Effects of tapentadol on mechanical hypersensitivity in rats with ligatures of the infraorbital nerve versus the sciatic nerve.

ABSTRACT

BACKGROUND: Convergent data showed that neuropathic pain has specific characteristics at cephalic versus extra-cephalic level, where single-targeted drugs differentially alleviate pain. Because the novel analgesic drug, tapentadol, is acting at two targets, µ-opioid receptors (as agonist) and noradrenaline reuptake (as inhibitor), we tested its effects on neuropathic pain at both cephalic and extra-cephalic levels. METHODS: Sprague-Dawley rats underwent unilateral constriction injury (CCI) to the infraorbital nerve (ION; cephalic territory) or the sciatic nerve (SN; extra-cephalic territory), and alleviation of nerve lesion-induced mechanical allodynia/hyperalgesia was assessed after acute or repeated (for 4 days) treatment with tapentadol compared with morphine and/or reboxetine (noradrenaline reuptake inhibitor) 2 weeks after surgery. Possible changes in the expression of the neuroinflammatory markers activating transcription factor 3 (ATF3), interleukin-6 (IL-6) and brain-derived neurotrophic factor (BDNF) by repeated tapentadol treatment were quantified by real-time reverse transcription polymerase chain reaction in ganglia and central tissues. RESULTS: Acute administration of tapentadol (1-10 mg/kg, i.p.) significantly reduced allodynia in both CCI-SN and CCI-ION rats. Although morphine (3 mg/kg, s.c.) or reboxetine (10 mg/kg, i.p.) alone was only marginally active, the combination of both drugs produced supra-additive effects like those observed with tapentadol. In contrast to repeated morphine whose effects vanished, the anti-allodynic effects of tapentadol remained unchanged after a 4-day treatment. However, the latter treatment with tapentadol did not affect nerve lesion-evoked overexpression of ATF3, IL-6 and BDNF transcripts. CONCLUSIONS: The dual synergistic pharmacological properties of tapentadol, which result in clear-cut anti-neuropathic pain effects at both cephalic and extra-cephalic levels, probably involve mechanisms downstream of nerve injury-induced neuroinflammatory reaction.