Plasticity of catecholaminergic neurons in aged rat brain: reinnervation and functional recovery after axotomy

Regenerative growth at the lesion site, reinnervation of a target nucleus and functional manifestations of recovery were studied in aged (20 and 30 months old) rats subjected to long-term transection of catecholaminergic (CA) fibers which contact and influence neurons of the supraoptic nucleus (SON). Small bilateral knife cuts were placed stereotaxically just caudal and medial to the SON. CA histofluorescence, induced by formaldehyde-glutaraldehyde (FAGLU) or aluminum-formaldehyde (ALFA) methods, was examined in hypothalamus at 2, 14, 21 and 60 days postsurgically. Water consumption, and urine volume and osmolality, were monitored presurgically, and through survival times. Subtotal CA denervation in the SON, and typical axonal transmitter "pile-up" at the lesion site, were evident two days after surgery. Among these degenerative profiles, which persisted for up to three weeks, fine-sized new fibers were apparent at the lesion, beginning between 2 and 14 days, and persisting throughout the period studied. At 21 days, and progressively thereafter, SON neurons were rimmed with fluorescent varicosites. Water consumption initially was depressed, but returned to presurgical mean levels by nine days. Urine volume returned to normal by 32 days. Urine osmolality showed a recovery by approximately three weeks. These functional parameters rebounded to levels higher than presurgical means among 20 month old, but not 30 month old, rats beyond 6 weeks survival, concurrent with a morphological hyperinnervation. The results reaffirm morphological regeneration, and support reinnervation and functional recovery, which extend considerably into the aging process.