Real Time Multiplicative Memory Amplification Mediated by Whole-Cell Scaling of Synaptic Response in Key Neurons
[['Traditionally , increased synaptic response is believed to reflect activity dependent synaptic plasticity , in which the association between different inputs is strengthened through synaptic-specific potentiation .', 'We have previously shown that acquiring the skill to perform in a particularly difficult olfactory-discrimination task [1–4] results in a robust enhancement of excitatory and inhibitory synaptic connectivity to and within the piriform cortex that lasts for days after training [5–8] .', 'The synaptic enhancement was observed few days after the rats were last trained and thus indicates long term induced synaptic modifications .', 'In particular , we suggested that this increase in synaptic strength results from a process where in a subset of cells all AMPA and GABAA receptors double their strength through a whole-cell two-fold increase of their single channel conductance .', 'This process results in a two-fold increase in the strength of all excitatory and inhibitory synapses in the cell [9] , and thus can support the previously observed increase in synaptic strength [5–8] .', 'Using a preliminary study of a computational network model [9] we suggested that if this mechanism is induced in a group of cells that compose a memory-pattern , it can substrate as a memory specific amplification mechanism [9] .', 'A memory amplification mechanism should largely increase the spiking response of the cells that compose the memory pattern when the memory pattern is active , and have a minor effect when not active .', 'In this work we aim to computationally establish the role of this mechanism in memory amplification by studying its effect on the net synaptic current and on the spiking response of a single cell .', 'Moreover , through characterization of the amplification properties such as speed and stimulus dependencies we aim to obtain a further characterization of the functional effect of this mechanism .', 'The memory amplification model extends beyond the well-studied activity-dependent synaptic plasticity that was suggested to underlie memory formation in several key ways: ( 1 ) It amplifies the response of an already formed memory-pattern rather than forming a new association between the different inputs .', '( 2 ) The increased synaptic excitation that underlies the memory amplification is not synapse specific and is mediated by the increased strength of virtually all the excitatory synapses in the cell .', '( 3 ) The increase in synaptic strength is not mediated by an increased number of AMPA receptors , as was shown for LTP [10–12] , but rather is mediated by increased AMPA receptor channel conductance .', '( 4 ) Importantly , unlike activity-dependent synaptic plasticity , memory amplification requires a parallel and similar increase in the strength of synaptic inhibition .', 'These characteristics enable this mechanism to be independent of memory formation and thus enable the promotion of an already learned memory into a dominant memory .', 'Several studies have shown that CaMKII-mediated phosphorylation of the AMPA channel in ser831 causes a two-fold increase in AMPA channel conductance [13–15] , which is similar to the increase found in our setting .', 'A co-increase in excitation and inhibition is vital for whole-cell balanced amplification .', 'This suggests that both increased excitation and increased inhibition are induced by a shared process for which CaMKII is a potential candidate that doubles the conductance of all AMPA and GABAA receptors in the cell .', 'Indeed we recently showed that the increase in inhibitory and excitatory synaptic transmission that was induced by learning the task is CaMKII dependent and that this increase is the result of increased conductance of the GABAA and AMPA channels [7 , 8] .', 'In these studies the blocker effect on each cell was measured by comparing the averaged synaptic event amplitude before and after the CaMKII blocker was applied .', 'In the current study we further examined whether the task learning induced reduction in averaged synaptic event amplitude is mediated through the effect on memory amplification mechanism , namely through reversal of the two-fold multiplication of the strength of all GABAA and AMPA mediated events in a subset of cells .', 'In these studies , miniature inhibitory and excitatory post-synaptic currents were recorded in trained and control animals before and after application of CaMKII blockers [7 , 8] .', 'The generation of a miniature synaptic event is triggered by a spontaneous release of one vesicle in the presynaptic site which activates synaptic receptors that are densely packed in a nanodomain cluster opposite to the release site [16] .', 'Each nanodomain cluster is 70 nm in length , contains 20 synaptic receptors and has an average distance of 250 nm from other nanodomain clusters in the same PSD [16] .', 'The binding of the CaMKII molecule to the PSD in the cytoplasmatic site [17 , 18] enable it to phosphorylate the synaptic receptors .', 'The nanoscale size of the cluster and the auto-phosphorylation characteristics of CaMKII can enable a single CaMKII molecule located