Ganglionic distribution of inputs and outputs of C-PR, a neuron involved in the generation of a food-induced arousal state in Aplysia

Variables controlling entry into and exit from the steady-state, one of two modes of feeding in Aplysia

Background

Aplysia feeding is a model system for examining the neural mechanisms by which changes in motivational state control behavior. When food is intermittently present, Aplysia eat large meals controlled by a balance between food stimuli exciting feeding and gut stimuli inhibiting feeding. However, when food is continuously present animals are in a state in which feeding is relatively inhibited and animals eat little. We examined which stimuli provided by food and feeding initiate steady-state inhibition of feeding, and which stimuli maintain the inhibition.

Results

Multiple stimuli were found to control entry into the steady-state inhibition, and its maintenance. The major variable governing entry into the steady-state is fill of the gut with bulk provided by food, but this stimulus cannot alone cause entry into the steady-state. Food odor and nutritional stimuli such as increased hemolymph glucose and L-arginine concentrations also contribute to inhibition of feeding leading to entry into the steady-state. Although food odor can alone cause some inhibition of feeding, it does not amplify the effect of gut fill. By contrast, neither increased hemolymph glucose nor L-arginine alone inhibits feeding in hungry animals, but both amplify the inhibitory effects of food odor, and increased glucose also amplifies the effect of gut fill. The major variable maintaining the steady-state is the continued presence of food odor, which can alone maintain the steady-state for 48–72 hrs. Neither increased glucose nor L-arginine can alone preserve the steady-state, although they partially preserve it. Glucose and arginine partially extend the effect of food odor after 72 hrs.

Conclusions

These findings show that control of Aplysia feeding is more complex than was previously thought, in that multiple inhibitory factors interact in its control.

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.

Gastric and pyloric motor pattern control by a modulatory projection neuron in the intact crab Cancer pagurus

Neuronal release of modulatory substances provides motor pattern generating circuits with a high degree of flexibility. In vitro studies have characterized the actions of modulatory projection neurons in great detail in the stomatogastric nervous system, a model system for neuromodulatory influences on central pattern generators. Less is known about the activities and actions of modulatory neurons in fully functional and richly modulated network settings, i.e., in intact animals. It is also unknown whether their activities contribute to the motor patterns in different behavioral conditions. Here, we show for the first time the activity and effects of the well-characterized modulatory projection neuron 1 (MCN1) in vivo and compare them to in vitro conditions. MCN1 was always spontaneously active, typically in a rhythmic fashion with its firing being interrupted by ascending inhibitions from the pyloric motor circuit. Its activity contributed to pyloric motor activity, because 1) the cycle period of the motor pattern correlated with MCN1 firing frequency and 2) stimulating MCN1 shortened the cycle period while 3) lesioning of the MCN1 axon reduced motor activity. In addition, gastric mill motor activity was elicited for the duration of the stimulation. Chemosensory stimulation of the antennae moved MCN1 away from baseline activity by increasing its firing frequency. Following this increase, a gastric mill rhythm was elicited and the pyloric cycle period decreased. Lesioning the MCN1 axon prevented these effects. Thus modulatory projection neurons such as MCN1 can control the motor output in vivo, and they participate in the processing of exteroceptive sensory information in behaviorally relevant conditions.

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.

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.

Regeneration of cerebral-buccal interneurons and recovery of ingestion buccal motor programs in Aplysia after CNS lesions

