Induction Of Synaptic Plasticity Research Articles

Type 1 Inositol Trisphosphate Receptor Regulates Cerebellar Circuits by Maintaining the Spine Morphology of Purkinje Cells in Adult Mice

The structural maintenance of neural circuits is critical for higher brain functions in adulthood. Although several molecules have been identified as regulators for spine maintenance in hippocampal and cortical neurons, it is poorly understood how Purkinje cell (PC) spines are maintained in the mature cerebellum. Here we show that the calcium channel type 1 inositol trisphosphate receptor (IP3R1) in PCs plays a crucial role in controlling the maintenance of parallel fiber (PF)-PC synaptic circuits in the mature cerebellum in vivo. Significantly, adult mice lacking IP3R1 specifically in PCs (L7-Cre;Itpr1(flox/flox)) showed dramatic increase in spine density and spine length of PCs, despite having normal spines during development. In addition, the abnormally rearranged PF-PC synaptic circuits in mature cerebellum caused unexpectedly severe ataxia in adult L7-Cre;Itpr1(flox/flox) mice. Our findings reveal a specific role for IP3R1 in PCs not only as an intracellular mediator of cerebellar synaptic plasticity induction, but also as a critical regulator of PF-PC synaptic circuit maintenance in the mature cerebellum in vivo; this mechanism may underlie motor coordination and learning in adults.

Bidirectional NMDA receptor plasticity controls CA3 output and heterosynaptic metaplasticity

N–methyl–d–aspartate glutamate receptors (NMDARs) are classically known as coincidence detectors for the induction of long–term synaptic plasticity, and have been implicated in hippocampal CA3–dependent spatial memory functions that likely rely on dynamic cellular ensemble encoding of space. The unique functional properties of both NMDARs and mossy fiber (MF) projections to CA3 pyramidal cells place MF–NMDARs in a prime position to influence CA3 ensemble dynamics. By mimicking pre and postsynaptic activity patterns observed in–vivo, we report a burst timing–dependent paradigm for bidirectional long–term NMDAR plasticity at MF–CA3 synapses in rat hippocampal slices. This form of plasticity imparts bimodal control of MF–driven CA3 burst–firing and spike temporal fidelity. Moreover, we show that MF–NMDARs mediate heterosynaptic metaplasticity between MF and associational/commissural synapses. Thus, bidirectional NMDAR plasticity at MF–CA3 synapses could significantly contribute to the formation, storage, and recall of CA3 cell assembly patterns.

Reactivation of the Same Synapses during Spontaneous Up States and Sensory Stimuli

In the mammalian brain, calcium signals in dendritic spines are involved in many neuronal functions, particularly in the induction of synaptic plasticity. Recent studies have identified sensory stimulation-evoked spine calcium signals in cortical neurons invivo. However, spine signaling during ongoing cortical activity in the absence of sensory input, which is essential for important functions like memory consolidation, is not well understood. Here, by using invivo two-photon imaging of auditory cortical neurons, we demonstrate that subthreshold, NMDA-receptor-dependent spine calcium signals are abundant during up states, but almost absent during down states. In each neuron, about 500 nonclustered spines, which are widely dispersed throughout the dendritic field, are on average active during an up state. The same subset of spines is reliably active during both sensory stimulation and up states. Thus, spontaneously recurring up states evoke in these spines "patterned" calcium activity that may control consolidation of synaptic strength following epochs of sensory stimulation.

Extracellular Signal–Regulated Kinase in the Ventromedial Hypothalamus Mediates Leptin-Induced Glucose Uptake in Red-Type Skeletal Muscle

Leptin is a key regulator of glucose metabolism in mammals, but the mechanisms of its action have remained elusive. We now show that signaling by extracellular signal–regulated kinase (ERK) and its upstream kinase MEK in the ventromedial hypothalamus (VMH) mediates the leptin-induced increase in glucose utilization as well as its insulin sensitivity in the whole body and in red-type skeletal muscle of mice through activation of the melanocortin receptor (MCR) in the VMH. In contrast, activation of signal transducer and activator of transcription 3 (STAT3), but not the MEK-ERK pathway, in the VMH by leptin enhances the insulin-induced suppression of endogenous glucose production in an MCR-independent manner, with this effect of leptin occurring only in the presence of an increased plasma concentration of insulin. Given that leptin requires 6 h to increase muscle glucose uptake, the transient activation of the MEK-ERK pathway in the VMH by leptin may play a role in the induction of synaptic plasticity in the VMH, resulting in the enhancement of MCR signaling in the nucleus and leading to an increase in insulin sensitivity in red-type muscle.

