The 5-HT_1A receptor agonist F 13640 attenuates mechanical allodynia in a rat model of trigeminal neuropathic pain

The effects of acute intraperitoneal injections of the 5-HT_1A receptor agonists F 13640 [(3-chloro-4-fluoro-phenyl)-[4-fluoro-4-([(5-methyl-pyridin-2-ylmethyl)- amino]-methyl)piperidin-1-yl]-methadone] and F 13714 [3-chloro-4-fluorophenyl-(4-fluoro-4-[[(5-methyl-6-methylamino-pyridin-2- ylmethyl)-amino]-methyl]-piperidin-1-yl-methanone] were studied in comparison with those of baclofen and morphine on responsiveness to von Frey hair stimulation after chronic constriction injury to the rat's infraorbital nerve (IoN-CCI). Following IoN-CCI, an ipsilateral hyperresponsiveness developed that remained stable in control rats throughout the period of drug testing. F 13640, F 13714, baclofen and morphine dose-dependently decreased the hyperresponsiveness; normalization of the response occurred at doses 0.63, 0.04, 5 and 10 mg/kg, respectively. Confirming earlier data, baclofen's effects further validate IoN-CCI as a model of trigeminal neuralgia. The effects of F 13640 and F 13714 are initial evidence that 5-HT_1A receptor agonists produce profound analgesia in the IoN-CCI model. The present data extend recent evidence that high-efficacy 5-HT_1A receptor activation constitutes a new mechanism of central analgesia the spectrum of which may also encompass trigeminal neuropathic pain.