- Email: [email protected]
Do NMDA receptors have a critical function in visual cortical plasticity?
Acknowledgements This study was supported by grant A4T-5781 of the Medical Research Council of Canada to A. Parent, and by a studentship from the Fonds de Recherche en SantE du QuEbec to L-N. Hazrati.
16 Park, M. R., Falls, W. M. and Kitai, S. T. (1982) J. Comp. Neurol. 211,284-294 17 Franqois, C., Yelnik, J. and Percheron, G. (1987) J. Comp. Neurol. 265, 473-493 18 Chang, H. T., Wilson, C. J. and Kitai, S. T. (1981) Science213, 915-918 19 Wilson, C. J. and Phelan, K. D. (1982) Brain Res. 243, 354-359 20 Kawaguchi, Y., Wilson, C. J. and Emson, P. C. (1990) J. Neurosci. 10, 3421-3438 21 Parent, A., Bouchard, C. and Smith, Y. (1984) Brain Res. 303, 385-390 22 Parent, A., Smith, Y., Filion, M. and Dumas, J. (1989) Neurosci. Left. 96, 140-144 23 Selemon, L. D. and Golclman-Rakic, P. S. (1990) J. Comp. Neurol. 297, 359-376 24 Haber, S. N. and Elde, R. (1981) Neuroscience 6, 1291-1297 25 Hedreen, J. C. and DeLong, M. R. (1991) J. Comp. Neurol. 304, 569-595 26 Parent, A. and De Beflefeuille, L. (1982) Brain Res. 245,
201-214 27 Ilinsky, I. A., Jouandet, M. L. and Goldman-Rakic, P. S. (1985) J. Comp. NeuroL 236, 315--330 28 IlJnsky, I. A. and Kultas-Ilinsky, K. (1987) J. Comp. Neurol. 262, 331-364 29 Kita, H. (1992) Brain Res. 589, 84-90 30 Smith, Y., Hazrati, L-N. and Parent, A. (1990) J. Comp. Neurol. 294, 306-323 31 Carpenter, M. B. etaL (1981) J. Comp. NeuroL 197, 579-603 32 Kita, H., Chang, H. T. and Kitai, S. T. (1983) J. Comp. NeuroL 215, 245-257 33 Bergman, H., Wichmann, T. and DeLong, M. (1990) Science 249, 1436-1438 34 Hartmann-von Monakow, K., Akert, K. and K0nzle, H. (1978) Exp. Brain Res. 33, 395-403 35 Afsharpour, S. (1985) J. Comp. Neurol. 236, 14-28 36 Hazrati, L-N., Parent, A., Mitchell, S. and Haber, S. N. (1990) Brain Res. 533, 171-175 37 Smith, Y., Parent, A., SEgu~la, P. and Descarries, L. (1987) J. Comp. Neurol. 259, 50-55
Do NMDA receptorshave a critical functionin visualcorticalplasticity? K. F o x a n d N. W . D a w
I( Fox is at the Dept of Physiology, University of 44innesota, 6-255, 44illardHa//, 435 Delaware St SE, 44inneapolis, 44N 55455, USA, and N. W. Daw is at the Dept of Ophthalmology and VisualScience, YaleUniversity, 330 CedarSt, New Haven, CT06510, USA.
The theoreticalframework, by which we understand the function of NMDA receptors, is derived, in large part, from work conducted on the hippocampal slice preparation, where NMDA receptors are crucial for a form of synaptic plasticity known as long-term potentiation (L TP). Establishing their role in plasticity mechanisms in the neocortex is proving to be far more difficult than originally envisaged, in part due to the fact that the operation of NMDA receptors is different in the intact animal than in vitro. For example, NMDA receptors are activated at low levels of sensory input in intact animals but only by high levels of input in slice preparations. Recent results suggest that a re-evaluation of the role of NMDA receptors in neocortical plasticity is required. Here we discuss some of the issues and introduce four criteria by which any factor supposedly involved in plasticity can be judged. NMDA receptors fulfill more of these criteria than any of the other factors so far investigated in the visual cortex, but maybe this is because they have been studied more intensively. Some 30 years ago, Wiesel and Hubel 1 introduced a paradigm whereby long-lasting anatomical and physiological changes occur in the visual cortex in response to altering the balance of sensory output from the two eyes. The cortex is only affected early in development during what is known as the 'critical period'. The experiment is done by suturing closed one eyelid for a short period of time (as little as eight hours during the peak of the critical period is sufficient to cause substantial changes2). When the animal's experience is altered in this way, changes occur in the synaptic connections of the visual cortex, a process referred to these days as experience-dependent synaptic plasticity (see Box 1). This paradigm has features in common with learning, where experience is thought to have a lasting effect on synaptic connections in the cortex and so enable memory formation and modification of behavior. Ocular dominance plasticity is the best experimental model we have at present for
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studying how synaptic plasticity occurs in the neocortex. Studies on neocortical long-term potentiation (LTP) are, by comparison, in their infancy. Moreover, LTP paradigms have not been clearly linked to the function of the cortex in the intact animal, as ocular dominance plasticity has. W h y NMDA receptors? Ever since the discovery that NMDA receptors have a critical function in the induction of synaptic modification in the hippocampus 3, it has seemed natural to hypothesize that NMDA receptors are also involved in neocortical plasticity. Although obvious differences exist in the expression of the two forms of plasticity (LTP involves a rapid change in synaptic efficacy, while ocular dominance plasticity involves changes in axonal arborization patterns as well), the induction mechanisms seemed likely to be similar. The first experiments to suggest that NMDA receptors might be involved in ocular dominance plasticity were based on procedures that antagonized NMDA receptors during monocular deprivation. Under these conditions ocular dominance plasticity was reduced 4'5, satisfying a preliminary condition for the involvement of NMDA receptors in this process. However, a number of issues complicate the interpretation of these results. Experimental issues. Other less specific effects on cortical physiology were also reported to occur after chronic NMDA receptor blockade, such as a general loss of responsiveness to visual stimuli and loss of receptive field specificity. Given this result, there is a concern that the drug being applied causes a general depression of visual input. Nonspecific receptive fields and poorly responding cells are also common when tetrodotoxin (TTX) is applied to the cortex, or animals are reared with binocular lid suture, known as binocular deprivation (BD) 6'7. This is because both BD and infusion of TTX prevent patterned visual information reaching the cortex from the eyes, and this is known to be necessary for refining the TINS, Vol. 16, No. 3, 1993
Box 1. The monocular deprivation paradigm viously closed eye input occurs f, demonstrating that this form of plasticity involves increases in synaptic efficacy as well as decreases (Fig. C). Reverse suture experiments suggest that NMDA receptors could be involved in potentiation of weak eye inputs g.
There are a number of good reviews of the extensive literature on ocular dominance plasticity a-c. In essence, closing one eye of a binocular animal during the critical
period causes a long-term decrease in responsiveness to the closed eye. The change can be measured physiologically by assaying ocular dominance: a number of microelectrode penetrations are made through the cortex to sample the preference of single cortical cells for stimulation of the right or left eyes. Normally, most cortical cells are binocular with the number of cells responding to left and right eye stimulation being approximately equal. This result is usually illustrated by an ocular dominance histogram (Fig. A). Monocular deprivation leads to a shift in the balance toward cells that respond preferentially to the open eye: at the peak of cortical sensitivity, the shift can almost be complete (Fig. B). Monocular deprivation has an effect during a critical period that, in the cat, lasts for about one year for cells in extragranular layers d and peaks between four to six weeks of agee. If monocular deprivation is reversed within this critical period, some recovery of the pre-
a Mitchell, D. E. and Timney, B. (1984) in Handbook of Physiology. The Nervous System: Sensory Processes, pp. 507-555, Am. Physiol. Soc. b Movshon, J. A. and Van Sluyters, R. C. (1981) Annu. Rev. PsychoL 32,477-522 c Shatz, C. J. (1990) Neuron 5, 745-756 d Daw, N. W., Fox, K., Sato, H. and Czepita, D. (1992) J. NeurophysioL 67, 197-202 e Olsen, C. R. and Freeman, R. D. (1980) Exp. Brain Res. 39, 17-21 f Blakemore, C. and Van Sluyters, R. C. (1974) J. PhysioL 237, 195-216 g Gu, Q., Bear, M. F. and Singer, W. (1989) Brain Res. 47, 281-288
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The effect of monocular deprivation on ocular dominance. (A) The ocular dominance histogram for a normal binocular cat is composed mainly of binocular cells (groups 2-6) with a small contralateral bias. Group 1 contains cells responding only to the contralateral eye; group 7, cells responding only to the ipsilateral eye; and group 4, cells that respond equally well to both eyes. (B) Monocular deprivation, imposed at two weeks, causes an almost total shift of ocular dominance to the open eye (contralateral eye) by five weeks. (C) Reversing eye closure from the contralateral to the ipsilateral side at five weeks causes a reversal of the ocular dominance shift by 14 weeks. (Adapted from Blakemore and Van Sluytersf.)
