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γ‐Aminobutyric acid is present in a spatially discrete subpopulation of hair cells in the crista ampullaris of the toadfish Opsanus tau

Although γ‐aminobutyric acid (GABA) and glutamate are known to be present in the vestibular sensory epithelia of a variety of species, the functional relationship between these two transmitters is not clear. The present study addresses the three‐dimensional spatial distribution of GABA and glutamate immunoreactivity in the vestibular labyrinth of the oyster toadfish by using whole end organs labeled by immunofluorescence with monoclonal anti‐GABA and/or antiglutamate antibodies and visualized as whole mounts by multiphoton confocal microscopy. We find glutamate‐immunoreactive hair cells present throughout the sensory epithelium. In contrast, prominent GABA immunoreactivity is restricted to a small population of hair cells located in the central region of the crista. Double immunofluorescence reveals two distinct staining patterns in GABA‐labeled hair cells. Most (∼80%) GABA‐labeled cells show trace levels of glutamate, appropriate for the metabolic/synthetic role of cytoplasmic glutamate. The remainder of the GABA‐stained cells contain substantial levels of both GABA and glutamate, suggesting transmitter colocalization. In the toadfish utricle, glutamatergic hair cells are present throughout the macula. GABA‐immunoreactive hair cells follow the arc of the striola, and most GABA‐labeled receptor cells coexpress glutamate. The localization of GABA was explored in other species as well. In the pigeon, GABAergic hair cells are present throughout the crista ampullaris. Our findings demonstrate that multiple, neurochemically distinct types of hair cells are present in vestibular sensory epithelia. These observations, together with the excitatory activity generally associated with 8th nerve afferent fibers, strongly suggest that GABA serves an important, specific, and complex role in determining primary afferent response dynamics. J. Comp. Neurol. 471:1–10, 2004. © 2004 Wiley‐Liss, Inc.