Responsiveness of sympathetic and sensory iridial nerves to NGF treatment in young and aged rats

Altered neuronal responses to trophic factors may play a role in neuronal m aintenance in adulthood and may also be involved in neuronal atrophy in old age. We have investigated this issue in the rat iris, studying responsiven ess of sympathetic and sensory iridial nerves to a range of NGF concentrati ons in mature and aged rats. We show here that growth responses of sensory nerves to NGF, as measured by quantitative immunohistochemistry and image a nalysis, were unchanged in old rats. In contrast, there was a small but sig nificant reduction in responsiveness of aged sympathetic neurons. The shape s of the dose-response curves for sensory and sympathetic neurons were diff erent, with a larger response over a narrower range of concentrations in se nsory neurons. Lower levels of p75 immunoreactivity were observed in iridia l nerves from old compared to young rats. NGF treatment had no effect on re ceptor staining in young rats but restored 'young' levels of p75 staining i n old rats. Our results do not support the hypothesis of a primary role for NGF in maintenance or atrophy of nerves in ageing. (C) 2001 Elsevier Scien ce Inc. All rights reserved.