Diverse synaptic connections between peptidergic radula mechanoafferent neurons and neurons in the feeding system of Aplysia

Repeated stimulation of feeding mechanoafferents in Aplysia generates responses consistent with the release of food

How does repeated stimulation of mechanoafferents affect feeding motor neurons? Monosynaptic connections from a mechanoafferent population in the Aplysia buccal ganglia to five motor followers with different functions were examined during repeated stimulus trains. The mechanoafferents produced both fast and slow synaptic outputs, which could be excitatory or inhibitory. In contrast, other Aplysia mechanoafferents produce only fast excitation on their followers. In addition, patterns of synaptic connections were different to the different motor followers. Some followers received both fast excitation and fast inhibition, whereas others received exclusively fast excitation. All followers showed strong decreases in fast postsynaptic potential (PSP) amplitude within a stimulus train. Fast and slow synaptic connections were of net opposite signs in some followers but not in others. For one follower, synaptic contacts were not uniform from all subareas of the mechanoafferent cluster. Differences in properties of the buccal ganglia mechanoafferents and other Aplysia mechanoafferents may arise because the buccal ganglia neurons innervate the interior of the feeding apparatus, rather than an external surface, and connect to motor neurons for muscles with different motor functions. Fast connection patterns suggest that these synapses may be activated when food slips, biasing the musculature to release food. The largest slow inhibitory synaptic PSPs may contribute to a delay in the onset of the next behavior. Additional functions are also possible.

Specific Plasticity Loci and Their Synergism Mediate Operant Conditioning

Despite numerous studies examining the mechanisms of operant conditioning (OC), the diversity of OC plasticity loci and their synergism have not been examined sufficiently. In the well-characterized feeding neural circuit of Aplysia, in vivo and in vitro appetitive OC increases neuronal excitability and electrical coupling among several neurons leading to an increase in expression of ingestive behavior. Here, we used the in vitro analog of OC to investigate whether OC reduces the excitability of a neuron, B4, whose inhibitory connections decrease expression of ingestive behavior. We found OC decreased the excitability of B4. This change appeared intrinsic to B4 because it could be replicated with an analog of OC in isolated cultures of B4 neurons. In addition to changes in B4 excitability, OC decreased the strength of B4's inhibitory connection to a key decision-making neuron, B51. The OC-induced changes were specific without affecting the excitability of another neuron critical for feeding behavior, B8, or the B4-to-B8 inhibitory connection. A conductance-based circuit model indicated that reducing the B4-to-B51 synapse, or increasing B51 excitability, mediated the OC phenotype more effectively than did decreasing B4 excitability. We combined these modifications to examine whether they could act synergistically. Combinations including B51 synergistically enhanced feeding. Taken together, these results suggest modifications of diverse loci work synergistically to mediate OC and that some neurons are well suited to work synergistically with plasticity in other loci.SIGNIFICANCE STATEMENT The ways in which synergism of diverse plasticity loci mediate the change in motor patterns in operant conditioning (OC) are poorly understood. Here, we found that OC was in part mediated by decreasing the intrinsic excitability of a critical neuron of Aplysia feeding behavior, and specifically reducing the strength of one of its inhibitory connections that targets a key decision-making neuron. A conductance-based computational model indicated that the known plasticity loci showed a surprising level of synergism to mediate the behavioral changes associated with OC. These results highlight the importance of understanding the diversity, specificity and synergy among different types of plasticity that encode memory. Also, because OC in Aplysia is mediated by dopamine (DA), the present study provides insights into specific and synergistic mechanisms of DA-mediated reinforcement of behaviors.

Background calcium induced by subthreshold depolarization modifies homosynaptic facilitation at a synapse in Aplysia

Some synapses show two forms of short-term plasticity, homosynaptic facilitation, and a plasticity in which the efficacy of transmission is modified by subthreshold changes in the holding potential of the presynaptic neuron. In a previous study we demonstrated a further interactive effect. We showed that depolarizing changes in the presynaptic holding potential can increase the rate at which facilitation occurs. These experiments studied synaptic transmission between an Aplysia sensory neuron (B21) and its postsynaptic follower, the motor neuron (B8). We have also shown that subthreshold depolarizations of B21 produce widespread increases in its [Ca²⁺]_i via activation of a nifedipine-sensitive current. To determine whether it is this change in ‘background’ calcium that modifies synaptic transmission we compared the facilitation observed at the B21-B8 synapse under control conditions to the facilitation observed in nifedipine. Nifedipine had a depressing effect. Other investigators studying facilitation have focused on Ca_res (i.e., the calcium that remains in a neuron after spiking). Our results indicate that facilitation can also be impacted by calcium channels opened before spiking begins.

Robustness, flexibility, and sensitivity in a multifunctional motor control model

Motor systems must adapt to perturbations and changing conditions both within and outside the body. We refer to the ability of a system to maintain performance despite perturbations as "robustness," and the ability of a system to deploy alternative strategies that improve fitness as "flexibility." Different classes of pattern-generating circuits yield dynamics with differential sensitivities to perturbations and parameter variation. Depending on the task and the type of perturbation, high sensitivity can either facilitate or hinder robustness and flexibility. Here we explore the role of multiple coexisting oscillatory modes and sensory feedback in allowing multiphasic motor pattern generation to be both robust and flexible. As a concrete example, we focus on a nominal neuromechanical model of triphasic motor patterns in the feeding apparatus of the marine mollusk Aplysia californica. We find that the model can operate within two distinct oscillatory modes and that the system exhibits bistability between the two. In the "heteroclinic mode," higher sensitivity makes the system more robust to changing mechanical loads, but less robust to internal parameter variations. In the "limit cycle mode," lower sensitivity makes the system more robust to changes in internal parameter values, but less robust to changes in mechanical load. Finally, we show that overall performance on a variable feeding task is improved when the system can flexibly transition between oscillatory modes in response to the changing demands of the task. Thus, our results suggest that the interplay of sensory feedback and multiple oscillatory modes can allow motor systems to be both robust and flexible in a variable environment.

Repetition priming-induced changes in sensorimotor transmission

When a behavior is repeated performance often improves, i.e., repetition priming occurs. Although repetition priming is ubiquitous, mediating mechanisms are poorly understood. We address this issue in the feeding network ofAplysia Similar to the priming observed elsewhere, priming inAplysiais stimulus specific, i.e., it can be either "ingestive" or "egestive." Previous studies demonstrated that priming alters motor and premotor activity. Here we sought to determine whether sensorimotor transmission is also modified. We report that changes in sensorimotor transmission do occur. We ask how they are mediated and obtain data that strongly suggest a presynaptic mechanism that involves changes in the "background" intracellular Ca(2+)concentration ([Ca(2+)]i) in primary afferents themselves. This form of plasticity has previously been described and generated interest due to its potentially graded nature. Manipulations that alter the magnitude of the [Ca(2+)]iimpact the efficacy of synaptic transmission. It is, however, unclear how graded control is exerted under physiologically relevant conditions. In the feeding system changes in the background [Ca(2+)]iare mediated by the induction of a nifedipine-sensitive current. We demonstrate that the extent to which this current is induced is altered by peptides (i.e., increased by a peptide released during the repetition priming of ingestive activity and decreased by a peptide released during the repetition priming of egestive activity). We suggest that this constitutes a behaviorally relevant mechanism for the graded control of synaptic transmission via the regulation of the [Ca(2+)]iin a neuron.

