Hyperbaric oxygen (HBO 2 ) therapy is reported to cause pain relief in several conditions of chronic pain. A single 60-min session of HBO 2 treatment produced a prolonged antinociceptive effect in mice that persisted for 90 min after cessation of treatment. The HBO 2 -induced antinociception was significantly attenuated by pretreatment prior to HBO 2 exposure with the opioid antagonist naltrexone, the non-specific nitric oxide synthase (NOS)-inhibitor N G -nitro-L-arginine methyl ester (L-NAME) and the selective neuronal NOS-inhibitor S-methyl-L-thiocitrulline (SMTC) but not the selective endothelial NOS-inhibitorThe antinociception was also significantly reduced by central pretreatment with a rabbit antiserum against dynorphin 1-13 but not by rabbit antisera against either β-endorphin or methionine-enkephalin. The prolonged antinociceptive effect at 90 min after HBO 2 -induced treatment was also significantly attenuated by naltrexone but not L-NAME administered 60 min following HBO 2 treatment but prior to nociceptive testing. These findings indicate that the antinociception that persists for 90 min after HBO 2 exposure is mediated by nitric oxide (NO) and opioid mechanisms but that the NO involvement is critical during the HBO 2 treatment and not at the time of nociceptive testing. These results are consistent with the concept that HBO 2 may induce an NO-dependent release of opioid peptide to cause a long-acting antinociceptive effect. Words for IndexingHyperbaric Oxygen; Nitric Oxide; Opioid; Antinociception; Mice Corresponding Author: Dr. Raymond M. Quock, Department of Pharmaceutical Sciences, College of Pharmacy, Washington State University, P.O. Box 646534, Pullman,. Perspective This article present evidence of a persistent antinociceptive effect of hyperbaric oxygen treatment that is mediated by opioid and nitric oxide mechanisms. Further elucidation of the underlying mechanism could potentially identify molecular targets to cause a longer-acting activation of endogenous pain-modulating systems.

Hyperbaric oxygen (HBO 2 ) therapy is approved by the FDA for limited clinical indications but is reported to produce pain relief in several chronic pain conditions. However, there have been no studies to explain this apparent analgesic effect of HBO 2 . Research conducted in our laboratory demonstrates that four daily 60-min HBO 2 treatments at 3.5 ATA induced an unparalleled antinociceptive response that consists of 1) an early phase that lasted at least six hours after the HBO 2 treatment before dissipating; and 2) a late phase that emerged about 18 hours after the early phase and lasted for up to three weeks. The early phase was sensitive to antagonism by acutely intracerebroventricular (i.c.v.)-administered opioid antagonist naltrexone and the nitric oxide synthase (NOS)-inhibitor L-NAME. The late phase was inhibited by treatment with i.c.v. naltrexone or L-NAME during the four HBO 2 treatments but was not antagonized by either naltrexone or L-NAME following acute pretreatment two weeks after HBO 2 treatment. These experimental results implicate a novel mechanism that is activated by HBO 2 , resulting in an antinociceptive response of unusually long duration that is of potential interest in the clinical management of pain.Perspective-Hyperbaric oxygen treatment of mice can induce a two-phase antinociceptive response of unusually long duration. Nitric oxide and opioid receptors appear to initiate or mediate both phases of the antinociceptive response. Further elucidation of the underlying mechanism may potentially identify molecular targets that cause long-lasting activation of endogenous analgesic systems.

We have previously shown that the antinociceptive effect of nitrous oxide (N 2 O) in the rat hot plate test is sensitive to antagonism by antisera against the endogenous opioid peptide β-endorphin. Moreover, N 2 O-induced antinociception is reduced by inhibition of nitric oxide (NO) production in the brain. To test the hypothesis that N 2 O might stimulate an NO-dependent neuronal release of β-endorphin, we conducted a ventricular-cisternal perfusion with artificial cerebrospinal fluid (aCSF) in urethane-anesthetized Sprague Dawley rats. Ten-min fractions of aCSF perfusate were collected from separate groups of room air-exposed rats, N 2 O-exposed rats, and L-NAME-pretreated, N 2 Oexposed rats; they were then analyzed for their content of NO metabolites and β-endorphin. Compared to room air control, exposure to 70% N 2 O increased perfusate levels of the NO metabolites nitrite and nitrate as well as β-endorphin. Pretreatment of rats with L-N G -nitro arginine methyl ester, an inhibitor of NO synthase, prevented the N 2 O-induced increases in nitrite, nitrate and β-endorphin. These findings demonstrate in an in vivo rat model that N 2 O may stimulate an NO-dependent neuronal release of β-endorphin.

Exposure to N2O has been shown to cause neuronal release of β‐EP from rat hypothalamic cells (Zuniga et al., Brain Res. 420:66, 1987). The antinociceptive of N2O in rodents is antagonized by inhibition of NO production (McDonald et al., JPET 269: 601, 1994). The present study was carried out to determine whether NO activity was increased concomitantly with the release of β‐EP in VC‐perfused rats. Male Sprague Dawley rats, 200‐250 g body weight, were anesthetized with urethane and mounted in a stereotaxic headholder. An inflow cannula was inserted into the lateral cerebral ventricle and an outflow cannula was inserted into the cisternum magnum. Artificial cerebrospinal fluid (aCSF) was infused into the lateral ventricle; perfusate fractions were collected from the cisternum and analyzed for NO metabolites and β‐EP content. Exposure to 70% N2O increased perfusate levels of nitrite and nitrate, compared to fractions collected from rats under room air. Exposure to 70% N2O also increased the concentration of β‐EP, compared to levels in fractions collected under room air. These findings indicate a functional link between increased NO activity and neuronal release of β‐EP in response to N2O and support our hypothesis that N2O may stimulate an NO‐dependent neuronal release of β‐EP. (Supported in part by State of Washington Initiative Measure No. 171 and the SURF Program in Pharmacology and Toxicology.)

HBO2 is approved by the FDA for limited clinical indications but not for chronic pain. Earlier we showed that HBO2 treatment produces an antinociceptive effect mediated by nitric oxide (NO) and opioid mechanisms [Zylstra et al., FASEB J 22, 711.16 & 711.17, 2008]. In this study, we assessed the antinociceptive effect of multiple HBO2 treatments. Male NIH Swiss mice (20‐30 g) were exposed to HBO2 at 2.5 ATA for 4×60‐min then returned to their home cages for different time intervals before testing with the acetic acid‐induced abdominal constriction test. Results revealed a robust antinociception of at least 6 hr after the last HBO2 treatment. This response was followed a day later by an emerging and equally robust late‐phase antinociceptive effect that surprisingly lasted for two weeks following the last HBO2 treatment. The early phase was antagonized by pretreatment with naltrexone (NTX) and the late phase was antagonized by the selective 5‐HT1A antagonist WAY‐100635 but not NTX. Continuous treatment with NTX or the NO synthase‐inhibitor L‐NAME during the 4 days of HBO2 treatment also reduced the late‐phase response. Based on these findings, we suggest that the early‐phase antinociception involves an opioid pathway, while the longer‐acting late‐phase antinociception involves an NO‐opioid‐5‐HT1A pathway. (Supported in part by NIH Grant GM‐77153, the College of Pharmacy and the Chico Hyperbaric Center.)