NEUROMODULATORY INPUTS MAINTAIN EXPRESSION OF A LOBSTER MOTOR PATTERN-GENERATING NETWORK IN A MODULATION-DEPENDENT STATE - EVIDENCE FROM LONG-TERM DECENTRALIZATION IN-VITRO

Neuromodulatory inputs play a critical role in governing the expressio n of rhythmic motor output by the pyloric network in the crustacean st omatogastric ganglion (STG). When these inputs are removed by cutting the primarily afferent stomatogastric nerve (stn) to the STG, pyloric neurons rapidly lose their ability to burst spontaneously, and the net work falls silent, By using extracellular motor nerve recordings from long-term organotypic preparations of the stomatogastric nervous syste m of the lobster Jesus lalandii, we are investigating whether modulato ry inputs exert long-term regulatory influences on the pyloric network operation in addition to relatively short-term neuromodulation, When decentralized (stn cut), quiescent STGs are maintained in organ cultur e, pyloric rhythmicity gradually returns within 3-5 d and is similar t o, albeit slower than, the triphasic motor pattern expressed when the stn is intact. This recovery of network activity still occurred after photoinactivation of axotomized input terminals in the isolated STG af ter migration of Lucifer yellow, The recovery does not depend on actio n potential generation, because it also occurred in STGs maintained in TTX-containing saline after decentralization. Resumption of rhythmici ty was also not activity-dependent, because recovery still occurred in STGs that were chronically depolarized with elevated K+ saline or wer e maintained continuously active with the muscarinic agonist oxotremor ine after decentralization. We conclude that the prolonged absence of extraganglionic modulatory inputs to the pyloric network allows expres sion of an inherent rhythmogenic capability that is normally maintaine d in a strictly conditional state when these extrinsic influences are present.