Developmental changes in the susceptibility to long-term potentiation of neurones in rat visual cortex slices

43

Developmental Brain Research, 60 (1991) 43-50 t~ 1991 Elsevier Science Publishers B.V. 0165-3806/91/$03.50 ADONIS 016538069151257Y BRESD 51257

Developmental changes in the susceptibility to long-term potentiation of neurones in rat visual cortex slices Nobuo Kato*, Alain Artola and Wolf Singer Max-Planck-lnstitute for Brain Research, Frankfurt am Main (ER. G.) (Accepted 15 January 1991)

Key words: Long-term potentiation; Plasticity; N-Methyl-D-aspartate; Visual cortex; Rat

We investigated with intracellular recordings from rat visual cortex slices whether the susceptibility to undergo long-term potentiation (LTP) is age-dependent and whether it is correlated with the expression of synaptic responses mediated by N-methyl-D-aspartate (NMDA) receptors. Test and tetanic stimuli were applied to the white matter and post-tetanic modifications of the amplitude of postsynaptic potentials (PSPs) were assessed in regular spiking cells of supragranular layers. At 2 weeks of age, the amplitudes of early (8-10 ms post-stimulus) and late (20 ms post-stimulus) PSP-components increased after tetanic stimulation to 137.1 + 13.4% and 141.3 + 12.1% of the pretetanic controls, respectively. At 3 weeks, potentiation of both PSP-components was less pronounced but still significant, the late component being on average more potentiated than the early one. At 4 weeks, PSPs were no longer potentiated. Bath application of 25/~M OL-2-amino-5-phosphonovalerate (APV), an NMDA receptor antagonist, blocked LTP induction both at 2 and at 3 weeks. We also studied developmental changes of two synaptic responses known to influence the susceptibility of cortical neurones to LTP, the NMDA receptor-mediated excitatory PSP (EPSP) and the initial inhibitory PSP (ilPSP). The amplitude of the APV-sensitive EPSP decreased with age and reached adult values in 4-week-old animals. The ilPSPs were pronounced already at 2 weeks and showed no marked change during further development. The results suggest a close correlation between the susceptibility to undergo LTP and the extent to which NMDA receptor-gated conductances contribute to the synaptic response. We propose that the decline of experience-dependent malleability that is observed in vivo during postnatal development is causally related to the age-dependent reduction of NMDA receptor-mediated responses. INTRODUCTION

MATERIALS AND METHODS

D u r i n g a critical p e r i o d of early p o s t n a t a l d e v e l o p ment,

neurons

in t h e

mammalian

visual c o r t e x

are

susceptible to e x p e r i e n c e - d e p e n d e n t m o d i f i c a t i o n s of t h e i r r e s p o n s e p r o p e r t i e s 2°'37. R e c e n t l y , e v i d e n c e b e c a m e a v a i l a b l e that t h e s e a d a p t i v e c h a n g e s r e q u i r e activation o f N - m e t h y l - D - a s p a r t a t e ( N M D A ) r e c e p t o r s 6'17'22 as is t h e case for u s e - d e p e n d e n t l o n g - t e r m p o t e n t i a t i o n (LTP) of synaptic t r a n s m i s s i o n in the h i p p o c a m p u s a3'18'38 and t h e n e o c o r t e x 2'3'2L33'34 o f adult animals. This s u g g e s t e d that t h e two f o r m s of a c t i v i t y - d e p e n d e n t synaptic malleability m i g h t d e p e n d on similar m e c h a n i s m s . In addition t h e r e is e v i d e n c e f r o m field p o t e n t i a l r e c o r d i n g s that the LTP susceptibility of s u p r a g r a n u l a r layers is also s u b j e c t to d e v e l o p m e n t a l c h a n g e s 32. T h u s , LTP can be u s e d as a m o d e l for t h e i n v e s t i g a t i o n of the m e c h a n i s m s u n d e r l y i n g a g e - d e p e n d e n t c h a n g e s o f n e u r o n a l plasticity. T h e goal of this study was to c o n f i r m the a g e - d e p e n d e n c y o f LTP with i n t r a c e l l u l a r r e c o r d i n g s and to analyze the causes of t h e s e d e v e l o p m e n t a l c h a n g e s .

