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.M orphine is one of the most prescribed and effective drugs used for treatment of postoperatory and acute severe pain. Nevertheless, its use is frequently limited by undesirable side effects including respiratory depression, tolerance, and addiction. The discovery that morphine can also produce peripheral analgesia in the setting of inflammatory pain opened the possibility of developing peripheral restricted opioids devoid of central side effects (1).Morphine peripheral analgesia was discovered by its direct effect on already established inflammatory hypernociception induced by prostaglandin E 2 (PGE 2 ) injected in rat hind paws (1). Therefore, in contrast to aspirin-like drugs whose analgesic mechanism depends on prevention of nociceptor sensitization by inhibiting synthesis of prostaglandins, opioids are able to directly block ongoing nociceptor sensitization. However, the molecular mechanisms triggered by morphine to promote this action have not been fully elucidated. The present study reports on a series of experiments using behavioral, biochemical, and electrophysiological approaches to address this issue. The following major findings are reported herein: (i) the activation of peripheral opioid receptors in primary nociceptive neurons by morphine triggers a cascade of intracellular signaling events initiated by PI3Kγ/Protein kinase B (AKT); (ii) this is accompanied by activation of neuronal nitric oxide synthase (nNOS) and nitric oxide (NO) production, which (iii) induces an increase in K ATP channel currents; and (iv) it causes a hyperpolarization of nociceptive neurons. Results and DiscussionBased on the evidence that cAMP was the key intracellular second messenger involved in PGE 2 -induced nociceptor sensitization (2) and that opioid-receptor activation in vitro was coupled to adenylyl-cyclase inhibition (3), it was initially suggested that these drugs counteracted inflammatory hypernociception directly through inhibition of PGE 2 -induced adenylyl-cyclase activation (1, 4). Subsequent in vitro studies, which confirmed the ability of opioids to inhibit ad...

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BackgroundDescending pronociceptive pathways may be implicated in states of persistent pain. Paw skin incision is a well-established postoperative pain model that causes behavioral nociceptive responses and enhanced excitability of spinal dorsal horn neurons. The number of spinal c-Fos positive neurons of rats treated intrathecally with serotonin, noradrenaline or acetylcholine antagonists where evaluated to study the descending pathways activated by a surgical paw incision.ResultsThe number of c-Fos positive neurons in laminae I/II ipsilateral, lamina V bilateral to the incised paw, and in lamina X significantly increased after the incision. These changes: remained unchanged in phenoxybenzamine-treated rats; were increased in the contralateral lamina V of atropine-treated rats; were inhibited in the ipsilateral lamina I/II by 5-HT1/2B/2C (methysergide), 5-HT2A (ketanserin) or 5-HT1/2A/2C/5/6/7 (methiothepin) receptors antagonists, in the ipsilateral lamina V by methysergide or methiothepin, in the contralateral lamina V by all the serotonergic antagonists and in the lamina X by LY 278,584, ketanserin or methiothepin.ConclusionsWe conclude: (1) muscarinic cholinergic mechanisms reduce incision-induced response of spinal neurons inputs from the contralateral paw; (2) 5-HT1/2A/2C/3 receptors-mediate mechanisms increase the activity of descending pathways that facilitates the response of spinal neurons to noxious inputs from the contralateral paw; (3) 5-HT1/2A/2C and 5-HT1/2C receptors increases the descending facilitation mechanisms induced by incision in the ipsilateral paw; (4) 5-HT2A/3 receptors contribute to descending pronociceptive pathways conveyed by lamina X spinal neurons; (5) α-adrenergic receptors are unlikely to participate in the incision-induced facilitation of the spinal neurons.

A modification of the Bennett and Xie chronic constriction injury model of peripheral painful neuropathy was developed in rats. Under tribromoethanol anesthesia, a single ligature with 100% cotton glace thread was placed around the right sciatic nerve proximal to its trifurcation. The change in the hind paw reflex threshold after mechanical stimulation observed with this modified model was compared to the change in threshold observed in rats subjected to the Bennett and Xie or the Kim and Chung spinal ligation models. The mechanical threshold was measured with an automated electronic von Frey apparatus 0, 2, 7, and 14 days after surgery, and this threshold was compared to that measured in sham rats. All injury models produced significant hyperalgesia in the operated hind limb. The modified model produced mean ± SD thresholds in g (19.98 ± 3.08, 14.98 ± 1.86, and 13.80 ± 1.00 at 2, 7, and 14 days after surgery, respectively) similar to those obtained with the spinal ligation model (20.03 ± 1.99, 13.46 ± 2.55, and 12.46 ± 2.38 at 2, 7, and 14 days after surgery, respectively), but less variable when compared to the Bennett and Xie model (21.20 ± 8.06, 18.61 ± 7.69, and 18.76 ± 6.46 at 2, 7, and 14 days after surgery, respectively). The modified method required less surgical skill than the spinal nerve ligation model.