Functional organization of the ventral lateral geniculate complex of the tree shrew (Tupaia belangeri): I. Nuclear subdivisions and retinal projections.

This is the first of two papers describing the organization and connections of the ventral lateral geniculate complex (GLv) in the tree shrew. Using a combination of Nissl, Golgi, histochemical, and immunocytochemical methods, we have identified two major divisions (lateral and medial) of GLv, both of which can be further subdivided. The lateral division contains three subdivisions, external, internal and intergeniculate leaflet. The medial division contains two subdivisions, medio-rostral and medio-caudal. All three lateral subdivisions receive input from the retina, the densest terminations being in the external subdivision and intergeniculate leaflet. These projections originate primarily from small retinal ganglion cells, although a few large retinal ganglion cells also project to GLv by way of collateral branches. Each subdivision of GLv has a distinct cytoarchitectonic and immunocytochemical make-up. In general, the level of immunoreactive endings for glutamic acid decarboxylase (GAD), leuenkephalin (ENK), and choline acetyltransferase (ChAT) parallels the distribution of retinal projections. Thus, all three markers are particularly dense in the external subdivision and the intergeniculate leaflet. Cell bodies immunoreactive for ENK are restricted to the external and intergeniculate leaflet subdivisions. The medial subdivisions stain relatively poorly for GAD, ENK, and ChAT, although each has other cytological features that differentiate them from the lateral subdivisions and the adjacent thalamic reticular nucleus.