Motor neuronal activity varies least among individuals when it matters most for behavior

How does motor neuronal variability affect behavior? To explore this question, we quantified activity of multiple individual identified motor neurons mediating biting and swallowing in intact, behaving Aplysia californica by recording from the protractor muscle and the three nerves containing the majority of motor neurons controlling the feeding musculature. We measured multiple motor components: duration of the activity of identified motor neurons as well as their relative timing. At the same time, we measured behavioral efficacy: amplitude of grasping movement during biting and amplitude of net inward food movement during swallowing. We observed that the total duration of the behaviors varied: Within animals, biting duration shortened from the first to the second and third bites; between animals, biting and swallowing durations varied. To study other sources of variation, motor components were divided by behavior duration (i.e., normalized). Even after normalization, distributions of motor component durations could distinguish animals as unique individuals. However, the degree to which a motor component varied among individuals depended on the role of that motor component in a behavior. Motor neuronal activity that was essential for the expression of biting or swallowing was similar among animals, whereas motor neuronal activity that was not essential for that behavior varied more from individual to individual. These results suggest that motor neuronal activity that matters most for the expression of a particular behavior may vary least from individual to individual. Shaping individual variability to ensure behavioral efficacy may be a general principle for the operation of motor systems.

Extracellularly identifying motor neurons for a muscle motor pool in Aplysia californica

In animals with large identified neurons (e.g. mollusks), analysis of motor pools is done using intracellular techniques. Recently, we developed a technique to extracellularly stimulate and record individual neurons in Aplysia californica. We now describe a protocol for using this technique to uniquely identify and characterize motor neurons within a motor pool. This extracellular technique has advantages. First, extracellular electrodes can stimulate and record neurons through the sheath, so it does not need to be removed. Thus, neurons will be healthier in extracellular experiments than in intracellular ones. Second, if ganglia are rotated by appropriate pinning of the sheath, extracellular electrodes can access neurons on both sides of the ganglion, which makes it easier and more efficient to identify multiple neurons in the same preparation. Third, extracellular electrodes do not need to penetrate cells, and thus can be easily moved back and forth among neurons, causing less damage to them. This is especially useful when one tries to record multiple neurons during repeating motor patterns that may only persist for minutes. Fourth, extracellular electrodes are more flexible than intracellular ones during muscle movements. Intracellular electrodes may pull out and damage neurons during muscle contractions. In contrast, since extracellular electrodes are gently pressed onto the sheath above neurons, they usually stay above the same neuron during muscle contractions, and thus can be used in more intact preparations. To uniquely identify motor neurons for a motor pool (in particular, the I1/I3 muscle in Aplysia) using extracellular electrodes, one can use features that do not require intracellular measurements as criteria: soma size and location, axonal projection, and muscle innervation. For the particular motor pool used to illustrate the technique, we recorded from buccal nerves 2 and 3 to measure axonal projections, and measured the contraction forces of the I1/I3 muscle to determine the pattern of muscle innervation for the individual motor neurons. We demonstrate the complete process of first identifying motor neurons using muscle innervation, then characterizing their timing during motor patterns, creating a simplified diagnostic method for rapid identification. The simplified and more rapid diagnostic method is superior for more intact preparations, e.g. in the suspended buccal mass preparation or in vivo. This process can also be applied in other motor pools in Aplysia or in other animal systems.

Functional differentiation of a population of electrically coupled heterogeneous elements in a microcircuit

Although electrical coupling is present in many microcircuits, the extent to which it will determine neuronal firing patterns and network activity remains poorly understood. This is particularly true when the coupling is present in a population of heterogeneous, or intrinsically distinct, circuit elements. We examine this question in the Aplysia californica feeding motor network in five electrically coupled identified cells, B64, B4/5, B70, B51, and a newly identified interneuron B71. These neurons exhibit distinct activity patterns during the radula retraction phase of motor programs. In a subset of motor programs, retraction can be flexibly extended by adding a phase of network activity (hyper-retraction). This is manifested most prominently as an additional burst in the radula closure motoneuron B8. Two neurons that excite B8 (B51 and B71) and one that inhibits it (B70) are active during hyper-retraction. Consistent with their near synchronous firing, B51 and B71 showed one of the strongest coupling ratios in this group of neurons. Nonetheless, by manipulating their activity, we found that B51 preferentially acted as a driver of B64/B71 activity, whereas B71 played a larger role in driving B8 activity. In contrast, B70 was weakly coupled to other neurons and its inhibition of B8 counteracted the excitatory drive to B8. Finally, the distinct firing patterns of the electrically coupled neurons were fine-tuned by their intrinsic properties and the largely chemical cross-inhibition between some of them. Thus, the small microcircuit of the Aplysia feeding network is advantageous in understanding how a population of electrically coupled heterogeneous neurons may fulfill specific network functions.

Monitoring changes in the intracellular calcium concentration and synaptic efficacy in the mollusc Aplysia

It has been suggested that changes in intracellular calcium mediate the induction of a number of important forms of synaptic plasticity (e.g., homosynaptic facilitation). These hypotheses can be tested by simultaneously monitoring changes in intracellular calcium and alterations in synaptic efficacy. We demonstrate how this can be accomplished by combining calcium imaging with intracellular recording techniques. Our experiments are conducted in a buccal ganglion of the mollusc Aplysia californica. This preparation has a number of experimentally advantageous features: Ganglia can be easily removed from Aplysia and experiments use adult neurons that make normal synaptic connections and have a normal ion channel distribution. Due to the low metabolic rate of the animal and the relatively low temperatures (14-16 °C) that are natural for Aplysia, preparations are stable for long periods of time. To detect changes in intracellular free calcium we will use the cell impermeant version of Calcium Orange which is easily 'loaded' into a neuron via iontophoresis. When this long wavelength fluorescent dye binds to calcium, fluorescence intensity increases. Calcium Orange has fast kinetic properties and, unlike ratiometric dyes (e.g., Fura 2), requires no filter wheel for imaging. It is fairly photo stable and less phototoxic than other dyes (e.g., fluo-3). Like all non-ratiometric dyes, Calcium Orange indicates relative changes in calcium concentration. But, because it is not possible to account for changes in dye concentration due to loading and diffusion, it can not be calibrated to provide absolute calcium concentrations. An upright, fixed stage, compound microscope was used to image neurons with a CCD camera capable of recording around 30 frames per second. In Aplysia this temporal resolution is more than adequate to detect even a single spike induced alteration in the intracellular calcium concentration. Sharp electrodes are simultaneously used to induce and record synaptic transmission in identified pre- and postsynaptic neurons. At the conclusion of each trial, a custom script combines electrophysiology and imaging data. To ensure proper synchronization we use a light pulse from a LED mounted in the camera port of the microscope. Manipulation of presynaptic calcium levels (e.g. via intracellular EGTA injection) allows us to test specific hypotheses, concerning the role of intracellular calcium in mediating various forms of plasticity.

Nitric oxide as a regulator of behavior: new ideas from Aplysia feeding

Nitric oxide (NO) regulates Aplysia feeding by novel mechanisms, suggesting new roles for NO in controlling the behavior of higher animals. In Aplysia, (1) NO helps maintain arousal when produced by neurons responding to attempts to swallow food; (2) NO biases the motor system to reject and reposition food that resists swallowing; (3) if mechanically resistant food is not successfully swallowed, NO mediates the formation and expression of memories of food inedibility; (4) NO production at rest inhibits feeding, countering the effects of food stimuli exciting feeding. At a cellular level, NO-dependent channels contribute to the resting potential of neurons controlling food finding and food consumption. Increases in L-arginine after animals eat act as a post-feeding inhibitory signal, presumably by modulating NO production at rest. NO also signals non-feeding behaviors that are associated with feeding inhibition. Thus, depending on context, NO may enhance or inhibit feeding behavior. The different functions of NO may reflect the evolution of NO signaling from a response to tissue damage that was then elaborated and used for additional functions. These results suggest that in higher animals (1) elicited and background transmitter release may have similar effects; (2) NO may be produced by neurons without firing, influencing adjacent neurons; (3) background NO production may contribute to a neuron's resting potential; (4) circulating factors affecting background NO production may regulate spatially separated neurons; (5) L-arginine can be used to regulate neural activity; (6) L-arginine may be an effective post-ingestion metabolic signal to regulate feeding.

