neuroscience, 4e
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Neurotransmitters and Receptors
March 18, 2010
Neurotransmitters
Classes of Neurotransmitters
• Small Molecules Amino Acids Biogenic Amines Acetylcholine Purines
• Peptides
• Unconventional
Small-molecule Neurotransmitters
Small-molecule Neurotransmitters
Biogenic Amines
Peptide Neurotransmitters
Neurotransmitters
Synthesis
• Precursors
• Rate limiting steps
• Location (Cell types)
Inactivation
Post-synaptic receptors
• Structure
• Subtypes
Neurotransmitter Receptors
Ionotropic – This week• Electrical response to neurotransmitter binding• Large, 4-5 subunit protein forms channel• Impermeable in the absence of transmitter• Rapid onset, rapidly reversible
Metabotropic – Next week• G protein coupled receptor (GPCR)• Single polypeptide receptor• Slow onset, long duration
Terminology
Agonist• Molecule that binds to and activates receptor or
channel
Antagonist• Molecule that binds to and inhibits receptor or channel
Desensitization• Transition to closed state in presence of
neurotransmitter• Limits ion influx
Allosteric binding sites• Different than binding site of ligand• Modulates receptor or channel properties
Acetylcholine
Synthesis• Acetyl from acetyl coA is transferred to choline
by choline acetyl transferase (ChAT) ChAT is rate limiting step
• Acetyl coA precursor Derived from pyruvate (glucose
metabolism) Must exit mitochondria to gain access to
ChAT• High affinity Na+/Choline transporter moves
choline into neuron
Acetylcholine
Packaging
• Vesicular cholinergic transporter
• Moves 10,000 molecules into vesicles
Inactivation
• Primarily enyzmatic by acetylcholinesterase (AChE) in synaptic cleft
5000 molecules/sec
• Choline is conserved by re-uptake
Acetylcholine in Cholinergic Nerve Terminals
Clinical Applications
Nerve gas and organophosphate insecticides target AChE
• Removal of inactivation causes muscle defibrillation and then inactivation of muscle
Neostigmine, inhibits AChE which increases ACh in synapse
• Compensates for decreased Ach receptors due to auto-antibodies in Myasthenia gravis
Myasthenia Gravis
End-plate potentials are smaller
Improved with neostigmine
Neurotoxins that Act on Postsynaptic Receptors
Causing Paralysis:• Bungarotoxin
From Bungarus multicinctus High affinity and specificity for nAChR Used to purify receptor
• Curare Turbocurarine Used on arrow tips by South
American Indians From Chondodendron tomentosum
Neurotoxins that Act on Postsynaptic Receptors
Plant alkaloids
• Nicotinia tabacum Activates Nicotinic AChR
• Muscarine Poisonous red mushroom, Amanita
Muscaria Activates muscarinic AChR
• Stimulants, producing nausea, vomiting, mental confusion
Nicotinic Acetylcholine Receptor Structure
5 subunits form functional channel
• 2 subunits ACh binding site
• 3 other subunits In neurons, 3 subunits In muscle, combination of
subunits
Each subunit has 4 transmembrane domains
Structure of the nACh receptor/channel
Long extracellular amino terminal has ACh binding site
• Pore formed by 2nd TM domain
• 0.6 nm diameter pore opening
The structure of the nACh receptor/channel
Receptor Types
IonotropicMetabotropic
Structure of Ligand-gated Receptor Channels
Five subunits
• nAChR
• GABAA
• Glycine
• Serotonin
Four subunits
• Glutamate Receptors
Structure of Ligand-gated Receptor Channels
Some have four TM domains
Some have three TM domains and a pore loop
Subunit Subtypes of Ligand-gated receptors
GlutamateGABAA
Structure of Metabotropic Receptors
Seven TM domains
Single subunit
Intracellular segment and 3-4 loop binds to GTP binding protein
Extracellular loops 2-3 and 6-7 bind to neurotransmitter
Subtypes of Metabotropic Receptors
All but biogenic amines have ionotropic receptors
Most will be discussed next week ACh
Amino Acids
Excitatory
• Glutamate
• Aspartate
Inhibitory
• amino butyric acid (GABA)
• glycine
Major neurotransmitters in CNS
Amino Acid Transmitters
Synthesis• Derived from glucose metabolism• -keto glutarate is formed by Tricarboxylic
acid cycle• Transaminated to glutamate by GABA
oxoglutarate transaminase (GABA-T)• Glutamic acid decarboxylase (GAD) forms
GABA from glutamate
Alternative Synthesis• Glutamate is formed directly from glutamine
Glutamine produced in glia, then transported into nerve terminals
Amino Acid Transmitters
Vesicular Storage• Vesicular Glutamate transporter • GABA vesicular transporter
