Nitrous Oxide: Mechanism of Its Antinociceptive Action

Nitrous oxide (N2O) is an anesthetic gas known to produce an analgesic effect at sub-anesthetic concentrations. This analgesic property of N2O can be clinically exploited in a broad range of conditions where pain relief is indicated. The mechanism of this analgesic effect was long thought to be nonspecific in nature, but a landmark study by Berkowitz and others in 1976 first implicated an opioid mechanism of action, possibly via N2O-stimulated neuronal release of endogenous opioid peptides to activate opioid receptors. N2O-induced release of opioid peptide has been demonstrated in both in vivo and in vitro preparations. Reversal of N2O-induced antinociception in animals by narcotic antagonists has been reported by a number of laboratories. Subsequent studies have utilized more selective opioid antagonists to identify the opioid receptor subtypes involved in the antinociceptive effect of N2O. Extensive pharmacological testing in the mouse abdominal constriction and rat hot plate paradigms have established that N2O-induced antinociception is mediated by κ-opioid receptors in the former and by µ- and -opioid receptors in the latter. Current studies focus on two recent developments. The poor responsiveness of the DBA/2J mouse strain to N2O has led to pharmacogenetic studies that hope to identify the underlying genetic basis for antinociceptive responsiveness to N2O. Other research suggests an involvement of nitric oxide (NO) in mediating the antinociceptive effects of N2O in both rats and mice

Detection and Mapping of Quantitative Trait Loci that Determine Responsiveness

Exposure to 70% N2O evokes a robust antinociceptive effect in C57BL/6 (B6) but not in DBA/2 (D2) inbred mice. This study was conducted to identify quantitative trait loci (QTL) in the mouse genome that might determine responsiveness to N2O. Offspring from the F2 generation bred from B6 and D2 progenitors exhibited a broad range of responsiveness to N2O antinociception as determined by the acetic acid-induced abdominal constriction test. QTL analysis was then used to dissect this continuous trait distribution into component loci, and to map them to broad chromosomal regions. To this end, 24 spleens were collected from each of the following four groups: male and female F2 mice responding to 70% N2O in oxygen with 100% response (high-responders); and male and female F2 mice responding with 0% response (low-responders). Genomic DNA was extracted from the spleens and genotyped with simple sequence length polymorphism MapPairs markers. Findings were combined with findings from the earlier QTL analysis from BXD recombinant inbred mice [Brain Res 725 (1996) 23]. Combined results revealed two significant QTL that influence responsiveness to nitrous oxide on proximal chromosome 2 and distal chromosome 5, and one suggestive QTL on midchromosome 18. The chromosome 2 QTL was evident only in males. A significant interaction was found between a locus on chromosome 6 and another on chromosome 13 with a substantial effect on N2O antinociception

Advances in Understanding the Actions of Nitrous Oxide

Nitrous oxide (N 2 O) has been used for well over 150 years in clinical dentistry for its analgesic and anxiolytic properties. This small and simple inorganic chemical molecule has indisputable effects of analgesia, anxiolysis, and anesthesia that are of great clinical interest. Recent studies have helped to clarify the analgesic mechanisms of N 2 O, but the mechanisms involved in its anxiolytic and anesthetic actions remain less clear. Findings to date indicate that the analgesic effect of N 2 O is opioid in nature, and, like morphine, may involve a myriad of neuromodulators in the spinal cord. The anxiolytic effect of N 2 O, on the other hand, resembles that of benzodiazepines and may be initiated at selected subunits of the γ-aminobutyric acid type A (GABA A ) receptor. Similarly, the anesthetic effect of N 2 O may involve actions at GABA A receptors and possibly at N-methyl-D-aspartate receptors as well. This article reviews the latest information on the proposed modes of action for these clinicaleffects of N 2 O

Increased Apomorphine-Induced Hypothermia Precedes Development of Hypertension in SHRs

Apomorphine produced a greater hypothermic response in spontaneously hypertensive rats (SHRs) than in normotensive Wistar-Kyoto rats (WKYs). Experiments were conducted in SHRs and WKYs of three age groups to determine whether the increased hypothermie responsiveness to apomorphine occurs prior to the development of hypertension. The mean systolic blood pressures (SBPs) of SHRs and WKYs were comparable at 4–6 weeks of age. The mean SBP of SHRs were significantly greater than that of WKYs at both 8–10 and 12–15 weeks of age. Yet SHRs responded to apomorphine with significantly greater hypothermia than WKYs at all three ages. These findings indicate that the hyperresponsiveness of SHRs to apomorphine-induced hypothermia precedes the development of hypertension. This sequence of events is consistent with the hypothesis that central DA systems play a role in development of hypertension in SHRs

