Meeting Abstract

120.5  Saturday, Jan. 7  Is GABA an Inhibitory Neurotransmitter in the Neural Circuit Regulating Metamorphosis in Ilyanassa obsoleta? BISCOCHO, D. *; LEISE, E.M.; University of North Carolina Greensboro; University of North Carolina Greensboro

Metamorphosis in the marine mud snail, Ilyanassa obsoleta, is an irreversible biological event by which larvae metamorphose into sexually immature juveniles that reside on coastal mudflats. I. obsoleta undergoes transformations that include loss of the apical ganglion, reorganization of brain ganglia, and disappearance of the velar lobes that are essential in mobility and feeding. The neurotransmitter γ-aminobutyric acid (GABA) can induce metamorphosis in the abalone Haliotis rufescens (Morse, et al 1979. Science 204, 407-410), but results of our experiments with I. obsoleta have revealed an inhibitory action for this compound. Bath application of GABA to larval I. obsoleta does not elicit metamorphosis. Earlier work on I. obsoleta demonstrated that the neurotransmitters serotonin (5-HT) and nitric oxide function as a promoter and repressor of metamorphosis, respectively. GABA can inhibit serotonergically induced metamorphosis, suggesting that GABA acts either downstream of, or directly on serotonergic neurons. We are currently exploiting reagents that modify the function of GABA transport, synthesis and metabolism. In bath application, muscimol, a GABAA agonist, inhibits spontaneous metamorphosis for 48 hours. Bath application of isoguvacine, another GABAA agonist or aminooxyacetic acid (AOA), a major inhibitor of GABA-Transaminase can repress metamorphosis in 5-HT-induced larvae. Currently, experiments in which we inject reagents into competent larvae allow us to drastically decrease chances of interactions between neuroactive compounds and epidermal sensory receptors. These experiments should further support the idea that GABAergic activity occurs internally within the larval central nervous system, perhaps in the apical ganglion. Together, our data will help to elucidate the role of GABA in this molluscan metamorphic pathway.