The role of competition in the refinement of the projections of sympathetic neurons to the rat eye during development

Within the iris, the extent of the nerve plexuses derived from the sympathetic neurons of the superior cervical ganglion (SCG) and the sensory, substance P (SP) neurons of the trigeminal ganglion depend on competition for target tissue derived growth factors. During the postnatal period when these plexuses are initially established, many sympathetic neurons are known to extend transient collateral projections to various targets within the eye. We have investigated the role of neuronal competition in the withdrawal of these transient projections by removing the sensory SP fibres using neonatal capsaicin treatment (50 mg/kg) on days 2, 10 and 17, or day 2 only. At 2, 4 and 7 weeks, fast blue was injected into the anterior chamber or posteriorly into the vitreous to retrogradely label sympathetic neurons in the SCG. Capsaicin treatment resulted in a transient retention of the projections of supernumerary neurons to the eye early during development and a maintenance of the collaterals of some of these sympathetic neurons to adulthood, but only when rats received multiple capsaicin injections. The retention of collaterals in these animals was reflected in an increase in the density of the sympathetic nerve plexus within targets such as the iris. Immunohistochemistry for SP showed that a single injection of capsaicin was less effective than multiple injections in removing the SP-containing nerve fibres from the iris and in causing long-lasting changes to the sympathetic projections. We conclude that some form of interaction between different neuronal populations within the eye plays an important role in the refinement of collateral projections of sympathetic neurons, but has no long-term effect in influencing the final number of neurons which project to the eye.