Cortical target depletion and the developing lateral geniculate nucleus: implications for trophic dependence.

The dependence of the developing dorsal lateral geniculate nucleus (LGd) on visual cortex for survival has been well documented. Complete removal of visual cortex during early postnatal development results in degeneration of the LGd. To further explore the nature of this trophic relationship, we depleted variable proportions of the principal targets of geniculocortical axons, layer IV neurons, and also variable proportions of the supragranular neurons by intraperitoneal injections of different dosages of a mitotic inhibitor MAM (methylazoxymethanol acetate) into pregnant hamsters at the time when these neurons were being generated in the ventricular zone. We demonstrate that after more than 75% loss of layer IV there is no reduction in cell number in the LGd. HRP (horseradish peroxidase) injections into the LGd in adult animals reveal an essentially normal pattern of termination without evidence of rerouting of geniculocortical axons to other cortical areas, nor compensatory increase in arborization in layer VI and VIb (subplate). Geniculocortical axons terminate principally in the middle stratum of the depleted cortex above layer V, with obvious reduction in both the extent and density of arborization. After higher dosages of MAM treatment resulting in more severe cell loss in layers II-IV with the apparent loss of layer IV, the extent and density of geniculocortical arborization are further reduced. Reduction in size as well as total number of geniculate neurons become detectable. Above depletions of 75% of layer IV neurons, the number of surviving LGd neurons is linearly related to the total number of remaining layer II-IV neurons in the cortex. These findings are discussed in light of the possible trophic mechanisms that match cell populations in number during development.