrent excitation-inhibition, repetition priming is indirect as it is preferentially implemented via modulation of the output of CPGs. In contrast, in motoneurons that are driven by alternating excitation-inhibition, direct modulation of motoneurons may be preferentially used.

Evolving concepts of arousal: insights from simple model systems

Arousal states strongly influence behavioral decisions. In general, arousal promotes activity and enhances responsiveness to sensory stimuli. Earlier work has emphasized general, or nonspecific, effects of arousal on multiple classes of behaviors. However, contemporary work indicates that arousal has quite specific effects on behavior. Here we review studies of arousal-related circuitry in molluscan model systems. Neural substrates for both general and specific effects of arousal have been identified. Based on the scope of their actions, we can distinguish two major classes of arousal elements: localized versus general. Actions of localized arousal elements are often limited to one class of behavior, and may thereby mediate specific effects of arousal. In contrast, general arousal elements may influence multiple classes of behaviors, and mediate both specific and nonspecific effects of arousal. One common way in which general arousal elements influence multiple behaviors is by acting on localized arousal elements of distinct networks. Often, effects on distinct networks have different time courses that may facilitate formation of specific behavioral sequences. This review highlights prominent roles of serotonergic systems in arousal that are conserved in gastropod molluscs despite extreme diversification of body forms, diet and ecological niches. The studies also indicate that the serotonergic elements can act as either localized or general arousal elements. We discuss the implications of these findings across animals.

Composite modulatory feedforward loop contributes to the establishment of a network state

Feedforward loops (FFLs) are one of many network motifs identified in a variety of complex networks, but their functional role in neural networks is not well understood. We provide evidence that combinatorial actions of multiple modulators may be organized as FFLs to promote a specific network state in the Aplysia feeding motor network. The Aplysia feeding central pattern generator (CPG) receives two distinct inputs-a higher-order interneuron cerebral-buccal interneuron-2 (CBI-2) and the esophageal nerve (EN)-that promote ingestive and egestive motor programs, respectively. EN stimulation elicits a persistent egestive network state, which enables the network to temporarily express egestive programs following a switch of input from the EN to CBI-2. Previous work showed that a modulatory CPG element, B65, is specifically activated by the EN and participates in establishing the egestive state by enhancing activity of egestion-promoting B20 interneurons while suppressing activity and synaptic outputs of ingestion-promoting B40 interneurons. Here a peptidergic contribution is mediated by small cardioactive peptide (SCP). Immunostaining and mass spectrometry show that SCP is present in the EN and is released on EN stimulation. Importantly, SCP directly enhances activity and synaptic outputs of B20 and suppresses activity and synaptic outputs of B40. Moreover, SCP promotes B65 activity. Thus the direct and indirect (through B65) pathways to B20 and B40 from SCPergic neurons constitute two FFLs with one functioning to promote egestive output and the other to suppress ingestive output. This composite FFL consisting of the two combined FFLs appears to be an effective means to co-regulate activity of two competing elements that do not inhibit each other, thereby contributing to establish specific network states.

Spike integration and cellular memory in a rhythmic network from Na+/K+ pump current dynamics

The output of a neural circuit results from an interaction between the intrinsic properties of neurons within the circuit and the features of the synaptic connections between them. The plasticity of intrinsic properties has been primarily attributed to modification of ion channel function and/or number. In this study, we demonstrate a mechanism for intrinsic plasticity in rhythmically active Drosophila neurons that is not conductance-based. Larval motor neurons show a long lasting sodium-dependent afterhyperpolarization (AHP) following bursts of action potentials that is mediated by the electrogenic activity of Na⁺/K⁺ ATPase. This AHP persists for multiple seconds following volleys of action potentials and is able to function as a pattern-insensitive integrator of spike number that is independent of external calcium. This current also interacts with endogenous Shal K⁺ conductances to modulate spike timing for multiple seconds following rhythmic activity, providing a cellular memory of network activity on a behaviorally relevant time scale.

