Pharmacology
Chemical CNS Neurotransmission
Review general neurotransmission Axonal conductance Neurotransmitter (NT) synthesis Ca++-dependent release Post-synaptic receptor activation NT degradation NT reuptake and transport
Understanding CNS neurotransmission
The year 2000 Nobel Prize winning lectures provide a brief historical perspective on the research to understand signal transduction in the central nervous system by Carlsson, Greengard, and Kandel (http://www.nobel.se/medicine/laureates/2000/index.html). Chart neurotransmitter-containing pathways Localize receptors in brain slices or living animals Identify regional brain activity by electrophysiologic stimulation and recording Study animal and human behaviors Correlate cognitive, affective, and perceptual deficits with abnormal levels of neurotransmitters, CNS proteins, or physical brain abnormalities
CNS active neurotransmitters Amino Acids
Amino acids are the major neurotransmitters in the brain by abundance of molecules and number of neuronal pathways Excitatory Amino Acid Transmitters:
Dicarboxylic Glutamate and Aspartate
Inhibitory Amino Acid Transmitters:
Monocarboxyls GABA and Glycine
Glutamate
Major excitatory NT in the brain, utilized by 25-30% of neurons Precursor to GABA (major inhibitory NT) Ubiquitous throughout the brain Highest levels in cerebral cortex, hippocampus
Intro CNS page 8
Glutamate synthesis and degradation Synthesized from intermediates in the Kreb's cycle or recycled from glutamine by glutaminase
Synthesis is not rate limiting
Actively transported into synaptic vesicles for storage Ca++-dependent release into the synapse Metabolized back to glutamine in glial cells by glutamine synthetase Metabolized to GABA by decarboxylation Transaminated to glycine Glutamate Re-uptake Distinct family of transporters (EAAT1-EAAT5 cloned) with 6, 8, or10 putative transmembrane spanning domains Different subtypes exist on glial cells, neuronal plasma membranes, and vesicles
Glutamate Receptors
Both ion channels and G protein-coupled receptor subtypes
Ionotropic (ion channel) glutamate receptors
N-methyl-D-aspartate (NMDA), AMPA and Kainate-selective subtypes Composed of four heteromeric subunits Cationic channel (Na+, K+, and Ca++) opened upon ligand binding in presence of glycine NMDA (R1, R2A, R2B, R2C, R2D) Receptors
Functionally contain at least one R1 and one R2 subunit Mediates slower EPSPs Activation of NMDA receptors is essential for learning and memory NMDA receptors are involved in long lasting enhancement of synaptic transmission associated with memory (longterm potentiation)
Intro CNS page 9
Coincidence detection: Many events must occur for an NMDA channel to open 1. Glutamate binding site is extracellular and glutamate is required for channel opening 2. Glycine is a co-agonist and must be present for the channel to open (strychnineinsensitive glycine binding sites) Kynurenic acid is a glycine-site antagonist selective for NMDA receptors 3. Depolarization must also accompany binding. At rest, the cation channel is blocked by Mg++, blockade is voltage sensitive and reversed by depolarization
NMDA Channel blockers Bind inside the channel to block ion flow Phencyclidine (PCP), Ketamine, Dextromethorphan
4. Several other compounds such as Zn++ and polyamines also bind to the receptor and modulate activity
Excitotoxicity Over-activation of NMDA receptors causes cell death due to too much Ca++ influx into the neuronal cytoplasm NMDA antagonists are neuroprotective; there is active pharmaceutical interest in NMDA antagonists for protection against stroke, epilepsy, head trauma, or neurodegenerative damage. Unfortunately, many of these compounds are also highly psychogenic, like PCP AMPA-preferring Glutamate Receptors (GluR1, GluR2, GluR3, GluR4) Mediate fast EPSPs Kainate preferring Glutamate Receptors (GluR5, GluR6, GluR7, KA1, KA2) Domoic acid is a potent Kainate receptor agonist and shellfish contaminant causing amnesia
Intro CNS page 10
Metabotropic Glutamate Receptors (mGluR1-mGluR6)
G protein coupled glutamate receptors mGluR1 and mGluR5 couple through Gq to activation of phospholipase C mGluR2, R3, R4, and R6 couple to inhibition of adenylyl cyclase Long-term potentiation A means by which metabotropic and ion channel glutamate receptors cooperate to form a strengthened synaptic connection. With a brief release of glutamate, only AMPA receptors are activated and a brief depolarization occurs. With sustained glutamate release, AMPA receptors produce a sustained depolarization that opens NMDA receptors; mGluR1 or mGluR5 receptors are also activated. Increased intracellular Ca++ (from mGluR and NMDA activation) activates Ca++/calmodulin-dependent kinase II (CaM kinase II) to stimulate nitric oxide synthase (NOS) to produce NO. NO diffuses back to the presynaptic neuron to increase glutamate release in a positive feedback loop. Signal transduction is potentiated and the synapse is strengthened
