GABA-MEDIATED SYNAPTIC INTERACTION BETWEEN THE VISUAL AND VESTIBULAR PATHWAYS OF HERMISSENDA

The synaptic convergence of the eyes and the vestibular hair cells in the nudibranch mollusc Hermissenda has been shown previously to mediat e the learning of simple visual-vestibular associations. The neurotran smitter mediating this interaction between the visual and vestibular o rgans was characterized. HPLC chromatography, confirmed by mass spectr oscopic analysis, demonstrated endogenous GABA in the statocysts, in a concentration approximately 150 times greater than in the whole CNS. Additional confirmation was provided by immunocytochemical localizatio n of GABA in hair cell axons and branches that converge with photorece ptor terminal branches. Depolarization of the hair cells in the caudal region of the statocyst in response to positive current injection or vibratory stimulation caused a hyperpolarization and a cessation of th e type B photoreceptor impulse activity. The inhibition of the B cell was unaffected by addition to the artificial sea water bath of the adr energic antagonist yohimbine (250 muM), the cholinergic antagonist atr opine (250 muM), and the serotonergic antagonist imipramine (50 muM). In contrast, the GABA(A) antagonist bicuculline (250 muM) significantl y reduced the inhibitory interaction. Moreover, the GABA reuptake inhi bitor guvisine (250 muM) increased the hyperpolarization. Pressure mic roapplication of GABA (12.5 or 25 muM) onto the terminal branches of t he B cell resulted in a concentration-dependent hyperpolarization and cessation of spikes in the B cell. Depolarization of the caudal hair c ell, or direct GABA application, decreased input resistance across the B cell soma membrane. Moreover, removal of chloride from the extracel lular solution reduced inhibition of the B cell induced by GABA applic ation or hair cell stimulation. Furthermore, application of the GABA(B ) agonist baclofen hyperpolarized the type B cell and reduced or elimi nated spontaneous impulse activity at the resting membrane potential. The reversal potentials for inhibition induced in all three procedures ranged from -70 to -80 mV and were consistent with mixed Cl- and K+ c onductances. These results implicate GABA as the endogenous neurotrans mitter mediating visual-vestibular interactions in this animal, and su ggest a possible role of GABA in visual-vestibular associative learnin g.