GABAergic inhibition upon auditory response properties of neurons in the dorsal cochlear nucleus of the rat.

It is well known that the superficial layers of the dorsal cochlear nucleus (DCN) are rich in GABAergic neurons. We investigated the effects of topical application of GABA receptor agonists and/or antagonists upon the auditory response properties of DCN neurons in rats anesthetized with alpha chloralose-urethane. Auditory stimuli consisted of 20 ms tone bursts presented in a free field. Response properties of DCN neurons were studied before and during iontophoretic application of GABA, bicuculline methiodide (BIC) and muscimol (MUS) alone and GABA with MUS or BIC through triple barrel electrodes glued to the recording microelectrode. Of 68 DCN neurons studied, 27 were sensitive to topical application of the GABA agonists or antagonist. In these neurons, BIC enhanced spontaneous activity as well as auditory responses and decreased the Q-30 quality factor values. MUS reduced auditory responses. BIC often increased the width of the turning curve but GABA and/or MUS reduced it. Without drug application, GABA sensitive neurons tended to have longer response latencies and larger tuning widths at 30 dB above threshold as well as larger Q-30 values as compared with neurons that were insensitive to GABA. These findings suggest that: 1) GABAergic neurons determine the width of the tuning curve in neurons with GABA receptors by curtailing the excitatory response area, and 2) such neurons receive tonic inhibition from intrinsic GABAergic neurons.