Impaired motor cortex plasticity in patients with Noonan syndrome

ObjectiveNoonan syndrome (NS; OMIM 163950) is a developmental disorder caused by activating mutations in various components of the RAS-MAPK pathway. Recent in vitro studies demonstrated impairment of synaptic plasticity caused by RAS-MAPK pathway hyperactivity. Induction of synaptic plasticity critically depends on the level of attention. We therefore studied the induction of synaptic plasticity in patients with NS and healthy volunteers under different conditions of attention using transcranial magnetic stimulation. MethodsWe investigated 10 patients with NS and healthy controls (HC) using paired associative stimulation (PAS) with different attention levels (unspecific, visual and electrical attention control). Changes in motor evoked potential (MEP) amplitudes were assessed immediately after as well as 30 and 60min after PAS. ResultsWe demonstrated that MEP amplitudes of healthy controls significantly increased from 1.00±0.17 to 1.74±0.50mV (p=0.001), which was not seen in patients with Noonan-Syndrome (0.88±0.09 to 1.10±0.48mV, p=0.148) and there was a significant difference between both groups (p=0.003) when using an unspecific attention control. Under specific electrical attention control, MEP amplitudes decreased significantly in patients with NS, whereas a visual attention focus diminished synaptic plasticity in healthy controls. ConclusionOur study provides evidence that synaptic plasticity is impaired in patients with NS, which is probably a consequence of constitutive activity of the RAS-MAPK pathway. The induction of synaptic plasticity in these patients critically depends on attention. SignificanceThis is the first study that indicates reduced synaptic plasticity in patients with a RAS-pathway disorder. Our results may have direct implications for learning and memory strategies in patients with a RAS-pathway disorder.

Neurophysiological and Neurochemical Mechanisms of Behavioral Disorders in Rats with Chronic Inflammation of Back Tissues

We observed modifications of behavior in rats with experimentally induced chronic aseptic inflammation of back tissues; these were increases in the levels of depressiveness and anxiety. Such shifts were manifested in increases in the time of immobilization in the forced swimming test and decreases in the time of stay of animals in open branches of the elevated X-like labyrinth. We also observed disorders in training for and realization of the conditioned active avoidance reaction (increases in the latency of this reaction and in the number of stimulation combinations necessary for providing its high stable level). Analysis of the effects of inflammation-derived products on neurons of layer V of the medial prefrontal cortex showed that chronic peripheral inflammation induces suppression of basal glutamatergic synaptic transmission combined with enhancement of the amplitudes of NMDA components of field EPSPs (fEPSPs) and decrease in the sensitivity of the latter to the influences of nonselective (ketamine) and selective with respect to NR2A subunits (zinc sulfate) blockers of NMDA receptors; the sensitivity to blockers of NR2B subunits (haloperidol) did not change. Induction of long-term potentiation (LTP) and long-term depression (LTD) of synaptic transmission were suppressed. In experiments on hippocampal slices, we observed weakening of the suppressive effect of serotonin (and not of those of buspirone and noradrenaline) on the amplitude of antidromic field APs of pyramidal neurons of the CA1 area. These facts demonstrate that the processes of serotonin (but not of noradrenaline) reuptake in varicosities of axons of monoaminergic neurons are intensified in inflammation. Considering the data obtained, we hypothesize that the increased level of depressiveness under conditions of chronic inflammation is determined, to a considerable extent, by enhanced functional activity of NMDA receptors and by weakening of serotonergic influences on cortical neurons. The increased anxiety level can be related to the above-mentioned weakening of serotonergic effects and/or to intensification of functional activity of high-threshold L-type calcium channels. Inflammation-related disorders of mnestic processes can result from a metaplastic shift of the threshold of induction of synaptic plasticity determined by the increase in functional activity of NMDA receptors and the above-mentioned calcium channels.