Immunohistological localization of serotonin in the CNS and feeding system of the stable fly Stomoxys calcitrans L. (Diptera: Muscidae)

Serotonin, or 5-hydroxytryptamine (5-HT), plays critical roles as a neurotransmitter and neuromodulator that control or modulate many behaviors in insects, such as feeding. Neurons immunoreactive (IR) to 5-HT were detected in the central nervous system (CNS) of the larval and adult stages of the stable fly, Stomoxys calcitrans, using an immunohistological technique. The location and pattern of the 5-HT IR neurons are described and compared for these two different developmental stages. Anatomical features of the fly feeding system were analyzed in third instar larvae and adult flies using a combination of histological and immunohistological techniques. In third instar larvae, the cibarial dilator muscles were observed within the cibarial pump skeleton and innervated by 5-HT IR neurons in nerves arising from the brain. There were four pairs of nerves arising from the frontal surface of the larval brain that innervate the cibarial pump muscles, pharynx, and muscles controlling the mouth hooks. A strong serotoninergic innervation of the anterior stomatogastric system was observed, which suggests 5-HT may play a role in the coordination of different phases of food ingestion by larvae. Similarly, many 5-HT IR neurons were found in both the brain and the thoracico-abdominal ganglia in the adult, some of which innervate the cibarial pump dilator muscles and the stomatogastric muscles. This is tnhe first report describing neuromuscular structures of the stable fly feeding system. The results reported here suggest 5-HT may play a critical role in feeding behaviors of stable fly larvae and adults.

Effect of presynaptic membrane potential on electrical vs. chemical synaptic transmission

The growing realization that electrical coupling is present in the mammalian brain has sparked renewed interest in determining its functional significance and contrasting it with chemical transmission. One question of interest is whether the two types of transmission can be selectively regulated, e.g., if a cell makes both types of connections can electrical transmission occur in the absence of chemical transmission? We explore this issue in an experimentally advantageous preparation. B21, the neuron we study, is an Aplysia sensory neuron involved in feeding that makes electrical and chemical connections with other identified cells. Previously we demonstrated that chemical synaptic transmission is membrane potential dependent. It occurs when B21 is centrally depolarized prior to and during peripheral activation, but does not occur if B21 is peripherally activated at its resting membrane potential. In this article we study effects of membrane potential on electrical transmission. We demonstrate that maximal potentiation occurs in different voltage ranges for the two types of transmission, with potentiation of electrical transmission occurring at more hyperpolarized potentials (i.e., requiring less central depolarization). Furthermore, we describe a physiologically relevant type of stimulus that induces both spiking and an envelope of depolarization in the somatic region of B21. This depolarization does not induce functional chemical synaptic transmission but is comparable to the depolarization needed to maximally potentiate electrical transmission. In this study we therefore characterize a situation in which electrical and chemical transmission can be selectively controlled by membrane potential.

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.

Comparative neurobiology of feeding in the opisthobranch sea slug, Aplysia, and the pulmonate snail, Helisoma: evolutionary considerations

The motor systems that generate feeding-related behaviors of gastropod mollusks provide exceptional opportunities for increasing our understanding of neural homologies and the evolution of neural networks. This report examines the neural control of feeding in Helisoma trivolvis, a pulmonate snail that ingests food by rasping or scraping material from the substrate, and Aplysia californica, an opisthobranch sea slug that feeds by using a grasping or seizing motion. Two classes of neurons that are present in the buccal ganglia of both species are considered: (1) clusters of peptidergic mechanoafferent cells that transmit sensory information from the tongue-like radula/odontophore complex to the central motor circuit; and (2) sets of octopamine-immunoreactive interneurons that are intrinsic to the feeding network. We review evidence that suggests homology of these cell types and propose that their roles have been largely conserved in the control of food-scraping and food-grasping consummatory behaviors. We also consider significant differences in the feeding systems of Aplysia and Helisoma that are associated with the existence of radular closure in Aplysia, an action that does not occur in Helisoma. It is hypothesized that a major adaptation in the innervation patterns of analogous, possibly homologous muscles could distinguish the food-scraping versus food-grasping species. It appears that although core CPG elements have been largely conserved in this system, the neuromuscular elements that they regulate have been more evolutionarily labile.

Distinct inhibitory neurons exert temporally specific control over activity of a motoneuron receiving concurrent excitation and inhibition

Recent work suggests that concurrent excitation and inhibition originating in central pattern generators (CPGs) may be used to control rhythmic motoneuronal activity. The specific roles that the inhibition plays in such cases are not well understood, however, in part because of the lack of identification of presynaptic inhibitory neurons. Here we demonstrate that, in the Aplysia feeding CPG, inhibitory inputs may be critical for flexible control of the activity of motoneurons in different forms of behavior. The feeding CPG generates ingestive and egestive motor programs, differing in the high and low activity, respectively, of the motoneuron B8 during the retraction phase of the programs. We show that, during retraction, B8 receives concurrent excitation and inhibition that produces a high-conductance state. The inhibition originates in two types of CPG neurons, B4/5 and B70, that are more active in egestion than ingestion and play a role in suppressing B8 activity during egestion. In turn, the activities of both B4/5 and B70 are suppressed by the ingestion-promoting descending interneuron CBI-3 (for cerebral-buccal interneuron 3). Thus, concurrent excitation and inhibition may be an effective means of controlling motoneuronal activity in a behavior-dependent manner. More detailed analyses reveal, furthermore, that B4/5 and B70 exert complementary actions by acting preferentially in the early and late part of retraction, respectively. Thus, the use of multiple neurons to generate inhibitory inputs to motoneurons that receive concurrent excitation and inhibition brings an additional level of flexibility that allows a temporally specific control of motoneuronal activity within a single phase of motor programs.

Two distinct mechanisms mediate potentiating effects of depolarization on synaptic transmission

Two distinct mechanisms mediate potentiating effects of depolarization on synaptic transmission. Recently there has been renewed interest in a type of plasticity in which a neuron's somatic membrane potential influences synaptic transmission. We study mechanisms that mediate this type of control at a synapse between a mechanoafferent, B21, and B8, a motor neuron that receives chemical synaptic input. Previously we demonstrated that the somatic membrane potential determines spike propagation within B21. Namely, B21 must be centrally depolarized if spikes are to propagate to an output process. We now demonstrate that this will occur with central depolarizations that are only a few millivolts. Depolarizations of this magnitude are not, however, sufficient to induce synaptic transmission to B8. B21-induced postsynaptic potentials (PSPs) are only observed if B21 is centrally depolarized by >or=10 mV. Larger depolarizations have a second impact on B21. They induce graded changes in the baseline intracellular calcium concentration that are virtually essential for the induction of chemical synaptic transmission. During motor programs, subthreshold depolarizations that increase calcium concentrations are observed during one of the two antagonistic phases of rhythmic activity. Chemical synaptic transmission from B21 to B8 is, therefore, likely to occur in a phase-dependent manner. Other neurons that receive mechanoafferent input are electrically coupled to B21. Differential control of spike propagation and chemical synaptic transmission may, therefore, represent a mechanism that permits selective control of afferent transmission to different types of neurons contacted by B21. Afferent transmission to neurons receiving chemical synaptic input will be phase specific, whereas transmission to electrically coupled followers will be phase independent.

