Levels of NGF protein do not correlate with changes in innervation of the rat iris in old age.

We have examined the hypothesis that alterations in target levels of NGF protein lead to selective changes in peripheral nerve fibres in maturity and old age. NGF levels were assayed using 2-site ELISA in the iris of young and old rats. NGF levels in the iris showed no significant change in old age and did not correlate with age-related changes in the density of NGF-sensitive irideal sympathetic and sensory nerve fibres. Following sympathectomy, NGF levels increased substantially in old but not in young irises, possibly because the lower density of aged sensory nerves resulted in reduced capacity for NGF uptake. Content and uptake of NGF in sympathetic (SCG) neurones was shown to be reduced in old age, while NGF content of the sensory (trigeminal) ganglion increased in old age. Thus NGF levels in neurones, as well as targets, fail to correlate with peripheral nerve fibre growth or atrophy.