Evidence that cholinergic axons from the parabrachial region of the brainstem are the exclusive source of nitric oxide in the lateral geniculate nucleus of the cat.

We investigated the source of axons and terminals in the cat's lateral geniculate nucleus that stain positively for NADPH-diaphorase. The functional significance of such staining is that NADPH-diaphorase is identical to the enzyme nitric oxide synthetase, and thus it is thought to reveal cells and axons that use nitric oxide as a neuromodulator. Within the lateral geniculate and adjacent perigeniculate nuclei, a dense network of axons and terminals is labeled for NADPH-diaphorase. The pattern of NADPH-diaphorase staining here is remarkably similar to that of choline acetyltransferase (ChAT) staining, suggesting that the source of these axons and terminals might be the parabrachial region of the brainstem because this provides the major cholinergic input to the lateral geniculate nucleus. In other areas of the brain to which parabrachial axons project, there is also a similar staining pattern for NADPH-diaphorase and ChAT. Furthermore, the patterns of cell staining within the parabrachial region for NADPH-diaphorase and ChAT are virtually identical. However, the relationship between ChAT and NADPH-diaphorase staining for the parabrachial region is not a general property of cholinergic neurons. Other cholinergic cells and axons, such as the trochlear nerve, the oculomotor nerve and nucleus, and the parabigeminal nucleus, which all label densely for ChAT, stain poorly or not at all for NADPH-diaphorase. It is significant that the parabigeminal nucleus, which provides a cholinergic input to the lateral geniculate nucleus, has no cells that label for NADPH-diaphorase. We used double labeling methods to identify further the source of NADPH-diaphorase staining in the lateral geniculate nucleus. We found that parabrachial cells co-localize NADPH-diaphorase and ChAT. Noradrenergic and serotoninergic cells in the brainstem also innervate the lateral geniculate nucleus, but we found that none of these co-localize NADPH-diaphorase. Finally, by combining NADPH-diaphorase histochemistry with retrograde labeling of cells that project to the lateral geniculate nucleus, we found that the cholinergic cells of the parabrachial region are essentially the sole source of NADPH-diaphorase in the lateral geniculate nucleus. We thus conclude that cells from the parabrachial region that innervate the lateral geniculate nucleus use both acetylcholine and nitric oxide for neurotransmission, and that this is virtually the only afferent input to this region that uses nitric oxide.