Unusual features of GABA responses in layers IV-V neurons of neocortex.

Perikaryal application of GABA produced a hyperpolarization and increased the input conductance in neurons of layers IV-V of neocortex (guinea pig). This response faded during brief applications and had a long duration when the application period was increased from 4s to greater than 10 s. The sensitivity of the first response to blockade by the selective antagonist, bicuculline, indicated a mediation by gamma-aminobutyric acid-A (GABAA) receptors. The longer duration response was mimicked to some extent by the GABAB agonist, baclofen. Dendritic application of GABA induced a depolarization and a conductance increase - a response which was not particularly sensitive to antagonism by bicuculline. The depolarizing response also did not have a clearly defined reversal potential and may be a consequence of complex changes in membrane conductance, possibly for Cl and Ca or Na. Fading in both types of responses may result from a concomitant postsynaptic activation of a Cl conductance with Na-dependent GABA uptake.