The mechanism by which physiological synaptic NMDA receptor activity promotes neuronal survival is not well understood. Here, we show that that an episode of synaptic activity can promote neuroprotection for a long time after that activity has ceased. This long-lasting or "late phase" of neuroprotection is dependent on nuclear calcium signaling and cAMP response element (CRE)-mediated gene expression. In contrast, neuroprotection evoked acutely by ongoing synaptic activity relies solely on the activation of the phosphatidylinositol 3-kinase/Akt pathway. This "acute phase" does not require nuclear calcium signaling and is independent of activation of the CRE-binding protein (CREB) family of transcription factors. Thus, activity-dependent neuroprotection comprises two mechanistically distinct phases that differ in their spatial requirements for calcium and in their reliance on the CREB family.

this action potential-dependent route. In contrast, NMDA doses on the other side of the toxicity threshold do not favor synaptic NMDA receptor activation because they strongly suppress firing rates below baseline. The classic bell-shaped curve depicting neuronal fate in response to NMDA dose can be viewed as the net effect of two antagonizing (synaptic vs extrasynaptic) curves: via increased firing the synaptic signaling dominates at low doses, whereas firing becomes suppressed and extrasynaptic signaling dominates as the toxicity threshold is crossed.

Since the observation that nitric oxide (NO) can act as an intercellular messenger in the brain, the past 25 years have witnessed the steady accumulation of evidence that it acts pre-synaptically at both glutamatergic and GABAergic synapses to alter release-probability in synaptic plasticity. NO does so by acting on the synaptic machinery involved in transmitter release and, in a coordinated fashion, on vesicular recycling mechanisms. In this review, we examine the body of evidence for NO acting as a retrograde factor at synapses, and the evidence from in vivo and in vitro studies that specifically establish NOS1 (neuronal nitric oxide synthase) as the important isoform of NO synthase in this process. The NOS1 isoform is found at two very different locations and at two different spatial scales both in the cortex and hippocampus. On the one hand it is located diffusely in the cytoplasm of a small population of GABAergic neurons and on the other hand the alpha isoform is located discretely at the post-synaptic density (PSD) in spines of pyramidal cells. The present evidence is that the number of NOS1 molecules that exist at the PSD are so low that a spine can only give rise to modest concentrations of NO and therefore only exert a very local action. The NO receptor guanylate cyclase is located both pre- and post-synaptically and this suggests a role for NO in the coordination of local pre- and post-synaptic function during plasticity at individual synapses. Recent evidence shows that NOS1 is also located post-synaptic to GABAergic synapses and plays a pre-synaptic role in GABAergic plasticity as well as glutamatergic plasticity. Studies on the function of NO in plasticity at the cellular level are corroborated by evidence that NO is also involved in experience-dependent plasticity in the cerebral cortex.

A total of twelve synaptic connections between pairs of pyramidal neurones in layer 2/3 of slices of rat visual cortex maintained in vitro was investigated using whole‐cell voltage recordings under visual control. The connections varied widely in strength, with the mean peak amplitudes of the resulting excitatory postsynaptic potentials (EPSPs) ranging between approximately 40 μV and 2 mV at 23°C. The smaller mean amplitudes included a substantial proportion of apparent failures of transmission. The properties of these EPSPs were examined over a range of temperatures between 13 and 36°C. All the connections became more reliable, in that they showed fewer apparent failures of transmission, and showed less trial‐to‐trial variability at the higher temperatures. These changes appeared to be due primarily to an increase in the mean number of transmitter quanta released per presynaptic action potential. At 36°C most connections were relatively reliable, with a mean failure rate of only 16%. Five connections showed virtually no failures (1% or fewer) at this temperature. We conclude that quantal transmitter release is temperature dependent at these synapses, and that experiments performed at room temperature could lead to an exaggerated impression of the unreliability of transmission at central excitatory synapses.

In this study, we investigated the mechanisms underlying synaptic plasticity at the layer IV to II/III pathway in barrel cortex of mice aged 6 -13 weeks. This pathway is one of the likely candidates for expression of experience-dependent plasticity in the barrel cortex and may serve as a model for other IV to II/III synapses in the neocortex. We found that postsynaptic autocamtide-2-inhibitory peptide is sufficient to block long-term potentiation (LTP) (IC 50 of 500 nM), implicating postsynaptic calcium/calmodulin-dependent kinase II in LTP induction. AMPA receptor subunit 1 (GluR1) knock-out mice also showed LTP in this pathway, but potentiation was predominantly presynaptic in origin as determined by paired-pulse analysis, coefficient of variation analysis, and quantal analysis, whereas wild types showed a mixed presynaptic and postsynaptic locus. Quantal analysis at this synapse was validated by measuring uniquantal events in the presence of strontium. The predominantly presynaptic LTP in the GluR1 knock-outs was blocked by postsynaptic antagonism of nitric oxide synthase (NOS), either with intracellular N--nitro-L-arginine methyl ester or N-nitro-L-arginine, providing the first evidence for a retrograde transmitter role for NO at this synapse. Antagonism of NOS in wild types significantly reduced but did not eliminate LTP (group average reduction of 50%). The residual LTP formed a variable proportion of the total LTP in each cell and was found to be postsynaptic in origin. We found no evidence for silent synapses in this pathway at this age. Finally, application of NO via a donor induced potentiation in layer II/III cells and caused an increase in frequency but not amplitude of miniature EPSPs, again implicating NO in presynaptic plasticity.