Motor outputs in a multitasking network: relative contributions of inputs and experience-dependent network states

Network outputs elicited by a specific stimulus may differ radically depending on the momentary network state. One class of networks states-experience-dependent states-is known to operate in numerous networks, yet the fundamental question concerning the relative role that inputs and states play in determining the network outputs remains to be investigated in a behaviorally relevant manner. Because previous work indicated that in the isolated nervous system the motor outputs of the Aplysia feeding network are affected by experience-dependent states, we sought to establish the behavioral relevance of these outputs. We analyzed the phasing of firing of radula opening motoneurons (B44 and B48) relative to other previously characterized motoneurons. We found that the overall pattern of motoneuronal firing corresponds to the phasing of movements during feeding behavior, thus indicating a behavioral relevance of network outputs. Previous studies suggested that network inputs act to trigger a response rather than to shape its characteristics, with the latter function being fulfilled by network states. We show this is an oversimplification. In a rested state, different inputs elicited distinct responses, indicating that inputs not only trigger but also shape the responses. However, depending on the combination of inputs and states, responses were either dramatically altered by the network state or were indistinguishable from those observed in the rested state. We suggest that the relative contributions of inputs and states are dynamically regulated and, rather than being fixed, depend on the specifics of states and inputs.

Presynaptic inhibition selectively weakens peptidergic cotransmission in a small motor system

The presence and influence of neurons containing multiple neurotransmitters is well established, including the ability of coreleased transmitters to influence the same or different postsynaptic targets. Little is known, however, regarding whether presynaptic regulation of multitransmitter neurons influences all transmission from these neurons. Using the identified neurons and motor networks in the crab stomatogastric ganglion, we document the ability of presynaptic inhibition to selectively inhibit peptidergic cotransmission. Specifically, we determine that the gastropyloric receptor (GPR) proprioceptor neuron uses presynaptic inhibition to selectively regulate peptidergic cotransmission from the axon terminals of MCN1, a projection neuron that drives the biphasic (retraction, protraction) gastric mill (chewing) rhythm. MCN1 drives this rhythm via fast GABAergic excitation of the retraction neuron Int1 and slow peptidergic excitation of the lateral gastric (LG) protraction neuron. We first demonstrate that GPR inhibition of the MCN1 axon terminals is serotonergic and then establish that this serotonergic inhibition weakens MCN1 peptidergic excitation of LG without altering MCN1 GABAergic excitation of Int1. At the circuit level, we show that this selective regulation of MCN1 peptidergic cotransmission is necessary for the normal GPR regulation of the gastric mill rhythm. This is the first demonstration, at the level of individual identified neurons, that a presynaptic input can selectively regulate a subset of coreleased transmitters. This selective regulation changes the balance of cotransmitter actions by the target multitransmitter neuron, thereby enabling this neuron to have state-dependent actions on its target network. This finding reveals additional flexibility afforded by the ability of neurons to corelease multiple neurotransmitters.

Parallel regulation of a modulator-activated current via distinct dynamics underlies comodulation of motor circuit output

The cellular mechanisms underlying comodulation of neuronal networks are not elucidated in most systems. We are addressing this issue by determining the mechanism by which a peptide hormone, crustacean cardioactive peptide (CCAP), modulates the biphasic (protraction/retraction) gastric mill (chewing) rhythm driven by the projection neuron MCN1 in the crab stomatogastric ganglion. MCN1 activates this rhythm by slow peptidergic (CabTRP Ia) and fast GABAergic excitation of the reciprocally inhibitory central pattern generator neurons LG (protraction) and Int1 (retraction), respectively. MCN1 synaptic transmission is limited to the retraction phase, because LG inhibits MCN1 during protraction. Bath-applied CCAP also excites both LG and Int1, but selectively prolongs protraction. Here, we use computational modeling and dynamic-clamp manipulations to establish that CCAP prolongs the gastric mill protractor (LG) phase and maintains the retractor (Int1) phase duration by activating the same modulator-activated inward current (I(MI)) in LG as MCN1-released CabTRP Ia. However, the CCAP-activated current (I(MI-CCAP)) and MCN1-activated current (I(MI-MCN1)) exhibit distinct time courses in LG during protraction. This distinction results from I(MI-CCAP) being regulated only by postsynaptic voltage, whereas I(MI-MCN1) is also regulated by LG presynaptic inhibition of MCN1. Hence, without CCAP, retraction and protraction duration are determined by the time course of I(MI-MCN1) buildup and feedback inhibition-mediated decay, respectively, in LG. With I(MI-CCAP) continually present, the impact of the feedback inhibition is reduced, prolonging protraction and maintaining retraction duration. Thus, comodulation of rhythmic motor activity can result from convergent activation, via distinct dynamics, of a single voltage-dependent current.

