Drugs and the Nervous System

The activity of the nervous system is mediated by many kinds of interneurons releasing one or another neurotransmitter such as
Link to discussion of synapses and their neurotransmitters.

Presynaptic neurons

Postsynaptic neurons display receptors to which the neurotransmitter binds.

All of this machinery provides many targets for alteration by exogenous chemicals; that is, psychoactive chemicals introduced into the body. These drugs fall into several distinct families. We shall briefly examine the

Stimulants

The most widely used stimulants are All of these drugs mimic the stimulation provided by the sympathetic nervous system.
Link to discussion of the sympathetic nervous system.

Nicotine binds to a subset of acetylcholine (ACh) receptors. ACh is a neurotransmitter at synapses early in the pathways of sympathetic stimulation. Although a weak drug in one sense, nicotine is strongly addictive. The use of chewing gum and skin patches containing nicotine is designed to satisfy the craving for nicotine while avoiding the serious health effects of other ingredients in cigarette smoke.

Amphetamines and cocaine bind to transporters used for the reuptake of dopamine into presynaptic neurons. This causes the level of dopamine to rise in the synapses. High levels of dopamine in an area of the brain called the nucleus accumbens appear to mediate the pleasurable effects associated with these (as well as other) psychoactive drugs.

Some amphetamines:
Generic name Trade name
dextroamphetamine sulfate Dexedrine
methyl phenidate Ritalin
pemoline Cylert
mixture of 4 amphetamines Adderall

The chief medical uses for amphetamines are At first glance, this second use seems counterproductive. This controversial procedure seems to work by increasing the alertness of the child so that it can focus its energies more effectively on the tasks in front of it.

Fen-Phen

Fen-Phen refers to a mixture of two amphetamine-like drugs that were prescribed for losing weight. Because of reports of occasional very serious side effects, the mixture is no longer available and fenfluramine has been removed from the U.S. market.

Cocaine

Cocaine has been used for thousands of years by certain tribes in the Andes of South America. Cocaine and some of its derivatives have legitimate medical uses as local anesthetics (e.g., "Novocain"). However, the widespread recreational use of cocaine has created serious social problems.

Immunity to cocaine addiction? In order to achieve its effects, cocaine must cross the so-called blood-brain barrier. If antibodies are bound to the cocaine molecule, it cannot cross. This has raised the possibility of immunizing people against cocaine. It works in mice.

Sedatives

Sedatives induce sleep.

They include

Ethanol

Ethyl alcohol (ethanol) is, by a wide margin, the most widely used drug in most of the world. Its popularity comes not from its sedative effect but from the sense of well-being that it induces at low doses. Perhaps low doses sedate those parts of the brain involved with, for example, tension and anxiety and in this way produce a sense of euphoria. However, higher doses depress brain centers involved in such important functions as pain sensation, coordination, and balance. At sufficiently high doses, the reticular formation can be depressed enough to cause loss of consciousness.

Barbiturates

Barbiturates are often prescribed to prevent seizures as well as sleeping pills.

Barbiturates mimic some of the action of alcohol, particularly in their ability to depress the reticular formation (thus promoting sleep) and, in high doses, the medulla oblongata (thus stopping breathing). Barbiturates and alcohol act additively, the combination producing a depression greater than either one alone. The combination is a frequent cause of suicide, both accidental and planned.

Barbiturates bind to a subset of GABA receptors designated GABAA receptors. These are ligand-gated channels that enhance the flow of chloride ions (Cl-) into the postsynaptic neuron, thus increasing its resting potential and making it less likely to fire. By binding to the GABAA receptor, barbiturates (and ethanol) increase the natural inhibitory effect of GABA synapses.

Meprobamate

Meprobamate is prescribed as a tranquilizer, but its action is quite different from the tranquilizers discussed below. Its molecular activity is like that of other sedatives and in combination with them can produce a lethal overdose. All sedatives produce two related physiological effects: These traits are also shared with nicotine, opiates, and other psychoactive drugs.

Inhalational Anesthetics

Most of these are volatile hydrocarbons. Ether and chloroform are seldom used today, having been replaced by safer alternatives such as isofluorane, an fluorinated hydrocarbon. They act by binding to GABAA receptors thus decreasing the sensitivity of the postsynaptic neurons.

Opiates

These are substances isolated from the opium poppy or synthetic relatives. Examples: Opiates depress nerve transmission in sensory pathways of the spinal cord and brain that signal pain. This explains why opiates are such effective pain killers.

Opiates also inhibit brain centers controlling coughing, breathing, and intestinal motility. Both morphine and codeine are used as pain killers, and codeine is also used in cough medicine.

Opiates are exceedingly addictive, quickly producing tolerance and dependence. Although heroin is even more effective as a painkiller than morphine and codeine, it is so highly addictive that its use is illegal. Methadone is a synthetic opiate that is used to break addiction to heroin (and replace it with addiction to methadone).

Opiates bind to so-called mu (µ) receptors . These are located on the subsynaptic membrane of neurons involved in the transmission of pain signals. Their natural ligands are two pentapeptides (containing five amino acids):

Release of enkephalins suppresses the transmission of pain signals. (Little is to be gained by having the perception of pain increase indefinitely in proportion to the amount of damage done to the body. Beyond a certain point, it makes sense to have a system that decreases its own sensitivity in the face of massive, intractable pain.)