Selective spike propagation in the central processes of an invertebrate neuron

Within a neuron, spike propagation can occur in a complex manner, with spikes propagating into some processes but not others. We study this phenomenon in an experimentally advantageous mechanoafferent in Aplysia, neuron B21. B21 has two processes within the CNS. One is ipsilateral to the soma and is referred to as the lateral process. The second travels into the contralateral hemiganglion and is referred to as the contralateral process. Previously we characterized spike propagation to the lateral process, which is an output region that contacts follower motor neurons. Spikes fail to actively propagate to the lateral process when B21 is peripherally activated at its resting potential. This propagation failure can be relieved if the medial regions of B21 are centrally depolarized during peripheral activation. This study examines spike propagation to the contralateral process. We show that, unlike the lateral process, active spike propagation in the contralateral process occurs when B21 is peripherally activated at its resting membrane potential. Thus spike propagation occurs selectively, favoring the contralateral process. Interestingly, the contralateral process of one B21 is immediately adjacent to the medial region of the bilaterally symmetrical cell. The B21 neurons are electrically coupled, suggesting that spikes propagating in the contralateral process of one cell could modify propagation in the sister neuron. Consistent with this idea, we show that lateral process propagation failures observed when a single B21 is peripherally activated can be relieved by central coactivation of the contralateral cell. These results imply that stimuli that coactivate the B21 neurons bilaterally are more apt to generate afferent activity that is transmitted to followers than stimuli that activate one cell.

Feeding CPG in Aplysia directly controls two distinct outputs of a compartmentalized interneuron that functions as a CPG element

In the context of motor program generation in Aplysia, we characterize several functional aspects of intraneuronal compartmentalization in an interganglionic interneuron, CBI-5/6. CBI-5/6 was shown previously to have a cerebral compartment (CC) that includes a soma that does not generate full-size action potentials and a buccal compartment (BC) that does. We find that the synaptic connections made by the BC of CBI-5/6 in the buccal ganglion counter the activity of protraction-phase neurons and reinforce the activity of retraction-phase neurons. In buccal motor programs, the BC of CBI-5/6 fires phasically, and its premature activation can phase advance protraction termination and retraction initiation. Thus the BC of CBI-5/6 can act as an element of the central pattern generator (CPG). During protraction, the CC of CBI-5/6 receives direct excitatory inputs from the CPG elements, B34 and B63, and during retraction, it receives antidromically propagating action potentials that originate in the BC of CBI-5/6. Consequently, in its CC, CBI-5/6 receives depolarizing inputs during both protraction and retraction, and these depolarizations can be transmitted via electrical coupling to other neurons. In contrast, in its BC, CBI-5/6 uses spike-dependent synaptic transmission. Thus the CPG directly and differentially controls the program phases in which the two compartments of CBI-5/6 may transmit information to its targets.

Regulatory roles of Ca2+/calmodulin-dependent protein kinase II and protein phosphatase 2A on the quisqualic acid-induced K+-current response in identified neurons of Aplysia

In identified B6 neurons of Aplysia buccal ganglia under voltage-clamp, application of quisqualic acid (QA) induces a unique slow K(+)-current response independent of G-protein. The response was augmented by raising the temperature in a similar fashion to the Phe-Met-Arg-Phe-NH(2)-induced K(+)-current response mediated by Gi/o. The QA-induced K(+)-current response markedly increased during the perfusion with Ca(2+)-free solution or after the application of W-7, a calmodulin (CaM) inhibitor. It was also enhanced by intracellular application either of H-7, a serine/threonine protein kinase inhibitor, or of KN-93, a Ca(2+)/CaM-dependent kinase II (CaMKII) inhibitor. Furthermore, the QA-induced response was markedly augmented by pre-treatment with 2,3-butanedione monoxime, an inorganic phosphatase. Intracellular application of protein phosphatase 2A (PP2A) significantly augmented the QA-induced response although neither protein phosphatase 1 nor protein phosphatase 2B altered the response. Application of either okadaic acid or calyculin A, protein phosphatase inhibitors, only slightly depressed the QA-induced response. Surprisingly, W-7 had no augmenting effect on the QA-induced response when examined after the application of either okadaic acid or calyculin A. These results suggest that the K(+)-current response is reciprocally but sequentially regulated by PP2A and CaMKII, the response of which the former is facilitating and the latter is inhibiting.

Mechanoafferent Neuron With An Inexcitable Somatic Region: Consequences for the Regulation of Spike Propagation and Afferent Transmission

In the Aplysia mechanoafferent B21, afferent transmission is in part regulated via the control of active spike propagation. When B21 is peripherally activated at its resting membrane potential, spikes fail to propagate to an output process, and afferent transmission does not occur. In this report, we show that the propagation failure is in part a result of the fact that the somatic region of B21 is relatively inexcitable. We isolate this region and demonstrate that net currents evoked by depolarizing pulses are outward. Furthermore, we show that all-or-none spikes are not triggered when current is injected. Previous reports have, however shown that spiking is triggered when current is somatically injected and cells are intact. We demonstrate that spikes evoked under these circumstances do not originate in the soma. Instead they originate in an adjacent part of the neuron that is excitable (the medial process). In summary, we show that the mechanoafferent B21 consists of excitable input and output processes separated by a relatively inexcitable somatic region. A potential advantage of this arrangement is that somatic depolarization can be used to modify spike propagation from the input to the output processes without altering the encoding of peripherally generated activity.

Mechanical reconfiguration mediates swallowing and rejection in Aplysia californica

Muscular hydrostats, such as tongues, trunks or tentacles, have fewer constraints on their degrees of freedom than musculoskeletal systems, so changes in a structure's shape may alter the positions and lengths of other components (i.e., induce mechanical reconfiguration). We studied mechanical reconfiguration during rejection and swallowing in the marine mollusk Aplysia californica. During rejection, inedible material is pushed out of an animal's buccal cavity. The grasper (radula/odontophore) closes on inedible material, and then a posterior muscle, I2, pushes the grasper toward the jaws (protracts it). After the material is released, an anterior muscle complex (the I1/I3/jaw complex) pushes the grasper toward the esophagus (retracts it). During swallowing, the grasper is protracted open, and then retracts closed, pulling in food. Grasper closure changes its shape. Magnetic resonance images show that grasper closure lengthens I2. A kinetic model quantified the changes in the ability of I2 and I1/I3 to exert force as grasper shape changed. Grasper closure increases I2's ability to protract during rejection, and increases I1/I3's ability to retract during swallowing. Motor neurons controlling radular closure may therefore affect the behavioral outputs of I2's and I1/I3's motor neurons. Thus, motor neurons may modulate the outputs of other motor neurons through mechanical reconfiguration.

Proprioceptor regulation of motor circuit activity by presynaptic inhibition of a modulatory projection neuron

Phasically active sensory systems commonly influence rhythmic motor activity via synaptic actions on the relevant circuit and/or motor neurons. Using the crab stomatogastric nervous system (STNS), we identified a distinct synaptic action by which an identified proprioceptor, the gastropyloric muscle stretch receptor (GPR) neuron, regulates the gastric mill (chewing) motor rhythm. Previous work showed that rhythmically stimulating GPR in a gastric mill-like pattern, in the isolated STNS, elicits the gastric mill rhythm via its activation of two identified projection neurons, modulatory commissural neuron 1 (MCN1) and commissural projection neuron 2, in the commissural ganglia. Here, we determine how activation of GPR with a behaviorally appropriate pattern (active during each gastric mill retractor phase) influences an ongoing gastric mill rhythm via actions in the stomato gastric ganglion, where the gastric mill circuit is located. Stimulating GPR during each retractor phase selectively prolongs that phase and thereby slows the ongoing rhythm. This selective action on the retractor phase results from two distinct GPR actions. First, GPR presynaptically inhibits the axon terminals of MCN1, reducing MCN1 excitation of all gastric mill neurons. Second, GPR directly excites the retractor phase neurons. Because MCN1 transmitter release occurs during each retractor phase, these parallel GPR actions selectively reduce the buildup of excitatory drive to the protractor phase neurons, delaying each protractor burst. Thus, rhythmic proprioceptor feedback to a motor circuit can result from a global reduction in excitatory drive to that circuit, via presynaptic inhibition, coupled with a phase-specific excitatory input that prolongs the excited phase by delaying the onset of the subsequent phase.

