Localization of GABA-like immunoreactivity in the central nervous system of Aplysia californica

Gamma-aminobutyric acid (GABA) is present in the central nervous system of Aplysia californica (Gastropoda, Opisthobranchia) where its role as a neuro transmitter is supported by pharmacological, biochemical, and anatomical in vestigations. In this study, the distribution of GABA-immunoreactive (GABAi ) neurons and fiber systems in Aplysia was examined by using wholemount imm unohistochemistry and nerve backfill methods. GABAi neurons were located in the buccal, cerebral, and pedal ganglia. Major commissural fiber systems w ere present in each of these ganglia, whereas more limited fiber systems we re observed in the ganglionic connectives. Some of the interganglionic fibe rs were found to originate from two unpaired GABAi neurons, one in the bucc al ganglion and one in the right pedal ganglion, each of which exhibited bi lateral projections. No GABAi fibers were found in the nerves that innervat e peripheral sensory, motel; or visceral organs. Although GABAi cells were not observed in the pleural or abdominal ganglia, these ganglia did receive limited projections of GABAi fibers originating from neurons in the pedal ganglia. The distribution of GABAi neurons suggests that this transmitter s ystem may be primarily involved in coordinating cel tain bilateral central pattern generator (CPG) systems related to feeding and locomotion. In addit ion, the presence of specific interganglionic GABAi projections also sugges ts a role in the regulation or coordination of circuits that produce compon ents of complex behaviors. (C) 1999 Wiley-Liss, Inc.