Eunhee Chung

A prolonged nitric oxide-dependent, opioid-mediated antinociceptive effect of hyperbaric oxygen in mice.

UNLABELLED Hyperbaric oxygen (HBO(2)) therapy is reported to cause pain relief in several conditions of chronic pain. A single 60-minute session of HBO(2) treatment produced a prolonged antinociceptive effect in mice that persisted for 90 minutes after cessation of treatment. The HBO(2)-induced antinociception was significantly attenuated by pretreatment before HBO(2) exposure with the opioid antagonist naltrexone, the nonspecific 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-inhibitor N(5)-(1-iminoethyl)-L-ornithine (L-NIO). The antinociception was also significantly reduced by central pretreatment with a rabbit antiserum against dynorphin(1-13) but not by rabbit antisera against either beta-endorphin or methionine-enkephalin. The prolonged antinociceptive effect at 90 minutes after HBO(2)-induced treatment was also significantly attenuated by naltrexone but not L-NAME administered 60 minutes after HBO(2) treatment but before nociceptive testing. These findings indicate that the antinociception that persists for 90 minutes 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. PERSPECTIVE This article presents evidence of a persistent antinociceptive effect of hyperbaric oxygen treatment that is mediated by opioid and NO mechanisms. Further elucidation of the underlying mechanism could identify molecular targets to cause a longer-acting activation of endogenous pain-modulating systems.

Nitrous oxide-antinociception is mediated by opioid receptors and nitric oxide in the periaqueductal gray region of the midbrain

Previous studies have shown that nitrous oxide (N(2)O)-induced antinociception is sensitive to antagonism by blockade of opioid receptors and also by inhibition of nitric oxide (NO) production. The present study was conducted to determine whether these occur within the same brain site. Mice were stereotaxically implanted with microinjection cannulae in the periaqueductal gray (PAG) area of the midbrain. In saline-pretreated mice, exposure to 70% N(2)O resulted in a concentration-dependent antinociceptive effect in the mouse abdominal constriction test. Pretreatment with an opioid antagonist in the PAG significantly antagonized the antinociceptive effect. Pretreatment with an inhibitor of NO production in the PAG also significantly antagonized the antinociceptive effect. These findings suggest that N(2)O acts in the PAG via an NO-dependent, opioid receptor-mediated mechanism to induce antinociception.

Hyperbaric Oxygen Treatment Induces a 2-Phase Antinociceptive Response of Unusually Long Duration in Mice

UNLABELLED 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 4 daily 60-minute HBO(2) treatments at 3.5 absolute atmospheres induced an unparalleled antinociceptive response that consists of 1) an early phase that lasted at least 6 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 3 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 4 daily HBO(2) treatments but was not antagonized by either naltrexone or L-NAME following acute pretreatment 2 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 2-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.

Dynorphin-mediated antinociceptive effects of L-arginine and SIN-1 (an NO donor) in mice

The antinociceptive response of mice to the amino acid L-arginine (L-ARG) has been attributed to either an opioid mechanism or a non-opioid but nitric oxide (NO)-dependent mechanism. Earlier it was reported that the mechanism of nitrous oxide-induced antinociception involved opioid components and was also dependent on brain NO. This study was designed to determine whether the antinociceptive effects of L-ARG and the NO donor 3-morpholinosydnoimine (SIN-1) might be mediated by brain mechanisms similar to those that are responsible for nitrous oxide (N(2)O) antinociception. L-ARG and SIN-1 were administered to mice intracerebroventricularly (i.c.v.), and antinociception was assessed using the acetic acid abdominal constriction test. Both L-ARG and SIN-1 caused dose-related antinociceptive effects that were blocked by naloxone and norbinaltorphimine. The antinociceptive effects of both SIN-1 and L-ARG were also blocked to a greater extent by i.c.v. administration of a rabbit antiserum against rat dynorphin 1-13 than an antiserum against methionine-enkephalin, suggesting that the SIN-1 and L-ARG effects may be related to stimulated release of dynorphin. The antinociceptive effect of L-ARG was antagonized by an inhibitor of neuoronal NO synthase enzyme, indicating that L-ARG had to be converted to NO for its antinociceptive action. These findings indicate that the mechanisms of antinociceptive action of L-ARG and SIN-1 are both mediated by dynorphin and dependent on NO.

Antagonism of the antinociceptive effect of nitrous oxide by inhibition of enzyme activity or expression of neuronal nitric oxide synthase in the mouse brain and spinal cord.

Previous studies have implicated nitric oxide (NO) in the antinociceptive response to the anesthetic gas nitrous oxide (N(2)O). The present study was conducted to confirm this NO involvement using pharmacological and gene knockdown and knockout strategies to inhibit the supraspinal and spinal production of NO. Antinociceptive responsiveness to 70% N(2)O was assessed using the acetic acid (0.6%) abdominal constriction test in NIH Swiss mice following intracerebroventricular (i.c.v.) or intrathecal (i.t.) pretreatment with the NOS-inhibitor l-N(G)-nitro arginine methyl ester (L-NAME) or an antisense oligodeoxynucleotide (AS-ODN) directed against neuronal NOS (nNOS). Experiments were also conducted in mice homozygous for a defective nNOS gene (nNOS(-/-)). Mice that were pretreated i.c.v. or i.t. with L-NAME (1.0 microg) both exhibited 80-90% reduction in the magnitude of the N(2)O-induced antinociceptive response. Mice that were pretreated i.c.v. or i.t. with nNOS AS-ODN (3 x 25microg) exhibited a 60-80% antagonism of the antinociceptive response. Compared to wild-type mice, nNOS knockout mice showed a 60% reduction in N(2)O-induced antinociception. These findings consistently demonstrate that transient or developmental suppression of nNOS expression significantly reduces antinociceptive responsiveness to N(2)O. NO of both supraspinal and spinal origin, therefore, plays an important role in the antinociceptive response to N(2)O.