Responses of cerebral GABA-containing CBM neuron to taste stimulation with seaweed extracts in Aplysia kurodai

Aplysia kurodai distributed along Japan feeds well on Ulva pertusa but rejects Gelidium amansii with distinctive patterned movements of the jaws and radula. On the ventral side of the cerebral M cluster, four cell bodies of higher order neurons that send axons to the buccal ganglia are distributed (CBM neurons). We have previously shown that the dopaminergic CBM1 modulates basic feeding circuits in the buccal ganglia for rejection by firing at higher frequency after application of the aversive taste of seaweed such as Gelidium amansii. In the present experiments immunohistochemical techniques showed that the CBM3 exhibited gamma-aminobutyric acid (GABA)-like immunoreactivity. The CBM3 may be equivalent to the CBI-3 involved in changing the motor programs from rejection to ingestion in Aplysia californica. The responses of the CBM3 to taste stimulation of the lips with seaweed extracts were investigated by the use of calcium imaging. The calcium-sensitive dye, Calcium Green-1, was iontophoretically introduced into a cell body of the CBM3 using a microelectrode. Application of Ulva pertusa or Gelidium amansii extract induced different changes in fluorescence in the CBM3 cell body, indicating that taste of Ulva pertusa initially induced longer-lasting continuous spike responses at slightly higher frequency compared with that of Gelidium amansii. Considering a role of the CBM3 in the pattern selection, these results suggest that elongation of the initial firing response may be a major factor for the CBM3 to switch the buccal motor programs from rejection to ingestion after application of different tastes of seaweeds in Aplysia kurodai.

Rapid Dopaminergic Signaling by Interneurons That Contain Markers for Catecholamines and GABA in the Feeding Circuitry of Aplysia

Consummatory feeding behaviors in Aplysia californica are controlled by a polymorphic central pattern generator (CPG) circuit. Previous investigations have demonstrated colocalization of markers for GABA and catecholamines within two interneurons, B20 and B65, that participate in configuring the functional output of this CPG. This study examined the contributions of GABA and dopamine (DA) to rapid synaptic signaling from B20 and B65 to follower cells that implement their specification of motor programs. Pharmacological tests did not substantiate the participation of GABA in the mediation of the excitatory postsynaptic potentials (EPSPs) from either B20 or B65. However, GABA and the GABAB receptor agonist baclofen were found to modify these signals in a target-specific manner. Several observations indicated that DA acts as the neurotransmitter mediating fast EPSPs from B20 to two radula closer motor neurons B8 and B16. In both motor neurons, application of DA produced depolarizing responses associated with decreased input resistance and increased excitation. B20-evoked EPSPs in both follower cells were occluded by exogenous dopamine and blocked by the DA antagonist sulpiride. While dopamine occlusion and sulpiride block of convergent signaling to B8 from B65 resembled that of B20, both of these actions were less potent on the rapid signaling from B65 to the multifunctional and widely acting interneuron B4/5. These findings indicate that dopamine mediates divergent (B20 to B16 and B8) and convergent (B20 and B65 to B8) rapid EPSPs from two influential CPG interneurons in which it is colocalized with GABA-like immunoreactivity.

Regulation of afferent transmission in the feeding circuitry of Aplysia

Although feeding in Aplysia is mediated by a central pattern generator (CPG), the activity of this CPG is modified by afferent input. To determine how afferent activity produces the widespread changes in motor programs that are necessary if behavior is to be modified, we have studied two classes of feeding sensory neurons. We have shown that afferent-induced changes in activity are widespread because sensory neurons make a number of synaptic connections. For example, sensory neurons make monosynaptic excitatory connections with feeding motor neurons. Sensori-motor transmission is, however, regulated so that changes in the periphery do not disrupt ongoing activity. This results from the fact that sensory neurons are also electrically coupled to feeding interneurons. During motor programs sensory neurons are, therefore, rhythmically depolarized via central input. These changes in membrane potential profoundly affect sensori-motor transmission. For example, changes in membrane potential alter spike propagation in sensory neurons so that spikes are only actively transmitted to particular output regions when it is behaviorally appropriate. To summarize, afferent activity alters motor output because sensory neurons make direct contact with motor neurons. Sensori-motor transmission is, however, centrally regulated so that changes in the periphery alter motor programs in a phase-dependent manner.

Inhibition of afferent transmission in the feeding circuitry of aplysia: persistence can be as important as size

We are studying afferent transmission from a mechanoafferent, B21, to a follower, B8. During motor programs, afferent transmission is regulated so that it does not always occur. Afferent transmission is eliminated when spike propagation in B21 fails, i.e., when spike initiation is inhibited in one output region-B21's lateral process. Spike initiation in the lateral process is inhibited by the B52 and B4/5 cells. Individual B52 and B4/5-induced inhibitory postsynaptic potentials (IPSPs) in B21 differ. For example, the peak amplitude of a B4/5-induced IPSP is four times the amplitude of a B52 IPSP. Nevertheless, when interneurons fire in bursts at physiological (i.e., low) frequencies, afferent transmission is most effectively reduced by B52. Although individual B52-induced IPSPs are small, they have a long time constant and summate at low firing frequencies. Once IPSPs summate, they effectively block afferent transmission. In contrast, individual B4/5-induced IPSPs have a relatively short time constant and do not summate at low frequencies. B52 and B4/5 therefore differ in that once synaptic input from B52 becomes effective, afferent transmission is continuously inhibited. In contrast, periods of B4/5-induced inhibition are interspersed with relatively long intervals in which inhibition does not occur. Consequently, the probability that afferent transmission will be inhibited is low. In conclusion, it is widely recognized that afferent transmission can be regulated by synaptic input. Our experiments are, however, unusual in that they relate specific characteristics of postsynaptic potentials to functional inhibition. In particular we demonstrate the potential importance of the IPSP time constant.

Different sensory systems share projection neurons but elicit distinct motor patterns

Considerable research has focused on issues pertaining to sensorimotor integration, but in most systems precise information remains unavailable regarding the specific pathways by which different sensory systems regulate any single central pattern-generating circuit. We address this issue by determining how two muscle stretch-sensitive neurons, the gastropyloric receptor neurons (GPRs), influence identified projection neurons that regulate the gastric mill circuit in the stomatogastric nervous system of the crab and then comparing these actions with those of the ventral cardiac neuron (VCN) mechanosensory system. Here, we show that the GPR neurons activate the gastric mill rhythm in the stomatogastric ganglion (STG) via their excitation of two identified projection neurons, modulatory commissural neuron 1 (MCN1) and commissural projection neuron 2 (CPN2), in the commissural ganglion. Support for this conclusion comes from the ability of the modulatory proctolin neuron (MPN), a projection neuron that suppresses the gastric mill rhythm via its inhibitory actions on MCN1 and CPN2, to inhibit the GPR-elicited gastric mill rhythm. Selective elimination of MCN1 and CPN2 access to the STG also prevents GPR activation of this rhythm. The VCN neurons also elicit the gastric mill rhythm by coactivating MCN1 and CPN2, but the GPR-elicited gastric mill rhythm is distinct. These distinct rhythms are likely to result partly from different MCN1 activity levels under these two conditions and partly from the presence of additional GPR actions in the STG. These results support the hypothesis that different sensory systems differentially regulate neuronal circuit activity despite their convergent actions on a single subpopulation of projection neurons.

