Biology for Engineers: Cellular and Systems Neurophysiology

1. Biology for Engineers: Cellular and Systems Neurophysiology Christopher Fiorillo BiS 521, Fall 2009 042 350 4326, fiorillo@kaist.ac.kr Part 5: Neurotransmitters, Receptors, and Signal Transduction Reading: Bear, Connors, and Paradiso Chapter 6

2. Neurotransmitters • Conventional transmitters • The basic excitatory and inhibitory transmitters • Glutamate, GABA, glycine • Modulatory transmitters • acetylcholine, norepinephrine, dopamine, serotonin, histamine • Peptides • Many types, example: endorphin • Unconventional transmitters • Membrane permeable; not released from vesicles • May not have specific, dedicated receptors • Examples: • Endocannabinoids • Nitric Oxide • Retrograde transmission • These may sometimes be referred to as “inter-cellular messengers” rather than “neurotransmitters”

3. Distribution of Major Modulatory Neurotransmitters

4. Peptide Neurotransmitters • Peptides are often cotransmitters: they are released together with a small transmitter • Release of peptides typically requires a high-frequency train of stimuli • Peptides act on slow metabotropic receptors. There are not peptide-gated ion channels • There are a great divesity of peptides • Examples: • Opioid peptides • Endorphin, enkephalin, dynorphin • Substance P • Orexin • The functions of peptides are generally not well understood • They can have excitatory or inhibitory effects • They are best thought of as modulatory

5. Synthesis of Neurotransmitters • Each neurotransmitter has its own specific synthetic enzyme or enzymes • In some cases, the synthetic enzyme is found exclusively in neurons that release that neurotransmitter • Thus it serves as a marker for those neurons • Example: Tyrosine hydroxylase for norepinephrine and dopamine containing neurons • In other cases, the enzymes are found in all cells, but are expressed at higher levels in neurons that use that neurotransmitter • Example: glutamate

6. Receptor Pharmacology • Ligands are molecules that bind to a receptor • Natural or artificial • Agonists bind and activate a receptor • Antagonists bind to a receptor and prevent its activation by agonists

7. Major Receptor Classifications for Major Neurotransmitters Ionotropic, ligand-gated ion channels Metabotropic, G-protein coupled receptors • Glutamate • AMPA, NMDA, kainate • mGluR1 - mGluR8 • GABA • GABAA • GABAB • Norepinephrine • Alpha1, Alpha2, Beta • Dopamine • D1 - D5 • Serotonin (5-HT) • 5-HT3 • 5-HT1 5-HT2 5-HT4 5-HT5 • Acetylcholine • Nicotinic • Muscarinic (M1 - M5) • These are only major divisions. Finer distinctions have been made. • All of the ionotropic receptors have multiple types of subunits, and different types of subunits combine to make a receptor / ion channel.

8. Ligand-gated ion channels • Most are pentamers • Glutamate receptors are tetramers • Most require two transmitter molecules to bind in order to open the channel • Some have binding sites for modulators • GABAA is modified by several commonly used classes of drugs • These drugs enhance the effect of GABA, and reduce anxiety

9. Ionotropic Receptors (Ligand-gated Ion Channels) • Fast kinetics (a few ms) • Excitatory or inhibitory, depending on the channel’s ion selectivity • Most common types: • Glutamate-gated • Cation channels (permable to both Na+ and K+) • Excitatory • GABA-gated • Cl- channels • Inhibitory

10. Glutamate EPSCs Fast Excitatory and Inhibitory Postsynaptic Potentials • Mediated by Ionotropic Receptors (ligand-gated ion channels) • Fast GABA IPSPs (~30 ms) typically last longer than fast glutamate EPSPs (~5 ms) (contrary to the drawing below and in the textbook) • PSCs are faster than PSPs due to the membrane capacitance (which usually has a time constant of 1-30 ms).

11. Metabotropic Receptors (G-protein-coupled receptors) • Modifies “effectors” though G-proteins • G-proteins metabolize GTP to GDP. Since they use energy, these receptors are called “metabotropic” • Often modulatory rather than simply excitatory or inhibitory • One receptor may alter multiple types of ion channels and other effectors • Slow kinetics (> 100 ms)

12. G-proteins • G-proteins couple G-protein-coupled receptors to their effectors • In its inactive state, the alpha subunit of a G-protein binds to GDP • When a G-protein-coupled receptor binds to neurotransmitter, it induces the G-protein to release GDP and bind GTP • The G-protein splits into two parts, alpha and beta-gamma. Each of these diffuses in the membrane and is able to activate effector proteins • The alpha subunit is at GTPase. It hydrolyzes GTP to GDP. This typically occurs after about 1 second. • The GDP-bound alpha subunit is inactive, and binds to beta-gamma.

13. Three types of G-proteins and signalling cascades • Gs: • alpha subunit activates adenylyl cyclase, which synthesizes cAMP. cAMP activates PKA • Gi: • alpha subunit inhibits adenylyl cyclase • Beta-gamma subunits activate potassium channels and inhibit calcium channels • Gq: • Alpha subunit activates phospholipase C, which produces IP3 and diacylglycerol (DAG). IP3 opens ion channel on the endoplasmic reticulum, whish release calcium. DAG activates PKC. • Each G-protein coupled receptor activates just one type of G-protein

14. GABAB IPSP Direct activation of K+ channels by G-proteins • Gi Beta-gamma subunits activate potassium channels • This is the basis for the GABAB IPSP • They also inhibit Ca2+ channels

15. The Gq / Phosphoinositide pathway • Gq Alpha subunit activates phospholipase C, which produces IP3 and diacylglycerol (DAG). IP3 opens ion channel on the endoplasmic, whish releases calcium. DAG activates PKC. Calcium activates a kinase, potassium channels, and other effectors. • The next slide shows a slow IPSP mediated by glutamate activation of mGluR1, a receptor that activates this pathway. The calcium activates a K+ channel.`

16. GABAB IPSP Inhibition by glutamate (mediated by mGluR1 and Ca2+ activated K+ channels) Inhibition and excitation by the same glutamate receptor (mGluR1) Slow Postsynaptic Potentials Mediated by Metabotropic Receptors • Metabotropic PSPs typically require multiple stimuli at high frequencies • 10 stimuli at 66 Hz (15 ms intervals) in these examples • PSPs start after about 100-300 ms and last for a second or longer • There are many other effects of metabotropic receptors besides PSPs • Modification of ion channels • Modification of gene expression

17. Second Messengers • The “first” messenger is the neurotransmitter, which mediates intercellular communication between cells • A second messenger is a small molecule that carries information within a cell (through diffusion). It mediates intracellular communication. • Examples: • cAMP • cGMP • IP3 • DAG • Calcium • Calcium • The most common second messenger • Three main sources • Voltage-gated calcium channels • NMDA receptors (gated by glutamate) • Intracellular calcium stores in the endoplasmic reticulum • Calcium is released when IP3 or calcium opens channels in the ER

18. Protein Kinases and Phosphatases • The function of proteins (including ion channels) is modulated through phosphorylation • A phosphate group is attached to a serine, threonine or tyrosine residue • Kinases add phosphate groups; phosphatases remove them • Best known kinases: • Protein kinase A (PKA) • Protein kinase C (PKC) • Calcium-calmodulin-dependent protein kinase (cam-kinase) • Each kinase or phosphatase modifies a variety of different proteins (effectors) • The set of effector proteins depends on the kinase / phosphatase