The histaminergic innervation of the mesencephalic nucleus of the trigeminal nerve in rat brain: a light and electron microscopical study.

Histaminergic fibers in the mesencephalic nucleus of the trigeminal nerve of Long-Evans rats were examined by light and electron microscopy after peroxidase-antiperoxidase immunocytochemical staining for histidine decarboxylase (HDC) as a marker. By light microscopy, neurons in the mesencephalic nucleus of the trigeminal nerve were seen to be surrounded by a number of HDC-like immunoreactive (HDCI) fibers, suggesting the presence of axo-somatic contact. This finding was supported by immunoelectron microscopic demonstration of synaptic contact of some HDCI fibers with the soma of the neurons in this nucleus. These findings indicate that histamine is involved in the sensory regulation of movement of the masticatory muscles at the level of the trigeminal mesencephalic nucleus.