Neural control exploits changing mechanical advantage and context dependence to generate different feeding responses in Aplysia

How does neural control reflect changes in mechanical advantage and muscle function? In the Aplysia feeding system a protractor muscle's mechanical advantage decreases as it moves the structure that grasps food (the radula/odontophore) in an anterior direction. In contrast, as the radula/odontophore is moved forward, the jaw musculature's mechanical advantage shifts so that it may act to assist forward movement of the radula/odontophore instead of pushing it posteriorly. To test whether the jaw musculature's context-dependent function can compensate for the falling mechanical advantage of the protractor muscle, we created a kinetic model of Aplysia's feeding apparatus. During biting, the model predicts that the reduction of the force in the protractor muscle I2 will prevent it from overcoming passive forces that resist the large anterior radula/odontophore displacements observed during biting. To produce protractions of the magnitude observed during biting behaviors, the nervous system could increase I2's contractile strength by neuromodulating I2, or it could recruit the I1/I3 jaw muscle complex. Driving the kinetic model with in vivo EMG and ENG predicts that, during biting, early activation of the context-dependent jaw muscle I1/I3 may assist in moving the radula/odontophore anteriorly during the final phase of protraction. In contrast, during swallowing, later activation of I1/I3 causes it to act purely as a retractor. Shifting the timing of onset of I1/I3 activation allows the nervous system to use a mechanical equilibrium point that allows I1/I3 to act as a protractor rather than an equilibrium point that allows I1/I3 to act as a retractor. This use of equilibrium points may be similar to that proposed for vertebrate control of movement.

Afferent-induced changes in rhythmic motor programs in the feeding circuitry of aplysia

A manipulation often used to determine whether a neuron plays a role in the generation of a motor program involves injecting current into the cell during rhythmic activity to determine whether activity is modified. We perform this type of manipulation to study the impact of afferent activity on feeding-like motor programs in Aplysia. We trigger biting-like programs and manipulate sensory neurons that have been implicated in producing the changes in activity that occur when food is ingested, i.e., when bites are converted to bite-swallows. Sensory neurons that are manipulated are the radula mechanoafferent B21 and the retraction proprioceptor B51. Data suggest that both cells are peripherally activated during radula closing/retraction when food is ingested. We found that phasic subthreshold depolarization of a single sensory neuron can significantly prolong radula closing/retraction, as determined by recording both from interneurons (e.g., B64), and motor neurons (e.g., B15 and B8). Additionally, afferent activity produces a delay in the onset of the subsequent radula opening/protraction, and increases the firing frequency of motor neurons. These are the changes in activity that are seen when food is ingested. These results add to the growing data that implicate B21 and B51 in bite to bite-swallow conversions and indicate that afferent activity is important during feeding in Aplysia.

Frequency-Dependent Regulation of Afferent Transmission in the Feeding Circuitry of Aplysia

During rhythmic behaviors, sensori-motor transmission is often regulated so that there are phasic changes in afferent input to follower neurons. We study this type of regulation in the feeding circuit of Aplysia. We characterize effects of the B4/5 interneurons on transmission from the mechanoafferent B21 to the radula closer motor neuron B8. In quiescent preparations, B4/5-induced postsynaptic potentials (PSPs) can block spike propagation in the lateral process of B21 and inhibit afferent transmission. B4/5 are, however, active during the retraction phase of motor programs, i.e., when mechanoafferent transmission to B8 presumably occurs. To determine whether mechanoafferent transmission is necessarily inhibited when B4/5 are active, we characterize the B4/5 firing frequency during retraction and show that, for the most part, it is low (below 15 Hz). There is, therefore, a low probability that spike propagation will be inhibited. The relative ineffectiveness of low frequency activity is not simply a consequence of insufficient PSP magnitude, because a single PSP can block spike propagation. Instead, it is related to the fact that PSPs have a short duration. When B4/5 fire at a low frequency, there is therefore a low probability that afferent transmission in the lateral process of B21 can be inhibited. In conclusion, we demonstrate that afferent transmission will not always be affected when a neuron that exerts inhibitory effects is active. Although a cell may be ineffective when it fires at a low frequency, ineffectiveness is not necessarily a consequence of spike frequency per se. Instead it may be due to spike timing.

Long-lasting activation of rhythmic neuronal activity by a novel mechanosensory system in the crustacean stomatogastric nervous system

Sensory neurons enable neural circuits to generate behaviors appropriate for the current environmental situation. Here, we characterize the actions of a population (about 60) of bilaterally symmetric bipolar neurons identified within the inner wall of the cardiac gutter, a foregut structure in the crab Cancer borealis. These neurons, called the ventral cardiac neurons (VCNs), project their axons through the crab stomatogastric nervous system to influence neural circuits associated with feeding. Brief pressure application to the cardiac gutter transiently modulated the filtering motor pattern (pyloric rhythm) generated by the pyloric circuit within the stomatogastric ganglion (STG). This modulation included an increased speed of the pyloric rhythm and a concomitant decrease in the activity of the lateral pyloric neuron. Furthermore, 2 min of rhythmic pressure application to the cardiac gutter elicited a chewing motor pattern (gastric mill rhythm) generated by the gastric mill circuit in the STG that persisted for < or =30 min. These sensory actions on the pyloric and gastric mill circuits were mimicked by either ventral cardiac nerve or dorsal posterior esophageal nerve stimulation. VCN actions on the STG circuits required the activation of projection neurons in the commissural ganglia. A subset of the VCN actions on these projection neurons appeared to be direct and cholinergic. We propose that the VCN neurons are mechanoreceptors that are activated when food stored in the foregut applies an outward force, leading to the long-lasting activation of projection neurons required to initiate chewing and modify the filtering of chewed food.

Regulation of spike initiation and propagation in an Aplysia sensory neuron: gating-in via central depolarization

Afferent transmission can be regulated (or gated) so that responses to peripheral stimuli are adjusted to make them appropriate for the ongoing phase of a motor program. Here, we characterize a gating mechanism that involves regulation of spike propagation in Aplysia mechanoafferent B21. B21 is striking in that afferent transmission to the motor neuron B8 does not occur when B21 is at resting membrane potential. Our data suggest that this results from the fact that spikes are not actively propagated to the lateral process of B21 (the primary contact with B8). When B21 is peripherally activated at its resting potential, electrotonic potentials in the lateral process are on average 11 mV. In contrast, mechanoafferent activity is transmitted to B8 when B21 is centrally depolarized via current injection. Our data suggest that central depolarization relieves propagation failure. Full-size spikes are recorded in the lateral process when B21 is depolarized and then peripherally activated. Moreover, changes in membrane potential in the lateral process affect spike amplitude, even when the somatic membrane potential is virtually unchanged. During motor programs, both the lateral process and the soma of B21 are phasically depolarized via synaptic input. These depolarizations are sufficient to convert subthreshold potentials to full-size spikes in the lateral process. Thus, our data strongly suggest that afferent transmission from B21 to B8 is, at least in part, regulated via synaptic control of spike initiation in the lateral process. Consequences of this control for compartmentalization in B21 are discussed, as are specific consequences for feeding behavior.

A population of pedal-buccal projection neurons associated with appetitive components of Aplysia feeding behavior

Backfills of the cerebral-buccal connective (CBC) of Aplysia californica revealed a cluster of five to seven pedal-buccal projection neurons in the anterolateral quadrant of the ventral surface of each pedal ganglion. Intra- and extracellular recordings showed that the pedal-buccal projection neurons shared common electrophysiological properties and synaptic inputs. However, they exhibited considerable heterogeneity with respect to their projection patterns. All pedal-buccal projection neurons that were tested received a slow excitatory postsynaptic potential from the ipsi- and contralateral cerebral-pedal regulator (C-PR) neuron, a cell that is thought to play a key role in the generation of a food-induced arousal state. Tests were conducted to identify potential synaptic follower neurons of the pedal-buccal projection neurons in the cerebral and buccal ganglia, but none were detected. Finally, nerve recordings revealed projections from the pedal-buccal projection neurons in the nerves associated with the buccal ganglion. In tests designed to determine the functional properties of these peripheral projections, no evidence was obtained supporting a mechanosensory or proprioceptive role and no movements were observed when they were fired. It is proposed that peripheral elements utilized in consummatory phases of Aplysia feeding may be directly influenced by a neuronal pathway that is activated during the food-induced arousal state.

