1. Academic Half-Day The Chemical Basis for Neuronal Communication
Marie-Pierre Thibeault-Eybalin, R4
November 5th, 2008

2. Introduction 100 billion (1011) neurons in the brain
Up to 100,000 terminal contacts / neuron
1016 connections between neurons / brain
Connections = Synapses
Chemical messenger is released at pre-synaptic membrane of axon or dendrite terminal
It travels across synaptic cleft
It binds onto its receptor on post-synaptic membrane of other neuron
It activates effector system
Chemical messenger may be released at non-synaptic locations to influence distant neurons

3. Criteria to define chemical messenger as neurotransmitter
Localization: A putative neurotransmitter must be localized to the presynaptic elements of an identified synapse and must be present also within the neuron from which the presynaptic terminal arises.
Release: The substance must be shown to be released from the presynaptic element upon activation of that terminal and simultaneously with depolarization of the parent neuron.
Identity: Application of the putative neurotransmitter to the target cells must be shown to produce the same effects as those produced by stimulation of the neurons in question.

4. Examples of neurotransmitters

5. Synaptic transmission
Variable synaptic delay from pre-synaptic neurotransmitter release to excitation or inhibition of post-synaptic neuron
Synaptic delay depends on complexity of transduction mechanisms at post-synaptic membrane
Synaptic delay
Receptor
Neurotransmitter
Effector system
Fast
Few msec
Ligand-gated ion channel
Small molecule
Flux of ions to generate transmembrane electrical potential (EPSP, IPSP ± AP if reach threshold)
Slow
Hundreds of msec
G-protein-coupled
Neuropeptide
Indirect effect on ion channel
Enzyme activation to produce 2nd chemical messenger (intracellular)

6. Sequence of events

7. Regulatory mechanisms
To regulate amount of neurotransmitter release
Pre-synaptic receptor-mediated autoregulation
Neurotransmitter in synaptic cleft binds to pre-synaptic receptor
Inhibitory feedback mechanism
Retrograde transmission
2nd chemical messenger diffuses from post-synaptic to pre-synaptic membranes, e.g. NO

8. Secretory vesicles Small = Synaptic vesicles
For small molecules
Synthesized within vesicles, e.g. NE or uploaded by high-affinity ATP-proton-coupled transporters in terminals, e.g. ACh
Recycled
50 nm diameter
Cluster in active zones
Large dense-cored vesicles
For neuropeptides, "built-in" in neuronal soma ± co-stored small molecule
Not-recycled
nm diameter
Found in intraneuronal locations + terminals, less numerous
Neurosecretory vesicles
Hypothalamic neuron terminals in neurohypophysis
For neurohormones
nm diameter

9. Secretory vesicles

10. Exocytic release
Fusion pore
Docking complex

11. Signal transduction Most receptors are transmembrane glycoproteins
Binding of neurotransmitter to receptor induces conformational change
4 transduction mechanisms
Ligand-gated ion channels
G-protein-coupled receptors
Enzymes e.g. tyrosine kinase
Ligand-dependent regulators of nuclear transcription e.g. testosterone
Receptors often named after family of neurotransmitters they bind e.g. cholinergic and adrenergic receptors
Multiple subtypes based on response
Nicotinic ACh receptors usually excitatory
Muscarinic ACh receptors usually inhibitory
Individual neurotransmitter family members of have different potency
Rank order of potency according to EC50%
Concentration of individual neurotransmitter required to reach 50% of maximal response expected
The same neurotransmitter may have excitatory or inhibitory responses depending on receptor type

12. Structure of neurotransmitter receptors
Ligand-gated ion channels
Multiple subunits = transmembrane glycoproteins connected via intra-and extra-cellular loops
Cylindrical
Binding site in transmembrane portion
Conformation changes opens gate inside channel
Selectively pass small ions
2 genetic families based on AA sequence homology
Nicotinic ACh, serotonin, GABA, glycine
Glutamate

13. G-protein-coupled receptors
Glycoprotein chains with multiple transmembrane loops
-helices
β-pleated sheets
Binding site in transmembrane or extra-cellular portion
3 components
Receptor
GTP-binding heterotrimer
Effector protein (enzyme or ion channel)
Examples
Rhodopsin
Odorants
Biogenic amines
Bioactive peptides
β2-adrenergic receptor
      
                 

14. G-protein action

15. Examples of effector proteins
Enzyme
2nd intracellular messenger
Adenylyl cyclase
cAMP
Guanylate cyclase
cGMP
Phospholipase C
IP3 and DAG
Phospholipase A2
Members of the eicosanoid family

16. Receptor regulation Desensitization
Reduction in receptor agonist-induced response after seconds to minutes of stimulation mediated by conformational changes
Homologous
Heterologous
Phosphorylation of intracellular portion of receptor altering its binding affinity
Downregulation of receptor number at post-synaptic membrane
Internalization of receptor by invagination of post-synaptic membrane

17. Maintenance of synaptic environment
To reduce or eliminate neurotransmitters in synaptic cleft
Enzymatic degradation
ACh cleaved by acetylcholinesterase
Neuropeptides degraded by peptidases
Transporter-mediated reuptake of small molecules (not neuropeptides) by pre-and post-synaptic neuron or glia (extraneuronal monoamine transport; EMT)
NET for norepinephrine
DAT for dopamine
SERT for serotonin
After reuptake, neurotrasmitter either recycled or degraded by mitochondria (MAO)
COMT for norepinephrine

18. Pharmacologic modification of synaptic transmission
Drugs may affect:
Neurotransmitter synthesis
Vesicular uptake and storage
Depolarization-induced exocytosis
Neurotransmitter receptor binding
Termination of neurotransmitter action
Post-synaptic effector system

19. Metyrosine for
pheochromocytoma
-Methyldopa
VAMT Reserpine
MAO inhibitors
Yohimbine
Guanethidine
Cocaine
Propranolol

20. Synopsis of clinical points
Many drugs function by altering chemical transmission at the synaptic cleft.
Neuropeptides play a role in the body's response to stress.
Some drugs must traverse the plasma membrane to access receptors.
Epinephrine is used in cardiopulmonary resuscitation and to treat anaphylactic reactions.
The excess production of catecholamines, seen in tumors such as pheochromocytoma, can be treated by the drug metyrosine.
Reserpine is sometimes used to treat hypertension.
Reserpine may precipitate Parkinson-like symptoms or galactorrhea, or worsen clinical depression.
α-Methyldopa is effective for managing hypertension during pregnancy.
The side effects of guanethidine include reduced heart rate, nasal congestion, and orthostatic hypotension.
Propranolol is used in the management of angina pectoris, hypertension, and congestive heart failure.
Yohimbine may be effective in treating male impotence of vascular or psychogenic origin.
Amphetamines enhance motor performance and relieve fatigue; these are habit-forming if used inappropriately.