Neural analog of arousal: persistent conditional activation of a feeding modulator by serotonergic initiators of locomotion

We investigated how a neural analog of a form of arousal induced by a mildly noxious stimulus can promote two antagonistic responses, locomotion and feeding. Two pairs of cerebral serotonergic interneurons in Aplysia, CC9 and CC10, were persistently activated by transient noxious stimuli. Direct stimulation of CC9-10 activated locomotor activity that outlasted the stimulation and enhanced subsequent nerve-evoked locomotor programs. Thus, CC9-10 function both as initiators and as modulators of the locomotor network. CC9-10 also interacted with the feeding circuit but in a fundamentally different manner. CC9-10 did not directly trigger feeding activity or activate feeding command or pattern generating interneurons. CC9-10 did, however, elicit slow EPSPs in serotonergic cells that modulate feeding responses, the metacerebral cells (MCCs). CC9-10 persistently enhanced MCC excitability, but did not activate the MCCs directly. Previous work has demonstrated that the MCCs are activated during food ingestion via a sensory neuron C2. Interestingly, we found that CC9-10 stimulation converted subthreshold C2 mediated excitation of the MCC into suprathreshold excitation. Transient noxious stimuli also enhanced MCC excitability, and this was largely mediated by CC9-10. To summarize, CC9-10 exert actions on the feeding network, but their functional effects appear to be conditional on the presence of food-related inputs to the MCCs. A potential advantage of this arrangement is that it may prevent conflicting responses from being directly evoked by noxious stimuli while also facilitating the ability of food-related stimuli to generate feeding responses in the aftermath of noxious stimulation.

An input-representing interneuron regulates spike timing and thereby phase switching in a motor network

Despite the importance of spike-timing regulation in network functioning, little is known about this regulation at the cellular level. In the Aplysia feeding network, we show that interneuron B65 regulates the timing of the spike initiation of phase-switch neurons B64 and cerebral-buccal interneuron-5/6 (CBI-5/6), and thereby determines the identity of the neuron that acts as a protraction terminator. Previous work showed that B64 begins to fire before the end of protraction phase and terminates protraction in CBI-2-elicited ingestive, but not in CBI-2-elicited egestive programs, thus indicating that the spike timing and phase-switching function of B64 depend on the type of the central pattern generator (CPG)-elicited response rather than on the input used to activate the CPG. Here, we find that CBI-5/6 is a protraction terminator in egestive programs elicited by the esophageal nerve (EN), but not by CBI-2, thus indicating that, in contrast to B64, the spike timing and protraction-terminating function of CBI-5/6 depends on the input to the CPG rather than the response type. Interestingly, B65 activity also depends on the input in that B65 is highly active in EN-elicited programs, but not in CBI-2-elicited programs independent of whether the programs are ingestive or egestive. Notably, during EN-elicited egestive programs, hyperpolarization of B65 delays the onset of CBI-5/6 firing, whereas in CBI-2-elicited ingestive programs, B65 stimulation simultaneously advances CBI-5/6 firing and delays B64 firing, thereby substituting CBI-5/6 for B64 as the protraction terminator. Thus, we identified a neural mechanism that, in an input-dependent manner, regulates spike timing and thereby the functional role of specific neurons.

State dependence of spike timing and neuronal function in a motor pattern generating network

When sustained firing of a neuron is similar in different types of motor programs, its role in the generation of these programs is often similar. We investigated whether this is also the case for neurons involved in phase transition. In the Aplysia feeding central pattern generator (CPG), identified interneuron B64 starts firing at the transition between the protraction and the retraction phases of all types of motor programs, and its firing is sustained during the retraction phase. It was thought that B64 functions as a protraction terminator as it provides strong inhibitory input to protraction interneurons and motoneurons. Furthermore, premature activation of B64 can lead to premature termination of the protraction phase. Indeed, as we show here, B64 can terminate the protraction phase regardless of the type of motor program. However, B64 actually only functions as a protraction terminator in ingestive-like but not in egestive-like programs. This differential role of B64 results from a differential timing of the initiation of B64 spiking in the two types of programs. In turn, this differential timing of the initiation of B64 firing is determined by the internal state of the CPG. Thus, this study indicates the importance of the timing of initiation of firing in determining the functional role of a neuron and demonstrates that this role depends on the activity-dependent state of the network.