Loss of regulation by presynaptic dopamine D2 receptors of exogenous L-DOPA-derived dopamine release in the dopaminergic denervated striatum.

To determine whether dopamine release derived from exogenous l-DOPA is under inhibitory control of presynaptic dopamine D2 receptors in the dopaminergic denervated striatum, extracellular dopamine levels were measured in the striatum of 6-hydroxydopamine-lesioned rats using in vivo brain microdialysis. Quinpirole, a D2 agonist, dose-dependently (0.01-3 mg/kg s.c.) inhibited endogenous dopamine release both in the intact and dopaminergic denervated striatum. The dose-response curve obtained from the denervated striatum showed a shift to the right. Administration of l-DOPA (30 mg/kg i.p.) with carbidopa (25 mg/kg i.p.) increased dopamine release to 130% of basal levels in the intact striatum and 770% of basal levels in the denervated striatum. In the intact striatum, dopamine release was continuously inhibited by quinpirole pretreatment (1 mg/kg s.c.) even after l-DOPA administration. In the denervated striatum, l-DOPA-derived dopamine release was not affected by quinpirole pretreatment. These results suggest that, in the striatum with dopaminergic denervation, regulation by presynaptic D2 receptors is still operative on endogenous dopamine release but it does not work on dopamine release derived from exogenously administered l-DOPA.