Intraplantar injection of tetrahydrobiopterin induces nociception in mice.

Tetrahydrobiopterin (BH4) is implicated in the development and maintenance of chronic pain. After injury/inflammation, the biosynthesis of BH4 is markedly increased in sensory neurons, and the pharmacological and genetic inhibition of BH4 shows analgesic effects in pre-clinical animal pain models. Intrathecal injections of BH4 have been shown to induce and enhance pain-like behaviours in rats, suggesting that under chronic pain conditions BH4 may act by facilitating central sensitisation. So far it is unknown whether BH4 acts on peripheral sites of the somatosensory system or whether BH4 per se provokes nociceptive pain behaviours. The purpose of this study was therefore to investigate the acute nociceptive effects of intraplantar injection of BH4. BH4 was found to induce dose-dependent licking/biting of the paw lasting 5 min, which was not observed following an injection of biopterin (inactive BH4 metabolite). Paw swelling, measured as paw thickness and weight, was not observed after BH4 injection. To explore possible mechanisms of action of BH4, the effect of local pre-treatment with indomethacin, Nω-nitro-L-arginine methyl ester, Nω-nitro-L-arginine, capsazepine and ruthenium red was tested. Morphine served as a positive control. Intraplantar pre-injection of morphine dose-dependently inhibited BH4-induced nociception, while none of the other compounds showed any statistical significant antinociception. These results suggest that BH4 exhibits nociceptive properties at peripheral sites of the somatosensory system, proposing an as yet unexplored involvement of BH4 in peripheral nociceptive processes. However, this appears not to be mediated through nitric oxide and prostaglandin release or by activation of the transient receptor potential vanilloid 1.