in close proximity to phosphorylate all synaptic receptors in this cluster [19] , and thus to affect all synaptic receptors that mediate a single miniature synaptic event .', 'The miniature synaptic events are spontaneously evoked and therefore do not reflect a pathway-dependent activity .', 'This enables us to use these recordings to analyze the whole-cell effect of CaMKII and to study if this enhancement can be explained within the scope of the memory amplification mechanism .', 'Moreover , while in the previous study [9] we indirectly inferred the multiplication mechanism based on between-groups comparison , in the current study , the learning specific effect of CaMKII blocker allows us to directly establish the induction of the memory amplification mechanism based on within-cell comparison .', 'This enables us to further examine the segregated effect of learning and the induction of a cell-based two-fold multiplication through a whole-cell transduction mechanism .', 'If CaMKII phosphorylation is indeed the molecular mechanism that underlies memory amplification , it is expected that in the selected subset of cells CaMKII blocker will induce a two-fold reduction in all AMPA and GABAA mediated synaptic responses , through a two-fold decrease in the conductance of all AMPA and GABAA receptors .', 'Thus the memory amplification model has four testable predictions for both GABAA and AMPA mediated miniature synaptic events:', '( a ) the effect of the CaMKII blocker will be prominent in a sub-group of cells .', '( b ) In this subset of cells the effect will be mediated by a decrease in the single channel conductance .', '( c ) The effect of the CaMKII blocker should correspond to a reduction of each event amplitude by a factor of two , and', '( d ) the sub-group of cells that was affected by CaMKII blockers should be the sub-group of cells that was affected by task learning .', 'In this work we first analyze the experimental data in depth , to test the predictions of the model .', 'Then , using theory and single-cell biophysical simulations , we show how such a whole-cell increase in excitatory and inhibitory synaptic inputs can function as an instantaneous and linear amplifier of the cell spiking response .', 'These findings establish the basis for the memory amplification mechanism; namely , applying a cell specific linear amplifier on a subset of cells that constitutes a memory-pattern can lead to amplification of this memory when activated .'], ['In this part , we aim to explore the functional significance of whole-cell synaptic multiplication , in which the strength of all excitatory and all inhibitory synapses in a single cell was doubled .', 'We previously showed in a preliminary study , using integrate and fire neurons , that such a whole-cell two-fold multiplication in synaptic strength functions as a memory amplification mechanism [9] .', 'Here we attempt to bridge the gap between the simplified integrate and fire neuron that was used in the network simulations and a more realistic biophysical neuron .', "To that end , using biophysical simulations , we first characterize the effect of a whole-cell , two-fold increase in synaptic strength on the neurons' firing response .", 'A detailed characterization of the effect of the whole-cell amplification mechanism on the number of spikes should enable a better understanding of its effect on memory amplification .', 'An amplifier whose main effect on the single cell firing response is at the near-threshold level should have a different effect on memory amplification than an amplifier whose main effect is at the supra-threshold level .', 'Moreover , an amplifier which instantaneously follows the changes in synaptic input is likely to exert a different effect than a history dependent amplifier .', 'We used a single cell with a simplified morphology and realistic biophysical properties ( see Materials and Methods ) , where realistic post-synaptic excitatory and inhibitory responses were implemented along the basal and apical dendritic cylinders .', 'In order to simulate synaptic bombardment as recorded in vivo we mimicked the firing of populations of inhibitory and excitatory pre-synaptic neurons using an independent random Poisson process such that the reversal potential of the net synaptic current ( see Material and Methods ) during background activity was about -37 mV [20] .'], ['In this study we verified the hypothesis in which in a subset of cells , learning the task induced a whole-cell , CaMKII dependent process , in which all the inhibitory and excitatory synapses doubled their strength through a two-fold increase in the single channel current of the AMPA and GABAA receptors .', 'All the predictions derived from this hypothesis were confirmed using two different blockers of CaMKII for both inhibition and excitation .', 'Validation of the first and second predictions indicates that only a subset of the cells underwent a task learning induced CaMKII dependent phosphorylation that significantly increased the synaptic single channel current and as a consequence the average event amplitude .', 'The clustered effect could not be explained using within-animal variation since for almost every cell in the affected cluster there