DTIC ADA242652: Free Radicals Accelerate the Decay of Long-Term Potentiation in Field CA1 of Guinea-Pig Hippocampus

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Peilmar et al.
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OFFREPORT OF THISP AGE OFA BSTRACT ABSTRACT
I
UNCLASSIFIED UNCLASSIFIED
NSN7 540401-2WO5500 Standard Form 298, (Rev.2 .89)
hwr~4" bvANdSS .1
X- ornr, Vol u. yNo2 .p p 35-39. t99t uoc-.us22Sjo3iS C00+0 0
Pomrdin ( treaBt ta IOA RMEF ORCERSA DIOBOLeGIPruso rs
IaafanaitEPuOT IBRO
SR138
FREE RADICALS ACCELERATE THE DECAY OF
LONG-TERM POTENTIATION IN FIELD CAI OF
GUINEA-PIG HIPPOCAMPUS
T. C PELLMAR*t G. E HOLLINDEN*.* ard J. M. SARVEY§
*hysilogy DepartmentA. rmedF orcesR adiobiology ResearcIhn stitute. Bethesda.
MD 20889-514U5,. SA .
§Department of PharmsacoloUgyn.if orned ServiceUs niversity of the HealthS ctencesB.e thesda.
MD 20814-479U9,S A
Abstract-Fee radicalsh aveh eroi mplicatedin a nunmbeorf pathologicaclo nditions To evaloareth e
neuroprysiologiccaolnl sequences of freer adicael xposures,li ceso f hippocampuisso latedfr omg uinea-igs
%seree xposetdo hydrogenp eroxides shich reactsw itlht issueir on to generatehy droxyl free radicals
Long-termp otentiation, a sustaineidn creasein synapticr esponsews as elicitedin fieldC AI by high
frequencys timulationo f ana fferenpt athwayW e foundt hat0 002% peroxidedi d not directlya ffeett he
responseesv ohedb %s tirrnx!.ivoon Lc siferentp athwayb ut did preseotm aintenancoef long-term
poteotiation Sb--.-rs potentiation and paired-pulsefa cilitation were not affectedb y peroxide
treatmentP eroxidwe asl esse ffectisief removefdo llowing highf requencsyt imulaion andw asI neffective
if appliedo nlya fterh ighf requencsyt imulation Input/output analysiss howetdh att hei ncreasien synaptic
efficacyw asr educewd ith peroxidetr eatmentC hangeisn the enhanceadb ility of the synapticp otential
to generatae spikew erel essa pparent
Thesed ata showt hat thei nterferencoef freer adicalsw ith long-termp otentiatianm ayc ontribute to
pathologicadle ficitsI t is possiblteh at intracellular calciumre gulationis disruptedb yp eroxidetr eatument
A numbeor f seconmd essengseyr stemisn solvedw ith lung-teempo tentiation arep otentiatla rgetsfo r free
radicaal ttack
Lungtercan potentiation (LTP) is a persistent increase reactive frce radical Hydroxyl radicals attack
in munosynapttc efficacy following a high frequency membrane lipids and cellular proteins. which dis-
train Becauset he potentiation can last fur hours ur rupts cell function Exposure of a hippocampal
esend ays in rIto,"' this eleetrophystolugical phenom- slic to peroxide (>-0005%) decreases synaptic
enon has beenc onsidered to hea correlate of mercury responses,d ecreaseso rthodromic spike generation"'
and learning. The biochemical changes that underlie and increases spike Frequtencayd aptation " Freec ad-
LIP are complex. possibly invulving a number of teal scavengers (dimelhylsulfoxide. Trolus-C) and an
second messengesry stems 11M.2s-ir2 on checlator( defecrosammrper)e vent most of the
Free radicals andi active oxygen conmpounds peroxide damage, suggesting that hydroxyl radicals.