properties of cortical receptive fields during development. Since NMDA receptors are involved in transmission of patterned visual information within kitten visual cortex 8, and infusion of D-2-amino-5-phosphonovalerate (APV) reduces visual responses 9, the simplest explanation for the results is that NMDA antagonists act by attenuating cortical responses to visual stimuli. This explanation removes the need to argue that NMDA receptors have a special role in cortical plasticity. Theoretical issues. A good deal is already known about the presynaptic activity requirements for ocular dominance plasticity. Somewhere in the pathway for plasticity, there must be a mechanism for detecting correlated presynaptic activity involving the postsynaptic element (see Box 2). Could the NMDA receptor possibly fulfill this role? Some have argued that it can in the hippocampus 1°' 11 and neocortex 12 on TINS, Vol. 16, No. 3, 1993
the basis that the NMDA receptor acts like a Hebbian mechanism (see Box 3). However, there are significant problems with this theory. First, since NMDA receptors have a high affinity for glutamate 13, extracelhlar glutamate is of sufficient concentration to bind to NMDA receptors even when at rest 14. This prevents the NMDA receptor from being a Hebbian mechanism. Second, because of the cable properties of dendritic spines 1S, even small inputs would be expected to cause a large depolarization at the spine head, sufficient to relieve the voltage-dependent block of the NMDA channel, whether correlated with other inputs or not. This prevents it from being a correlation detector. These expectations are supported by observations. First, NMDA receptors are found to be spontaneously active in a variety of structures 8,16,17. They conduct current in unstimulated preparations in vitro 117
Box 2. Conditions for ocular dominance plasticity To cause a shift in ocular dominance towards one eye, retinal activity must differ between the two eyes"-C: in particular the electrical spiking activity must differ between the two eyesc,d. Monocular deprivation causes an increase in the area occupied by the geniculocortical endings for the open eye and a decrease in the area occupied by the afferents driven by the closed eye (see the figure below). The synapses formed by the geniculocortical afferents of both the closed and the open eyes differ from normal in the number of synapses per bouton, bouton size and location of their terminations (see Ref. e). The temporal correlation of afferent spike activity determines how connections are grouped; correlated presynaptic activity causes aggregation of correlated inputs, uncorrelated activity causes segregation of uncorrelated inputs f'g. Extreme levels of uncorrelated activity from the two eyes can occur with strabismus, which leads to total segregation of geniculocortical h and intracortical axon arbors driven by the two eyes'. In addition to these presynaptic requirements, it is almost certain (but not known beyond doubt) that postsynaptic cortical activity is necessary for plasticity. If NMDA receptors are only present postsynaptically in visual cortex, the fact that plasticity is disrupted when NMDA receptors are blocked implies that some postsynaptic activity is required j. Another line of evidence supporting the necessity of postsynaptic activity comes from experiments where the orientations of experienced stimuli are restricted during monocular deprivation. This leads to only those cells that are responsive to the experienced orientation being involved in the shift in ocular dominance, while the other cells remain unaffected. Since orientation selectivity first arises postsynaptically in the cortex, this implies postsynaptic involvement k-m (but see Ref. n on interpretation of this result). The relationship between pre- and postsynaptic activity affects the sign with which the synaptic alteration occurs, and does so in a manner that appears to be described by the covariance rule. If pre- and postsynaptic activities are correlated, the functional connection between the two increases, but if they are uncorrelated, the functional connection decreases °.p. An analogous segregation process occurs in the tectum of the three-eyed frog. Two retinae are forced to innervate the same tectum, which results in a striated pattern of innervation much like that seen in mammalian cortex. Separation of retinal axons into eye-specific stripes is prevented by blocking NMDA receptors with antagonists q. Evidence suggests the NMDA receptors are postsynaptic r, so this is another example of a postsynaptic mechanism affecting presynaptic morphology.
References a b c d e f g h i j k I m
Wiesel, T. N. and Hubel, D. H. (1963) J. Neurophysiol. 26, 1003-1017 Wiesel, T. N. and Hubel, D. H. (1965) J. Neurophysiol. 28, 1029-1040 Stryker, M. P. and Harris, W. A. (1986) J. Neurosci. 6, 2117-2133 Chapman, B., Jacobson, M. D., Reiter, H. O. and Stryker, M. P. (1986) Nature 324, 154-156 Friedlander, M. J., Martin, K. A. and Wassenhove-McCarthy, D. (1991) J. Neurosci. 11, 3268-3288 Blasdel, G. G. and PeRigrew, J. D. (1979) J. Neurophysiol. 42, 1692-1710 Stryker, M. P. (1986) in The Biology of Change in Otolaryngology (Ruben, R. W., Van De Water, T. R. and Rubel, E. W., eds), pp. 211-244, Elsevier Shatz, C. J., Lindstrom, S. and Wiesel, T. N. (1977) Brain Res. 131, 103-116 Lowel, S. and Singer, W. (1992) Science 255, 209-212 Kleinschmidt, A., Bear, M. F. and Singer, W. (1987) Science 238, 355-358 Rauschecker,J. P. and Singer, W. (1979) Nature 280, 58-60 Cynacler, M. S. and Mitchell, D. E. (1977) Nature 270, 177-178 Carlson, M., Hubel, D. G. and Wiesel, T. N. (1986) Dev. Brain Res. 25, 71-81
and support spontaneous spike activity in vivo in the absence of sensory stimulation. Second, even small inputs just above threshold are sufficient to activate NMDA receptors in the visual cortex TM. These results show that the threshold for activating NMDA receptors is low and certainly not reserved for conditions of extreme correlation. The timing characteristics of the biological correlation detector are also important, but as yet remain unknown. NMDA receptors produce long-duration 118
The effect of monocular deprivation on geniculocortical afferents viewed by autoradiography. (A) In a 4J-year-old monkey reared with binocular vision during the critical period, the geniculocortical afferents occupy approximately equal areas of cortex. (B) In a l]-year-old monkey reared from two weeks with one eye closed, the geniculocortical afferents from the open eye occupy a greater area of cortex than normal. In both cases the ipsilateral eye was injected with radiolabelled tracer and collages made of sections through layer IVC (anterior up, medial left). (Adapted from Hubel, Wiesel and LeVayS.) n Movshon, J. A. and Van Sluyters, R. C. (1981)Annu. Rev. Psychol. 32,477-522 o Fregnac, Y., Shulz, D., Thorpe, S. and Bienenstock, E. (1992) J. NeuroscL 12, 1280-1300 p Shulz, D. and Fregnac, Y. (1992) J. NeuroscL 12, 1301-1318 q Cline, H. T., Debski, E. A. and Constantine-Paton, M. (1987) Proc. Natl Acad. ScL USA 84, 4342-4345 r Cline, H. T., McDonald, J. and Constantine-Paton, M. (1991) Soc. Neurosci. Abstr. 17, 794 s Hubel, D., Wiesel, T. N. and LeVay, S. (1977) Philos. Trans. R. Soc. London Ser. B 278, 377-409
excitatory postsynaptic potentials (EPSPs) 19, and therefore could correlate activity over periods of about 1-200ms. If they are involved in correlating inputs between the two eyes, cells responsive to both eyes (binocular ceils) should be retained if inputs from the left and fight eyes occur within this time window. At present, this issue is unresolved experimentally; one study shows that binocular cells occur if the eyes are stimulated as much as 10 s apart 2° and the other when only stimulated 3-500 ms apart 21. Consequently, TINS, Vol. 16, No. 3, 1993
Box 3. Hebbian synaptic mechanisms Hebb mechanism The figure shows the basic timing concept of a Hebbian mechanism. As originally conceived by Hebb a, the connection between pre- and postsynaptic elements of the synapse strengthen if the presynaptic site 'repeatedly or persistently takes part in firing' (the cell). In this case, the presynaptic event (Event 1) is the firing of the presynaptic cell, and the postsynaptic event (Event 2) is the target cell producing an action potential. The mechanism necessary to detect such an event would be activated only when both the pre- and postsynaptic elements are active (And). This coincidence signal would in turn cause a long-lasting change in the synaptic connection (Latch). Correlation detector The same basic Hebbian scheme is capable of detecting correlated firing of different synapses. For example, imagine Event 1 and Event 3 represent glutamate release from synapses that are incapable of depolarizing the neuron individually, but which do cause the cell to fire when they are activated simultaneously. In this modification of the Hebbian scheme, Event 2 occurs only when Events 1 and 3 occur together. This causes inputs from both eyes to be strengthened when their activity is highly correlated through binocular visual stimulation, and inputs from a single open eye to be strengthened when their activity is highly correlated by monocular visual stimulation. In some formulations of Hebb's proposal, the postsynaptic event is a certain level of depolarization of the postsynaptic cell, rather than firing of an action potential, but the principle remains the same. Biological substrate The NMDA receptor has been suggested as a possible candidate for a biological substrate for the modified Hebbian mechanism. Presynaptic activity results in release of glutamate that binds to the glutamate-binding site on the NMDA receptor. Postsynaptic activity produces depolarization that is detected by the voltage dependence of the NMDA channel. The voltage dependence of the NMDA channel results from a voltage-sensitive Mg 2 + -binding site within the conductive pore. At hyperpolarized membrane potentials, Mg 2÷ binds to and blocks the NMDA channel but at depolarized potentials the Mg 2+ blockade is greatly reduced, allowing conduction of other ions through the channel, including Ca2+. Obviously, the real biological versions of the processes illustrated in the figure will not involve square waves nor binary functions. However, a sharp nonlinear function is required somewhere in the sequence to distinguish normal synaptic transmission from that required to cause synaptic modifications (see main text). At present, there is insufficient experimental evidence to know what the time-base on this diagram should be for ocular dominance plasticity.