The effect of hyperbaric oxygen on regional brain and spinal cord levels of nitric oxide metabolites in rat

Hyperbaric oxygen (HBO(2)) therapy is reported to be beneficial in transient brain ischemia. The present study was conducted to determine the influence of HBO(2) on metabolites of nitric oxide (NO) in brain and spinal cord of rats. Rats were exposed to room air (RA), normobaric air (NBA), normobaric oxygen (NBO(2)), hyperbaric air (HBA) or HBO(2), the last two conditions at 2.5ATA (atmosphere absolute) for 60 min. The results demonstrate that, compared to the NBA control, oxygen alone generally reduced tissue levels of NO(x)(-) (nitrite plus nitrate). On the other hand, 2.5ATA alone tended to have a slight, if any, effect on tissue levels of NO(x)(-). The combination of oxygen and pressure (i.e., HBO(2)) generally led to an increase in tissue levels of NO(x)(-). Based on these findings, it is concluded that HBO(2) appears to markedly increase NO function most notably in the corpus striatum, brainstem, cerebellum and spinal cord.

Nitrous oxide-induced NO-dependent neuronal release of β-endorphin from the rat arcuate nucleus and periaqueductal gray.

Nitrous oxide (N(2)O)-induced antinociception is thought to result from nitric oxide (NO)-dependent neuronal release of endogenous opioid peptides in the central nervous system. The present study employed microdialysis to determine whether exposure to N(2)O stimulates proopiomelanocortin (POMC) neurons to release β-endorphin in the arcuate nucleus (ARC) of the hypothalamus and the periaqueductal gray (PAG) of the midbrain. Male Sprague-Dawley rats were stereotaxically implanted with microdialysis probes in the ARC or PAG. Exposure to 70% N(2)O significantly increased dialysate levels of oxidation products of NO as well as β-endorphin, compared to levels in fractions collected under room air. These increases in the ARC and PAG were abolished by systemic pretreatment with L-N(G)-nitro arginine methyl ester (L-NAME). These findings suggest an association between increased NO activity and the stimulated release of β-endorphin during exposure of rats to N(2)O.

Nitric oxide in hyperbaric oxygen-induced acute antinociception in mice.

Hyperbaric oxygen (HBO2) therapy induces analgesia in various conditions of pain in humans. In mice, HBO2 treatment evokes an acute antinociceptive response in the abdominal constriction test. To demonstrate the dependence of HBO2-induced antinociception on nitric oxide (NO), antinociceptive responsiveness to HBO2 was assessed after three different approaches that interfered with NO production. HBO2-induced antinociception was significantly attenuated by intracerebroventricular and intrathecal pretreatment with an inhibitor of NO synthase (NOS) enzyme and also by an antisense oligodeoxynucleotide directed against neuronal NOS. The antinociceptive effect was also significantly reduced in mice homozygous for a defective neuronal NOS gene. On the basis of these results, we conclude that neuronal NO is critical in the expression of the acute antinociceptive effect of HBO2.

Role of cyclic GMP in nitrous-oxide-induced anxiolytic-like behavior in the mouse light-dark exploration test.

Nitric oxide (NO) has been implicated in the anxiolytic-like behavioral effects of nitrous oxide (N2O). This study was conducted to determine whether NO activates a soluble guanylyl cyclase (sGC)-cyclic guanosine monophosphate (cGMP) pathway in the behavioral response to N2O in the light-dark exploration test. In mice pretreated with an sGC inhibitor, the increased light-compartment activity normally induced by N2O was significantly attenuated. Pretreatment with a cGMP phosphodiesterase inhibitor antagonized the anxiogenic effect of 15% N2O and enhanced the anxiolytic effect of 25% N2O, implying that cGMP reduces anxiety. These preliminary findings suggest that a signaling pathway involving NO and cGMP may mediate the behavioral effects of N2O.

Correlation of inbred mouse sensitivity to nitrous oxide antinociception with brain nitric oxide synthase activity following exposure to nitrous oxide

Inhibition of nitric oxide synthase (NOS) antagonizes nitrous oxide (N2O)-induced antinociception in mice. This study was conducted to compare brain NOS activity in high responding C57BL/6 mice, low responding DBA/2 mice and S5 mice selectively bred for low responsiveness to N2O. Exposure to 70% N2O suppressed acetic acid-induced abdominal constrictions in C57BL/6 mice but not DBA/2 or S5 mice. N2O exposure also elevated NOS activity in brains of C57BL/6 mice but not DBA/2 or S5 mice. The absence of these effects in DBA/2 or S5 mice is further support for the hypothesis that nitric oxide (NO) may play a critical role in N2O-induced antinociception in mice.