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Chapter 3

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PYSC62 – Chapter 3: Basic Principles of Pharmacology  There is no commonly accepted definition of the word “drug” o Any substance used in chemistry or medical practice  Today, many people use the word as a synonym for a narcotic agent or an illicit substance  A drug is a chemical that affects one or more biological processes however not all chemicals that affect biological processes are considered to be drugs o Ex. Protein, hormones, foods  According to an act, dietary supplements can now be in the market without being tested by the FDA. The FDA may not restrict the use of any such supplements until substantial harm has been proven o Ex. St. John’s wort is a herbal antidepressant that has evidence for its effectiveness in the treatment of mild to moderate cases of depression but there is also considerable evidence for its interacting with a variety of medications in adverse ways o These properties indicate that there is one or more ingredient in St. John’s wort that would be considered as a drug  If the FDA could prove that tobacco companies intended for cigarettes to provide nicotine to satisfy an addiction, the FDA would have the right to regulate them under the Federal Food, Drug, and Cosmetic Act which considers a product to be a drug if the vendor intends it to be one o But since tobacco companies know that nicotine is addictive thus they manipulated the levels of nicotine to enhance the enjoyment. o The Supreme Court ruled against allowing the FDA to regulate the tobacco products  The manufacturers of alcohol beverages prefer not to view alcohol as a drug. Thus, ethanol should be considered a drug since it is not of any nutritional value and it affects the normal biological activities of the body  Chemicals originating or produced in within an organism that are used to carry out the normal biological functions in the body are not usually thought of as drugs o Endogenous substances  Drug = nonfood, non-mechanical substance (usually a chemical substance) that exerts an effect upon a living system Basic Chemical Principles Related to Psychopharmacology  The largest chemical constituent of animals is water  Water molecule is polar  Water molecules are attracted to each other very powerfully = hydrogen bond o This bonding results in the fluid characteristics of water  Water is an excellent solvent for other substances that are polar or ionic  Because many of the salts that dissolve in sea water also are present in out body fluids, life first evolved in the sea and that terrestrial creatures could be said to carry the sea inside of us, in the form of water and many dissolved ions  A large component of living systems is in the aqueous phase and the other phase is the organic phase  The organic phase consists of hydrocarbons o Fats/lipids have a high degree of hydrocarbon content o Cholesterol is also made up of strings of hydrocarbons  The organic and aqueous phases are immiscible  Phospholipids are a type of diglyceride o The polar head is attracted to water while the tails are repelled by water o These characteristics lead to the important features of the cell membranes  A lipid bilayer provides a barrier to the movement of any polar and charged substances  Any drug that is highly lipid-soluble would easily be stored in adipose tissue and if it is not metabolized, it could stay there for a considerable time  The cell barriers between different compartments are cells and the cell membrane would present a barrier to the diffusion of any substance that is too water-soluble and is not transported across the membrane by proteins  The stomach and intestines are lined with cells and if a pill is introduced into the body, this would represent the first barrier to entry into the systemic circulation  Most drugs must pass into the brain o The BBB is made up of tight junctions between the endothelial cells that compose the outer lining of the blood vessels o This barrier can provide a significant resistance to the entry of drugs and other substances into the brain, and subtle alterations in the chemical structure of drugs can yield substantial differences in passage across the brain-blood barrier o Ex. Scopolamine and atropine are anticholinergic drugs that readily penetrate into the brain to produce therapeutic effects. However, there are 2 close structural analogs of these drugs known as methlyscopolamine and methlyatropine that are much less likely to penetrate into the brain  Ethanol is an example of a substance that is relatively soluble in water and lipid  In our body, these properties mean that ethanol easily passes from the stomach into widespread circulation, easily dissolves in the blood, and easily passes into the brain  Marijuana (THC) which is mostly hydrocarbon and it therefore highly lipid-soluble is rapidly absorbed when smoked and easily passes thought the BBB  Unlike ethanol, THC is not rapidly metabolized and is not very water-soluble so it deposits itself in adipose tissue for a very long time  In order for a drug to exert any effects on an organism, it must get to its sites of action in the nervous system  The effects of a particular drug depend heavily on the rate of accumulation and the concentration of the drug at its sites of action and the duration of contact at those sites o These are a function of amount of drug administered and pharmacokinetics  Pharmacokinetics = processes involved in the movement of drugs within biological systems with respect to the drug’s absorption , distribution, binding, or localization of tissues, metabolic alterations, and excretion from the body  Pharmacodynamics refers to the biochemical and physiological effects of drugs and their mechanisms of action Routes of Administration  Most common drug admission is through the mouth (per os or PO) so that the drug is absorbed in various parts of the GI