we cannot yet assign a time-base to the mechanism outlined in the figure in Box 3. A l t e r n a t i v e theories There are clearly holes in the present experimental evidence as well as inadequacies in current theories that attempt to explain exactly how NMDA receptors might be involved in ocular dominance plasticity. It had previously been thought that the correlation detector required a voltage element to link synaptic activity at different synaptic locations. However, discovery of the rapidly diffusing second messenger, nitric oxide (NO), has led to alternative theories of how correlations over relatively large distances might be linked22. On the other hand, if the slow correlation times that were suggested by the Blasdel and Pettigrew experiments 2° are corroborated, even a slowly diffusing substance would be sufficient (see Box 3). Where does this leave NMDA receptors? One notable gap in our knowledge is whether postsynaptic TINS, VoL 16, No. 3, 1993
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Reference a Hebb, D. O. (1949) The Organization of Behavior, Wiley
Ca2+-dependent mechanisms are critically involved in plasticity. [f it can be shown that they are, one resolution of the current evidence might be that the plasticity switch (Latch in Box 3) is in fact a postsynaptic second or third messenger system activated by the Ca 2÷ influx caused by sufficient NMDA receptor activation. The binding of Ca 2+ to calmodulin appears to be sufficiently nonlinear to fulfill the function of a switch for LTP in the hippocampus (while the voltage dependence of the NMDA receptor is inadequate for this on its own) 23'24.
A unique link? While this form of argument rescues the theory that NMDA receptors play some role in plasticity, it simultaneously opens up other possibilities. If a Ca2+activated second messenger system were to perform correlation detection, how could we rule out the possibility that other transmitter-receptor-second messenger systems were not involved in plasticity in the same way? For example, voltage-dependent Ca 2+ 119
required (though it is often treated this way). In order to include factors as candidates, this criterion must be linked with others and for two main reasons. Changes in spine First, from a technical point of n s ~ morphology? view, the experiment may block more than the single factor targeted. For example, the drug 6-hydroxydopamine, which depletes noradrenaline, abolishes Inl plasticity, but this may occur fro because of the general damaging oF effects of 6-hydroxydopamine e~ when used in high doses 25 or because it affects acetylcholine as well26. We include criterion (2) as one method of distinguishing between factors that cause a general deterioration of cortical function In from those which, if absent or fn langes in spine removed, specifically cause a loss cl )rphology? of plasticity. e~ Second, blocking the factor may affect a general process rather than one specifically related to plasticity. For example, anesthesia and paralysis both block plasticity 27, Fig. 1. The relationship between some of the steps possibly involved in ocular dominance plasticity. but it d,oes not follow that kittens Activity of N/VlDA receptors may be necessary to admit second messengers (Ca2+) postsynaptically. are more plastic than cats because Little is known at present about how this event might be finked to sprouting and retraction of they move their eyes around more, presynaptic elements. If diffusible factors are involved, possible targets include the afferent or because they are awake more terminals of the open eye, the afferent terminals of the closed eye and the dendritic spines of the closed eye. The rate and freedom of second messenger diffusion determine integration time and than cats. We include criteria (3) and (4) in order to distinguish the distance over which correlated synapses can be detected (see Ref. 25). factors that are necessary for normal cortical function at all ages channels or the metabotropic receptor could activate from those peculiar to ocular dominance plasticity. the same Ca2+-dependent mechanisms as NMDA re- (2) Reintroducing the factor into older animals restores plasticity. This is also a necessary criterion, with two ceptors do. One constraint on the possibilities is that the caveats. First, it assumes that there is a single crucial mechanism must be affected by what we know factor in a single crucial pathway that is lost during the instructs synaptic plasticity (i.e. correlated activity) aging process. If two factors are required in combiand it must also in turn affect systems that sequen- nation, then reintroducing just one of them will not tially end up altering the sprouting and retraction of work. Moreover, if there is a sequence of steps presynaptic elements (systems that remain to be dis- involved in plastic changes, and more than one of the covered). In other words, the candidate mechanism factors in the sequence of steps is missing in adults, needs to be on the pathway between retinal activity then again reintroducing one of the factors will not and synaptic alterations (see Fig. 1). So far, we have work. Second, whether an experiment will be succoncentrated on current evidence related to this cessful depends on knowing enough about the process single criterion. However, we propose that this is one so that the factor can be added back in at the right of at least four criteria that are required to test which time and the right place. For example, assuming factors, or steps in the sequence of processes, are NMDA receptors were crucial to plasticity, just crucial to plasticity. What follows is generally appli- adding them back to the adult system would not cable to any stage of the plasticity process, but we necessarily be enough: this might simply flood the concentrate here on the way the criteria apply to system and nonspecifically strengthen all synapses rather than those that were specific for coordinated current evidence on NMDA receptors. binocular function. Consequently this criterion has to be applied with intelligence, and has not been tested Requirements to demonstrate that a factor is for many candidate factors. crucial for plasticity (1) Removing the factor, or introducing an antagonist, (3) The factor is present during the critical period, prevents plasticity. This is a necessary criterion, but it including when this period is altered by dark rearing. is certainly not sufficient. That it is necessary hardly This criterion distinguishes the crucial factor from the requires explanation. Any factor which does not block others in the sequence of steps that are active at all plasticity when removed or antagonized obviously ages for a variety of processes besides plasticity. For does not have a critical function in the process. As the sake of argument, take the hypothesis illustrated such, this first criterion is an exclusionary criterion, in Fig. 2. This hypothesis arises from work on LTP in not one that by itself can be used to show a factor is the rat hippocampus 3°. It is controversial for LTP and 120
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is certainly not proved for plasticity in the cortex, nevertheless it can be used to illustrate the general point to be made. There are more than 11 steps in the sequence of processes in this hypothesis (Fig. 2). We know from experiments with TTX (Ref. 31) that geniculocortical activity is involved in plasticity. However, there is no evidence for a difference in geniculocortical activity between kittens and cats that can account for the end of the critical period. Similarly, resting levels of Ca 2+ are not thought to be different between kittens and cats. If the hypothesis turns out to be true, it is almost certain that one of the other steps in the sequence will be the crucial one. To some extent this criterion defines what we mean here by 'crucial'. Any factor that can be artificially removed to abolish plasticity is in some sense involved in plasticity, but the factor or factors that alter naturally during development to bring plasticity to an end are special cases. The timecourse of the critical period is delayed by rearing animals in the dark 32,33. Consequently, a corollary of this criterion is that the presence, concentration or efficacy of the crucial factor should remain elevated in animals reared in the dark for some time after the peak of the critical period for lightreared animals.
