Hyperthermic reactions induced by orexin A: role of the ventromedial hypothalamus

This experiment tested the involvement of the ventromedial hypothalamus (VMH) in the sympathetic and hyperthermic reactions induced by an intracerebroventricular (i.c.v.) injection of orexin A (1.5 nmol). In the first part of the experiment, the firing rate and cytochrome oxidase activity of the VMH neurons, and the colonic temperature were monitored in 12 urethane-anaesthetized rats before an i.c.v. injection of orexin and over a period of 2 h after the injection. Orexin induced an increase in the firing rate, colonic temperature and cytochrome oxidase activity. A group of 12 rats was used as a control: saline, but not orexin, was injected. No modifications in the firing rate, cytochrome oxidase reactivity and colonic temperature were noted. In the second part of the experiment, 12 rats were anaesthetized and lesioned bilaterally in the VMH with an injection of ibotenic acid. Sham lesions were carried out in 12 control rats. After 48 h, all animals were anaesthetized with ethyl-urethane. The firing rates of the sympathetic nerves to interscapular brown adipose tissue (IBAT), along with IBAT and colonic temperatures and heart rate were monitored before and over a period of 2 h after an i.c.v. injection of orexin or saline in the lesioned and sham-lesioned rats. Orexin increased the sympathetic firing rate, IBAT and colonic temperatures and heart rate in the sham-lesioned rats. These increases were reduced by lesion of VMH. Saline did not induce any modification. These findings indicate that the VMH is involved in the control of the orexin-induced hyperthermia.