Autonomic and sensory reinnervation of smooth muscle transplanted to the anterior chamber of the eye: effect of target postnatal age.

The ability of the nervous system to provide target innervation is greatest in early development, but decreases as a function of age. The objective of the present study was to determine if age-related changes occurring within the target tissue contribute to this decline. Periorbital tarsal smooth muscle from donor rats 6, 14, 30, and 48 days postnatal were transplanted to the anterior chamber of the eye of 84- to 90-day-old host rats. The tissue was removed at 3, 6, or 10 days post-transplant and immunostained for presumptive sympathetic nerves (dopamine beta-hydroxylase-immunoreactive, DBH-ir), sensory (calcitonin gene-related peptide-immunoreactive, CGRP-ir) or parasympathetic (vasoactive intestinal polypeptide-immunoreactive, VIP-ir). DBH-ir sympathetic fibers sprouted into target from donors of all ages. However, the rate of ingrowth was most rapid in tissue from 6-day-old donors. In contrast, CGRP-ir sensory fibers showed no age-related differences, but grew more rapidly than sympathetic fibers. However, the innervation density at 10 days was comparable for both types of nerves. No significant VIP-ir parasympathetic ingrowth could be demonstrated at any age. We conclude that smooth muscle target in developing animals can have selective effects on different populations of ingrowing fibers; the rate of sympathetic ingrowth declines with maturity, whereas ingrowth of sensory fibers is not altered.