Orexin A and B are hypothalamic peptides known to modulate arousal, feeding and reward via OX1 and OX2 receptors. Orexins are also antinociceptive in the brain but their mechanism(s) of action remain unclear. Here, we investigated the antinociceptive mechanism of orexin A in the rat ventrolateral periaqueductal gray (vlPAG), a midbrain region crucial for initiating descending pain inhibition. In vlPAG slices, orexin A (30-300 nM) depressed GABAergic evoked inhibitory postsynaptic currents (IPSCs). This effect was blocked by an OX1 (SB 334867), but not OX2 (Compound 29), antagonist. Orexin A increased the paired-pulse ratio of paired IPSCs, and decreased the frequency, but not amplitude, of miniature IPSCs. Orexin A-induced IPSC depression was mimicked by WIN 55,212-2, a cannabinoid 1 (CB1) receptor agonist. AM 251, a CB1 antagonist, reversed depressant effects by both agonists. Orexin A-induced IPSC depression was prevented by U73122 and tetrahydrolipstatin, inhibitors of phospholipase C (PLC) and diacylglycerol lipase (DAGL), respectively, and enhanced by URB602, which inhibits enzymatic degradation of 2-arachidonoylglycerol (2-AG). Moderate DAGLα, but not DAGLβ, immunoreactivity was observed in the vlPAG. Orexin A produced an overall excitatory effect on evoked postsynaptic potentials and hence increased vlPAG neuronal activity. Intra-vlPAG microinjection of orexin A reduced hot-plate nociceptive responses in rats in a manner blocked by SB 334867 and AM 251. Therefore, orexin A may produce antinociception by activating postsynaptic OX1 receptors, stimulating synthesis of 2-AG, an endocannabinoid, through a Gq-protein-mediated PLC-DAGLα enzymatic cascade culminating in retrograde inhibition of GABA release (disinhibition) in the vlPAG.
The nociceptin/orphanin FQ (N/OFQ) peptide (NOP) receptor is a non-opioid branch of the opioid receptor family implicated in several neurological and psychological disorders, such as pain, anxiety, depression, involuntary movement, addiction, seizure and dementia. Heterogeneity of NOP receptors has been proposed based on the findings of splicing variants and from binding and functional studies. We have previously reported that Ro 64-6198, a NOP receptor agonist, activated a subset, but not all, of N/OFQ-sensitive NOP receptors in midbrain ventrolateral periaqueductal grey (vlPAG). In this study, we found that a new NOP receptor ligand, (+)-5a Compound ((3aS, 6aR)-1-(cis-4-isopropylcyclohexyl)-5'-methyl-2'-phenylhexahydrospiro[piperidine-4,1'-pyrrolo[3, 4-c]pyrrole]), also activated a subset of NOP receptors in vlPAG neurons. (+)-5a Compound (0.1-30 μm) concentration-dependently activated G-protein-coupled inwardly-rectifying potassium (GIRK) channels mediated through the NOP receptors in about 35% of the recorded vlPAG neurons. (+)-5a Compound (EC50: 605 nm) was less potent (1/12) and efficacious (47%) than N/OFQ. In (+)-5a Compound-insensitive neurons, Ro 64-6198 was also ineffective, and vice versa, but N/OFQ activated GIRK channels through NOP receptors. In (+)-5a Compound-sensitive neurons, (+)-5a Compound precluded the effect of Ro 64-6198. Immunofluorecent and morphometric studies showed that most of the (+)-5a Compound-sensitive neurons were multipolar with intensive dendritic arborization and immunoreactive to glutamic acid decarboxylase-67. It is suggested that (+)-5a Compound activates a subset of NOP receptors, similar to the Ro 64-6198-sensitive subset, in the vlPAG neurons which are mostly GABAergic. These results further support the presence of functional heterogeneity of NOP receptors in the midbrain PAG.
Opioids are effective analgesics for pain relief, however, inappropriate use may cause risks. The aims of the study were to evaluate trends of opioid consumption for pain management in Taiwan and compare them among neighboring Asian countries. Opioid consumption data, including fentanyl, morphine, oxycodone, hydromorphone, codeine, and pethidine, were collected from the Controlled Drugs Management Information System of Taiwan Food and Drug Administration from 2008 to 2018. Data of different continents and neighboring Asian countries were retrieved from the WHO website. The major findings include: (1) In Taiwan, the total annual opioid consumption has gradually increased from 2008 to 2018, with fentanyl being the most frequently consumed opioid analgesic, followed by morphine. Codeine and pethidine consumption dropped significantly over the years. (2) In neighboring Asian countries, the opioid consumption in order from highest to lowest consumption were South Korea, Japan, Taiwan, Singapore, Hong Kong (China), and China. We concluded that, from 2008 to 2018, the total opioid consumption trend for pain management in Taiwan has slowly increased, with fentanyl and morphine being the most commonly used opioids. When compared with neighboring Asian countries, level of opioid consumption in Taiwan was between Japan and Singapore. The research results may provide a reference for healthcare professionals worldwide.
Background: Opioids are effective for severe pain; however, the safety issue is also a primary concern. To better understand the opioid use in Taiwan, we conducted this study. Methods: Data on patients with opioid prescriptions, including morphine, fentanyl, pethidine, codeine, oxycodone, hydromorphone, and buprenorphine were collected using the Taiwan National Health Insurance Database (NHID). Results: Our analysis of opioid prescriptions from 2008 to 2018 in Taiwan indicated that (1) A slow increase in prevalence of opioid prescription was found during the study period. Among the drugs studied, morphine accounted for the majority of the prescriptions written, with a gradual increase annually. Pethidine prescriptions showed a significant and rapid decline over the years; (2) medical centers prescribed the largest number of opioids, followed by regional hospitals, local hospitals, and clinics; (3) the number of prescriptions per year per capita in cancer group was much higher than that in noncancer group. In noncancer group, most of the prescriptions were used in acute pain service (98.7%); and (4) use of opioids increased with age in both cancer and noncancer patients. Conclusion:The total number of opioid prescriptions in Taiwan gradually increased annually, among which morphine was the most commonly used opioid. Cancer patients consumed more opioid prescriptions than noncancer patients and most of the prescriptions in noncancer patients were used in acute pain service. The number of prescriptions increased with the age of the patients in both cancer and noncancer patients. The low prescription rate of opioids in chronic pain in Taiwan is not similar as those in high opioid-consuming countries, such as United States.
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