Activation of nociceptive neurons in T9 and T10 in cerulein pancreatitis

Mechanisms of pain transduction in acute pancreatitis are poorly understood. Increased Fos expression in the spinal cord is a marker of activation of nociceptive neurons. We hypothesized that cerulein pancreatitis leads to increased Fos expression at T9 and T10, which receive sensory input from the pancreas. Rats were injected with cerulein (100 microg/kg, s.c.) or saline carrier (NS). Endpoints at 4, 6, and 10 h were serum amylase, myeloperoxidase activity (MPO), and spinal cord Fos expression (number of immunoreactive nuclei/section dorsal gray matter). Fos-like immunoreactivity (FLI) at T9-T10 was compared to internal controls (T6, T12). An average of 20 spinal cord histologic sections were evaluated per rat. Some animals were injected with the mu-opioid receptor agonist, buprenorphine (90 microg/kg, s.c.), 3 h after cerulein, and their endpoints were measured at 6 h. Analysis of variance and t tests were used for statistical analysis. Results are means +/- SEM. As expected, cerulein induced edematous pancreatitis, with a 4-fold increase in serum amylase at 6 h [cer (n = 8): 14,000 +/- 1,300 U/ml versus NS (n = 10): 3,700 +/- 300, P < 0.005)] and a 2-fold increase in MPO activity (0.25 +/- 0.05) activity units/dry wt versus 0.13 +/- 0.02, P < 0.05). Cerulein induced nearly a 2-fold increase in FLI at T9 and T10 [n = 10 (cer) and n = 13 (NS): T9, 14 +/- 1.5 versus 7.8 +/- 0.88; T10, 15 +/- 1.7 versus 8.3 +/- 0.70; P < 0.05]. Peak effects of cerulein on FLI occurred at 6 h and were greatest at T9/T10 with relative sparing of T6/T12. T6/T12 expression was similar in experimental and control groups. Buprenorphine significantly reduced both serum amylase and FLI and T9/T10. Cerulein-induced acute pancreatitis in rat increases visceral nociceptive signaling at spinal cord levels T9 and T10, with a peak at 6 h. Blockade of this effect by the mu-opioid receptor agonist buprenorphine could occur either by direct activation of central opioid receptors and/or an anti-inflammatory mechanism. FLI is a useful tool for studying the pathophysiology of pain in experimental acute pancreatitis.     

Involvement of Pro‐Nociceptive 5‐HT2A Receptor in the Pathogenesis of Medication‐Overuse Headache

(Headache 2010;50:185‐197) Objectives.— To determine the involvement of 5‐HT_2A (5‐HT_2A) receptor in the process of trigeminal plasticity induced by chronic analgesic exposure and in the process of inflammatory‐induced thermal hyperalgesia. Background.— Derangement in 5‐HT_2A serotonin receptor has been reported to implicate in pathogenesis of medication‐overuse headache. No clear explanation concerning the precise roles of these receptors in the process. Methods.— Wistar rats were daily administered with paracetamol (200 mg/kg) for 30 days. On the next day, ketanserin, a 5‐HT_2A antagonist, or saline was given prior to cortical spreading depression (CSD) induction. Electrocorticogram, cortical blood flow, Fos and 5‐HT_2A‐immunoreactivity in cortex and trigeminal pathway were studied. In the other experiment, complete Freund's adjuvant was injected into the rat hind paw to induce tissue inflammation. Three days later, ketanserin was given and noxious heat was applied to both inflamed and noninflamed paws. The response between 2 sides was compared by measuring paw withdrawal latency. Results.— Chronic paracetamol exposure led to an increase in CSD frequency and CSD‐evoked Fos expression in cerebral cortex indicating the increase in neuronal excitability. Prolonged medication exposure also facilitated trigeminal nociception as evident by an increase in CSD‐evoked Fos expression in trigeminal nucleus caudalis. The expression of 5‐HT_2A receptor in cerebral cortex and trigeminal ganglia was enhanced by chronic paracetamol administration. Pretreatment with ketanserin significantly attenuated these effects. The second experiment showed that ketanserin was able to lengthen the paw withdrawal latency in the inflamed side but did not alter nociceptive response in the noninflamed side. Conclusion.— These findings suggest that up‐regulation of pro‐nociceptive 5‐HT_2A receptor is an important step in the process of cortical hyper‐excitation and nociceptive facilitation induced by chronic analgesic exposure.     

