High-efficacy 5-hydroxytryptamine 1A receptor activation counteracts opioid hyperallodynia and affective conditioning

Pain may become intractable as tolerance develops to opioids and the opioids, paradoxically, induce pain. We examined the hypothesis that the analgesia produced by the novel analgesic and high-efficacy 5-hydroxytryptamine ( 5-HT)(1A) receptor agonist ( 3-chloro-4-fluoro-phenyl)-[ 4-fluoro-4-{[( 5-methyl-pyridin-2-ylmethyl)- amino] methyl} piperidin-1-yl] methanone, fumaric acid salt ( F 13640) may counteract opioid-induced pain. In studies of the somatosensory quality of pain in infraorbital nerve-injured rats, morphine infusion ( 5 mg/day) by means of osmotic pumps initially caused analgesia ( i.e., decreased the behavioral response to von Frey filament stimulation), followed by hyperallodynia and analgesic tolerance. Infusion of F 13640 ( 0.63 mg/day) prevented the development of opioid hyperallodynia and reversed opioid hyperallodynia once established. In studies of the affective/ motivational quality of pain, F 13640 both prevented and reversed the conditioned place aversion induced by naloxone ( 0.04 mg/kg i.p.) in morphine-infused rats; F 13640 also prevented and reversed the conditioned place preference induced by morphine injections ( 7.5 mg/kg i.p.). The data confirm that opioids produce bidirectional hypo- dand proalgesic actions, and offer initial evidence that high-efficacy 5-HT1A receptor activation counteracts both the sensory and the affective/ motivational qualities of opioid-induced pain. The data also indicate that F 13640 may be effective with opioid-resistant pain. It further is suggested that opioid addiction may represent self-therapy of opioid-induced pathological pain.