Effects of the combined continuous administration of morphine and the high-efficacy 5-HT1A agonist, F 13640 in a rat model of trigeminal neuropathic pain.

F 13640 is a recently discovered high-efficacy 5-HT1A receptor agonist that has demonstrated robust anti-allodynic efficacy in a rat model of trigeminal neuropathic pain upon acute and continuous administration. In this model, continuous morphine infusion (5 mg/day) was shown to be effective during the first week of its administration but became almost completely ineffective by the end of the second week; F 13640's effectiveness (0.63 mg/day) remained unchanged during two weeks. Here, we examined the effects of combining F 13640 infusion with that of morphine. During the first week, the combination of the two agents produced a magnitude of effect that was similar to that of morphine when given alone and larger than that of F 13640 alone. During the second week, the combination produced an effect that was similar to that of F 13640 alone, and more effective than that of morphine alone. The latter data suggest that the 5-HT1A agonist, F 13640, inhibits the development of tolerance to morphine in this model. However, it is also possible that little, if any, interaction occurred between the different mechanisms initiated by opioid and 5-HT1A receptor activation, and that the anti-allodynic effect that remained by the end of the two-week treatment period is due solely to 5-HT1A receptor activation. The stable effects of F 13640 during the second week of treatment surpassed those of morphine and were not improved by the addition of morphine to F 13640.