Loss of GABAergic Inhibition in Hypothalamus in Chronic Stress

Abstract

Chronic stress increases activity of hypothalamic‐pituitary‐adrenal (HPA) axis. CRH‐expressing neuron in the PVN is an essential component of HPA axis and plays a critical role in stress response. The activity of CRH neurons is regulated by excitatory and inhibitory GABAergic synaptic inputs. We hypothesized that chronic stress diminishes GABAergic inhibition, which contributes to hyperactivity of PVN‐CRH neurons. We identified PVN‐CRH neurons by expressing green fluorescent protein (GFP) driven by CRH promoter. Chronic unpredictable mild stress (CUMS) significantly increased plasma corticosterone levels. Blocking GABAA receptor significantly increased plasma corticosterone levels in unstressed rats. Furthermore, it significantly increased firing activity of PVN‐CRH neurons in unstressed rats but did not significant change in CUMS rats, while GABAA receptor significantly decreased firing activity of PVN‐CRH neurons in unstressed rats but slightly increased in CUMS rats. CUMS treatment significantly increased Na+‐K+‐2Cl‐ cotransporter (NKCC1) protein and mRNA levels while decreased K+‐2Cl‐ (KCC2) protein and mRNA levels in the PVN tissue. Immunocytochemical staining showed that they are expressed on CRH immunoreactivity positive PVN neurons. In addition, treatment with NKCC1 specific inhibitor decreased the basal firing activity of PVN‐CRH neurons in CUMS rats, but not in unstressed rats. Furthermore, it restored gabazine‐induced excitatory effect on PVN‐CRH neurons in CUMS rats. These data suggest that GABAergic inhibition in the PVN is diminished due to upregulation of NKCC1 in chronic stress.