A Glutamatergic Hypothalamomedullary Circuit Mediates Thermogenesis, but Not Heat Conservation, during Stress-Induced Hyperthermia.

Stress elicits a variety of autonomic responses, including hyperthermia (stress fever) in humans and animals. In this present study, we investigated the circuit basis for thermogenesis and heat conservation during this response. We first demonstrated the glutamatergic identity of the dorsal hypothalamic area (DHA(Vglut2)) neurons that innervate the raphe pallidus nucleus (RPa) to regulate core temperature (Tc) and mediate stress-induced hyperthermia. Then, using chemogenetic and optogenetic methods to manipulate this hypothalamomedullary circuit, we found that activation of DHA(Vglut2) neurons potently drove an increase in Tc, but surprisingly, stress-induced hyperthermia was only reduced by about one-third when they were inhibited. Further investigation showed that DHA(Vglut2) neurons activate brown adipose tissue (BAT) but do not cause vasoconstriction, instead allowing reflex tail artery vasodilation as a response to BAT-induced hyperthermia. Retrograde rabies virus tracing revealed projections from DHA(Vglut2) neurons to RPa(Vglut3), but not to RPa(GABA) neurons, and identified a set of inputs to DHA(Vglut2) → RPa neurons that are likely to mediate BAT activation. The dissociation of the DHA(Vglut2) thermogenic pathway from the thermoregulatory vasoconstriction (heat-conserving) pathway may explain stress flushing (skin vasodilation but a feeling of being too hot) during stressful times.