The Aplysia gill-withdrawal reflex revisited: components of the network

Attempts to understand how changes at identified synapses contribute to the behavioral changes that constitute learning in the GWR have been complicated by the complexity of gill innervation. In addition to the well-studied circuit between siphon sensory neurons and identified gill motor neurons of the PVG, both PNS and as yet unidentified CNS pathways are also involved in the control of gill movement. In this study we combine an anatomical study of the PNS with physiological and behavioral analyses of the CNS's contribution to the GWR. We tested the possibility that altering the activity of an identified gill motor neuron is sufficient to alter the GWR. The results show that altering activity of GMNs has no demonstrable effect on the GWR in the suppressed behavioral state. Furthermore, activity in identified MNs may vary in response to a uniform stimulus and is not a good predictor of gill behavior. Immunohistochemical staining in gill and siphon showed discrete and well localized serotonin and SCPB-like reactivity. This is the first report of serotonin and SCPB-like immunoreactivity in the PNS of Aplysia siphon.