Thomas Rowley

The role of adrenergic and cholinergic transmission in volatile anesthetic-induced pain enhancement

Volatile anesthetic drugs have a biphasic effect on pain transmission. At very small concentrations they enhance pain sensitivity whereas at larger subanesthetic concentrations they have an analgesic effect. Previous work has suggested that nicotinic inhibition could mediate the pronociceptive action of isoflurane. Furthermore, activation of nicotinic receptors facilitates the release of norepinephrine in the spinal cord. We hypothesize that nicotinic modulation of norepinephrine release in the spinal cord mediates isofluranes pronociceptive action. We used hindpaw withdrawal latency as a measure of pain sensitivity after inhibition of adrenergic activity or treatment with nicotine in mice. Isofluranes effect on pain is separable by concentration. The 50% effective concentration for pain enhancement is 0.16% isoflurane whereas the 50% effective concentration for the antinociceptive action of isoflurane is 0.8%. Depletion of systemic norepinephrine with the neurotoxin DSP-4 caused a reduction in baseline withdrawal latencies and prevented isoflurane pronociception. Baseline latency was also reduced by intrathecal yohimbine. After treatment with yohimbine, isoflurane had no additional pronociceptive effect. Nicotine administered through intracerebroventricular injection increased baseline latency but did not prevent isoflurane pronociception. Conversely, intrathecal applications of nicotine caused a slight reduction in baseline latency and prevented isofluranes pronociceptive effect. We conclude that spinal noradrenergic transmission seems to be necessary for isoflurane pronociception to occur. Isoflurane may act by inhibiting tonically active nicotinic receptors that modulate the release of norepinephrine in the spinal cord.

The antinociceptive response to nicotinic agonists in a mouse model of postoperative pain.

BACKGROUND:Nicotine, the prototypical broad spectrum agonist at central nicotinic receptors, has analgesic action after surgery. Various subtype-specific nicotinic agonists have antinociceptive effects in animal models, but the response is highly dependent on the model tested. In an effort to determine what nicotinic subtypes might be targeted in future clinical studies, we tested agonists selective for α4β2 and α7 containing nicotinic receptors in a mouse model of postoperative pain. METHODS:After paw incision, mice were tested for heat latency and pressure threshold before and after treatment with a dose range of ligands selective for α4β2 and α7 containing nicotinic receptors. To demonstrate that nicotine reduced nociceptive input in this model, the lumbar spinal cords of a subgroup of these mice were stained for the phosphorylated form if CREB. RESULTS:Nicotine and metanicotine (α4β2 selective) were fully effective as an analgesic in heat and pressure testing. The α7 partial agonist GTS-21 significantly increased the heat latency after surgery, but did not alter pressure threshold. The α7 selective antagonist methyllicaconitine decreased the efficacy of nicotine to increase heat latency but did not affect pressure threshold. The number of cells in the superficial dorsal horn with nuclei that stained for pCREB was double on the surgical side and the ratio was reduced by nicotine in a dose-dependent manner. CONCLUSIONS:Our findings suggest that nicotine reduced nociceptive input to the superficial and deep dorsal horn. It also provides support for α4β2 and α7 nicotinic-mediated antinociceptive actions.

Isoflurane Prevents Nicotine-Evoked Norepinephrine Release from the Mouse Spinal Cord at Low Clinical Concentrations

BACKGROUND:Volatile anesthetics inhibit nicotinic acetylcholine receptors at subanesthetic concentrations. In both animal and human studies, similar concentrations of volatile anesthetics have been associated with increased sensitivity to pain. Nicotinic analgesia is thought to involve the enhanced release of norepinephrine. These studies are intended as a “proof of concept” that alteration of the nicotinic facilitation of norepinephrine release is a potential mechanism for isoflurane-induced pronociception. METHODS:We conducted our study using a murine lumbar spinal cord slice model. We evoked norepinephrine release with nicotine in the presence and absence of isoflurane. To identify the type of nicotinic receptor involved, we studied the effect of receptor and subtype-specific ligands and genetically engineered mice, which lacked the gene expression for the nicotinic &bgr;2 subunit. The amount of [3H]-norepinephrine released was measured under the different conditions. RESULTS:Nicotine-facilitated norepinephrine release was significantly and maximally inhibited by isoflurane at concentrations that enhance pain sensitivity in vivo (0.38%). Facilitation of norepinephrine release was mimicked by the &agr;7 selective agonist choline and inhibited in the presence of &agr;-bungarotoxin, an &agr;7-nicotinic selective antagonist. Facilitation of norepinephrine release was not different in animals lacking &bgr;2 subunits compared with matched controls. CONCLUSIONS:Nicotinic facilitation of norepinephrine release in the spinal cord is inhibited by isoflurane at low clinically relevant concentrations. Because the net effect of noradrenergic tone in the spinal cord is inhibitory, the removal of this mechanism might be responsible for the enhanced pain sensitivity seen at these concentrations of isoflurane.