(c g. peroxide, superoxide and hydroxyl radicals) are and nut the peroxide itself, are the reactive oxygen
normally generated with cellslar metabolism but speciesC olton ei a! ' useda similar model and found
are well controlled by intrinsic enzyme systemsst hat peroxide reduced the potentiationt occurrtng
and anliuxidants '10i"" Under certain pathologtcal 15min after high freqor-scy stimulation
conditions, this delicate balance can be disrupted This paper examines the actions of free radicals
Free radicals aret hought 10 contrihute soa numeber on LTP
of diseases such as isehemic, injury. aluminum
toxicity, Alzheimer's disease and Down's syn. EXPERIMEFNPTRAOL.C EDUIRES
drome" '11 " all of which affect cognitive pro. Male Hlartley (Itarlau Sprague-DawleIyn.c . Indiana-
cesses polis. IN) guinea-pig(s2 50-300g)w erea nesthetizewdi th
Previous studies have shown that free radicals isofluranea nde uthanirebd yc ervicadl islocationT heb rain
can interfere with neuronal clcerophystolugy 21m a wasr emoveadn dc hilledb ys ubmersioinn seec olda rtificial
Hlydruxyl radicals can be generated itt tisr cerebrospinfalul id( aCSFN aCt1 24m ht. KCI3 m M. CaCti
24mM. MgSO,I 3mM. KPO, 1 24. NalIC0, 26mM.
through the Fenton reactton, peroxisdere acts with glucosel~ mM, equilibrated w5ith 95%/ ri5%CO) Htippo-
tron intrtnsic to the tissue to produce this very camp %ere dissectedo ur, slicedo n a Mcllwain tissue
chopperto a nominatlh icknesosf 415p mann di ncubateda t
- -~--- ah oldingc hambeart reamt emperaturfeo ra t leas9t 0min
fiTo whomc orrespondenscheo uldh ea ddressed Peroxides olutionsw erem adef resh daily from 50%
tPresen addressP atenat ndT rAdemarOk ffive.G roup 120, concentrate(F isher) Most experimentuss eda peroxide
2011J effersoDn avis IlighuvayA.r lington. VA 22202. concentratioonf 0002% (72OPMsP) reviousst udmes"Sox
US A hippocampasl licesu sedc oncentratins between0 005%V
Abbet-iaimvna CSF.a rtifivial terebeospinal fluid. It15. (180mMa) nd0 010 / (3 6mM) The0 0 02% concentration
nigh frequencys timulation.1 /0. input/output. LTP. wasc hosenf or the presenst tudy becausea t this lesel
long-termp otentiation.P SP. postsynapt potential. Peroxidhea dn o directe ffectso n electrophysiotogiP'Ainl to-
SIP, short-termp otentiation tials in the brain slice Thesec oncentrationosf peroxide
353
p --.
354 T. C PEr.Luaeinta !
arc comparable to thoseu seda s a modelo f freer adical ageo f thef irstp opulation spikFea cilitation vasev aluated
damaget o leectrophysiologiceavle ntsi n cardiact issue inn ormal aCSFa nd 30mm after perfusionw ith 001%
(O0,oM-IO tMMl)l- peroxide
For electrophystological recordinga, slicew asp lacedi n Data are expresseads mean %alues± standard errors
a lamirar flow submersioc hamber( Z'zued esign)"a nd Student'sc -teswt asu sedf or comparsons of too predeer-
perfused with aCSFat 30 C. A bipolar stmulating electrode msd setso fdata Analyss of'variance wasu sedto evaluate
was postoned in thes tratumr adiatun of fieldC AI to differenceasm ong moret han too treatmengt roupsF or all
stimulaatffeer ent pathwaysG lassm crorlectrodes (2N staistical analysesa, probability leselo f P<005 was
NaCI)w erep lacedi nt hes radiatumo f fieldC AI to record consideretdo reflects ignificance
thea fferenvt olley and thep opulastopno stsynaptpica ten.
Ial (populaton PSP)a ndi n the s pyramidalteo record
Shep opulation spike The magnitudeo f thep opulation
spikew as calculatedfr om them eano f thee arlya nd late RESULTS
posiv.ties minust he maximaln egativtty. The population Peroxide at a concentration of 0002% had no
PSP sias quantifiedf ron tiem asimum negatve slope
earlyi n thes ynapic rsponse Signalsf rom shem icso- direct effecto n thea mplitude of the population spike.
