Exposure to a hot environment can activate spinally projecting and nitrergic neurones in the lower brainstem in the rat

Reflex responses to hyperthermia include sweating, salivation and a redirection of blood flow from the viscera to the periphery, and involve changes in peripheral nerve activity mediated by the central nervous system (CNS), including specific areas of the ventral lower brainstem. The lower brainstem contains nitrergic neurones and neurones that project to intermediolateral cell column; however, it is not known whether these populations of neurones in the lower brainstem are activated following hyperthermia. The aims of the present study were to determine whether lower brainstem neurones activated by acute hyperthermia are nitrergic and/or whether they also project to the spinal cord. Retrogradely transported rhodamine‐tagged beads were microinjected into the spinal cord. The rats were heated (environmental temperature 39°C) for 1 h. Following perfusion/fixation, brain sections were processed to detect Fos (a marker of neuronal activation) and NADPH‐diaphorase activity (a marker of nitrergic neurones). The results showed a significant increase in activated neurones in the mid‐line (by fivefold), ventromedial (by eightfold) and ventrolateral lower brainstem (by ninefold). Some of these neurones were nitrergic, particularly in the ventromedial lower brainstem (5% of the activated neurones in this region were nitrergic). A small proportion of activated neurones were spinally projecting neurones (2–3% of activated neurones were spinally projecting). There were no triple‐labelled neurones at any level of the lower brainstem examined. These findings indicate that only a small proportion of nitrergic neurones and spinally projecting neurones are activated by hyperthermia.