In the sea slug Aplysia, rhythmic biting is eliminated after bilateral cerebral-buccal connective (CBC) crushes and recovers within 14 days postlesion (dpl). The ability of cerebral-buccal interneuron-2 (CBI-2) to elicit ingestion buccal motor programs (iBMPs; i.e., fictive rhythmic ingestion) and to regenerate synaptic connections with target buccal neurons was assessed with intracellular recordings and dye injections. Isolated central ganglia were obtained from control animals and from lesioned animals at selected times after bilateral CBC crushes. Within 3 wk postlesion, transected CBI-2 axons sprouted at least 10 fine neurites confined to the core of the CBC that projected across the crush site toward the buccal ganglia. When fired with depolarizing current steps, CBI-2 was not observed to elicit iBMPs in preparations until 14 dpl. Thereafter a progressive enhancement in CBI-2's ability to elicit iBMPs was observed with time postlesion. By 40 dpl, CBI-2-elicited iBMPs were indistinguishable from those of controls. CBI-2 regenerated monosynaptic connections with appropriate buccal premotor- and motorneurons by 14 dpl, and the strength of these connections increased with time postlesion. Dramatic frequency facilitation was exhibited by the regenerating CBI-2 buccal synapses; for instance, at early postlesion times, no observable excitatory postsynaptic potentials (EPSPs) were obtained with 1- Hz stimulation of CBI-2, while at 7 Hz, a dramatic increase in EPSP amplitude was obtained with successive spikes. The present study shows that the time course of axonal and synaptic regeneration by command-like interneuron CBI-2 is correlated with the recovery of ingestion buccal motor programs elicited by CBI-2. These results parallel our previous findings of functional neural regeneration in the feeding system and suggest that functional neural regeneration is at least in part mediated by regeneration of specific synaptic pathways.

Intrinsic and extrinsic modulation of a single central pattern generating circuit

Intrinsic and extrinsic neuromodulation are both thought to be responsible for the flexibility of the neural circuits (central pattern generators) that control rhythmic behaviors. Because the two forms of modulation have been studied in different circuits, it has been difficult to compare them directly. We find that the central pattern generator for biting in Aplysia is modulated both extrinsically and intrinsically. Both forms of modulation increase the frequency of motor programs and shorten the duration of the protraction phase. Extrinsic modulation is mediated by the serotonergic metacerebral cell (MCC) neurons and is mimicked by application of serotonin. Intrinsic modulation is mediated by the cerebral peptide-2 (CP-2) containing CBI-2 interneurons and is mimicked by application of CP-2. Since the effects of CBI-2 and CP-2 occlude each other, the modulatory actions of CBI-2 may be mediated by CP-2 release. Although the effects of intrinsic and extrinsic modulation are similar, the neurons that mediate them are active predominantly at different times, suggesting a specialized role for each system. Metacerebral cell (MCC) activity predominates in the preparatory (appetitive) phase and thus precedes the activation of CBI-2 and biting motor programs. Once the CBI-2s are activated and the biting motor program is initiated, MCC activity declines precipitously. Hence extrinsic modulation prefacilitates biting, whereas intrinsic modulation occurs during biting. Since biting inhibits appetitive behavior, intrinsic modulation cannot be used to prefacilitate biting in the appetitive phase. Thus the sequential use of extrinsic and intrinsic modulation may provide a means for premodulation of biting without the concomitant disruption of appetitive behaviors.

C-PR neuron of Aplysia has differential effects on "Feeding" cerebral interneurons, including myomodulin-positive CBI-12

Head lifting and other aspects of the appetitive central motive state that precedes consummatory feeding movements in Aplysia is promoted by excitation of the C-PR neuron. Food stimuli activate C-PR as well as a small population of cerebral-buccal interneurons (CBIs). We wished to determine if firing of C-PR produced differential effects on the various CBIs or perhaps affected all the CBIs uniformly as might be expected for a neuron involved in producing a broad undifferentiated arousal state. We found that when C-PR was fired, it produced a wide variety of effects on various CBIs. Firing of C-PR evoked excitatory input to a newly identified CBI (CBI-12) the soma of which is located in the M cluster near the previously identified CBI-2. CBI-12 shares certain properties with CBI-2, including a similar morphology and a capacity to drive rhythmic activity of the buccal-ganglion. Unlike CBI-2, CBI-12 exhibits myomodulin immunoreactivity. Furthermore when C-PR is fired, CBI-12 receives a polysynaptic voltage-dependent slow excitation, whereas, CBI-2 receives relatively little input. C-PR also polysynaptically excites other CBIs including CBI-1 and CBI-8/9 but produces inhibition in CBI-3. In addition, firing of C-PR inhibits plateau potentials in CBI-5/6. The data suggest that activity of C-PR may promote the activity of one subset of cerebral-buccal interneurons, perhaps those involved in ingestive behaviors that occur during the head-up posture. C-PR also inhibits some cerebral-buccal interneurons that may be involved in behaviors in which C-PR activity is not required or may even interfere with other feeding behaviors such as rejection or grazing, that occur with the head down.