Involvement of a NO–cyclic GMP–PKG signaling pathway in nitrous oxide-induced antinociception in mice

The antinociceptive effect of nitrous oxide (N 2O) is dependent on nitric oxide (NO); however, the next step in the pathway activated by NO is undetermined. The present study was conducted to test the hypothesis that a N 2O action involves sequential activation of NO synthase, soluble guanylyl cyclase and protein kinase G to induce an antinociceptive effect in mice. The antinociceptive responsiveness of male NIH Swiss mice to N 2O was assessed using the acetic acid abdominal constriction test. Different groups of mice were pretreated with either saline, the NO scavenger 2-(4-carboxyphenyl)-4,5-dihydro-4,4,5,5-tetramethyl-1H-imidazolyl-1-oxy-3-oxide (carboxy-PTIO), the guanylyl cyclase-inhibitor 1H-[1,2,4]-oxadiazolo-[4,3-a]quinoxalin-1-one (ODQ), the protein kinase G-inhibitor Rp-isomer of 8-(4-chlorophenylthio)-guanosine-3′,5′-cyclic monophosphorothioate (Rp-8-pCPT-cGMPS) or the selective phosphodiesterase V-inhibitor 1,2-dihydro-2-[(2-methyl-4-pyridinyl)methyl]-1-oxo-8-(2-pyrimidinylmethoxy)-4-(3,4,5-trimethoxyphenyl)-2,7-naphthyridine-3-carboxylic acid methyl ester hydrochloride (T 0156). Vehicle (saline)-pretreated mice responded to N 2O in a concentration-dependent manner. This antinociceptive effect was antagonized by systemic pretreatment with carboxy-PTIO and ODQ and central pretreatment with Rp-8-pCPT-cGMPS. In each case, the dose–response curve for N 2O was progressively shifted to the right by increasing the dose of each pretreatment drug. On the other hand, N 2O-induced antinociception was enhanced by systemic pretreatment with T 0156; the dose–response curve for N 2O was shifted to the left. The ATP-sensitive potassium channel blocker glibenclamide was without influence on the antinociceptive effect of N 2O. These results support the hypothesis that N 2O-induced antinociception in mice is mediated by a NO–cyclic GMP–PKG pathway. [Display omitted

Do inhalation general anesthetic drugs induce the neuronal release of endogenous opioid peptides?

The antagonism of some effects of inhalation general anesthetic agents by naloxone suggests that there may be an opioid component to anesthetic action. There is evidence that this opioid action component is due to neuronal release of endogenous opioid peptides. The strongest evidence is provided by studies that monitor changes in the concentration of opioid peptides in the perfused brain following inhalation of the anesthetic. Indirect or circumstantial evidence also comes from studies of anesthetic effects on regional brain levels of opioid peptides, antagonism of selected anesthetic effects by antisera to opioid peptides and anesthetic-induced changes radioligand binding to opioid receptors. It is likely that some inhalation general anesthetics (e.g., nitrous oxide) can induce neuronal release of opioid peptides and that this may contribute to certain components of general anesthesia (e.g., analgesia). More definitive studies utilizing in vivo microdialysis or autoradiography in selected areas of the brain during induction and successive states of general anesthesia have yet to be conducted

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

Nitrous oxide (N 2O)-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 2O 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 2O 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 2O. ►N 2O increases extracellular levels of nitrite and nitrate in the ARC and PAG. ►N 2O increases extracellular levels of β-endorphin in the ARC and PAG. ►These increases are both reversed by pretreatment with a NOS-inhibitor

Exposure to nitrous oxide stimulates a nitric oxide-dependent neuronal release of β-endorphin in ventricular-cisternally-perfused rats

We have previously shown that the antinociceptive effect of nitrous oxide (N 2O) in the rat hot plate test is sensitive to antagonism by antisera against the endogenous opioid peptide β-endorphin. Moreover, N 2O-induced antinociception is reduced by inhibition of nitric oxide (NO) production in the brain. To test the hypothesis that N 2O 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-minute fractions of aCSF perfusate were collected from separate groups of room air-exposed rats, N 2O-exposed rats, and L-NAME-pretreated, N 2O-exposed rats; they were then analyzed for their content of NO metabolites and β-endorphin. Compared to room air control, exposure to 70% N 2O 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 2O-induced increases in nitrite, nitrate and β-endorphin. These findings demonstrate in an in vivo rat model that N 2O may stimulate an NO-dependent neuronal release of β-endorphin