Visual experience and deprivation bidirectionally modify the composition and function of NMDA receptors in visual cortex

Extrasynaptic alpha 7-nicotinic acetylcholine receptor expression in developing neurons is regulated by inputs, targets, and activity

Alpha7-nicotinic acetylcholine receptors (nAChRs) are widely expressed in the vertebrate nervous system. alpha7-nAChR functions include postsynaptic transmission, modulating neurotransmitter release, reinforcing nicotine addiction, and a role in neurological disorders, such as schizophrenia and Alzheimer's disease. In chick parasympathetic ciliary ganglion (CG) neurons, alpha7-nAChRs are excluded from the synapse and localize perisynaptically. Despite their extrasynaptic distribution, the highly Ca2+-permeable alpha7-nAChRs have important synapse-related Ca2+-dependent signaling functions in the CG. We show here that the synaptic partners regulate alpha7-nAChR expression during synapse formation in embryonic CG neurons in situ. The absence of inputs and target tissues cause reductions in alpha7-nAChR mRNA and protein levels that primarily resemble those seen for synaptic alpha3-nAChRs. However, there is a difference in their regulation. alpha7-nAChR levels are downregulated by reduced activity, whereas alpha3-nAChR levels are not. We propose that the activity-dependent regulation of extrasynaptic alpha7-nAChR levels may be an important mechanism for postsynaptic CG neurons to detect changes in presynaptic activity levels and respond with Ca2+-dependent plasticity changes in gene expression.

A unified model of NMDA receptor-dependent bidirectional synaptic plasticity

Synapses in the brain are bidirectionally modifiable, but the routes of induction are diverse. In various experimental paradigms, N-methyl-d-aspartate receptor-dependent long-term depression and long-term potentiation have been induced selectively by varying the membrane potential of the postsynaptic neurons during presynaptic stimulation of a constant frequency, the rate of presynaptic stimulation, and the timing of pre- and postsynaptic action potentials. In this paper, we present a mathematical embodiment of bidirectional synaptic plasticity that is able to explain diverse induction protocols with a fixed set of parameters. The key assumptions and consequences of the model can be tested experimentally; further, the model provides the foundation for a unified theory of N-methyl-d-aspartate receptor-dependent synaptic plasticity.

L-type calcium channel-mediated plateau potentials in barrelette cells during structural plasticity

Development and maintenance of whisker-specific patterns along the rodent trigeminal pathway depends on an intact sensory periphery during the sensitive/critical period in development. Barrelette cells of the brain stem trigeminal nuclei are the first set of neurons to develop whisker-specific patterns. Those in the principal sensory nucleus (PrV) relay these patterns to the ventrobasal thalamus, and consequently, to the somatosensory cortex. Thus PrV barrelette cells are among the first group of central neurons susceptible to the effects of peripheral damage. Previously we showed that membrane properties of barrelette cells are distinct as early as postnatal day 1 (PND 1) and remain unchanged following peripheral denervation in newborn rat pups (Lo and Erzurumlu 2001). In the present study, we investigated the changes in synaptic transmission. In barrelette cells of normal PND 1 rats, weak stimulation of the trigeminal tract (TrV) that was subthreshold for inducing Na(+) spikes evoked an excitatory postsynaptic potential-inhibitory postsynaptic potential (EPSP-IPSP) sequence that was similar to the responses seen in older rats (Lo et al. 1999). Infraorbital nerve transection at birth did not alter excitatory and inhibitory synaptic connections of the barrelette cells. These observations suggested that local neuronal circuits are already established in PrV at birth and remain intact after deafferentation. Strong stimulation of the TrV induced a sustained depolarization (plateau potential) in denervated but not in normal barrelette neurons. The plateau potential was distinct from the EPSP-IPSP sequence by 1) a sustained (>80 ms) depolarization above -40 mV; 2) a slow decline slope (<0.1 mV/ms); 3) partially or totally inactivated Na(+) spikes on the plateau; and 4) a termination by a steep decay (>1 mV/ms) to a hyperpolarizing membrane level. The plateau potential was mediated by L-type Ca(2+) channels and triggered by a N-methyl-D-aspartate (NMDA) receptor-mediated EPSP. gamma-aminobutyric acid-A (GABA(A)) receptor-mediated IPSP dynamically regulated the latency and duration of the plateau potential. These results indicate that after neonatal peripheral damage, central trigeminal inputs cause a large and long-lasting Ca(2+) influx through L-type Ca(2+) channels in barrelette neurons. Increased Ca(2+) entry may play a key role in injury-induced structural remodeling, and/or transsynaptic cell death.

