Neuronal and vascular localization of histamine N-methyltransferase in thebovine central nervous system

Histamine N-methyltransferase (HMT) (EC 2.1.1.8) plays a crucial role in th e inactivation of the neurotransmitter histamine in the CNS. However, the l ocalization of HMT remains to be determined. In the present study, we inves tigated immunohistochemical localization of HMT in the bovine CNS using a p olyclonal antibody against bovine HMT. The HMT-like immunoreactivity was ob served mainly in neurons. Strongly immunoreactive neurons were present in t he oculomotor nucleus and ruber nucleus in the midbrain, the facial nucleus in the pons, the dorsal vagal nucleus and hypoglossal nucleus in the medul la oblongata and in the anterior horn as well as intermediolateral zone of the spinal cord. Intermediately immunoreactive neurons were present in the piriform cortex and the inferior olivary nucleus. The grey matter of the fo rebrain regions was diffusely and faintly stained. In the cerebellum and th e striatum, the nerve fibres in the white matter were positive. The tuberom ammillary nucleus, where histaminergic neurons are present, were weakly pos itive. The other immunoreactive structures in the CNS were blood vessels. A lmost all of the blood vessel walls, irrespective of whether they were arte rial or venous, were variably stained. The glial fibrillary acidic protein- (GFAP-) immunoreactive astrocytes were not stained. These findings indicat ed that histamine released from histaminergic nerve terminals or varicose f ibres is methylated mainly in postsynaptic or extrasynaptic neurons rather than in astrocytes. The localization of HMT in the blood vessel wall may me an that blood-borne histamine and histamine released from mast cells associ ated with the blood vessels are catabolized in this structure.