(4) The factor is instructive, not just permissive for plasticity. This criterion distinguishes factors that are on the pathway from those that feed into it. As an example, there is some evidence that a combination of the noradrenaline and acetylcholine pathways may affect plasticity26. However, neither noradrenaline nor acetylcholine pathways arise from neurons receiving specific visual signals from the retina. Consequently, as it is the degree of correlation between retinal signals that instructs synaptic modification, neither pathway receives the signal that specifies the nature of the synaptic change. At most they can only permit changes specified through a different mechanism. Thus blocking both acetylcholine and noradrenaline pathways might affect plasticity even if the direct sequence of steps involved in plasticity turned out to be the sequence described in Fig. 2, although neither pathway comprises any of those steps. H o w far do NMDA receptors fit t h e s e four criteria? NMDA receptors have been tested more extensively than any other factor in terms of whether they fit these criteria. Antagonists to NMDA receptors reduce or abolish ocular dominance shifts in the visual cortex 4'5'34. Criterion (1) is therefore satisfied. However, as discussed above, this by itself is not a sufficient test. Criterion (2) has not yet been tested, probably because of the difficulties in designing and interpreting an experiment to make the test. Criterion (3) has been examined; NMDA receptors are more abundant during the critical period for plasticity3s-3s, and their contribution to the visual response decreases between three and six weeks of age in layers IV, V and VI of the visual cortex 8'39. This drop in the contribution to the visual response can be delayed by rearing animals in the dark 39,4°. The decrease in NMDA receptors in layer IV correlates with the period of geniculocortical afferent segregation4t and so may correlate with the critical period for this layer, which has not yet been defined in the cat. Thus criterion TINS, VoL 16, No. 3, 1993
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Retraction of some geniculocorticat terminals, sprouting of others Fig. 2. A possible role for NMDA receptors m ocular dominance plasticity. This currently popular hypothesis is one way in which the pathways illustrated in Fig. 1 could work. The rapid and freely diffusible substance is nitric oxide, which may play a role in hippocampal L TP (Refs 28, 29) and is under test for the visual cortex. There is reasonable evidence for the first five steps in the sequence, but tittle evidence exists for the stages finking step five to the changes in presynaptic terminals (indicated by question marks).
(3) may be satisfied for some layers of the cortex. Criterion (4) is difficult to satisfy because it requires an overall knowledge of the sequence of plasticity mechanisms. So far, the input side of the sequence has been determined; correlated retinal activity is known to play an instructive role, though we are unsure of the exact timing constraints. Cortical NMDA receptors are certainly driven by retinal activity and in that sense receive the instructive signal, but have not been shown to be involved in detecting the degree to which presynaptic activity is correlated. The output side of the instructive sequence has not been determined but is widely supposed to stem from the postsynaptic entry of Ca 2+. If it can be shown that a Ca2+-activated second messenger system controlled by NMDA receptors is necessary for plasticity, we will know that it is transmitting the instructive signal as well. One can therefore conclude that, so far, NMDA receptors partly fulfill three out of the four criteria listed. This is more than any other factor currently under investigation (primarily because NMDA receptors have been more extensively tested than any other factor). It is clear that NMDA receptors have 121
other roles besides developmental plasticity: they are found in layers II and III of the adult visual cortex 8'42 where ocular dominance plasticity does not occur. Here, they may be necessary for whatever function LTP subserves 43'44, or they may simply function in a computational role that is unrelated to plasticity 18'45. Nevertheless, NMDA receptors are still the best candidate that we have at the present time for a factor that makes kittens plastic while adults are not, at least for the neurons in cortical layer IV.
Selected references 1 Wiesel, T. N. and Hubel, D. H. (1963) J. Neurophysiol. 26, 1003-1017 2 Freeman, R. D. and Olsen, C. (1982) J. Neurophysiol. 47, 139-150 3 Collingridge, G. L., Kehl, S. J. and McKlennan, H. (1983) J. Physiol. 334, 33-46 4 Kleinschmidt, A., Bear, M. F. and Singer, W. (1987) Science 238, 355-358 5 Bear, M. F., Kleinschmidt, A., Gu, Q. and Singer, W. (1990) J. Neurosci. 10, 909-925 6 Reiter, H., Waitzman, D. M. and Stryker, M. P. (1988) Exp. Brain Res. 65, 182-188 7 Mower, G. D., Berry, D., Burchfiel, J. L. and Duffy, F. H. (1981) Brain Res. 220, 255-267 8 Fox, K., Sato, H. and Daw, N. W. (1989) J. Neurosci. 9, 2443-2454 9 Miller, K. D., Chapman, B. and Stryker, M. P. (1989) Proc. Natl Acad. Sci. USA 86, 5183-5187 10 Cotman, C. W., Monaghan, D. T. and Ganong, A. H. (1988) Annu. Rev. Neurosci. 11, 61-80 11 Brown, T. H., Change, V. C., Ganong, A. H., Keenan, C. L. and Kelso, S. R, (1988) in Long-term Potentiation: From Biophysics to Behavior (Lanfield, P. W. and Deadwyler, S. A., eds), pp. 201-264, Alan Liss 12 Bear, M. F., Cooper, L. N. and Ebner, F. F. (1987) Science 237, 42-48 13 OIverman, H. J., Jones, A. W. and Watkins, J. C. (1988) Neuroscience 26, 1-15 14 Lerrna, J., Herranz, A. S., Herraras, O., Abraira, V. and Martin del Rio, R. (1986) Brain Res. 384, 145-155 15 Koch, C. and Poggio, T. (1983) Philos. Trans. R. Soc. London Ser. B 218, 455-477 16 Sah, P., Hestrin, S. and Nicoll, R. A. (1989) Science 246, 81 5-818 17 Blanton, M. G., LoTurco, J. J. and Kriegstein, A. R. (1990) Proc. Natl Acad. Sci. USA 87, 8027-8030
18 Fox, K., Sato, H. and Daw, N. W. (1990) J. Neurophysiol. 64, 1413-1428 19 Howe, J. R., Colquhoun, D. and Cull-Candy, S. G. (1988) Philos. Trans. R. Soc. London Ser. B 233,407--422 20 Blasdel, G. G. and Pettigrew, J. D. (1979)J. Neurophysiol. 42, 1692-1710 21 Altmann, L., Luhman, H. J., Greuel, J. M. and Singer, W. (1987) J. Neurophysiol. 58, 965-980 22 Gaily, J. A., Montague, P. R., Reeke, G. N. and Edelman, G. M. (1990) Proc. NatlAcad. Sci. USA 87, 3547-3551 23 Gamble, E. and Koch, C. (1987) Science 236, 1311-1315 24 Zador, A., Koch, C. and Brown, T. H. (1990) Proc. NatlAcad. Sci. USA 87, 6718-6722 25 Gordon, B., Allen, E. E. and Trombley, P. Q. (1988) Prog. NeurobioL 30, 171-191 26 Bear, M. F. and Singer, W. (1986) Nature 320, 172-176 27 Freeman, R. D. and Bonds, A. B. (1979) Science 206, 1093-1095 28 Schuman, E. M. and Madison, D. V. (1991) Science 254, 1503-1506 29 O'Dell, T. J., Hawkins, R. D., Kandel, E. R. and Arancio, O. (1991) Proc. Natl Acad. Sci. USA 88, 11285-11289 30 Nowak, R. (1992) J. NIH Res. 4, 49-55 31 Stryker, M. P, and Harris, W. A. (1986) J. Neurosci. 6, 2117-2133 32 Cynader, M. S. and Mitchell, D. E. (1980) J. Neurophysiol. 43, 1026-1040 33 Mower, G. D. (1991) Dev. Brain Res. 58, 151-158 34 Rauschecker, J, P., Egert, U. and Kossel, A. (1990) Int. J. Dev. Neurosci. 8, 425-435 35 Bode-Greuel, K. M. and Singer, W. (1989) Dev. Brain Res. 46, 197-204 36 Gordon, B., Daw, N. W. and Parkinson, D. (1991) Dev. Brain Res. 62, 61-68 37 Reynolds, I. J. and Bear, M. F. (1991) Exp. Brain Res. 85, 611-615 38 Tsumoto, T., Haigihara, K., Sato, H. and Hata, Y. (1987) Nature 327, 513-514 39 Fox, K., Daw, N., Sato, H. and Czepita, D. (1991) Nature 350, 342-344 40 Fox, K., Daw, N. W., Sato, H. and Czepita, D. (1992) J. Neurosci. 12, 2672-2684 41 LeVay, S., Stryker, M. P. and Shatz, C. J. (1978) J. Comp. Neurol. 179, 223-244 42 Monaghan, D. T., Olverman, H. J., Nguyen, L., Watkins, J. C. and Cotrnan, C. W. (1988) Proc. Natl Acad. Sci. USA 85, 9836-9840 43 Artola, A. and Singer, W. (1987) Nature 330, 649-652 44 Tsumoto, T. (1990) Jpn J. Physiol. 40, 573-593 45 Fox, K. and Daw, N. W. (1992) Neur. Comput. 4, 59-83