Subunit-specific NMDA receptor trafficking to synapses

To elucidate mechanisms controlling the number and subunit composition of synaptic NMDA-Rs in hippocampal slice neurons, the NR1, NR2A, and NR2B subunits were optically and electrophysiologically tagged. The NR2 subunit directs delivery of receptors to synapses with different rules controlling NR2A and NR2B. Synaptic incorporation of NR2B-containing receptors is not limited by synaptic transmission nor enhanced by increased subunit expression. NR2A-containing receptors whose expression normally increases with age replace synaptic NR2B-containing receptors. Replacement is enhanced by increased NR2A expression and requires synaptic activity. Surprisingly, spontaneously released transmitter acting on synaptic NMDA-Rs is sufficient for replacement and reduces NMDA-R responses. Thus, as with AMPA-Rs, synaptic trafficking of NMDA-Rs is tightly regulated and has subunit-specific rules with functionally important consequences.

Developmental and hormonal regulation of NR2A mRNA in forebrain regions controlling avian vocal learning

Developmental changes in the composition of NMDA receptors can alter receptor physiology as well as intracellular signal transduction cascades, potentially shifting thresholds for neural and behavioral plasticity. During song learning in zebra finches, NMDAR currents become faster, and transcripts for the modulatory NR2B subunit of this receptor decrease in lMAN, a region in which NMDAR activation is critical for vocal learning. Using in situ hybridization, we found that NR2A transcripts change reciprocally, increasing significantly in both lMAN (59%) and in another song region, Area X (38%), between posthatch day (PHD) 20 and 40, but not changing further at PHD60 or 80. In adjacent areas not associated with song learning, NR2A mRNA did not change between PHD20-80. Although early song deprivation (which extends the sensitive period for song learning) delays changes in NR2B gene expression and NMDAR physiology within the lMAN, it did not alter NR2A mRNA levels measured at PHD40, 45, or 60. Early testosterone (T) treatment, which disrupts vocal development and accelerates the maturation of both NR2B levels and NMDAR physiology in lMAN, also significantly increased NR2A transcripts measured at PHD35 in lMAN. In Area X, a similar effect of T approached significance. Together with our previous studies, these results show that in a pathway critical for vocal plasticity, the ratio of NR2A:NR2B mRNA rises abruptly early during the sensitive period for song learning. Furthermore, androgen regulation of NMDAR gene expression may alter thresholds for experience-dependent synaptic change.

Extended plasticity of visual cortex in dark-reared animals may result from prolonged expression of cpg15-like genes

cpg15 is an activity-regulated gene that encodes a membrane-bound ligand that coordinately regulates growth of apposing dendritic and axonal arbors and the maturation of their synapses. These properties make it an attractive candidate for participating in plasticity of the mammalian visual system. Here we compare cpg15 expression during normal development of the rat visual system with that seen in response to dark rearing, monocular blockade of retinal action potentials, or monocular deprivation. Our results show that the onset of cpg15 expression in the visual cortex is coincident with eye opening, and it increases until the peak of the critical period at postnatal day 28 (P28). This early expression is independent of both retinal activity and visual experience. After P28, a component of cpg15 expression in the visual cortex, lateral geniculate nucleus (LGN), and superior colliculus (SC) develops a progressively stronger dependence on retinally driven action potentials. Dark rearing does not affect cpg15 mRNA expression in the LGN and SC at any age, but it does significantly affect its expression in the visual cortex from the peak of the critical period and into adulthood. In dark-reared rats, the peak level of cpg15 expression in the visual cortex at P28 is lower than in controls. Rather than showing the normal decline with maturation, these levels are maintained in dark-reared animals. We suggest that the prolonged plasticity in the visual cortex that is seen in dark-reared animals may result from failure to downregulate genes such as cpg15 that could promote structural remodeling and synaptic maturation.

Development of excitatory synapses in cultured neurons dissociated from the cortices of rat embryos and rat pups at birth

