NERVE GROWTH-FACTOR (NGF) UPTAKE AND TRANSPORT FOLLOWING INJECTION INTHE DEVELOPING RAT VISUAL-CORTEX

Recent investigations have shown that cortical nerve growth factor (NG F) infusions during the critical period inhibit ocular-dominance plast icity in the binocular portion of the rat visual cortex. The mechanism s underlying the effects of NGF on visual cortical plasticity are stil l unclear. To investigate whether during normal development intracorti cal and/or extracortical cells possess uptake/transport mechanisms for the neurotrophin, we injected I-125-NGF into the occipital cortex of rats at different postnatal ages. Within the cortex, only a few labell ed cells were observed. These cells were confined to the vicinity of t he injection site and their number depended on the animal's age at the time of injection. Labelled cells were absent at postnatal day (PD) 1 0 but could be detected between PD 14 and PD 18. They then decreased i n number over the following period and were not detected in adult anim als. Outside the cortex, neurons of the lateral geniculate nucleus (LG N) were not observed to take up and retrogradely transport NGF at any age after birth. In contrast, retrogradely labelled neurons were found in the basal forebrain. Labelled cells were first observed here at PD 14 and then increased in number until reaching the adult pattern. Our results show that intrinsic and extrinsic neurons are labelled follow ing intracortical injections of iodinated NGF. In both neuronal popula tions, the uptake and transport of NGF is present over a period corres ponding to the critical period for visual cortical plasticity. These f indings suggest that NGF may play a role, both intra and extracortical ly, in plasticity phenomena.