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16 Park, M. R., Falls, W. M. and Kitai, S. T. (1982) J. Comp. Neurol. 211,284-294 17 Franqois, C., Yelnik, J. and Percheron, G. (1987) J. Comp. Neurol. 265, 473-493 18 Chang, H. T., Wilson, C. J. and Kitai, S. T. (1981) Science213, 915-918 19 Wilson, C. J. and Phelan, K. D. (1982) Brain Res. 243, 354-359 20 Kawaguchi, Y., Wilson, C. J. and Emson, P. C. (1990) J. Neurosci. 10, 3421-3438 21 Parent, A., Bouchard, C. and Smith, Y. (1984) Brain Res. 303, 385-390 22 Parent, A., Smith, Y., Filion, M. and Dumas, J. (1989) Neurosci. Left. 96, 140-144 23 Selemon, L. D. and Golclman-Rakic, P. S. (1990) J. Comp. Neurol. 297, 359-376 24 Haber, S. N. and Elde, R. (1981) Neuroscience 6, 1291-1297 25 Hedreen, J. C. and DeLong, M. R. (1991) J. Comp. Neurol. 304, 569-595 26 Parent, A. and De Beflefeuille, L. (1982) Brain Res. 245,
201-214 27 Ilinsky, I. A., Jouandet, M. L. and Goldman-Rakic, P. S. (1985) J. Comp. NeuroL 236, 315--330 28 IlJnsky, I. A. and Kultas-Ilinsky, K. (1987) J. Comp. Neurol. 262, 331-364 29 Kita, H. (1992) Brain Res. 589, 84-90 30 Smith, Y., Hazrati, L-N. and Parent, A. (1990) J. Comp. Neurol. 294, 306-323 31 Carpenter, M. B. etaL (1981) J. Comp. NeuroL 197, 579-603 32 Kita, H., Chang, H. T. and Kitai, S. T. (1983) J. Comp. NeuroL 215, 245-257 33 Bergman, H., Wichmann, T. and DeLong, M. (1990) Science 249, 1436-1438 34 Hartmann-von Monakow, K., Akert, K. and K0nzle, H. (1978) Exp. Brain Res. 33, 395-403 35 Afsharpour, S. (1985) J. Comp. Neurol. 236, 14-28 36 Hazrati, L-N., Parent, A., Mitchell, S. and Haber, S. N. (1990) Brain Res. 533, 171-175 37 Smith, Y., Parent, A., SEgu~la, P. and Descarries, L. (1987) J. Comp. Neurol. 259, 50-55
Do NMDA receptorshave a critical functionin visualcorticalplasticity? K. F o x a n d N. W . D a w
I( Fox is at the Dept of Physiology, University of 44innesota, 6-255, 44illardHa//, 435 Delaware St SE, 44inneapolis, 44N 55455, USA, and N. W. Daw is at the Dept of Ophthalmology and VisualScience, YaleUniversity, 330 CedarSt, New Haven, CT06510, USA.
The theoreticalframework, by which we understand the function of NMDA receptors, is derived, in large part, from work conducted on the hippocampal slice preparation, where NMDA receptors are crucial for a form of synaptic plasticity known as long-term potentiation (L TP). Establishing their role in plasticity mechanisms in the neocortex is proving to be far more difficult than originally envisaged, in part due to the fact that the operation of NMDA receptors is different in the intact animal than in vitro. For example, NMDA receptors are activated at low levels of sensory input in intact animals but only by high levels of input in slice preparations. Recent results suggest that a re-evaluation of the role of NMDA receptors in neocortical plasticity is required. Here we discuss some of the issues and introduce four criteria by which any factor supposedly involved in plasticity can be judged. NMDA receptors fulfill more of these criteria than any of the other factors so far investigated in the visual cortex, but maybe this is because they have been studied more intensively. Some 30 years ago, Wiesel and Hubel 1 introduced a paradigm whereby long-lasting anatomical and physiological changes occur in the visual cortex in response to altering the balance of sensory output from the two eyes. The cortex is only affected early in development during what is known as the 'critical period'. The experiment is done by suturing closed one eyelid for a short period of time (as little as eight hours during the peak of the critical period is sufficient to cause substantial changes2). When the animal's experience is altered in this way, changes occur in the synaptic connections of the visual cortex, a process referred to these days as experience-dependent synaptic plasticity (see Box 1). This paradigm has features in common with learning, where experience is thought to have a lasting effect on synaptic connections in the cortex and so enable memory formation and modification of behavior. Ocular dominance plasticity is the best experimental model we have at present for
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studying how synaptic plasticity occurs in the neocortex. Studies on neocortical long-term potentiation (LTP) are, by comparison, in their infancy. Moreover, LTP paradigms have not been clearly linked to the function of the cortex in the intact animal, as ocular dominance plasticity has. W h y NMDA receptors? Ever since the discovery that NMDA receptors have a critical function in the induction of synaptic modification in the hippocampus 3, it has seemed natural to hypothesize that NMDA receptors are also involved in neocortical plasticity. Although obvious differences exist in the expression of the two forms of plasticity (LTP involves a rapid change in synaptic efficacy, while ocular dominance plasticity involves changes in axonal arborization patterns as well), the induction mechanisms seemed likely to be similar. The first experiments to suggest that NMDA receptors might be involved in ocular dominance plasticity were based on procedures that antagonized NMDA receptors during monocular deprivation. Under these conditions ocular dominance plasticity was reduced 4'5, satisfying a preliminary condition for the involvement of NMDA receptors in this process. However, a number of issues complicate the interpretation of these results. Experimental issues. Other less specific effects on cortical physiology were also reported to occur after chronic NMDA receptor blockade, such as a general loss of responsiveness to visual stimuli and loss of receptive field specificity. Given this result, there is a concern that the drug being applied causes a general depression of visual input. Nonspecific receptive fields and poorly responding cells are also common when tetrodotoxin (TTX) is applied to the cortex, or animals are reared with binocular lid suture, known as binocular deprivation (BD) 6'7. This is because both BD and infusion of TTX prevent patterned visual information reaching the cortex from the eyes, and this is known to be necessary for refining the TINS, Vol. 16, No. 3, 1993
Box 1. The monocular deprivation paradigm viously closed eye input occurs f, demonstrating that this form of plasticity involves increases in synaptic efficacy as well as decreases (Fig. C). Reverse suture experiments suggest that NMDA receptors could be involved in potentiation of weak eye inputs g.
