Loss of striatal mu₁ opiate binding by substantia nigra lesions in the rat

Opiate receptors have been identified within the striatum and some have been localized presynaptically to nigrostriatal neurons. Using unilateral ablative lesions of the substantia nigra, we examined binding in the ipsilateral and contralateral striata. Lesions significantly lowered both ³H[D-Ala², MePhe⁴, Gly(ol)⁵] enkephalin (DAGO) and ³H[D-Ala², Leu⁵] enkephalin (DADL) binding. The inclusion of competitors in these assays revealed a decrease in both mu₁ and mu₂ receptors. Mu₁ binding was slightly more sensitive to the lesioning than mu₂ binding. Selective mu₁ and mu₂ binding assays supported these observations. No change in delta binding was observed in the lesioned striata. These studies raise the possibility that both mu₁ and mu₂, but not delta, receptors are localized presynaptically on nigrostriatal neurons.