was a cell from the same rat that was in the non effected cluster .', 'Moreover , the recordings were taken from identified pyramidal neurons in layer II of the piriform cortex; therefore the clustering effect could not be ascribed to different types of cells being different [7 , 8] .', 'The blocker effect was mainly apparent in cells from the trained group ( 30–40% of the cells ) , present to a lesser extent in the pseudo-trained group ( 0–10% ) and absent in the naïve group ( 0% ) .', 'Thus , learning the task clearly induced the amplification process , even though the recordings were made 4–5 days after training ended .', 'The third prediction stated that CaMKII blocker would reduce the amplitude of each event by a factor of two .', 'This prediction required a uniform synaptic event-based effect rather than an average effect .', 'We approached this issue by computationally reversing the effect of the CaMKII blocker on the histogram of event amplitudes through a uniform two-fold multiplication of the events amplitudes .', 'In each cell in the affected cluster , this manipulation yielded a full computational reversal of the blocker effect , thus confirming that the CaMKII blocker reduced the amplitude of each synaptic event by a factor of two .', 'Due to the moderate concentrations of CaMKII blockers , the blockers did not yield a full effect on the cells in the affected cluster ( see results ) .', 'The effect of tatCN21 ( inhibition 147±7%; excitation 148±3% ) was significantly smaller than the effect of KN93 ( inhibition 165±9%; excitation 173±21% ) .', 'The similar effect of each blocker on inhibition and excitation supports the notion that indeed the discrepancy between the expected effect of the twofold reduction and the experimentally observed effect is due to a partial effect of the blocker .', 'For each cell , the fraction of blocker affected events was used when calculating the reversal of the blocker effect on the amplitude distribution .', 'This reversed blocker effect cannot be attributed to the calculated fraction since when assuming different processes we did not achieve a full reversal of the blocker effect on the events amplitude histogram , even though the effect on the averaged amplitude was reversed .', 'The fourth prediction stated that the cells that were affected by CaMKII blocker are the cells that were affected by the task learning; thus in these cells , there was a process that caused all the synaptic AMPA and GABAA receptors to multiply their single channel current by a factor of two .', 'This prediction was confirmed by demonstrating that the single channel current in these cells was twice as high as in the controls , and that their amplitude distribution curve could be reconstructed by two-fold multiplication of all the events in the control group .', 'The affected cells could be discriminated from the non- affected cells solely based on their single channel current , which was indicative of their difference before the CaMKII blocker was applied .', 'In addition to demonstrating the task learning effect on the CaMKII blocker-affected cells , we argued that for inhibition , the multiplication process might account for virtually all the measured learning induced modifications of the synaptic events .', 'We showed that the two-fold multiplication of the average inhibitory event amplitude in a randomly selected 35% of the cells could fully account for the learning induced modification on the cumulative frequency curve of all the cells .', 'However we could not fully explain the learning induced modification in the cumulative frequency curve of excitatory events .', 'Task learning induced a shift in the cumulative curve that could not be accounted for by the multiplication process in a subset of cells .', 'Thus learning the task induced an additional process that caused an almost constant increase in the average excitatory events amplitude in all cells in the trained group .', 'Phosphorylation of the AMPA receptor by CaMKII is known to double the single channel conductance of AMPA receptors [13–15] .', 'These findings consolidate our basic premise that CaMKII induces a two-fold increase of the AMPA receptor conductance .', 'Although a two-fold increase in the AMPA channel conductance as a result of CaMKII dependent phosphorylation is believed to be a universal property of the AMPA receptor , currently there are no other findings that demonstrate the induction of the memory amplification mechanism in other brain regions .', 'However since the amplification mechanism is a whole-cell mechanism and thus appears to be independent of circuit property , the memory amplification mechanism is likely to be induced also on pyramidal cells in other brain regions .', 'Future demonstration of the induction of this mechanism in other regions can further increase its impact , as learning is known to be supported by parallel modifications in several regions .', 'As expected by a mechanism that is induced through whole-cell transduction mechanism , the reversal of the memory amplification mechanism using CaMKII blockers was rapid ( several minutes passed between the application of the drug in the perfusion solution and the effect of CaMKII blocker ) .', 'In this set