Before
describing the various drugs of abuse and the possible mechanisms of action, it
is important to define terms. The
management of drug abuse and addiction must be individualized according to the
drugs involved and the associated psychosocial problems of the individual
patient. An understanding of the pharmacology of the drug or combination of
drugs ingested by the patient is essential to rational and effective treatment.
Addiction: A
primary chronic neurobiological disease, with genetic, psychosocial, and
environmental factors influencing its development and manifestations. It is
characterized by behaviors that include one or more of the following 5Cs:
chronicity, impaired control over drug use, compulsive use, continued use
despite harm, and craving.
Physical
dependence: A state of adaptation that is manifested by a drug class–specific
withdrawal syndrome that can be produced by abrupt cessation, rapid dose
reduction, decreasing blood level of the drug, and/or administration of an
antagonist.
Drug
dependence and the older term drug addiction describe the human condition in which drug
taking becomes compulsive, taking precedence over other needs, often with
serious adverse consequences. This generally implies a state of physical, as
well as psychological, dependence. Drug dependence (physical dependence) can be
viewed as a reversible pharmacological phenomenon, readily induced in animals,
whereas addiction(psychological
dependence)is a chronic, relapsing human condition, as distinct from an acute
illness that can be cured by abstinence. Drug
abuse and substance abuse
are more general terms, meaning any recurrent use of substances that are
illegal or that cause harm to the individual, including drugs in sports. Tolerance-the decrease in
pharmacological effect on repeated administration of the drug-often accompanies
the state of dependence, and it is possible that related mechanisms account for
both phenomena. Withdrawal syndrome
or abstinence syndrome
describes the adverse effects, both physical and psychological and lasting for
a few days or weeks, of stopping taking a drug. Several psychotropic drugs,
including antidepressant and antipsychotic agents, produce withdrawal symptoms
but are not addictive, so it is important to distinguish this type of commonly
observed 'rebound' phenomenon from true dependence.
Neurobiological
basis of drug addiction
Reward pathways:
Virtually all
dependence-producing drugs so far tested, including opioids, nicotine, amphetamines,
ethanol and cocaine; activate the reward
pathway-the mesolimbic dopaminergic pathway, which runs, via the medial
forebrain bundle, from the ventral tegmental area of the midbrain to the
nucleus accumbens and limbic region. Even though their primary sites of action
are generally elsewhere in the brain, all these drugs increase the release of
dopamine in the nucleus accumbens, as shown by microdialysis and other
techniques.
Synaptic Plasticity &
Addiction:
Long-term
potentiation (LTP) is a form of experience-dependent synaptic plasticity that
is induced by activating glutamate receptors of the N- methyl-D-aspartate
(NMDA) type. Since NMDA receptors are blocked by magnesium at negative
potentials, their activation requires the concomitant release of glutamate
(presynaptic activity) onto a receiving neuron that is depolarized
(post-synaptic activity). Correlated pre- and postsynaptic activity durably
enhances synaptic efficacy and triggers the formation of new connections.
Because associativity is a critical component, LTP has become a leading
candidate mechanism underlying learning and memory. LTP can be elicited at
glutamatergic synapses of the mesolimbic reward system and is modulated by
dopamine. Drugs of abuse could therefore interfere with LTP at sites of
convergence of dopamine and glutamate projections (e.g. ventral tegmental area
[VTA], nucleus accumbens, or prefrontal cortex). Interestingly, exposure to an
addictive drug triggers LTP at excitatory afferents and reduces GABAA
receptor-mediated inhibition of the VTA, thus increasing the excitability of
dopamine neurons
Role of DOPAMINE in addiction:
To understand the
long-term changes induced by drugs of abuse, their initial molecular and
cellular targets must be identified. A combination of approaches in animals and
humans, including functional imaging, has revealed the mesolimbic dopamine
system as the prime target of addictive drugs. This system originates in the ventral
tegmental area (VTA), a tiny structure at the tip of the brain stem, which
projects to the nucleus accumbens, the amygdala, the hippocampus, and
the prefrontal cortex. Most projection neurons of the VTA are
dopamine-producing neurons. When the dopamine neurons of the VTA begin to fire
in bursts, large quantities of dopamine are released in the nucleus accumbens
and the prefrontal cortex. Direct application of drugs into the VTA also acts
as a strong reinforcer, and systemic administration of drugs of abuse causes
release of dopamine. As a general
rule, all addictive drugs activate the mesolimbic dopamine system. Since each addictive drug has a specific molecular target
that engages distinct cellular mechanisms to activate the mesolimbic system,
three classes can be distinguished. A first group binds to Gio
protein-coupled receptors, a second group interacts with ionotropic
receptors or ion channels, and a third group targets the dopamine
transporter.
