Operant behavioural and neurochemical effects after neonatal 6-hydroxydopamine treatment.

Newborn rats were treated at 1 and 2 days after birth with 100 mg/kg 6-hydroxydopamine (60HDA), s.c. Testing on several operant behavioural tasks was begun at 6 months of age. On a fixed ratio 30 (FR 30) schedule of food reinforcement, the neonatal 60HDA treated rats responded at a significantly higher rate. Further analysis of the FR 30 response pattern indicated that the higher rate was due to a decrease in the amount of time spent pausing after the receipt of each reinforcer. The 60HDA treatment failed to alter the rat's behaviour during the extinction of the FR 30 response and on the progressive ratio or variable interval schedules of food reinforcement. Biochemical analysis of several brain areas at 9 months of age showed a decrease in noradrenaline (NA) levels in the cerebral cortex and hippocampus, while in the pons-medulla NA content was doubled. The tyrosine hydroxylase activity in these same brain areas was not significantly altered, but there appeared to be some decrease in the activity of this enzyme in the hippocampus. Comparison of the operant behavioural effects seen after various lesioning procedures in this and other studies, suggest the effects on FR performance are a result of destruction of NA neurons in the hippocampus and/or the apparent regeneration of neurons in the pons-medulla.