There are a number of good reviews of the extensive literature on ocular dominance plasticity a-c. In essence, closing one eye of a binocular animal during the critical
period causes a long-term decrease in responsiveness to the closed eye. The change can be measured physiologically by assaying ocular dominance: a number of microelectrode penetrations are made through the cortex to sample the preference of single cortical cells for stimulation of the right or left eyes. Normally, most cortical cells are binocular with the number of cells responding to left and right eye stimulation being approximately equal. This result is usually illustrated by an ocular dominance histogram (Fig. A). Monocular deprivation leads to a shift in the balance toward cells that respond preferentially to the open eye: at the peak of cortical sensitivity, the shift can almost be complete (Fig. B). Monocular deprivation has an effect during a critical period that, in the cat, lasts for about one year for cells in extragranular layers d and peaks between four to six weeks of agee. If monocular deprivation is reversed within this critical period, some recovery of the pre-
a Mitchell, D. E. and Timney, B. (1984) in Handbook of Physiology. The Nervous System: Sensory Processes, pp. 507-555, Am. Physiol. Soc. b Movshon, J. A. and Van Sluyters, R. C. (1981) Annu. Rev. PsychoL 32,477-522 c Shatz, C. J. (1990) Neuron 5, 745-756 d Daw, N. W., Fox, K., Sato, H. and Czepita, D. (1992) J. NeurophysioL 67, 197-202 e Olsen, C. R. and Freeman, R. D. (1980) Exp. Brain Res. 39, 17-21 f Blakemore, C. and Van Sluyters, R. C. (1974) J. PhysioL 237, 195-216 g Gu, Q., Bear, M. F. and Singer, W. (1989) Brain Res. 47, 281-288
B
A
C
26
!6 !4
"5
References
36
!2' !0' 18' 16' 14' i2' 10'
8' 6 4 2' 0
, 1
2
3
4
5
6
Ocular dominance group
7
1
2
Contra
I 3
4 Equal
5
6
7
1
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Ipsi
The effect of monocular deprivation on ocular dominance. (A) The ocular dominance histogram for a normal binocular cat is composed mainly of binocular cells (groups 2-6) with a small contralateral bias. Group 1 contains cells responding only to the contralateral eye; group 7, cells responding only to the ipsilateral eye; and group 4, cells that respond equally well to both eyes. (B) Monocular deprivation, imposed at two weeks, causes an almost total shift of ocular dominance to the open eye (contralateral eye) by five weeks. (C) Reversing eye closure from the contralateral to the ipsilateral side at five weeks causes a reversal of the ocular dominance shift by 14 weeks. (Adapted from Blakemore and Van Sluytersf.)
properties of cortical receptive fields during development. Since NMDA receptors are involved in transmission of patterned visual information within kitten visual cortex 8, and infusion of D-2-amino-5-phosphonovalerate (APV) reduces visual responses 9, the simplest explanation for the results is that NMDA antagonists act by attenuating cortical responses to visual stimuli. This explanation removes the need to argue that NMDA receptors have a special role in cortical plasticity. Theoretical issues. A good deal is already known about the presynaptic activity requirements for ocular dominance plasticity. Somewhere in the pathway for plasticity, there must be a mechanism for detecting correlated presynaptic activity involving the postsynaptic element (see Box 2). Could the NMDA receptor possibly fulfill this role? Some have argued that it can in the hippocampus 1°' 11 and neocortex 12 on TINS, Vol. 16, No. 3, 1993
the basis that the NMDA receptor acts like a Hebbian mechanism (see Box 3). However, there are significant problems with this theory. First, since NMDA receptors have a high affinity for glutamate 13, extracelhlar glutamate is of sufficient concentration to bind to NMDA receptors even when at rest 14. This prevents the NMDA receptor from being a Hebbian mechanism. Second, because of the cable properties of dendritic spines 1S, even small inputs would be expected to cause a large depolarization at the spine head, sufficient to relieve the voltage-dependent block of the NMDA channel, whether correlated with other inputs or not. This prevents it from being a correlation detector. These expectations are supported by observations. First, NMDA receptors are found to be spontaneously active in a variety of structures 8,16,17. They conduct current in unstimulated preparations in vitro 117
Box 2. Conditions for ocular dominance plasticity To cause a shift in ocular dominance towards one eye, retinal activity must differ between the two eyes"-C: in particular the electrical spiking activity must differ between the two eyesc,d. Monocular deprivation causes an increase in the area occupied by the geniculocortical endings for the open eye and a decrease in the area occupied by the afferents driven by the closed eye (see the figure below). The synapses formed by the geniculocortical afferents of both the closed and the open eyes differ from normal in the number of synapses per bouton, bouton size and location of their terminations (see Ref. e). The temporal correlation of afferent spike activity determines how connections are grouped; correlated presynaptic activity causes aggregation of correlated inputs, uncorrelated activity causes segregation of uncorrelated inputs f'g. Extreme levels of uncorrelated activity from the two eyes can occur with strabismus, which leads to total segregation of geniculocortical h and intracortical axon arbors driven by the two eyes'. In addition to these presynaptic requirements, it is almost certain (but not known beyond doubt) that postsynaptic cortical activity is necessary for plasticity. If NMDA receptors are only present postsynaptically in visual cortex, the fact that plasticity is disrupted when NMDA receptors are blocked implies that some postsynaptic activity is required j. Another line of evidence supporting the necessity of postsynaptic activity comes from experiments where the orientations of experienced stimuli are restricted during monocular deprivation. This leads to only those cells that are responsive to the experienced orientation being involved in the shift in ocular dominance, while the other cells remain unaffected. Since orientation selectivity first arises postsynaptically in the cortex, this implies postsynaptic involvement k-m (but see Ref. n on interpretation of this result). The relationship between pre- and postsynaptic activity affects the sign with which the synaptic alteration occurs, and does so in a manner that appears to be described by the covariance rule. If pre- and postsynaptic activities are correlated, the functional connection between the two increases, but if they are uncorrelated, the functional connection decreases °.p. An analogous segregation process occurs in the tectum of the three-eyed frog. Two retinae are forced to innervate the same tectum, which results in a striated pattern of innervation much like that seen in mammalian cortex. Separation of retinal axons into eye-specific stripes is prevented by blocking NMDA receptors with antagonists q. Evidence suggests the NMDA receptors are postsynaptic r, so this is another example of a postsynaptic mechanism affecting presynaptic morphology.
