HISTAMINE-IMMUNOREACTIVE NEURONS AND THEIR INNERVATION OF VISUAL REGIONS IN THE CORTEX, TECTUM, AND THALAMUS IN THE PRIMATE MACACA-MULATTA

The histaminergic system is involved in the control of arousal in the brain and may impact significantly on visual processing. However, litt le is known about the histaminergic innervation of visual areas, or th e histamine system in the primate brain, in general. We examined in Ma caca mulatta the location of histamine-immunoreactive neurons and the innervation of important-cortical and subcortical visual areas by hist amine-immunoreactive axons. Brain sections were treated with an antibo dy to histamine and processed with standard immunohistological procedu res. Histamine-immunoreactive neurons (20-45 mu m in diameter) were lo calized bilaterally in the hypothalamus, particularly in ventral, late ral, posterior, and perimammillary hypothalamic areas. These hypothala mic cells appear to provide the sole neural source of histamine in the macaque brain. A plexus of varicose histamine-immunoreactive axons wa s present throughout the superior colliculus, the dorsal and ventral l ateral geniculate nuclei of the thalamus, the reticular nucleus of the thalamus, the lateral posterior/pulvinar complex, and the visual cort ex, including areas 17, 18, and the nearby extrastriate cortex. The ax ons nearly homogeneously innervated every region and layer in these st ructures, except for an increase in density in layer 1 of the visual c ortex and in the superficial-most layers of the superior colliculus. H istaminergic axons broadly innervated every visual region examined. In comparison with the other aminergic and the cholinergic projection sy stems, which show considerable projection specificity, the histaminerg ic projection exhibited great homogeneity. The breadth of the distribu tion of histaminergic axons ensures that virtually all levels of visua l processing in the primate can be influenced, either directly or indi rectly, by the neuromodulatory effects of histamine. (C) 1996 Wiley-Li ss, Inc.