The effect of altered neuronal activity on the development of layers in the lateral geniculate nucleus.

The main objective of this study was to examine the role of neural activity in the development of cell layers in the dorsal lateral geniculate nucleus (LGN). We studied this relationship in postnatal tree shrews either by completely blocking retinal ganglion cell activity with TTX or by selectively blocking activity to the developing ON-center LGN layers (1 and 2) with 2-amino-4-phosphonobutyric acid (APB), using unilateral and bilateral eye injections. All manipulations were carried out from birth (P0), when no LGN cell layers are evident, to or past the point when layers are recognizable (i.e., 1-2 weeks). Nissl-stained and cytochrome oxidase (CO)-reacted material was examined for all cases. Our results show that in the absence of activity produced by bilateral TTX injections, interlaminar spaces between cell layers do begin to develop. Retinal afferents, which are segregated at birth, remain segregated, and differential CO staining between matched sets of LGN layers is evident. The normal pace of LGN development, however, is slowed significantly: LGN cells are smaller and interlaminar spaces are narrower than are seen in age-matched controls. Unilateral TTX injections produce similar, but more dramatic and asymmetric, effects on LGN cells, perhaps because cells are at a competitive disadvantage relative to their normally innervated counterparts in cortex. Combining unilateral eye enucleation at P0 with subsequent TTX treatment of the other eye clearly demonstrates that axons from the remaining eye are capable of producing their normal complement of LGN layers. The development of the LGN ON-center layers, 1 and 2, and the interlaminar space between them are more affected by TTX treatment than are the other layers. By contrast, APB eye injections do not selectively affect the development of the ON-center layers, but do result in some slowing of overall LGN development. Taken together, these results suggest that activity of retinal afferents is not essential for initiating interlaminar space formation, but is important for the normal pace of maturation of LGN cell layers.