Brain Stem Ppioidergic and GABAergic Neurons Mediate Nitrous Oxide Induced Antinociceptive Effect in Fischer Rats

Background The study hypothesizes that nitrous oxide (N(2)O) releases opioid peptide in the brain stem, which results in inhibition of gamma-aminobutyric acid-mediated (GABAergic) neurons that tonically inhibit the descending noradrenergic inhibitory neurons (DNIN), resulting in activation of DNIN. In the spinal cord, activation of DNIN leads to the release of norepinephrine, which inhibits nociceptive processing through direct activation of alpha2 adrenoceptor and indirect activation of GABAergic neurons through alpha1 adrenoceptor. Arising from this hypothesis, it follows that GABAergic neurons will modulate the antinociceptive effect of N(2)O in diametrically opposite directions at supraspinal and spinal levels. The authors have tested this tenet and further examined the effect of midazolam, a GABA-mimetic agent, on N(2)O-induced antinociceptive effect. Methods Adult male Fischer rats were administered muscimol (GABA(A) receptor agonist) intracerebroventricularly (icv), gabazine (GABA(A) receptor antagonist) intrathecally (intrathecal), or midazolam intraperitoneally (intraperitoneal). Fifteen minutes later, they were exposed to air or 75% N(2)O and were subjected to the plantar test after 30 min of gas exposure. In some animals administered with midazolam, gas exposure was continued for 90 min, and the brain and spinal cord were examined immunohistochemically. Results The N(2)O-induced antinociceptive effect, which was attenuated by icv muscimol, intrathecal gabazine, and intraperitoneal midazolam. Midazolam inhibited N(2)O-induced c-Fos expression (a marker of neuronal activation) in the pontine A7 and spinal cord. Conclusions The GABAergic neurons modulate the antinociceptive effect of N(2)O in opposite directions at supraspinal and spinal levels. The pronociceptive effects of enhancement at the supraspinal GABAergic site predominate in response to systemically administered midazolam.

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Nitrous Oxide Activates GABAergic Neurons in the Spinal Cord in Fischer Rats

Anesthesiology ◽

10.1097/00000542-200108000-00031 ◽

2001 ◽

Vol 95 (2) ◽

pp. 463-469 ◽

Cited By ~ 21

Author(s):

Toshikazu Hashimoto ◽

Mervyn Maze ◽

Yoko Ohashi ◽

Masahiko Fujinaga

Keyword(s):

Spinal Cord ◽

Nitrous Oxide ◽

Glutamic Acid ◽

Glutamic Acid Decarboxylase ◽

Antinociceptive Effect ◽

Gabaergic Neurons ◽

Acid Decarboxylase ◽

Noradrenergic Neurons ◽

Neuronal Activation ◽

Fischer Rats

Background Findings to date indicate that nitrous oxide exerts its antinociceptive effect by activating descending noradrenergic neurons. The mechanism whereby descending inhibitory neurons, including noradrenergic neurons, produce antinociceptive effect remains unclear. Using c-Fos protein as a marker for neuronal activation, we examined whether spinal cord neurons activated by nitrous oxide are y-aminobutyric acid-mediated (GABAergic) neurons. Methods Adult male Fischer (a strain in which nitrous oxide shows strong antinociceptive properties) and Lewis (a strain in which nitrous oxide lacks antinociceptive properties) rats were exposed to either air (control) or nitrous oxide. Frozen sections of the spinal cord were either stained for c-Fos or double-stained for c-Fos and glutamic acid decarboxylase (a rate-limiting enzyme for GABA synthesis) and analyzed by standard or confocal microscopy. Results In Fischer rats, 90 min of 75% N2O administration increased the number of c-Fos-positive cells in the spinal cord approximately threefold as compared with the control group. The c-Fos-positive cells induced by nitrous oxide were almost entirely colocalized with glutamic acid decarboxylase-positive cells. In contrast, exposure did not change the number of c-Fos-positive cells in the spinal cord in Lewis rats. Conclusions Exposure to nitrous oxide activates GABAergic neurons in the spinal cord. The dose-dependence of GABAergic neuronal activation in the Fischer rats and its absence in the Lewis rat correlate with antinociceptive responses previously reported in these same circumstances. Together, we interpret these data to indicate that activation of GABAergic neurons in the spinal cord are involved in the antinociceptive action of nitrous oxide.

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Evidence for the Involvement of Spinal Cord α1Adrenoceptors in Nitrous Oxide–induced Antinociceptive Effects in Fischer Rats

Anesthesiology ◽

10.1097/00000542-200212000-00018 ◽

2002 ◽

Vol 97 (6) ◽

pp. 1458-1465 ◽

Cited By ~ 21

Author(s):

Ryo Orii ◽

Yoko Ohashi ◽

Tianzhi Guo ◽

Laura E. Nelson ◽

Toshikazu Hashimoto ◽

...

