Spinal-Supraspinal and Intrinsic m-Opioid Receptor Agonist-Norepinephrine Reuptake Inhibitor (MOR-NRI) Synergy of Tapentadol in Diabetic Heat Hyperalgesia in Mice

Tapentadol is a m-opioid receptor (MOR) agonist and norepinephrine reuptake inhibitor (NRI) with established efficacy in neuropathic pain in patients and intrinsic synergistic interaction of both mechanisms as demonstrated in rodents. In diabetic mice, we analyzed the central antihyperalgesic activity, the occurrence of site-site interaction, as well as the spinal contribution of opioid and noradrenergic mechanisms in a hotplate test. Tapentadol (0.1–3.16 mg/animal) showed full efficacy after intrathecal as well as after intracerebroventricular administration (ED50 0.42 mg/animal i.t., 0.18 mg/animal i.c.v.). Combined administration of equianalgesic doses revealed spinal-supraspinal synergy (ED50 0.053 mg/animal i.t. 1 i.c.v.). Morphine (0.001–10 mg/animal) also showed central efficacy and synergy (ED50 0.547 mg/animal i.t., 0.004 mg/animal i.c.v., 0.014 mg/animal i.t. 1 i.c.v.). Supraspinal potencies of tapentadol and morphine correlated with the 50-fold difference in their MOR affinities. In contrast, spinal potencies of both drugs were similar and correlated with their relative systemic potencies (ED50 0.27 mg/kg i.p. tapentadol, 1.1 mg/kg i.p. morphine). Spinal administration of the opioid antagonist naloxone or the a2-adrenoceptor antagonist yohimbine before systemic administration of equianalgesic doses of tapentadol (1 mg/kg i.p.) or morphine (3.16 mg/kg i.p.) revealed pronounced influence on opioidergic and noradrenergic pathways for both compounds. Tapentadol was more sensitive toward both antagonists than was morphine, with median effective dose values of 0.75 and 1.72 ng/animal i.t. naloxone and 1.56 and 2.04 ng/animal i.t. yohimbine, respectively. It is suggested that the antihyperalgesic action of systemically administered tapentadol is based on opioid spinal-supraspinal synergy, as well as intrinsic spinally mediated MOR-NRI synergy.