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Suzanne Erb

Chapter 2 Basic Principles of Pharmacology Drug (definition) -A drug originally referred to any substance used in chemistry or medical practice. -Any agent used in medicine or any ingredient in medicines. -Synonym for a narcotic agent or illicit substance None of this actually defines what a drug is. -It is a chemical that affects one or more biological processes. -However, not all chemicals that affect biological processes are considered drugs. -Substances that are commonly used for nutritional purposes eg salt, vitamins and minerals etc are generally considered drugs, because they are necessary for carrying out the normal biological functions of the body. -Chemicals originating or produced within an organism that are used to carry out the normal bioligcal functions in the body are not usually thought of as drugs= endogenous substances -Drugs are exogenous substances -Biochemists and neuroscientists have isolated a variety of substances found to be important in the function of the body, have extracted or synthesized them, and have administered them in purified form to reverse neurological deficits. The use of L-dopa, a chemical necessary for the production of an important bran chemical, in the treatment of Parkinsons disease and L-dopa is a drug. -Consider Aspirin, which is an effective treatment for several kinds of pain. It is believed that most of its action is due to its ability to inhibit the activity of prostaglandins, a class of chemicals found throughout the body that play a vital role in almost every life process, including respiration, reproduction, and circulation. -From a pharmacological perspective, it seems most appropriate to define a DRUG as a non-food chemical that alters one of more normal biological processes in living organism. -The most important factors in determining a drugs effect are the concentration of the drug at its site(s) of action and the rate of accumulation there. -These factors, in turn, can be affected by many other factors, most commonly the drug dose- that is, the quantity of drug administered at one time. -Dose of drug is expressed in terms of unit of drug per unit of body weight of the organism, such as milligrams per kilogram. Drug dosage refers to administrations of the drug per unit of time, such as 10mgkg four times a day for three days -We will see that there is higher concentration in a rat than in a human and will exert a much greater effect in that rat than in the human. -There can be large differences in the rate and degree of drugs absorption, distribution, metabolization, and excretion across species and in different ages. -Also, animal species differ with respect to the amounts of tissue for drug storage. fact that drugs are not generally dispersed evenly throughout the body and are usually taken up in tissues other than the site of drug action can affect the drugs action. Drugs and Receptors -Pharmacodynamics refers to the biochemical and physiological effects of drugs and their mechanisms of action. -The vast majority of durgs act very specific receptor sites. -These are fairly large molecules usually proteins which comprise the sites wehre biologically active chemicals of the body often called ligands, induce their effects. -Eg. Insulin, testosterone, estrogen, adrenaline, neurotransmitter, neurohormones, and neuromodulators. -If the receptor then starts some biological activity, it is said to be activated -In most cases, binding is temporary, or reversible, and when the chemical leaves the receptor, it is said to dissociate from the receptor. -Affinity refers to the relative capacity of the compound to maintain contact with or be bound to a receptor. -Efficacy refers to the degree of biological activity or relative capability of a compound to activate the receptor after being bound to it. -It should be noted that a compounds efficacy and affinity are generally independent of each other, although without receptor affinity, a compound is unlikely to exhibit efficacy. -It should be noted that each neuroactive ligand in the nervous system generally had several different types of receptors on which it acts, each with its own separate function. -Drugs may mimic the actions of a particular ligand at one of more of its receptor types and may have no actions or act in an opposing fashion at others. -Agonist are compounds with both an affinity for and a capability of activating a receptor=has efficacy. -If the compound activating the receptor is endogenour (naturally occurring in the body, it is typically called a ligand. -If it is exogenous (produced outside the body), it is a drug and sometimes drugs can be referred to as ligands. -Both drugs and ligands can be agonists. -In some cases, drugs do not combine directly with a receptor, but enhance the amount of the endogenous ligands available for the receptor which is called an indirect agonists. -Some drugs exert their effects by blocking the action of agonists and are thus called antagonists. -Partial agonists are drugs that display intermediate efficacy in receptor activation between the efficacy of a full agonist and an antagonist. A partial agonist may actually have a greater affinity for a particular receptor than a full agonist. It cannot achieve the maximal response of the full agnonist at those receptors, it can reduce the effects of a full agonist, because it reduces the full agonists accessibility to its receptors. -Thus, partial agnosits may be therapeutically useful in conditions in which one wants to reduce excessive receptor activity but without totally eliminating receptor activity.
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