In
the VTA, the action of these drugs is preferentially on the g-aminobutyric acid (GABA) neurons that act as local
inhibitory interneurons. Addictive drugs that bind to ionotropic receptors and
ion channels can have combined effects on dopamine neurons and GABA neurons,
eventually leading to enhanced release of dopamine. Finally, addictive drugs
that interfere with monoamine transporters block reuptake or stimulate
nonvesicular release of dopamine, causing an accumulation of extracellular
dopamine in target structures. Since neurons of the VTA also express
somatodendritic transporters, which normally clear dopamine released by the
dendrites. This is consistent with the observations that antidepressants that
block serotonin and norepinephrine uptake, but not dopamine uptake, do not
cause addiction even after prolonged use.
Drug dependence:
With
chronic exposure to addictive drugs, the brain shows signs of adaptation. For
example if morphine is used for pain at regular intervals, the dose has to be
progressively increased to maintain the analgesic effect.
Many
variables or factors operate simultaneously to influence that a given person
would become drug addict. These variables are classified in to three
categories:
Ø
Drug dependent
Ø
User dependent
Ø
Environment
dependent
Drug
dependent
|
User
dependent
|
Environment
dependent
|
·
Availability
·
Cost
·
Purity/potency
·
Mode of
administration
|
·
Heredity
·
Innate tolerance
·
Psychiatric
symptoms
·
Prior
experience
propensity toward drug
|
·
Social settings
·
Community
attitude
·
Peer influences
·
Employment and
educational opportunities
|
Pharmacological basis of Drug Abuse
Since all addictive drugs
increase dopamine concentrations in target structures of the mesolimbic
projections, we classify them on the basis of their molecular targets and the
underlying mechanisms.
I-Drugs that activate G10 –coupled receptors:
The first group contains the Opioids,
cannabinoids, g-hydroxybutyric acid (GHB),
and the hallucinogens, which all exert their action through Gio
protein-coupled receptors.
II-Inotropic receptors activators:
Second group includes nicotine, alcohol, the benzodiazepines, dissociative
anesthetics, and some inhalants, which interact with ionotropic
receptors or ion channels.
III-Drugs act through biogenic amine transporters:
The last group comprises cocaine, amphetamines,
and ecstasy, which all bind to monoamine transporters. The non-addictive
drugs are classified using the same criteria.
I-Drugs That Activate Gio-Coupled Receptors:
a-Opioids:
Although Opioids may have been the
first drugs to be abused (preceding stimulants), they are still among the most
commonly used for nonmedical purposes.
Mechanism of addiction:
Opioids comprise a large family of
endogenous and exogenous agonists at three G protein-coupled receptors: the μ-,
Ҡ-, and δ-Opioids receptors. Although all three receptors couple to inhibitory
G proteins (i.e. they all inhibit adenylyl 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 Ҡ-Opioids
receptors are expressed on and inhibit dopamine neurons. This may explain why μ-Opioids
agonists cause euphoria, whereas Ҡ-agonists induce dysphoria.
Opioid drugs:
The most commonly abused μ-Opioids
include morphine, heroin (diacetylmorphine, which is rapidly metabolized
to morphine), codeine, and oxycodone. Meperidine abuse is common among health
professionals. All of these drugs induce strong tolerance and dependence.
