From hunger to satiety: reconfiguration of a feeding network by Aplysia neuropeptide Y.

A shift in motivational state often produces behavioral change, but the underlying mechanisms are poorly understood. In the marine mollusc, Aplysia californica, feeding-induced transition from a hunger to satiation state leads to a slowdown and an eventual termination of feeding. Because the multifunctional feeding network generates both ingestion and the competing response, egestion, it is possible that the transition from a hunger to a satiety state is associated with network reconfiguration that results in production of fewer ingestive and more egestive responses. Chronic electrophysiological recordings in free-feeding Aplysia showed that as the meal progressed, food elicited fewer ingestive responses and simultaneously increased the number of egestive responses. Injections of Aplysia neuropeptide Y (apNPY) reduced food intake and slowed down the rate of ingestion. apNPY was localized to buccal-ganglion afferents originating in the gut-innervating esophageal nerve (EN), a nerve involved both in satiation and in the generation of egestive programs. During EN stimulation, apNPY was released in the feeding circuit. Importantly, stimulation of the cerebral-buccal interneuron-2, a command-like interneuron that is activated by food and normally elicits ingestive responses, elicited egestive responses in the presence of apNPY. This was accompanied by increased activity of the egestion-promoting interneuron B20 and decreased activity in the ingestion-promoting interneuron B40. Thus, apNPYergic reconfiguration of the feeding central pattern generator plays a role in the gradual transition from hunger to satiety states. More generally, changes in the motivational states may involve not only simple network inhibition but may also require network reconfiguration.

Cycle-to-cycle variability of neuromuscular activity in Aplysia feeding behavior.

Aplysia consummatory feeding behavior, a rhythmic cycling of biting, swallowing, and rejection movements, is often said to be stereotyped. Yet closer examination shows that cycles of the behavior are very variable. Here we have quantified and analyzed the variability at several complementary levels in the neuromuscular system. In reduced preparations, we recorded the motor programs produced by the central pattern generator, firing of the motor neurons B15 and B16, and contractions of the accessory radula closer (ARC) muscle while repetitive programs were elicited by stimulation of the esophageal nerve. In other similar experiments, we recorded firing of motor neuron B48 and contractions of the radula opener muscle. In intact animals, we implanted electrodes to record nerve or ARC muscle activity while the animals swallowed controlled strips of seaweed or fed freely. In all cases, we found large variability in all parameters examined. Some of this variability reflected systematic, slow, history-dependent changes in the character of the central motor programs. Even when these trends were factored out, however, by focusing only on the differences between successive cycles, considerable variability remained. This variability was apparently random. Nevertheless, it too was the product of central history dependency because regularizing merely the high-level timing of the programs also regularized many of the downstream neuromuscular parameters. Central motor program variability thus appears directly in the behavior. With regard to the production of functional behavior in any one cycle, the large variability may indicate broad tolerances in the operation of the neuromuscular system. Alternatively, some cycles of the behavior may be dysfunctional. Overall, the variability may be part of an optimal strategy of trial, error, and stabilization that the CNS adopts in an uncertain environment.

Fast synaptic connections from CBIs to pattern-generating neurons in Aplysia: initiation and modification of motor programs.

Consummatory feeding movements in Aplysia californica are organized by a central pattern generator (CPG) in the buccal ganglia. Buccal motor programs similar to those organized by the CPG are also initiated and controlled by the cerebro-buccal interneurons (CBIs), interneurons projecting from the cerebral to the buccal ganglia. To examine the mechanisms by which CBIs affect buccal motor programs, we have explored systematically the synaptic connections from three of the CBIs (CBI-1, CBI-2, CBI-3) to key buccal ganglia CPG neurons (B31/B32, B34, and B63). The CBIs were found to produce monosynaptic excitatory postsynaptic potentials (EPSPs) with both fast and slow components. In this report, we have characterized only the fast component. CBI-2 monosynaptically excites neurons B31/B32, B34, and B63, all of which can initiate motor programs when they are sufficiently stimulated. However, the ability of CBI-2 to initiate a program stems primarily from the excitation of B63. In B31/B32, the size of the EPSPs was relatively small and the threshold for excitation was very high. In addition, preventing firing in either B34 or B63 showed that only a block in B63 firing prevented CBI-2 from initiating programs in response to a brief stimulus. The connections from CBI-2 to the buccal ganglia neurons showed a prominent facilitation. The facilitation contributed to the ability of CBI-2 to initiate a BMP and also led to a change in the form of the BMP. The cholinergic blocker hexamethonium blocked the fast EPSPs induced by CBI-2 in buccal ganglia neurons and also blocked the EPSPs between a number of key CPG neurons within the buccal ganglia. CBI-2 and B63 were able to initiate motor patterns in hexamethonium, although the form of a motor pattern was changed, indicating that non-hexamethonium-sensitive receptors contribute to the ability of these cells to initiate bursts. By contrast to CBI-2, CBI-1 excited B63 but inhibited B34. CBI-3 excited B34 and not B63. The data indicate that CBI-1, -2, and -3 are components of a system that initiates and selects between buccal motor programs. Their behavioral function is likely to depend on which combination of CBIs and CPG elements are activated.

Identification of a GABA-containing cerebral-buccal interneuron-11 in Aplysia californica.

The cerebral-buccal interneurons (CBIs) in Aplysia are a group of inter-ganglionic projection neurons that regulate feeding motor programs. In this study, electrophysiological and immunocytological methods were used to identify a previously uncharacterized CBI, designated CBI-11. CBI-11 is a gamma-aminobutyric acid (GABA)-immunoreactive neuron located in the G cluster of the cerebral ganglion. Firing CBI-11 produced fast picrotoxin-sensitive inhibitory postsynaptic potentials (IPSPs) in buccal motor neuron B3. Local application of GABA to B3 produced a picrotoxin-sensitive hyperpolarization that reversed at the same membrane potential as the IPSPs elicited by stimulation of CBI-11. Together, these observations indicate that CBI-11 utilizes GABA as its transmitter. Finally, stimulation of CBI-11 elicited rhythmic motor programs in quiescent buccal ganglia. Thus, CBI-11 is a GABAergic CBI that can function as a motor program initiator.

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.