Slow and fast synaptic inhibition evoked by pattern-generating neurons
of the gastric mill network in spiny lobsters.
Elson, Robert C. and Allen I. Selverston.
Department of Biology 0357, University of California at San Diego, 9500
Gilman Drive, La Jolla, CA 92093-0357
APStracts 2:0214N, 1995.
SUMMARY AND CONCLUSIONS
1 . In this paper we begin an assessment of the role of synaptic properties,
especially synaptic time-course, in the function of the central pattern
generator circuit (CPG) that controls rhythmical movements of the gastric mill
in the foregut of spiny lobsters ( Panulirus interruptus ). 2. The majority of
neurons in the gastric CPG are motor neurons (MNs) which innervate striated
muscles of the gastric mill, but which also make electrical and inhibitory,
chemical interconnections within the neuropil of the stomatogastric ganglion
(STG). We studied the ionic dependence, pharmacology and time-course of
inhibitory postsynaptic potentials (IPSPs) evoked by two such MNs -- the
dorsal gastric (DG) and lateral gastric (LG) -- in their central synaptic
partners. In the periphery, LG and DG are thought to release glutamate (Marder
1987). 3 . LG and DG evoke two types of IPSP in follower neurons. The first,
fast type of IPSP rises rapidly (the graded component within 100-300 ms, the
spike-mediated components within a few tens of ms), is mediated by increased
chloride and potassium conductances, and is blocked by ¯< 10 [mu] M picrotoxin
(PTX). These fast IPSPs closely resemble the glutamatergic IPSPs described in
the pyloric circuit of the same ganglion (Eisen and Marder 1982; Marder and
Eisen 1984). 4. The second, slow type of IPSP has a long rise-time (1-2 s), is
mediated by increased conductance to potassium (with little or no involvement
of chloride), and is not blocked by 10 [mu] M PTX, 5mM tetraethyl ammonium
chloride (TEA) or 0.1 mM scopolamine. These properties distinguish slow IPSPs
from the forms of glutamatergic and cholinergic inhibition that have been
described in the pyloric circuit. 5. Fast inhibition occurs alone at
connections from DG and LG to powerstroke MNs (median gastric, MG, and gastric
mill, GM). Slow inhibition occurs in parallel with fast (producing dual-
component responses) at connections from LG to returnstroke neurons (lateral
posterior gastric MNs, LPG, and interneuron 1). DG seems to evoke only a slow
IPSP in LPG. 6. The transmitter mediating the fast IPSPs is likely to be
glutamate. We discuss possible mechanisms for the slow IPSP but have no
evidence at present concerning the transmitter(s) involved. Slow inhibition is
likely to be an important synaptic "building block" (Getting 1989) in the
gastric CPG; it is "tuned" to the duration of gastric bursts and may
contribute to the long cycle period of gastric rhythms.
Received 8 December 1994; accepted in final form 30 June 1995.
APS Manuscript Number J768-4.
Article publication pending J. Neurophysiol.
ISSN 1080-4757 Copyright 1995 The American Physiological Society.
Published in APStracts on 10 August 1995.