Brae Re.search. 32(~ ( 1985 ) 251 260 25 1 Elsevier

BRE lll-ltJt)

Axon Terminals of GABAergic Chandelier Cells Are Lost At Epileptic Foci

CHARLES E. RIBAK

l)epartment ~{['A natomy. University o,t CaliJornia. lrvine, ( A 92717 ( U. S.,4. )

(Accepted May 8th, 1984)

K<'r words': chandelier cells ecrct~ral cortex GABA ncurons - - epilcps 3

Axon terminals of chandelier cells were analvzed in monkcvs with cortical local epilepsy produced hy alumina gel to determine il this type of GABAergic terminal is lost at epileptic loci. These tcrminals form a dcnse plexus with the axon initial scgments of pyrami- dal neurons, especially those in layers II and II1. Axon initial segments of pyramidal neurons were traccd for at lea';l 4(i um in serial thin sections and hewed this point were observed to become mvelinated. In single sections, 10- 15 axon terminals were lound to form symmetric synapses throughout the entire length of the axon initial segments irom noncpileptic preparations and were observed to synapsc with only these structures and not adjacent dendritcs or spines. In epileptic cortex, the axon initial scgmenls el pyramidal neu- rons ~crc apposcd by glial profiles that contamcd cluslers of filaments typical of reactivc astrocytes. Only a few, small axon termimtls were observed to form symmetric synapses with these axon initial segments. Thus, thc chandelier cell axons appeared to degenerate in epileptic cortex. The highly strategic site of GABAergic inhibitory synapses on axon initial segments suggests that they cxcrt a strong influence on the output of pyramidal cells. The near absence of thcsc chandclicr cell axons in epileptic foci most likely conlributcs to the hyperexcitability of neurons.

INTRODt r('TI()N

It is now apparen t that in some var ie t ies of epi lep-

sy, a severe deficit occurs in the cort ical G A B A e r g i c

system. N u m e r o u s studies of expe r imen ta l models

that r esemble pos t - t r aumat ic epi lepsy have shown a

loss of G A B A e r g i c te rmina ls in ep i lep t ic foei e-' ca. A

quant i ta t ive assessment at the e lec t ron microscopic

level has indicated that this loss is p re fe ren t ia l for

G A B A e r g i c , symmet r i c synapses2e. B iochemica l

studies also suppor t this f inding in that indices for

o ther neuro t r ansmi t t e r s were not r educed as se-

verely as those for G A B A I,e,.<. These findings in ani-

mal models have rece ived co r robo ra t i on f rom bio-

chemical results of human epi lept ic loci >. T h e r e f o r e ,

a loss of G A B A e r g i c te rmina ls appears to be a hall-

mark of epi lept ic loci.

The initial i n m m n o c y t o c h e m i c a l study on the mor-

phology of cort ical G A B A c r g i c neurons revea led an

abundant var ie ty of s ta ined cell types d is t r ibuted

th roughout all layers of cor tex -q. This study localized

the synthesizing enzyme of G A B A , g lu tamate decar-

boxvlase ( G A D ) . A l t h o u g h most of these neurons

were classified as aspinous and sparsely spinous stel-

late cells, subsequen t Golgi studies have revea led

major subclasses of this ca tegory based on cell body

shape, dendri t ic o r ien ta t ion and axonal distr ibu-

tion 13,2°,2<*,>. O n e cell type, the basket cell, was sug-

gested to be G A B A e r g i c because its axonal plexus

that forms per icel lu lar nests a round pyramidal cells

in laver V was G A D - p o s i t i v e in i m m u n o c y t o c h e m i -

cal preparationsS,le,-'t,2L This ident i f icat ion was given

fur ther suppor t f rom recent studies that uti l ized col-

ch ic ine- t rea ted immt inocy tochemica l p repara t ions

that displayed the cell bodies of this neurona l

type li,iz. Since the terminals of this plexus form sym-

metr ic axosomat ic synapses, they are readily quant i -

fied and were shown to be reduced by S()c~ al epi lep-

tic foci e-~. These data indicated that a basket cell defi-

cit occurs at epi lept ic foci and as a result the laver V

pyramidal cells at such sites arc probably more hy-

percxci table than normal .