electrodemse rer ecordeds ith WPI high gain d c amplifiers even after a Xt mm exposure (Fig. I) (n u 4) Yet. this
andt hn digitizeds, toreda nda nalysed on an LSI 11/23 concentration of peroxide signtficantly impaired the
minicomputer. ability of HFS to produce LTP In untreated slices
For 30mm, baselinere cordingssi ere madet o ensure (n - 17)I,I FSc auseda n immediatien creasien the
stablit! of thet issUeI f ther ecordings devtatseudbs tantially
from the initial valueso r developesde condarpyo pulation population spike to 285± 14% of control amplitude
spikes, thee xpenment wast erminated. Thes lices nass timu- Within IS mi, this amplitude fell to 238 ± 13% of
lted (0 2 Hiz)a t an itensity sufficientt o producea half control and sustained that levelf or the remainder of
maximal responseA verageosf four response%s eres tored thee xperiment All 17 slies shooed potentiation of
at 5-mrinn tervals
Inm oste xperimentsp,e rfusiono f peroxide% iass tarted at least 130% of control 60 rmin following HFS In
aftetrh iesq uilibration peinol qndc ontinuedfo r the dur slices treated with peroxtde (i 16). there was also
ation of thee xperimenAt fter 30mm,a highf requency an early enhancement of the amplitude (270± 15%)
stimulus( IFS. 100li z. Is) was delhcred at the half which was not significantly different from poten.
maximasl timulusa mplitudeF ollowing |IFSd,a ta collec.
tion continuedfo r anothe6i0 mm In moste xpenments, tiatton in untreated slices (t-test, P >0 05) In con.
datac ontinuedto be storede very5 min In expenments used trast to untreated tissue. however, by 30mi the
for the exponentiaclu rsef itting. data mcrec ollectedm ore amplitude was only 185± 13% and by 60 mm only
frequentlayn d% eren ot aseraged. is ordert oallowaccurale 124± 14% The amplitude of population spikes in
representatioonf the limc ourseI ns omee xpenimenttsh. e peroxide-treated slices did not establish a plateau
perfusiono f peroxide% as eitherd ela)edt o follow lIFS
or terminatd immediatelay fterIF S Somes liceso ere but slowly fell throughout them easurement period
untreated lie neseer xposetdo p eroxide)I n control exper- (Fig IA) Only five of 16 slices showed population
iments, no IFS %as dhred andt hea ctionso f peroxide spike amplitudes of 130% of control or greater at
alone ore follioed The timing of thee xpermentss as 60rin post-HFS At 60mtn post-HFS. the popu-
identicailn all other respets In too of the 78s lices,s srs
falied to causee ven shortter a potentiation (STP. i e latton spike amplitudes of treated and untreated slices
potentiation that developism mediately andd ecayso ithin vcre significantly different from one another (c-test.
55m in), theset oo expenmeots aeren oti ncludedi nt he P < 005). The inset in Fig IA shoos sample traces
analysis from treated and untreated slices The population
}|IFnSipn u pter/ooxiudt(e-p/tru0et)tce du rae nsd eunrtrege aetende srlaictbeseeu dfs ionnrg eaa rafntgee r snp'iike in untreated slices shooed a substantial increase
of stimulusi ntensities( 0 0 5m A. 200j s) Three reia- a amplitude that vas sustained for 60 mi. Poten-
tionships .,ereeam ined using the ascragesfr om eight ttation vas not sustained in peroxitdc-trcated slices
experiments population spike is afferenvt ollep.o pulation and the population spikes before and 60m after
PSP xsa fferent volley and population spikes s popu UFS are nearly the same size
latioPnS P The third relationship (population spikei s
population PSP)v iasal soe valuatedfo r eachi ndisidual A similar pattern was evident with the population
experiment SOc urvesv erege nerate3d0 mis poor to tsFS PSP( Fig I B) In untreated stites the magnitude of
and 60mi foltootog IFS The timing %as designetd o the early slope of the population PSPw as increased
presenta ny influenceo f STP. sometimeps roducedb y to 284± 26% of control with a decline to 203± 20%
generatintgh el iOcur.oen ,t he LTP eoked by lFS Since of control within about 15m in The increase was
the pre-||FSt ime point coincided ith the application of
peroxidet,h e peroxidec oncentrationsi asn ot at its peak sustained for the remainder of the experiment
This is nota concerns incep eroxidea t the concentration (202 ± 28% at 60 mix post-lIFSA)t 60 mix after
usedd oesn otc hangeth e1 0 courve(dsa ta nots hoon)D ata lIFS.mi 1 3 of 17 sltcest he population PSPw as at
for the 10 cursesm ere analyseda s previousldy escribe0d least 130% of control amplitude Sample traces
In briefb, est-fits igmoidc ursess iered eterminedf orbo th beforea nd 60m following IIPSi nu ntreatseldi ces
pre. andp ost-tIFS data Fore achc vrc, a parametesri as
computedfr om them aximal y-valie dvidedb yt he -value show a sustained enhartcement of the synaptic
at half maximaly Changeisn this parametehr aseb een response Treatment with 0 002% peroxide prevented
effrctise in evalaatingc hangeisn | IOc aries the maintained increasine t heP SP By 60 min post.
