Activation of spinally projecting and nitrergic neurons in the PVN following heat exposure

1 School of Medical Sciences, RMIT University; and 2 Howard Florey Institute, University of Melbourne, Victoria, Australia Submitted 18 September 2005 ; accepted in final form 17 February 2006 The present study investigated the effect of acute thermal stimulation in conscious rats on the production of Fos, a marker of increased neuronal activity, in spinally projecting and nitrergic neurons in the hypothalamic paraventricular nucleus (PVN). The PVN contains a high concentration of nitrergic neurons, as well as neurons that project to the intermediolateral cell column (IML) of the spinal cord that can directly influence sympathetic nerve activity (SNA). During thermal stimulation, the PVN is activated, but it is unknown whether spinally projecting PVN neurons and the nitrergic neurons are involved. Compared with controls, rats exposed to an environmental temperature of 39°C for 1 h had a 10-fold increase in the number of cells producing Fos in the PVN (133 ± 23 vs. 1,336 ± 43, respectively, P < 0.0001). Of the spinally projecting neurons in the PVN of heated rats (98 ± 10), over 20% expressed Fos. Additionally, of the nitrergic neurons (NADPH-diaphorase positive) located in the parvocellular PVN (723 ± 17), 40% also expressed Fos ( P < 0.0001 compared with controls). Finally, there was a significant increase in the number of spinally projecting neurons in the PVN that were nitrergic and expressed Fos after heat exposure (12%) compared with controls (0.1%) ( P < 0.0001). These results suggest that spinally projecting and nitrergic neurons in the PVN may contribute to the central pathways activated by thermal stimulation. Fos immunohistochemistry; spinally projecting Address for reprint requests and other correspondence: E. Badoer, School of Medical Sciences, RMIT Univ., PO Box 71, Bundoora 3083, Melbourne, Victoria, Australia (e-mail: emilio.badoer{at}rmit.edu.au )