Light-dark Adaptation of Channelrhodopsin C128T Mutant

Channelrhodopsins are microbial type rhodopsins that operate as light-gated ion channels. Largely prolonged lifetimes of the conducting state of channelrhodopsin-2 may be achieved by mutations of crucial single amino acids, i.e. cysteine 128. Such mutants are of great scientific interest in the field of neurophysiology because they allow neurons to be switched on and off on demand (step function rhodopsins). Due to their slow photocycle, structural alterations of these proteins can be studied by vibrational spectroscopy in more detail than possible with wild type. Here, we present spectroscopic evidence that the photocycle of the C128T mutant involves three different dark-adapted states that are populated according to the wavelength and duration of the preceding illumination. Our results suggest an important role of multiphoton reactions and the previously described side reaction for dark state regeneration. Structural changes that cause formation and depletion of the assumed ion conducting state P520 are only small and follow larger changes that occur early and late in the photocycle, respectively. They require only minor structural rearrangements of amino acids near the retinal binding pocket and are triggered by all-trans/13-cis retinal isomerization, although additional isomerizations are also involved in the photocycle. We will discuss an extended photocycle model of this mutant on the basis of spectroscopic and electrophysiological data.

Reduced d-serine levels in the nucleus accumbens of cocaine-treated rats hinder the induction of NMDA receptor-dependent synaptic plasticity

Cocaine seeking behaviour and relapse have been linked to impaired potentiation and depression at excitatory synapses in the nucleus accumbens, but the mechanism underlying this process is poorly understood. We show that, in the rat nucleus accumbens core, D-serine is the endogenous coagonist of N-methyl-D-aspartate receptors, and its presence is essential for N-methyl-D-aspartate receptor-dependent potentiation and depression of synaptic transmission. Nucleus accumbens core slices obtained from cocaine-treated rats after 1 day of abstinence presented significantly reduced D-serine concentrations, increased expression of the D-serine degrading enzyme, D-amino acid oxidase, and downregulated expression of serine racemase, the enzyme responsible for D-serine synthesis. The D-serine deficit was associated with impairment of potentiation and depression of glutamatergic synaptic transmission, which was restored by slice perfusion with exogenous D-serine. Furthermore, in vivo administration of D-serine directly into the nucleus accumbens core blocked behavioural sensitization to cocaine. These results provide evidence for a critical role of D-serine signalling in synaptic plasticity relevant to cocaine addiction.

Wavelet Transform-Based De-Noising for Two-Photon Imaging of Synaptic Ca2+ Transients

Postsynaptic Ca2+ transients triggered by neurotransmission at excitatory synapses are a key signaling step for the induction of synaptic plasticity and are typically recorded in tissue slices using two-photon fluorescence imaging with Ca2+-sensitive dyes. The signals generated are small with very low peak signal/noise ratios (pSNRs) that make detailed analysis problematic. Here, we implement a wavelet-based de-noising algorithm (PURE-LET) to enhance signal/noise ratio for Ca2+ fluorescence transients evoked by single synaptic events under physiological conditions. Using simulated Ca2+ transients with defined noise levels, we analyzed the ability of the PURE-LET algorithm to retrieve the underlying signal. Fitting single Ca2+ transients with an exponential rise and decay model revealed a distortion of τrise but improved accuracy and reliability of τdecay and peak amplitude after PURE-LET de-noising compared to raw signals. The PURE-LET de-noising algorithm also provided a ∼30-dB gain in pSNR compared to ∼16-dB pSNR gain after an optimized binomial filter. The higher pSNR provided by PURE-LET de-noising increased discrimination accuracy between successes and failures of synaptic transmission as measured by the occurrence of synaptic Ca2+ transients by ∼20% relative to an optimized binomial filter. Furthermore, in comparison to binomial filter, no optimization of PURE-LET de-noising was required for reducing arbitrary bias. In conclusion, the de-noising of fluorescent Ca2+ transients using PURE-LET enhances detection and characterization of Ca2+ responses at central excitatory synapses.

A common framework of signal processing in the induction of cerebellar LTD and cortical STDP

Cerebellar long-term depression (LTD) and cortical spike-timing-dependent synaptic plasticity (STDP) are two well-known and well-characterized types of synaptic plasticity. Induction of both types of synaptic plasticity depends on the spike timing, pairing frequency, and pairing numbers of two different sources of spiking. This implies that the induction of synaptic plasticity may share common frameworks in terms of signal processing regardless of the different signaling pathways involved in the two types of synaptic plasticity. Here we propose that both types share common frameworks of signal processing for spike-timing, pairing-frequency, and pairing-numbers detection. We developed system models of both types of synaptic plasticity and analyzed signal processing in the induction of synaptic plasticity. We found that both systems have upstream subsystems for spike-timing detection and downstream subsystems for pairing-frequency and pairing-numbers detection. The upstream systems used multiplication of signals from the feedback filters and nonlinear functions for spike-timing detection. The downstream subsystems used temporal filters with longer time constants for pairing-frequency detection and nonlinear switch-like functions for pairing-numbers detection, indicating that the downstream subsystems serve as a leaky integrate-and-fire system. Thus, our findings suggest that a common conceptual framework for the induction of synaptic plasticity exists despite the differences in molecular species and pathways.

