Evidence for an involvement of striatal D1 receptors in
levodopa-induced
dyskinesia has been presented whereas the contribution of striatal D2 receptors remains controversial. In addition, whether D1 and D2 receptors located in the substantia nigra reticulata shape the response to
levodopa remains unknown. We therefore used dual probe microdialysis to unravel the impact of striatal and nigral D1 or D2 receptor blockade on abnormal
involuntary movements (AIMs) and striatal output pathways in unilaterally
6-hydroxydopamine lesioned dyskinetic rats. Regional perfusion of D1/D5 (
SCH23390) and D2/D3 (
raclopride) receptor antagonists was combined with systemic administration of
levodopa.
Levodopa-induced AIMs coincided with a prolonged surge of
GABA and
glutamate levels in the substantia nigra reticulata. Intrastriatal
SCH23390 attenuated the
levodopa-induced AIM scores (~50%) and prevented the accompanying neurochemical response whereas
raclopride was ineffective. When perfused in the substantia nigra, both antagonists attenuated AIM expression (~21-40%). However, only intranigral
SCH23390 attenuated
levodopa-induced nigral
GABA efflux, whereas
raclopride reduced basal
GABA levels without affecting the response to
levodopa. In addition, intranigral
raclopride elevated
amino acid release in the striatum and revealed a (mild) facilitatory effect of
levodopa on striatal
glutamate. We conclude that both striatal and nigral D1 receptors play an important role in
dyskinesia possibly via modulation of the striato-nigral direct pathway. In addition, the stimulation of nigral D2 receptors contributes to
dyskinesia while modulating
glutamate and
GABA efflux both locally and in the striatum.