Effects of 6-hydroxydopamine on central noradrenaline neurons during ontogeny.

The effects of the catecholamine neurotoxic compound, 6-hydroxydopamine (6-OHDA) have been investigated on central noradrenaline (NA) neurons after neonatal administration. In agreement with previous studies this treatment (1-3 X 100 mg/kg) led to a pronounced reduction of the in vitro uptake of [3H]NA and the endogenous NA in the cerebral cortex, while these parameters were markedly augmented in the pons and medulla oblongata, regions containing the NA perikarya. The 6-OHDA induced changes in the cerebral cortex and the pons-medulla could be completely prevented by the 'membrane pump' blocker desipramine, indicating that the effects are associated with a specific neurotoxic action of 6-OHDA on the NA neurons. Consistently, 6-OHDA acutely (within 2 h) produced a marked reduction of the [3H]NA uptake in both the cerebral cortex and pons-medulla. In the cerebral cortex the nadir (approximately 75% reduction) was reached within 6 h and remained so, while in the pons-medulla the [3H]NA uptake rapidly recovered, being maximally elevated after 14 days (50-80% increase) and remained so for at least 6 months. The [3H]NA uptake in the pons-medulla from 6-OHDA treated rats had the same kinetic and pharmacological properties as that of control. Thus the observed differences in [3H]NA uptake are most likely quantitatively related to actual changes in the number of NA nerve terminals. Treatment with lower 6-OHDA doses (10 or 50 mg/kg) resulted in less pronounced reduction of [3H]NA uptake initially, and there was a gradual recovery of tuptake with time in the cerebral cortex, which was more pronounced after the lower dose. These results are indications of regenerative growth, which may be possible when a critical part of the axon is spared from the neurotoxic effect of 6-OHDA. Administration of 6-OHDA on various days after birth disclosed that both the reduction of [3H]NA uptake in the cerebral cortex and the increase of [3H]NA uptake in the pons-medulla did not appear as permanent phenomena when 6-OHDA was given later than on the seventh postnatal day. This is most likely associated with the postnatal development of the blood-brain barrier. It may be concluded that the neonatal 6-OHDA treatment causes a marked NA denervation in the forebrain, e.g. the cerebral cortex, and an increased outgrowth of NA nerve terminals in the pons-medulla, which is preceded by a partial damage. This partial NA denervation is then followed by a regeneration (regenerative and/or collateral sprouting) and a stimulated outgrowth of NA nerve terminals.