NSC 4352 1st EditionExam # 3 Study Guide Lectures: 12-16Lecture 12: Neurotransmitters I—Chemical signaling consists of... • A molecular signal (neurotransmitter) • A receptor molecule (transduces information provided by the signal) •A target molecule (ion channel) that is altered to cause electrical response in the postsynaptic cell.Two types of Receptors:Ionotropic • ligand-gated channels receptor IS channel Metabotropic • G-protein-coupled receptors receptor modulates channel, or other intracellular effects Ionotropic ReceptorsMembrane spanning region forms ion channel. Comprised of 4-5 protein subunits. Mediate rapid postsynaptic effects (millisecond time scale) Glutamate receptors (NMDA, AMPA/Kainate) nicotinic acetylcholine receptor (nAChR) 5-HT3 receptor GABAA receptor Glycine receptor PurinergicreceptorsGeneral features of metabotropic receptorG-protein coupled receptors Monomeric proteins consisting of 7 transmembrane domains. Domains II, III, VI, and VII make up the neurotansmitter bindG-proteins bind to both the loop between domains V and VI andthe C-terminus (blue) Acetylcholine – the nicotinic AChR Consists of 5 subunits: • Each subunit consists of 4in muscle αα:β:γ:δ transmembranespanning α-helices in neurons ααα:ββ• α-subunits bind acetylcholine •M2 subunit (blue) lines pore •Binding of acetylcholine to the twoalpha subunits results in the 15° rotation of all M2 helices.The cytoplasm side of the nAChR receptor has rings of high negative charge that determine the cation specificity of the receptor and remove the hydration shell of the passing ions.•nAChR is non-selective cation channel •Higher driving force for Na+ typically results in an inward current and an EPSP Acetylcholine metabolismPrecursorsAcetyl coenzyme A and choline Enzyme choline acetyltransferase (ChAT) catalyzes Acetylcholine A vesicular ACh transporter loads ACh into vesicles (~10.000 per vesicle) After release, Acetylcholinesterase breaks up ACh into acetate and choline. ACh-esterase is the target of nerve gases/pesticides A Na+/choline transporter takes choline back up into the presynaptic terminal Irreversible Acetylcholinesterase inhibitorsInsecticides (so-called organophosphates), and nerve gases (e.g. Sarin, Soman) Irreversible AChE-inhibitors completely inhibit ACh breakdown The lethal effect results from “overstimulation” (persistent depolarization) of the postsynaptic membrane, particularly muscle cells. The main effect is neuromuscular paralysis (leading to respiratory failure within 5 min), preceded by cognitive and severe autonomic symptoms.Treatment involves combined administration of a muscarinic receptor antagonist (e.g. atropine) and the AChE antagonist pralidoxime, which paradoxically restores AChE function(Pralidoxime attaches to the site where the cholinesterase inhibitor has attached, then attaches to the inhibitor, removing the organophosphate from cholinesterase, allowing it to work normally again) Glutamate:most prevalent excitatory transmitter (>half of all synapses) Precursor: glutamine, released by glia cells Enzyme glutaminase catalyzes glutamate from glutamine VGLUT = vesicular glutamate transporter EAAT = excitatory amino acid transporter 5 different types – some on presynaptic terminals, others on glia cellsLecture 13: Neurotransmitters IIBiogenic Amines – the Monoamines”Catecholamines: • Dopamine • Norepinephrine (Noradrenaline) • Epinephrine (Adrenaline) Serotonin (5-HT) Histamine DA as final common pathway in addiction:• Nearly all drugs, directly or indirectly, target the brain’s reward system by flooding the circuit with dopamine. Drugs of abuse typically release 2 to 10 times the amount of dopamine that natural rewards do. •Chronic drug use changes monoamine metabolites and uptake transporters.Dopamine D2 receptors are down-regulated to compensate for their drug-induced overstimulation. Thus the brain's capacity to experience pleasure is diminished tolerance Cocaine and amphetamines inhibit the re-uptake of dopamine. • Cocaine blocks the dopamine transporter (DAT), i.e. blocks dopamine re-uptake. This increases the lifetime of dopamine in the synaptic cleft. • Amphetamines increase dopamine release and block re-uptake.Amphetamines enter the presynaptic terminal (via DAT, or through direct diffusion) and release dopamine from the vesicles by making the dopamine transporters work in reverse.Serotonin (5-HT) •Precursor: Tryptophan = essential amino acid •Regulates Mood and SleepImplicated in depression, anxiety disorders, schizophrenia. •(i.e. can not be synthesized by body, has to be part of diet) plentiful in turkey (poultry), chocolate, bananas, milk... •Depression, anxiety disorders. •Many antidepressants (Prozac; Zoloft) are SSRIs(Selective Serotonin Reuptake Inhibitors) •block the serotonin transporter (SERT) •SERT is target for MDMA (ecstasy) •Degraded by MAO •Most 5-HT receptors are metabotropic (only 5-HT3 receptor is ligand- gated ion channel.)Lecture 14: Intracellular SignalingCells alter internal chemistry in response to external signals Signal transduction: is the process by which extracellular signals are communicated to specific domains within the cell Requires 3 steps: Signal, receptor, and target molecule that mediates the cellular response Signal Amplification and temporal control • Most signal transduction pathways serve not only to communicate signals to specific intracellular domains but to amplify them as well. •In addition, different signaling pathways allow for control of cellular responses on various time scales Extracellular signaling molecules: •Small soluble organic moleculese.g. Neurotransmitters, amino acids, sugars, nucleotides •Inorganic ions (e.g. Zn2+) •Diffusable gases (e.g. CO &NO) •Peptides •Lipophilic organic molecules (e.g. endocannabinoids)•Cell-surface expressed proteins G-Protein Coupled Receptors (GPCR) •The human genome encodes roughly 800 G protein-coupled receptors, which detect photons (light), hormones, growth factors, drugs, and other endogenous ligands.Approximately 150 of the GPCRs found in the human genome have unknown functions. •Whereas G proteins are activated by G protein-coupled receptors, they are inactivated by RGSproteins. Receptors stimulate GTP binding. RGS proteins (Regulators of G protein signaling, orGAPs - GTPase-Activating Protein)stimulateGTP hydrolysis via