DIFFERENTIAL-EFFECTS OF A MOLTING HORMONE, 20-HYDROXYECDYSONE, ON THENEUROMUSCULAR-JUNCTIONS OF THE CLAW OPENER AND ABDOMINAL FLEXOR MUSCLES OF THE AMERICAN LOBSTER

Intracellular recordings at the neuromuscular junction of the dactyl o pener muscle and the abdominal phasic flexor muscle of intermolt lobst ers were made in the presence and absence of the steroid hormone, 20-h ydroxyecdysone (20-HE). Evoked excitatory junctional potentials (EJPs) were recorded from both muscles; spontaneous miniature excitatory jun ctional potentials (MEJPs) were recorded from the claw. In the opener muscle, 20-HE caused an increase in EJP amplitudes and MEJP frequency. In the abdomen 20-HE caused EJP amplitudes to become significantly sm aller than controls. These results are consistent with changes in the relative activity of the two muscles over the molt cycle. In premolt l obsters where the titers of 20-HE are highest, there is an increased t endency towards aggressive behaviors (which include the meral spread) and a corresponding reduction of escape swimming. The results also sup port our earlier findings that one or more molt-related blood-borne fa ctors can modulate peripheral synaptic transmission. (C) 1998 Elsevier Science Inc. All rights reserved.