IMMUNOHISTOCHEMICAL ORGANIZATION OF THE VENTRAL LATERAL GENICULATE-NUCLEUS IN THE GROUND-SQUIRREL

The ventral lateral geniculate nucleus (vLGN) of the thirteen-lined gr ound squirrel (Citellus tridecemlineatus) is a highly differentiated n ucleus that is divisible into five major subdivisions on the basis of retinal projections and cytoarchitecture. To pursue the likelihood tha t these subdivisions (the dorsal cap, intergeniculate leaflet, externa l magnocellular lamina, internal magnocellular lamina, and parvicellul ar segment) correlate with the functional diversity of this complex, t he present study examined the neurochemical composition of the vLGN wi th regard to substances that have previously proved useful in distingu ishing functionally distinct subregions within nuclei (i.e., neuropept ide Y (NPY), substance P (SP), leucine and methionine enkephalins, gam ma-aminobutyric acid (GABA), cytochrome oxidase (CO), acetyleholineste rase (AChE), and NADPH-diaphorase). The results showed a clear differe ntial neurochemical distribution within the nucleus. Neuropeptide Y im munoreactive perikarya were found predominantly in the intergeniculate leaflet and external magnocellular lamina, with only a few present in the internal magnocellular lamina and dorsal cap, and none observed i n the parvicellular segment. NPY+ fibers, however, were present in all divisions except the parvicellular segment. The highest concentration of SP immunoreactive cells was observed in the internal magnocellular lamina, and substantial numbers also were scattered in the external m agnocellular lamina and parvicellular segment. SP+ fibers were seen pr edominantly in the intergeniculate leaflet and the magnocellular lamin ae. The heaviest concentration of enkephalinergic fibers occurred in t he internal magnocellular lamina and dorsal cap, but fibers were also observed in the external magnocellular lamina and intergeniculate leaf let. GABA reactivity was widespread throughout the vLGN, with the dors al cap and external magnocellular lamina most heavily labeled, followe d by the intergeniculate leaflet and the internal magnocellular lamina . Cytochrome oxidase, AChE, and NADPH-diaphorase histochemistry reveal ed rich reactivity within the dorsal cap, and external and internal ma gnocellular laminae and paler reactivity in the intergeniculate leafle t and parvicellular segment. The external magnocellular lamina was mor e reactive for CO and NADPH-diaphorase than AChE, while the internal m agnocellular lamina showed the opposite pattern of reactivity. In addi tion, NADPH-diaphorase reactive cells were present in caudal intergeni culate leaflet and lateral external magnocellular lamina. These local differences in the neurochemical character of the vLGN support its par cellation into multiple subdivisions. Taken in conjunction with the di fferences in cytoarchitecture and retinal projections, these results s uggest substantial functional diversity within the ventral lateral gen iculate complex.