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Neuroscience Letters, 149 (1993) 185-188 185 1993 Elsevier Scient if ic Publ ishers I rela nd Ltd. A l l r ights reserved 0304-3940/93/$ 06.00

NS L 09235

H igh conc entrat ions of g lycine induce long- lasting chang es in synapticeff icacy in rat hippocampal sl ices

K a v i a n S h a h i, J u a n - C a r l o s M a r v i z o n a n d M i c h el B a u d r yNeuroscience Program, U niversity of Southern C alifornia, Los Angeles, CA 90089-2520 (U SA )

(Received 6 August 1992; Revised version received 14 October 1992; Accepted 14 October 1992)Key words." Hippocam pus ; N M D A r ecep to r; P l as ti ci ty ; L ong- t e r m po ten t i a t i on ; L ong- t e r m dep r ess ion; E p i l epsy

Brief per fusion o f adul t rat h ipp ocam pal sl ices wi th high conc entrat ions of glycine resul ts in a slowly developing, long- last ing increase in synapt icresponses in field CA1. Tw o observat ion s indicated that th e ef fect requires the act ivat ion of NM D A receptors by glycine. F irst , the glycine- inducedpo ten t i a t i on i s r educed by ke t amine , an N M DA r ecep to r channe l b locker . S econd , g lyc ine po t en t i a te s t he N M DA r ecep to r - med ia t ed ep il ep t i fo r mac t iv i ty r eco r ded in t he p r esence o f lo w magnes ium concen t r a t i on andp ic r o tox in . I n s l i ces p r epa r ed f r om r a t pups( 5 8 pos tna t a lda y ) ,pe r f us ion wi thglycine resul ts in a slowly developing , long- last ing depression o f EPSP am pli tude. These resul ts provid e a new way of producing p otent iat io n ofsynapt ic eff icacy and suggest new proper t ies of NM D A recep tors.

S e v e r a l m a n i p u l a t i o n s h a v e b e e n f o u n d t o p r o d u c e along- las t ing poten t i a t ion of synap t i c e f fi cacy a t exc i t a -tory synap ses . These inc lude the we l l desc r ibed h igh f re -quen cy e lec tr i ca l s timula t ion tha t resu l t s in wh a t i s no wgenera l ly de f ined as long- te rm poten t i a t ion (LTP) [6] andwhich requi res the inf lux of ca lc ium in pos t syn apt i cs t r u c tu r e s f o l lo w i n g th e a c t iv a t i o n o f N M D A r e c e p t o r s[18] . Var ian t s o f th i s form of poten t i a t ion involve repe t i -t iv e p a i r in g o f p r e s y n a p t i c s t i m u l a t i o n w i t h p o s t s y n a p t i cd e p o l a r i z a t io n [ 1 2 ]. A n o t h e r f o r m o f p o t e n t i a t io n d o e sn o t r e q u i re N M D A r e c e p t o r a ct i v a ti o n , b u t r a th e r t h ea c t i v a ti o n o f v o l t a g e - d e p e n d e n t c a l c iu m c h a n n e ls a n dcan be pro duc ed by e lec t ri ca l s t imula t ion a t f requenc iesh i g h e r t h a n t h a t r e q u i r e d f o r L T P [ 1 0] o r b y b l o c k a d e o fp o t a s s i u m c h a n n e l s w i t h t e t r a e t h y l a m m o n i u m ( T E A )[ 1] . M o r e r e ce n t ly , a n o t h e r f o r m o f p o t e n t i a ti o n h a s b e e nd e s c r ib e d w h i c h i n v o l v e s a c t i v a ti o n o f m e t a b o t r o p i c g l u -t a m a t e r e c e p t o r s f o l l o w i n g N M D A r e c e p t o r a c t i v a t i o n[20] . In th i s case , the po tent i a t ion deve lop s s lowly over ap e r i o d o f te n s o f m i n u t e s, a s c o m p a r e d t o t h e f e w m i n -u tes tha t a re requ i red to fu l ly es t ab l i sh s t ab le LT P [2 , 11] .I t i s ye t unc lea r whe ther or not these d i f fe rent forms ofp o t e n t i a t i o n s i m p l y re f le c t t h e a c t i v a ti o n o f a c o m m o nsequence o f in t race l lu la r event s .

