In vivo measurement of spontaneous release and metabolism of dopamine from intrastriatal nigral grafts using intracerebral dialysis.

Spontaneous release and metabolism of dopamine (DA) from intrastriatal grafts of fetal mesencephalic DA neurons was measured by intracerebral dialysis. Mesencephalic DA cell suspensions were implanted into the head of the caudate-putamen in rats with unilateral 6-hydroxydopamine (6-OHDA) lesions of the mesostriatal DA pathway. Four months later, when tests for amphetamine-induced turning behaviour showed that the grafts had become functional, loops of dialysis tubing were implanted into the striatum on the grafted side and the contralateral non-lesioned side of the grafted rats, and in a similar position in the denervated caudate-putamen of 6-OHDA lesioned control rats. Dialysis perfusates collected from the 6-OHDA lesioned striata showed a reduction of about 95-98% in DA and its metabolites 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA). In the grafted animals these levels had recovered to about 40% of control for DA and to 12-16% of control for HVA and DOPAC. In addition, the serotonin metabolite 5-hydroxyindoleacetic acid (5-HIAA) was increased in the grafted striata compared to both the lesioned and non-lesioned controls. Amphetamine had little or no effect on DA release in the 6-OHDA lesioned rats, but caused a marked increase in DA release in the grafted rats, this response being proportional to that seen in intact striata. Since the subsequent histochemical analysis showed that the dialysis probe had been located in the transplant-reinnervated part of the caudate-putamen, the results provide additional evidence that the grafted DA neurons exert their functional effects through a continuous active transmitter release from their newly-established terminals in the reinnervated host target.