We studied the development of excitatory synapses in cultured neurons dissociated from the cortices of rat embryos at the 18th day of gestation (E18) and rat pups at birth (P0). Between 7 and 14 days in vitro (DIV), large increases in the amplitudes and frequencies of the spontaneous excitatory postsynaptic currents (EPSCs) of both cultured E18 and P0 neurons were observed. The EPSCs of E18 neurons were mediated primarily by alpha-amino-3-hydroxy-5-methyl-4-iso-xazole-propionic acid (AMPA) receptors at 7 DIV and by both N-methyl-D-aspartate (NMDA) and AMPA receptors at 14 DIV. Consistently, immunostaining indicated significant increases in the proportion of the clusters of NR1, an NMDA receptor subunit, which were associated with the accumulation of synaptophysin, a presynaptic marker, in cultured E18 neurons between 7 and 14 DIV. The proportion of NR1 clusters residing in synaptic regions and the proportion of synapses that colocalized with NR1 clusters in 7-day-old P0 neurons were not different statistically from those found in 7-day-old E18 neurons. However, cultured P0 neurons at 7 DIV displayed clear EPSCs mediated by NMDA receptors. Our results suggest that the targeting of NMDA receptors to synaptic regions lag behind the synaptic clustering of AMPA receptors during the in vitro development of cultured rat E18 cortical neurons. The results further suggest that the cortical neurons at P0 differ from those at E19 in certain cellular properties; as a result, the currents mediated by the synaptic NMDA receptors in 7-day-old P0 neurons are larger than those mediated by the synaptic NMDA receptors in 7-day-old E18 neurons.

Nicotine exposure during a postnatal critical period alters NR2A and NR2B mRNA expression in rat auditory forebrain

Chronic nicotine exposure (CNE) can alter brain development and is thought to produce deficits in auditory function. Previously, we found that CNE during the second postnatal week, but not before or after, increases the duration of excitatory postsynaptic potentials (EPSPs) mediated by N-methyl-D-aspartate receptors (NMDARs) in rat auditory cortex. It was proposed that a potential mechanism underlying increased EPSP duration could be over-stimulation of presynaptic nicotinic acetylcholine receptors, leading to prolonged glutamate release. Since glutamatergic activity regulates levels of postsynaptic NMDAR subunits, here we examine the effects of CNE on mRNA expression for the NR2A and NR2B subunits in auditory cortex and thalamus. Two days of CNE (postnatal days 8-9), produced no effects, but 5 days (postnatal days 8-12) enhanced cortical NR2A mRNA levels and reduced thalamic NR2B mRNA levels for up to 2 weeks. These effects are consistent with the hypothesis that CNE during a postnatal critical period disrupts auditory cortex development by over-stimulating glutamatergic synapses.

Forebrain-specific calcineurin knockout selectively impairs bidirectional synaptic plasticity and working/episodic-like memory

Calcineurin is a calcium-dependent protein phosphatase that has been implicated in various aspects of synaptic plasticity. By using conditional gene-targeting techniques, we created mice in which calcineurin activity is disrupted specifically in the adult forebrain. At hippocampal Schaffer collateral-CA1 synapses, LTD was significantly diminished, and there was a significant shift in the LTD/LTP modification threshold in mutant mice. Strikingly, although performance was normal in hippocampus-dependent reference memory tasks, including contextual fear conditioning and the Morris water maze, the mutant mice were impaired in hippocampus-dependent working and episodic-like memory tasks, including the delayed matching-to-place task and the radial maze task. Our results define a critical role for calcineurin in bidirectional synaptic plasticity and suggest a novel mechanistic distinction between working/episodic-like memory and reference memory.

Barrel cortex critical period plasticity is independent of changes in NMDA receptor subunit composition

The regulation of NMDA receptor (NMDAR) subunit composition and expression during development is thought to control the process of thalamocortical afferent innervation, segregation, and plasticity. Thalamocortical synaptic plasticity in the mouse is dependent on NMDARs containing the NR2B subunit, which are the dominant form during the "critical period" window for plasticity. Near the end of the critical period there is a gradual increase in the contribution of NR2A subunits that happens in parallel to changes in NMDAR-mediated current kinetics. However, no extension of the critical period occurs in NR2A knockout mice, despite the fact that NMDA subunit composition and current kinetics remain immature past the end of the critical period. These data suggest that regulation of NMDAR subunit composition is not essential for closing the critical period plasticity window in mouse somatosensory barrel cortex.

Effect of transgenic overexpression of NR2B on NMDA receptor function and synaptic plasticity in visual cortex

The NMDA receptor (NMDAR) is a heteromer comprised of NR1 and NR2 subunits. Mice that overexpress the NR2B subunit exhibit enhanced hippocampal LTP, prolonged NMDAR currents, and improved memory ( Tang et al., 1999). In the current study, we explored visual cortex plasticity and NMDAR function in NR2B overexpressing transgenic mice. Unlike the hippocampus, in vitro synaptic plasticity of the visual cortex was unaltered by NR2B overexpression. Consistent with the plasticity findings, NMDAR excitatory postsynaptic current (EPSC) durations from layer 2/3 pyramidal cells were similar in wild-type (wt) and transgenic (tg) mice. Furthermore, temporal summation of NMDAR EPSCs to 10, 20, and 40 Hz stimulation did not differ between cells from wt and tg mice. Finally, although in situ studies clearly demonstrate overexpression of NR2B mRNA in visual cortex, we failed to observe a significant elevation in the synaptic expression of NR2B protein. We conclude that the synaptic ratio of NR2B over NR2A in the NMDA receptor complex in the visual cortex is not significantly influenced by the transgene overexpression. These data suggest that mRNA availability is not a limiting factor for the synthesis of NR2B protein in the visual cortex, and support the hypothesis that levels of NR2A, rather than NR2B, normally determine the subunit composition of NMDARs in visual cortex.

