Neurotransmission and Neuromodulation Flashcards
Structure of a neuron
· Dendrites - recipient of information from other neurons, large receptive field
· Soma (cell body) - contains the machinery that controls processing in the cell and integrates information
· Axon - carries information (action potential) from the soma to the terminal boutons and hence to other cells. Axons can branch to contact multiple neurons
- Terminal Boutons - found at the end of the axon, location of the synapse, communication point with other neuron
neuronal membrane
· Boundary of soma, dendrites, axon and terminal boutons.
· Separates the extracellular environment from the intracellular environment
· Membrane - lipid bilaver (5nm)
· Protein structures:
- Detect substances outside of the cell
- Allow access of certain substances into the cell (gated - chemical or electrical) cytoskeletal
the synapse
· Types of synapse:
1. Electrical synapse:
- very rare in adult mammalian neurons (e.g., found in retina)
- Junction between the neurons is very small (3nm - gap junction)
- Gap is spanned by proteins (connexins) which are used to communicate between the neurons (ions move freely)
2. Chemical synapses:
- Common in adult mammalian neurons
- Junction between the neurons 20-50nm (synaptic cleft)
- Chemicals (neurotransmitters) are released from the presynaptic neuron to communicate with the postsynaptic neurons.
Early experimental evidence for chemical transmission:
· First demonstrated by Loewi in the 1920s
· Application of fluid following vagus nerve stimulation slowed down heart rate
· Substance was ‘sufficient’ to change heart activity
- Acceptance as primary means of communication in the brain in ’60s
Overview of chemical transmission:
- Neurotransmitter (NT) synthesis, transport and storage
- Depolarisation (action potential)
- Open voltage-gated Ca2+ channel
- Ca2+ influx
- Movement and docking of vesicles
- Exocytosis-diffusion
- And 8 interact with receptors
· In/deactivation of NTs
Neurotransmitters:
· Chemical that is used to transmit information from the presynaptic neuron to the postsynaptic neuron
· Criteria for neurotransmitter:
1. Chemical synthesised presynaptically
2. Electrical stimulation leads to the release of the chemical
3. Chemical produces physiological effect
4. Terminate activity
Postsynaptic action of the neurotransmitter:
- Neurotransmitter binds to receptors on the postsynaptic membrane, which affects the activity of the postsynaptic cell.
- The configuration of the receptors make them specific for different neurotransmitters.
- Ionotropic receptor - opening of an ionic channel (typically)
- Metabotropic receptor - activates an internal 2nd messenger systems that goes on to affect the functioning of the postsynaptic cells.
Action of neurotransmitters at receptors:
- Pharmacology - what transmitter binds to the receptor and how drugs interact
- Receptors vary in their pharmacology:
- Agonist - a drug (or endogenous ligand/neurotransmitter) that can combine with a receptor on a cell to produce a cellular reaction.
- Antagonist - a drug that reduces or completely blocks the activity of the agonist or endogenous ligand, no cellular effect after interacting with receptor.
Actions of neurotransmitters at receptors 2:
· Receptors vary in their:
- Kinetics - rate of transmitter binding and channel gating determine the duration of effects
- Selectivity - what ions are fluxed (Na+, Cl-, K+ and/or Ca2+)
- Conductance - the rate of flux
Ionotropic receptors:
· Fast transmission - ion movement leads to an immediate change in the postsynaptic cell
· Excitatory fast transmission:
- Ion channels opens
- Movement of positive ions into the neurone (Na+)
- E.g., glutamate receptors
- Depolarisation
- Excitatory post synaptic potential (EPSP)
· Inhibitory fast transmission:
- Ion channels opens
- Movement of negative ions into the neurone (Cl-)
- E.g., GABAa receptors
- Hyperpolarisation
- Inhibitory post synaptic potential (IPSP)
Metabotropic receptor:
· Activation of a G-protein coupled receptor:
1. Neurotransmitter binds to receptor and activates the G-protein (exchange GDP for GTP)
2. G protein splits and activates other enzymes
3. The breakdown of GTP turns off G protein activity
4. Series of chemical reactions that leads to an amplification of the signal - second messenger system
Amplification:
- Example of amplification of signal through G-protein coupled receptor activation
Neurotransmitter deactivation:
- Neurotransmitters must be inactivated after use to remove them from the synaptic cleft.
