Innervation of histamine neurons in the caudal part of the arcuate nucleus of hypothalamus and their activation in response to food deprivation under scheduled feeding.

It has been well established that histaminergic neurons innervate densely the anterior hypothalamus and regulate several functions through the histamine H₁ receptor (H1R). However, the physiological function of the histaminergic neurons in other regions including the posterior hypothalamus has not been fully investigated. Recently, we have found a selective c-Fos expression in the caudal part of the arcuate nucleus of the hypothalamus (cARC) by food deprivation under scheduled feeding in rats. In this study, we histochemically examined the correlation of this c-Fos expression with the activation of histaminergic neurons in this region using an anti-H1R antibody. Strong H1R immunoreactivity was observed in the perikarya of the c-Fos positive cells. Abundant histamine-containing fibers were also found in the cARC and in the area between the cARC and the tuberomammillary nucleus (TM), where the histaminergic neuronal cell bodies are exclusively distributed. Our morphological observations suggest that c-Fos expression in the cARC by food deprivation under scheduled feeding is caused by the activation of histaminergic neurons projected from the TM.