Histamine-immunoreactive neurons of the tuberomammillary nucleus are innervated by alpha-melanocyte stimulating hormone-containing axons. Generation of a new histamine antiserum for ultrastructural studies.

Leptin regulates the release of histamine in the hypothalamus, however, the histaminergic neurons contain few leptin receptors. To reveal that alpha-melanocyte stimulating hormone (alpha-MSH)-containing neurons of the arcuate nucleus may mediate the effects of leptin to the histaminergic neurons, we studied the putative innervation of histamine-immunoreactive (IR) neurons by alpha-MSH-containing axons using double-labeling immunocytochemistry. In order to analyze the relationship of alpha-MSH- and histamine-IR elements, we also generated an antiserum against histamine that is compatible with acrolein-based fixatives commonly used for immuno-electron microscopic studies. The apposition of alpha-MSH-IR axons to histaminergic neurons was observed in all five subnuclei of the tuberomammillary nucleus. Both axo-somatic and axo-dendritic contacts were found. At the ultrastructural level, silver-intensified colloidal gold particles identified the histaminergic neurons, whose ultrastructure was well preserved after fixation with acrolein demonstrating that the new antiserum is a useful tool for the ultrastructural examination of the histaminergic system. The histamine-IR cells received synaptic inputs from alpha-MSH-IR axon terminals visualized by diaminobenzidine. These data indicate that alpha-MSH-synthesizing neurons innervate histaminergic neurons in the tuberomammillary nucleus and may relay the hormonal influence of leptin to the histaminergic system.