Albino rats of various ages were obtained from a laboratory colony. Preparation of slices, recordings and data analysis were carried out as described previously3. After decapitation, the brains were rapidly removed and placed into cold oxygenated standard medium (containing, in mM: NaCl 124, KC1 5, NaPO, 1.25, MgSO 4 2, CaCI 2 2, NaHCO 3 26, D-glucose 10 and H~O 2 0.002%, bubbled with 95% O z, 5% CO2) for a few minutes. Visual cortex was sectioned at 350-400/~m on a vibratome. Slices were allowed to recover in the incubation medium at room temperature for at least 1-2 h before recording was attempted. A single slice was then transferred to the recording chamber. Recordings were performed at 29-30°C in a submerged type chamber. In some experiments, DL-2-amino°5-phosphonovalerate (APV; 25 ~M) was added to the perfusate. A bipolar stimulating electrode consisting of two insulated tungsten wires was inserted into the white matter below the recording site. Intracellular recordings were obtained from cells in layers II-IV with a glass micropipette filled with 3 M potassium-acetate. Postsynaptic potentials (PSPs) were evoked by subthreshold stimuli (70-80% of threshold intensity for the elicitation of a spike) applied to the white matter at low frequency (0.07 Hz). When test responses were stable for at least 30 min (control period) tetanic stimulation was applied through the same stimulating electrode. The tetanic stimulus consisted of five 2-s long trains (200 ks pulses at 50 Hz) that were repeated at intervals of 10 s. For the tetanus stimulation intensity

* Present address: Institute for Brain Research, Faculty of Medicine, Kyoto University, 606 Kyoto, Japan. Correspondence: W. Singer, Max-Planck-Institute for Brain Research, Deutschordenstr. 46, D-6000 Frankfurt am Main 71, ER.G.

44 TABLE I

Developmental changes in resting membrane potential (Vmr) and input resistance (Ri) of the cells Mean _+S.D. ; n, number of cells.

Vmr(mV) Ri (MW)

2 weeks

3 weeks

4 weeks

Adult

-68.5+5.2 (n = 17) 48.4+25.0 (n = 14)

-71.7_+6.1 (n = 18) 46.4_+12.8 (n = 15)

-78.3_+6.2 (n = l l ) 42.9+14.0 (n = 10)

-72.2_+4.0 (n = 20) 42.8+18.1 (n = 20)

was raised to 1.5-2 times threshold intensity. Recording of test responses was then continued for up to 60 min. Responses were digitized at a rate of 5 kHz, averaged and stored on disk. Post-tetanic modifications were quantified by comparing averaged (from 5 successive responses, 0.07 Hz) responses measured 15 rain post-tetanus with averaged pretetanic controls. Changes were expressed as percentage of control. Values in the text, figures and legends, are given as mean + standard deviation unless mentioned

II

| Control ~

A

T

III

15 min

otherwise. Student's or Welch's tests were used to compare the means. To assess age-dependent changes in the strength of the initial inhibitory PSP (ilPSP), response conductance 15 was measured at the peak of the ilPSP. Current pulses (400 ms duration) of variable intensity were applied starting 100 ms before the white matter stimulus. We measured the current-voltage relationship at the latency of the negative peak of the ilPSP (20-22 ms post-stimulus) and calculated the corresponding membrane conductance from the slope of this relationship close the the resting membrane potential. This value was then subtracted from the resting membrane conductance to obtain the response conductance.

RESULTS I n t r a c e l l u l a r r e c o r d i n g s w e r e o b t a i n e d f r o m n e u r o n s in t h e s u p e r f i c i a l l a y e r s ( I I - I V ) o f v i s u a l c o r t e x slices o f r a t s w h o s e a g e r a n g e d f r o m 2 t o 10 w e e k s . T h e v a s t m a j o r i t y of the recorded

cells h a d p r o p e r t i e s

IV

3 0 rain

characteristic of

V

i !,

T.

', I~

\ i

I

'

i

B C

160

1

TET.

150 i

140130-

" o *"

oo

peak • +20 ms post-stimulus ~ S.E.M.

120 ~ 110-

~) 1 0 0 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . o o~ 90~ q,.