CB2 cannabinoid receptor activation produces antinociception by stimulating peripheral release of endogenous opioids

CB(2) cannabinoid receptor-selective agonists are promising candidates for the treatment of pain. CB(2) receptor activation inhibits acute, inflammatory, and neuropathic pain responses but does not cause central nervous system (CNS) effects, consistent with the lack of CB(2) receptors in the normal CNS. To date, there has been virtually no information regarding the mechanism of CB(2) receptor-mediated inhibition of pain responses. Here, we test the hypothesis that CB(2) receptor activation stimulates release from keratinocytes of the endogenous opioid beta-endorphin, which then acts at opioid receptors on primary afferent neurons to inhibit nociception. The antinociceptive effects of the CB(2) receptor-selective agonist AM1241 were prevented in rats when naloxone or antiserum to beta-endorphin was injected in the hindpaw where the noxious thermal stimulus was applied, suggesting that beta-endorphin is necessary for CB(2) receptor-mediated antinociception. Further, AM1241 did not inhibit nociception in mu-opioid receptor-deficient mice. Hindpaw injection of beta-endorphin was sufficient to produce antinociception. AM1241 stimulated beta-endorphin release from rat skin tissue and from cultured human keratinocytes. This stimulation was prevented by AM630, a CB(2) cannabinoid receptor-selective antagonist and was not observed in skin from CB(2) cannabinoid receptor-deficient mice. These data suggest that CB(2) receptor activation stimulates release from keratinocytes of beta-endorphin, which acts at local neuronal mu-opioid receptors to inhibit nociception. Supporting this possibility, CB(2) immunolabeling was detected on beta-endorphin-containing keratinocytes in stratum granulosum throughout the epidermis of the hindpaw. This mechanism allows for the local release of beta-endorphin, where CB(2) receptors are present, leading to anatomical specificity of opioid effects.     

Understanding and controlling the enteric nervous system

The enteric nervous system or the 'Little Brain' of the gut controls gastrointestinal motility and secretion, and is involved in visceral sensation. In this chapter, new developments in understanding the function of the enteric nervous system are described. In particular, the interaction of this system with the interstitial cells of Cajal, the pacemaker cells of the gut, is highlighted. The importance of the interaction between the enteric nervous system and the immune system is discussed, especially in relation to functional bowel disorders and post-operative ileus. Evidence is also provided that neurones can change their function and phenotype, a phenomenon called neuronal plasticity, which contributes to the pathogenesis of visceral hypersensitivity. Finally, new developments in stem cell transplantation are described. All these new insights should lead to a better understanding of the enteric nervous system and hopefully to better ways of controlling it.     

Morphine peripheral analgesia depends on activation of the PI3Kγ/AKT/nNOS/NO/KATP signaling pathway

Morphine is one of the most prescribed and effective drugs used for the treatment of acute and chronic pain conditions. In addition to its central effects, morphine can also produce peripheral analgesia. However, the mechanisms underlying this peripheral action of morphine have not yet been fully elucidated. Here, we show that the peripheral antinociceptive effect of morphine is lost in neuronal nitric-oxide synthase null mice and that morphine induces the production of nitric oxide in primary nociceptive neurons. The activation of the nitric-oxide pathway by morphine was dependent on an initial stimulation of PI3Kγ/AKT protein kinase B (AKT) and culminated in increased activation of K_ATP channels. In the latter, this intracellular signaling pathway might cause a hyperpolarization of nociceptive neurons, and it is fundamental for the direct blockade of inflammatory pain by morphine. This understanding offers new targets for analgesic drug development.     

