A dendritic GABA(A)-mediated IPSP regulates facilitation of NMDA-mediated responses to burst stimulation of afferent fibers in piriform cortex

Studies in a number of cortical systems have shown that the NMDA component of the EPSP is strongly regulated by GABA(A)-mediated inhibition. The present study explored the possibility that specificity in inhibitory circuitry could allow such regulation to occur during normal function without increasing the propensity for epileptiform bursting, which occurs with indiscriminate GABA(A) blockade. Specifically, the hypothesis was tested that a dendritic GABA(A)-mediated IPSP is present which strongly modulates the NMDA component and can be activated independently of the somatic IPSP. The experiments were performed on slices of piriform cortex in which the NMDA component of the EPSP was pharmacologically isolated by bath-applied 6,7-dinitroquinoxaline-2,3-dione. A facilitation of NMDA responses to burst stimulation of afferent fibers is described, which required GABA, blockade and sewed as an assay for the presence of a functionally significant GABA(A) input. When bicuculline was applied focally in the somatic region, the feedback IPSP was blocked with little or no increase in the NMDA component of the response to burst stimulation of afferent fibers. In contrast, when bicuculline was applied focally in the dendritic region, the NMDA-mediated response to burst stimulation was facilitated with minimal effect on the somatic IPSP, confirming the hypothesis.