Other ways of regulating synaptic transmission - autoreceptors:
- Located on the presynaptic terminal
- Respond to neurotransmitter in the synaptic cleft
- Generally they are G-protein coupled - don’t directly open ion channels
- Regulate internal process controlling the synthesis and release of neurotransmitter
- Negative feedback mechanism
Neurotransmitters :
- Major classes
- Integration of excitatory and inhibitory signals
Fast synaptic transmission:
- Glutamate - ionotropic receptors in general flux Na+ which causes an EPSP (excitatory post synaptic potential) depolarising the postsynaptic neuron - excitatory
- GABA - ionotropic receptors flux Cl-, which causes as IPSP (inhibitory post synaptic potential) hyperpolarising the postsynaptic neuron - inhibitory
- Acetylcholine, serotonin and ATP also activate ionotropic receptors
Glutamate:
- Major fast excitatory neurotransmitter in the CNS
- Very widespread through the CNS
- Activates different types of receptors - mGluR, NMDA, AMPA, Kainate
Glutamate synthesis, storage, release and reuptake:
- Synthesised in nerve terminals from glucose or glutamine
- Loaded and stored in vesicles by vesicular glutamate transporters
- Released by exocytosis (Ca2+ dependent mechanism)
- Acts at glutamate receptors on postsynaptic membrane
- Reuptake by excitatory amino acid transporters (EAATs) in the plasma membrane of presynaptic cell and surrounding glia
Glutamate receptor diversity:
- Based on their pharmacology, three types of ionotropic receptor have been described that respond to glutamate - NMDA, AMPA, and Kainate
- They are named based on the agonists selective for them
AMPA receptor:
- Ionotropic receptor
- Binding of glutamate leads to the opening of a Na+ channel (slight K+ permeability) and hence depolarisation
- Selective agonists - AMPA
- Antagonists - CNQX, DNQX
NMDA receptor:
- Ionotropic receptor
- Permeable to Na+, K+ and Ca2+
- Binding of glutamate - nothing happens
- Voltage dependent blockade:
- At resting membrane potential (-65mV):
- Glutamate binds
- Channel opens
- Blocked by Mg2+
- Depolarised membrane (-30mV):
- Mg2+ pushed out of pore
- Channel is open
- Ion movement
- Further depolarisation
· Different ‘kinetics’ from AMPA receptor (open much longer)
Selectivity and conductance of glutamate receptors:
· AMPA (and kainate) receptors:
- Fast opening channels permeable to Na+ and K+
· NMDA receptors:
1. Slow opening channels - permeable to Ca2+ as well as Na+ and K+
· BUT also
2. Requires glycine as a cofactor (no glycine, no activation)
3. And gated by membrane voltage
· NMDA receptors are only activated in an already depolarised membrane in the presence of glutamate
NMDA receptors - dysregulation:
· NMDA receptors and schizophrenia?:
- NMDA receptors also blocked by phencyclidine (PCP, angel dust) and MK801 which both bind in the open pore
- Blockade of NMDA receptors in this way produces symptoms that resemble the hallucinations associated with schizophrenia (associated with reduced NMDAR function)
- Certain antipsychotic drugs enhance current flow through NMDA channels
· Glutamate excitotoxicity:
- Caused by excessive Ca2+ influx into the cell which activates calcium dependent proteases and phospholipases that damage the cell
- This kind of cell damage occurs after stroke and chronic stress
GABA:
· GABA (gamma aminobutyric acid)
· Major inhibitory neurotransmitter
· Activates an ionotropic receptor (GABAa receptor) which opens a chloride channel (Cl-) leading to hyperpolarisation (IPSP)
GABA synthesis, storage, release and reuptake:
- GABA is synthesized from glutamate
- GABA is loaded and stored into synapses by a vesicular GABA transporter
