Dopamine-independent motor behavior following microinjection of rimorphin in the substantia nigra.

The motor-activating effects of rimorphin, an opioid peptide derived from prodynorphin, were examined in the substantia nigra pars reticulata of rats. Unilateral microinjections of rimorphin produced dose-dependent contralateral rotational behavior that was antagonized by naloxone, suggesting that these effects were mediated by opiate receptors. Lesions of midbrain dopamine cells with 6-hydroxydopamine (6-OHDA) produced a 95% or greater depletion of tyrosine hydroxylase in the striatum ipsilateral to the lesion, but failed to reduce the number of circles made by the rats. In addition to an overall preservation of rimorphin-induced circling in animals with 6-OHDA lesions, 50% of these rats exhibited circling that was at least 2 standard deviations above the mean of animals without lesions. The motor activating effects of rimorphin, thus, appear to occur independently of the nigrostriatal dopamine system; these effects may instead be mediated by GABAergic efferents in the pars reticulata.