Participation of GABA in establishing behavioral hierarchies in the terrestrial snail

GABA-immunoreactive fibers were observed in the neuropile of each ganglion of Helix lucorum, while GABA-immunoreactive neural somata were found only i n the buccal, cerebral, and pedal ganglia. Bath application of 10(-5) M GAB A to the preparation "buccal mass-buccal ganglia" elicited a sequence of ra dula movements characteristic of feeding behavior. Corresponding bursts of activity were recorded in the buccal nerves under GABA application and in t he buccal neurons recorded optically. In preparations of isolated central n ervous system, the bath applications of GABA (10(-5) to 10(-4) M) elicited no changes in synaptic input of the premotor interneurons involved in the w ithdrawal behavior. However, a significant decrease in amplitude of the syn aptic input and in the number of spikes in responses elicited by the test n erve stimulation was observed in metacerebral serotonergic neurons involved in modulating the feeding behavior. GABA application inhibited the spontan eous spike activity in some pedal serotonergic neurons involved in the netw ork underlying withdrawal responses and evoked bursting activity in the oth er neurons of this functional group. The effects of GABA application on mec hanically isolated serotonergic neurons suggest that the primary effect of GABA is inhibition. Thus, our results give evidence of the putative role of GABA in activating the feeding behavior and in the synergistic suppression of serotonergic modulation of the withdrawal behavior and serotonergic mod ulation of feeding, which has corresponded to the observed behavioral suppr ession of withdrawal reactions during feeding.