Regulation of rat sympathetic nerve density by target tissues and NGF in maturity and old age

Previous studies in our laboratory using a transplantation model have shown that target tissues of some autonomic neurons, including cerebral blood vessels, exert a controlling influence on nerve fibre loss in old age. The present study was undertaken in order to discover whether the influence of targets extends to controlling age changes in specific populations of nerves. In old rats, we have demonstrated a significant decrease of approximately 50% in the sympathetic innervation of middle cerebral arteries, using tyrosine hydroxylase-like immunoreactivity. Following transplantation, tyrosine hydroxylase-like immunoreactive nerve density on both young and old implanted middle cerebral arteries mirrored the nerve densities seen in normal, non-transplanted vessels. Furthermore, implanted tissue from old donors became reinnervated with a nerve density approximately 50% less than that of young implanted vessels. Treatment of transplants with nerve growth factor, however, was able to reverse these age changes and restore the sympathetic innervation of aged middle cerebral arteries to levels above those seen in young middle cerebral arteries. These results suggest that the pattern and density of sympathetic innervation that the middle cerebral artery receives is determined by the target rather than by the neurons supplying the tissue. The ability of nerve growth factor to induce regrowth in sympathetic neurons innervating ageing target tissues implies that age-related neuronal atrophy may be due to reduced synthesis or availability of target-derived neurotrophic factors.