Changes in spontaneous activity and emotional responses of rats treated with 6-hydroxydopamine at the suckling age

Spontaneous activity and emotional responses in mature rats treated with 6-hydroxydopamine (6-OHDA: 100 microgram X 2) injected into their left and right lateral ventricles during the suckling age (12th and 15th day of birth) were investigated. The 6-OHDA treated rats showed normal growth and no significant change in the norepinephrine levels of peripheral organs despite a marked and long-lasting decrease of catecholamine levels in the central nervous system. In the open field test, ambulation and rearing within 3 min were not altered by 6-OHDA treatment whereas a significant decrease in ambulation was noted in the treated rats during continuous 30 min observation. The running-wheel activity of the treated rats was significantly lower than that of control rats during 10 days of continuous observation. Methamphetamine (1 mg/kg s.c.) produced a marked increase of activity in both groups, but the activity of the treated rats was significantly higher than that of the control rats. In the emotional response test, an increased startle response to tail pinching was observed in the 6-OHDA treated rats. Thr results showed that compensatory mechanisms following destruction of catecholamine neurons developed to a considerable extent in the rats injected with 6-OHDA intraventricularly at their suckling age. The possible role of increased activity in the remaining intact or regenerated catecholamine neurons on the compensatory mechanisms was discussed.