80

-2'0

-1'0

()

1'0

2'0

3'0 min

Fig. 1. Effect of high frequency stimulation on synaptic responses recorded from cells in slices of 2-week-old rats. A,B: PSPs elicited by t e s t stimuli (0.07 Hz) in 2 different cells before (I), 15 min (If) and 30 min (III) after tetanic stimulation. Pre- and post-tetaaic responses are superimposed at the same time scale in IV. PSPs in I and II are superimposed at expanded time scale in V to show the change in initial slope. Calibrations in B are for A and B. All traces represent averages of 5 successive test responses. The trace B I I I is missing because the cell was lost 25 rain after tetanus. C: time course of post-tetanic amplitude changes of early ( 0 ) and late ( A ) PSP-components. Values are averages from 11 cells expressed as percent of controls taken just before tetanus. Vertical bars indicate + S.E.M.

45

I Control i\

II TET.

I

,\

V

IV

III 40 rain

15 rain

\

,

Control

I

I

15 min

i

30 min

/

-1

i,~

1 2O ms

C

2ms

140® ,D o,-,

130-

TET.

C

120-

ID

®

=-

110-

"

100-

.__________________'_...... peak • +20 ms post-stimulus I S.E.M.

C

o u

90

~

8o 70

-2'0

-1'0

0

1'0

2'0

3'0 min

Fig. 2. Effect of high frequency stimulation on synaptic responses recorded from cells in slices of 3-week-old rats. A,B: PSPs elicited by test stimuli (0.07 Hz) in 2 different cells before (I), 15 min (II) and 30 (40) min (III) after tetanic stimulation. Pre- and post-tetanic responses are superimposed in IV. The PSPs in I and II are superimposed in V at expanded time scale to show the change in the initial slope. Calibrations in B are for A and B. All traces are averages from 5 successive responses (0.07 Hz). C: time course of post-tetanic amplitude changes of early (0) and late (&) PSP-components. Values are averages from 9 cells expressed as percent of controls taken just before tetanus. Vertical bars indicate + S.E.M.

regular spiking cells27 but especially in slices of the youngest rats (2 weeks of age) bursting cells27 were also e n c o u n t e r e d . In this study we only included regular spiking cells with stable m e m b r a n e potential below - 5 5 mV and overshooting spikes. Resting m e m b r a n e potential and input resistance of the cells were similar to those r e p o r t e d previously in slice p r e p a r a t i o n s 23'28 and showed no a g e - d e p e n d e n t variation (Table I). This is in accordance with the results of M c C o r m i c k and Prince 2~ who found that the most significant maturational changes in passive m e m b r a n e p r o p e r t i e s occur during the first postnatal week. As described previously for the neocortex of adult rats 14"15, white m a t t e r stimulation elicited a characteristic p a t t e r n of PSPs, i.e. an excitatory PSP (EPSP) followed by two IPSPs, the initial IPSP (ilPSP) and the late IPSP (IIPSP), respectively. In addition, two c o m p o n e n t s could

be dissociated in the EPSP on the basis of their differential susceptibility to APV, a selective antagonist of N M D A receptors, and to 6-cyano-7-nitroquinoxaline2,3-dione ( C N Q X ) , a selective antagonist of alphaamino-3-hydroxy-5-methyl-4-isoxazole-propionic acid

1

TET

"IT

-,r

nr

I~' 3 0 rain. jI" "\

t

v)

5 mV

Fig. 3. PSPs obtained from a cell in a slice of a 4-week-old rat before and after tetanic stimulation. Conventions are the same as in previous figures.

46

A~

150-

c

Ill U)

Z

o

• pea# values Ovaluea at 20 ms

140-

* I)
O. U) UJ ..J

on . iz O

(..)

A I~

m e l i l t 1.96 S.O. of the mean

T

130-

ontrol

20 ms TET

11"

~

;' ~\

11T 1 1rain.