Al though i t has been shown tha t g lyc ine ac t s as a co-a g o n i s t o f g l u t a m a t e a t t h e N M D A r e c e p to r , t h e p h y s io -Correspondence: K. S hah i , Neur osc i ence P r og r am, HNB 311 , US C,Los Ang eles, Ca 90089-2520, US A. Fax: (1) (213) 746-2863.

logical s ignif icance of the glycine s i te st il l remain s p uzz-l ing as the aff ini ty o f the s i te for glycine is so high th at i ti s l ike ly to be a lways sa tura ted [13 , 24] . Indeed , mos ta t t empts to f ind d i rec t e f fec t s of exogenous g lyc ine org l y ci n e a g o n i s ts o n N M D A r e c e p t o r - m e d i a t e d p h y s i o -logica l event s have proven unsucces s fu l . Only whenglyc ine an tagoni s t s a re used i s i t pos s ib le to observeb l o c k a d e o f N M D A r e c e p t o r f u n c ti o n w h i c h is t h e n r e-versed by the add i t ion of exogen ous g lyc ine [4 , 19 , 23].We pos tu la ted tha t the re might ex i s t g lyc ine s it e s wi th ana f f i n i t y l o w e n o u g h t h a t t h e y h a d n o t b e e n d e t e c t e d b yl igand b inding t echniques or tha t they had no t been s tim-ula ted by the re l a t ive ly low concent ra t ions of g lyc ineused in previous s tudies . In the present work , we repor ttha t h igh co ncent ra t ion s o f g lyc ine prod uce a s lowly de-ve loping , long- las t ing potent i a t ion of synapt i c t ransmis -s ion and present ev idence tha t the e f fec ts requi re the ac t i -v a t io n o f N M D A r e ce p to r s.

E x p e r i m e n t s w e r e p e r f o r m e d o n h i p p o c a m p a l s l i c e s( 4 0 0 / l m t h ic k ) p r e p a r e d f r o m a d u l t ( 1 5 0 - 2 0 0 g) a n d n e o -na ta l (5-8 pos tna ta l days ) Sprague-Dawley ra t s . S l i cesw e r e p l a c e d i n a n i n t e r f a c e c h a m b e r a n d s u b f u s e d a t ar a t e o f 1 m l /m i n w i t h m e d i u m c o n t a i n i n g ( in m M ) : N a C 1124, KCI 3, KH2PO4 1.25, CaCI2 3, MgCI2 1, NaHCO326, L-glucose 10, L-asco rbate 2. In expe rime nts analyz ingrhythm ic burs t ing ac t iv i ty , p i c ro toxin (50 / IM) wasa d d e d a n d t h e c o n c e n t r a t i o n o f M g 2+ w a s l o w e r e d t o 5 0~ tM. S l i ces were main ta ined a t 35C and oxygena tedwi th a m ixture O2:CO2 (95:5) . Af te r a t l eas t 1 h of inc ub a-

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[ 3 C o n t r o lt ion , a g las s r ecor d ing e lec t r od e (1 5 M D , f i l l ed w i th 2 MN a C 1 ) w a s p o s i t i o n e d i n t h e d e n d r i t i c l a y e r o f fi e ld C A 1t o r e c o r d e x t r a c e l l u l a r f ie l d p o t e n t i a l s . S c h a f f e r -c o m m i s -s u r a l f i b e r s w e r e a c t i v a t e d b y b i p o l a r s t i m u l a t i n g e l e c -t r o d e s p o s i t i o n e d i n s t r a t u m r a d i a t u m . W h e n r e c o r d i n gs p o n t a n e o u s b u r s t i n g a c t i v i t y , n o s t i m u l a t i o n w a s g i v e n .A c o n c e n t r a t e d s t o c k s o l u t i o n o f g l y c i n e w a s a d j u s t e d t op H 7 .4 w i th N a O H , a n d w a s a d d e d t o t h e p e r f u s ed b u f f e rf o r t h e in d i c a t e d p e r i o d s o f ti m e . P o p u l a t i o n E P S P s w e r er e c o rd e d , a n a l y z e d , a n d s t o r e d o n c o m p u t e r . E P S P a m -p l i t u d e s a n d s l o p e s w e r e c a l c u l a t e d a n d a m p l i t u d e d a t aa r e pr esen ted in the r esu l t s s ec t ion ( s imi la r r esu l t s w er eo b t a i n e d u s i n g s l o p e d a t a ) .

