Serotonergic modulation of nonspiking local interneurones in the terminal abdominal ganglion of the crayfish.

The modulatory effect of serotonin on local circuit neurones forming the uropod motor control system of the crayfish Procambarus clarkii Girard was analysed electrophysiologically. Bath application of 10 micro mol l(-1) serotonin caused a decrease in the tonic spike activity of the exopodite reductor motor neurone. The inhibitory effect of serotonin on the motor neurone was dose-dependent and its spike discharge was completely suppressed for long periods by 1 mmol l(-1) serotonin perfusion. Nonspiking local interneurones in the terminal abdominal ganglion showed either a membrane depolarization (N=6) or hyperpolarization (N=9) of 10-30 mV in amplitude when 100 micro mol l(-1) serotonin was perfused for 3-5 min. By contrast, spiking local interneurones and intersegmental ascending interneurones showed no observable excitatory responses to the perfusion of serotonin but instead some showed a small membrane hyperpolarization of 2-5 mV. These results indicate that the nonspiking interneurones could contribute substantially to the level of tonic excitation of the uropod motor neurones. Sensory stimulation elicited depolarizing or hyperpolarizing potentials in the nonspiking interneurones and excitatory postsynaptic potentials (EPSPs) and spikes in the spiking interneurones. The sensory responses of spiking interneurones increased during bath application of serotonin and were reduced after 20-30 min of washing with normal saline. In the nonspiking interneurones, the amplitude of both depolarizing and hyperpolarizing potentials increased without any direct correlation with the serotonin-mediated potential change. This effect of serotonin was long-lasting and continued to enhance the responses of the nonspiking interneurones after washing. This postserotonin enhancement persisted for over 1 h.