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Receptors and transduction 1Receptors and transduction 1
References:Chapter 11 – Neuron by Levitan & Kaczmarek ORChapter 6 – Neuroscience by Purves et al
1. K. Tsuzuki and T. Ozawa. Glutamate receptors www.els.net2. Madden, D. Structure and function of glutamate receptor ion
channels (2002). Nature Revs Neurosci. 3, 91.
Dr. MV HejmadiDr. MV Hejmadi
SynapseSynapse: the point where electrical conduction is converted to chemical conduction (mostly)
The Chemical The Chemical SynapseSynapse1) AP invades
2) Ca++ influx 3) Depolarization
release of NT 4) NT diffusion
5) Ligand binding6) Depolarization7) transmitter recycling8) vesicular membrane recycling
Action potential
Calcium channels
Calcium ions
L-glutamate vesicles
L-glutamate release
AMPAR
Na+
NMDAR
Mg2+
Synaptic cleft
Presynaptic Terminal
Postsynaptic spine
Excitatory synaptic transmission by L-glutamate
GlutamateAcetylcholine etc
GABA
Specificity of responsesSpecificity of responses
Following activation by a NT, how does a neuron know what the response should be?
Depending on the type of NT (excitatory or inhibitory), key receptors transduce this signal and dictate the nature of the response
GluGlu
DADA
AChACh
Substance P
GABA
Output
• i.e. glutamatergic, GABAergic, i.e. glutamatergic, GABAergic, cholinergic etccholinergic etc
Each neuron uses one NT to send a signal Each neuron uses one NT to send a signal (usually)(usually)
Synaptic inputs to a GABA-ergic Synaptic inputs to a GABA-ergic medium spiny neuronemedium spiny neurone
Each NT can activate diverse Each NT can activate diverse receptorsreceptors
e.g. acetylcholine can activate 2 classes of receptors, nicotinic or muscarinic
Nicotinic AChRNicotinic AChR• Mimicked by nicotine
(agonist)
• Only in certain tissues (skeletal muscle, parts of the CNS and PNS)
Muscarinic AChRMuscarinic AChR• Mimicked by
muscarine (agonist)
• Cardiac muscle, cholinergic synapses of the CNS
How do you measure specificity? How do you measure specificity? Pharmacological toolsPharmacological tools
Nicotinic AChRNicotinic AChR
blocked by antagonists
-bungarotoxin
curare
Muscarinic AChRMuscarinic AChR
Blocked by antagonists like
atropine
Bungarus multicinctus
Henbane plant
Receptors categorised based on their Receptors categorised based on their transduction mechanismstransduction mechanisms
A)A) Ionotropic receptorsIonotropic receptorsIonotropic receptors are directly coupled to ion channels and are activated by neurotransmitters which open the channel pore to allow movement of specific ions, creating postsynaptic potentials
B) Metabotropic receptorsB) Metabotropic receptorsThese receptors are not directly coupled to their ion channels and transduce the signal via guanyl nucleotide-binding proteins (G-proteins) that activate intracellular second messenger pathways
FAST DIRECT
SLOW INDIRECT PSPs
Fig 5.22; Purves et al
FAST DIRECT PSPs
SLOW INDIRECT PSPs
Ionotropic receptorsIonotropic receptors
Fig 11.8; Levitan and Kaczmarek
Ionotropic receptor typesIonotropic receptor types
pentameric
glutamateglutamate
tetrameric
Assembly of either similar subunits (homomeric) or different subunits (heteromeric)
Heterogeneity of nAChRin mammalian tissues
BrainBrainaddictionanalgesia
anxietyattentioncognition
AutonomicAutonomicGangliaGanglia
cardiovasculargastro-intestinal
MuscleMusclecontraction
Ionotropic receptors - Generic structureExample:
Nicotinic acetylcholine receptor (pentameric)
SubunitsEach subunit has four hydrophobic regions
(TMI – IV)
glutamateglutamate
Ionotropic receptors-Generic Ionotropic receptors-Generic structurestructure
Unlike other iR, iGluR have a re-entrant loop inTMII domain
extracellular N-terminus and intracellular C-terminus
Long loop between TMIII and IV forms part of the binding domain with the C-terminal half of the N-terminus
The modular nature of iGluR subunits
Extracellular NTD followed by S1 - sequence between NTD and TMIS2 - sequence between M3 and M4TMIII and TMIV domains with an intervening re-entrant P loop,. C terminus intracellular
S1 and S2 half-domains form the ligand-binding domain.
