GABA and responses to GABA in the stomatogastric ganglion of the crab Cancer borealis

The multifunctional neural circuits in the crustacean stomatogastric gangli on (STG) are influenced by many small-molecule transmitters and neuropeptid es that are co-localized in identified projection neurons to the STG, We de scribe the pattern of gamma-aminobutyric acid (GABA) immunoreactivity in th e stomatogastric nervous system of the crab Cancer borealis and demonstrate biochemically the presence of authentic GABA in C, borealis, No STG somata show GABA immunoreactivity but, within the stomatogastric nervous system, GABA immunoreactivity co-localizes with several neuropeptides in two identi fied projection neurons, the modulatory proctolin neuron (MPN) and modulato ry commissural neuron 1 (MCN1). To determine which actions of these neurons are evoked by GABA it is necessary to determine the physiological actions of GABA on STG neurons. We therefore characterized the response of each typ e of STG neuron to focally applied GABA, All STC neurons responded to GABA. In some neurons, GABA evoked a picrotoxin-sensitive depolarizing, excitato ry response with a reversal potential of approximately -40mV, This response was also activated by muscimol, In many STG neurons, GABA evoked inhibitor y responses with both K+-and Cl--dependent components. Muscimol and beta-gu anidinopropionic acid weakly activated the inhibitory responses, but many o ther drugs, including bicuculline and phaclofen, that act on vertebrate GAB A receptors were not effective. In summary, GABA is found in projection neu rons to the crab STC and can evoke both excitatory and inhibitory actions o n STG neurons.