Inactivation is via re-uptake by glia and neurons
• 3 types of GABA transporters (GAT)• Excitatory amino acid transporters for
glutamate Glia re-synthesize glutamine from
glutamate
Glutamate Synthesis and Inactivation
Glutamatergic Neurons
Ubiquitous, excitatory transmitter
• Pyramidal neurons of cortex and hippocampus
• Granule cells of cerebellum
• Thalamus
Difficult to distinguish glutamate from aspartate
Glutamate Receptor Subtypes and Agonists
All have four subunits per channel
Subtypes distinguished by affinity of agonist
All have reversal potential of 0 mV
NMDA type Glutamate Receptors
Glycine is co-agonist
Magnesium blocks pore unless depolarized
Calcium permeates channel
NMDA type Glutamate Receptors
Mg++ blocks current below -40 mV
• Without Mg++, linear IV curve
Glycine required for current
NMDA and AMPA/kainate Receptors
AMPA has linear IV curve
AMPA is not permeable to calcium
AMPA response is faster than NMDA
Drugs Acting at Glutamate Receptors
NMDA Receptor• AP5 and AP7 bind to and block glutamate site
Hallucinogenic• Open channel blocker (Allosteric)
MK801 (dizocilpine) Phencyclidine (PCP) Become trapped when closed, difficult to
wash out
AMPA receptor • DNQX and CNQX used experimentally
Excitatotoxicity
Caused by abnormally high levels of glutamate
• Dendrites of target neurons are swollen
• Effect blocked by glutamate antagonists
Observed after ischemia, e.g. due to stroke
• Clinical trials using glutamate antagonists were disappointing
Treatment may occur too late
Synthesis, release, reuptake of GABA
Pyridoxal Phosphate derived from vitamin B6
Synthesis, release, reuptake of glycine
GABAergic Neurons
Local circuit interneurons• Cortex• Hippocampus• Striatum
Projection neurons • Cerebellar Purkinje Cells• Spiny projection neurons of striatum• Globus pallidus and Substantia Nigra pars
Reticulata
Glycine• Predominant inhibitory transmitter in spinal
cord
Ionotropic GABAA receptors
Chloride permeable channels
• Chloride influx produces IPSP
• Stop firing
• Decrease firing rate
Drugs acting on GABAA Receptors
Benzodiazepines
• Valium, Librium
• Enhances GABA currents
Barbiturates
• Phenobarbital – anti-epileptic
• Pentobarbital – anesthetic
Steroid metabolites of testosterone, corticosterone, progesterone
Drugs acting on GABA and Glycine Receptors
Strychnine
• From seeds of Strycnos nux-vomica
• Blocks glycine receptors
• Overexcitation of brainstem and cord
• Seizures
Picrotoxin
• From Anamerta cocculs
• Blocks GABAA channels
• Used experimentally
Ionotropic GABAA receptors
Two GABA binding sites
Two subunits
Excitatory Actions of GABAA in Developing Brain
Developing brain has higher K/Na/Cl transporter
• Higher intracellular chloride
Older brains have higher K/Cl transporters
• Lower intracellular chloride
Excitatory Actions of GABAA in Developing Brain
Developing brain:
• ECl is greater than AP threshold
• GABA is Excitatory
Older brain:
• ECl is lower than AP threshold
• GABA is Inhibitory
Catecholamines
Molecule with Catechol nucleus
• Benzene Ring with 2 adjacent hydroxyl substitutions plus amine group
Types
• Dopamine (DA)
• Epinephrine (Epi or Adrenaline)
• Norepinephrine (NE or Noradrenaline)
Act as neurotransmitters in CNS, PNS and hormonal function
Catecholamine Synthesis
Precursor
• Tyrosine General large amino acid transporter;
energy dependent mechanism to cross BBB
Rate limiting step
• Tyrosine Hydroxylase
• Converts tyrosine to DOPA
Biosynthetic Pathway for the Catecholamines
DOPA decarboxylase has extremely rapid action
• L-DOPA crosses BBB, rapidly converted to DA
• L-DOPA is treatment for Parkinson’s
Biosynthetic Pathway for the Catecholamines
Dopamine hydroxylase only in NE producing neurons
Phenylethanolamine N-methyltransferase (PNMT) on in Epi producing neurons
Catecholamine Storage
Vesicular Monoamine Transporter (VMAT1 and 2)
• Also used for serotonin
• Will transport other amines, including amphetamines
Blocked by Reserpine
• Depletes stores of serotonin, DA, NE
• Used to treat psychosis of Schizophrenia
Catecholamine Inactivation
Dopamine Transporter (DAT)• Binds to dopamine, transports it into pre-
synaptic terminal for re-use• Methylphenidate inhibits the DAT• Cocaine inhibits the DAT
Norepinephrine Transporter (NET)• Binds to NE and dopamine, transports
them into pre-synaptic terminal for re-use
• Tricyclic anti-depressants inhibit the NET
