Neurokinin B/NK3 receptors exert feedback inhibition on L-DOPA actions in the 6-OHDA lesion rat model of Parkinson's disease.

Neurokinin B (NKB) and substance P (SP) act via NK(3) and NK(1) receptors. Using the unilateral 6-hydroxydopamine (6-OHDA) lesion rat model of Parkinson's disease (PD), it was found that chronic, but not acute, administration of L-DOPA increases striatal NKB expression in the dopamine-depleted hemisphere. In contrast, both acute and chronic administrations of L-DOPA restore reduced levels of SP mRNA. Co-treatment with the NK(3) receptor antagonist, SB222200, and L-DOPA increased contralateral rotations compared to L-DOPA alone in L-DOPA primed rats. The NK(3)R agonist, senktide, increased the phosphorylation of tyrosine hydroxylase (TH) at Ser(19)-TH, a CaMKII site, and of Thr(286)-CaMKII in striatal slices. Senktide had no effect on P-Ser(31)-TH, a MAPK site, but reduced P-Ser(217/221)-MEK. Amperometry demonstrated that senktide increased evoked dopamine release. SB222200 blocked the effects of senktide. In striatal slices prepared from 6-OHDA-lesioned rats repeatedly treated with L-DOPA, senktide no longer increased P-Thr(286)-CaMKII, suggesting a role of NK(3)R on dopamine terminals under normal conditions. SB222200 increased P-Ser(217/221)-MEK only in dopamine-depleted slices, indicating an increased NK(3)R tone under Parkinsonism conditions. Altogether, these data demonstrate a differential regulation of NKB and SP by L-DOPA in an animal model of PD and indicate a unique role of NKB in long-term effects of L-DOPA. Behavioural, biochemical and amperometric data indicate that NKB/NK(3)R signalling stimulates dopamine transmission at the presynaptic site, but inhibits it at the postsynaptic site. The inhibitory influence of NKB/NK(3)R on dopamine transmission dominates in an animal model of PD and provides a feedback inhibition on actions mediated via L-DOPA.