Neurotransmitters contained in the subcortical extraretinal inputs to the monkey lateral geniculate nucleus

The lateral geniculate nucleus (LGN) is the thalamic relay of retinal infor mation to cortex. An extensive complement of nonretinal inputs to the LGN c ombine to modulate the responsiveness of relay cells to their retinal input s, and thus control the transfer of visual information to cortex. These inp uts have been studied in the most detail in the cat. The goal of the presen t study was to determine whether the neurotransmitters used by nonretinal a fferents to the monkey LGN are similar to those identified in the cat. By c ombining the retrograde transport of tracers injected into the monkey LGN w ith immunocytochemical labeling for choline acetyl transferase, brain nitri c oxide synthase, glutamic acid decarboxylase, tyrosine hydroxylase, or the histochemical nicotinamide adenine dinucleotide phosphate (NADPH)-diaphora se reaction, we determined that the organization of neurotransmitter inputs to the monkey LGN is strikingly similar to the patterns occurring in the c at. In particular, we found that the monkey LGN receives a significant chol inergic/nitrergic projection from the pedunculopontine tegmentum, gamma-ami nobutyric acid (GABA)ergic projections from the thalamic reticular nucleus and pretectum, and a cholinergic projection from the parabigeminal nucleus. The major difference between the innervation of the LGN in the cat and the monkey is the absence of a noradrenergic projection to the monkey LGN. The segregation of the noradrenergic cells and cholinergic cells in the monkey brainstem also differs from the intermingled arrangement found in the cat brainstem. Our findings suggest that studies of basic mechanisms underlying the control of visual information flow through the LGN of the cat may rela te directly to similar issues in primates, and ultimately, humans. (C) 2000 Wiley-Liss, Inc.