GABA CAN IMPROVE ACOUSTIC CONTRAST IN THE RAT VENTRAL COCHLEAR NUCLEUS

The effect of microiontophoretically applied gamma-aminobutyric acid ( GABA) and its agonists and antagonists on the response pattern of sing le units in the ventral cochlear nucleus (VCN) of the rat was examined in order to study GABA's physiological function in auditory processin g. The effects of the drugs were judged by changes of spontaneous and sound-evoked activity in peristimulus-time histograms (PSTHs) of at le ast 20 consecutive presentations of acoustic stimuli. GABA inhibited t he discharge activity of the majority of neurons. All response types f ound in the VCN except onset-I responders were sensitive to GABA. The GABAergic inhibition is most probably mediated by GABA(A) receptors, s ince the GABA(A)-receptor agonist muscimol, but not the GABA(B)-recept or agonist baclofen, mimicked the effect of GABA. The GABA(A)-receptor antagonists, bicuculline and picrotoxin, had an excitatory effect on the neurons' spontaneous activity, suggesting a tonic endogeneous rele ase of GABA which exerts a permanent inhibition on VCN neurons. Althou gh inhibitory, iontophoresis of GABA emphasized the response to stimul us onset in the PSTHs by means of a stronger inhibition of spontaneous activity. When using iontophoretical currents which did not suppress the neuronal activity completely, a strong inhibition of spontaneous a ctivity was accompanied by only a small inhibition of tone-evoked acti vity. Under these conditions, the response to tone onset was frequentl y not inhibited at all. Therefore, GABA's physiological function is po ssibly to improve the contrast between transient acoustic signals and ongoing background activity. In order to test this hypothesis, the tes t tone was masked by continuous background noise. Indeed, GABA reduced the noise-evoked discharge more than the tone-evoked discharge, leavi ng the onset peak in the PSTHs almost unchanged. Thus, GABAergic input improves the signal-to-noise ratio for acoustic transients in VCN neu rons. Our data suggest that a functional role of GABA in the VCN is to act as a transmitter within a descending inhibitory feedback loop of the auditory brainstem which serves to improve the transmission of rel evant acoustic signals in constant background noise.