P e r f u s i o n o f 1 0 m M g l y c i n e f o r 1 0 r a i n i n a M g 2+-c o n t a i n i n g b u f f e r d i d n o t p r o d u c e a n y s i g n i f i c a n t a l t e r a -t i o n i n th e a m p l i t u d e o r s l o p e o f t h e E P S P ( F i g . 1 A ), n o rd id i t a f f ec t the f ibe r vo l ley , an index of a f f e r en t r ec r u i t -m e n t . U p o n w a s h o u t o f g ly c in e , a s lo w l y d e v e lo p i n g i n -c r e a s e in E P S P w a s o b s e r v e d ; t h e i n c r ea s e r e a c h e d a p l a -t e a u a f t e r a b o u t 1 h w h e n i t r e p r e s e n t e d a 9 4 + 1 7% in -c r e a s e in E P S P a m p l i t u d e ( m e a n + S . E . M . o f 6 e x p e r i-m e n t s ) a n d l a s t e d u p t o 3 h . T h e g t y c i n e - i n d u c e d p o t e n t i -a t i o n w a s m a r k e d l y r e d u c e d i n s l i c e s i n c u b a t e d i n t h ep r e s e n c e o f k e t a m i n e (2 5 0 p M ) ( F i g . I A ) ; u n d e r t h e s ec o n d i t i o n s t h e m a x i m u m i n c r e a s e i n E P S P a m p l i t u d er e p r e s e n t e d a 3 4 + 1 1 % i n c r e a s e ( m e a n + S . E . M . o f 9 ex -p e r i m e n t s ) . W e c o m p a r e d t h e e f f e c t o f k e t a m i n e o ng l y c i n e -i n d u c e d p o t e n t i a t i o n t o t h a t o b s e r v e d o n L T Pe l i ci t e d b y t h e t a b u r s t s t i m u l a t i o n ( F i g . 1 B ) . K e t a m i n es i m i l a r l y r e d u c e d t h e e x t e n t o f L T P , r e d u c i n g t h e i n -c r e a s e i n E P S P a m p l i t u d e f r o m 5 2 + 1 2% t o 1 6 + 5 %( m e a n s + S . E . M . o f 4 - 5 e x p e r i m e n t s ) .G l y c i n e - i n d u c e d p o t e n t i a t i o n w a s n o t a c c o m p a n i e d b ya c h a n g e i n t h e e x t e n t o f p a i r e d - p u l s e f a c i l i t a t i o n , a ni n d e x o f t r a n s m i t t e r r e l ea s e ( d a t a n o t s h o w n ) . W h i l e n oe f f e c t s o f g l y c i n e o n t h e a m p l i t u d e a n d s l o p e o f E P S P sw e r e o b s e r v e d d u r i n g i ts a p p l i c a t i o n i n n o r m a l M g 2+-c o n t a i n i n g b u f f e r , a s m a l l d e p r e s s i o n o f t h e s e p a r a m e t e r sw a s p r e s e n t d u r i n g t h e a p p l i c a t i o n i n l o w M g 2 + - c o n ta i n -i n g b u ff e r . T o f u r t h e r d o c u m e n t t h e p o s s i b i l i t y t h a t h i g hc o n c e n t r a t i o n s o f g l yc i n e c o u l d a c t i v a te N M D A r e ce p -t o r s , w e d e v e l o p e d a m o d e l o f s p o n t a n e o u s e p i l e p t i c ac -t i v it y . I n t h e p r e s en c e o f p i c r o t o x i n ( 5 0 / I M ) a n d l o wM g 2+ c o n c e n t r a t i o n ( 5 0 / ~ M ) , e x t r a c e l l u l a r r e c o r d i n g s o ff ie l d p o t e n t i a l s i n t h e d e n d r i t i c l a y e r o f f ie l d C A 1 r e-v e a l e d s p o n t a n e o u s b u r s t i n g a c t i v i t y ( F i g . 2A ) . T h i sb u r s t in g a c t i v it y w a s d e p e n d e n t o n N M D A r e c e p t o r ac -t i v a t i o n a s i t w a s t o t a l l y b l o c k e d b y k e t a m i n e ( 1 0 0 / ~ M )( F i g . 2 C ) o r b y M g 2+ ( 1 4 m M ) ( d a t a n o t s h o w n ) . P e r f u -s i on o f h i g h c o n c e n t r a t i o n s o f g l y c in e (5 - 1 0 m M ) r e-s u l t e d i n a s i g n i f ic a n t in c r e a s e i n t h e f r e q u e n c y o f t h eb u r s t i n g a c t i v i t y ( F i g . 2 B ) , e v e n w h e n C A 3 a f f e r e n t s t oC A 1 w e r e i n t e r r u p t e d ; t h i s e ff e c t w a s r e v e r s ib l e u p o n

