Regulation of N-methyl-d-aspartate (NMDA) receptor function during the rearrangement of developing neuronal connections

J. van Pelt, M.A. Corner H.B.M. Uylings and F.H. Lopes da Silva (Eds.) Pmgress in Bmin Research, Vol 102 0 1994 Elsevier Science BV. All rights reserved.

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CHAPTER 18

Regulation of N-methyl-D-aspartate ( NMDA) receptor function during the rearrangement of developing neuronal connections Magdalena Hofer and Martha Constantine-Paton Department of Bwlogy, Yak UniuersQ,New Hauen, CT 06511, U SA.

Introduction

The development of mature synaptic connectivity in the central nervous system is the result of a multistage process. Cell-surface cues and initial synaptic contacts set the stage for a dynamic phase of synaptogenesis during which contacts within a developing neuropil are rearranged in a pattern partially imposed by neuronal activity. The final sharpening and fine-tuning of synaptic contacts involves elimination of incorrect and redundant terminals and stabilization of appropriate ones. Donald Hebb first proposed, as part of his theory for associative learning, that synapses can undergo strengthening whenever activity in presynaptic cells occurs simultaneously with that in the postsynaptic cell (Hebb, 1949). The N-methyl-Daspartate (NMDA) type of glutamate receptor is believed to play an important role in the process of activity-dependent synaptic restructuring (Constantine-Paton et al., 1990). As it gets activated only when the postsynaptic cell membrane is already depolarized (Mayer et al., 1984; Mayer and Westbrook, 1985; MacDermott et al., 1986) it is capable of detecting correlated afferent activity. CaZ+influx through the integral ion channel may initiate the selective stabilization of simultaneously active synapses by a Hebbian-like mecha-

nism (Constantine-Paton et al., 1990; Shatz, 1991; Cline and Tsien, 1991; Yuste and Katz, 1991). The potential of synapses to reorganize is, in most neuronal systems, restricted to a short period of time in early life. The question therefore arises: what are the developmental mechanisms that permit synaptic plasticity and that terminate it? What is the molecular basis for the critical difference between the pronounced structural plasticity of the developing brain and the less plastic properties of many regions in the mature brain? The answer to these questions is largely unknown. Accumulating evidence, however, suggests that the molecular properties of NMDA receptors become altered during development, and these changes may define periods of developmental plasticity. This paper reviews some of the data illustrating developmental changes in the properties of the NMDA receptor complex that may reflect alterations in the ability of synapses to rearrange. It will focus on the visual system of amphibia and rodents for a discussion of cellular and molecular changes that may be involved in the mutability of synaptic connections. Developmental changes in NMDA receptors and synaptic plasticity If NMDA receptors are involved in developmental stabilization and sorting of synapses, then some

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aspects of their function should change in parallel with anatomical and physiological alterations in developmental plasticity. Although transient increases in glutamate receptor function and density appear to be common during development, the correlation with periods of enhanced synaptic plasticity is less clear. One of the systems in which a relationship between NMDA receptor efficacy and some form of synaptic plasticity was demonstrated is the cat visual cortex (reviewed by Fox and Daw, 1993). A decrease of the NMDA receptor component of the visual response occurs with increasing age in layers IV, V and VI of cat visual cortex (Tsumoto et al., 1987; Fox et al., 1989). This down-regulation of NMDA receptor efficacy correlates with the end of the period of segregation of geniculate axon terminals into ocular dominance columns in layer IV (LeVay et al., 1978) but not with the reduced plasticity of layer IV as measured in the monocular deprivation paradigm (Olson and Freeman, 1980). The relationship between NMDA receptor protein levels, as measured in receptor binding assays, and critical periods for monocular deprivation in cat visual cortex also remains perplexing. Increases in the levels of NMDA receptor binding appear to parallel the critical period for monocular deprivation (Bode-Greuel and Singer, 1989; Reynolds and Bear, 1991; Gordon et al., 1991), but they do not correspond to the period of sensitivity of the visual response of neurons in the deep cortical layers to NMDA receptor antagonists (Fox et al., 1989). Rearing kittens in the dark, a procedure which prolongs physiological plasticity (Cynader and Mitchell, 1980) but not the anatomically analyzed formation of ocular dominance columns (Mower et al., 1983, does not delay the decrease in NMDA binding sites (Bode-Greuel and Singer, 1989; Gordon et al., 1991). This treatment does, however, prevent the decline in the NMDA receptor potency (Fox et al., 1991). These discrepancies may, in part, be caused by technical difficulties associated with receptor binding studies. For example, L3H1MK-801binding of cortical homogenates addresses changes in the number of

