PSYB65H3 Chapter Notes - Chapter 7: Tricyclic Antidepressant, Dopamine Agonist, Dopamine Antagonist

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13 Mar 2014
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Chapter 7:The Influence on Drugs and Hormones on Behaviour
Psychopharmacology=study of how drugs affect the nervous system and behaviour.
Principles of Psychopharmacology
A drug is a chemical compound that is administered to bring about some desired change in the body.
Usually drugs are used to diagnose, treat, or prevent illness, to relieve pain and suffering, or to improve an
adverse physiological condition.
-On the other hand, throughout human history, drugs have also been used as food substances, for recreation,
and even as poisons.
-Today, they are also used as research tools.
Psychoactive drugs—substances that act to alter mood, thought, or behavior and are used to manage
neuropsychological illness.
Many psychoactive drugs are also substances of abuse.
-That is, people take them for nonmedical reasons or recreationally to the point that their functioning
becomes impaired.
Many psychoactive drugs promote craving and can produce addiction.
-Some can also act as toxins, producing sickness, brain damage, or death.
Routes of Drug Administration
To be effective, a psychoactive drug has to reach its target in the nervous system.
The way in which a drug enters and passes through the body to reach that target is called its route of
administration.
Many drugs are taken orally—the most natural and generally the safest way to consume a substance.
Drugs can also be inhaled, administered through rectal suppositories, absorbed from patches applied to the
skin, or injected into the bloodstream, into a muscle, or even into the brain.
Taking a drug by mouth is convenient, but not all drugs can withstand the acidity of gastric secretions or are
able to penetrate the digestive-tract walls.
Generally, there are fewer barriers between a drug and its target if the drug is inhaled rather than
swallowed, and fewer still if it is injected into the blood.
The fewest obstacles are encountered if a drug is injected directly into the brain.
To reach the bloodstream, an ingested drug must first be absorbed through the lining of the stomach or
small intestine.
If the drug is liquid, it is absorbed more readily than if it is a solid.
Drugs taken in solid form are not absorbed unless they can be dissolved by the stomach’s gastric juices.
Absorption is also affected by other chemical properties of the drug.
If a drug is a weak acid, such as alcohol, it is readily absorbed across the stomach lining.
If it is a weak base, it cannot be absorbed until it passes through the stomach and into the intestine, by
which time the digestive juices may have destroyed it.
The drug must next enter the bloodstream.
This part of the journey presents a different set of barriers.
Blood has a high water concentration, and so a drug must be hydrophilic to mix with it. A hydrophobic
substance will be blocked from entering the bloodstream.
 If a drug does make its way into the circulatory system, it becomes diluted by the blood’s 6-liter volume.
To reach a neurological target, a drug must also travel from the blood into the extracellular fluid.
This part of the journey requires that molecules of the drug be small enough to pass through the pores of
capillaries, the tiny vessels that carry blood to the body’s cells.
Even if the drug makes this passage, it encounters other obstacles.
The extracellular fluid’s volume of roughly 35 liters of water dilutes the drug even further, and, if it passes
through cell membranes, the drug is at risk of being modified or destroyed by various metabolic processes
taking place in the cells.
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Routes of Drug Removal
Soon after a drug is taken, the body begins to remove it.
Drugs are metabolized throughout the body, but particularly in the kidneys, liver, and bile.
They are excreted in urine, feces, sweat, breast milk, and exhaled air.
Drugs manufactured for therapeutic purposes are usually designed to optimize their chances of reaching
their targets and to prolong their survival in the body.
The body has trouble removing some substances, however, and these substances are potentially dangerous
because, with repeated exposure, they can build up in the body and become poisonous.
Many metals, such as mercury, are not easily eliminated from the body and, when they accumulate there,
they can cause severe neurological problems.
In 1956 in Minamata, Japan, many people suffered physical and psychiatric effects from eating fish caught
near a factory that released mercury into the sea.
This case gave rise to improved rights for Japanese citizens affected by industrial
by-products and to a new term for mercury poisoning—Minamata disease.
Because mercury can accumulate in the food chain, especially in fish, pregnant women are advised not to
eat tuna, a fish that accumulates mercury by eating other fish.
Mercury can produce neurological damage in the fetus as well as in young children.
Revisiting the Blood–Brain Barrier
You know that many substances that can affect the body are prevented from entering the brain by the
blood–brain barrier.