GABA (g-aminobutyric acid)
Major inhibitory NT in the brain, utilized by 40% of neurons High concentration in CNS and spinal cord, low concentrations in periphery GABA agonists have analgesic, muscle relaxant (Baclofen), anti-convulsant (carbamazepine), and hypnotic/sedative (benzodiazepines and barbiturates) properties GABA antagonists cause convulsions
Important GABAergic pathways Predominantly interneurons that modulate (inhibit) many pathways in CNS and spinal cord Some projection pathways
Intro CNS page 11
GABA synthesis and degradation Decarboxylation of Glutamate to GABA by glutamic acid decarboxylase (GAD) Stored in synaptic vesicles and released by Ca++-dependent means Primary metabolism by GABA transaminase (GABA-T) localized pre- and postsynaptically in the mitochondria (similarly to MAO)
GABA metabolizing enzymes are targets for anti-seizure drugs e.g. Valproate
GABA receptors (Dr. Collins will cover this in detail) Ligand-gated ion channels (GABAA) and G protein coupled receptors (GABAB) GABAA
Ligand-gated channels ion (Cl- conductance) Composed of five heteromeric subunits (a, b, g, d, r; similary to nicotinic receptor) in a 2a, 2b, and g or d configuration Multiple subtypes of each subunit (4 transmembrane domains each) identified allowing for up to 300 different combinations Produce fast IPSPs Mediate the anti-convulsant and hypnotic/sedative effects of GABA agonists Multiple binding sites on the ion channel proteins GABA binds to the extracellular surface opening the channel for inward flow of Cl- and hyperpolarization Agonists (e.g. Muscimol) open the channel Antagonists (e.g. Bicuculline) block opening of the channel
Intro CNS page 12
Convulsant toxin interactions Picrotoxin is a potent convulsant agent Bind within the channel and block the flow of Cl- ions Act as GABA antagonists which are convulsants Benzodiazepines Benzodiazepines bind at an extracellular site separate from GABA Increase frequency of the channel opening Enhance the effects of agonists (e.g. GABA) allosterically Benzodiazepine site antagonists inhibit the effects of GABA, e.g. Flumazenil, (Romazicon) used for benzodiazepine overdose Benzodiazepine antagonists are actually inverse agonists, decrease channel opening frequency Barbiturates Barbiturates bind along the membrane portion of the receptor protein Increase the duration (not frequency) of channel opening directly (don't require GABA) Some clinical usefulness in treating acute epileptic seizures No antidote for overdose, low therapeutic index Neuroactive steroids have a barbiturate-like effect Alcohol and inhalational anaesthetics may also affect this site
Intro CNS page 13
GABA-B or GABAB
G protein coupled receptor, 7 transmembrane spanning domains Exists as a highly unusual heterodimer (GABAB1, and GABAB2) with extremely long amino termini where GABA binds Coupled to Gi/Go to inhibit adenylyl cyclase Produces an IPSP by increasing K+ conductance Present on pre-synaptic nerves termini in the CNS and spinal cord Baclofen (an anti-spasmodic and analgesic) is a selective GABAB receptor agonist Baclofen inhibits excitatory amino acid release onto motor neurons in the ventral horn of the spinal cord Baclofen decreases release of Substance P and glutamate in dorsal horn afferent neurons, inhibiting pain transmission
GABA transport Primary method of termination 12 transmembrane-spanning family of transporters common to many neurotransmitters (except glutamate) Selective for GABA on neuronal cells Glial cells and vesicles also have isotypes of GABA transporters that are less selective Major inhibitory NT in the spinal cord Glycine-selective receptors exist in addition to glycine binding sites on NMDA receptors Strychnine is a potent antagonist at glycine (non-NMDA) receptors Glycine antagonists (e.g. strychnine) cause lethal convulsions
Glycine
Intro CNS page 14
Biogenic Amine Neurotransmitters
Localized to specific tracts in the brain, unlike amino acid NTs. Acetylcholine Catecholamines: Epinephrine, Norepinephrine and Dopamine Monoamines: Serotonin, Melatonin, and Histamine
Acetylcholine (ACh)
Important peripheral neurotransmitter in the ANS (review sites of action) Review for yourself acetylcholine synthesis, release, and degradation
Cholinergic Receptors
Nicotinic receptors (Review) Ligand-gated ion channels Pentameric (5) subunit structure
Each subunit (2a,b,g, or d) composed of 4 transmembrane spanning domains 2 ACh molecules bind per ion channels (largely at a subunits)
Muscular nicotinic (Nm) receptors
Localized to skeletal muscle (a1)2b1gd pentamers a-bungarotoxin sensitive (binds to a1 subtype)
Neuron...