Received December 19, 2006
Revised January 15, 2007
Accepted after revision February 16, 2007

¹ RMIT University

^* To whom correspondence should be addressed. E-mail: emilio.badoer{at}rmit.edu.au.

	Abstract

Reflex responses to hyperthermia include sweating, salivation and a redirection of blood flow from the viscera to the periphery and involves 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 neurons and neurons that project to intermediolateral cell column, however, it is not known whether these populations of neurons in the lower brainstem are activated following hyperthermia. The aims of the present study were to determine whether lower brainstem neurons 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 39oC) for one hour. Following perfusion / fixation, brain sections were processed to detect Fos (a marker of neuronal activation) and NADPH-diaphorase activity (a marker of nitrergic neurons) The results showed a significant increase in activated neurons in the midline (by five-fold), ventromedial (by eight-fold) and in the ventrolateral (by nine-fold) lower brainstem. Some of these neurons were nitrergic, particularly in the ventromedial lower brainstem (5% of the nitrergic neurons in this region). A small proportion of activated neurons were spinally-projecting neurons (2-3% of spinal projecting neurons). There were no triple labeled neurons at any level of the lower brainstem examined. These findings indicate that only a small proportion of nitrergic neurons and spinally- projecting neurons are activated by hyperthermia.

Key Words: Autonomic circulation, Brainstem, Cardiovascular