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Chapter 3 – Basic Principles of Pharmacology
- The psychological effects of a drug rely on many factors: The individuals set and setting, the way
the drug changes the CNS, the route of administration, the amount, the frequency, etc.
- There is no commonly accepted definition of the word drug. It was originally used to reference
any substance used in chemistry or medical practice, but later on referred solely to substances
used in medicines. Today it is frequently used to refer to a narcotic agent or illicit substance.
- In general, it is a chemical that affects one or more biological processes. However this does not
apply to substances commonly used for nutritional purposes like salt, water, proteins, fat etc.
- Many people would prefer to not think of alcohol and nicotine as drugs, and chemicals
originating or produced within an organism that are used to carry out the normal biological
functions are not usually thought of as drugs (endogenous vs exogenous).
- Pharmacologists define a drug as a non-food, non-mechanical substance (usually a chemical
substance) that exerts an effect upon a living system.
Basic Chemical Principles Related to Psychopharmacology
- To understand how drugs interact with living systems, it is important to review some of the
basic chemical properties of living things.
o The largest chemical constituent of animals is water. It is electrically neutral overall but
the oxygen end of the molecule has a partial negative charge and the hydrogen end has
a partial positive charge. This means it is polar. Water molecules are attracted to each
other very powerfully which creates hydrogen bonds. It is an excellent solvent for other
substances that are polar or ionic. In its entirety it is referred to as the aqueous phase.
o In the organic phase, organic refers to the chemistry of carbon based substances. The
major constituents of the organic phase are hydrocarbons which are the substances that
contain hydrogen and carbon bound together covalently.
o An important fat we need to consider is phospholipids. They are a type of diglyceride
which has a polar head and two fatty acid carbon tails. This lipid bilayer provides a
barrier to the movement of any polar and charged substances. Some of the barriers
include the lipid bilayer as just mentioned, the stomach and intestines, entry into the
circulatory system, the brain (the blood brain barrier BBB). This provides significant
resistance to the entry of drugs into the brain.
o The effects of a particular drug on an organism depend heavily on the rate of
accumulation and the concentration of the drug at its sites of action, as well as the
duration of contact. These are a function of not only drug administration, but also
pharmacokinetics – The dynamic process involved in the movement of drugs within
biological systems with respect to the drugs absorption, distribution, binding or
localization in tissues, metabolic alterations and excretion from the body.
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o Pharmacodynamics - refers to the biochemical and physiological effects of drugs and
their mechanisms of action.
Routes of Administration
1. The most common route of injection is the mouth (Per Os). It is generally the safest, cheapest
and most convenient way. It is then absorbed through various parts of the GI tract. Lots of
factors can influence the absorption of drugs from the GI tract which can greatly alter the rate of
drug accumulation, concentration and duration at the sites of action.
o Most drugs that pass through the GI mucosa by a process of passive diffusion, which is
limited mostly by their lipid solubility. Many drugs are weak bases and this often results
in their being ionized (a state in which an atom or a molecule has a net electrical
charge). Therefore to facilitate their absorption, such drugs are commonly administered
in the form of a salt. However because the components of a salt dissociate in a solution,
the drug may exist as both a nonionized or an ionized species.
o The pH of the local area determines the ration of ionized or nonionized drug in the area.
The pH value represents the negative log of the hydrogen ion concentration. Solutions
with a pH of 7.0 are neutral, less is acid and more are basic.
2. The introduction of a drug directly to the blood is most commonly done by intravenous (IV)
injection, results in rapid onset of drug action and relatively intense effects. Drugs ingested
intravenously must also be in a solution or microsuspension and must have an aqueous vehicle.
Injecting drugs that are not water soluble can result in clogging of capillaries to important
organs, and cause organ failure. There is also a high incidence of allergic reaction.
3. Because of the relatively good blood supply surrounding muscles, intramuscular (IM) injection
generally results in more rapid absorption than does the PO route.
4. Because the lining of the inside of the lungs provides a large surface area in proximity to many
blood vessels, drug inhalation leads to rapid onset of drug action and intense effects. This route
eliminates drug loss through first pass metabolism by the liver. However you may acquire
pneumonia or cancer.
5. The injection of a drug underneath the skin into the tissue between the skin and the muscle
(body fat) is referred to as subcutaneous (SC) drug injection. This often produces a slow and
even absorption, but it is dependent on the form on the drug.
6. Sublingual Buccal administration is the absorption through the oral mucosa under the tongue,
or between the cheek and gum, and may be used with drugs that are destroyed in the stomach.
7. Others include: intra-arterial, bone marrow, rubbing drugs over a large surface of skin,
intranasal, vaginally, urethral, and through the eye. In animals, intraperitoneal injection is
sometimes used. These methods involve systemic administration (general distribution all over
8. Other ways administer it directly to the nervous system: intrathecal, subdural places of the
spinal cord, and intracerebroventricular injection.
9. Intracranial administration involves direct infusion of a drug to a discrete brain area.
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