Keyword(s):

Spinal Cord ◽

Nitrous Oxide ◽

Receptor Antagonist ◽

Serotonin Receptor ◽

Antinociceptive Effect ◽

Gabaergic Neurons ◽

Adrenoceptor Antagonist ◽

Fischer Rats ◽

Fos Expression ◽

Alpha2 Adrenoceptor

Background In a previous study, the authors found that nitrous oxide (N2O) exposure induces c-Fos (an immunohistochemical marker of neuronal activation) in spinal cord gamma-aminobutyric acid-mediated (GABAergic) neurons in Fischer rats. In this study, the authors sought evidence for the involvement of alpha1 adrenoceptors in the antinociceptive effect of N2O and in activation of GABAergic neurons in the spinal cord. Methods Adult male Fischer rats were injected intraperitoneally with alpha1 adrenoceptor antagonist, alpha2 adrenoceptor antagonist, opioid receptor antagonist, or serotonin receptor antagonist and, 15 min later, were exposed to either air (control) or 75% N2O. In some animals, nociception was investigated with the plantar test after 30 min of exposure, while in other animals, gas exposure was continued for 90 min and the spinal cord was examined for c-Fos immunostaining. In a separate experiment, animals were exposed to the above gases alone, after which the spinal cords were examined immunohistochemically for c-Fos and alpha1 adrenoceptor by double-staining methods. Results The antinociceptive effect of N2O was attenuated by prazosin (an alpha1 adrenoceptor antagonist), yohimbine (an alpha2 adrenoceptor antagonist), and naloxone (an opioid receptor antagonist) but not by methysergide and tropisetron (serotonin receptor antagonists). N2O exposure induced c-Fos expression in the spinal cord, which was blocked by prazosin and naloxone but not by other drugs. N2O-induced c-Fos expression was colocalized with alpha1 adrenoceptor immunoreactivity in laminae III-IV. Conclusions These findings support the hypothesis that N2O activates GABAergic interneurons through alpha1 adrenoceptors to produce its antinociceptive effect.

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Brain Stem Opioidergic and GABAergic Neurons Mediate the Antinociceptive Effect of Nitrous Oxide in Fischer Rats

Anesthesiology ◽

10.1097/00000542-200310000-00030 ◽

2003 ◽

Vol 99 (4) ◽

pp. 947-954 ◽

Cited By ~ 31

Author(s):

Yoko Ohashi ◽

Tianzhi Guo ◽

Ryo Orii ◽

Mervyn Maze ◽

Masahiko Fujinaga

Keyword(s):

Spinal Cord ◽

Brain Stem ◽

Antinociceptive Effect ◽

Opioid Peptide ◽

Periaqueductal Gray ◽

Gabaergic Neurons ◽

Aminobutyric Acid ◽

Gamma Aminobutyric Acid ◽

Gray Area ◽

Fos Expression

Background Recent studies have revealed that N2O exerts its antinociceptive effect by inducing opioid peptide release in the brain stem, thereby activating the descending noradrenergic inhibitory neurons, which modulate pain processing in the spinal cord. However, the precise neuronal pathways that mediate these events remain to be determined. Methods Using immunohistochemical and behavioral techniques in adult male Fischer rats, the authors studied the involvement of brain stem opioidergic and gamma-aminobutyric acid-mediated (GABAergic) neurons in the N2O-induced antinociceptive effect using discrete microinjections of an opioid receptor antagonist or GABAergic activator into the periaqueductal gray area and pontine noradrenergic nuclei. They used c-Fos expression as an immunohistochemical mark of neuronal activation induced by N2O and the plantar test as the behavioral paradigm for nociception. Results Microinjection of either naloxone (an opioid receptor antagonist) or muscimol (a gamma-aminobutyric acid receptor type A agonist) into the ventrolateral periaqueductal gray area inhibited N2O-induced c-Fos expression in the spinal cord and pontine noradrenergic nuclei, particularly in the A7. Microinjection of either naloxone or muscimol into the A7 nuclei also inhibited N2O-induced c-Fos expression in the spinal cord and the N2O-induced antinociceptive effect by the plantar test. Conclusions These results support the hypothesis that both opioidergic and GABAergic neurons mediate the antinociceptive effect of N2O at the periaqueductal gray area and A7 in the brain stem. The authors postulate that N2O-induced opioid peptide release leads to inhibition of GABAergic neurons via opioid receptors. The descending noradrenergic inhibitory pathways, which are tonically inhibited by these gamma-aminobutyric acid neurons, are thereby activated (disinhibited) and modulate pain processing in the spinal cord.

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