Gamma-aminobutyric acid is a neurotransmitter in the auditory pathway of oyster toadfish, Opsanus tau

Binaural computations involving the convergence of excitatory and inhibitory inputs have been proposed to explain directional sharpening and frequency tuning documented in the brainstem of a teleost fish, the oyster toadfish (Opsanus tau). To assess the presence of inhibitory neurons in the ascending auditory circuit, we used a monoclonal antibody to GABA to evaluate immunoreactivity at three levels of the circuit: the first order descending octaval nucleus (DON), the secondary octaval population (dorsal division), and the midbrain torus semicircularis. We observed a subset of immunoreactive (IR) cells and puncta distributed throughout the neuropil at all three locations. To assess whether contralateral inhibition is present, fluorescent dextran crystals were inserted into dorsal DON to fill contralateral, commissural inputs retrogradely prior to GABA immunohistochemistry. GABA-IR somata and puncta co-occurred with retrogradely filled, GABA-negative auditory projection cells. GABA-IR projection cells were more common in the dorsolateral DON than in the dorsomedial DON, but GABA-IR puncta were common in both dorsolateral and dorsomedial divisions. Our findings demonstrate that GABA is present in the ascending auditory circuit in the brainstem of the toadfish, indicating that GABA-mediated inhibition participates in shaping auditory response characteristics in a teleost fish as in other vertebrates.