Naloxone prolongs cutaneous nociceptive block by lidocaine in rats.

We aimed to investigate the local anesthetic properties of naloxone alone or as an adjunct for the local anesthetic lidocaine. After the block of the cutaneous trunci muscle reflex (CTMR) with drugs delivery by subcutaneous infiltration, cutaneous nociceptive block was tested on the ratsꞌ backs. We demonstrated that naloxone, as well as lidocaine, elicited cutaneous analgesia dose-dependently. The relative potency in inducing cutaneous analgesia was lidocaine [22.6 (20.1 - 25.4) μmol/kg] > naloxone [43.2 (40.3 - 46.4) μmol/kg] (P < 0.05). On an equianesthetic basis [50% effective dose (ED50 ), ED25 , and ED75 ], naloxone displayed a greater duration of cutaneous analgesic action than lidocaine (P < 0.01). Coadministration of lidocaine (ED95 or ED50 ) and ineffective-dose naloxone (13.3 μmol/kg) intensifies sensory block (P < 0.01) with prolonged duration of action (P < 0.001) compared with lidocaine (ED95 or ED50 ) alone or naloxone (13.3 μmol/kg) alone on infiltrative cutaneous analgesia. The preclinical data showed that naloxone is less potent than lidocaine as an infiltrative anesthetic, but its analgesic duration was longer than that of lidocaine. Furthermore, naloxone prolongs lidocaine analgesia, acting synergistically for nociceptive block.