The brain has a rich capillary network.
-In fact, none of its neurons is farther than about 50 lm away from a capillary.
Many drugs cannot enter the brain through the blood– brain barrier, whereas other drugs can.
The single layer of endothelial cells that compose brain capillaries is surrounded by the end feet of
astrocyte glial cells, covering about 80% of a capillary’s outer surface.
The glial end feet play only minor roles in the blood–brain barrier.
The glial cells’ main function is to provide a route for the exchange of food and waste between the
capillaries and the brain’s extracellular fluid and from there to other cells.
But astrocytes may also play a role in maintaining the tight junctions between endothelial cells and in
making capillaries dilate to increase blood flow to areas of the brain in which neurons are very active.
Thus, substances that can pass through the endothelial cells’ junctions in the body cannot do so in the
brain.
Many substances—for instance, oxygen, glucose, and amino acids (the building blocks of proteins)—must
routinely travel from the blood to brain cells, just as carbon dioxide and other waste products must
routinely be excreted from brain cells into the blood.
molecules of these substances cross the blood–brain barrier in two ways:
1. Small molecules such as oxygen and carbon dioxide, which are not ionized and so are fat soluble, can
pass through the capillary wall.
2. Molecules of glucose, amino acids, and other nutrients can be carried across the capillary by active-
transport systems, which are pumps, such as the sodium–potassium pump described in Chapter 4, that are
specialized for the transport of a particular substance.
A few brain regions lack tight junctions between the cells of capillary walls and so lack a blood–brain
barrier.
The pituitary gland of the hypothalamus is a source of many hormones that are secreted into the blood,
and their release is triggered in part by other hormones carried to the pituitary gland by the blood.
The absence of a blood–brain barrier at the area postrema of the lower brainstem allows toxic substances
in the blood to trigger a vomiting response.
The pineal gland also lacks a blood–brain barrier and is therefore open to the hormones that modulate the
day–night cycles controlled by this structure.
Drug Routes and Dosage
Drugs that can make the entire trip from the mouth to the brain have certain chemical properties.
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The most effective consist of molecules that are small in size, weakly acidic, water or fat soluble, potent in
small amounts, and not easily degraded.
Given the many obstacles that psychoactive drugs encounter on their journey from mouth to brain, it is
clear why inhaling a drug or injecting it into the bloodstream has advantages: these routes of administration
bypass the obstacle of the stomach.
In fact, with each obstacle eliminated on the route to the brain, the dosage of a drug can be reduced by a
factor of 10 and the drug will still have the same effects.
For example, 1 milligram (1000 lg) of amphetamine, a psychomotor stimulant, produces a noticeable behavioral
change when ingested orally.
-If inhaled into the lungs or injected into the blood, thereby circumventing the stomach, 100 lg of the drug (1000 lg
10) produces the same results.
- Similarly, if amphetamine is injected into the cerebrospinal fluid, thereby bypassing the stomach and the blood, 10
lg is enough to produce an identical outcome, as is 1 lg if dilution in the CSF also is skirted and the drug is injected
directly onto target neurons.
Drugs that can be inhaled or injected intravenously are much cheaper to use because the doses required are
a fraction of those needed for drugs taken by mouth.
Drug Actions in Synapses
Most psychoactive drugs work by influencing the chemical reactions at synapses.
Scientists and pharmaceutical companies continue to develop many forms of each drug in attempts to
increase penetration to the brain, increase effectiveness, and reduce side effects.
As an understanding of synaptic activity in the brain advances, drugs that have a more selective action in
their therapeutic effects can be designed.
At the same time, this research helps explain the psychoactive effects of drugs and their potential benefits
and harm.
Thus, to understand the psychoactive effects of drugs, we must explore the ways in which they modify
synaptic activity.
Steps in Synaptic Transmission
Synthesis of the neurotransmitter (1) can take place in the cell body, axon, or terminal. The
neurotransmitter may then be (2) stored in storage granules or in vesicles until it is (3) released from the
terminal’s presynaptic membrane to (4) act on a receptor embedded in the postsynaptic membrane. Excess
neurotransmitter in the synapse is either (5) deactivated or (6) taken back into the presynaptic terminal for
(7) reuse. The synapse also has mechanisms for degrading excess neurotransmitter and removing unneeded
by-products from the synapse.
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