of experiments the recordings were made at a fixed interval of 4–5 days after training termination [7 , 8] .', 'This interval allows us to suggest that the memory amplification mechanism comprise a long-term effect of learning , nevertheless it does not enable us to determine the time scale of the induction of this mechanism .', 'Resolving this issue is significant for understanding the behavioral role of the amplification mechanism in different settings .', 'Using simulations we have shown that whole-cell balanced amplification acts mainly via multiplicative amplification of the average and standard deviation of the net synaptic current; as such its effect is correlated with the size and the polarity of the net synaptic current .', 'Moreover , we showed using simulations and analytical study that in the balanced state this modification leads to a linear amplification of the number of spikes with a fairly stable multiplication factor .', 'This trait suggests that the amplification mechanism is a robust and stable mechanism for gain modulation .', 'Quite a few studies have attempted to describe the mechanism of gain modulation .', 'It has been shown that tonic inhibition creates a divisive operation during physiological excitation in which the cell is under continuous bombardment of synaptic activity [26–30] .', 'In other studies [22 , 29 , 31–33] , it was shown that depolarizing the membrane potential in the presence of synaptic noise increases the response gain .', 'In both mechanisms ( i . e . increase in tonic inhibition and depolarization of the membrane potential ) , the change has an additive effect on the input-output function , and in both , the amplitude of the baseline noise determines the shift from additive to multiplicative operations [26 , 31] .', 'Indeed , theoretical studies [31 , 32] have shown that in the presence of noise , the firing frequency becomes a power-law function of the membrane potential but only for a limited range of values , and that this is primarily visible at low firing frequencies .', 'Due to this limitation , these mechanisms cannot be considered a robust driver of gain modulation at supra-threshold conditions [34] .', 'The mechanism we suggest here is inherently multiplicative and cell selective , and hence can cause robust cell specific gain modulation .', 'We found that both the amplitude and the standard deviation of the net synaptic current were multiplied through a whole-cell balanced multiplication by approximately the same constant for all simulations .', 'The simulations spanned a large space of several parameters , which effectively produced different initial conditions for the inhibition excitation ratio , and a different activation curve of the cell .', 'We confirmed these results analytically and showed that in the balanced state whole-cell multiplication induces a linear amplifier on the number of spikes .', 'Moreover , this analysis showed that when the cell is not in the threshold regime , the multiplication factor is fairly stable .', 'Theoretical works suggested that gain modulation can be mediated by changes in intrinsic excitability [35 , 36] , and more specifically through modulation of the currents that mediate the firing frequency adaptation; however using simulations these studies indicated that the induced gain is not robust at lower firing frequencies [36] .', 'Moreover , since pyramidal cells differ in their firing adaptation properties , the resulting gain modulation varies considerably across cells .', 'Gain modulation mechanisms that act through modulation of the firing frequency adaptation are dependent on the past spiking activity of the cell .', 'In the balanced regime spiking activity is inherently irregular and therefore a consistent gain modulation effect of such mechanisms can be achieved by averaging , or by long time integration of the spiking activity .', 'Whole-cell balanced amplification modifies the synaptic input directly and as such is not dependent on past spiking activity .', 'Moreover its underlying multiplicative process enables it to respond in particular to large deviations in the membrane potential .', 'These characteristics enable it to act instantaneously to rapid changes in the synaptic input .', 'We showed that whole cell balanced multiplication could reliably amplify the number of spikes in 75% of the voltage bumps that are caused by a transient , brief elevation of excitatory synaptic activity , despite ongoing differences in synaptic noise .', 'These characteristics enable a reliable instantaneous amplification during transient periods of brief orchestrated elevations of the membrane potential that results from a brief external stimulus such as touch , sound and visual stimulation [25 , 37–39] .', 'To summarize , to our knowledge the memory amplification mechanism is currently the only candidate for a real-time amplifier that creates a high impact also at highly supra-threshold values .', 'While a mechanism that works through depolarizing the membrane potential is real time , it exerts its main impact mainly at near threshold , and mechanism that works through firing frequency adaptation is history dependent and thus does not react instantaneously to changes in the membrane potential .', 'Amplification of a subset of cells that comprise a memory-pattern would primarily amplify the response of these cells when memory is activated .', 'Due to the multiplicative nature , the net effect of the amplification during baseline activity is much smaller than when the cell is tuned to the input .', 'This tendency is increased by the larger multiplication factor of the net synaptic current at membrane potentials that are considerably more depolarized than the GABAA reversal potential .', 'This enables the amplification to have a large net effect mainly when the memory-pattern is activated and the cells receive large excitatory input .', 'In this fashion , the amplification mechanism does not modify the information carried by the memory pattern , but only amplifies it .', 'We showed previously [9] using a simple computational model that this mechanism induces selective memory amplification .', 'The current work explains the underlying cellular basis of this property .', 'A parallel increase in inhibition and excitation was previously suggested to function as a network-wide gain increase mechanism [40 , 41] .', 'This effect is mediated by the non-specific effect of neuro-modulators which increases the membrane potential of both the excitatory and inhibitory neurons [41] .', 'While such a mechanism can affect coding by generally impacting synaptic circuits between different regions [40] , the mechanism presented here is cell-specific and thus can affect coding by amplifying an input specific response .', 'A pattern of cells that underlies a critical behavioral response should reliably respond to the input , even during brief periods .', 'Increasing the reliability of eliciting a spike in each of these transient periods can create a consistent response in such states .', 'We showed that the effect of whole-cell multiplication on the probability of eliciting a spike possesses the following properties', '( a ) at threshold values the probability of eliciting a spike was increased nearly to one and', '( b ) the increase in probability was prominent only if the ability of the event to elicit a spike was not zero .', 'These properties enable whole-cell multiplication to induce reliable coding without modifying the information carried by the synaptic input .', 'The whole-cell increase in synaptic strength observed after learning the task is not the result of homeostatic synaptic scaling [42 , 43]; Learning-induced increase in synaptic strength occurs when the spike firing rate is enhanced [44–46] rather than decreased [42 , 43] .', 'Thus , while homeostatic synaptic scaling is a negative feedback control mechanism , learning-induced whole-cell synaptic strengthening appears to be a positive feedback mechanism .', 'Moreover , while the memory amplification mechanism appears to induce a uniform scaling of all synapses , homeostatic synaptic scaling appears to cause a general increase in synaptic strength which does not appear to be uniform .', 'Indeed , increase in synaptic strength that is supported by a pseudo binary increase in single channel conductance as we showed for the memory amplification mechanism allows such a uniform increase , while increase in synaptic strength which is supported by addition of synaptic receptors appears to be more general and less uniform [43] .', 'An across-the-board increase in the strength of all excitatory synapses in a sub-group of cells cannot be explained by the classical learning theories that underlie memory formation in which only synapses relevant to the storage of a memory are strengthened .', 'Moreover , while we show that doubling the channel conductance underlies the increase in synaptic strength; activity-dependent synaptic enhancement is thought to be mediated by an increase in the number of AMPA channel receptors [47 , 48] .', 'Indeed , memory amplification should not modify the composition of the subset of cells that compose the memory and thus should be uniformly applied on all synapses in this subset .', 'Moreover , since memory formation is tuned by the correlation between presynaptic and postsynaptic activity in each synapse , it should support a continuous increase in synaptic strength and thus can be implemented by addition of AMPA channels .', 'In contrast , a linear amplification of a memory can be induced by multiplying its firing response by a single factor and thus can be implemented by a uniform two-fold increase of the synaptic strength .', 'We presented a cell-specific amplification mechanism that can induce a memory selective amplification process when induced on a subset of cells that comprise a memory-pattern .', 'This memory enhancement is independent of memory formation .', 'A memory of vital importance needs to be enhanced to dominate subsequent behavior [49 , 50] .', 'When the memory becomes less crucial it should be de-enhanced to balance its weight with the weights of other memories .', 'The induction of whole cell multiplication via a whole cell transduction mechanism and its action through channel conductance enables it to be readily toggled-on when needed and off when no further needed .', 'Such properties may underlie the extinction and relapse seen in our behavioral settings .', 'Thus whole-cell