Analgesia induced by morphine injected into the pallidum

Bilateral microinjections of morphine hydrochloride (10; 20; 30 micrograms/0.5 microliter/side) or saline were aimed at three different regions of the rat globus pallidus: dorsal, medial, ventral. Before and at various intervals after intrapallidal morphine (15; 30; 60; 90; 180 min), estimation of pain threshold was made by the hot plate procedure. Dose-dependent morphine analgesia was elicited from all three regions injected. Differences between the pallidal areas as to the intensity and duration of the drug's effect were noticed. Pretreatment with subcutaneous naloxone (1 mg/kg, s.c.) inhibited the morphine (20 micrograms) analgesia elicited from the medial and dorsal pallidum; it decreased and delayed the effect of morphine injected into the ventral pallidum. The results suggest that the three pallidal areas tested are involved to a different degree (medial/dorsal greater than ventral) in the morphine analgesia mediated by opiate receptors.