DEPRESSION OF SYNAPTIC EFFICACY AT INTERMOLT IN CRAYFISH NEUROMUSCULAR-JUNCTIONS BY 20-HYDROXYECDYSONE, A MOLTING HORMONE

This report demonstrates that ecdysteroids can reduce synaptic transmi ssion at an intermolt stage of a crustacean tonic neuromuscular juncti on by acting at a presynaptic site. The steroid molting hormone, 20-hy droxyecdysone (20-HE), appears to act through a rapid, nongenomic mech anism that decreases the probability of synaptic vesicle release and r educes the number of release sites. Quantal analysis revealed that few er vesicles were released for a given stimulus when 20-HE was present, and this in turn accounted for the seduced synaptic efficacy. Reduced synaptic efficacy produced smaller evoked postsynaptic currents and s maller excitatory postsynaptic potentials (EPSPs) across the muscle fi ber 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 beco me quiescent during the premolt periods as do insects. The effects of 20-HE can be reversed with the application of the crustacean neuromodu lator serotonin, which enhances synaptic transmission.