Development of the diencephalic relay structures of the visual thalamofugal system in pigeons.

To compare the developmental pattern of the visual tecto- and thalamofugal pathways in the altricial pigeon, we examined the posthatch differentiation of the retinothalamic system. Choleratoxin was injected into the left and right eye to visualize the retinal innervation pattern of the lateral geniculate nucleus of the thalamus (GLd). The calcium-binding proteins parvalbumin and calbindin and GABA(Abeta) receptors were used as indicators for the functional development of the GLd. Although all retinorecipient thalamic target structures were invaded by retinal fibers directly after hatching, density of the projection increased during the first week. While the adult GLd was characterized by a substantial number of cells displaying calbindin-immunoreactivity and by a sparse innervation by parvalbumin-immunoreactive fibers, after hatching no labelling for calcium-binding proteins could be detected. Calbindin-immunoreactivity appeared not before posthatching day 7, while parvalbumin-immunoreactive fibers were detected only after the third week. In contrast, a dense but diffuse GABA(Abeta) receptor-labelling was present from hatching onwards that decreased during development. The delayed expression of calbindin as well as changes in the density of GABA(Abeta) receptors indicate that maturation of GLd neurons extends long into the posthatch period. It is likely that the GABAergic interneurons mainly develop within this posthatch timeframe. Combined with the delayed development of the parvalbumin-positive innervation, the developmental pattern of GLd neurons suggests that the thalamofugal networks are immature after hatching and therefore still sensitive to modulations of posthatch visual experience.