Hardingham NR, Hardingham GE, Fox KD, Jack JB. Presynaptic efficacy directs normalization of synaptic strength in layer 2/3 rat neocortex after paired activity. J Neurophysiol 97: [2965][2966][2967][2968][2969][2970][2971][2972][2973][2974][2975] 2007. First published January 31, 2007; doi:10.1152/jn.01352.2006. Paired neuronal activity is known to induce changes in synaptic strength that result in the synapse in question having different properties to unmodified synapses. Here we show that in layer 2/3 excitatory connections in young adult rat cortex paired activity acts to normalize the strength and quantal parameters of connections. Paired action potential firing produces long-term potentiation in only a third of connections, whereas a third remain with their amplitude unchanged and a third exhibit long-term depression. Furthermore, the direction of plasticity can be predicted by the initial strength of the connection: weak connections potentiate and strong connections depress. A quantal analysis reveals that changes in synaptic efficacy were predominantly presynaptic in locus and that the key determinant of the direction and magnitude of synaptic modification was the initial release probability (P r ) of the synapse, which correlated inversely with change in P r after pairing. Furthermore, distal synapses also exhibited larger potentiations including postsynaptic increases in efficacy, whereas more proximal inputs did not. This may represent a means by which distal synapses preferentially increase their efficacy to achieve equal weighting at the soma. Paired activity thus acts to normalize synaptic strength, by both pre-and postsynaptic mechanisms. I N T R O D U C T I O NPaired bursts of pre-and postsynaptic action potentials (APs) are believed to be a physiological mechanism of plasticity at many central synapses (e.g., Markram and Tsodyks 1996;Paulsen and Sejnowski 2000). Paired recordings from hippocampal cultures and cortical slices suggest that the direction of synaptic plasticity that paired activity produces is dependent on the order of the presynaptic and postsynaptic spikes (Bi and Poo 1998;Markram et al. 1997). Pairing presynaptic spikes shortly before postsynaptic spikes produces long-term potentiation (LTP), whereas pairing postsynaptic spikes before presynaptic spikes produces long-term depression (LTD), with less temporal spike constraint (Bi and Poo 1998;Feldman 2000;Markram et al. 1997). The initial strength of the synapse may also dictate whether a synapse potentiates, with weaker synapses potentiating preferentially over stronger ones (Bi and Poo 1998). The relative timing of the presynaptic and postsynaptic spikes could be reflected by both the amplitude and kinetics of calcium transients in spines, with larger, more transient calcium signals producing LTP and smaller, longer-lasting ones producing LTD (Cormier et al. 2001;Hansel et al. 1997;Ismailov et al. 2004;Koester and Sakmann 1998;Yang et al. 1999).Pairing presynaptic before postsynaptic spikes has been shown to induce both LTP and...

Experience-dependent plasticity can be induced in the barrel cortex by removing all but one whisker, leading to potentiation of the neuronal response to the spared whisker. To determine whether this form of potentiation depends on synaptic plasticity, we studied long-term potentiation (LTP) and sensory-evoked potentials in the barrel cortex of alpha-calcium/calmodulin-dependent protein kinase II (alphaCaMKII)T286A mutant mice. We studied three different forms of LTP induction: theta-burst stimulation, spike pairing, and postsynaptic depolarization paired with low-frequency presynaptic stimulation. None of these protocols produced LTP in alphaCaMKIIT286A mutant mice, although all three were successful in wild-type mice. To study synaptic plasticity in vivo, we measured sensory-evoked potentials in the barrel cortex and found that single-whisker experience selectively potentiated synaptic responses evoked by sensory stimulation of the spared whisker in wild types but not in alphaCaMKIIT286A mice. These results demonstrate that alphaCaMKII autophosphorylation is required for synaptic plasticity in the neocortex, whether induced by a variety of LTP induction paradigms or by manipulation of sensory experience, thereby strengthening the case that the two forms of plasticity are related.

At many central synapses, the presynaptic bouton and postsynaptic density are structurally correlated. However, it is unknown whether this correlation extends to the functional properties of the synapses. To investigate this, we made recordings from synaptically coupled pairs of pyramidal neurons in rat visual cortex. The mean peak amplitude of EPSPs recorded from pairs of L2/3 neurons ranged between 40 V and 2.9 mV. EPSP rise times were consistent with the majority of the synapses being located on basal dendrites; this was confirmed by full anatomical reconstructions of a subset of connected pairs. Over a third of the connections could be described using a quantal model that assumed simple binomial statistics. Release probability (P r ) and quantal size (Q), as measured at the somatic recording site, showed considerable heterogeneity between connections. However, across the population of connections, values of P r and Q for individual connections were positively correlated with one another. This correlation also held for inputs to layer 5 pyramidal neurons from both layer 2/3 and neighboring layer 5 pyramidal neurons, suggesting that during development of cortical connections presynaptic and postsynaptic strengths are dependently scaled. For 2/3 to 2/3 connections, mean EPSP amplitude was correlated with both Q and P r values but uncorrelated with N, the number of functional release sites mediating the connection. The efficacy of a cortical connection is thus set by coordinated presynaptic and postsynaptic strength.