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 last no longer than a few days,
individuals who have received Opioids as analgesics, only rarely develop
addiction. In contrast, when taken for recreational purposes, Opioids are
highly addictive.
Treatment:
The Opioids 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 Opioids. In the treatment
of Opioids addiction, a long-acting Opioids (eg, methadone, buprenorphine)
is often substituted for the shorter-acting, more rewarding, Opioids (e.g.,
heroin). Using a partial agonist (buprenorphine) and the much longer half-life
(methadone and buprenorphine) may also have some beneficial effects (e.g.
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.
b-Cannabinoids:
Endogenous
cannabinoids that act as neurotransmitters include 2-arachidonyl glycerol
(2-AG) and anandamide, both of which bind to CB1 receptors. Exogenous
cannabinoids, e.g. in marijuana, include several pharmacologically
active substances including 9-tetrahydrocannabinol (THC), a
powerful psychoactive substance.
Mechanism of
addiction:
These very lipid-soluble compounds
are released at the postsynaptic somatodendritic membrane, and diffuse through
the extracellular space to bind at presynaptic CB1 receptors, where they
inhibit the release of either glutamate or GABA. Because of such backward
signaling, endocannabinoids are called retrograde messengers. In the
hippocampus, release of endocannabinoids from pyramidal neurons selectively
affects inhibitory transmission and may contribute to the induction of synaptic
plasticity during learning and memory formation. Like Opioids, THC causes
disinhibition of dopamine neurons, mainly by presynaptic inhibition of GABA
neurons in the VTA.
Pharmacological effects:
The most prominent effects are
euphoria and relaxation. Users also report feelings of well-being, grandiosity,
and altered perception of passage of time. Dose-dependent perceptual changes
(e.g., visual distortions), drowsiness, diminished coordination, and memory
impairment may occur. Cannabinoids can also create a dysphoric state and, in rare
cases following the use of very high doses, may result in visual
hallucinations, depersonalization, and frank psychotic episodes. Additional
effects of THC, e.g. increased appetite, attenuation of nausea, decreased
intraocular pressure, and relief of chronic pain, have led to the use of
cannabinoids in medical therapeutics.
Withdrawal syndrome: that includes restlessness, irritability, mild
agitation, insomnia, nausea, and cramping, Sleep EEG disturbance.
C-Gamma-Hydroxybutyric
Acid:
Gamma-hydroxybutyric
acid (GHB) is produced during the metabolism of GABA, but the function of this
endogenous agent is unknown at present. The pharmacology of GHB is complex
because there are two distinct binding sites. The low-affinity binding site has
been identified as the GABAB receptor.
GHB causes euphoria,
enhanced sensory perceptions, a feeling of social closeness, and amnesia. These
properties have made it a popular "club drug" that goes by colorful
street names such as "liquid ecstasy," "grievous bodily
harm," or "date rape drug." As the latter name suggests, GHB has
been used in date rapes because it is odorless and can be readily dissolved in
beverages.
Although
GABAB receptors are expressed on all neurons of the VTA, GABA
neurons are much more sensitive to GHB than dopamine neurons. Because GHB is a
weak agonist, only GABA neurons are inhibited at the concentrations typically
obtained with recreational use. At higher doses, however, GHB also
hyperpolarizes dopamine neurons, eventually completely inhibiting dopamine
release. Such an inhibition of the VTA may in turn preclude its activation by
other addictive drugs and may explain why GHB might have some usefulness as an
"anticraving" compound.
II-Drugs
that Mediate Their Effects via Ionotropic Receptors:
a-Nicotine:
In terms of numbers affected,
addiction to nicotine exceeds all other forms of addiction, touching more than
50% of all adults in some countries. Nicotine exposure occurs primarily through
smoking of tobacco, which causes associated diseases that are responsible for
many preventable deaths. The chronic use of chewing tobacco and snuff tobacco
is also addictive.