The present stud,,,' was unde r t aken to de t e rmine if

ano ther cort ical G A B A e r g i c cell lype was reduced in

function at epi lept ic foci. The cell type chosen for this

analysis was the chande l i c r cell. Szcntf igothai and

('orrevl?omh'nce. Charles E. Ribak, Department of Anatomy, University of California, hvine, ( 'A 92717, U .S. ,,k.

Ilil06-S993i85 $03.31i ~ IriS5 El~cxicr Science Publishers B.V

252

/\rbib :~ first described this neuron in Golgi prepara-.

tions. This neuron is characterized by a small soma

with aspinous dendrites in layers I1 and 111. The chan-

delier cell's axon forms numerous vertical chains tha!

appear to be apposed to pyramidal cell apical den-

drites in light microscopic preparations Is,~3. Howe~-

cr, Somogyi > subsequently demonstrated in eleclron

microscopic preparations that these axons form swn-

metric synapses with the axon initial segment,~ of p~-

ramidal cells in layers 11 and 111. Further studies have

confirmed these electron microscopic obserw~tions in

numerous species anti cortical regions +'`>.> including

the hippocampus ~1. The morphology of these lermi-

nals and the localization of GAD-posit ive reaction

product within these terminals s,>.et,~: have indicated

that they are GABAergic . These data suggest that

chandelier cell axon terminals exert a strong inhibito-

ry influence on the output of pyramidal cells. A loss

of such a plexus of axons might also contribute to ~hc hyperexcitability of neurons in an epileptic focus.

MATERIALS AND METHODS

The present study utilized specimens obtained

from 3 of the 5 experimental monkeys from a prc-

vious study-'~. All of the monkeys had received alumi-

na gel applications to the left cerebral hemispheres to

produce seizure foci. Two of these monkeys (animals

3 and 5 from Ribak et al.>) received intracortical in-

jections directly into both pre- and postcentral gyri.

The remaining experimental monkey (animal 2 from

Ribak et al. ~3) had an injection of alumina gel limited

to the subarachnoid space in the area of the central

sulcus. Electrocorticography of all experimental ani-

mals verified epileptic foci > and subsequently, the

monkeys were fixed by intracardiac perfusions of a

mixture of two aldehydes. The fixative solution con-

tained 4% paraformaldehyde, 0.1% glutaraldehyde. and 0.002% CaCI~ in a 0.12 M phosphate buffer.

Blocks of cortical tissue were obtained from the epileptic focus and the homologous area in the con-

tralateral nonepileptic cortex. All blocks were sec- tioned on a Sorvall TC-2 tissue sectioner at a thick- ness of 150 urn. Specimens that contained the entire cortical thickness were cut from these sections.

These specimens were postfixed in 2% OsOa for I h. dehydrated in ethanol and embedded in Epon. For light microscopy, semithin 1 um sections were cut

from the embedded specimens and stained with

(I.05cf toluidine blue. Specimens were oriented in

the ultramicrotome to obtain SeCtions thai yielded

the longest segments of apical dendrites. The ~dja-

cent thin sections from these specimens usually had

the most number of identified axon initial segments

These sections were then stained with uranyl acetate

and lead citrate and examined ot~ ~ × 2 mm formvar,

coated slot grids with the electron microscopc,

RESUI:I-S

All observations were obtained from electron mi-

croscopic preparations of monkey sensorimotor cor-

tex. The analysis in the present study was limited t o

the axon initial segments of pyramidal cells in layers

11, IlI and V. Since the findings from the nonepiteptic

hemisphere were similar to those described in a pre-

vious study of normal primate sensorimotor cortex>,

they will be presented first. Then, the data obtained

from epileptic cortex will be compared with the data

from the nonepileptic cortex.