CaMKII mediated phosphorylation can function as a “software switch” unlike a hardware switch that requires morphological modulation as was suggested in processes that accompanies memory formation .'], ['In this study we have used data from previously published experiments in which training- induced modification of inhibitory [8] and excitatory [7] synaptic transmission was studied .', 'For sake of clarity we present here a brief description of the animal training and the experimental recording setup .', 'More details can be found in previously published works ( behavioral settings: [1 , 2 , 51]; Slice preparation and recordings: [7 , 8] ) Animal training: Rats designated to the trained group were rewarded with water when they chose the correct positive cue .', 'Rats designated to the pseudo-trained group were rewarded with water when choosing any odor in a random manner , and rats in the naive group were left in their home cages and were water deprived only .', 'Training consisted of 20 trials per day for each rat [2] .', 'Two learning phases can be seen during training [1 , 6]: in the initial phase of "rule learning" , which requires 7–8 consecutive training days , the rat forms a strategy for executing the olfactory discrimination task , and in the second phase the rat exhibits an enhanced learning ability , and can learn novel odors within 1–2 training days .', 'Once the rats have reached the criterion for olfactory discrimination , they were allowed to rest for 4–5 days and then brain slices were prepared .', 'Recordings [7 , \u200e8]: Whole cell recordings of spontaneous events where done in the presence of 1 μM tetrodotoxin and were performed 10–15 min after membrane rupture and lasted for up to 45–50 min .', 'Miniature excitatory post-synaptic currents ( mEPSCs ) were recorded at holding potential of −80 mV causing miniature inhibitory synaptic events to be exceedingly small due to the small driving force and leaving most voltage-dependent channels closed and NMDA receptors seldom activated .', 'To record GABAA-mediated miniature IPSCs ( mIPSCs ) , the recording electrode contained 140 mM cesium chloride solution in order to reach a reversal potential of 0 mV , yielding a strong GABAA-mediated currents at holding potential of −60 mV .', 'The perfusion solution also contained DNQX ( 20 μM ) and APV ( 50 μM ) , to block glutamatergic synaptic transmission via AMPA receptors , thus allowing recording of pure IPSCs .', 'CaMKII blockers: the CaMKII inhibitor KN93 ( 10 μM ) and the CaMKII cell-penetrating peptide inhibitor , tatCN21 ( 5 μM; GL Biochem , Shanghai Ltd , China ) were used in order to inhibit the active state of CaMKII .', 'Estimate of the average single channel current and the average number of active channels were obtained using a peak-scaled non-stationary fluctuation analysis ( NSFA ) of synaptic events [13] .', 'The NSFA was applied on the events that were electrotonically nearby ( 10–90% rise-times < 1 . 5 ms ) .', 'Using Mini analysis software ( Synaptosoft Inc . ) , events ( 80–300 per each cell ) were scaled and aligned by their peak , and their decay phase was divided to 30 bins .', 'The single channel current ( i ) and the number of channels ( N ) were calculated ( using Mini Analysis Software ) by fitting the theoretical relationship for the peak scaled variance ( σ2 ) after subtraction of the background variance σb2:', 'σ2 ( t ) =i·I ( t ) –I ( t ) 2/N+σb2', 'Only cells in which the fitting of the equation yielded an R>0 . 85 were incorporated in the analysis .', 'Under the assumption of a model in which the blocker effect ( averaged events amplitude before the blocker divided by the averaged events amplitude after the blocker ) on the event amplitude is a division by a factor of a , given that a fraction x of the events was affected by the blocker then the blocker effect y should be calculated as follows:', 'y= ( 1−x ) +x*a', 'Some algebra enables as to calculate the fraction of the events that was modified by the blocker ( x ) as', 'x=y−1a−1 This study examined the effect of whole-cell balanced multiplication on the spiking activity of neurons , using computer simulations .', 'We used a conductance based cell with simplified morphology and a realistic generation and adaptation of action potentials as was observed in recordings from pyramidal cells in the piriform cortex [2] .', 'The uniform whole-cell property where the strength of all synapses is doubled makes whole-cell balanced amplification relatively insensitive to the locality of the synaptic responses and thus relatively independent of the precise morphology of the cells .', 'This enabled us to implement the modeled cell morphology as a simplification of a pyramidal cell where the apical and basal dendritic trees were reduced to long cylinders connected to the soma [52] .', 'Both the morphology and the passive properties were constructed such that the membrane decay time constant ( 12 ms ) and the size of a single excitatory synaptic current ( 1 mV ) would approximate previously measured parameters in-vitro [1 , 53] .', 'In our model cell , the input resistance ( 114 MΏ ) was in the high range of previously reported values [1] , in order to compensate for the leak introduced by the sharp