Mechanism of addiction:
Nicotine is a selective agonist of the nicotinic acetylcholine receptor (nAChR)
that is normally activated by acetylcholine. The rewarding effect of nicotine
requires involvement of the VTA in which nAChRs are expressed on dopamine
neurons. When nicotine excites projection neurons, dopamine is released in the
nucleus accumbens and the prefrontal cortex, thus fulfilling the dopamine
requirement of addictive drugs.
Withdrawal symptoms:
Nicotine withdrawal is mild compared
with Opioids withdrawal and involves irritability and sleep problems. However,
nicotine is among the most addictive drugs and relapse after attempted
cessation is very common.
Treatment:
Treatments for nicotine addiction
include nicotine itself in forms that are slowly absorbed and several other
drugs. Nicotine that is chewed, inhaled, or transdermally delivered can be
substituted for the nicotine in cigarettes, thus slowing the pharmacokinetics
and eliminating the many complications associated with the toxic substances
found in tobacco smoke. The antidepressant bupropion is approved for
nicotine cessation therapy. It is most effective when combined with behavioral
therapies.
b-Benzodiazepines:
Benzodiazepines
are commonly prescribed as anxiolytics and sleep medications. They represent a
moderate risk for abuse, which has to be weighed against their beneficial
effects. Benzodiazepines are abused by some persons for their euphoriant
effects, but most often abuse occurs concomitant with other drugs, e.g. to
attenuate anxiety during withdrawal from Opioids.
Barbiturates, which preceded benzodiazepines as the most commonly
abused sedative hypnotics (after ethanol), are now rarely prescribed to
outpatients.
Withdrawal Symptoms:
Include irritability, insomnia,
phono- and photophobia, depression, muscle cramps, and even seizures.
Typically, these symptoms taper off within 1–2 weeks.Benzodiazepines
are positive modulators of the GABAA receptor, increasing both
single-channel conductance and open-channel probability. GABA receptors on
dopamine neurons of the VTA lack α1, a subunit that is typically
present in GABA neurons. In addition, GABAA receptors are expressed
in much higher density on interneurons, so that a disinhibition of the
mesolimbic dopamine system may explain the rewarding effects of
benzodiazepines.
Alcohol’s
mechanism of action is complex, and no single receptor mediates all of its
effects. On the contrary, alcohol alters the function of several receptors and
cellular functions, including GABAA receptors, adenosine reuptake
(through the equilibrative nucleoside transporter, ENT1), glycine receptor,
NMDA receptor, and 5-HT3 receptor. They are all, with the exception
of ENT1, either ionotropic receptors or ion channels. It is not clear which of
these targets is responsible for the increase of dopamine release from the
mesolimbic reward system.
Withdrawal syndrome:
That may include tremor (mainly of
the hands), nausea and vomiting, excessive sweating, agitation, and anxiety. In
some individuals, this is followed by visual, tactile, and auditory
hallucinations 12–24 hours after cessation. Generalized seizures may manifest
after 24–48 hours. Finally, 48–72 hours after cessation, an alcohol withdrawal
delirium (delirium tremens) may become apparent in which the person
hallucinates, is disoriented, and shows evidence of autonomic instability.
Delirium tremens is associated with 5–15% mortality.
Treatment:
Treatment of ethanol withdrawal is
supportive and relies on benzodiazepines. As in the
treatment of all chronic drug abuse problems, heavy reliance is placed on psychosocial
approaches to alcohol addiction. The pharmacologic
treatment of alcohol addiction is limited, although several compounds, with
different goals, have been used.
Inhalant
abuse is defined as recreational exposure to chemical vapors, such as nitrates,
ketones, and aliphatic and aromatic hydrocarbons. These
substances are present in a variety of household and industrial products that
are inhaled by "sniffing," "huffing," or
"bagging." Sniffing refers to inhalation from an open container,
huffing to the soaking of a cloth in the volatile substance before inhalation,
and bagging to breathing in and out of a paper or plastic bag filled with
fumes. It is common for novices to start with sniffing and progress to huffing
and bagging as addiction develops. Inhalant abuse is particularly prevalent in
children and young adults.
