B-50 (GAP-43) IMMUNOREACTIVITY IS RARELY DETECTED WITHIN INTACT CATECHOLAMINERGIC AND SEROTONERGIC AXONS INNERVATING THE BRAIN AND SPINAL-CORD OF THE ADULT-RAT, BUT IS ASSOCIATED WITH THESE AXONS FOLLOWING LESION

The persistence of high levels of B-50 (GAP-43) in fibers innervating various regions of the adult central nervous system is generally thoug ht to characterize neuronal systems capable of undergoing morphologica l plasticity. In a recent series of in situ hybridization studies, it has been shown that most catecholaminergic and serotonergic neurons of the adult rat brain express high levels of B-50 mRNA. The present stu dy addresses the question whether high expression of B-50 mRNA in the catecholaminergic and serotonergic perikarya corresponds with detectab le high levels of the B-50 protein in the efferent axonal fibers that innervate various regions of the adult rat brain and spinal cord. For this purpose, vibratome sections were doubly immunostained for B-50 an d for tyrosine hydroxylase or serotonin and were analyzed by laser sca nning confocal microscope. Colocalizations were investigated either (1 ) in regions of intact rat brain and spinal cord in which particular c oncentrations of B-50 immunoreactive fibers appeared codistributed wit h catecholaminergic or serotonergic fibers or (2) in intrahypothalamic portions of the medial forebrain bundle in which a surgical lesion wa s made. In the intact brain, frequent colocalizations of B-50 and tyro sine hydroxylase were detected in fibers innervating both the mediobas al hypothalamus and the neurointermediate hypophysial lobe. In all the other regions examined, the analysis of thin optical sections demonst rated that immunoreactivity to B-50 was only rarely associated with ax onal profiles immunoreactive to tyrosine hydroxylase or to serotonin. By contrast, in the lesioned medial forebrain bundle B-50 immunoreacti vity was found to be associated with numerous catecholaminergic and se rotonergic axonal sprouts that regenerate around the surgical lesion. These data indicate that the majority of intact catecholaminergic and serotonergic axons innervating the adult rat brain and spinal cord con tains low levels of B-50. However, following axotomy, B-50 is immunocy tochemically detectable in the regenerating sprouts produced by both t ypes of axonal fibers. This suggests that under basal conditions the r elatively high content of B-50 mRNA in monoaminergic perikarya does no t lead to appreciable accumulation of B-50 within corresponding axonal fibers and terminals, whereas conditions of morphological reorganizat ion induce increased production of B-50 that accumulates within monoam inergic axonal sprouts. (C) 1995 Academic Press, Inc.