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University of Toronto Scarborough
Suzanne Erb

Chapter 2- Basic Principles of Pharmacology In general, the definition of a drug is a chemical that affects one or more biological processes. Endogenous substances are chemicals that originate or are produced within an organism for the use of normal biological processes. Exogenous substances are drugs. The most important determinants of a drugs effects are its concentration at the site of action as well as the rate of accumulation at that point. Drug dose is expressed in terms of unit of drug per unit of body weight of the organism. Drug dosage refers to the administrations of the drug per unit of time. DRUGS AND RECEPTORS: Pharmacodynamics refers to the biochemical and physiological effects of drugs and their mechanisms of action. Receptors, which are fairly large molecules, comprise of sites at which biologically active chemicals (ligands) induce their effects. Affinity refers to the relative capacity of a compound to maintain contact with or be bound to a receptor. Efficacy refers to the degree of biological activity or relative a capability of a compound to activate the receptor after being bound to it. Agonists are compounds with both an affinity for and capability if activating a receptor (efficacy). There are 4 types of agonists: Indirect- a drug that doesnt bind to the receptor itself but enhances the amount of the endogenous ligand to bind to a receptor. Partial- drugs that display intermediate efficacy in receptor activation between the efficacy of a full agonist and an antagonist. E.g. when these types of drugs bind to a receptor they hinder the full agonists ability to bind completely to the receptor. Hence, reducing the effect of that full agonist but not eliminating it; can be useful in schizophrenia where there is too much receptor activity. However, there is a disadvantage to these types of agonists; since they block the full activity of the full agonist they may induce prolonged inactivity of the receptor which can cause some side effects. Recently though, a drug known as aripiprazole (abilify) partially blocks dopamine from binding to its receptors but still stimulates the receptors to a certain degree do that the side effects of prolonged inactivity dont occur. Inverse- a drug that appears to act through the same receptor as an agonist but produces effects opposite to those of the agonist. Mixed agonist-antagonist- when a drug acts as an agonist by itself, but blocks the activity of another agonist in the same system. www.notesolution.com There are two broad classes of receptors: ionotropic and metabotropic receptors. Ionotropic- produce rapid changes in neuronal activity via a pore that allows for the flow of certain ions that alter the excitability of the neuron. Most are composed of five subunits (,,,) which contain binding sites that change the conformation of the receptor to increase of decrease the passage of ions through the channel. Metabotropic- produce relatively slow-developing and long-lasting effects on neurons. They are composed of a long string of amino acids that weave in and out of the neuronal membrane and are attached to a G-protein. Neuronal transmission occurs in these receptors via the G-protein and production of second messengers. There are two main factors that effect ligand binding: the ligands molecular shape and the location of positive and negative electrical charges. Varied responsiveness to drugs in the human race is due to polymorphisms within multiple genes and those in the discipline of pharmacogenetics attempt to discover differential effects of drugs in variations of genes. Those in the discipline of pharmacogenomics attempt to discover differential effects of compounds on gene expression among all genes that expressed in humans. DOSE-RESPONSE RELATIONSHIPS: Dose-response function- expresses the relationship between the dose administered and the response observed. Active placebo- a substance that mimics some of the noticeable physiological characteristics of the drug being evaluated but without the effects on the brain that the researcher is interested in. Biphasic functions are those that display that substantial concentrations of a drug may cause even less stimulation in the subject than if no drug was administered at all. Therapeutic window- a specific range of drug concentration at which the optimal therapeutic effect in patients can be obtained. The plasma half-life of drugs can be an important factor in determining the desirable drug doses and the time interval in which they should be administered to maintain the optimal therapeutic concentration level. In summary several observations can be made regarding dose-response relationships: www.notesolution.com
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