GABA REGULATES THE BUCCAL MOTOR OUTPUT OF HELISOMA BY 2 PHARMACOLOGICALLY DISTINCT ACTIONS

GABA was tested for its effects on patterned motor activity (PMA) unde rlying feeding. Using buccal motoneuron B19 to monitor PMA through int racellular recordings, GABA was found to exert effects at two levels. First, GABA stimulated rhythmic patterned activity resembling fictive feeding, which is under the control of the buccal CPG. In addition, GA BA produced a direct inhibition of neuron B19. Both effects were obser ved when the buccal ganglia were studied in isolation from the rest of the central nervous system, suggesting local interactions with GABA r eceptors of buccal neurons. Furthermore, these two actions of GABA wer e found to be pharmacologically distinguishable. The direct hyperpolar ization of neuron B19 was mimicked by muscimol, but not baclofen, and involved an increased chloride conductance, which was blocked by picro toxin. Baclofen duplicated CPG activation by GABA. Picrotoxin had no e ffect on GABA- or baclofen-induced PMA. These results demonstrate that the Helisoma buccal ganglia have two GABA receptor types which resemb le, pharmacologically, mammalian GABA, and GABA, receptors, and that G ABA plays a key role in feeding patterned motor activity in Helisoma.