Differential effects of a molting hormone, 20-hydroxyecdysone, on the neuromuscular junctions of the claw opener and abdominal flexor muscles of the American lobster

Abstract

Intracellular recordings at the neuromuscular junction of the dactyl opener muscle and the abdominal phasic flexor muscle of intermolt lobsters were made in the presence and absence of the steroid hormone, 20-hydroxyecdysone (20-HE). Evoked excitatory junctional potentials (EJPs) were recorded from both muscles; spontaneous miniature excitatory junctional 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 smaller than controls. These results are consistent with changes in the relative activity of the two muscles over the molt cycle. In premolt lobsters where the titers of 20-HE are highest, there is an increased tendency towards aggressive behaviors (which include the meral spread) and a corresponding reduction of escape swimming. The results also support our earlier findings that one or more molt-related blood-borne factors can modulate peripheral synaptic transmission.