There are several effects of opioids on the central nervous system. One of the most common effects of opioids is the occurrence of nausea and vomiting because of the direct stimulation of the chemoreceptor trigger zone (CTRZ) on the floor of the fourth ventricle. Different classes of anti-emetics have been used to treat nausea and vomiting associated with opioid use. These include anticholinergics such as scopolamine, serotonin antagonists such as ondansetron, and the antidopaminergics such as droperidol and metoclopramide. Opioids can relieve the sensation of pain without affecting other sensations, such as temperature and pressure. They can, however, change the affective response and can produce dysphoria (activation of k) and euphoria (activation of |x).
Confusion, delirium, and seizures can be seen with high doses of morphine in animal models. Seizures have been seen with meperidine in elderly patients and in patients with renal failure (in the latter group, the seizures are related to meperidine's metabolite, normeperi-dine). A common finding with opioid overuse is the presence of miosis by the stimulation of the parasympathetic Edinger-Westphal nucleus of the occulomotor nerve (Koyyalagunta 2006). Miosis can be seen with opioids having mu and kappa agonist effects. Muscle rigidity is another unwanted side effect seen with opioid administration. The mechanism is unknown, but studies hypothesize that it is located in the striatum, which has numerous opioid receptors (Monk 1998).
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