Cerebral CBM1 neuron contributes to synaptic modulation appearing during rejection of seaweed in Aplysia kurodai

The Japanese species Aplysia kurodai feeds well on Ulva but rejects Gelidium with distinctive rhythmic patterned movements of the jaws and radula. We have previously shown that the patterned jaw movements during the rejection of Gelidium might be caused by long-lasting suppression of the monosynaptic transmission from the multiaction MA neurons to the jaw-closing (JC) motor neurons in the buccal ganglia and that the modulation might be directly produced by some cerebral neurons. In the present paper, we have identified a pair of catecholaminergic neurons (CBM1) in bilateral cerebral M clusters. The CBM1, probably equivalent to CBI-1 in A. californica, simultaneously produced monosynaptic excitatory postsynaptic potentials (EPSPs) in the MA and JC neurons. Firing of the CBM1 reduced the size of the inhibitory postsynaptic currents (IPSCs) in the JC neuron, evoked by the MA spikes, for >100 s. Moreover, the application of dopamine mimicked the CBM1 modulatory effects and pretreatment with a D1 antagonist, SCH23390, blocked the modulatory effects induced by dopamine. It could also largely block the modulatory effects induced by the CBM1 firing. These results suggest that the CBM1 may directly modulate the synaptic transmission by releasing dopamine. Moreover, we explored the CBM1 spike activity induced by taste stimulation of the animal lips with seaweed extracts by the use of calcium imaging. The calcium-sensitive dye, Calcium Green-1, was iontophoretically loaded into a cell body of the CBM1 using a microelectrode. Application of either Ulva or Gelidium extract to the lips increased the fluorescence intensity, but the Gelidium extract always induced a larger change in fluorescence compared with the Ulva extract, although the solution used induced the maximum spike responses of the CBM1 for each of the seaweed extracts. When the firing frequency of the CBM1 activity after taste stimulation was estimated, the Gelidium extract induced a spike activity of ~30 spikes/s while the Ulva extract induced an activity of ~20 spikes/s, consistent with the effective firing frequency (>25 spikes/s) for the synaptic modulation. These results suggest that the CBM1 may be one of the cerebral neurons contributing to the modulation of the basic feeding circuits for rejection induced by the taste of seaweeds such as Gelidium.

A kinematic model of swallowing inAplysia californicabased on radula/odontophore kinematics andin vivomagnetic resonance images

A kinematic model of the buccal mass of Aplysia californica during swallowing has been developed that incorporates the kinematics of the odontophore, the muscular structure that underlies the pincer-like grasping structure, the radula. The model is based on real-time magnetic resonance images (MRIs) of the mid-sagittal cross section of the buccal mass during swallowing. Using kinematic relationships derived from isolated odontophores induced to perform feeding-like movements, the model generates predictions about movement of the buccal mass in the medio-lateral dimension during the feeding cycle that are well-matched to corresponding coronal MRIs of the buccal mass during swallowing. The model successfully reproduces changes in the lengths of the intrinsic (I) buccal muscles I2 and I3 measured experimentally. The model predicts changes in the length of the radular opener muscle I7 throughout the swallowing cycle, generates hypotheses about the muscular basis of radular opening prior to the onset of forward rotation during swallowing and suggests possible context-dependent functions for the I7 muscle, the radular stalk and the I5 (ARC) muscle during radular opening and closing.

Identification and characterization of the feeding circuit-activating peptides, a novel neuropeptide family of aplysia

We use a multidisciplinary approach to identify, map, and characterize the bioactivity of modulatory neuropeptides in the circuitry that generates feeding behavior in Aplysia. Matrix-assisted laser desorption/ionization time-of-flight mass spectrometry of the cerebral-buccal connective (CBC), a nerve containing axons of many interneurons that control feeding behavior of Aplysia, was used to identify neuropeptides that may participate in generation and shaping of feeding motor programs. Using this functionally oriented search, we identified a novel family of peptides that we call the feeding circuit-activating peptides (FCAPs). Two peptides with masses identical to those observed in the CBCs (molecular weight 1387 and 1433) were purified from buccal ganglia and partially sequenced using mass spectrometry. The amino acid sequence was then used to clone the FCAP precursor, which encodes multiple copies of eight different FCAPs. The two FCAPs present in highest copy number correspond to those observed in the CBC. The distribution of FCAP expression was mapped using Northern analysis, whole-mount in situ hybridization, and immunocytochemistry. Consistent with our initial findings, FCAP-immunopositive axons were observed in the CBC. Furthermore, we found that FCAP was present in some cerebral-buccal and buccal-cerebral interneurons. As their name suggests, FCAPs are capable of initiating rhythmic feeding motor programs and are the first neuropeptides with such activity in this circuit. The actions of FCAPs suggest that these peptides may contribute to the induction and maintenance of food-induced arousal. FCAPs were also localized to several other neuronal systems, suggesting that FCAPs may play a role in the regulation of multiple behaviors.

Egestive feeding responses in Aplysia persist after sectioning of the cerebral-buccal connectives: evidence for multiple sites of control of motor programs

Ingestive and egestive behaviors in Aplysia are generated by motor neurons and interneurons chiefly located in the buccal ganglion, but cerebral ganglion neurons appear to contribute to both ingestive and egestive motor programs. We investigated if the cerebral ganglion input to the buccal ganglion is necessary for the generation of buccal ingestive and egestive behaviors in free-moving animals. We confirmed a prior study that showed that animals with lesions of the cerebro-buccal connectives (CBCs) do not exhibit rhythmic biting following seaweed stimulation of the lips, but do show swallowing of seaweed inserted into the buccal cavity. We found that CBC-lesioned animals also exhibited rejection of a tube inserted into the buccal cavity and esophagus. The programs for swallowing and rejection behaviors were similar to those observed before lesioning the CBCs, although the rate of swallowing was slower. These results suggest that the cerebral input to the buccal ganglion is necessary for generating biting responses, but is not required for producing swallowing or rejection responses.

Colocalization of gamma-aminobutyric acid-like immunoreactivity and catecholamines in the feeding network of Aplysia californica

Functional consequences of neurotransmitter coexistence and cotransmission can be readily studied in certain experimentally favorable invertebrate motor systems. In this study, whole-mount histochemical methods were used to identify neurons in which gamma-aminobutyric acid (GABA)-like immunoreactivity (GABAli) was colocalized with catecholamine histofluorescence (CAh; FaGlu method) and tyrosine hydroxylase (TH)-like immunoreactivity (THli) in the feeding motor circuitry (buccal and cerebral ganglia) of the marine mollusc Aplysia californica. In agreement with previous reports, five neurons in the buccal ganglia were found to exhibit CAh. These included the paired B20 buccal-cerebral interneurons (BCIs), the paired B65 buccal interneurons, and an unpaired cell with projections to both cerebral-buccal connectives (CBCs). Experiments in which the FaGlu method was combined with the immunohistochemical detection of GABA revealed double labeling of all five of these neurons. An antibody generated against TH, the rate-limiting enzyme in the biosynthesis of catecholamines, was used to obtain an independent determination of GABA-CA colocalization. Biocytin backfills of the CBC performed in conjunction with TH immunohistochemistry revealed labeling of the rostral B20 cell pair and the unpaired CBI near the caudal surface of the right hemiganglion. THli was also present in a prominent bilateral pair of caudal neurons that were not stained with CBC backfills. On the basis of their position, size, shape, and lack of CBC projections, the lateral THli neurons were identified as B65. Double-labeling immunohistochemical experiments revealed GABAli in all five buccal THli neurons. Finally, GABAli was observed in individual B20 and B65 neurons that were identified using electrophysiological criteria and injected with a marker (neurobiotin). Similar methods were used to demonstrate that a previously identified catecholaminergic cerebral-buccal interneuron (CBI) designated CBI-1 contained THli but did not contain GABAli. Although numerous THli and GABAli neurons and fibers were present in the cerebral and buccal ganglia, additional instances of their colocalization were not observed. These findings indicate that GABA and a catecholamine (probably dopamine) are colocalized in a limited number of interneurons within the central pattern generator circuits that control feeding-related behaviors in Aplysia.

