Effect of GABA on the processing of interaural time differences in nucleus laminaris neurons in the chick

Neurons in the avian nucleus laminaris (NL) are the first to receive binaural information and are presumed to play a role in encoding interaural time differences (ITD). NL not only receives excitatory projections from the ipsi- and contralateral nucleus magnocellularis, but also receives inhibitory (GABAergic) input. This study investigates how GABA (gamma-aminobutyric acid) influences ITD coding in NL. Intracellular responses of chick NL neurons were studied in a brain slice preparation. Both excitatory inputs to NL were electrically activated and the delay between trains of bilateral stimuli (simulated-interaural time difference [s-ITD]) was varied. The resulting s-ITD functions were recorded in the presence of 0-75 microM GABA. The discharge rate of NL neurons varied with s-ITD. Cells responded maximally using s-ITDs at which the peak of the ipsi- and contralateral excitatory postsynaptic potentials occurred simultaneously (favourable s-ITD). At unfavourable s-ITDs, the discharge rates usually fell below unilateral levels. GABA had contrary effects on the s-ITD functions depending on the drug concentration. A low GABA dose enhanced excitability at favourable s-ITD, but not at unfavourable s-ITDs. In contrast, higher GABA concentrations diminished excitability. Moderate GABA concentrations had no consistent effect. These results suggest that the GABAergic input to NL will either increase or decrease the excitability of the NL neuron depending on the degree to which this GABAergic input is activated. A gain control hypothesis is presented in which the GABAergic input makes ITD processing in NL independent of the stimulus intensity by adjusting the excitability of NL neurons.