120-

UJ

s

2:

I-14. O l'Z I,U t.,) nUJ

110-

1OO-

. . . . . . . . . . . . . . . . . . . . . . .

n~_g_ . . . . .

adult

AGE OF A N I M A L S Fig. 4. Developmental changes in LTP amplitudes. Values represent averaged PSP amplitudes from the different age groups measured 15 min after tetanus and expressed in percent of pre-tetanic control values (n = number of cells). Filled and open circles refer to early and late PSP-components, respectively. These values were compared to corresponding values measured previously in slices of adult rats which were investigated under identical conditions and showed no post-tetanic response modification 3. In these experiments posttetanic amplitudes of early and late PSP-components were 97.5 + 5.5% and 97.1 _+ 9.3% of pretetanic controls, respectively. This comparison reveals that amplitude changes differ significantly (P < 0.001 or 0.01) from those in the adult controls in 2- and 3-week-old, but not in 4-week-old animals.

(AMPA) receptors. The AMPA receptor-mediated EPSP had a large amplitude and a short peak latency (8-10 ms) and the NMDA receptor-mediated EPSP a smaller amplitude and a longer peak latency (Figs. 5 and 6; see also ref. 3). Because of the composite nature of the synaptic response, amplitude changes were measured at two post-stimulus delays: first, at the peak of the PSP

2 weeks

3 weeks

Fig. 5. The effect of A P V application on PSP amplitude and LTP induction. A: APV-sensitive EPSP in a neuron from a slice of a 3-week-old rat. The upper two traces show PSPs before (control) and after A P V application (APV). The APV-sensitive EPSP was obtained by subtracting the response recorded after A P V application from control and is shown at the bottom. B: PSPs recorded from the same cell before (I) and after (II) tetanic stimulation in the presence of 25 g M APV. PSPs in I and II are superimposed in III. All traces are averages from 5 successive test responses (0.07 H_z).

which is usually coincident with the peak of the AMPA receptor-mediated EPSP and second, at a delay of 20 ms, where both the NMDA receptor-mediated EPSP and the iIPSP peak. These two amplitude values will be referred to as amplitudes of early and late PSP-components, respectively. All measurements were done on averaged (n = 5) PSPs recorded before (control) and 15 min after the tetanus and changes were expressed as percentage of control.

Developmental changes in the susceptibility to undergo LTP LTP was readily inducible in slices of 2-week-old animals (n = 11) (Fig. 1). Potentiation appeared after a short post-tetanic depression (2-3 rain) and was stable throughout the remaining recording period (up to 60 min after the tetanus) (Fig. 1C). It was associated with an 4 weeks

Adult

T Fig. 6. APV-sensitive EPSPs recorded in neurons from slices of rats of different ages. Conventions are as in Fig. 5A.

47 increase of the amplitude and the initial slope of the PSP (see column V in Fig. 1A,B). The average amplitudes of the early and the late PSP-components had increased to a similar extent and were 137.1 + 13.4% and 141.3 + 12.1% of the controls, respectively. However, in individual cells amplitude changes of early and late PSPcomponents were not correlated. Strong potentiation of the early PSP-component could be associated with a weak enhancement of the late PSP-component (Fig. 1A) and vice versa (Fig. 1B). Post-tetanic modifications of PSP amplitude were still inducible in slices of 3-week-old rats (n = 9) (Fig. 2) but the changes were smaller than in slices of 2-week-old rats. Now the amplitude changes of the late PSP-component were significantly larger (P < 0.05) than those of the early PSP (Fig. 2C). The post-tetanic amplitudes of the two components were 121.2 + 18.9% and 106.8 + 7,5% of the pretetanic controls, respectively. On occasions the late PSP-component was potentiated without any detectable modification of the early component (Fig. 2B). In slices of 4-week-old (n = 7) and older rats (from 6 to 10 weeks; n = 11), post-tetanic PSP modifications were no longer apparent (Fig. 3). In the 4-week-old rats, average early and late PSP-components were 104.9 + 12.0% and 101.1 + 16.3% of the pre-tetanic controls. These values do not differ significantly from those obtained in the older rats of this study and from adult rats analyzed previously with identical techniques 3. These age-dependent PSP modifications are summarized in Fig. 4. Effect o f N M D A

receptor blockade on L T P induction

In slices of adult rat visual cortex, LTP induction requires the activation of N M D A receptors 2'3,21. To test whether this is also the case in the visual cortex of young rats, we applied the same tetanic stimuli as before to slices of 2- to 3-week-old rats but added 25/~M APV to the bath. This prevented the induction of LTP in all cases (n = 7) (Fig. 5). The amplitudes of early and late PSP-components were 95.6 + 9.0% and 88.6 + 13.5% of the controls, respectively. A similar result was obtained when APV was washed out (30 min after tetanus) and the PSP-amplitude (15 min after wash-out) was compared to that before APV-application (n = 2). Developmental changes o f the N M D A receptor-mediated EPSP

