Effect of Fasting on c-Fos Expression in the Hypothalamic Nuclei and Nucleus of the Solitary Tract in Male Rats: Time Course Study and the Role of Testosterone.

Abstract

We have shown that 48-h fasting suppresses luteinizing hormone (LH) secretion in female rats which is enhanced by an estrogenic milieu and mediated by noradrenergic neurons. A similar fasting protocol yielded a suppression of LH secretion in male rats in a testosterone-dependent manner. In this experiment, we intended to identify specific nuclei in the hypothalamus and caudal brainstem that are activated during fasting for 6, 24, 30 or 48 h in castrated male rats with or without chronic testosterone treatment. Fasting started at 1300 h (lights-off at 1900 h). In testosterone-treated castrates, Fos-like-immunoreactive (FL-ir) cells were significantly increased in the paraventricular nucleus, A2 region of the nucleus of the solitary tract, supraoptic nucleus and amygdala 6 h after the onset of fasting but not 24, 30 or 48 h after the onset. On the other hand, FL-ir cells were not significantly different in those areas among fasted castrates compared with unfasted controls. In the A2 region of testosterone-treated castrates, tyrosine hydroxylase- and dopamine-β-hydroxylase-immunoreactive (TH-ir, DBH-ir) cells did not exhibit Fos-like immunoreactivity (FLI). Our results suggest that the absence of food may activate neurons in discrete hypothalamic and brainstem nuclei in male rats at the beginning of fasting and this activation requires an androgenic milieu. The absence of FLI in TH- or DBH-ir cells in the A2 region would indicate that transmitter systems other than catecholaminergic neurons may be involved during the very early stage of 48-h fasting-induced suppression of LH secretion.