The Nicotine Receptor ebooksclub.org Tobacco or Health Physiological and Social Damages Caused by Tobacco Smoking Second Edition
LangleyandAnderson [1]
andDale [2]
.Detailedanalysesofthereceptoranditssubunits weremadepossibleinthewakeofstudiesoftheelectricorganoftheTorpedoandthe
identiicationofa-neurotoxins. nAChRsconsistofpentamericLGICs,arefoundinthecentralnervoussystemCNS
andinperipheralnervestructures,andcompriseana-andab-subunitwithalargenumber ofvariantsa2–a9,b2–b4,g,d,eFig.
4.1
[3,4] .Incontrasttotheircentralroleinauto-
nomicneurotransmissionandthetriggeringofmusclecontraction,nAChRsintheCNS displayseveralmodulatoryreactions
[3] .TheimportanceofnAChRsinthepathophysiol-
ogyofvariousdisorders,suchasAlzheimer’sdisease,Parkinson’sdisease,schizophrenia, Tourettesyndrome,etc.,isnowestablished
[5] .Furthermore,theyalsoplayakeyrolein
smokingcessation,analgesia,anxiolysisandneuroprotection [5]
.Importantsubtypesof nAChRarefoundinthecentralandperipheralnervoussystemandanothersubtypeis
foundinbothsystemsFig. 4.1
. ThenAChRofmammalianbrainconsistspredominantlyabout90of2a
4
and3b
2
subunits
[6] andbinds[H]-cytisineandnicotinewithhighafinity
[7,8] Table
4.1 [10]
. AnotherformofnAChRoccurringbothintheCNSandtheperipheralnervoussystemis
madeupexclusivelyofa
7
subunits [11]
Fig. 4.1
.Theasubunitscontainacysteinepair inpositions192–193oftheCloopFigs.
4.2 and
4.3 .Theseloopsaccommodatethebind-
ingsiteforagonistsandalsocontainaromaticside-chainstryptophan,tyrosine,which triggercationicp-interactionswiththeagonists
[13] .ThepolypeptidechainofthenAChR
subunitscontainsfourhydrophobictransmembranedomainsM1–M4,whichspanthe plasmamembraneFig.
4.2 .M2isana-helixthatincludesthecationchannel.
BindingafinityK
i
nM FunctionalpotencyEC
50
µM a
4b2 a
7 a
3b4 a
4b2 a
7 a
3b4 –-Nicotine
1–11 400–8,900
300–475 0.3–15
18–91 5–410
–-Cytisine 0.14–2.7
1,400–3,883 56–195
0.019–71.4 n.d. 72–134
–-Epibatidine 0.01–0.06 3.1–350 n.d.
0.004–0.02 1.1–2.2 0.009
Acetylcholine 6.8–57
4,000–10,830 560–881
0.48–3 79–316
53–210 Choline
112,000 2,380,000
n.d. n.d.
1,600 n.d.
Lobeline 4–50
11,000–13,100 480
n.d. noactiv.
n.d. Carbachol
207–582 18,000–580,000 3,839
2.5–29 296
n.d. DMPP
9.4–400 160–2,300
1,300 0.07–18
19–75 10–92
Table 4.1
Binding afinities K
i
and functional potencies EC
50
reported at brain thalamic synaptosomesa
4
b
2
-nAChRsorheterogenouslyexpressednAChRsfornicotinicagonistsatnative orrecombinanta
4
b
2
-anda
7
-nAChRsandrecombinanta
3
b
4
-nAChRs [9]
DMPPdimethylphenylpiperazine;n.d.notdetermined;noactiv.noactivation CNSPNS
CNS PNS
α4 α4
β2 β2
β2 α3
α5 α3
β4 βx
α7 α7
α7 α7
α7
Fig. 4.1
Schematic illustrationofthestructureof
nAChRsinthecentraland peripheralnervoussystem
NH
2
N-Terminal
Glycosylation
C-Terminal COOH
128 142
M1
M2 M4
Phosphorylation s
s 192-193
M3 s-s
Fig. 4.2
Structureofa nAChRsubunitsegments
M1toM4 [12]
Channel mouth and outer vestibule
Non-competitive antagonist interacting with extracellular
domain
Positive allosteric modulator
Protein-lipid Interface: Local anesthetic blockage,
allosteric steroid blockage
Channel blocking non-competive
antagonist
Modulatory sites e. g. Pi: Abnormal aggregation,
defective interaction with nonreceptor proteins
Channel inner vestibule Cytoplasmatic domain
Channel “synaptic” half
Agonist binding site
AgonistCompetive antagonist
Extracellular domain: ligand recognition sites
Ca2+Na+ K+
Fig. 4.3
Structural overview of a nAChR with one subunit removed revealing channel lumen. Demonstratedarebindingsitesforagonists,competitiveandnon-competitiveantagonistsandpositive
allostericmodulatorse.g.AChesteraseinhibitors [5]
Astudypublishedin2004inScienceanalysedtheidentityofnicotinicreceptorsubtypes suficienttoelicitboththeacuteandchroniceffectsofnicotinedependenceisunknown.
