Neuroscience, 4e

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