Alterations in target innervation and collateral sprouting in the aging sympathetic nervous system.

The ability to sustain appropriate target innervation and to undergo collateral sprouting following losses of related neural inputs may favor the maintenance of normal function in adult and aged organisms. Young (4 month old) rats underwent a unilateral sympathetic denervation of the pineal gland and 1 day later exhibited an approximately 50% decrease in the area fraction represented by tyrosine hydroxylase (TH)-immunoreactive profiles in this target tissue. Ten days after this lesion, the density of TH immunoreactivity increased to over 80% of control values. In aged (25 month old) animals, endogenous fluorescence produced by the presence of lipofuscin was subtracted from the captured image, revealing a more than 50% decrease in innervation density to this target tissue with aging. The density of TH-immunoreactive profiles decreased by approximately one-half in aged animals lesioned 1 day earlier. However, 10 days after a unilateral denervation it was still approximately one-half of that obtained in control aged rats, providing morphologic support for a failure in collateral sprouting with aging in this system.