A Hypothalamic Inhibitory Circuit Encoding the Scalability of Stress Responses

Abstract

An appropriate stress response is essential for properly responding to, coping with, and subsequently recovering from disturbing environmental stimuli. However, how the brain dynamically encodes the scalability of stress responses remains poorly understood. Here, we found that, GABAergic neurons in the arcuate nucleus (Arc, denoted as ArcGABA neurons) send direct inputs to corticotropin-releasing hormone (CRH) neurons in the paraventricular nucleus of the hypothalamus (PVH, denoted as PVHCRH neurons), the primary regulators of the hypothalamic-pituitary-adrenal (HPA) axis. Although PVHCRH neurons exhibited time-locked activation in response to various environmental stressors, both GABA release onto PVHCRH neurons and the activity of PVHCRH-projecting ArcGABA neurons were selectively reduced during exposure to prolonged, high-intensity stressors, but not following exposure to transient, low-intensity stressors. Notably, GABA release onto PVHCRH neurons was positively correlated with PVHCRH-projecting ArcGABA neuron activity, yet anticorrelated with PVHCRH neuronal activity in response to the same prolonged, high-intensity stressors. Selective silencing of PVHCRH-projecting ArcGABA neurons was sufficient to elevate HPA axis activity and stress levels, phenocopying the effect of direct of PVHCRH neuron activation. Conversely, selective activation of PVHCRH-projecting ArcGABA neurons reduced both HPA axis activity and stress levels, this effect was completely abolished by concurrent excitation of PVHCRH neurons. Molecular identity screening further revealed that these PVHCRH-projecting ArcGABA neurons are not subsets expressing agouti-related peptide (AgRP) and tyrosine hydroxylase (TH) markers. Collectively, these findings indicate that the non-AgRP/TH ArcGABA PVHCRH neurocircuit serves as a critical neural substrate that directly encodes the scalability of stress responses to environmental stressors by modulating inhibitory GABA release in a stimulus intensity-dependent manner.