Effects of stress on the GABAergic system in the hippocampal formation and medial prefrontal cortex of the adult male rat

Abstract

Stress facilitates the development of psychiatric disorders in vulnerable individuals. Previous studies have well documented the impact of stress on glutamatergic excitatory transmission in limbic brain regions, but little is known about stress effects on the GABAergic network. The aims of the present thesis were 1) to investigate effects of acute as well as chronic restraint stress on activities of interneurons in the hippocampal formation and the medial prefrontal cortex (mPFC) of adult male rats; and 2) to identify the neurophysiological mechanisms which may contribute to potential pathogenetic processes of chronic stress. In the first part of this thesis, effects of stress and the synthetic glucocorticoid dexamethasone (DEX) on hippocampal GABAergic transmission and network function were studied, focusing on two perisomatic interneurons, the parvalbumin (PV) and the cholecystokinin (CCK)-expressing interneurons. Whole-cell voltage clamping of CA1 pyramidal neurons of acute hippocampal slices revealed a rapid increase of spontaneous inhibitory postsynaptic currents (sIPSCs) following bath application of the potent glucocorticoid receptor agonist DEX. This effect of DEX was mediated by a nongenomic glucocorticoid receptor which evoked nitric oxide (NO) release from pyramidal neurons. Retrograde NO signaling caused the augmentation of GABA release from the interneurons and additionally increased CCK release from CCK interneurons which in turn further enhanced the activity of the PV interneurons. Chronic restraint stress also resulted in a Ca2+-dependent increase in sIPSCs in CA1 pyramidal neurons and an additional DEX application elicited no further effect. Concomitantly, chronic stress reduced the number of PV-immunoreactive cells and impaired rhythmic sIPSCs originating from the PV-positive interneurons. In contrast, these parameters of CCK interneurons remained unaffected. Because CCK specifically stimulates the PV interneurons it is proposed here that, in addition to the immediate effect, the sustained activation of nongenomic glucocorticoid receptors during chronic stress may eventually injure the PV-interneuron network and lead to its functional impairment observed in this study. Furthermore, chronic restraint stress resulted in a dysfunction in CB1 mediated modulation of GABAergic transmission in the hippocampus. In acute hippocampal slices, depolarization induced suppression of inhibition (DSI; a form of short term plasticity at the GABAergic synapses known to be CB1 mediated and suggested to be involved in hippocampal information encoding) was impaired by chronic restraint stress. Because hippocampal CB1 receptors locate almost exclusively on the CCK interneurons, these data demonstrate altered functioning of the CCK interneurons following chronic stress. Taken together, chronic stress had differential impacts on the structural and functional integrity of the CCK and PV interneurons in the hippocampus and, probably as a consequence, resulted in an imbalance in perisomatic inhibition mediated by the PV and CCK interneurons. The stress-induced dysfunctional inhibitory network may in turn impair rhythmic oscillations and thus lead to cognitive deficits that are common in animal models as well as in patients with stress-related psychiatric disorders. The second part of this thesis describes effects of restraint stress in the mPFC with a discrimination of the right and left hemisphere. Three weeks of chronic stress decreased the number of PV-immunoreactive interneurons in the right mPFC while the number of PV cells in the left mPFC was unaffected. Whole-cell voltage clamp recording in layer II/III pyramidal neurons in acute PFC slices showed that 30 min of acute stress increased sIPSC frequency specifically in the right mPFC but induced DSI sensitivity specifically in the left mPFC. Both effects of acute stress could be abolished in the presence of a dopamine D1 receptor antagonist. Furthermore, acute application of dopamine mimicked the observed effect in a dose-dependent manner. Whereas a low concentration of dopamine evoked enhanced sIPSC frequency, mimicking the stress effect in the right mPFC, a higher concentration induced DSI sensitivity, mimicking the stress effect in the left mPFC. This is in consistence with previous reports showing that a brief restraint stress resulted in a left>right increase in dopamine release in the mPFC. These findings provide preliminary physiological evidences suggesting how the lateralized stress-induced dopamine release may contribute to the lateralized mPFC functioning which is probably necessary for efficient coping with acute environmental stressors. In summary, the present thesis describes in detail the effects of chronic and acute restraint stress on the GABAergic system in hippocampal formation and mPFC. The results increase the understanding of how changes in the activity of the interneuron network may contribute to the responses to stress of varying durations.