ABSTRACT
Objective To investigate whether the μ- and δ-opioid receptors were involved in mediating the a ntinociceptive effect of opioid injection into the nucleus submedius (Sm). Methods Nociceptive behavior produced by subcutaneous injection of formalin (65 mmol/L, 50 μL) into the hind paw of the rat was assessed quantitatively using an automated movement detection system. The effects of morphine and selectiveμ- and δ-opioid receptor antagonists microinjected unilaterally into the Sm were determined in the awake rats. Results Morphine (31 mmol/L, 0. 5 μL) depressed the nociceptive behavior elicited by formalin, and this effect was antagonized completely by the selective μ-receptor antagonist β-funaltrexamine (β-FNA, 0. 4 mmol/L, 0. 5 μL) and naloxonazine (0.8 mmol/L, 0.5 μL), and partly by the δ-receptor antagonist naltrindole (0.4 mmol/L, 0.5μL).Administration of morphine into thalamic regions more than 0. 5 mm dorsal to the Sm had no effect on the nociceptive behavior. Conclusion Antinociceptive effects produced by opioid acting on Sm neurons are mediated mainly by μ-opioid receptors, and partly by δ-receptors.