Further evidence for an involvement of nociceptin/orphanin FQ in the pathophysiology of Parkinson's disease: a behavioral and neurochemical study in reserpinized mice.

The contribution of nociceptin/orphanin FQ (N/OFQ) to reserpine-induced parkinsonism was evaluated in mice. A battery of motor tests revealed that reserpine caused dose-dependent and long-lasting motor impairment. Endogenous N/OFQ sustained this response since N/OFQ peptide (NOP) receptor knockout (NOP-/-) mice were less susceptible to the hypokinetic action of reserpine than wild-type (NOP+/+) animals. Microdialysis revealed that reserpine elevated glutamate and reduced GABA levels in substantia nigra reticulata, and that resistance to reserpine in NOP-/- mice was accompanied by a milder increase in glutamate and lack of inhibition of GABA levels. To substantiate this genetic evidence, the NOP receptor antagonist 1-[(3R,4R)-1-cyclooctylmethyl-3-hydroxymethyl-4-piperidyl]-3-ethyl-1,3-dihydro-2H benzimidazol-2-one (J-113397) simultaneously reduced akinesia and nigral glutamate levels in reserpinized NOP+/+ mice, being ineffective in NOP-/- mice. Moreover, repeated J-113397 administration in reserpinized mice resulted in faster recovery of baseline motor performance which was, however, accompanied by a loss of acute antiakinetic response. The short-term beneficial effect of J-113397 was paralleled by normalization of nigral glutamate levels, whereas loss of acute response was paralleled by loss of the ability of J-113397 to inhibit glutamate levels. We conclude that endogenous N/OFQ contributes to reserpine-induced parkinsonism, and that sustained NOP receptor blockade produces short-term motor improvement accompanied by normalization of nigral glutamate release.