receptors as well as in agonist affinity. However, this technique does not resolve individual cortical layers (Reynolds and Bear, 1991; Gordon et al., 1991). Layer by layer changes in NMDA receptor binding have been resolved by receptor autoradiography using [3H]glutamatedisplaced by APV (Bode-Greuel and Singer, 1989). It seems likely that some of these inconsistencies will be reconciled as the subunit composition of NMDA receptors in different layers and at different ages become defined. Examples from other systems indicate that developmental changes in the efficacy of the receptor, their density and their function are widespread. In hippocampus, the effectiveness of the NMDA receptor is enhanced in young animals compared with adult (Hamon and Heinemann, 1988; McDonald et al., 1988). NMDA receptor number as assayed by ligand binding was shown to increase transiently during postnatal days 6-10 and then decrease to adult levels by day 13 in rat hippocampus (Tremblay et al., 1988). However, NMDA receptor-mediated hippocampal long-term potentiation (LTP: a long lasting change in synaptic efficacy) persists throughout adulthood. In cerebellum, a transiently enhanced sensitivity of Purkinje and granule cell responses to NMDA corresponds to the period of synapse elimination during development (Dupont et al., 1987; Garthwaite et al., 1987). Other functional properties of the NMDA receptor are changed during development. The Mg2+ sensitivity and voltage dependence of the NMDA receptor have been shown to be lower in hippocampal neurons from immature rats than those from adult ones (Ben-Ari and Cherubini, 1988; Bowe and Nadler, 1990; Morrisett et al., 1990; Kleckner and Dingledine, 1990, whereas the sensitivity for glycine is higher in young rats than in adult ones (Kleckner and Dingledine, 1991). These studies suggest that NMDA receptor from immature animals are more easily excitable and may pass more current in comparison with adult animals. Recently, a developmental change in the duration of NMDA receptor mediated currents was observed in collicular neurons: in 10- to 15-day-old

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animals the NMDA currents are several times longer than in 23- to 33-day-old animals (213 & 57 msec vs. 85 f 37 msec) (Hestrin, 1992). A similar change in NMDA channel open time was observed in the rat visual cortex. Moreover, this change is apparently activity-dependent, as it can be delayed by either dark rearing or 'ITX treatment (Carmignoto and Vicini, 1992). This change in NMDA receptor channel kinetics is similar to the observation at the neuromuscular junction where the time course of the embryonic end-plate current is slow compared with that of the adult (Sakmann and Brenner, 1978). In the muscle nicotinic acetylcholine receptor, a subunit replacement results in the transition from the fetal to the adult form, leading to a decrease in the mean open time of the ion channel (Mishina et al., 1986). An analogous alteration in the subunit composition of the NMDA receptor may define its channel open time in a similar manner. In an attempt to obtain information about the postulated changes in the NMDA receptor subunit composition, Williams et al. (1993) measured the affinity of rat brain NMDA receptors to the non-competitive antagonist ifenprodil, that may modulate polyamine sites. They found a uniformly high affinity in neonatal animals whereas, in 21-day-old or adult animals, a second population of receptors with a 100-fold lower affinity appears. Complementary voltage-clamp recording of Xenopus oocytes expressing various stoichiometric configurations of NMDA receptor subunits revealed potent inhibition by ifenprodil of responses of homomeric NR1 and heteromeric NRl/NR2B receptors, but not of NRl/NR2A receptors, suggesting that differences in subunit composition are the basis for the observed change in antagonist affinity (Williams et al., 1993). In addition, in situ hybridization studies have revealed differential developmental regulation of the spatial and temporal expression pattern of mRNA coding for various NMDA receptor subunits, supporting the notion that changes in the subunit composition of the NMDA receptor channel complex take place during development (Watanabe et al., 1992). Taken together, these

findings from many different brain regions make clear that a number of aspects of NMDA receptor function are developmentally regulated. Whether these alterations correspond to structural changes and the ability of synapses to reorganize still remains to be demonstrated. Regulation of NMDA receptor efficacy in the frog retinotectal system

