Current pharmaceutical design
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Opiates lack potent analgesic efficacy in neuropathic pain although it is now generally accepted that the poor effect of these drugs reflects a reduction in their potency. Reduction of morphine antinociceptive potency was postulated to be due to the fact that nerve injury altered the activity of opioid systems or opioid specific signaling. ⋯ Opioid peptides biosynthesis and opioid receptors density in the nociceptive pathways and their functions change under various conditions associated with neuropathic pain following damage to the spinal cord and injury of peripheral nerves. Identification of a role of opioid systems in neuropathic pain and molecular and cellular mechanisms underlying these processes are of importance to understanding of the opioid action in neuropathic pain that will hopefully facilitate development of therapeutic strategies in which effectiveness of opioids in alleviation neuropathic pain is increased.
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NMDA (N-methyl-D-aspartate) receptors are one class of ionotropic receptor for the ubiquitous excitatory neurotransmitter L-glutamate. The receptor is made up of four protein subunits combined from a larger library of proteins, which gives this receptor a great deal of variability. This explains the large number of modulatory sites, a variety of sites at which antagonists can interact, and therefore a number of potential drug targets. ⋯ This review summarises the preclinical rationale, based on animal models, and the clinical evidence on the use of NMDA antagonists in pain states. It also summarises the details of the receptor so as to explain the rationale for targeting either specific sites on the receptor, or exploiting anatomical differences in subtype expression, so as to provide the beneficial effects of NMDA receptor block with an improved side effect profile. In particular, agents that are selective for receptors that include the NR2B subunit preclinically have a substantially better profile for treating neuropathic pain than do current NMDA antagonists; some emerging clinical evidence supports this view.
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Subtypes of tetrodotoxin resistant voltage-gated sodium channels are involved in the development of certain types of neuropathic pains. After nerve injury hyperexcitability and spontaneous firing develop at the site of injury and also in the dorsal root ganglion cell bodies. This hyperexcitability results at least partly from accumulation of sodium channels at the site of injury. ⋯ Its use is limited by the fact that it cannot be administered orally. An oral local anesthetic type sodium channel blocker, mexiletine is an antiarrhythmic agent that is effective in neuropathic pain. However, effective doses may be difficult to achieve because of adverse effects.
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Postoperative nausea and vomiting (PONV) are distressing and frequent adverse events of anesthesia and surgery, with a relatively high incidence after laparoscopic cholecystectomy. Numerous antiemetics have been studied for the prevention and treatment of PONV in patients scheduled for laparoscopic cholecystectomy. Traditional antiemetics, including anticholinergics (e.g., scopolamine), antihistamines (e.g., dimenhydrinate), phenothiazines (e.g., promethazine), butyrophenones (e.g., droperidol), and benzamide (e.g., metoclopramide), are used for the control of PONV. ⋯ The efficacy of a combination of serotonin receptor antagonists (ondansetron and granisetron) and droperidol is superior to monotherapy with a serotonin receptor antagonist or droperidol. Similarly, adding dexamethasone to ondansetron or granisetron improves antiemetic efficacy in PONV. Knowledge regarding antiemetics is necessary to completely prevent and treatment of PONV in patients scheduled for laparoscopic cholecystectomy.
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Neuropathic pain can be difficult to treat clinically, as current therapies involve partial effectiveness and significant adverse effects. Following the development of preclinical models for neuropathic pain, significant advances have been made in understanding the neurobiology of neuropathic pain. ⋯ Preclinical studies provide evidence that peripheral applications of opioids, alpha-adrenergic agents, and antidepressants also may be beneficial in neuropathic pain, and some clinical reports provide support for topical applications of such agents. An appreciation of the ability of drug application, to sites remote from the site of injury, to alleviate aspects of neuropathic pain will provide a significant impetus for the further development of novel topical analgesics for this condition.