- GABA released by exocytosis (Ca2+ dependent mechanism)
- GABA acts at ionotropic GABAA and metabotropic GABAB receptors on postsynaptic membrane
- GABA cleared from synapse by reuptake using transporters on glia and neurons including non-GABAergic neurons
GABA receptor diversity:
- Two main families of GABA receptor:
- GABAa ionotropic receptors:
- Ligand gated Cl- channel
- Fast IPSPs
- GABAb metabotropic receptors:
- G protein coupled receptors
- Indirectly coupled to K+ or Ca2+ channel through 2nd messengers
- Opens K+ channel, closes Ca2+ channel
- Slow IPSPs
Regulation of amino acid transmitter release:
· Too much glutamate / too little GABA
- hyperexcitability – epilepsy
- excitotoxicity
· Too much GABA
- sedation/coma
- (At right dose, drugs which increase GABA transmission can be used to treat epilepsy)
· Cerebral ischemia
- the metabolic events that retain the electrochemical gradient are abolished
- reversal of the Na+ K+ gradient
- transporters release glutamate from cells by reverse operation
- excitotoxic cell death.
· GHB gamma-hydroxybutyrate (date rape drug)
- a GABA metabolite that can be converted back to GABA by transamination
- Increases amount of available GABA
- Moderate dose like alcohol, but too much leads to unconsciousness and coma
Unconsciousness, coma, and death also from excessive alcohol:
· Quite drunk - BAC of 0.2 - 0.3
· Death - BAC of 0.35 - 0.5
· But typically one passes out before ingesting lethal dose
GABAa receptors and drugs:
· Complex receptor with multiple binding sites
· Drugs binding at GABA binding site
· Muscimol – agonist
· Bicuculine, picrotoxin – antagonist
· “They eat certain fungi in the shape of fly-agarics, and thus they become drunk worse than on vodka, and for them that’s the very best banquet.” -
· Drugs binding elsewhere on the receptor (no competition with GABA)
- Benzodiazepine
- Barbiturates
- Ethanol
- Neurosteroids
· net result more inhibitory Cl- current, stronger IPSPs and behavioural consequences of enhanced inhibition
Drugs increasing GABA activity reduce anxiety (anxiolytic):
· Agonists - alcohol, barbiturates
· Indirect agonist - benzodiazepines (BDZ)
Drugs decreasing GABA activity increase anxiety (anxiogenic):
· Antagonist - flumazenil
· These drugs all act at the GABAa ionotropic receptor
Barbiturates:
· Problems:
a. General (non-specific) depression of neuronal activity - includes vital functions like breathing
b. Poor therapeutic ratio. Small difference between therapeutic dose and overdose. High suicide risk in emotionally unstable patients.
c. Long-term treatment leads to dependence and withdrawal
d. Thus, only used for severe insomnia, seizures.
Benzodiazepines:
- Discovered in 1960s
- First benzodiazepine was chlordiazepoxide (Librium)
- Shortly after that diazepam (Valium) became the major treatment for anxiety disorders
- Act as:
- Anxiolytic
- Anticonvulsant
- sedative
- Muscle relaxant
- amnestic
- Advantages:
- Good, fast acting anxiolytics
- Large therapeutic window
- Disadvantages:
- May cause dependence
- Effects potentiated by alcohol
Other classical neurotransmitter:
· Dopamine
· Serotonin
· Acetylcholine
Neurotransmission vs neuromodulation (rule of thumb):
· Primary neurotransmitters
- Glutamate and GABA - the main workhorses of the brain
- Directly mediate the transmission of information between neurons either via activation (excitation, EPSPs) or inactivation (inhibition, IPSPs) of post-synaptic targets
· Some neurotransmitters are ‘Neuromodulators’
- Affect the response properties of a neuron (e.g. release, excitability) - do not carry primary information themselves.