(D

~ cno

c L - - J

2 5 0

2 0 0

1 5 0

100

5 0

%

186

Gl y10 mM

l b 2 ' o 3'o 5'o 6'0 7'oT i m e ( m i n )

0 ControlB Ketamine 200 laM

2 5 0

2 0 0

1 5 0

1 0 0

5 0 TB SI0 5 I I I10 15 20

T i m e ( m i n )25

Fig. 1. Effect of ketamine on glyc ine-induced (A) or TBS-induced (B)potentiation of synaptic responses in CAl. Hippocampal slices wereprepared and population EPSPs were evoked and recorded as describedunder Methods in the absence (open squares) or presence (closedsquares) of ketamine (250 yM) . A: glycine (10 mM) was added to theperfusion buffer during the pe riod indicated by the horizontal bar. B: atthe time indicated by the arrow, theta burst stimulation (TBS) wasdelivered to the stimulating electrode. Results represent the amplitudeof the population EPSP (similar results were obtained on EP SP slopes)and are expressed as percentage of the mean base-line value obta inedduring 10 min before glycine or TBS application. Me ans _+S.E.M. of

4 9 experimen ts.

w a s h o u t o f g l y c i n e . M o r e o v e r , n o e f f e c t o f g l y c i n e w a so b s e r v e d w h e n s l i ce s w e r e p r e t r e a t e d w i t h k e t a m i n e ( F i g .2 D ) .

S e v e r al s t u d i e s h a v e s h o w n t h a t L T P i s a b s e n t i n s l i ce sp r e p a r e d f r o m r a t p u p s b e f o r e p o s t n a t a l d a y 1 0 - 1 2 [ 5 ,2 1 ]. P e r f u s i o n o f 1 0 m M g l y c i n e i n s li c es p r e p a r e d f r o m5 to 8 - d a y - o l d r a t p u p s i n i t i a l l y p r o d u c e d a d e p o l a r i z a -t i o n a s r e f l e c t e d b y t h e d e c r e a s e i n E P S P s l o p e a n d a m -p l i t u d e d u r i n g t h e p e r f u s i o n i n l o w M g 2 + - c o n t a i n i n gb u f f e r (F i g . 3 ). U p o n w a s h o u t o f g l y c i n e , t h e p a r a m e t e r so f t h e e v o k e d r e s p o n s e s r e t u r n e d t o p r e - g l y c i n e v a l u e sb e f o r e e x h i b i t i n g a s l o w l y d e v e l o p i n g a n d l o n g - l a s t i n gd e p r e s s i o n . L i k e t h e g l y c i n e - i n d u c e d p o t e n t i a t i o n s e e n i ns li c es f r o m a d u l t a n i m a l s , t h e g l y c i n e - i n d u c e d d e p r e s s io nr e a c h e d a p l a t e a u a t a b o u t 1 h f o l l o w i n g g ly c i n e a p p l i c a -

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18 7

Control

~ . ~ , ~ _ , ~ ~ ~ K e t a m m eKetamlne+ Gl y

F i g . 2 . E f f e c t o f v a r i o u s c o m p o u n d s o n s p o n t a n e o u s b u r s t i n g a c t i v it y .E x t r a c e l l u la r r e c o r d i n g e l e c t r o d e s w e r e p l a c e d i n s t r a t u m r a d i a t u m o fa r e a C A 1 o f h i p p o c a m p a l s l i ce s m a i n t a i n e d i n a m e d i u m c o n t a i n i n g 5 0/ I M M g > a n d 5 0 / I M p i c r o t o x i n . F i e l d p o t e n t i a l s w e r e r e c o r d e d i n t h ea b s e n c e o f a n y s t i m u l a t i o n a n d u n d e r c o n t r o l c o n d i t i o n s ( A ) , i n t h ep r e s e n c e o f 1 0 m M g l y c i n e ( B ) , i n t h e p r e s e n c e o f k e t a m i n e ( 1 0 0 / a M )( C ) , a n d i n t h e p r e s e n c e o f g l y c in e a n d k e t a m i n e ( D ) . B a r = 1 m V , 1 s .T h e e x p e r i m e n t w a s r e p e a t e d 3 4 t i m e s w i t h e s se n t i a ll y s im i l a r r e s u lt s .

t i on , a t which p oin t i t represented a 30 + 6% decrease ina m p l i t u d e o f th e E P S P s .