The visual system of cold-blooded vertebrates has served as an excellent model for studying the chemospecific and activity-dependent processes that lead to a precisely organized projection. The retinotectal projection in amphibians and fish retains structural plasticity well into maturity as retinal terminals are constantly shifting over the tectal surface in order to maintain retinotopy throughout larval development, despite asymmetric patterns of retinal and tectal cell proliferation. A preparation that anatomically assays the operation of the mechanisms that mediate synaptic competition has been developed using Rana pipiens (leopard frog) embryos. By implanting a third eye primordium into a tadpole embryo and forcing two afferent projections to share one tectal lobe, a series of alternating eye-specific afferent termination zones or stripes is created in the optic tectum (Constantine-Paton and Law, 1978; Law and Constantine-Paton, 1981). NMDA receptors have been shown to be required for the formation and maintenance of these eye-specific stripes, as chronic treatment of the tecta of three-eyed tadpoles with the specific NMDA receptor blocker APV for 4-6 weeks results in the desegregation of the normal striped pattern (Cline et al., 1987). In contrast, chronic treatment of three-eyed tadpoles with NMDA (the agonist) sharpens stripe boundaries (Cline and Constantine- Paton, 1990). Even though the initial formation of the retinotectal projection is not activitydependent in cold-blooded animals (Harris, 1980; Harris, 1984; Stuermer et al., 1990), treatment of normal two-eyed tadpoles with APV dramatically disrupts the topography of the retinotectal map (Cline and Constantine-Paton, 1989).

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Complementing these anatomical studies, Debski et al. (1991) used a physiological approach in order to investigate the state of NMDA receptors in animals chronically treated with NMDA, and found that chronic treatment with NMDA decreases the electrophysiologically measured sensitivity of the optic tectum to applied NMDA (Debski et al., 1991). Receptor binding autoradiography on similarly treated tissue failed to reveal any change in binding site density. Thus, there appears to be a change in the effectiveness of individual NMDA receptors rather than a decrease in receptor number. The lowered sensitivity to NMDA is a long-term change, and remains stable for hours even after the agonist has been removed, thus differing from the rapid desensitization reported by others (Trussell et al., 1988; Mayer et al., 1989). This experimentally induced decrease in NMDA receptor effectiveness may be analogous to the naturally occurring reduction in receptor function that is correlated with the end of some periods of visual plasticity. It may be reflected anatomically by the sharpening of stripes, that represent a further restriction of the intermingling of axon branches from the two eyes. A similar limiting of synapse stabilization to areas where afferent activity is most highly correlated must occur as a young brain matures in order to ensure that only highly correlated, efficient contacts persist into adulthood. If NMDA receptor activation is indeed the critical trigger for such synapse stabilization, then a decrease in its effectiveness with age would be sufficient to restrict the stabilization of new contacts in older brain. Thus, new contacts would be less likely to be established in mature brains, and synaptic plasticity would be reduced. Recent whole-cell patch clamp analysis of tadpole tectal neurons have demonstrated that these neurons have both non-NMDA and NMDA receptor-mediated currents, and that NMDA receptors on these tectal neurons are not responsible for the bulk of normal excitatory transmission (Hickmott and Constantine-Paton, 1993) . The same study also examined GABAergic inhibitory currents in tectal neurons and suggested that

inhibition might modulate NMDA receptor-mediated excitatory responses. Similarly, in rat cortex a transient manifestation of strong NMDA receptor-mediated potentials is a consequence of the relative immaturity of GABA-mediated inhibition. Further development is accompanied by a decrease in NMDA receptor mediated excitation, thus providing a temporal window of enhanced sensitivity for LTP induction (Luhmann, 1990; Luhmann and Prince, 1990). These observations have implications for the control of synaptic plasticity during development: the relative contribution of NMDA and non-NMDA receptors to postsynaptic responses appears to change depending upon alterations in inhibitory input and these changes may affect the ability of synapses to rearrange during development. Regulation of NMDA receptors at the mRNA level in the rat superior colliculus To get insight into the mechanisms of topographic map formation in warm-blooded animals, we have studied the development of retinocollicular projections in rats. The period of rearrangement of retinal terminals in the superior colliculus is restricted to the first 2 weeks of postnatal development, and several clearly defined stages of this process have been described (Simon and O’Leary, 1992) (see Fig. 1). The developing retinmllicular projection therefore provides a more suitable system for studying factors that may control the onset and the termination of plasticity than does the amphibian retinotectal projection. Developing rat retinal axons do not grow directly to their topographically appropriate location in the SC, but initially mistarget widely (Simon and O’Leary, 1992). This is in contrast to the development of retinal projections in coldblooded vertebrates where a precisely organized retinotopic map is present from early stages, and coherence of this projection is maintained throughout a subsequent period of synapse sorting and retinal and tectal growth (Gaze et al., 1979; Holt and Harris, 1983; Constantine-Paton and Reh, 1985; Stuermer, 1988). Disruption of