References a b c d e f g h i j k I m
Wiesel, T. N. and Hubel, D. H. (1963) J. Neurophysiol. 26, 1003-1017 Wiesel, T. N. and Hubel, D. H. (1965) J. Neurophysiol. 28, 1029-1040 Stryker, M. P. and Harris, W. A. (1986) J. Neurosci. 6, 2117-2133 Chapman, B., Jacobson, M. D., Reiter, H. O. and Stryker, M. P. (1986) Nature 324, 154-156 Friedlander, M. J., Martin, K. A. and Wassenhove-McCarthy, D. (1991) J. Neurosci. 11, 3268-3288 Blasdel, G. G. and PeRigrew, J. D. (1979) J. Neurophysiol. 42, 1692-1710 Stryker, M. P. (1986) in The Biology of Change in Otolaryngology (Ruben, R. W., Van De Water, T. R. and Rubel, E. W., eds), pp. 211-244, Elsevier Shatz, C. J., Lindstrom, S. and Wiesel, T. N. (1977) Brain Res. 131, 103-116 Lowel, S. and Singer, W. (1992) Science 255, 209-212 Kleinschmidt, A., Bear, M. F. and Singer, W. (1987) Science 238, 355-358 Rauschecker,J. P. and Singer, W. (1979) Nature 280, 58-60 Cynacler, M. S. and Mitchell, D. E. (1977) Nature 270, 177-178 Carlson, M., Hubel, D. G. and Wiesel, T. N. (1986) Dev. Brain Res. 25, 71-81
and support spontaneous spike activity in vivo in the absence of sensory stimulation. Second, even small inputs just above threshold are sufficient to activate NMDA receptors in the visual cortex TM. These results show that the threshold for activating NMDA receptors is low and certainly not reserved for conditions of extreme correlation. The timing characteristics of the biological correlation detector are also important, but as yet remain unknown. NMDA receptors produce long-duration 118
The effect of monocular deprivation on geniculocortical afferents viewed by autoradiography. (A) In a 4J-year-old monkey reared with binocular vision during the critical period, the geniculocortical afferents occupy approximately equal areas of cortex. (B) In a l]-year-old monkey reared from two weeks with one eye closed, the geniculocortical afferents from the open eye occupy a greater area of cortex than normal. In both cases the ipsilateral eye was injected with radiolabelled tracer and collages made of sections through layer IVC (anterior up, medial left). (Adapted from Hubel, Wiesel and LeVayS.) n Movshon, J. A. and Van Sluyters, R. C. (1981)Annu. Rev. Psychol. 32,477-522 o Fregnac, Y., Shulz, D., Thorpe, S. and Bienenstock, E. (1992) J. NeuroscL 12, 1280-1300 p Shulz, D. and Fregnac, Y. (1992) J. NeuroscL 12, 1301-1318 q Cline, H. T., Debski, E. A. and Constantine-Paton, M. (1987) Proc. Natl Acad. ScL USA 84, 4342-4345 r Cline, H. T., McDonald, J. and Constantine-Paton, M. (1991) Soc. Neurosci. Abstr. 17, 794 s Hubel, D., Wiesel, T. N. and LeVay, S. (1977) Philos. Trans. R. Soc. London Ser. B 278, 377-409
excitatory postsynaptic potentials (EPSPs) 19, and therefore could correlate activity over periods of about 1-200ms. If they are involved in correlating inputs between the two eyes, cells responsive to both eyes (binocular ceils) should be retained if inputs from the left and fight eyes occur within this time window. At present, this issue is unresolved experimentally; one study shows that binocular cells occur if the eyes are stimulated as much as 10 s apart 2° and the other when only stimulated 3-500 ms apart 21. Consequently, TINS, Vol. 16, No. 3, 1993
Box 3. Hebbian synaptic mechanisms Hebb mechanism The figure shows the basic timing concept of a Hebbian mechanism. As originally conceived by Hebb a, the connection between pre- and postsynaptic elements of the synapse strengthen if the presynaptic site 'repeatedly or persistently takes part in firing' (the cell). In this case, the presynaptic event (Event 1) is the firing of the presynaptic cell, and the postsynaptic event (Event 2) is the target cell producing an action potential. The mechanism necessary to detect such an event would be activated only when both the pre- and postsynaptic elements are active (And). This coincidence signal would in turn cause a long-lasting change in the synaptic connection (Latch). Correlation detector The same basic Hebbian scheme is capable of detecting correlated firing of different synapses. For example, imagine Event 1 and Event 3 represent glutamate release from synapses that are incapable of depolarizing the neuron individually, but which do cause the cell to fire when they are activated simultaneously. In this modification of the Hebbian scheme, Event 2 occurs only when Events 1 and 3 occur together. This causes inputs from both eyes to be strengthened when their activity is highly correlated through binocular visual stimulation, and inputs from a single open eye to be strengthened when their activity is highly correlated by monocular visual stimulation. In some formulations of Hebb's proposal, the postsynaptic event is a certain level of depolarization of the postsynaptic cell, rather than firing of an action potential, but the principle remains the same. Biological substrate The NMDA receptor has been suggested as a possible candidate for a biological substrate for the modified Hebbian mechanism. Presynaptic activity results in release of glutamate that binds to the glutamate-binding site on the NMDA receptor. Postsynaptic activity produces depolarization that is detected by the voltage dependence of the NMDA channel. The voltage dependence of the NMDA channel results from a voltage-sensitive Mg 2 + -binding site within the conductive pore. At hyperpolarized membrane potentials, Mg 2÷ binds to and blocks the NMDA channel but at depolarized potentials the Mg 2+ blockade is greatly reduced, allowing conduction of other ions through the channel, including Ca2+. Obviously, the real biological versions of the processes illustrated in the figure will not involve square waves nor binary functions. However, a sharp nonlinear function is required somewhere in the sequence to distinguish normal synaptic transmission from that required to cause synaptic modifications (see main text). At present, there is insufficient experimental evidence to know what the time-base on this diagram should be for ocular dominance plasticity.
we cannot yet assign a time-base to the mechanism outlined in the figure in Box 3. A l t e r n a t i v e theories There are clearly holes in the present experimental evidence as well as inadequacies in current theories that attempt to explain exactly how NMDA receptors might be involved in ocular dominance plasticity. It had previously been thought that the correlation detector required a voltage element to link synaptic activity at different synaptic locations. However, discovery of the rapidly diffusing second messenger, nitric oxide (NO), has led to alternative theories of how correlations over relatively large distances might be linked22. On the other hand, if the slow correlation times that were suggested by the Blasdel and Pettigrew experiments 2° are corroborated, even a slowly diffusing substance would be sufficient (see Box 3). Where does this leave NMDA receptors? One notable gap in our knowledge is whether postsynaptic TINS, VoL 16, No. 3, 1993
Event1
Event2
Event3
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The Hebbian synaptic mechanism requires an element to detect coincident events (Event 1, Event 2). The ideal process for this would be an 'And' function. A further mechanism is required that switches on when the coincidence is detected by the And function, translating a transient event into a lasting change (Latch).
Reference a Hebb, D. O. (1949) The Organization of Behavior, Wiley
Ca2+-dependent mechanisms are critically involved in plasticity. [f it can be shown that they are, one resolution of the current evidence might be that the plasticity switch (Latch in Box 3) is in fact a postsynaptic second or third messenger system activated by the Ca 2÷ influx caused by sufficient NMDA receptor activation. The binding of Ca 2+ to calmodulin appears to be sufficiently nonlinear to fulfill the function of a switch for LTP in the hippocampus (while the voltage dependence of the NMDA receptor is inadequate for this on its own) 23'24.