Mechanism of addiction:
The exact mechanism of action of most
volatile substances remains unknown. Altered function of ionotropic receptors
and ion channels throughout the central nervous system has been demonstrated
for a few. Nitrous oxide, for example, binds to NMDA receptors and fuel
additives enhance GABAA receptor function. Most inhalants produce euphoria;
increased excitability of the VTA has been documented for toluene and may
underlie its addiction risk. Other substances, such as amyl nitrite
("poppers"), primarily produce smooth muscle relaxation and enhance
erection, but are not addictive. With chronic exposure to the aromatic
hydrocarbons (e.g. benzene, toluene), toxic effects can be observed in many
organs, including white matter lesions in the central nervous system.
Management of overdose remains supportive.
III-Drugs
That Bind to Transporters of Biogenic Amines:
Cocaine is an alkaloid found in the leaves of Erythroxylon
coca, a shrub indigenous to the Andes. For more than 100 years, it has been
extracted and used in clinical medicine, mainly as a local anesthetic and to dilate
pupils in ophthalmology. Sigmund Freud famously proposed its use to treat
depression and alcohol dependence, but addiction quickly brought an end to this
idea.
Mechanism of addiction:
In the peripheral nervous system,
cocaine inhibits voltage-gated sodium channels, thus blocking initiation and
conduction of action potentials. This effect, however, seems responsible for
neither the acute rewarding nor the addictive effects. In the central nervous
system, cocaine blocks the uptake of dopamine, noradrenalin, and serotonin
through their respective transporters. The block of the dopamine transporter
(DAT), by increasing dopamine concentrations in the nucleus accumbens, has
been implicated in the rewarding effects of cocaine. The activation of the
sympathetic nervous system results mainly from blockage of the norepinephrine
transporter (NET) and leads to an acute increase in arterial pressure,
tachycardia, and often, ventricular arrhythmias. Users typically lose their
appetite, are hyperactive, and sleep little. Cocaine exposure increases the
risk for intracranial hemorrhage, ischemic stroke, myocardial infarction, and
seizures. Cocaine overdose may lead to hyperthermia, coma, and death.
Susceptible individuals may become dependent and addicted after only a few
exposures to cocaine. Although a withdrawal syndrome is reported, it is not as
strong as that observed with Opioids.
b-Amphetamines
Amphetamines
are a group of synthetic, indirect-acting sympathomimetic drugs that cause the
release of endogenous biogenic amines, such as dopamine and noradrenaline.
Mechanism of action:
Amphetamine, methamphetamine, and
their many derivatives exert their effects by reversing the action of biogenic
amine transporters at the plasma membrane. Amphetamines are substrates of these
transporters and are taken up into the cell. Once in the cell, amphetamines
interfere with the vesicular monoamine transporter (VMAT) depleting synaptic
vesicles of their neurotransmitter content. As a consequence, levels of
dopamine (or other transmitter amine) in the cytoplasm increase and quickly
become sufficient to cause release into the synapse by reversal of the plasma
membrane DAT. Normal vesicular release of dopamine consequently decreases
(because synaptic vesicles contain less transmitter), whereas nonvesicular
release increases. Similar mechanisms apply for other biogenic amines
(serotonin and norepinephrine).
In general, amphetamines lead to elevated
catecholamine levels that increase arousal and reduce sleep, whereas the
effects on the dopamine system mediate euphoria but may also cause abnormal
movements and precipitate psychotic episodes. Effects on serotonin transmission
may play a role in the hallucinogenic and anorexigenic functions as well as in
the hyperthermia often caused by amphetamines. Unlike many
other abused drugs, amphetamines are neurotoxic. The exact mechanism is not
known, but neurotoxicity depends on the NMDA receptor and affects mainly
serotonin and dopamine neurons. Hypertensive crisis and
vasoconstriction may lead to stroke.