In visual cortex slices of adult rats the susceptibility to undergo LTP can be raised by reducing G A B A A receptor-mediated inhibition 2'3'21, by adding cholinergic and noradrenergic agonists 12 or by depolarizing the postsynaptic cell 1. In all cases there is evidence that this facilitation results from an augmentation of N M D A

TABLE II Developmental changes of the APV-sensitive EPSP

Mean+ S.D. ; n = number of ceils. 2 weeks (n = 9)

3 weeks (n = 6)

4 weeks (n = 4)

Adult (n = 8)

Peak amplitude 3.2+0.69* 3.4+1.2" 2.1+0.27 2.0+0.7 (mV) Time to peak (ms) 38.3-+12.3 35.9-+13.6 27.4+9.9 26.4-+9.4 Time to half-decay (ms) 85.6+16.7 76.7+34.6 84.2+34.1 56.0+8.5 Integral of amplitude (mY x ms) 416.3+130.1" 420.9+181.2" 197.8+52.2 163.6+45.2 * Indicates that the value is significantly (P < 0.01) different from the corresponding value of the adult controls documented in Artola and Singer3.

receptor-mediated EPSPs during the response to the tetanus. This suggested the possibility that the increased susceptibility to LTP of slices obtained from young rats might also be due to augmented N M D A receptor-gated conductances. We therefore investigated whether there are age-dependent changes in the extent to which N M D A receptor-mediated EPSPs contribute to the PSP. N M D A receptor-mediated EPSPs were isolated by subtracting PSPs recorded during APV application from control PSPs (Fig. 6). Amplitude and time course of these APV-sensitive EPSPs were measured and averaged for each age group (Table II). To make measurements from different slices comparable, stimulus intensity was always set to just subthreshold levels for spike elicitation. The amplitude integral of the APV-sensitive EPSPs was significantly larger (P < 0.01) at 2 and 3 weeks of age than at later stages of development. This was due to both a larger amplitude and a longer duration of this EPSP. Since G A B A A receptor-mediated IPSPs antagonize very effectively the N M D A receptor-gated conductances in neurones of the neocortex 2'3"21 and the hippocampus 16' 19, we wondered whether the age-dependent decrease of N M D A receptor-mediated EPSPs was paralleled by an increase of G A B A A receptor-mediated inhibition. Therefore we measured the response conductance at the peak of the ilPSP in 2-week-old and in adult rats (see Methods). This conductance was 66.0 +__27.0 nS (n = 5) in the former and 63.9 + 28.0 nS (n = 8) in the latter, DISCUSSION The present results show that susceptibility to LTP of neurons in superficial layers of the rat visual cortex

48 decreases rapidly from 2 to 4 weeks of age. This agrees with the field potential study of Perkins and Teyler32 who obtained similar results in supragranular but not in deep layers. Our data indicate further, that (1) LTP susceptibility decreases faster for the early than for the late PSP-component, (2) LTP induction requires also in the immature cortex the activation of NMDA receptor-gated conductances and (3) the age-dependent reduction of susceptibility to LTP is paralleled by a decrease of the extent to which the NMDA receptor-mediated EPSP contributes to the synaptic response.

