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

SUMMARY The modulatory effect of serotonin on local circuit neurones forming the uropod motor control system of the crayfish Procambarus clarkiiGirard was analysed electrophysiologically. Bath application of 10 μmol 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 μmol l-1serotonin 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.

Processing of mechanosensory signals in local reflex pathways of the locust

The processing of mechanosensory signals responsible for the reflex adjustment of the posture or movement of the legs of the locust is described in terms of the actions and connections of identified neurones. Signals can be followed from the major classes of exteroceptors of a leg, through their various integrative stages in the central nervous system to their emergence as specific patterns in known motor neurones. Particular emphasis is placed on the integrative roles of two classes of local interneurones. The spiking local interneurones map the leg as a series of overlapping receptive fields and reverse the sign of the afferent input. The nonspiking local interneurones control the output of the motor neurones by the graded release of chemical transmitter and can adjust the gain of a local reflex depending on the position and movements of the joints of that leg. The reflex movements of one leg must not impair the stability of the animal and must therefore be influenced by events at the other legs. Populations of intersegmental interneurones convey sensory information from one segment to another to ensure such coordination. These interneurones do not produce stereotyped intersegmental reflexes but, instead, alter the performance of a local reflex in a distant leg by making synaptic connections with nonspiking local interneurones. These connections change the effectiveness of the outputs to the motor neurones and consequently the local reflex. The local interneurones therefore play a crucial role both in the production of local reflexes and in the integration of these actions with the movements of the other legs.