THE DISTRIBUTION OF HISTAMINERGIC AXONS IN THE SUPERIOR COLLICULUS OFTHE CAT

The histaminergic projection from the hypothalamus to the superior col liculus was examined immunohistochemically in the cat brain using an a ntibody to histamine. The source of histaminergic fibers in the brain is a group of neurons in the posterior hypothalamus, located primarily in ventrolateral and periventricular regions and collectively referre d to as the tuberomammillary nucleus. All laminae of the superior coll iculus-including the superficial, intermediate, and deep layers, as we ll as the central gray-were blanketed with histamine-immunoreactive ax onal fibers. Overall, labeling in the superior colliculus was moderate ly dense compared to other locations in the cat brain, with some varia tion in fiber density. Individual labeled fibers resembled histaminerg ic fibers described previously in the brain. Labeled axonal fibers sho wed infrequent branching and were beaded with numerous en passant vari cosities that were typically 1 mum or smaller, but as large as 2.5 mum in diameter. Varicosity size differed significantly at different dept hs in the colliculus. The histaminergic projection appears to be separ ate from a previously reported, apparently non-histaminergic projectio n from neurons in the dorsal hypothalamic area to discrete regions of intermediate and deep colliculus. These results indicate that the hist aminergic projection from the tuberomammillary nucleus of the hypothal amus projects extensively throughout the superior colliculus. Histamin e, which is believed to act as a neuromodulator in the brain, is in a position to influence sensory and motor-related processes in every lay er of the cat superior colliculus.