Abdelkader Ashour, Ph.D. 7th Lecture Pharmacology-1 PHL 351 Abdelkader Ashour, Ph.D. 7th Lecture

Desensitization (Tachyphylaxis) and Tolerance The loss of a drug’s effect, when it is given continuously or repeatedly On a short time-scale, such as a few minutes, this situation is called desensitization or tachyphylaxis and on a longer time-scale, such as days or weeks, the term tolerance is preferred. Receptor-mediated responses to drugs and hormonal agonists often desensitize with time, when they are given continuously or repeatedly After reaching an initial high level, the response (e.g., cellular cAMP accumulation, Na+ influx, contractility, etc) gradually diminishes over seconds or minutes, even in the continued presence of the agonist This is usually reversible; a second exposure to agonist, if provided a few minutes after termination of the first exposure, results in a response similar to the initial response Example: chronic salbutamol (b2 agonist) can cause internalisation of receptors → less receptors available for stimulation (down-regulation) → decreased bronchodilation How about Chronic antagonist administration? Can this lead to UP REGULATION??? ……………………………..YES Example: chronic propranolol (b blocker) can cause increased synthesis of β1 receptors in the heart → less antagonism → decreased drug effect (increased heart rate and blood pressure) Why desensitization? Many receptor-effector systems incorporate desensitization mechanisms for preventing excessive activation when agonist molecules continue to be present for long periods

Idiosyncrasy A structural or behavioral characteristic peculiar to an individual or group. Idiosyncratic drug reaction is a qualitatively abnormal, and usually harmful, drug effect that occurs in a small proportion of individuals In many cases, genetic materials are responsible. Example: individuals with G6PD (an enzyme that maintains the content of GSH in red cells, and thus prevent hemolysis) deficiency cannot tolerate primaquine or some sulfonamide drugs (well tolerated in most individuals) Those individuals will suffer from hemolysis leading to severe anemia Primaquine and related substances reduce red cell GSH harmlessly in normal cells, but enough to cause hemolysis in G6PD-deficient cells

Drug Mechanisms (How Drugs Act?) Receptor mechanisms: -Most drugs exert their effects by binding to receptors -This has the effect of either mimicking the body’s own (endogenous) substances binding to receptors or preventing their binding or actions Non-receptor mechanisms These include: - Actions on enzymes - Carrier Molecules, e.g. uptake proteins - Changing Cell Membrane Permeability - Changing Physical Properties - Combining With Other Chemicals - Anti-metabolites

Receptors Serve as recognition sites for specific endogenous compounds such as: 1. Neurotransmitters, e.g. noradrenaline (NA) 2. Hormones, e.g. adrenaline (released from the adrenal medulla and acts on the heart) 3. Local Hormones /Autacoids (released and act upon the same/nearby tissue, e.g. prostaglandins) Receptor-Effector Coupling -When a receptor is occupied by an agonist, the resulting conformational change is only the first of many steps usually required to produce a pharmacologic response. -The transduction process between occupancy of receptors and drug response is often termed coupling.

Receptor Family Summary and Examples

Types of Receptor Location Effector Coupling Examples Ligand-Gated Ion Channels G-protein-coupled receptors Kinase-Linked Receptors Nuclear Receptors   Location Membrane Intracellular Effector Ion channel Channel or Enzyme enzyme Gene transcription Coupling Direct G-protein Via DNA Examples nAChR, GABAA mAChR, adrenoceptors Insulin, growth factor, cytokine receptors Steroid, thyroid hormone receptors

Action Potential Repolarization Depolarization

Ligand-gated Ion Channels They incorporate a ligand-binding (a receptor) site, usually in the extracellular domain and they are activated by binding of a ligand (agonist) to the receptor on the channel molecule. Binding of the agonist causes a conformational change in the receptor which leads to ion channel opening. Involved in fast synaptic transmission They control the fastest synaptic events in the nervous system, in which neurotransmitter acts on the postsynaptic membrane of a nerve or muscle cell and transiently increases its permeability to particular ions

Ligand-gated Ion Channels nACh Receptor

G-protein-Coupled Receptors The largest family: G-protein (guanine nucleotide binding regulatory proteins) families: Gs ,Gi and Gq Examples: mAChR, adrenoceptors, glutamate receptors, GABAB receptors Actions: fast (seconds) Structure: Seven transmembrane a-helices G-protein consists of 3 subunits, a, b, g. Guanine nucleotides bind to the a-subunit which has enzymatic activity (GTP  GDP) The b and g subunits remain together as b, g-complex

G-protein-Coupled Receptors Mechanism: binding of the agonist to the GPCR  activation of the GPCR  G-protein activation (G-GDP  G-GTP) : activation of enzyme with subsequent generation of second messengers (e.g. cAMP, IP3) → biological effect or opening or closing of an ion channel “The activation of the effector tends to be self-limiting”?? ------GTPase (Cholera and pertussis toxins?? Gi vs Gs) Amplification? Opposite functional effects may be produced at the same cell type by GPCRs (e.g., mAChR and b-adrenoceptors in cardiac cells)

G-protein-Coupled Receptors, Targets PIP2 PIP2: phosphatidylinositol-4,5-bisphosphate IP3: inositol-1,4,5-trisphosphate DAG: 1,2-diacylglycerol

G-protein-Coupled Receptors, An Example, FYI Binding of a hormone to its receptor may lead to activation of the G protein (Gq), which in turn activates phospholipase C by a mechanism analogous to activation of adenylyl cyclase. Phospholipase C then cleaves PIP2 to IP3 and DAG. The IP3 diffuses through the cytosol and interacts with IP3-sensitive Ca2+ channels in the membrane of the endoplasmic reticulum, causing release of stored Ca2+ ions, which mediate various cellular responses. Release of intracellular Ca2+ stores promotes influx of extracellular Ca2+ via store-operated channels. A rise in cytosolic Ca2+ recruits protein kinase C (PKC) from the cytosol to the membrane where it is activated by DAG. The activated kinase then phosphorylates several cellular enzymes and receptors, thereby altering their activity.