Opioids may have been
the first drugs to be abused (preceding stimulants), and are still among the
most commonly used for nonmedical purposes.
As
described, opioids comprise a large family of endogenous and exogenous agonists
at three G protein-coupled receptors: the μ-, κ-, and δ-opioid receptors. Although all
three receptors couple to inhibitory G proteins (ie, they all inhibit ade-nylyl
cyclase), they have distinct, sometimes even opposing effects, mainly because
of the cell type-specific expression throughout the brain. In the VTA, for
example, μ-opioid
receptors are selectively expressed on GABA neurons (which they inhibit),
whereas κ-opioid
receptors are expressed on and inhibit dopamine neu-rons. This may explain why μ-opioid
agonists cause euphoria, whereas κ agonists induce dysphoria.
In
line with the latter observations, the rewarding effects of morphine are absent
in knockout mice lacking μ receptors but persist when either
of the other opioid receptors are ablated. In the VTA, μ opioids cause an inhibition of
GABAergic inhibitory interneurons, which leads eventually to a disinhibition of
dop-amine neurons.
The
most commonly abused μ opioids include morphine,heroin (diacetylmorphine,
which is rapidly metabolized to mor-phine), codeine, and oxycodone.
Meperidine abuse is common among health professionals. All of these drugs
induce strong toler-ance and dependence. The withdrawal syndrome may be very
severe (except for codeine) and includes intense dysphoria, nausea or vomiting,
muscle aches, lacrimation, rhinorrhea, mydriasis, piloerection, sweating,
diarrhea, yawning, and fever. Beyond the withdrawal syndrome, which usually
lasts no longer than a few days, individuals who have received opioids as
analgesics only rarely develop addiction. In contrast, when taken for
recreational pur-poses, opioids are highly addictive. The relative risk of
addiction is 4 out of 5 on a scale of 1 = nonaddictive, 5 = highly
addictive.
The opioid antagonist naloxone reverses the effects of a dose
of morphine or heroin within minutes. This may be life-saving in the case of a
massive overdose. Naloxone administration also provokes an acute withdrawal
(precipitated abstinence) syndrome in a dependent person who has recently taken
an opioid.
In the treatment of
opioid addiction, a long-acting opioid (eg, methadone, buprenorphine) is often substituted for the
shorter-acting, more rewarding, opioid (eg, heroin). For substitution therapy,
methadone is given orally once daily, facilitating super-vised intake. Using a
partial agonist (buprenorphine) and the much longer half-life (methadone and
buprenorphine) may alsohave some beneficial effects (eg, weaker drug
sensitization, which typically requires intermittent exposures), but it is
important to realize that abrupt termination of methadone administration
invariably precipitates a withdrawal syndrome; that is, the subject on
substitution therapy remains dependent. Some countries (eg, Switzerland,
Netherlands) even allow substitution of heroin by heroin. A follow-up of a
cohort of addicts who receive heroin injections in a controlled setting and
have access to counseling indicates that addicts under heroin substitution have
an improved health status and are better integrated in society.
Related Topics
Privacy Policy, Terms and Conditions, DMCA Policy and Compliant
Copyright © 2018-2024 BrainKart.com; All Rights Reserved. Developed by Therithal info, Chennai.