Differential effects of maternal and sibling presence of hyperactivity of 6-hydroxydopamine-treated developing rats.

The influence of test environment on the expression of hyperactivity produced by neonatal 6-hydroxydopamine (6-OHDA) administration was assessed in rat pups at 15, 19, and 22 days of age. The 6-OHDA pups and an equal number of controls were tested in one of five different social circumstances: (a) isolation, (b) mixed groupings of two treated and two control pups, (c) mixed groupings with their anesthetized mothers, (d) mixed groupings with an anesthetized sibling, and (e) homogeneous groupings of all treated and all control siblings. Social factors had striking differential effects on activity, particularly at 19 days of age. The 6-OHDA pups were hyperactive relative to controls in isolation. Both treated and control pups were equally active in the mixed grouping, and they were hyperactive relative to control isolation levels. The addition of an anesthetized mother sharply attenuated the activity of both types of pups, whereas the addition of an anesthetized sibling attenuated the activity of the controls only. Pups in both types of homogeneous groupings were less active than pups in mixed groupings, and under homogeneous test conditions the treated pups were even less active than the controls. Thus, social factors strongly influence the behavior of rat pups with whole brain dopamine depletion produced by 6-OHDA and, within the confines of this animal model of hyperactivity, exert greater attenuating effects on their activity than previously observed with stimulant medication.