Pairedp ulsef acittation WAss tudiedb yu singt oo ideati- UPFSth e population PSP was nearly the same as
cal stimuli (200Ds) separatedb y intervals from 10 to
200ms The stimuluss trength wasa djustedt o prod.re during the control period The initial mctiase was
approximately a half maximapl opulation spikeA mplitude 261 ± 30% and declined to1 29± 17% after 60m m
of the seconpdo pulation pikeo ase xpressaesda percent. Only five of 16 slicess howed population PSPst hat
___________
1
Freer adicalsa ndl ong-termp otentiation 355
A. maximal at an interstiaxulus interval of 30mis,
300 -u~viiat302:E50% in untreated slices and 297±28% in
~,*
300 urv.t peroxide-treated slices In agreement with Colton
8a a et al.6 facilitation was unaffected by peioxide at
~ all intcr"mulus intervals tested( 10-200ms)
S 20 -~ a Perox.., might prevent UTPb yI nterfering with its
inductioonr its expression. To distinguish between
taO these possibilities, peroxide was applied at different
" _times during the process Table I illustrates the
. 100 L .&UU-'I-IsV change in population spike 60m after HFS with
0 20 40 6 S0 four experimental treatments. (I) untreated (n= 17),
Tme. (M) (2) treated with peroxide throughout the experimen.
B. -tal period (n- 16), (3) treated with peroxide before
300 .. "" and up to1 0 min after IIFS (n- 16) and (4) treated
with peroxide only after HFS (5-60min post-HFS,
250 -irt".1. n -9) Analysis of variance showed that potentiation
U W in slices treated with peroxide throughout (treat.
2o20 0 o ment 2) was significantly lesst han in untreated slices
ISO P.O.W. (atfrteeart mHeFnSt (1tr,e Pat m< 0en0t5 )3 )W,ph oepnu lpaetiroonxi dsep ikwea asmr epmliotuvdede
was not maintained at the level of untreated slices but
to * .W Uoo'VI.oaa 3U decayed more slowly than in slices with continued
20 40 60 to exposure to peroxide (treatment 2),p otentiation was
Tice( MO) not statistically different from that in either of the
Fig I H)drogen peroxide(0 002%)p rvents the mainten. first two treatments When peroxide was applied only
anceof LTP (A) Amplituodfe p opulation spikee xpressed after HFS (treatment 4). potentiatin of the popu.
as percentagoet control plotted Ns time Error barss how lation spike was sustained as in untreated slices
standarderrorofthe meanfo r eacht impeo int Open circles Analysis of synaptic potentials at 60 min post-HFS
popuation spikei n tissue treated iith peroxideb utn ot showed a similar pattern (data not shown) per-
stimulated with IIFS (n. 4) Opens quaresp eroxidper e.
sent,l IFS applieda t3 0min time point (n= 16) Closed oxide following HFS was ineffective while peroxide
circlesu ntreatedti ssue.I PS atm -30m (n- 17)I nset,r emoved after HFS was only partly effective in
shows amplet racftrosm indisidual expenments Light trace reducing LTP,
isf rom beforHeF S whileb oldt raceo ccurred6 0m ix after In thep revious serieso f experiments, te poten-
IFS (i -90m en) Calibration I mV,2 n's (B) Changeisn
population PSPA th time Symbolss amea s is A Insets tationf ollowinHgF S was evaluateadt onlyo ne
shows ample population PSPfi rom sameex pexnmeiaas si stimulus intensitkye,pt constantth roughout the
A tight tracef rom befcrel IFS,bo ld trace from Som experiment In an effort to evaluate the change
post.IIFS Sameca hbration as in A in the response to a range of stimulus strengths.
1/O curves were generated 30min prior to HFS
werea t least 130% of control Sample traces show and 60 min after IIFS. As seeni n Fig 2. in untreated
very ittldeif ference between synaptic responses slices(n - 8).H FS increased the ability of the afferent
recorded prior to HFS and responses6 0 min follow, volley to elicit a population spike throughout the
ing IFS in peroxidetreated slices At 60rin post.