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activity patterns interferes with the maintenance and regeneration of these maps in amphibia and fish, but it does not affect the initial organization of retinal projections in these species (Harris, 1980, 1984; Stuermer et al., 1990). In rats, the establishment of retinocollicular order requires a major remodeling of the early, diffuse retinal projection prior to eye opening and pattern vision. A large-scale elimination of aberrantly positioned branches and arbors, along with an increase in branching and arborization at topographically correct locations in the SC, occurs during the first 2 postnatal weeks (Simon and O'Leary, 1992). This refinement of the retinotopic map appears to depend upon normal NMDA receptor function: chronic blockade of NMDA receptors in the SC from birth disrupts the remodeling of the retinocollicular projection so that mistargeted axons remain and arborize at topographically incorrect sites (Simon et al., 1992). Since NMDA receptors are involved in retinotopic map formation in the SC, we asked whether developmental changes in their properties would parallel this process. As a first step in addressing this issue, we investigated the regulation of mRNA coding for NMDA and non-NMDA receptor subunits in the developing rat retinocollicular system in order to see whether NMDA receptor expression and the period of topographic map refinement in the SC are temporally correlated. Figure 2 shows the temporal expression pattern

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Fig. 1. Development of the rat retinocollicular projection, as described by Simon and OLeary (1992). The horizontal bars mark the approximate duration of events.

of mRNA coding for various glutamate receptor subunits in the developing rat superior colliculus. It is obvious that the mRNAs coding for the NMDA receptor subunits NR1 and NR2B, the AMPA receptor subunit GluR2 and the metabotropic receptor mGRl are regulated differentially, suggesting different roles in the development of the retinocollicular projection. The levels of mRNA coding for the NMDA receptor subunit NR1 are low during the first postnatal week (Fig. la) with a marked rise of NR1 mRNA occurring between postnatal day (P) P6 and P12. This increase in NR1 gene expression coincides with the completion of the refinement of the retino- and corticocollicular projections. It also seems to parallel an increase in synaptic density within the superficial layers of the SC (Warton and McCart, 1989) and the development of electrophysiological activity in the SC (Molotchnikoff and Itaya, 1993). If NMDA receptors are involved in the process of retinocollicular map development, we may expect their density to be elevated during this period. However, we find that NR1 mRNA expression reaches high levels only during the final stages of synaptic rearrangements of afferent projections to the SC. Assuming that mRNA levels of the NR1 subunit reflect the density of the NMDA receptor complex, our results may indicate that NMDA receptors are less important for the early stages of map develop ment, which involve widespread collateralization and the onset of synaptogenesis, than for the later stages when mistargeted projections are completely withdrawn. The expression pattern of the NMDA receptor subunit NR2B (Monyer et al., 1992) differs from the NR1 mRNA (Fig. lb). Fairly high levels are present at early postnatal ages followed by a decrease to adult levels. It is conceivable that the high NR2B mRNA levels during the period of retinocollicular map refinement provide a condition permissive for synapses to rearrange, and that the drop in NR2B mRNA levels restricts this ability. In contrast to NR1 and NR2B mRNA, the mRNA expression pattern of the non-NMDA receptor subunit GluR2 in the developing SC was

282

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Fig. 2. Expression patterns of mRNA coding for the NMDA receptor subunits NR1 (a) and NR2B (b), the AMPA receptor subunit GluR2 (c) and the metabotropic receptor mGRl (d). RNA from the superJicial superior colliculus was analyzed by Northern blots and quantitative measurements of mRNA levels were obtained by densitometric scanning of autoradiographic signals.

