Comparison of cholinergic and histaminergic axons in the lateral geniculate complex of the macaque monkey

The cholinergic and histaminergic projections have important neuromodulator y functions in the ascending visual pathways, so we compared the pattern an d mode of innervation of the two projections in the lateral geniculate comp lex (dorsal lateral geniculate nucleus and pregeniculate nucleus) of the ma caque monkey. Brain tissue from macaques was immunoreacted by means of anti bodies to choline acetyltransferase (ChAT) or to histamine and processed fo r light and electron microscopy. A dense plexus of thin, highly branched ChAT-immunoreactive axons laden wit h varicosities was found in all layers of the dLGN including the koniocellu lar laminae and in the pregeniculate nucleus. ChAT label was more dense in magnocellular layers 1 and 2 than in parvocellular layers 3-6 and relativel y sparse in the interlaminar zones. Varicosities associated with the cholin ergic axons had an average of three conventional asymmetric synapses per va ricosity, and these appeared to contact dendrites of both thalamocortical c ells and interneurons. Histamine-immunoreactive axons were distributed homogeneously throughout al l laminar and interlaminar zones of the dLGN, but mere denser in the pregen iculate nucleus than in the dLGN. Histaminergic axons branched infrequently and were typically larger in caliber than cholinergic axons. The overwhelm ing majority of varicosities were found en passant and rarely displayed con ventional synapses, despite the abundance of synaptic vesicles, and were no t associated preferentially with specific cellular structures. The innervation of the macaque dLGN complex by cholinergic and histaminergi c systems is consistent with their proposed role in state dependent modulat ion of thalamic activity. The dense and highly synaptic innervation by chol inergic axons supports the proposal of additional involvement of these axon s in functions related to eye movements. Anat Rec 255:295-305, 1999. (C) 19 99 Wiley-Liss, Inc.