Nonepileptic cortex

A previous study 22 from this laboratory described

the features of nonepileptic monkey sensorimotor

cortex in electron microscopic preparanons. De-

scnptions of layer V pyramidal neurons and adjacenl

neuropil regions were provided. Briefly, layer V py-

ramidal neurons have multiangular-shaped somata

formed by an array of basal dendrites, a single apical

dendrite and a single axon usually located between

the basal dendrites. Much of the soma is occupied by a large, rounded nucleus wh~se nucleoplasm con-

tains a relatively homogeneous sprinkling of electron- opaque chromatin and a centrally located nucleo- lus ]~.22. Pyramidal neurons contain numerous orga-

nelles in the perikaryal cytoplasm including both free ribosomes and cisternae of granular endoptasmic re-

uculum. Terminals thai form axosomatic s}napses with these neurons make only ,~ymmetnc synapses. Pyramidal neurons in layers II and III are somewhat smaller than those in layer V. bul they share the same ultrastructural characteristics, Since our prewous ul- trastructural studv 22 provided details on the somata

and dendrites of pyramidal neurons and the axon ter- minals thai formed synapses with these structures, the present study will involve primarily an analysis

of the synaptic contacts \̀ ̀itll aXOil in i t ia l Scglnenls of

pyranl idal netlrons.

The axon ini t ia l segmeill {irises from the axon hi l l -

ock region of the neuronal cell hod\'. Peters ct ill. >

provide a COlllplctc description of the feattll-es of

~lxon initial segments . Such features will bl-icfh' bc

describcd lind attent ion wi l l bc given to those slttlC-

ttlres and rehi i ionships th{it appe:,lr d i f fercnt in tilL'

epileptic cortex.

Axon in i t ial segments usually appear al t i le base of

file p_vranlidai cell bed) (Pig. 1). The bot lnd{ l r \ be-

tweeI1 those two port ions of tile l l e u r o l l is : . lpp{i i+el l t l i t

1o+`̀ magnif icat ion because ti le cisternao of the ~rantl-

hlr endoplasmic rct iculum or Nissl bodies that reside

wi th in the soma do i1Ot enter the ~iX,Oll. The {iXOll hi l l -

ock region is conici i l . \vhcl+eas the axon ini t ia l seg-

i l lel] l has ii const{inl cvlmdrical shape that is main-

tainud thl-oughc}ul its lcl lgth. This ]at lcr lealul-c

stands in COlltrasl to dendrites which tistiallv Ila\C dif-

fereil l cross-section diameters over lheir entire

length. l h e most disial part of lhe ini t ial segillenl is

ci lari lcterizcd b\ the paranodal region of il in \o l in

sheath (Figs. I alld _~ I. Thcsc major features t)f aXOll

in i t ia l SeglllelltS <If cortical pyramidal cells {ire often

observed il l it si l lglc thin section that is properl>

oriented perpendicular 1o tilL' pial surface.

AXOI1 initial seglllClltS tire idenl if ied by t``\o olhel"

characteristics. ()no of thcsc is the increased lltlIllbel

and aggregalion of lnicrotubules, and the other is the

dense undercoating of the axc)lenuna (Pig, 31. Both

fealtire,, tire l }p ical for tlxon ini t ial segilleilts {lild al-

low for lheir ident i f icat ion in sections where the``

ill{i,, not bu COlltintious witt~ the cell body. Other or-

ganelles frequent l} obser``ed in mJtial segnlel l lS in-

clude cisleinac of i i~ ia l lu l l i r endoplasmic re l icu lu ln,

neurof ih in lenls, elol lgated nmochondr ia, occasional

frec ribosomes and cistcrnal organelles.

l-he terminals that conlacl {IXt)ll ini t i { l l segments e l

pyramidal cells in lii,,crs i l and 111 form s\n ln lc t r ic

~)llapses I t:ig. 31. l hesc terlnJnals that fern1 s\ i l -

{lpses with axon initial scglllents tit) llOt appear 1o

contact oil ier shuc tu rcs in the adjacenl neuropi l .