Catecholamine Inactivation
Degradation
• Monoamine oxidase (MAO) After re-uptake In mitochondria
• Catechol-o-methyltransferase (COMT) In cytoplasm
• Both are targets of anti-depressant drugs
Dopamine Neurons and their Projections
Substantia Nigra pars Compacta projects strongly to striatum
• Degenerates in Parkinson’s
VTA projects strongly to Nucleus Accumbans and Prefrontal Cortex
Role in reward and addiction
Norepinephrine Neurons and their Projections
Locus Coeruleus produces NE
Wide and diffuse projection
Role in
• attention
• Sleep-wake cycles
Epinephrine Neurons and their Projections
Brain: medullary epinephrine neurons
• Project to thalamus, hypothalamus, medulla
Periphery: adrenal medulla
• Part of adrenal gland
• Endocrine organ near kidneys
• Fight or Flight
Synthesis of Histamine
Produced by mast cells in the blood stream
• Role in inflammationLoaded into vesicles
with VMAT
Degradation by histamine methyltransferase and MAO
Histamine Neurons and their Projections
Role in arousal and attention
Reactivity of vestibular system
Histamine Receptors
Three types (metabotropic)
• Antagonists to H1 prevent motion sickness
• Antagonists to H2 reduce gastric acid secretion
• Diphenhydramine crosses BBB, acts as sedative
Synthesis of Serotonin
Indoleamine
• Indole structure similar to LSD
Precursor
• Tryptophan
Rate limiting step
• Tryptophan hydroxylase
Serotonin Receptors and Inactivation
Serotonin Transporter (SERT)
• Binds to serotonin, transports it into pre-synaptic terminal for re-use
• Inhibited by Fluoxetine (Prozac)
Loaded into vesicles by VMAT
Fenfluramine, MDMA, ecstatsy
• Inhibits both VMAT and SERT
Serotonin Neurons and their Projections
Regulates sleep-wake cycles
Implicated in psychiatric disorders
Only one ionotropic receptor
• 5-HT3
• Non-selective cation channel
• ER = 0 mV
Purines
Two main types• ATP: co-released by all vesicles• Adenosine: generated from ATP by
extracellular enzymes
Three classes of receptors• Ionotropic
Nonselective cation channel Two transmembrane domain
• Metabotropic Adenosine preferring
blocked by caffeine and theophylline ATP preferring
Neuropeptides
Pre-propeptides synthesized in soma (rough ER) by protein translation
Propeptide created by cleavage of signal sequence (in RER), secreted
Peptide created by processing in Golgi
• Proteolytic cleavage
• Glycosylation, phosphorylation, disulfide bond formation
• Packaging into vesicles
Proteolytic processing of pre-proenkephalin A
Large propeptides can be cleaved into multiple active peptides
Proteolytic processing of pre-proopiomelanocortin
All act on G protein coupled receptors
Neuropeptides contain 3 to 36 amino acids
Five categories
• Brain-gut: found in brain and gut
• Opioid: morphine-like activity
Neuropeptides contain 3 to 36 amino acids
Five categories
• Hypothalamic: release pituitary peptide hormones
Opioid Receptors
Distributed throughout the brain
• Co-localized with GABA and 5HT receptors
• Analgesic
• Depressant
• Behaviors: sexual attraction and aggression/submission
Involved in addiction
Unconventional Neurotransmitters
Why unconventional?
• Not stored in vesicles
• Released from post-synaptic terminals
• Act on pre-synaptic terminals
Two classes
• Endocannabinoids
• NO
Endocannabinoid Molecules
Phosphatidyl-ethanolamine is a membrane phospholipid
Endocannabinoid Molecules
Phosphatidylinositol is membrane phospholipid
Endocannabinoid Molecules
Unsaturated fatty acid with polar head group
Production stimulated by rise in calcium
Diffuse from post-synaptic neuron to pre-synaptic terminal to bind to CB1 receptors
Inhibits release of GABA neurotransmitter
Two inhibitors
Endocannabinoid-mediated inhibition of GABA
Depolarization leads to calcium influx, endocannabinoid production, inhibition of GABA release, smaller IPSC
• IPSC inhibition is blocked by rimonabant
Endocannabinoid Receptors
Receptors • Cortex, cerebellum, hippocampus• Enriched caudate putamen and substrantia
nigra Brain regions involved in addiction
Marijuana and the Brain
Marijuana acts on endocannabinoid receptors
• Active ingredient is 9-tetrahydrocannabinol
Synthesis, release, and termination of NO
NO synthase produces nitric oxide
• NO synthase activated by calcium-calmodulin
Synthesis, release, and termination of NO
NO freely diffuses through membranes to activate pre- and post-synaptic terminals
• Spontaneously decays within seconds