Journal of Neuroscience, Vol 14, 3565-3577, Copyright © 1994 by Society for Neuroscience
Identification and characterization of pleural neurons that inhibit tail sensory neurons and motor neurons in Aplysia: correlation with FMRFamide immunoreactivity
Y Xu, LJ Cleary and JH Byrne
Department of Neurobiology and Anatomy, University of Texas Medical School, Houston 77225.
Neurons on the rostral 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, RPI4 and RPI5,
according to their affinity for an antibody directed against FMRFamide.
RPI4 was not FMRFamide immunoreactive, and RPI5 was. RPI4 and RPI5 were
found to have different electrophysiological profiles. The summated IPSPs
in sensory neurons produced by RPI4 developed more rapidly and had a
shorter duration than those produced by RPI5. In addition, RPI4 produced
IPSPs in the tail motor neurons, whereas RPI5 did not. Both RPI4 and RPI5
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 RPI4, the excitatory synaptic inputs
were followed by short and transient hyperpolarization, whereas in RPI5,
the excitatory synaptic inputs were followed by slow and long-lasting
hyperpolarization. Excitatory inputs elicited in RPI4 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 RPI4 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.
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