not closely correlated with synaptic changes (Fig. lc). We chose to examine the expression of GluR2 mRNA because GluR2, when coexpressed with the AMPA receptor subunits GluRl or Glum in Xenopus oocytes, decreases the Ca" permeability of the channel formed by these subunits (Hollman et al., 1991). Thus, low levels of GluR2 subunits could endow non-NMDA ionotropic receptors with a high Ca2+-permeabilitysimilar to that of NMDA receptors, but without the voltage-gating property that is presumably essential for detection of correlated activity (Constantine-Paton et al., 1990, Bourne and Nicoll, 1993). The observation that the time course of GluR2

mRNA expression appears to be unrelated to the anatomically defined periods of synaptic rearrangement and topographic map refinement in the SC suggests that this subunit is involved more generally in synaptic transmission than is the more tightly regulated NMDA receptor subunit NR1. A similar result was found with the regulation of the mRNA coding for a metabotropic glutamate receptor, mGRl (Masu et al., 1991). There was no clear correlation between the mRNA expression pattern of mGRl in developing SC and synaptogenic events (Fig. Id). After it was shown that chronic AFV-blockade of NMDA receptors in the SC disrupts the devel-

283

opment of the retinotopic map in SC (Simon et al., 1992) we examined whether NR1 mRNA levels are affected by this treatment as well. Our results showed that NR1 mRNA levels are dramatically reduced at P12 and P19 after chronic APV application. In fact, NR1 mRNA levels in treated animals were similar to the levels measured in normal PO or P6 animals. Thus, chronic NMDA receptor blockade appears to prevent the developmental increase of NR1 mRNA levels. If the NR1 subunit is a common element of the native NMDA receptor, as suggested by its widesread distribution (Kutsuwada et al., 1992; Monyer et al., 1992), and if its transcript levels reflect the density of the whole NMDA receptor complex, our findings indicate that chronic exposure to APV causes a reduction in the number of NMDA receptors. During early postnatal development of the SC, neuronal activity, mediated through NMDA receptors, may therefore be necessary to trigger the developmental rise of NR1 mRNA, while disruption of activity may prevent the normal developmental increase of NR1 mRNA expression. Our result indicates that the suppression of the normal increase of the subunit NR1 may, in part, account for the disruption of the refinement of the retinocollicular projection. Thus, the current data support the hypothesis that the NMDA receptor is specifically involved in structural modifications of the developing nervous system, and that activity-dependent regulation of NMDA receptor subunits at the mRNA level may be an important factor in this process. Summary

There is evidence from a number of studies that the molecular and biophysical properties of NMDA receptors are altered during normal development. A temporal correlation with changes in NMDA receptor efficacy and periods of synaptic plasticity has been demonstrated in several systems, suggesting that NMDA receptors have a critical function in determining periods of synaptic plasticity. Data from our laboratory demon-

strate reduced NMDA sensitivity of the tectal evoked potential following chronic application of NMDA to the tadpole tectum, a treatment that may mimic a naturally occurring mechanism for limiting neuronal plasticity to certain stages of development. Our analysis of the expression pattern of mRNA coding for various glutamate receptor subunits in the rat retinocollicular system establishes that differential regulation of NMDA receptor subunits at the mRNA level could be a molecular basis for changes in biophysical and pharmacological properties of the NMDA receptor complex. However, even though the NMDA receptor is the best studied candidate to function as a ‘plasticity switch’, there are large gaps in our understanding of the complete set of factors that control the ability of synapses to rearrange during development. Acknowledgments

The authors would like to thank Dr. J. Boulter for the GluR2 clone, Dr. S. Nakanishi for the NR1 and mGRl cDNA and Dr. P. Seeburg for the NR2B cDNA. This work was supported by EMBO longterm fellowship to M.H., and by NEI grant EY06039 to M.C.-P. References Ben-Ari, Y. and Cherubini, E. (1988) Changes in voltage dependence of NMDA currents during development. Neurosci Lett., 94: 88-92. Bode-Greuel, K. and Singer, W. (1989) The development of N-methy1-D-aspartate receptors in cat visual cortex. Dev. Brain Res., 46: 197-204. Bourne, H.R. and Nicoll, R. (1993) Molecular machines integrate coincident synaptic signals. Cell /Neuron, 72/10 (Suppl.> 65-75. Bowe, M.A. and Nadler, J.V. (1990) Developmental increase in the sensitivity to magnesium of NMDA receptors on CA1 hippocampal pyramidal cells. Dev. Bruin Res., 56: 55-61. Carmignoto, G. and Vicini, S. (1992) Activity-dependent decrease in NMDA receptor responses during development of the visual cortex. Science, 258: 1007-1011. Cline, H.T. and Constantine-Paton, M. (1989) NMDA receptor antagonists disrupt the retinotectal topographic map. Neuron, 3: 413-426.

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