HFS. treated and untreated population PSPsw ere
statistically different from one another (i-test, Table I Timing of peroxide application affectsa mounto f
P < 0 05) potentiation resultfironmg high frequencsyt inmulaton
A higher concentration of peroxide (0.005%) was Percentaginec reasien
tested with IIFS on three slices. As previously Treatment population spike( ±SE M
observed,- this concentration had a direct effect to (I) Untreated 137 5± 145
decreaseb oth the synaptic response and the popu. (2) Peroxideth roughout 240 ± 136
latio, spike HFS elicited early potentiation of both (3) Peroxideb eforea nd
population spike amplitude and population PSP duangtitprsF5S 5164
slope Within about 30 rminh,o wever, the responses Peroxde after US 1361± 164
were back to control level and continued to decline Measoremcnsthso wp ercentagien creains peo pulation spike
POm ina ter IFS comparetdo control The population
Population spike potentiation fell to only 105 + 28% spike from untreated tissues ho%ss ignificantp oten.
of control within 30 min At 45 rin post.-FS, the naion Treatmentw ith peroxide( 0002%) through-
average population spike was 58 ± 33% of control out the experment preventedsu staMiedp olentiation
The synaptic response was similarly reduced very *Anal)szsofvanarceP<005 Applieation ofperoxide
quickly beforea ndd urng sirs but washedo utw ithin 5 min
preventeds ome butn ot allp otentiation (P<005)
Paired.pulse facilitation was evaluated with 001% Application of peroxide only after IPS did nota ffect
peroxide (n - 3; data not shown) Facilitation was potentiation (P< 0 05)
356 T. C PIELLMfAl.ta
Untreated Peroxids
A.
7
'4
2
00 05 10 11 20 25 00 0.5 10 1.1 20 25S
Volley( mV) Volley(m y)
-;06
.4
a02
D0 0.5 10 IS 20 2S 00 05 10 15 20 20
VoNh(yM V) Volley( MVY)
7[C.
> 4
2
00 02 04 06 00 00 02 04 06 041
pop POPs lop. (nY/ms) PopP SPS tope(m Y/mn)
Fig 2 1/0 curse obtainedfr om tissuetr eatedw ithi0 002%p eroxide(n -8, right)a ndu ntreated(n -8,
left) Curveso btained3 0m ini prior to lIFS (opens quare$a)n d6 0mina ftcr tIFS (closedc ircles)in both
eonditions (A) Plot Of popsultion spike vs volleys hawss ignificant enhancemreonf tp opulation spike
following IPS in untreatedti ssuew ihichi s not evidenitn pecoside-treatteisds ue (Bt)P loto f population
PSPv s volleys howsse nhancemeonft synapticre sponsien untreatedti ssues whichis lessd ramaticw ith
proxide treatment(.C ) Ploto f popultin spikev s population PSPs loan onlym inimalc hangeiss E/S
coupling both in treateda ndu ntreatedti ssue
range of stimulus intensities (166± 9%) In tissue ous change in this relationship following LTP in thc
treated svith 00 02% peroside (n -8), Ibis enhance- average responseo f eight untreated slices (109± 5%).
incnt is greatty reduced (112± 7%). I-FS also in- yet four ofeght ofthese sices did show enhanced E/S
creasedt he ability of the afferent volley to evoke a coupling (i e. greatcr than 20% increase) in agee-
synaptic potential in untreated slices( 134 ±13%) ment with Andersen er af' In peroxide-treated slices.
(Fig 2B1) Peroxide decreasedt his Potentiation there is alto no E/S enhancement to the averaged
(105± II %) (Fig. 2B). The third seto f graphs (plot curves (94±t4%) and only two of eight treated slices
of population spike vs population PSP)p rovides an showed enhancement.