The present resu l ts ind ica te tha t pe r fus io n o f sl icesw i t h m i l l im o l a r c o n c e n t r a t i o n s o f g l y c in e p r o d u c e ss lowly deve loping , long- las t ing and opp os i t e changes insynapt i c e f f icacy in adul t a nd n eona ta l ra t s. W hi le poten-t i a t ion fo l lowed g lyc ine appl i ca t ions in adul t s , t he s amecondi t ions induced depres s ion in neona ta l ra t s . Severa l2 o 0 [

150[

E m 100

g ly10 mM

C L ~L I 5O

0 i I I I I t0 2 0 4 0 6 0 8 0 10 0 1 2 0T i m e (m i n )

F i g . 3 . G l y c i n e - i n d u c e d d e p r e s s i o n i n n e o n a t a l s l ic e s. H i p p o c a m p a ls l ic e s f r o m n e o n a t a l r a t s ( P N D 5 8 ) w e r e p r e p a r e d a n d m a i n t a i n e d i na m e d i u m c o n t a i n i n g l o w M g > c o n c e n t r a t io n ( 5 0 / I M ) a n d p o p u l a t i o nE P S P s w e r e r e c o r d e d a s d e s c r ib e d u n d e r M e t h o d s . G l y c i n e ( 10 m M )w a s a p p l i e d d u r i n g t h e p e r i o d i n d i c a t e d b y a n h o r i z o n t a l b a r . R e s u l t sr e p r e s e n t t h e a m p l i t u d e o f t h e E P S P s ( s i m i l a r r e s u lt s w e r e o b t a i n e d o nE P S P s l o p e s ) a n d a r e e x p r e s s e d a s p e r c e n t a g e o f t h e m e a n b a s e - l i n ev a l u e r e c o r d e d d u r i n g 1 0 r a i n b e f o r e g l y c i n e a p p l i c a t i o n . M e a n s

+ S . E . M . o f 5 e x p e r i m e n t s .