NMDA receptor-mediated EPSPs and LTP susceptibility The close correlation between the age-dependent decline of the susceptibility to undergo LTP and the reduction of NMDA receptor-mediated EPSPs in response to white matter stimulation suggests a causal relation. This is supported by the finding that blockade of NMDA receptors during the tetanus prevents LTP induction in slices of young rats. Thus, it appears that synaptic responses need to comprise a minimum amount of NMDA receptor-gated conductances in order to support the induction of LTP. This interpretation is compatible with data from adult animals which show that LTP induction requires a critical amount of postsynaptic depolarization. In superficial layers of visual cortex slices of adult rats, LTP can only be induced if GABAergic inhibition is reduced 2'3'21 or if the postsynaptic cell is additionally depolarized by current injection during tetanus 1. Correspondingly, in slices of the rat parietal c o r t e x 9 and in motor cortex of awake cats 4 LTP can be obtained if presynaptic input is paired with postsynaptic activation (but see ref. 33). Given the voltage-dependent Mg2+-block of the NMDA receptorgated channels 26'31, all these manipulations are likely to increase the contribution of NMDA receptor-mediated EPSPs to the synaptic response. For example, in disinhibited slices of adult rats, which are susceptible to LTP, the amplitude of the NMDA receptor-mediated EPSP is two-fold larger than in slices kept in normal medium, which are not susceptible to LTP3. Interestingly, in the disinhibited slices of adult rats the amplitudes of NMDA receptor-mediated EPSPs were in the same range (4.0 ___ 2.0 mV) as in the untreated slices of the 2- to 3-week-old rats investigated in this study (see Table II). Differential LTP susceptibility of early and late PSPcomponents The finding that the early and the late PSP-components could potentiate independently and that the susceptibility to LTP decayed more slowly with age for the late than for the early PSP-component suggests, first, that the potentiation of the two PSP-components depends on

different mechanisms, and second, that the LTP threshold is lower for the late than for the early PSPcomponent. Both the NMDA receptor-mediated EPSP and the ilPSP peak at about 20 ms post-stimulus. An increase of the amplitude of the response at this delay may thus result from (1) an increase of the NMDA receptormediated EPSP, (2) an increase of a polysynaptic EPSP, or (3) a decrease of the ilPSP. The latter possibility appears unlikely if one considers related results from adult rats. In the presence of the G A B A A antagonist, bicuculline, the late PSP-component can be potentiated independently of the early PSP-component and the threshold for the potentiation of the former is lower than for the latter 3. There is now increasing evidence from a variety of cortical structures that the NMDA receptormediated EPSP can undergo LTP in much the same way as the AMPA receptor-mediated EPSP 3'5"8"24. Thus, the most parsimonious interpretation of our results is that the potentiation of the late PSP-component reflects LTP of the NMDA receptor-mediated EPSP and that the LTP threshold of this EPSP is lower than that of the AMPA receptor-mediated EPSP.

The prevalence of NMDA-receptor mediated responses in young animals One reason for the enhanced expression of NMDA receptor-mediated responses in young animals could be reduced GABAergic inhibition because GABAA receptor-mediated IPSPs very effectively suppress NMDA receptor-mediated EPSPs in both neocortex2'3'21 and hippocampus 16'19. This possibility is particularly attractive because in the rat neocortex GABAergic inhibition has been reported to mature slowly and to increase during the first postnatal weeks 25. Furthermore, the slices of young rats resembled those of adult rats in which inhibition was reduced: in both cases, NMDA receptormediated EPSPs contribute substantially to the synaptic response and the susceptibility to undergo LTP is high 2'3. This interpretation of reduced inhibition is in conflict with our finding that the response conductance at the peak of the ilPSP was similar in 2-week-old and adult rats. However, there is the possibility that the applied current pulses did not modify the membrane potential evenly in all compartments of the neurones. Thus, no strong conclusions can be drawn from those measurements and therefore reduced GABAergic inhibition remains an attractive possibility to explain the enhanced LTP susceptibility of immature cortex. Other reasons for the age-dependent reduction of NMDA receptor-mediated responses could be developmental changes of the NMDA receptors. Two observations support this possibility. First, while an age-depen-