indication of the ability of the synaptic potential to Potentiation following HFS can be resolved into
evokeca spike, also called E/S coupling Other components by evaluating the time constants of
authors"," have shown that E/S coupling is some- decay of the responsbeo ck ts baseline " in eacho f
ttihmaet s oennlyh a5nc0e%d o wfi thth eLiTr Ps liAcnedsse hrsoewne de t thW1is* prehpeonrotmed- tehxep eeriimghetn tt reaabteodv e,a ndda teai gahtt iudndtirtetaotnead l stilmicee sip no inthtes
enon In the present experiment, herew asn o ohot- werec ollected to provide a more aceurate represen-
Freer adicalsa nd long-termpo tentialao 357
and 4.16 min in peroxide-treated slicesI.t is obvious
Untreated Peloilde that the slower time ccnstant is decrease by peroxide
200 T, 249XI0't 5852 treatment, whereas he faster time constant is not
IT "s s 416 I mreuspcohn saeff eschteodw. eTd l'a.s s tiimmiela rc oenffsetcat.n tIsn fourn trtehaet esdy nsalpicteics
bS0 - the calculated time constants were 158 x 10s and
5.55 mn and in peroxide-treated slices they were
i61.83 and 4.2min Again, the slow time constant is
greatly decreased by treatment with peroxide while
the faster time constant is not. Thus. the late phase
of LTP appears to be selectively altered byp eroxide,
so0I 1Ln 2L0 3L0 4L0 50 60
Time( Inse DISCUSSiON
B Untreated P, e-roide poTtehnet iaellsec tirno hphipypsoioclaomgipcaall eslfifceecsts h oafv pee broexeidnes hoonw fni etlod
2n0 -. t.Saxio" 6583 be mediated by free radicals.Y In the present study,
V, 55 482 we demonstrate that peroxide interferes with LTP at
ISO concentrations that do not affect unpotentilated
* osynaptic transmission. The I/O curves show that free
.1 0 00 radicals predominantly impair potentiation of the
. 00 synaptic response and have much lesse ffect on spike
sa generation (E/S coupling) This contrasts with higher
so iconcentrations of peroxide, which significantly
reduced both synaptic efficacy and E/S coupling "
10 20 30 40 SO 6o The decreased synaptic potentials in previous studies
Time( min) were hypothesized to be a consequence of reduced
Fprigo ce3s sCe%usrr vtie fsid tteeds ctori btihnegt d haet as ufomr othf e tdweoc aeyxo pf opnoetennti.a l tar ancosmmpitlteexr rneelcuarosnea.2l sFp reroec ersasd. icaml aye ffeincvtso lvoen oLtThePr,
tiation of the population spike(A ) andt he population PSP mechanisms
(B) b th ith (opens quares) and without (openc ircles) HFS induces at least two phaseso f potentiation,
pcteoexrnots xtPraiednoept sirnt er(tsemsaer tniem)t pteivrsie th soeleIwrnrFaontSvir n emgb rniaavsrgee itsensfa rs sot mDh iioe metwaieignls fh o 0vte r f x Cc-p,uaoetilscriehcium uflaiergvtn eettOd sim ninee trtheisefes urlreae tdse ru tgoogn eea ssbtt heSaiTtgP L.LT5TP3'3P d.1 oT eOhsue n r eorate rdslyeu cltprseh maines neuth namtsre eabasteueerd-n
ably. While the calculated time constant may not be
an accurate assessmenot f the extended time course,
it does make the point that in untreated tissue, LTP
rsaptaikine a onf dch tahneg epso pinu lathteio anm PplSituPd wei tohf tthime ep oTphuela tpioern- itsi sas suuesdt,a einceady p orof ceLsTsP. Inh acso nat .atismt, ein c poenrsoxtaidnet-o trfe aotneldy
cwrPdeeexeinsStpshtuoac Plngrttabieemwnen etdtii han chc- ubrser0 yva e sasteth et(hiw pne oe pqrtieuumsaluaetp it ooioonnfn s HresfnwF oporaSi kts e (t hFpeailg on tdst3e u)dpm oT vphsoue fl attfi tomioaue nr, aoopI sfko 5oetPe mluyovnt m etteietrda onyoeit snictcoa Isaoantveyirti,nor vO esnoisnwt., i p vhoTwwoe ienc iitmthdhtho pt heathope raha e astatriw um ohraxeeta i ddtct eseitamo e M nneecsT s tNhiaatcniiaevsnotu ia tntg yoc sh oftocamt a on5nntpn-t o 6o" ma nmacfecrcpnno.aoo.ntul unl ennliiidydstss
% change a-xp(- t/r,)+bh xp(-i/r,), for much of the quantitative difference in the time
constant between his experiments and the present
wbheet re fit at,o bt,h e ite xapnedri mer2n taalr e daetav alwuaitthe d a foror utitnhee resAunltas.lysis of the time constants of decay revealed
(FITFUNCTION) in the analytical package RS/I that only the late phaseo f potentiation was affected
(BBN Software Product, Cambridge, MA). it and I, by peroxide while the early decay was unchanged
aproen tehnei tstoi mf teh ec ocnusrvtaen, trse spfoerc titvheel ys. loa wa nda nbd a fraes wt ecigohmt-- Pharevevi ovuesry sdtiuffdeireesn th maveec hasnhioswmns tSheavt erSalT aPu thaonrds isL"T"P
ing constants for the two exponentials and were have suggestedth at STP reflects an increase in the
insensitive to peroxide While the computed values probability of transmitter releasem, ost likely due to
seem to fit the data very well, ve do not assume an increase in presynaptic calcium Similarly, paired-
that they are necessarily unique solutions to the pulse facilitation is caused by an inciease in the
equation The time constants ofdecay of potentiation probability of transmitter release In contrast, it has
of the population spikes of the best fit curves were been suggested that enhanced calcium entry at
2 49 x 10I" and 6.16 mm in untreated tissue and 58 52 presynaptic teminals is not the mechanism for
