Antagonistic effects of phentolamine and octopamine on rhythmic motor output of crayfish thoracic ganglia

Spontaneous rhythmic motor output of crayfish thoracic ganglia consists of bursts of activity in antagonistic leg motor neurons (MNs), alternating wit h a rather slow cycle period (typically greater than or equal to 20 s). The most common pattern (77% of preparations) consists of long coral promotor bursts, the duration of which was correlated strongly with cycle period, an d relatively shea remoter bursts independent of cycle period. Octopamine, a t a concentration of 2-30 mu M reversibly retarded this rhythm, increasing both cycle period and promotor burst duration. Higher concentrations of oct opamine inhibited promotor nerve activity and abolished rhythmic bursting. Phentolamine (10-50 mu M) had the opposite effect of decreasing cycle perio d, mainly by decreasing promotor burst duration. Whereas in the presence of octopamine promotor bursts were lengthened and became even more strongly r elated to cycle period, phentolamine promoted a more symmetrical rhythm wit h shorter promotor bursts that were less dependent on cycle period. When oc topamine was applied in the presence of phentolamine, there was no signific ant increase in cycle period or burst duration, although high octopamine co ncentrations (100 mu M) were still capable of inhibiting promotor nerve act ivity. To our knowledge, pharmacological modulation of a spontaneous locomo tor rhythm by an amine antagonist (applied by itself) has not been reported previously. The results raise the testable possibility that phentolamine e xerts its modulatory effects by acting as an octopamine antagonist in crayf ish thoracic ganglia.