Depression of Synaptic Efficacy at Intermolt in Crayfish Neuromuscular Junctions by 20-Hydroxyecdysone, a Molting Hormone. Robin L. Cooper and Marvin E. Ruffner. Nerve-Muscle Group, Thomas Hunt Morgan School of Biological Sciences, University of Kentucky, Lexington, KY 40506-0225, USA.
APStracts 4:376N, 1997.
ABSTRACT
This report demonstrates that ecdysteroids can reduce synaptic transmission at an intermolt stage of a crustacean tonic neuromuscular junction by acting at a presynaptic site. The steroid molting hormone, 20-Hydroxyecdysone (20-HE), appears to act through a rapid, non-genomic mechanism that decreases the probability of synaptic vesicle release and reduces the number of release sites. Quantal analysis revealed that fewer vesicles were released for a given stimulus when 20-HE was present, and this in turn accounted for the reduced synaptic efficacy. Reduced synaptic efficacy produced smaller evoked postsynaptic currents and smaller excitatory postsynaptic potentials (EPSPs) across the muscle fiber membrane. The reduction in EPSPs was observed among muscle fibers that were innervated by high- or low-output terminals. The behavior of crustaceans/crayfish during the molt cycle, when 20-HE is high, may be explained by the reduction in synaptic transmission. Crustaceans become quiescent during the premolt periods as do insects. The effects of 20-HE can be reversed with the application of the crustacean neuromodulator serotonin (5-HT) that enhances synaptic transmission. 

Received 4 September 1997; accepted in final form 11 December 1997.
APS Manuscript Number J733-7.
Article publication pending J. Neurophysiol.
ISSN 1080-4757 Copyright 1997 The American Physiological Society.
Published in APStracts on 7 January 1998