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Narcotics
The narcotic analgesics act primarily on the CNS. The perception of
and emotional response to pain is modified when the narcotic analgesics bind
with stereospecific receptors in the CNS. Five major groups of opioid receptors
are known: mu, kappa, sigma, delta and epsilon. Narcotic analgesic activity
occurs at the mu, kappa and sigma receptors. Opioid agonists such as morphine
and others exert their activity mainly at the mu receptor. Mixed
agonist-antagonists such as butorphanol, nalbuphine and pentazocine act
primarily at the kappa receptors (thought to mediate analgesic effects) and
sigma receptors (may produce subjective and psychotomimetic effects).
As well as analgesia, opioid agonist activity in the CNS causes
suppression of the cough reflex, change in mood such as euphoria or dysphoria,
mental clouding and EEG changes. Nausea and vomiting, probably caused by
stimulation of the chemoreceptor trigger zone, can also occur. Peripheral
vasodilation, reduced peripheral resistance and the inhibition of baroreceptors
can result in orthostatic hypotension and fainting. The inhibition of
peristalsis can lead to constipation while increased bladder sphincter tone may
cause urinary retention.
Large doses may elicit excitation or seizures. Morphine and its
congeners cause miosis. In therapeutic doses they increase accommodation and
sensitivity to light reflex and decrease intraocular pressure in both normal
patients and those with glaucoma.
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