tract (this is the safest, cheapest, and most convenient way of administering drugs)  Most drugs are thought to penetrate the GI mucosa by a process of passive diffusion, which in turn is limited by their lipid solubility  Many drugs are weak bases or weak acids and this acidity results in their being ionized  But since proteins in the membranes have positive and negative charges, the ionization reduces solubility in the cellular membrane o In order to facilitate their absorption, they are administered in the form of salt o But since the salt dissociates in the solution, the drug may exist as both non-ionized and ionized species o The proportion of non-ionized to ionized drug molecules present in a given area is important for drug absorption  The pH of the local area determines the ratio of ionized to non-ionized drug in that area  Weak acids (aspirin) are less ionized in an acid medium and are therefore more lipid-soluble through the stomach  Alkaloids (heroin, morphine, cocaine) are poorly absorbed from the stomach  The small intestine environment favors the absorption of weak bases because of the greater surface area and longer duration of dug contact o This is the reason why people get intoxicated faster with carbonated alcoholic drinks; the carbonation forces the alcohol quickly out of the stomach and into the small intestine where it is absorbed more rapidly  IV results in rapid onset of drug action and relatively intense effects o If over dosage occurs, little can be done unless there is a specific antagonist readily available o Repeated injections can lead to clot formation, vessel irritation, or vessel collapse o There is a high incidence of allergic reaction, pronounced cardiovascular action, and side effects with this route  Because of the good blood supply surrounding the muscles, intramuscular injection (IM) generally results in more rapid absorption than does the PO route o Drugs dissolved in an aqueous vehicle are more rapidly absorbed through the IM route than when dissolved or suspended in oil  Because the lining of the inside of the lungs provides a large surface area, drug inhalation leads to rapid onset of drug action and intense effects o Irritants or oils can cause pneumonia and long term consequences with this route  Injection of a drug underneath the skin into the tissue between the skin and muscle is subcutaneous drug injection (SC) o This method can be used for non-irritating substances to produce fairly slow and even absorption o The rate of absorption can be controlled through the form of the drug  Sublingual or buccal administration may be used with drugs that are destroyed in the stomach or intestines (nitroglycerin, nicotine)  Other rarely used routes include… o Intra-arterial administration o Bone marrow administration o Rubbing drugs over the skin surface o Intranasal administration o Mucous membranes of the vagina or urethra o Administration through the eye o Intraperitoneal injection  Routes of administration that deliver drugs directly to the nervous system include… o Intrathecal administration (injection into subdural space of spinal cord) o Intracerebroventricular injection (injection into the ventricular spaces of the brain) o Intracranial administration (direct infusion of a drug into a discrete brain area; often done in animal research)  Several experimental approaches have been developed that facilitate a drug’s ability to cross the BBB  Drugs can be administered at pro drugs; the active compound is covalently bound to another chemical structure and is relatively inert when combined with the other substituent  Transdermal drug delivery is a useful alternative to conventional routes of administration because it avoids degradation in the GI tract and first-pass metabolism, allows steady delivery, and improves patient compliance Importance of the Blood-Brain Barrier in Psychopharmacology  The brain has evolved in preventing most non-nutritive substances from entering it and affecting nervous tissue  The BBB limits entry of substances into the brain and is a vital source of stability and defense  Rate of entry of potential abusable drugs into the brain, which can affect their rate of receptor occupancy, plays a significant role in their abuse/dependence o Faster the drug penetrates = greater the likelihood of its being abused and producing dependence  BBB is actually a feature of the physical structure of the capillaries supplying blood to brain tissue. It is the characteristic of capillary endothelial cells that constitute the primary barrier to drugs and other chemicals that are potentially toxic to the brain  Endothelial cells lining the vascular wall of brain capillaries are tightly linked with junctional complexes that eliminate gaps between cells and prevent any free diffusion of blood-borne substances into the brain extracellular space  Brain capillaries have very few pores and each brain capillary is in contact with long extensions of several glial cells (astrocytes)  Endothelial cells selectively transport nutrients into the brain by several carrier-mediated or active transport systems o The active transport systems evolved in a way that nutritive but nonlipid-soluble substances can get into the brain o Carrier-mediated transport system transport substances across biological membranes  In order for a drug to have access to CNS neurons, it must be lipid soluble to some degree or compatible with one of the several carrier-mediated or active transport systems  The capillaries atop the brain stem where the vomiting center is located are permeable to chemicals because the neurons there must monitor the blood for deadly poisons, detection of which induces vomiting  The BBB may temporarily break down as a result of injury or illness, allowing chemicals to penetrate it Dose-Response Relationships  The dose administered is the single most important variable to consider
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