- e.g. dopamine, serotonin, noradrenaline, acetylcholine (others: histamine, neuropeptides)
The diffuse modulatory systems:
· Specific populations of neurons that project diffusely and modulate the activity of glutamate and GABA neurons in their target areas
The dopaminergic system:
· Dopamine neurons
- cell bodies in the midbrain
- project into the forebrain
· Nigrostriatal system (substantia nigra projections to neostriatum (caudate and putamen) role in movement.
- dysfunction:
- Parkinson’s disease - destruction of DA projections from SN to basal ganglia
- Huntington’s disease - destruction of DA target neurons in striatum
· Mesolimbic system (ventral tegmental area projections to nucleus accumbens (NAcc) role in reinforcement (reward)
- dysfunction:
- Addiction - most drugs of abuse lead to enhanced DA release in the NAcc
· Mesocortical system (VTA projections to prefrontal cortex) role in functions such as working memory and planning.
- dysfunction: Schizophrenia
1 - dopamine synthesis:
· Tyrosine - essential amino acid obtained in diet
- Catalysed by tyronsine hydroxylase (TH)
- Rate limiting step (or slowest step)
· L-dopa
- Catalysed by dopa decarboxylase
· Dopamine
2 - Catecholamine storage:
· Loaded into vesicles
Drugs that affect dopamine synthesis and storage modulate behaviour:
· Reserpine impairs storage of monoamines in synaptic vesicles. (The vesicles remain empty resulting in no transmitter release upon activation)
· L-DOPA, the precursor of dopamine, is used as a treatment for Parkinson’s disease. (Bypasses rate-limiting TH step – Dopa decarboxylase converts it into dopamine increases the pool of releasable transmitter)
· AMPT (a-methyl-p-tyrosine) inactivates TH (used experimentally – not in treatment)
· Role and importance of neurotransmitter systems in behaviour revealed by drugs (e.g. Reserpine was used to treat high blood pressure but caused depression)
3 - dopamine release:
· depolarization of presynaptic membrane
· influx of Ca2+ through voltage gated Ca2+ channels
· Ca2+ dependent vesicle docking and Release (Ca2+ dependent exocytosis)
5 - Dopamine reuptake / metabolism
· Signal terminated by reuptake into the axon terminal by transporters powered by electrochemical gradient (Dopamine transporters (DATs))
· in the cytoplasm dopamine is:
- reloaded back into vesicles
- enzymatically degraded by Monoamine oxidases (MAOs) or Catechol-O-methyl-transferase (COMT)
Drugs that affect dopamine release and reuptake modulate behaviour:
· Cocaine, Amphetamine and Methylphenidate (Ritalin) - psychostimulants
- all block the reuptake of monoamines into terminals. More dopamine in synaptic cleft
- extended action of dopamine on postsynaptic neuron.
