GABA‐Mediated Synaptic Interaction Between the Visual and Vestibular Pathways of Hermissenda

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Abstract
The synaptic convergence of the eyes and the vestibular hair cells in the nudibranch mollusc Hermissenda has been shown previously to mediate the learning of simple visual‐vestibular associations. The neurotransmitter mediating this interaction between the visual and vestibular organs was characterized. HPLC chromatography, confirmed by mass spectroscopic analysis, demonstrated endogenous GABA in the statocysts, in a concentration approximately 150 times greater than in the whole CMS. Additional confirmation was provided by immunocytochemical localization of GABA in hair cell axons and branches that converge with photoreceptor 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 the type B photoreceptor impulse activity. The inhibition of the B cell was unaffected by addition to the artificial sea water bath of the adrenergic antagonist yohimbine (250 μM), the cholinergic antagonist atropine (250 μM), and the serotonergic antagonist imipramine (50 μM). In contrast, the GABAA antagonist bicuculline (250 μM) significantly reduced the inhibitory interaction. Moreover, the GABA reuptake inhibitor guvisine (250 μM)M) increased the hyperpolarization. Pressure microapplication of GABA (12.5 or 25 μM) onto the terminal branches of the B cell resulted in a concentration‐dependent hyperpolarization and cessation of spikes in the B cell. Depolarization of the caudal hair cell, or direct GABA application, decreased input resistance across the B cell soma membrane. Moreover, removal of chloride from the extracellular solution reduced inhibition of the B cell induced by GABA application or hair cell stimulation. Furthermore, application of the GABAB agonist baclofen hyperpolarized the type B cell and reduced or eliminated 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+ conductances. These results implicate GABA as the endogenous neurotransmitter mediating visual‐vestibular interactions in this animal, and suggest a possible role of GABA in visual‐vestibular associative learning.