Paradoxical effect of NKCC1 inhibition of GABAergic signaling in the paraventricular nucleus of rats subjected to chronic mild unpredictable stress

Classically, activation of gamma aminobutyric acid type A receptors (GABAA) inhibits neural excitation. Recent findings indicate that adaptations occur during pathological situations resulting in plasticity in GABAergic regulation. Depression may be such a condition. GABAA inhibition requires maintenance of low intracellular chloride (Cl-i) which depends upon extrusion of chloride ion by the potassium chloride co-transporter 2 (KCC2). In contrast, chloride ion is pumped into the cell by the sodium potassium two chloride co-transporter 1 (NKCC1). Downregulation of KCC2 or upregulation of NKCC1 would result in elevated Cl-i thereby rendering GABA less inhibitory or even excitatory. We hypothesized that inhibition of KCC2 would inhibit whereas inhibition NKCC1 would enhance GABAA inhibition within the paraventricular nucleus (PVN) of rats subjected to a chronic mild unpredictable stress (CMS) paradigm, a validated model of depression. Male and female Sprague Dawley rats were subjected to control or CMS conditions for 4 wks. MAP, heart rate and renal sympathetic nerve activity (RSNA) were assessed in conscious chronically instrumented rats at baseline and after PVN injection of 9 nmol muscimol or vehicle alone or after PVN injection of 20 pmol VU0240551 (VU; KCC2 inhibitor) or 20 pmol bumetanide (BMX, NKCC1 inhibitor). Muscimol decreased MAP and RSNA in control male and female rats but failed to do so in CMS rats. Pre-injection with VU prevented the decrease in MAP in both control and CMS rats. Unexpectedly, pre-injection with BMX also prevented the decrease in MAP in both male and female control and CMS rats. RSNA also did not decrease and actually increased in control male rats after NKCC1 inhibition (P < 0.025 vs baseline). The paradoxical response to GABAergic stimulation after NKCC1 inhibition of the PVN may be due to effects on an heterogenous population of neurons within the PVN with some neurons hyperpolarizing and others depolarizing in response to GABA stimulation. These findings are important in that they were performed in conscious animals; however, the studies are limited by the impact of inhibitors as well as muscimol on multiple neuronal populations. Future studies in individual neurons will be needed to untangle the complex relationship of neuronal elements within the PVN in both control and stress situations. Funded by VA Merit BX003480). This abstract was presented at the American Physiology Summit 2026 and is only available in HTML format. There is no downloadable file or PDF version. The Physiology editorial board was not involved in the peer review process.