49 dent increase of inhibition can account for the a m p l i t u d e reduction of the N M D A r e c e p t o r - m e d i a t e d EPSP, it cannot explain the o b s e r v e d modification of its time course, in particular not the reduction of the decay time. In the visual cortex of adult rats, reducing the ilPSP with bicucuUine increases the a m p l i t u d e of the N M D A recept o r - m e d i a t e d E P S P but does not alter its time course 3. This supports d e v e l o p m e n t a l changes o t h e r than m e r e modifications of inhibition. Second, there is evidence that N M D A r e c e p t o r - d e p e n d e n t mechanisms are m o r e prominent in the nervous system of young than of adult animals. In the cat visual cortex binding sites for ligands of N M D A receptors decrease in density and change their laminar distribution during postnatal d e v e l o p m e n t 1°. This agrees with the evidence that visual responses are m o r e strongly suppressed by A P V iontophoresis in the visual cortex of young than of adult cats 36 (but see ref. 30). Similar a g e - d e p e n d e n t changes have been o b s e r v e d in o t h e r structures of the nervous system. N M D A r e c e p t o r s are always m o r e n u m e r o u s 35 and N M D A r e c e p t o r - m e d i a t e d pharmacological effects are m o r e p r o n o u n c e d 29 in young than in adult animals. In addition, there is recent evidence that Mg 2+ antagonizes the depolarizing action of N M D A less potently in hippoREFERENCES 1 Artola, A., BrOcher, S. and Singer, W., Different voltagedependent thresholds for the induction of long-term depression and long-term potentiation in slices of the rat visual cortex, Nature, 347 (1990) 69-72. 2 Artola, A. and Singer, W., Long-term potentiation and NMDA receptors in rat visual cortex, Nature, 330 (1987) 649-652. 3 Artola, A. and Singer, W., The involvement of N-methyl; D-aspartate receptors in induction and maintenance of long-term potentiation in rat visual cortex, Eur. J. Neurosci., 2 (1990) 254-269. 4 Baranyi, A. and Szente, M.B., Long-lasting potentiation of synaptic transmission requires postsynaptic modifications in the neocortex, Brain Res., 423 (1987) 378-384. 5 Bashir, Z.I., Alford, S., Davies, S.N., Randall, A.D. and CoUingridge, G.L., Long-term potentiation of NMDA receptormediated synaptic transmission in the hippocampus, Nature, 349 (1990) 156-158. 6 Bear, M.F., Kieinschmidt, A., Gu, Q. and Singer, W., Disruption of experience-dependent synaptic modifications in striate cortex by infusion of an NMDA receptor antagonist, J. Neurosci., 10 (1990) 909-925. 7 Ben Ari, Y., Cherubini, E. and Krnjevic, K., Changes in voltage dependence of NMDA currents during development, Neurosci. Len., 94 (1988) 88-92. 8 Beretta, N., Berton, E, Bianchi, R., Brunelli, M., Capogna, M. and Francesconi, W., Long-term potentiation (LTP) of NMDA receptor-mediated EPSP in guinea pig hippocampal slices, Eur. J. Neurosci., submitted. 9 Bindman, L.J., Murphy, K.P.S.J. and Pockett, S., Postsynaptic control of the induction of long-term changes in efficacy of transmission at neocortical synapses in slices of rat brain, J. Physiol., 60 (1988) 1053-1065. 10 Bode-Greuel, K.M. and Singer, W., The development of N-methyl-D-aspartate in cat visual cortex, Dev. Brain Res., 46 (1989) 197-204.

c a m p a l neurones of developing than of adult rats 7"11. These changes are c o m p a t i b l e with the i n t e r p r e t a t i o n that the e n h a n c e d LTP susceptibility is at least in part due to the increased expression and/or efficiency of N M D A receptors in i m m a t u r e cortex. Relation to developmental plasticity in the kitten visual cortex In the kitten visual cortex u s e - d e p e n d e n t modifications

of neuronal response p r o p e r t i e s can be induced by manipulating visual experience. These changes can be induced only during a critical p e r i o d of early postnatal d e v e l o p m e n t 2° and as far as modifications of ocular d o m i n a n c e are c o n c e r n e d require for their induction the activation of N M D A receptors 6'17'22. Thus, the aged e p e n d e n t decline in e x p e r i e n c e - d e p e n d e n t malleability of visual cortex functions might, at least in part, be due to an a g e - d e p e n d e n t reduction of N M D A receptorm e d i a t e d responses.

Acknowledgements. We express our gratitude to Christa Ziegler for her pertinent technical assistance, to Renate Ruhl for graphics and to Irmi Pipacs for editorial assistance. Part of this work was supported by a grant from the H.S.EP. to W.S.

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