The effect of 6-hydroxydopamine on habituation of activity in the developing rat pup

The role of brain catecholaminergic mechanisms in habituation of activity was investigated in rat pups treated with 6-hydroxydopamine (6-OHDA). Intracisternal administration of this agent in the neonatal period resulted in a permanent and significant depletion of brain dopamine to 35.5% of controls while brain norepinephrine remained unchanged. Activity levels in normal developing rat pups increased rapidly between 15-22 days, then declined at maturity (26 days), while activity in 6-OHDA treated animals during this peak period of behavioral arousal increased to a significantly greater degree than that of their littermate controls. Habituation of activity, defined as the decrement of spontaneous activity, was calculated by regression over the first 30 min of observation. At both 5 and 8 days of age 6-OHDA and control rat pups exhibited low levels of activity whose decrease with time did not differ significantly and this pattern continued through 12 days of age. However, by 15 days of age activity in control animals declined by 19% each 10 min period compared to only a 10% decline found in 6-OHDA animals. At 19 days normal rat pups declined by 10% compared to a significantly reduced decrement of 3% found in treated animals, but these differences were no longer apparent by 22, 26, or 29 days of age. Our results are consistent with the notion that habituation of activity is a complex phenomenon mediated in part by catecholaminergic systems.