Synaptic inputs to histaminergic neurons in the rat posterior hypothalamus

Histamine is thought to be a neuroactive substance in the brain and have various physiological functions such as biological rhythms and autonomic regulation. It can be speculated, therefore, that synaptic inputs to histaminergic neurons may exert powerful influences upon those histaminergic functions. Synaptic connections between histaminergic neurons and substance P (SP) or neuropeptide Y (NPY) afferents in the caudal magnocellular nucleus (CM) of the hypothalamus were examined using an immunoelectron microscopic mirror method. SP-immunoreactive (SR-IR) and NPY-immunoreactive (NPY-IR) terminals made synaptic contacts with histidine decarboxylase immunoreactive (HDC-IR) neurons. Furthermore, the relationship between HDC-IR neurons and glycine receptor-immunoreactive (Gly-R-IR) profiles was examined in the CM by the same method as described above. A low to moderate density of dot-like Gly-R-IR profiles was seen at the light microscopic level. At the electron microscopic level, HDC-IR neurons were identified to have Gly-R immunoreactivity at postsynaptic sites on their somata. Our findings suggest that SP, NPY and possible glycinergic afferents exert monosynaptic influences on the central histaminergic neuronal system.