The enterins: a novel family of neuropeptides isolated from the enteric nervous system and CNS of Aplysia

To identify neuropeptides that have a broad spectrum of actions on the feeding system of Aplysia, we searched for bioactive peptides that are present in both the gut and the CNS. We identified a family of structurally related nonapeptides and decapeptides (enterins) that are present in the gut and CNS of Aplysia, and most of which share the HSFVamide sequence at the C terminus. The structure of the enterin precursor deduced from cDNA cloning predicts 35 copies of 20 different enterins. Northern analysis, in situ hybridization, and immunocytochemistry show that the enterins are abundantly present in the CNS and the gut of Aplysia. Using matrix-assisted laser desorption/ionization time-of-flight mass spectrometry we characterized the enterin-precursor processing, demonstrated that all of the precursor-predicted enterins are present, and determined post-translational modifications of various enterins. Enterin-positive neuronal somata and processes were found in the gut, and enterins inhibited contractions of the gut. In the CNS, the cerebral and buccal ganglia, which control feeding, contained the enterins. Enterin was also present in the nerve that connects these two ganglia. Enterins reduced the firing of interneurons B4/5 during feeding motor programs. Such enterin-induced reduction of firing also occurred when excitability of B4/5 was tested directly. Because reduction of B4/5 activity corresponds to a switch from egestive to ingestive behaviors, enterin may contribute to such program switching. Furthermore, because enterins are present throughout the nervous system, they may also play a regulatory role in nonfeeding behaviors of Aplysia.

Neural mechanisms of motor program switching in Aplysia

The Aplysia multifunctional feeding central pattern generator (CPG) produces at least two types of motor programs, ingestion and egestion, that involve two sets of radula movements, protraction-retraction and opening-closing movements. In ingestion, the radula closes during retraction to pull food in, whereas in egestion, the radula closes during protraction to push inedible objects out. Thus, radula closure shifts the phase in which it occurs with respect to protraction-retraction in the two programs. To identify the central switching mechanisms, we compared activity of CPG neurons during the two types of motor programs elicited by a higher-order interneuron, cerebral-buccal interneuron-2 (CBI-2). Although CPG elements (B63, B34, and B64) that mediate the protraction-retraction sequence are active in both programs, two other CPG elements, B20 and B4/5, are preferentially active in egestive programs and play a major role in mediating CBI-2-elicited egestive programs. Both B20 and B4/5 control the phasing of radula closure motoneurons (B8 and B16) to ensure that, in egestive programs, these motoneurons fire and produce radula-closing movements only during protraction. Elsewhere, another higher-order interneuron, CBI-3, was shown to convert CBI-2-elicited egestion to ingestion. We show that CBI-3 switches the programs by suppressing the activity of B20 and B4/5. CBI-3, active only during protraction, accomplishes this through fast inhibition of B20 during protraction and slow inhibition of B4/5 during retraction. The slow inhibition is mimicked and occluded by APGWamide, a neuropeptide contained in CBI-3. Thus, fast conventional and slow peptidergic transmissions originating from the same interneuron act in concert to meet specific temporal requirements in pattern switching.

Glutamate is the fast excitatory neurotransmitter of small cardioactive peptide-containing Aplysia radula mechanoafferent neuron B21

B21 is a radula mechanoafferent neuron in the mollusc Aplysia which likely plays a crucial role in integrating environmental cues into the feeding motor program. To facilitate understanding B21's interactions with its postsynaptic followers, we sought to identify its neurotransmitter. We find that B21 makes a chemical synapse onto the follower neuron B8. Although B21-induced excitatory postsynaptic potentials (EPSPs) in B8 paradoxically diminish in amplitude with B8 hyperpolarization, we show that an inwardly rectifying current is responsible. We conclude that these B21-induced EPSPs are likely glutamatergic as they are blocked by the glutamate antagonist DNQX. Furthermore, B8 exhibits a depolarizing response to exogenous glutamate, which is antagonized by DNQX. Finally, exogenous glutamate occludes B21-evoked EPSPs in B8.

A proprioceptive role for an exteroceptive mechanoafferent neuron in Aplysia

Afferent regulation of centrally generated activity is likely to be more complex than has been established. We show that a neuron that is an exteroceptor can also function as a proprioceptor. We study the Aplysia neuron B21. Previous data suggest that B21 functions as an exteroceptor during the radula closing/retraction phase of ingestive feeding. We show that the tissue innervated by B21, the subradula tissue (SRT), is innervated by a motor neuron (B66) and that B66-induced SRT contractions trigger centripetal spikes in B21. Thus, B21 is also a proprioceptor. To determine whether exteroceptive and proprioceptive activities occur during the same phase of ingestive feeding, we further characterize B66. We show that B66 stimulation does not close or retract the radula. Instead it opens it. Moreover, B66 is electrically coupled to other opening/protraction neurons. Finally, we elicit motor programs in semi-intact preparations and show that during radula opening/protraction we observe B66 activity, SRT contractions, and spikes in B21 that can be eliminated if B66 is indirectly hyperpolarized. B21 is, therefore, likely to act as an exteroceptor during one phase of ingestive feeding and as a proprioceptor during the antagonistic phase. Previous experiments have shown that centripetal spikes in B21 are only transmitted to one follower if they are "gated in" by depolarization. During ingestive programs B21 is centrally depolarized during closing/retraction, but it is not depolarized during opening/protraction. We sought to determine whether there are other followers that receive B21 input when it is not centrally depolarized. We found one such cell. Moreover, we found that stimulation of B21 during radula opening/protraction significantly decreases the duration of this phase of behavior. Thus, proprioceptive activity in B21 is likely to have an impact on motor programs.

Outputs of radula mechanoafferent neurons in Aplysia are modulated by motor neurons, interneurons, and sensory neurons

The gain of sensory inputs into the nervous system can be modulated so that the nature and intensity of afferent input is variable. Sometimes the variability is a function of other sensory inputs or of the state of motor systems that generate behavior. A form of sensory modulation was investigated in the Aplysia feeding system at the level of a radula mechanoafferent neuron (B21) that provides chemical synaptic input to a group of motor neurons (B8a/b, B15) that control closure and retraction movements of the radula, a food grasping structure. B21 has been shown to receive both excitatory and inhibitory synaptic inputs from a variety of neuron types. The current study investigated the morphological basis of these heterosynaptic inputs, whether the inputs could serve to modulate the chemical synaptic outputs of B21, and whether the neurons producing the heterosynaptic inputs were periodically active during feeding motor programs that might modulate B21 outputs in a phase-specific manner. Four cell types making monosynaptic connections to B21 were found capable of heterosynaptically modulating the chemical synaptic output of B21 to motor neurons B8a and B15. These included the following: 1) other sensory neurons, e.g. , B22; 2) interneurons, e.g., B19; 3) motor neurons, e.g., B82; and 4) multifunction neurons that have sensory, motor, and interneuronal functions, e.g., B4/5. Each cell type was phasically active in one or more feeding motor programs driven by command-like interneurons, including an egestive motor program driven by CBI-1 and an ingestive motor program driven by CBI-2. Moreover, the phase of activity differed for each of the modulator cells. During the motor programs, shifts in B21 membrane potential were related to the activity patterns of some of the modulator cells. Inhibitory chemical synapses mediated the modulation produced by B4/5, whereas excitatory and/or electrical synapses were involved in the other instances. The data indicate that modulation is due to block of action potential invasion into synaptic release regions or to alterations of transmitter release as a function of the presynaptic membrane potential. The results indicate that just as the motor system of Aplysia can be modulated by intrinsic mechanisms that can enhance its efficiency, the properties of primary sensory cells can be modified by diverse inputs from mediating circuitry. Such modulation could serve to optimize sensory cells for the different roles they might play.