Pharmacology
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The present study evaluated the role of ventrolateral periaqueductal gray (vlPAG)-located orphanin-FQ (OFQ) in the opioid tolerance induced by repeated microinjections of morphine (MOR) into vlPAG. Microinjection of MOR (5 microg/0.5 microl) into vlPAG caused antinociception as quantified with the tail flick and the hot plate tests. When MOR microinjection was repeated twice daily, the antinociceptive effect disappeared within 2 days (tolerance). ⋯ Furthermore, if OFQ (1 ng/0.5 microl) was microinjected into vlPAG, then a MOR microinjection administered 15 min later into vlPAG did not elicit antinociception. Finally, opioid tolerance induced by repeated systemic MOR injections (5 mg/kg, i.p.) was reversed by a single microinjection of NST into vlPAG. This emphasizes the central importance of vlPAG-located OFQ in the MOR tolerance.
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Using a new animal model of postoperative pain we recently developed, we investigated whether the selective cyclooxygenase-2 (COX-2) inhibitor parecoxib sodium, and the analgesic tramadol hydrochloride, attenuated mechanical primary hyperalgesia induced by minor surgery on the rat tail. ⋯ We have shown that the hyperalgesia in our model of postoperative pain is responsive to treatment with the analgesic tramadol, but it is not responsive to the selective COX-2 inhibitor parecoxib at the doses we used.