Long-term changes in dopaminergic innervation of caudate nucleus after continuous amphetamine administration.

Silicone pellets containing d-amphetamine base were implanted subcutaneously in rats. These pellets release amphetamine continuously for at least 10 days. Several days after implantation, swollen dopamine axons concomitant with large decreases in tyrosine hydroxylase activity were observed in the caudate nucleus. Decreased tyrosine hydroxylase activity was still present 110 days after pellet removal in the caudate but not in several other brain regions, nor in the caudate of rats injected with an equivalent amount of amphetamine in daily injections. This implies that continuous amphetamine administration has a selective neurotoxic effect on dopamine terminals in the caudate.