Peripheral substance P and neurokinin-1 receptors have a role in inflammatory and neuropathic orofacial pain models

There is accumulating evidence that substance P released from peripheral sensory neurons participates in inflammatory and neuropathic pain. In this study it was investigated the ability of substance P to induce orofacial nociception and thermal and mechanical hyperalgesia, as well as the role of NK₁ receptors on models of orofacial inflammatory and neuropathic pain. Substance P injected into the upper lip at 1, 10 and 100μg/50μL failed to induce nociceptive behavior. Also, substance P (0.1-10μg/50μL) injected into the upper lip did not evoke orofacial cold hyperalgesia and when injected at 1μg/50μL did not induce mechanical hyperalgesia. However, substance P at this latter dose induced orofacial heat hyperalgesia, which was reduced by the pre-treatment of rats with a non-peptide NK₁ receptor antagonist (SR140333B, 3mg/kg). Systemic treatment with SR140333B (3mg/kg) also reduced carrageenan-induced heat hyperalgesia, but did not exert any influence on carrageenan-induced cold hyperalgesia. Blockade of NK₁ receptors with SR140333B also reduced by about 50% both phases of the formalin response evaluated in the orofacial region. Moreover, heat, but not cold or mechanical, hyperalgesia induced by constriction of the infraorbital nerve, a model of trigeminal neuropathic pain, was abolished by pretreatment with SR140333B. Considering that substance P was peripherally injected (i.e. upper lip) and the NK₁ antagonist used lacks the ability to cross the blood-brain-barrier, our results demonstrate that the peripheral SP/NK₁ system participates in the heat hyperalgesia associated with inflammation or nerve injury and in the persistent pain evoked by formalin in the orofacial region.