They tire a \ e r age in size. conta in p leomorphic vesi-

cles and usmil l \ OllC or t\``o mitochondria. Il l single seclions, lit.-15 <~f these AXt)I1 terminals fol-nl s\n-

apses througl lou l tile enl i re length of the axon init ial

',t.'gments <H~tained ['rl)Ill nonepi lcp i ic preparations.

Il l c~mtraq, lhc inili~ll segmClltS of laver V pyramidal

253

cells have somewhat fewer terminals that synapse

with their initial segments . These f indings are consis-

tenl with a previous quant i ta t ive descript ion of axon

in i t ial segments of pyramidal ncclions ill prmlate seN-

soI imotor cortex 2s. In lhat sttldv, Sloper and Powell >

reported that the total length of all axon ini t ia l seg-

nlenl of a pyral l l idal cell in ]avers I1 or I11 received

13.4 synapses, on a\erage, whi le pyramidal ceils i l l

la\er V were contacted by 4.8 synapses per :.lX()ll ini- tial segment . It is i i l lportanl to note lllal these termi-

nals are not dis t r ibuted evenl} along tile lengfll of the

axon initial segment . A gradienl exists in that fewer

terminals arc present ad jacent to the pl-oxinlal axon

and nlore {ire fotlnd associated with tile dislal por t ion

of the aXOll ini t ial segment. This d ist r ihut ion is simi-

[;.tr to file one descr ibed for {iXOllS of chandel ie r cells

studied ill combined (hHgi-elociron microscopic

p re pa rat i o 11 s" ,20.2u.31i

f:t~ileptic corte.k

Thin sections of cortical tissue l ro ln the epileptic

focus disphiy similar s t ructures to those observed in

the nonepi lept ic cortex. Pyramidal ilCUl-Ons are read-

ih' identi f ied as well as axon hil locks and ini t ial seg-

ll lell lS that arise frotl l the bases of tl~eir somata

(Pigs. 4 sand 5). Most :.iXOll ini t ial segments have :,i

i lorma] appearance \ t i th the lSpieal fasciculati,,)n of

nl icrotubules and dense undercoat ing (Pigs. 4, 6 and

7). However, all occasional one displays a swell ing in

its nlost distal port ion where nunlcrous mitochondr ia

tla~e accumulated (Pig. 4). When til ls occurs, anoth-

el- di lat ion is found more distall\, i l l the mvelmated

part of the axoil beyond the par~inodal region. AI -

thouoh lt~e i l l tcr l la[ structures of nlost axon ini t ia l

segments from opi lcpi ic cortex appear normal, the

adjacen! slruclures ctiffel- from the nol-nlCll i l l two

ways: ( 1 i a loss of lernl inals lhat foi-m s \mnle l r ic syn-

apses: and (2) {ill increase ill gliosis.

Most of the axon te rminals thai normal ly form

s~ mmetl-ic S}llapScs with tile axon initial segments of

pyramidal l leurons il l la~ers 11, 111 and V are absent

(Figs. 5 7). The few Fenl ;A i l l i l l~ [ell l l i l l~l ls ~lrC much

smaller than nornlal , and lhe l lumber of these ler i l l i -

nals per ;.ixon initial sL'gll len[ for ii single section rang-

es from fl to 3 for the 20 axons examined ill this study.

In addit ion, lhe aXOll hi l lock lcgion of these same

nemOllS displa>s i /shn i lar loss of ternl inals that form

s\nln lotr ic S VilapSes. l h e smaller p> iamidal neurons

255

in layers 11 and I l l o f t en h a v e no terminals that form

synapses wth their axon initial segments. However ,

the larger pyramidal cells in layer V including two ex-

amined Betz cells have the highest number of initial

segment terminals and this finding probably results

from the much larger size of their axon initial scg-

m e l l t s .

The second difference observed in these epileptic

prepara t ives is the increased number of astrocvtic

processes that lie adjacent and orient parallel to the

axon mitial segments. Most of these astrocytic proc-

esses are packed with nunlerous filaments (Fig. 7).