358 T. C. PUInuANn at.
LTP.2" LTP doesn ot appear to entail an increase remove the blockers after injection, which compli-
in the probability of transmitter release, instead cates comparison with our experiments in which
McNaughton hypothesized that the presynaptic removal of peroxide was less effective than continued
component of synaptic enhancement in LTP could exposure. It is possible that peroxide is interfering
reflect an increase in the number of quanta available with one of the several second messenger systems
for release or an increase in the number of binding thought to be involved with LTP.
sites for vesicles Since STP and paired-pulse facili- Recent studiest'." suggest that the oxtdation/
tation are unaffected by peroxide exposure, it its reductionstateoftheN-methy.D.aspartateaffectsits
unlikely that free radicals are interfering with presyn- electrophysolog;cal response. In a number of neur-
aptic calcium entry In fact, voltage clamp studies onal preparations, dithiothreitol. a sulfhydryl reduc-
in hippocampal pyramidal cells have shown that ing agent, caused long-term eshancement of the
sustained high threshold calcium currentasnPd tran- response to N-methyl-D-aspartate. Tnis effect could
stenlto w threshold calcium currents (unpublished be reversed by oxidation with dithio-bis-nitrobenzoic
data) are insensitive to peroxide Other steps in the acid Tauck and Ashbeck" reported that dithiothreci-
release process need to be considered as possible tol, at a concentration that had no direct effects on
peroxide-sensitive sites the synaptic potential, was able to enhance LTP. It
Induction of LTP also requires calcium postsyn- is possible that the free radicals formed i, the pres-
apticallyU"'s but only within 5 min of HFS.Y Itis ents tudy oxidize the N-methyl-D-aspartate receptor,
possible that peroxide is interfering with a postsyn- decrease its contribution to the synaptic response
aptic calcium-dependent process Remote calcium even with HFS and thereby reduce the expression of
spikes in hippocampal neurons show an increased LTP.
threshold with exposure to peroxide" and calcium- Hydrogen peroxide reacts with tissue iron to gener-
dependent processes in a number of cell types are ate hydroxyl free radicals While free radicals are
reportedly sensitive to free radical damage".""' constantly formed in healthy tissue, the intrinsie
Peroxide must be present during induction of LTP antioxidant systems keep them in check. However.
to be effective. Yet itsac tion is to prevent mainten- under pathological conditions. frec radical generation
anceo f thep otentiation This suggests that peroxide can exceed the tissie's ability to control them Our
is interfering with some process during the induction study suggests that under such conditions, LTP, and
phase of LTP required to fully express the poten- perhaps memory processes, can be disrupted
tiation. Recent reportst --2-, show that a number of
second messenger s)stems must similarly be available
dring HFS for LTP to occur. Blocking postsynaptic Aclnouledenis-Ths work as supported by the
protein kinase C or calmodulin 11k inase prevents the Armed Forces Radsoblotogy Research Institute. Defense
induction of LTP, STP is evident, but by 30 mm Nuclear Agency.u nder work unit 00105 Viewsp resented
post-HFS the responses are back toc ontrol levels.0" int his papearr e those of the authors, no endorsement by
As with peroxide, the time of application of kinase the Defense Nuclear Agency has been given or should
inhibitors is critical. Intracetlilar injection of the be inferred Research was conducted according to the
prnciples enunciated is the" Gide for the Care and Use
blockers in the postsynapsic cell after HFS has no of Laboratory Animals" prepared by the lstitutofe
effect. In these experiments it is not possible to Laboratory Animal Resources. NationRaels earch Council
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