The structure of this domain is shown as a ribbon diagram. It consists of two lobes (lobe I, blue; lobe II, red), separated by a ligand-binding cleft.
Lobe 1- structural unit attached to the amino-terminal domainLobe 2 - structural unit that links to M1 and M3
Madden, D. Structure and function of glutamate receptor ion channels (2002). Nature Revs Neurosci. 3, 91.
Modular structure of iGluRsModular structure of iGluRs
Emerging structural explanations of ionotropic glutamate receptor
function ROBERT L. McFEETERS and
ROBERT E. OSWALDThe FASEB Journal. 2004;18:428-438
transmembrane topology, the two extracellular domains, and the carboxyl-terminal domain. Flip/flop refers to an alternatively spliced segment that plays a role in desensitization. S1 refers to the sequence between the amino-terminal domain and M1; S2 refers to the sequence between M3 and M4. Lobe 1 is the structural unit attached to the amino-terminal domain, and Lobe 2 is the structural unit that links to M1 and M3. Note that segments of S1 and S2 are found in both Lobes 1 and 2.
Monomers associate most strongly through interactions between their amino-terminal domains (NTDs) (star in middle figure). Dimers undergo a secondary dimerization, mediated by interactions in the S2 and/or transmembrane domains (stars in right-hand figure). The crystallographically observed S1S2 dimer probably corresponds to this secondary dimerization interaction.
The 'dimer-of-dimers' model of iGluR assembly
NMDA receptor(N-methyl- D-apartate)
AMPA receptor(-amino 3hydroxy 4methyl 5propionic acid)
Kainate receptors(kainic acid)
EpileptogenesisSynaptic plasticity?
Flip/flop region-spliced variants (exon 14 –flop and Exon 15 for flip)
Modulates LTPResults in calcium entry, changes in secondary messages altered protein synthesis
Calcium as a signal
The NMDA receptor is blocked by a Mg++ ion at resting potentialThe NMDA receptor is blocked by a Mg++ ion at resting potential
Glutamate directly gates NMDA and AMPAR. NMDAR controls movement of Ca 2+ Na+ and K+
binding sites for glycine, zinc, phencyclidine (angel dust) and Mg2+.
Under resting conditions Mg2+ blocks the site and keeps the receptors silent.
Upon depolarisation the Mg is released and there is movement of Ca into the cell.
NMDA receptorsNMDA receptorsGLYCINE GLUTAMATE
MgMK801
NR1-1a/b-4a/b
NR2ANR2BNR2CNR2D
NR3A
Ca Na
calmodulin
PSD proteins
CamKIIP P
NR1 NR2
2+ +
2+
OUT
IN
P = multiple sites for phosphorylation
Zn2+ POLYAMINE
NR1
NR1
NR2NR2
?
So what are the most important NT in the mammalian brain?
• glutamate and GABA are the most abundant which mediate synaptic transmission in the CNS via ionotropic receptors (LGICs)
• In general,
• GABA - inhibitory whereas
• glutamate - excitatory
Roles of glutamate in the CNSRoles of glutamate in the CNS
• Synaptic transmission (EPSPs)• Long term depression• Long term potentiation
– Dendritic sproutin– Synaptic modification– Control of gene expression
Excess of glutamate release can cause neuronal Excess of glutamate release can cause neuronal death (excitotoxicity)death (excitotoxicity)
e.g. during stroke, epilepsy, Parkinson’s diseasee.g. during stroke, epilepsy, Parkinson’s disease
Effects of glutamate-induced excitotoxicity
Nature Reviews Neuroscience 4; 399-414 (2003
GABA receptorsGABA - major inhibitory NT in the mammalian CNS GABA receptors are pentameric in structure3 classes GABAA and GABAC receptors are ionotropic, GABAB receptors are metabotropic
Cl-
GABA
Cl-Cl-
GABA
GABAA receptor binding sites
Benzodiazepine – allosteric agonist - tranquilisers / anticonvulsantsBarbiturates – prolong action of GABA - anaesthetics / hypnotics
So why have multiple iRSo why have multiple iR??• The post-synaptic response to stimulation can be
modulated in the short term (i.e. for hundreds of milliseconds) or for the long term (hours, days or even weeks!!).
• Synaptic strength is increased or decreased by altering the level of post-synaptic depolarisation. This is achieved through changing how well receptors respond to stimulation,– by altering the length of time they are active, – the number of receptors physically present or – by altering the amount of L-glutamate that is released
into the synaptic cleft