Long term expression of two interacting motor pattern generating networks in the stomatogastric system of freely behaving lobster. Stefan Clemens, Denis Combes, Pierre Meyrand, and John Simmers. Laboratoire de Neurobiologie des R‚seaux, CNRS et Universit‚ de Bordeaux I, UMR 5816, Place du Dr. Peyneau, F-33120 Arcachon, FRANCE.
APStracts 4:361N, 1997.
ABSTRACT
Rhythmic movements of the gastric mill and pyloric regions of the crustacean foregut are controlled by two stomatogastric neuronal networks that have been intensively studied in vitro. By using electromyographic recordings from the European lobster, Homarus gammarus, we have monitored simultaneously the motor activity of pyloric and gastric mill muscles for up to three months in intact and freely behaving animals. Both pyloric and gastric mill networks are almost continuously active in vivo, regardless of the presence of food. In unfed resting animals kept under "natural-like" conditions, the pyloric network expresses the typical triphasic pattern seen in vitro, but at considerably slower cycle periods (2.5 - 3.5 s instead of 1 - 1.5 s). Gastric mill activity occurs at mean cycle periods of 20 - 50 s compared to 5 - 10 s in vitro, but may suddenly stop for up to tens of minutes, then restart without any apparent behavioral reason. When conjointly active, the two networks express a strict coupling that involves certain but not all motor neurons of the pyloric network. The posterior pyloric constrictor muscles, innervated by a total of 8 PY motor neurons, are influenced by the onset of each gastric mill MG/LG neuron powerstroke burst, and for one cycle, PY neuron bursts may attain more than 300 % of their mean duration. However, the duration of activity in the lateral pyloric constrictor muscle, innervated by the unique LP motor neuron, remains unaffected by this perturbation. During this period following gastric perturbation, LP neuron and PY neurons thus express opposite burst-to-period relationships in that LP neuron burst duration is independent of the ongoing cycle period, whereas PY neuron burst duration changes with period length. In vitro, the same type of gastro-pyloric interaction is observed, indicating that it is not dependent on sensory inputs. Moreover, this interaction is intrinsic to the stomatogastric ganglion itself since the relationship between the two networks persists after suppression of descending inputs to the ganglion. Intracellular recordings reveal that this gastro-pyloric interaction originates from the gastric MG and LG neurons of the gastric network, which inhibit the pyloric pacemaker ensemble. As a consequence, the pyloric PY neurons, which are inhibited by the PD neurons of the pyloric pacemaker group, extend their activity during the time that PD neuron is held silent. Moreover, there is evidence for a pyloro-gastric interaction, apparently rectifying, from the pyloric pacemakers back to the gastric MG/LG neuron group. 

Received 18 August 1997; accepted in final form 3 December 1997.
APS Manuscript Number J680-7.
Article publication pending J. Neurophysiol.
ISSN 1080-4757 Copyright 1997 The American Physiological Society.
Published in APStracts on 12 December 1997