EVIDENCE FOR PRENATAL COMPETITION AMONG THE CENTRAL ARBORS OF TRIGEMINAL PRIMARY AFFERENT NEURONS - SINGLE AXON ANALYSIS

Previous studies from this laboratory have demonstrated that prenatal damage to vibrissae follicles results in significant increases in the brainstem representations of the remaining vibrissae as demonstrated b y staining for the mitochondrial enzyme cytochrome oxidase (CO). Becau se CO is primarily a postsynaptic marker, these results do not directl y address the question of whether there were changes in the projection s of primary afferent fibers. To address this issue, we made intra-axo nal recordings from individual vibrissa-related primary afferents in r ats that sustained damage to vibrissae follicles on embryonic day 17, and then injected horseradish peroxidase (HRP) into these axons to vis ualize their terminal arbors in the brainstem at the level of trigemin al subnucleus interpolaris (SpI). All vibrissae-related primary affere nts responded to deflection of one and only one vibrissa, and the term inal arbors of axons (N = 47) recovered from animals that sustained fe tal peripheral lesions were significantly larger than those (N = 23) f rom normal rats. Fibers from fetally damaged animals had increased tot al fiber lengths and numbers of branch points. These results indicate that reduced competition among primary afferent axons results in incre ases in the terminal arbors that remain. These increases occur without any significant alteration in their peripheral receptive fields. (C) 1994 Wiley-Liss, Inc.