Theresearchersengineeredmutantmicewitha4nicotinicsubunitscontainingasingle pointmutation,Leu9’→Ala9’,inthepore-formingM2domain,renderinga4receptors
hypersensitive to nicotine. Selective activation of a4 nicotinic acetylcholine receptors withlowdosesofagonistrecapitulatesnicotineeffectsthoughttobeimportantindepen-
dence,includingreinforcementinresponsetoacutenicotineadministration,aswellas toleranceandsensitisationelicitedbychronicnicotineadministration.Thedataindicated
thatactivationofa4receptorsissuficientfornicotine-inducedreward,toleranceand sensitisation
[14] .
M3andM4areseparatedfromeachotherbyalongintracellularloopthatcontains centresforthephosphorylationofserinethreoninekinases
[15] .Duringbindingofthe
agoniste.g.nicotine,thenAChRsundergoallostericmodulation [16]
inwhichthereisa transitionfromtherestingconformationtoanopenstateinwhichthecationsNa
+
,K
+
and Ca
2+
aretransported.WhennAChRsareintheopenstate,agonistsareboundwithlow afinityFigs.
4.2 and
4.3 .Thepermanentpresenceofanagonistresultsinchannelclosure
anddesensitisationofthereceptor,whichbecomesrefractoryforactivationFig. 4.4
[17]
. ThevariousnAChRsubtypesdifferconsiderablyintermsofextentofdesensitisationand
recovery:thea
7
nAChRbecomesdesensitisedveryrapidly [18]
andthepermanentpresence ofanagonistresultsininactivationfollowedbyonlyslowrecovery.Theneuronala
4
b
2
nAChRisverysusceptibletoinactivationinresponsetochronicnicotineexposure
[19] .
Thetransitionsfromtheresting,openandinactivestatesarereversible,andvariousligands areabletostabilisetheconformationalreceptorstate.Agonistsinitiallystabilisetheacti-
vated open state, whereas competitive antagonists preferentially stabilise the closed restingorinactivestateFig.
4.4 .
Tables 4.1
and 4.2
presentdetailsoftheactivityofasmallnumberofagonistsand antagonistsonvariousnAChRsubtypes.
Aswellasthemusclenicotinereceptor,theneuronalnicotinereceptorhasalsobeen largelycharacterised.ThenAChRsinthevariousbrainregionsdisplaydifferencesintheir
R
Resting Channel
closed Fast msec
Nicotine binds with low affinity
mM-µM
A
Active Channel open
There are multiple desensitised states
D
Desensitised channel
closed
I
Inactive Nicotine binds
with High affintiy µM-nM
Slow sec - min
Fig. 4.4
Relationship betweenmajorstates
occupiedbyanAChR
bindingkineticstonicotineandintheirreactivitytoelectrophysiologicalstimuli [3,4,20]
. Permanentdesensitisationofthereceptormayexplaintheoccurrenceoftachyphylaxis
Fig. 4.4
.Inaddition,up-regulationofnicotinereceptorsoccursinresponsetoprolonged nicotineexposure
[21,22] .