The distribution of histaminergic axons in the superior colliculus of the cat

The histaminergic projection from the hypothalamus to the superior colliculus was examined immunohistochemically in the cat brain using an antibody 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 referred to as the tuberomammillary nucleus. All laminae of the superior colliculus—including the superficial, intermediate, and deep layers, as well as the central gray—were blanketed with histamine-immunoreactive axonal fibers. Overall, labeling in the superior colliculus was moderately dense compared to other locations in the cat brain, with some variation in fiber density. Individual labeled fibers resembled histaminergic fibers described previously in the brain. Labeled axonal fibers showed infrequent branching and were beaded with numerous en passant varicosities that were typically 1 μm or smaller, but as large as 2.5 μm in diameter. Varicosity size differed significantly at different depths in the colliculus. The histaminergic projection appears to be separate from a previously reported, apparently non-histaminergic projection from neurons in the dorsal hypothalamic area to discrete regions of intermediate and deep colliculus. These results indicate that the histaminergic projection from the tuberomammillary nucleus of the hypothalamus projects extensively throughout the superior colliculus. Histamine, which is believed to act as a neuromodulator in the brain, is in a position to influence sensory and motor-related processes in every layer of the cat superior colliculus.