Identification and Characterization of Pleural Neurons that Inhibit Tail Sensory Neurons and Motor Neurons in Aplysia: Correlation with FMRFamide lmmunoreactivity

Neurons on the rostra1 edge of the ventral surface of the right pleural ganglion were identified as elements of the circuit mediating the defensive tail withdrawal reflex of Aplysia. These neurons produced IPSPs in tail sensory neurons and were classified into two groups, RPI, and RPI,, according to their affinity for an antibody directed against FMRFamide. RPI, was not FMRFamide immunoreactive, and RPI, was. RPI, and RPI, were found to have different electrophysiological profiles. The summated IPSPs in sensory neurons produced by RPI, developed more rapidly and had a shorter duration than those produced by RPI,. In addition, RPI, produced IPSPs in the tail motor neurons, whereas RPI, did not. Both RPI, and RPI, received excitatory synaptic inputs from stimulation of the pleural-abdominal connective as well as peripheral nerves P8 and P9, which innervate the tail and posterior part of the animal’s body. These inputs were sufficient to elicit spikes. In RPI,, the excitatory synaptic inputs were followed by short and transient hyperpolarization, whereas in RPI,, the excitatory synaptic inputs were followed by slow and long-lasting hyperpolarization. Excitatory inputs elicited in RPI, by stimulation of peripheral nerves appeared to be mediated, at least in part, by activation of tail sensory neurons. Intracellular stimulation of sensory neurons produced EPSPs in RPI, that appeared to be monosynaptic. These results suggest that inhibitory interneurons underlying the circuit of the tail withdrawal reflex may play roles in mediating or modulating neuronal responses to tail stimulation. By inhibiting tail sensory and motor neurons, these interneurons may reduce the effectiveness of an animal’s response to stimulation of the tail. [