Repetitive axonal discharges elicited by serotonin and intracellular adenosine 3',5'-cyclic monophosphate in the crayfish neuromuscular junction.

Experiments were carried out in the opener muscle of the claw of small crayfish. After pretreatment of the preparation with serotonin (5-HT), application of the membrane permeant analogue of adenosine 3',5'-cyclic monophosphate (cAMP), 8-bromoadenosine 3',5'-monophosphate was capable of evoking reversibly repetitive discharges in the inhibitory and excitatory axon. Reducing phosphodiesterase activity with application of either 3-isobutyl-1-methylxanthine or theophylline also elicited repetitive axonal discharges after 5-HT treatment. Moreover, application of forskolin dissolved in ethanol caused repetitive axonal discharges. The chemically induced presynaptic action potentials were detected mainly by their postsynaptic effects, i.e. by recording inhibitory and excitatory postsynaptic currents in voltage-clamped muscle fibres. In addition, nerve spikes were recorded extracellularly. It is concluded that 5-HT and intraaxonal cAMP alter membrane properties of the efferent axons innervating crayfish muscle.