A unique link? While this form of argument rescues the theory that NMDA receptors play some role in plasticity, it simultaneously opens up other possibilities. If a Ca2+activated second messenger system were to perform correlation detection, how could we rule out the possibility that other transmitter-receptor-second messenger systems were not involved in plasticity in the same way? For example, voltage-dependent Ca 2+ 119
required (though it is often treated this way). In order to include factors as candidates, this criterion must be linked with others and for two main reasons. Changes in spine First, from a technical point of n s ~ morphology? view, the experiment may block more than the single factor targeted. For example, the drug 6-hydroxydopamine, which depletes noradrenaline, abolishes Inl plasticity, but this may occur fro because of the general damaging oF effects of 6-hydroxydopamine e~ when used in high doses 25 or because it affects acetylcholine as well26. We include criterion (2) as one method of distinguishing between factors that cause a general deterioration of cortical function In from those which, if absent or fn langes in spine removed, specifically cause a loss cl )rphology? of plasticity. e~ Second, blocking the factor may affect a general process rather than one specifically related to plasticity. For example, anesthesia and paralysis both block plasticity 27, Fig. 1. The relationship between some of the steps possibly involved in ocular dominance plasticity. but it d,oes not follow that kittens Activity of N/VlDA receptors may be necessary to admit second messengers (Ca2+) postsynaptically. are more plastic than cats because Little is known at present about how this event might be finked to sprouting and retraction of they move their eyes around more, presynaptic elements. If diffusible factors are involved, possible targets include the afferent or because they are awake more terminals of the open eye, the afferent terminals of the closed eye and the dendritic spines of the closed eye. The rate and freedom of second messenger diffusion determine integration time and than cats. We include criteria (3) and (4) in order to distinguish the distance over which correlated synapses can be detected (see Ref. 25). factors that are necessary for normal cortical function at all ages channels or the metabotropic receptor could activate from those peculiar to ocular dominance plasticity. the same Ca2+-dependent mechanisms as NMDA re- (2) Reintroducing the factor into older animals restores plasticity. This is also a necessary criterion, with two ceptors do. One constraint on the possibilities is that the caveats. First, it assumes that there is a single crucial mechanism must be affected by what we know factor in a single crucial pathway that is lost during the instructs synaptic plasticity (i.e. correlated activity) aging process. If two factors are required in combiand it must also in turn affect systems that sequen- nation, then reintroducing just one of them will not tially end up altering the sprouting and retraction of work. Moreover, if there is a sequence of steps presynaptic elements (systems that remain to be dis- involved in plastic changes, and more than one of the covered). In other words, the candidate mechanism factors in the sequence of steps is missing in adults, needs to be on the pathway between retinal activity then again reintroducing one of the factors will not and synaptic alterations (see Fig. 1). So far, we have work. Second, whether an experiment will be succoncentrated on current evidence related to this cessful depends on knowing enough about the process single criterion. However, we propose that this is one so that the factor can be added back in at the right of at least four criteria that are required to test which time and the right place. For example, assuming factors, or steps in the sequence of processes, are NMDA receptors were crucial to plasticity, just crucial to plasticity. What follows is generally appli- adding them back to the adult system would not cable to any stage of the plasticity process, but we necessarily be enough: this might simply flood the concentrate here on the way the criteria apply to system and nonspecifically strengthen all synapses rather than those that were specific for coordinated current evidence on NMDA receptors. binocular function. Consequently this criterion has to be applied with intelligence, and has not been tested Requirements to demonstrate that a factor is for many candidate factors. crucial for plasticity (1) Removing the factor, or introducing an antagonist, (3) The factor is present during the critical period, prevents plasticity. This is a necessary criterion, but it including when this period is altered by dark rearing. is certainly not sufficient. That it is necessary hardly This criterion distinguishes the crucial factor from the requires explanation. Any factor which does not block others in the sequence of steps that are active at all plasticity when removed or antagonized obviously ages for a variety of processes besides plasticity. For does not have a critical function in the process. As the sake of argument, take the hypothesis illustrated such, this first criterion is an exclusionary criterion, in Fig. 2. This hypothesis arises from work on LTP in not one that by itself can be used to show a factor is the rat hippocampus 3°. It is controversial for LTP and 120
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is certainly not proved for plasticity in the cortex, nevertheless it can be used to illustrate the general point to be made. There are more than 11 steps in the sequence of processes in this hypothesis (Fig. 2). We know from experiments with TTX (Ref. 31) that geniculocortical activity is involved in plasticity. However, there is no evidence for a difference in geniculocortical activity between kittens and cats that can account for the end of the critical period. Similarly, resting levels of Ca 2+ are not thought to be different between kittens and cats. If the hypothesis turns out to be true, it is almost certain that one of the other steps in the sequence will be the crucial one. To some extent this criterion defines what we mean here by 'crucial'. Any factor that can be artificially removed to abolish plasticity is in some sense involved in plasticity, but the factor or factors that alter naturally during development to bring plasticity to an end are special cases. The timecourse of the critical period is delayed by rearing animals in the dark 32,33. Consequently, a corollary of this criterion is that the presence, concentration or efficacy of the crucial factor should remain elevated in animals reared in the dark for some time after the peak of the critical period for lightreared animals.
(4) The factor is instructive, not just permissive for plasticity. This criterion distinguishes factors that are on the pathway from those that feed into it. As an example, there is some evidence that a combination of the noradrenaline and acetylcholine pathways may affect plasticity26. However, neither noradrenaline nor acetylcholine pathways arise from neurons receiving specific visual signals from the retina. Consequently, as it is the degree of correlation between retinal signals that instructs synaptic modification, neither pathway receives the signal that specifies the nature of the synaptic change. At most they can only permit changes specified through a different mechanism. Thus blocking both acetylcholine and noradrenaline pathways might affect plasticity even if the direct sequence of steps involved in plasticity turned out to be the sequence described in Fig. 2, although neither pathway comprises any of those steps. H o w far do NMDA receptors fit t h e s e four criteria? NMDA receptors have been tested more extensively than any other factor in terms of whether they fit these criteria. Antagonists to NMDA receptors reduce or abolish ocular dominance shifts in the visual cortex 4'5'34. Criterion (1) is therefore satisfied. However, as discussed above, this by itself is not a sufficient test. Criterion (2) has not yet been tested, probably because of the difficulties in designing and interpreting an experiment to make the test. Criterion (3) has been examined; NMDA receptors are more abundant during the critical period for plasticity3s-3s, and their contribution to the visual response decreases between three and six weeks of age in layers IV, V and VI of the visual cortex 8'39. This drop in the contribution to the visual response can be delayed by rearing animals in the dark 39,4°. The decrease in NMDA receptors in layer IV correlates with the period of geniculocortical afferent segregation4t and so may correlate with the critical period for this layer, which has not yet been defined in the cat. Thus criterion TINS, VoL 16, No. 3, 1993
Presynaptic site
Postsynaptic site
Geniculocortical activity
Release of glutamate
Activation of NMDA receptors
Ca 2+ entry
Activation of calmodulin
;?
Activation of guanylate cyclase
Activation of NO synthase
Production of cGMP
Changes in proteins
Retraction of some geniculocorticat terminals, sprouting of others Fig. 2. A possible role for NMDA receptors m ocular dominance plasticity. This currently popular hypothesis is one way in which the pathways illustrated in Fig. 1 could work. The rapid and freely diffusible substance is nitric oxide, which may play a role in hippocampal L TP (Refs 28, 29) and is under test for the visual cortex. There is reasonable evidence for the first five steps in the sequence, but tittle evidence exists for the stages finking step five to the changes in presynaptic terminals (indicated by question marks).
(3) may be satisfied for some layers of the cortex. Criterion (4) is difficult to satisfy because it requires an overall knowledge of the sequence of plasticity mechanisms. So far, the input side of the sequence has been determined; correlated retinal activity is known to play an instructive role, though we are unsure of the exact timing constraints. Cortical NMDA receptors are certainly driven by retinal activity and in that sense receive the instructive signal, but have not been shown to be involved in detecting the degree to which presynaptic activity is correlated. The output side of the instructive sequence has not been determined but is widely supposed to stem from the postsynaptic entry of Ca 2+. If it can be shown that a Ca2+-activated second messenger system controlled by NMDA receptors is necessary for plasticity, we will know that it is transmitting the instructive signal as well. One can therefore conclude that, so far, NMDA receptors partly fulfill three out of the four criteria listed. This is more than any other factor currently under investigation (primarily because NMDA receptors have been more extensively tested than any other factor). It is clear that NMDA receptors have 121
other roles besides developmental plasticity: they are found in layers II and III of the adult visual cortex 8'42 where ocular dominance plasticity does not occur. Here, they may be necessary for whatever function LTP subserves 43'44, or they may simply function in a computational role that is unrelated to plasticity 18'45. Nevertheless, NMDA receptors are still the best candidate that we have at the present time for a factor that makes kittens plastic while adults are not, at least for the neurons in cortical layer IV.
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TINS, Vol. 16, No. 3, 1993