Calbindin immunoreactivity in the geniculo-extrastriate projection of the macaque

Purpose. Although the great majority of projection neurons in the primate dorsal lateral geniculate nucleus (dLGN) target striate cortex (VI), a small number project instead to extrastriate areas and have been implicated in the preserved vision that survives damage to VI. Moreover, the distribution of dLGN cells projecting to extrastriate bears a striking similarity to the distribution of dLGN neurons which stain for calbindin D-28K {CaBP}, a calcium-binding protein believed to be involved in regulating neuronal excitability. The purpose of these studies was to examine whether CaBP characterizes all or a subset of neurons making up the geniculo-extrastriate pathway. Methods, (a) In three normal adult macaques, we made multiple injections of the retrograde tracer CTB into the prelunate gyrus and examined the dLGN for neurons immunoreactive for both CTB and CaBP. fb) To see if the CaBP-immunoreactive dLGN population might potentially play a role in preserved vision after VI damage, we also examined the dLGN of a macaque which had sustained a unilateral lesion of VI at the age of 6 wk and survived until 4 yr. Results, (a) In all three injection cases we found a population of CTB-labelled neurons in the posterior half of the dLGN. In addition, in all animals a significant subset of these neurons displayed CaBP immunoreactivity. The proportion of retrogradely labelled neurons which were also immunoreactive for CaBP varied from less than 50% to over 80%, suggesting that the geniculo-extrastriate projection may be divisible into functionally distinct pathways which were differentially invoked by the specific placement of the prelunate injections in the different animals, (b) In the animal with VI damage, Urge, intensely CaBP-immunoreactive neurons were found scattered throughout the otherwise degenerated portions of the structure. Conclusions. These results suggest that CaBP immunoreactivity may be a critical feature of the pathway by which visual information reaches extrastriate cortex via the dLGN in the absence of VI.