Microdialysis reveals a morphine-induced increase in pallidal opioid peptide release

Freely moving rats were implanted with microdialysis probes in the globus pallidus/ventral pallidum, nucleus accumbens or caudate nucleus. Morphine (10 mg kg⁻¹ i.p.) induced an average 128% increase in extracellular opioid peptide levels in the pallidum over a 2 h period peaking 1 h after injection. No change was observed in the nucleus accumbens or caudate nucleus. Dose-response analysis showed a smaller (26%), non-significant, increase at a lower dose of 2 mg kg⁻¹ and no effect at a higher dose of 40 mg kg⁻¹. No evidence of significant acute tolerance was apparent following repeated administration of morphine (10 mg kg⁻¹, i.p.) at 3 h intervals. The magnitude of the morphine effect varied greatly between animals and was dependent on the pre-injection baseline opioid peptide levels.