arguments ind ica te tha t the g lyc ine- induced inc rease insynapt i c e f f i cacy in adu l t s li ces requi res the ac t iva t ion ofN M D A r e c e p to r s . F i r s t, t h e g l y c i n e- i n d u c ed p o t e n ti a -t ion was cons iderably reduced when s l i ces were pre -t r e a t e d w i t h t h e N M D A c h a n n e l b l o c k e r k e t a m i n e a n dt h e e x t e n t o f t h e r e d u c t i o n w a s t h e s a m e a s t h e r e d u c t i o nof LTP e l i c i ted by the ta bur s t s t imula t ion . In the absen ceof M g 2 and wi th inhib i t ion b lock ed by p ic ro toxin , ep-i l ep t i form ac t iv i ty charac te r i zed by repe t i t ive burs t ingac t iv i ty spontaneo us ly dev e loped in fi eld CAI of h ip-p o c a m p a l s li ce s e v e n w h e n t h e c o n n e c t i o n s b e t w e e n C A 3and C A1 were in te r rupted . T hi s burs t ing ac t iv i ty wasd u e t o t h e a c t i v a t io n o f N M D A r e c e p t o r a s it w a s t o t a ll yp r e v en t e d b y n o n - c o m p e t it iv e N M D A r e c e p to r a n t a g o-n i st s, s u c h a s k e t a m i n e a n d M g > . U n d e r t h e se c o n d i -t ions , g lyc ine inc reased the burs t ing f requency sugges t -ing tha t h igh concen t ra t ions of g lyc ine poten t i a t edN M D A receptor ac t iva t ion . F ina l ly , b inding exper i-men t s us ing the b inding of [3H]MK-801 to we l l -washedm e m b r a n e s a s a f u n c t i o n a l in d e x o f c h an n e l o p e n i n g a l s oshow ed tha t h igh concen t ra t ions o f g lyc ine s t imula ted[3H]MK-801 b inding ev en in the absence o f g lu tam ate[15] . However , whi l e these a rguments suppor t the ideat h a t g l y c i n e is c a p a b l e o f a c ti v a t in g N M D A r e c e p t o rs ,the re a re marked d i f fe rences be tween the e f fec t s ofg l y c i n e r e p o r t e d h e r e a n d t h o s e p r o d u c e d b y t h e d i r e c ta p p l i c a ti o n o f a n N M D A r e c e p t o r ag o n i s t su c h a sN M D A . U n d e r n o r m a l i o n ic c o n d i t io n s , p e r fu s i o n o fs lic es w it h m o d e r a t e c o n c e n t r a t i o n s o f N M D A ( 1 0 0 / I M )produced depola r i za t ion [8] and se izure ac t iv i ty , p roba-b l y a s a r e su l t o f th e r e g e n e r a ti v e p r o p e r t y o f t h e N M D Ar e c e p t o r s (i .e . N M D A r e c e p t o r a c t iv a t i o n l e a d s t o d e p o -l a r i z a t i o n m a k i n g n e i g h b o r i n g N M D A r e c e p t o r s l e s ssens it ive to b locka de b y M g 2+ and the re fore m ore pron eto ac t iva t ion) . F ur the rm ore , t rea tmen t o f s li ces wi thN M D A a l o n e ( ei th e r b y b a t h a p p l i c a ti o n o r b y io n t o -phores i s ) does not produce a long- las t ing potent i a t ion ofsynapt i c response but ra the r a shor t - l ived potent i a t ionlas ting a bo ut 30 min [3 , 14]. We thus h ave to co nc ludetha t i f g lyc ine ac tiva tes NM D A receptors , i t does i t ve rydi f fe rent ly than a fu l l agoni s t o f the receptors . Pe rhaps ,g lyc ine ac t s as a weak or pa r t i a l agoni s t which producesenou gh a c t iva t ion to t r igger a long- las t ing chan ge in syn-apt i c e f f i cacy bu t does n ot gen era te a s igni f icant depola r -i za t ion . In te res t ingly , we noted tha t under depola r i z ingc o n d i t i o n s p r o d u c e d b y t h e t a b u r s t s t i m u l a ti o n , g l y c in e -i n d u c e d p o t e n t i a t i o n d e v e l o p e d f a s t e r a n d a t a l o w e rglyc ine conce nt ra t ion than in the absence of h igh f re -q u e n c y s t i m u l a t io n ( d a t a n o t s h o w n ) .

In neona ta l ra t s , h igh f requency s t imula t ion doe s no ti n d u c e L T P a t l e a st n o t b e f o r e p o s t n a t a l d a y ( P N D ) 1 0 -12 [5 , 21] . How ever , long- las t ing de pres s ion of synap t i ct ransmis s ion i s much more readi ly observed in neona ta l

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188than in adult slices following brief periods (2-4 min) oflow frequency stimulation (Fitzpatr ick et al , unpublish eddata). Interestingly, glycine applica tion in slices of PND5-8 ani mals produced a long-lastin g depression whichrepresented a mirro r image of the potenti ati on observedin adult slices. Furthermore, we also observed oppositeeffects of PLA2 treatment of synaptic membranes on thebinding properties of the AM PA receptors in neonataland adult rat brain [17]. PLA2 treatment produced a de-crease in [3H]AMPA binding in neonatal membr anes andan increase in adult membranes. We have also arguedthat the PLA2-induced changes in receptor properties arepart o f the mechan isms under lyin g LTP [16]. As NM DAreceptor activat ion has been linke d to PLA 2 activity [7],these results suggest the possibility that the glycine-in-duced changes in synaptic efficacy are mediated throughPLA~ stimulation.

Finally, our results provide an additional way of pro-ducing long-lasting changes in synaptic transmission.Thus, in addition to high frequency stimulation, it hasbeen shown that increased calcium concentration [25],short period of K+-induced depolar izati on [9], TEA ap-plication [1], application of a combi nati on of NMDA ,glycine, calcium and spermine [22], and more recentlyactivation of NMD A and glutamate metabotropic recep-tors during low frequency stimulation [20], all produceincreased synaptic efficacy. Whether all these manipula-t ions produce the activation of a com mon f inal pathwayleading to a single modification of one element regulat-ing synaptic transmission or produce unique modif ica-tions of several regulatory elements remains to be deter-mined.

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