electrode used in these experiments .', 'The currents generated by the excitatory and inhibitory synapses were modeled by an alpha function with realistic rise times and decay times .', 'The excitatory and inhibitory synapses had characteristics of decay and rise times as measured for AMPA and GABAA receptor mediated currents [6] .', '350 Excitatory and 350 inhibitory synapses were uniformly distributed along the apical and basal dendrites .', 'The reversal potential of the net synaptic current resulted to a value of around -37mV , which similar to the value measured experimentally during UP-states [20] , and is theoretically expected at balanced state .', 'In our simulations , we measured the reversal potential of the net synaptic current by measuring the amplitudes of the averaged net synaptic current at various holding potentials .', 'The amplitudes of the excitatory synapses were chosen based on the excitatory miniature event amplitude distribution ( Fig 5B , naive ) and scaled to result in average amplitude of 70pA .', 'The scaling factor reflects the ratio of the average miniature excitatory synaptic event ( mepsc ) amplitude and the average synaptic response to one presynaptic action potential in piriform neurons 
['Introduction', 'Results', 'Discussion', 'Materials and Methods']
['Intense spiking response of a memory-pattern is believed to play a crucial role both in normal learning and pathology , where it can create biased behavior .', 'We recently proposed a novel model for memory amplification where the simultaneous two-fold increase of all excitatory ( AMPAR-mediated ) and inhibitory ( GABAAR-mediated ) synapses in a sub-group of cells that constitutes a memory-pattern selectively amplifies this memory .', 'Here we confirm the cellular basis of this model by validating its major predictions in four sets of experiments , and demonstrate its induction via a whole-cell transduction mechanism .', 'Subsequently , using theory and simulations , we show that this whole-cell two-fold increase of all inhibitory and excitatory synapses functions as an instantaneous and multiplicative amplifier of the neurons’ spiking .', 'The amplification mechanism acts through multiplication of the net synaptic current , where it scales both the average and the standard deviation of the current .', 'In the excitation-inhibition balance regime , this scaling creates a linear multiplicative amplifier of the cell’s spiking response .', 'Moreover , the direct scaling of the synaptic input enables the amplification of the spiking response to be synchronized with rapid changes in synaptic input , and to be independent of previous spiking activity .', 'These traits enable instantaneous real-time amplification during brief elevations of excitatory synaptic input .', 'Furthermore , the multiplicative nature of the amplifier ensures that the net effect of the amplification is large mainly when the synaptic input is mostly excitatory .', 'When induced on all cells that comprise a memory-pattern , these whole-cell modifications enable a substantial instantaneous amplification of the memory-pattern when the memory is activated .', 'The amplification mechanism is induced by CaMKII dependent phosphorylation that doubles the conductance of all GABAA and AMPA receptors in a subset of neurons .', 'This whole-cell transduction mechanism enables both long-term induction of memory amplification when necessary and extinction when not further required .']
['Amplifying the strength of a neuronal assembly that underlies a behavioral choice can lead to a particularly long lasting dominant memory .', 'We report experimental and theoretical evidence for a long-term mechanism that amplifies the response of a neuronal assembly which we termed “memory amplification mechanism” .', 'The amplification mechanism is mediated by doubling the strength of all inhibitory and all excitatory synapses in the cell and is induced by whole-cell phosphorylation of all inhibitory and excitatory synaptic receptors in a subset of cells , via a process that is distinct from memory formation .', 'Computationally , the inherent scaling of both excitation and inhibition yields a robust and stable amplifier of the neuron’s response .', 'When such an amplifier is induced in a set of cells that compose a memory-pattern , it can selectively amplify the response of this memory .', 'The memory amplification mechanism is independent from associative learning .', 'Thus , while associative learning forms a memory that encodes new associations , the amplification mechanism can promote an already formed memory to a dominant memory .']
['phosphorylation', 'learning', 'medicine and health sciences', 'action potentials', 'nervous system', 'membrane potential', 'social sciences', 'electrophysiology', 'neuroscience', 'learning and memory', 'cognitive psychology', 'mathematics', 'cognition', 'memory', 'distribution curves', 'statistical distributions', 'animal cells', 'proteins', 'probability theory', 'biochemistry', 'psychology', 'cellular neuroscience', 'anatomy', 'synapses', 'cell biology', 'post-translational modification', 'physiology', 'neurons', 'biology and life sciences', 'cellular types', 'physical sciences', 'cognitive science', 'neurophysiology']