The Journal of Neurophysiology Vol. 82 No. 6 December 1999, pp. 3586-3589
Copyright ©1999 by the American Physiological Society

RAPID COMMUNICATION

Antagonistic Effects of Phentolamine and Octopamine on Rhythmic Motor Output of Crayfish Thoracic Ganglia

School of Biological Sciences, Queen Mary and Westfield College, University of London, London E1 4NS, United Kingdom

Gill, Mark D. and Peter Skorupski. Antagonistic Effects of Phentolamine and Octopamine on Rhythmic Motor Output of Crayfish Thoracic Ganglia. J. Neurophysiol. 82: 3586-3589, 1999. Spontaneous rhythmic motor output of crayfish thoracic ganglia consists of bursts of activity in antagonistic leg motor neurons (MNs), alternating with a rather slow cycle period (typically 20 s). The most common pattern (77% of preparations) consists of long coxal promotor bursts, the duration of which was correlated strongly with cycle period, and relatively short remotor bursts independent of cycle period. Octopamine, at a concentration of 2-30 µM reversibly retarded this rhythm, increasing both cycle period and promotor burst duration. Higher concentrations of octopamine inhibited promotor nerve activity and abolished rhythmic bursting. Phentolamine (10-50 µM) had the opposite effect of decreasing cycle period, mainly by decreasing promotor burst duration. Whereas in the presence of octopamine promotor bursts were lengthened and became even more strongly related to cycle period, phentolamine promoted a more symmetrical rhythm with shorter promotor bursts that were less dependent on cycle period. When octopamine was applied in the presence of phentolamine, there was no significant increase in cycle period or burst duration, although high octopamine concentrations (100 µM) were still capable of inhibiting promotor nerve activity. To our knowledge, pharmacological modulation of a spontaneous locomotor rhythm by an amine antagonist (applied by itself) has not been reported previously. The results raise the testable possibility that phentolamine exerts its modulatory effects by acting as an octopamine antagonist in crayfish thoracic ganglia.