Preemptive analgesic effects of steroid anesthesia with alphaxalone in the rat formalin test : Evidence for differential GABAA receptor modulation in persistent nociception

Background : The role of preemptive treatment with volatile and intravenous anesthetics has been examined in previous studies using the rat formalin test. Evidence describing analgesic properties of the gamma-amino butyric acid-ergic (GABAergic) steroid anesthetics, such as alphaxalone, suggest that they may suppress the development of central sensitization to pain. This study examined the preemptive effects of alphaxalone in comparison with other GABAergic anesthetics, propofol and pentobarbital. Methods : The pain behavior of rats was evaluated (using the previously validated weighted scores method of behavioral rating) 15-60 min after subcutaneous hind paw injection of 50 μl 1.5% formalin. In each trial, anesthetics and their respective vehicles were administered by tail-vein injection either 0.5-10 min before or 5 min after, formalin injection. When analgesic effects were observed with any of these agents, further studies were conducted with a GABA A receptor antagonist in an attempt to confirm a specific receptor-mediated action of the agent. Results : Alphaxalone pretreatment produced transient analgesia in the early part of phase 2, which was not observed in rats posttreated with alphaxalone. The analgesic effect of alphaxalone was antagonized by picrotoxin, as well. Neither pentobarbital nor propofol showed any analgesic effects at the doses used in our study. Conclusions : Whereas alphaxalone was shown to produce preemptive analgesia through its action at the GABA A receptor, pentobarbital and propofol, which also are known to act at this site, showed no analgesic effects. The diversity of receptor subtypes and functional complexity of GABA A receptors is such that steroid anesthetics may have effects that are different from other GABAergic agents. Further research into the role of progesterone metabolites and steroid anesthetics in the prevention of central sensitization may have clinical implications for the treatment of acute or chronic pain.