Reduced Tonic Inhibition in the Dentate Gyrus Contributes to Chronic Stress-Induced Impairments in Learning and Memory

It is well established that stress impacts the underlying processes of learning and memory. The effects of stress on memory are thought to involve, at least in part, effects on the hippocampus, which is particularly vulnerable to stress. Chronic stress induces hippocampal alterations, including but not limited to dendritic atrophy and decreased neurogenesis, which are thought to contribute to chronic stress-induced hippocampal dysfunction and deficits in learning and memory. Changes in synaptic transmission, including changes in GABAergic inhibition, have been documented following chronic stress. Recently, our laboratory demonstrated shifts in E-GABA in CA1 pyramidal neurons following chronic stress, compromising GABAergic transmission and increasing excitability of these neurons. Interestingly, here we demonstrate that these alterations are unique to CA1 pyramidal neurons, since we do not observe shifts in E-GABA following chronic stress in dentate gyrus granule cells. Following chronic stress, there is a decrease in the expression of the GABA(A) receptor (GABA(A)R) delta subunit and tonic GABAergic inhibition in dentate gyrus granule cells, whereas there is an increase in the phasic component of GABAergic inhibition, evident by an increase in the peak amplitude of spontaneous inhibitory postsynaptic currents (sIPSCs). Given the numerous changes observed in the hippocampus following stress, it is difficult to pinpoint the pertinent contributing pathophysiological factors. Here we directly assess the impact of a reduction in tonic GABAergic inhibition of dentate gyrus granule cells on learning and memory using a mouse model with a decrease in GABA(A)R delta subunit expression specifically in dentate gyrus granule cells (Gabrd/Pomc mice). Reduced GABA(A)R delta subunit expression and function in dentate gyrus granule cells is sufficient to induce deficits in learning and memory. Collectively, these findings suggest that the reduction in GABA(A)R delta subunit-mediated tonic inhibition in dentate gyrus granule cells contributes, at least in part, to deficits in learning and memory associated with chronic stress. These findings have significant implications regarding the pathophysiological mechanisms underlying impairments in learning and memory associated with stress and suggest a role for GABA(A)R delta subunit containing receptors in dentate gyrus granule cells. (C) 2016 Wiley Periodicals, Inc.