PRESYNAPTIC VERSUS POSTSYNAPTIC LOCALIZATION OF MU-OPIOID AND DELTA-OPIOID RECEPTORS IN DORSAL AND VENTRAL STRIATOPALLIDAL PATHWAYS

Parallel studies have demonstrated that enkephalin release from nerve terminals in the pallidum (globus pallidus and ventral pallidum) can b e modulated by locally applied opioid drugs. To investigate further th e mechanisms underlying these opioid effects, the present study examin ed the presynaptic and postsynaptic localization of delta (DOR1) and m u(MOR1) opioid receptors in the dorsal and ventral striatopallidal enk ephalinergic system using fluorescence immunohistochemistry combined w ith anterograde and retrograde neuronal tracing techniques. DOR1 immun ostaining patterns revealed primarily a postsynaptic localization of t he receptor in pallidal cell bodies adjacent to enkephalin- or synapto physin-positive fiber terminals. MOR1 immunostaining in the pallidum r evealed both a presynaptic localization, as evidenced by punctate stai ning that co-localized with enkephalin and synaptophysin, and a postsy naptic localization, as evidenced by cytoplasmic staining of cells tha t were adjacent to enkephalin and synaptophysin immunoreactivities. In jections of the anterograde tracer Phaseolus vulgaris leucoagglutinin (PHA-L) or the retrograde tracer Texas Red-conjugated dextran amine (T RD) into the dorsal and ventral striatum resulted in labeling of stria topallidal fibers and pallidostriatal cell bodies, respectively. DOR1 immunostaining in the pallidum co-localized only with TRD and not PHA- L, whereas pallidal MOR1 immunostaining co-localized with PHA-L and no t TRD. These results suggest that pallidal enkephalin release may be m odulated by mu opioid receptors located presynaptically on striatopall idal enkephalinergic neurons and by delta opioid receptors located pos tsynaptically on pallidostriatal feedback neurons.