Such processes are found adjacent to the somata of

these same pyramidal cells as described previously -~-'.

Often, the same astrocytic process will appose a por-

tion of the cell bodx, the axon hillock and a l(I l;m

length of the axon inilial segment (Figs. 5 and ~3). Al-

though this orientat ion preference for the hmgitudi-

nal axis of the axon initial segment is observed most

frequentl,v (Fig. 7), somc glial processes arc or iented

transverse to this axis (double arrow in Fig. 4). This

apposit ion of glial processes is usually only one proc-

ess thick. However , inultiple layers of glial processes

are found adjacent to the larger axons of Betz cells.

DIS(t SSI()N

The major finding of this electron microscopic

study is that most of the axon terminals which form

symmetric synapses with axon initial segments of py-

ramidal netirons are lost at epileptic foci in monkeys.

Although previous studies have demonst ra ted a loss

of axosomatic synapses in epileptic loci 3-7,2-~,35. this

report is the first to document a loss of these initial

segment symmetric synapses that are formed by termi-

nals mainly derived from the chandel ier cell (Fig. 8).

Previous studies that utilized a combined G o l g i -

electron microscopic method have shown that a x o n

terminals forming synapses with laver I I - I11 pyranli-

dal neuron axon initial seglnents arise from a specific

cortical neuronal type, the chandcl icr cell < > . > , > .

A l t h o u g h a s imi la r study' o f the chande l ie r cel l has

not been made in the monkey scnsor imotor cortex, it

is likely that this cell exists in this region because So-

mogyi et al. x<> have dcmonst ra tcd chandel ier cells in

the motor cortex of cat alld in the m o n k e v s visual

cortex. In addit ion, the distribution of axon tcrrninals

alongside the axon initial segments of pyramidal cells

in laycrs 11 and III of the monkey sensor imotor cor-

tex es is very simihtr to the distribution of chandel ier

cell axons. Taken together, these dala mdicate that

most of the axon terminals that synapse with axon ini-

tial segments of pyramidal neurons arise from chan-

delier cells.

Recent resuhs from immunocvtochen~ical studies

which localize lhe G A B A synthesizing enzyme,

G A D , have ind ica ted that chande l i e r cells tire

G A B A e r g i c because most terminals that form sym-

metric synapses with axon initial segments contain

G A D imn3unoreac t i v i t v s. i l ,>. In fact. F reund et al. s

have p r o v i d e d d i rect cv ide i l ce fo r this no t ion by com-

b in ing Golgi impregnat ion with immuriocvtochemis-

try m the same preparat ion. In one fortuitous case

from cat visual cortex, they demonst ra ted that 11

Golgi- imprcgnated bout(ms derived from a single

chandelier cell contained OAD-pos i l ive reaction

product. The highly,' strategic site of these G A B A e r -

gic synapses on axon in i t ia l segments suggests that

they exert a strong inhibitory effect on the pyramidal

neurons which they contact. The large loss of these

GABAerg i c synapses in epileptic loci indicates a sig-

nificant reduction of inhibition because ( i A B A has an

inhibitory action tm cortical neuronsi4 i-. Thereforc ,

a probable result of this loss is a hypercxcitabil i ty of

neurons at the epileptic focus.

Thcse findings are consistent with our previous re-

sults that dcmonst ra ted a severe loss of GAI) -pos i -

t ive axon te rmina ls tit sites o f a lumin t ln l g e l - e d u c e d

epilepsy3L In addit ion, lhey coniplemeil t ~i previous

electron microscopic s t u d v o f ¢ i x o s o n l a l i c a n d a x e -

Fig. 1 [=lcctron micrograph el a laver I l l pyramidal soma and ~lxon initial scgillCtll lrom a nol'inal, nonepilepiic hemisphere, l'hc soma displap, ci round nucleus (iN) and several cislcrnao o[grariular cndoplasmic roticuhnn (E). Ihc axon hillock (H)is fori'ncd al the, base el lht.' SOll]{I :l[ItJ gives rise ~o the axon initial segment (nrrows) which Cili3 [It' followed to the point whert., ii acquires ~i mvclill sheath. This ~lXOrl is coi l tactcd b ~. ntllllc:l-ous Icrnl inals that form s,vmmctric synapscs (sce Fig. 2 I. ,x:500(I