Internalization of ionotropic glutamate receptors in response to mGluR activation

Activation of group 1 metabotropic glutamate receptors (mGluRs) stimulates dendritic protein synthesis and long-term synaptic depression (LTD), but it remains unclear how these effects are related. Here we provide evidence that a consequence of mGluR activation in the hippocampus is the rapid loss of both AMPA and NMDA receptors from synapses. Like mGluR-LTD, the stable expression of this change requires protein synthesis. These data suggest that expression of mGluR-LTD is at least partly postsynaptic, and that a functional consequence of dendritic protein synthesis is the regulation of glutamate receptor trafficking.

Electrical activity and development of neural circuits

A distinct feature of the nervous system is the intricate network of synaptic connections among neurons of diverse phenotypes. Although initial connections are formed largely through molecular mechanisms that depend on intrinsic developmental programs, spontaneous and experience-driven electrical activities in the developing brain exert critical epigenetic influence on synaptic maturation and refinement of neural circuits. Selective findings discussed here illustrate some of our current understanding of the effects of electrical activity on circuit development and highlight areas that await further study.

A biophysical model of bidirectional synaptic plasticity: dependence on AMPA and NMDA receptors

In many regions of the brain, including the mammalian cortex, the magnitude and direction of activity-dependent changes in synaptic strength depend on the frequency of presynaptic stimulation (synaptic plasticity), as well as the history of activity at those synapses (metaplasticity). We present a model of a molecular mechanism of bidirectional synaptic plasticity based on the observation that long-term synaptic potentiation (LTP) and long-term synaptic depression (LTD) correlate with the phosphorylation/dephosphorylation of sites on the alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor subunit protein GluR1. The primary assumption of the model, for which there is wide experimental support, is that postsynaptic calcium concentration and consequent activation of calcium-dependent protein kinases and phosphatases are the triggers for the induction of LTP/LTD. As calcium influx through the n-methyl-d-aspartate (NMDA) receptor plays a fundamental role in the induction of LTP/LTD, changes in the properties of NMDA receptor-mediated calcium influx will dramatically affect activity-dependent synaptic plasticity (metaplasticity). We demonstrate that experimentally observed metaplasticity can be accounted for by activity-dependent regulation of NMDA receptor subunit composition and function. Our model produces a frequency-dependent LTP/LTD curve with a sliding synaptic modification threshold similar to what has been proposed theoretically by Bienenstock, Cooper, and Munro and observed experimentally.

Plasticity: downstream of glutamate

Glutamate neurotransmission is an essential component of many forms of neuronal plasticity, however, the intracellular mechanisms that mediate plasticity are only beginning to be elucidated. The emerging image of the NMDA receptor complex reminds us that the similarity between mechanisms of plasticity in various model systems is greater than their apparent differences. For example, the cAMP-dependent protein kinase A signalling pathway is crucial for plasticity in a variety of neuronal systems and across a wide variety of species.

Emergence of input specificity of ltp during development of retinotectal connections in vivo

Input specificity of activity-induced synaptic modification was examined in the developing Xenopus retinotectal connections. Early in development, long-term potentiation (LTP) induced by theta burst stimulation (TBS) at one retinal input spreads to other unstimulated converging inputs on the same tectal neuron. As the animal develops, LTP induced by the same TBS becomes input specific, a change that correlates with the increased complexity of tectal dendrites and more restricted distribution of dendritic Ca(2+) evoked by each retinal input. In contrast, LTP induced by 1 Hz correlated pre- and postsynaptic spiking is input specific throughout the same developmental period. Thus, input specificity of LTP emerges with neural development and depends on the pattern of synaptic activity.

NMDA receptor subunits: diversity, development and disease

N-methyl-D-aspartate receptors (NMDARs) are present at many excitatory glutamate synapses in the central nervous system and display unique properties that depend on their subunit composition. Biophysical, pharmacological and molecular methods have been used to determine the key features conferred by the various NMDAR subunits, and have helped to establish which NMDAR subtypes are present at particular synapses. Recent studies are beginning to address the functional significance of NMDAR diversity under normal and pathological conditions.