MODULATION OF ELECTRICAL-ACTIVITY BY 5-HYDROXYTRYPTAMINE IN CRAYFISH NEUROSECRETORY-CELLS

The effect of 5-hydroxyptryptamine (5-HT) was tested in a population o f X organ neurosecretory cells in the eyestalk of the crayfish Procamb arus clarkii, Tests were conducted both irt situ and on isolated neuro nes kept in culture, The application of 5-HT induced action potentials in silent cells, In spontaneously active neurones, 5-HT increased the firing rate and either induced firing or enhanced bursting activity. The effect of 5-HT was dose-dependent within the range 1-100 mu mol l( -1) in cells of the intact organ, The effect persisted for 20-30min af ter 5-HT had been removed from the bathing solution, Successive applic ations of 5-HT onto the same neurone reduced responsiveness, suggestin g that desensitization had occurred. The effects of 5-HT were blocked by prior incubation with the 5-HT antagonist methysergide. In X organ cells whose axons and branches in the neuropile had been severed, 5-HT induced a depolarisation associated with a slow inward current, In X organ neurones isolated from the eyestalk and kept in culture, 5-HT wa s capable of evoking bursts of action potentials and elicited a slow i nward current. This effect was also blocked by methysergide (10(-4) mo l l(-1)). These results suggest a direct modulatory effect of 5-HT on the pattern of electrical activity in the X organ cells.