Fig. 2. Enlargcmcnt el the distal portion of lhc axon initial segmcnl shmvn m Fig. I. Numclous tcrmmals {|) arc located alongside this axon and manx term sx mmctric !-Vll~lpsos arrow, s). This axorl ht.'C(/ll3CS t31velimltcd at the [~ttl[Olll o[ the i~hotomiciogrlq~h, x I ",Ylltl.

:5,

dendritic SVlnlnctric synapses in the same nlonkev

preparations-'-L This latter study-'-" showed an 8()(~:

loss of axosonlatic symmetric synapses and a 5IV<;

loss of axodendritic synapses in cortical layer V of ep-

ileptic loci. Similar reductions in these two types of

synapses were also obscrved in the superficial corti-

cal layers where the present study was undertaken. It

is interesting to note that the loss of symmetric syn-

apses with axon initial seglncnts was lnorc similar to

lhc larger reduction of axosomatic synapses than the

smaller loss of axodendritic symlnetric synapses.

Thercf(tre, tile most severe loss of GABAergic . sym-

metric synapses at epileptic foci suggests a degenera-

tion of two cortical (]ALIA cell types, basket and

chandelier cells.

The terminals of basket and chandelier cells have

SOIllC similarities. They send their axons to the soma

and axon initial segment, respectively, of pyramidal

neurons, alld these two sites are most strategic for the

control of action potential generatiOl-l. Thus, both

cell types can dramatically influence the activity and

output of the cortical proiection neurons. Second,

these two types of terminals average more than onc

mitochondria per terminal. This fact has hecn doen-

nlented in nlanv previous studies for chandelier cell

terminals (sec also Fig. 3) and in a quantitat ive study

for the basket cell terminals -~-~. Thus, the chandelier

cell axonal plexus that forms initial segment Svlnnlet-

ric synapses lnay provide a tonically active inhibition

of cortical pnljcct ion neurons in a way proposed orig-

imlllx for the pcriccllular basket cell axonal plcxus :-~.

Furthcr support for this similar function of thesc txvo

cell types arises fronl their similar degree of malfunc-

tion in epileptic foci. Although a thor(/ugh quantita-

tive assessment was not made in the present s tud\ ,

one can conchlde from the electron microscopic data

of initial segment synapses in epileptic foci that they

are reduced m lnagnitudc to a similar amount as the

axOSOlllatic synapses as prc~ iously reportede-L

257

It is likely that the chandelier cells ha',e degener-

atcd in these epileptic foci because most of their axon

terminals are not present. Other cvMcnce to support

this notion is derived from a report that dcmon-

str:.tted neuronal degeneration in Fink -Hcimer prep-

aralions of alumina gel-treated epileptic i11Ollkevs'.

A c o m m o n sequel to degeneration of l l e u r ( ' H l s is the

hypertrophy and prol i feration of gila i". The results of

thc prcsent study showed till illcreasc of glial profiles

adiacent to the axon initial segments of pyramidal

neurons at epileptic loci. In fact, the s;tmc glial pro-

file was often found to apposc portions of the soma.

axon hillock and axon initial segment (Fig. h ). Thcsc

findings are consistent with previous results that

showed tip to a 5(Vi increase in glial proliferation at

epileptic foci and indicated degellCiution of net>

i(/ns x.~'-111.22̀ N,~5, a likely cause of this prolifcration of

glia alongside axon initial segments itl the alumina gel

model of epilepsy is the loss of GABAclg ic axon ter-

minals derived from chandelier cells.