- (Amphetamine reverses transporter so pumps out transmitter - uncontrolled release)
· Selegiline – Monoamine oxidase B inhibitor
· Entacapone - COMT inhibitor
- prevent the breakdown of catecholamines,
- increases the releasable pool
· These drugs can have antidepressant activity and can be used for treating Parkinson’s
The serotonergic system:
· Nine raphe nuclei (in brain stem) with diffuse projections
- each projects to a different part of the brain
· Descending projections to cerebellum and spinal cord (pain)
· Ascending projections (reticular activating system (with Locus Coeruleus))
· Dorsal and medial raphe project throughout the cerebral cortex
· raphe neurons:
- fire tonically during wakefulness
- quiet during sleep
· Function in:
- mood
- sleep
- pain
- emotion
- appetite
1 - serotonin synthesis:
· Tryptophan - essential amino acid
- Rate limiting
- E.g., tryptophan, hydroxylase
· 5-hydroxytryptophan (5-HTP)
- E.g., aromatic amino acid, decarboxylase
· Serotonin (5-hydroxytryptamine, 5-HT)
· Tryptophan and mood:
- Depletion diet - method of experimentally inducing a depressive state
- Enrichment - improving mood
2 - Serotonin storage:
· Loaded into vesicles
3 - Serotonin release:
· Ca2+ dependent exocytosis
5 - Serotonin reuptake / metabolism:
· signal terminated by reuptake by Serotonin transporters (SERTs) on presynaptic membrane
· degraded by MAOs in the cytoplasm
Drugs that affect serotonin release and reuptake modulate behaviour:
· Fluoxetine (Prozac) blocks reuptake of serotonin (SSRI – selective serotonin reuptake inhibitor) – antidepressant / anti-anxiety
· Fenfluramine causes the release of serotonin and inhibits its reuptake (has been used as an appetite suppressant in the treatment of obesity)
· MDMA 3,4-methylenedioxy-methamphetamine (ecstasy) causes noradrenaline and serotonin transporters (SERT) to work in reverse releasing neurotransmitter into synapse/extracellular space
· Monoamine oxidase inhibitors (boost monoamines)
The cholinergic system:
· In the periphery
- Acetylcholine (ACh) at neuromuscular junction (NMJ) and synapses in the autonomic ganglia
· In the brain:
- Basal forebrain complex
- Projections to hippocampus and cortex
· Pontomesencephalotegmental complex
- cholinergic link between the brain stem and basal forebrain complex
Cholinergic interneurons in the striatum and the cortex:
· Each interneuron innervates 1000’s of logical principal neurons and modulates their activity
Acetylcholine:
- Synthesis:
· made from choline amount of choline is rate limiting step- Storage:
* loaded into vesicles - Release:
* Ca2+ dependent exocytosis - 5 - Metabolism:
* rapidly degraded in synaptic cleft by acetylcholinesterase
* Choline is transported back into the presynaptic terminal and converted to acetylcholine
* (acetylcholinesterase is made by the cholinergic neuron, secreted into synaptic cleft and associated with the axonal membrane)
- Storage:
Drugs affecting acetylcholine release, storage and degradation modulate behaviour (or neurological function):
· Acetylcholinesterase inhibitors
- block the breakdown of ACh thus prolonging its actions in the synaptic cleft
· e.g. Physostigmine
· treatments for Alzheimer’s disease, Myasthenia gravis (autoimmune disorder, AchR’s destroyed)
Drugs affecting vesicle docking and release:
· ACh release at neuro-muscular junction, NMJ
· Botulinum and tetanus toxins
- (from bacteria Clostridium botulinum and tetani respectively)
- blocks the docking of vesicles by attacking SNAREs- no release
· Botox acts directly at synapse in NMJ
- The muscles lose all input and so become permanently relaxed.
Drugs affecting vesicle docking and release
· (ACh release at Neuro-Muscular Junction, NMJ)
· Tetanus toxin is retrogradely transported up at NMJ and works at inhibitory (Glycine) synapses on cholinergic motor neurons of spinal cord
- Also attacks SNARE proteins
- (Inhibiting the release of Glycine at these sites “disinhibits” the cholinergic neurons so they continuously fire resulting in permanent muscle contraction, ‘lock jaw’)
Disorders of the cholinergic system:
· Peripheral
- Myasthenia gravis - Autoimmune disease - destroys cholinergic receptors in the muscle - muscle weakness and eventual loss of muscle activity
· Brain
- Alzheimer’s disease
- Loss of cholinergic neurons in the basal ganglia - possibly underlies deficits in memory associated with the disease.
- →Drugs that increase acetylcholine help (e.g. donepezil)
· Addiction
- nicotine addiction
· Other psychiatric disorders
- Schizophrenia (Comorbidity with smoking)