Withdrawal symptoms:
Withdrawal consists of dysphoria,
drowsiness (in some cases, insomnia), and general irritability.
Ecstasy
is the name of a class of drugs that includes a large variety of derivatives of
the amphetamine-related compound methylene-dioxymethamphetamine (MDMA). MDMA
was originally used in some forms of psychotherapy but no medically useful effects
were documented.
Mechanism of action:
Similar to the amphetamines, MDMA
causes release of biogenic amines by reversing the action of their respective
transporters. It has a preferential affinity for the serotonin transporter
(SERT) and therefore most strongly increases the extracellular
concentration of serotonin. This release is so profound that there is a marked
intracellular depletion for 24 hours after a single dose. With repetitive
administration, serotonin depletion may become permanent, which has triggered a
debate on its neurotoxicity. Although direct proof from animal models for
neurotoxicity remains weak, several studies report long-term cognitive
impairment in heavy users of MDMA.
In
contrast, there is a wide consensus that MDMA has several acute toxic effects,
in particular hyperthermia, which along with dehydration (e.g., caused by an
all-night dance party) may be fatal. Other complications include serotonin
syndrome (mental status change, autonomic hyperactivity, and neuromuscular
abnormalities) and seizures.
Withdrawal symptoms:
Withdrawal is marked by a mood
"offset" characterized by depression lasting up to several weeks.
There have also been reports of increased aggression during periods of
abstinence in chronic MDMA users.
Non-addictive
Drugs of Abuse
Some
drugs of abuse do not lead to addiction. This is the case for substances that
alter perception without causing sensations of reward and euphoria, such as the
hallucinogens and the dissociative anesthetics. Unlike addictive drugs, which
primarily target the mesolimbic dopamine system, these agents primarily target
cortical and thalamic circuits. Lysergic acid diethylamide (LSD), for example,
activates the serotonin5-HT2A receptor in the prefrontal cortex,
enhancing glutamatergic transmission onto pyramidal neurons. These excitatory
afferents mainly come from the thalamus and carry sensory information of
different modalities, which may constitute a link to enhanced perception.
Phencyclidine (PCP) and Ketamine produce a feeling of separation of mind and
body (which is why they are called dissociative anesthetics) and, at higher
doses, stupor and coma. The principal mechanism of action is a use-dependent
inhibition of glutamate receptors of the N -methyl-D-aspartate
(NMDA) type.
Concurrent
effects on both thalamocortical and mesolimbic systems also exist for other
addictive drugs. Psychosis-like symptoms can be observed with cannabinoids,
amphetamines, and cocaine, which may reflect their effects on thalamocortical
structures. For example, cannabinoids, in addition to their documented effects
on the mesolimbic dopamine system, also enhance excitation in cortical circuits
through presynaptic inhibition of GABA release.Hallucinogens
and NMDA antagonists, even if they do not produce dependence or addiction, can
still have long-term effects. Flashbacks of altered perception can occur years
after LSD use. Moreover, chronic use of PCP may lead to an irreversible
schizophrenia-like psychosis.
References
Ø
Bertram
G. katzung, Susan B. Masters, Anthony J. Trevor, Basic and
clinical pharmacology, 11th edition.
Ø
Principles of
Pharmacology, The Pathophysiologic Basis of Drug Therapy 2nd
Edition. David E.
Golan MD, PhD, Armen H. Tashjian Jr. MD.
Ø
Applied therapeutics,The
clinical use of drugs(9th edition).koda kimble.
Ø Goodman & Gilman’s The Pharmacological Basis
of THERAPEUTICS eleventh
edition.
Ø Pharmacotherapy
Handbook Seventh Edition.Barbara
G. Wells, PharmD, FASHP, FCCP, BCPP.
Ø Pharmacology
4th Edition Lippincott Illustrated Reviews.