Differential Sensitivity to Morphine in Spontaneously Hypertensive and Normotensive Wistar-Kyoto and Wistar Rats

In spontaneously hypertensive rats (SHR) a diminished responsiveness for nociceptive stimuli is present, which might be associated with alterations in endogenous opioid peptide and receptor systems. In SHR, normotensive Wistar-Kyoto controls (WKY) and in Wistar rats the effects of acute morphine administration on pain sensitivity and body temperature have been investigated. Morphine potency decreases significantly in the rank order Wistar, SHR, WKY. Concerning body temperature, a low dose (2.5 mg/kg i.p) results in an initial and transient decrease in body temperature in SHR, which is not elicited in Wistar rats and WKY. After higher doses (10 and 20 mg/kg. i.p) in Wistar rats profound hypothermia occurs whilst SHR and WKY show a modest increase in body temperature. These differential effects in the three strains could not be explained by altered morphine concentrations in subcortical brain tissue.     

Temperature-dependent neuronal regulation of arterial blood flow in rat cranial dura mater

Several endogenous factors such as calcitonin gene-related peptide (CGRP), nitric oxide, and histamine have been found to affect meningeal blood flow. A possible regulation of meningeal blood flow by thermosensitive neurons has not been examined so far. We monitored meningeal arterial blood flow with laser Doppler flowmetry in anesthetized rats. A vortex thermode was used to control the temperature of the exposed dura mater. After fast heating from 36 degrees C to 45 degrees C, meningeal blood flow increased slowly within 5 min. Immediately after fast cooling to 36 degrees C, the flow transiently increased before it returned to the initial basal level. After the application of lidocaine onto the dura mater, the heating-induced flow increase was greater and the transient cooling-induced flow increase was reduced, indicating the involvement of neuronal mechanisms. Furthermore, after topical application of phenylephrine, the cooling-induced response was increased, and following topical application of phentolamine the heating-induced flow increase was higher. The flow changes evoked by thermal stimulation were unaffected by high concentrations of 1) cimetidine, 2) a vasoactive intestinal polypeptide antagonist and the CGRP receptor antagonists, 3) CGRP(8-37), and 4) BIBN4096BS. We conclude that activation of temperature-sensitive meningeal afferents can regulate the dura mater blood flow by a sympathetic reflex.     

Lignstrazine inhibits high voltage-gated Ca^2＋ and TTX-resistant Na^＋ channels of primary sensory neuron and thermal nociception in the rat： a study on peripheral mechanism

Objective Ligustrazine, also named as tetramethylpyrazine, is a compound purified from Ligusticum chuanxiong hort and has ever been testified to be a calcium antagonist. The present investigation was to determine the antinociceptive effect of ligustrazine and, if any, the peripheral ionic mechanism involved. Methods Paw withdrawal Latency （PWL） to noxious heating was measured in vivo and whole-cell patch recording was performed on small dorsal root ganglion （DRG） neurons. Results Intraplantar injection of ligustrazine （0.5 mg in 25 μl） significantly prolonged the withdrawal latency of ipsilateral hindpaw to noxious heating in the rat. Ligustrazine not only reversibly inhibited high-voltage gated calcium current of dorsal root ganglion （DRG） neuron in dose-dependent manner with IC50 of 1.89 mmol/L, but also decreased tetrodotoxin （TTX） -resistant sodium current in relatively selective and dose-dependent manner with IC50 of 2.49 mmol/L. Conclusion The results suggested that ligustrazine could elevate the threshold of thermal nociception through inhibiting the high-voltage gated calcium current and TTX-resistant sodium current of DRG neuron .in the rat.     

核苷酸和P2Y2受体与伤害性感受的研究进展

P2受体为以ATP、UTP及其类似物激活的受体，分为促离子型P2X受体和促代谢型P2Y受体两大类，其各自又分为许多亚型。许多证据显示，P2Y，特别是P2Y2受体，在痛觉信息调节和痛觉过敏中起着重要作用。ATP和UTP作为P2Y2受体的天然激动剂，是伤害性感受和痛觉过敏信息传递中的重要介质。P2Y2受体参与疼痛的调节可能与辣椒素受体有关。膜片钳分析小鼠背根神经节神经元证明了是P2Y2而非P2Y1受体在ATP诱发的TRPV1介导的热痛过敏反应中起作用。原位杂交组织化学研究结果也显示大鼠腰段脊髓背根神经节中TRPV1mRNA与P2Y2mRNA共同表达。这些证据证明了促代谢型P2Y，受体介导背根神经节神经元中核苷酸对VR1的增敏效应及参与伤害性感受的调节。