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.