LESION-INDUCED EXPRESSION OF LOW-AFFINITY NGF-BINDING PROTEIN (P75) IMMUNOREACTIVITY AFTER NEONATAL AND ADULT ASPIRATION LESIONS OF THE RATDORSOMEDIAL PREFRONTAL CORTEX

The present study was performed in order to examine whether or not NGF -mediated processes could be involved in the sparing of function obser ved after neonatal prefrontal cortex lesions. After unilateral neonata l aspiration lesions of the dorsomedial prefrontal cortex, fibers immu noreactive for the low-affinity NGF-binding protein (p75) with a devia nt morphology were observed in the severed hemisphere only. The morpho logy of these fibers was characterized by their large caliber, their l arge, often bulbous varicosities, and their curly appearance. These fi bers were present as soon as 24 h after the operation. Between 3 and 5 days after the operation, the greatest abundance of these fibers was found in the ventrorostral areas of the forebrain and along the pathwa ys of cortical projections of the cholinergic cell groups. After 7 day s, such fibers were no longer observed. After comparable lesions in ad ult animals, a similar type of fiber was observed in the lesioned hemi sphere. However, in these cases a response comparable to that observed in the neonatal animals was not observed until 5 days after the opera tion, with fewer fibers. Furthermore, in contrast to what was observed after neonatal lesions, in adult animals no indications of retrograde transport of p75 immunoreactive material towards the cholinergic cell s of the basal forebrain nuclei were found. From these findings it was concluded that the prompt upregulation of p75 expression in neonatal animals may contribute to the survival of the cholinergic cells of the basal forebrain, and may therefore be involved in the restoration of function of the medial prefrontal cortex.