The results of this study add further support for

the G A B A hypothesis of epilepsy that suggests a re-

duction of GABA-mcdia ted inhitqtion ill the brain

inav cause focal epilepsy e~. Biochemical sttidics of

cortical epileptic loci from hulnans and anilnal mod-

els have shown a sclective loss of ( iAI ) activity,

G A B A concentration and ( IABA receptor bind-

in, ,I-:.l".~a These data are consistent with thc results

from imlmUlOC\ tochemical studies that show a reduc-

tion of ( ;AD-conla in ing terminals at epileptic

foci<-':L Mcldrunli7 and Krnjcxid 1" ha~,c sumnlarized

in review chapters the physioh)gical and pharmaco-

logical data that sho,a' a (}ABA mechanism is in-

wllved in epileptic activity. Therefore, tl selective re-

duction of GABA-lnedia ted inhibition appears to he

a hallmark (if cortical epileptic loci. A possible cause

for this loss of (}ABAcrgic neurons tit epileptic loci is

hypoxia -~2,->'. Further studies nlust determine if the

Fig. ~. ElllargCIllCilI ()l aoolhc t axoll initial segment from a noncpilcptic hemisphere. The two characteristic feature', ol ~Lxon initial segments tire shown: a fasciculation of microtubules (M) and a dense undercoating that is continuous with the coaling o',cr a pinocytot- ic vesicle (V). Three terminals tire shown to form symmetric synapses (arrows). × 25,000.

Fig, 4. Electron nlicrograph of ~il1 a x o n initial segment oI a laver Ill pyramidal nCUFOn from an epileptic focus, lhc axon hillock (1 l) is located at the top oI this micrograph and the initial scgmcnt (arrows) in included in its cntirct~ up to and hlcludm~2 lhe point (',ffrow- heads) at which it HCqLliIC5 {I myelin sheath Both proximal and distal to this point, the axon displays slight dilation~ lh:lt Icalur¢ 11111iiv disoriented mitoch(mdrm NtlmCIOUS large pro f i l e s o f reactive ~ISIIOCVtCS ( A ) life 1(/1111¢] l l ca r fin', a x o n . ,~ > 0 0 0

S~

259

N O N E P I L E P T I C E P I L E P T I C

IS

[] GLIA [] TERMINALS THAT FORM [] S Y M M E T R I C SYNAPSES

G A B A h y p o t h e s i s is r e s p o n s i b l e fo r o t h e r t y p e s o f

epi lepsy; , such as t h o s e wi th g e n e t i c c ause s .

A( 'KN( )WLEDGEMENTS

This w o r k was s u p p o r t e d bv N I H G r a n t NS-15669

a n d a F e l l o w s h i p f r o m t h e K l i n g e n s t e i n F o u n d a t i o n .

T h e a u t h o r g r a t e f u l l y a c k n o w l e d g e s M. B r u n d a g e

a n d Y, J h u r a n i fo r t e c h n i c a l a s s i s t a n c e , A . B. H a r r i s

for t h e m o n k e y b r a i n s p e c i m e n s , a n d N. S e p i o n fo r

s e c r e t a r i a l a s s i s t a n c e .

Fig. 8. Summary diagram that shows the diftcrcnces observed for axon initial segments from nonepileptic and epileptic prepa- rations+ A reduction in the number of terminals that form sym+ metric synapses and an increase in gila arc the two major differ- ences. Some axon initial segments also displa} dilations of their distal portions. Such a loss of inhibitory, GABAergic terminals could contribute to the hyperexcitability of the ncunms at epi- leptic loci (see Discussion ).

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Fig. >. Another clcctron micrograph froin epileptic cortex that shows a layer II1 pyramidal soma, proximal apical dendritc (AP) and axon initial segment (arrows). The soma contains a large nucleus (N) and a perikaryal cytoplasm that contains the typical organclles as well as :m abundance ol lipofuscm granules. The initial segment is apposed to a gliaI profile that also contacts the axon hillock and soma. × 13.000

Fig. 6. Enlargement o[ the initial segment (IS) and adjacent reactive glial process (A) shown m Fig. 5. ×23.1)()0.

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