Laminar analysis of inhibition in the gerbil primary auditory cortex

Physiological and immunocytochemical evidence indicates a layer-dependent o rganization of inhibitory circuits in the neocortex. To investigate the con tribution of GABAergic inhibition to frequency tuning in the different cort ical layers, we recorded single and multiple units in near-radial penetrati ons before and during iontophoretic application of the GABA(A)-receptor ant agonist bicuculline in the auditory cortex of the lightly anesthetized gerb il. Bicuculline generally increased the spontaneous rate and enhanced and p rolonged onset activity. Application of bicuculline often resulted in a shi ft of best frequency and a decrease of threshold (5.5 dB). A broadening of the frequency tuning evident by lower Q(40dB) values was observed in 63% of the units. In units with multipeaked tuning curves or clearly separated re sponse areas, bicuculline application removed the inhibitory regions and cr eated single-peaked tuning curves. The influence of bicuculline on the rece ptive field size was not significantly layer-specific but tended to be most pronounced in lavers V and VI. In layer VI we frequently found "silent" ne urons that responded to sound only when GABAergic inhibition was antagonize d. From the analysis of postembedding GABA immunocytochemistry, the proport ion of GABAergic neurons was found to be maximal in layers I and V, and the number of GABAergic perisomatic puncta (axon terminals) on cell somata pea ked in layer V.