By binding to enkephalin receptors, opiates like morphine enhance the pain-killing effects of enkephalin neurons. Opiate tolerance can be explained, at least in part, as a homeostatic response that reduces the sensitivity of the system to compensate for continued exposure to high levels of morphine or heroin. When the drug is stopped, the system is no longer as sensitive to the soothing effects of the enkephalin neurons and the pain of withdrawal is produced.

Opiate antagonists

Opiate antagonists such as bind to µ receptors but instead of activating them, they prevent the binding of the opiates themselves. In fact, if the receptors are already occupied by, for example, heroin molecules, naloxone will push the heroin molecules off and quickly rescue the patient from a drug overdose. Naltrexone is used to help recovering heroin addicts stay drug-free.

Tranquilizers

Tranquilizers act like sedatives in reducing anxiety and tensions but do not have their sleep-inducing effect.

The group is often subdivided into the

Major tranquilizers

Examples: These are used to treat schizophrenia, a common and devastating mental disease. They act by binding to one class of receptors for the neurotransmitter dopamine. The drugs block the action of dopamine, and in a homeostatic response, the neurons increase their activity.

Minor tranquilizers

The minor tranquilizers are used to treat anxiety. Most belong to a group called benzodiazepines and include such commonly-prescribed drugs as Librium®, Valium®, Halcion®, and Xanax®.

The benzodiazepines act on interneurons that use the inhibitory neurotransmitter GABA. By binding to GABAA receptors on the postsynaptic membrane, they enhance the action of GABA at the synapse. [Further discussion]

This is the same receptor to which alcohol and barbiturates bind. Thus although Librium and Valium seem safe enough when used alone, combining them with alcohol or barbiturates can be (and often has been) lethal.

Antidepressants

Antidepressants fall into four chemical categories (of which we shall examine three). All share a common property: they increase the amount of serotonin at synapses that use it as a neurotransmitter.

Monoamine oxidase (MAO) inhibitors

These drugs act on a mitochondrial enzyme that breaks down monoamines such as noradrenaline and serotonin. By inhibiting the enzyme in presynaptic serotonin-releasing neurons, more serotonin is deposited in the synapse. Some examples: Parnate®, Nardil®, Marplan®. For several reasons, MAO inhibitors are not used much anymore.

Tricyclics

These drugs block the reuptake of both noradrenaline and serotonin causing an increase in the level of these neurotransmitters in the synapse . Some examples:
Generic name Trade name(s)®
Imipramine Tofranil
Amitriptyline Elavil, Endep
Trimipramine Surmontil
Doxepin Adapin, Sinequan
Desipramine Norpramin
Nortriptyline Aventyl, Pamelor
Amoxapine Asendin

Although tricyclics are still prescribed for pain relief, their role as antidepressants has largely been taken over by the selective serotonin reuptake inhibitors (SSRIs)

Selective Serotonin Reuptake Inhibitors (SSRIs)

As the name suggests, these drugs inhibit the reuptake of serotonin but not of noradrenaline.

Examples:
Generic name Trade name(s)®
Fluoxetine Prozac
Paroxetine Paxil
Sertraline Zoloft
Fluvoxamine Luvox

Although all these drugs quickly increase the amount of serotonin in the brain, there is more to the story than that. Unlike most psychoactive drugs, antidepressants do not relieve the symptoms of depression until a week or more after dosing begins. During this period, the number of serotonin receptors on the postsynaptic membranes decreases. How this translates into relief of symptoms is not yet understood.

Psychedelics

Psychedelic drugs distort sensory perceptions, especially sight and sound.

Some such as are natural plant products.

The photograph (courtesy of Dr. Richard Evans Schultes) shows the peyote cactus in flower. The cactus head contains several psychedelic chemicals, of which mescaline is the most important. Dried cactus heads ("mescal buttons") have been used since pre-Columbian times in the religious ceremonies of native peoples in Mexico. About a century ago, this religious use spread to some tribes in the United States and Canada who, in 1922, became incorporated into the Native American Church.

Other psychedelic drugs are synthetic. These include All the psychedelics have a molecular structure that resembles serotonin and probably bind to serotonin receptors on the postsynaptic membrane.

Phencyclidine (PCP)

PCP is used as an anesthetic in veterinary medicine.

Used (illicitly) by humans (called "crystal" or "angel dust"), it can produce a wide variety of powerful reactions resembling those of stimulants as well as psychedelics.

Unlike the other psychedelics, it binds to NMDA receptors (in the hippocampus and other parts of the forebrain).

Marijuana

The active ingredient in marijuana is delta-9-tetrahydrocannabinol (delta-9-THC). It binds to CB1 receptors that are present on postsynaptic membranes in portions of the forebrain (e.g., the hippocampus) as well as in the spinal cord.

THC produces

Unlike sedatives and opiates, however, tolerance and physical addiction to THC do not occur. In fact, the drug is excreted so slowly from the body that, with repeated use, a given response is achieved by a lower dose.

CB1 receptors in the central nervous system and their CB2 relatives in the peripheral nervous system are involved in blunting pain. Their natural ligands in the body are Both of these compounds are produced from phospholipids.
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2 November 1999