NMDA Receptor Subunits in the Adult Rat Hippocampus Undergo Similar Changes after 5 Minutes in an Open Field and after LTP Induction

NMDA receptor subunits change during development and their synaptic expression is modified rapidly after synaptic plasticity induction in hippocampal slices. However, there is scarce information on subunits expression after synaptic plasticity induction or memory acquisition, particularly in adults. GluN1, GluN2A and GluN2B NMDA receptor subunits were assessed by western blot in 1) adult rats that had explored an open field (OF) for 5 minutes, a time sufficient to induce habituation, 2) mature rat hippocampal neuron cultures depolarized by KCl and 3) hippocampal slices from adult rats where long term potentiation (LTP) was induced by theta-burst stimulation (TBS). GluN1 and GluN2A, though not GluN2B, were significantly higher 70 minutes –but not 30 minutes- after a 5 minutes session in an OF. GluN1 and GluN2A total immunofluorescence and puncta in neurites increased in cultures, as evaluated 70 minutes after KCl stimulation. Similar changes were found in hippocampal slices 70 minutes after LTP induction. To start to explore underlying mechanisms, hippocampal slices were treated either with cycloheximide (a translation inhibitor) or actinomycin D (a transcription inhibitor) during electrophysiological assays. It was corroborated that translation was necessary for LTP induction and expression. The rise in GluN1 depends on transcription and translation, while the increase in GluN2A appears to mainly depend on translation, though a contribution of some remaining transcriptional activity during actinomycin D treatment could not be rouled out. LTP effective induction was required for the subunits to increase. Although in the three models same subunits suffered modifications in the same direction, within an apparently similar temporal course, further investigation is required to reveal if they are related processes and to find out whether they are causally related with synaptic plasticity, learning and memory.

From channelrhodopsins to optogenetics

We did not expect that research on the molecular mechanism of algal phototaxis or archaeal light‐driven ion transport might interest readers of a medical journal when we conceived and performed our experiments a decade ago. On the other hand, it did not escape our attention that channelrhodopsin is helping an ever‐increasing number of researchers to address their specific questions. For example, the channelrhodopsin approach is used to study the molecular events during the induction of synaptic plasticity or to map long‐range connections from one side of the brain to the other, and to map the spatial location of inputs on the dendritic tree of individual neurons. The current applications have been summarized in a number of recent reviews (Fenno et al, 2011; Yizhar et al, 2011; Zhang et al, 2011). Here, we give personal insight into the history of the discovery of channelrhodopsin and a biophysical perspective on this remarkable class of proteins that has been the main topic of our research since the 1990s. The discovery of channelrhodopsin is based on two quite different research fields, studies on living algae and experiments on reconstituted microbial rhodopsins. A number of researchers have characterized the swimming behaviour and light responses of motile microalgae over at least 140 years (Fig 1A). Early studies on green microalgae root back to L.G. Treviranus (Treviranus, 1817) and behavioural responses were described by A. Famintzin from St. Petersburg University in 1878 (Famintzin, 1878). During helical swimming of the green alga Chlamydomonas , its orange eye signals to the flagella to alter the flagellar beating plane (Mast, 1916). Researchers at Stanford University implicated Mg2+ and Ca2+ in the behavioural responses and identified the role of Ca2+ influx in flagellar beat frequency changes (Halldal, 1957, Schmidt & Eckert, 1976). Then Oleg Sineshchekov …

Actin Dynamics within Single Dendritic Spine Investigated by Two Photon Fluorescence Correlation Spectroscopy during Synaptic Plasticity

Dendritic spines are the major site of excitatory synaptic input to pyramidal neurons of the hippocampus. The activity-dependent changes in the structure of spine structure have been clearly related to learning and memory processes. Actin is known to regulate cell shape and motility and in mature neurons is highly concentrated within dendritic spines (1). Indeed, the integrity as well as the changes in the spine structure have been shown to depend on the actin cytoskeleton dynamics. However, how actin filament dynamics are regulated during activity-dependent structural changes at synapses is up to now largely unexplored. Our study, by combining fluorescence microscopy (FM) with two-photon fluorescence correlation spectroscopy (2P-FCS)(2), allows us to simultaneously monitor the morphological changes and the dynamic behavior of actin filaments within single dendritic spine of neurons expressing actin-eGFP before and after chemical induction of synaptic plasticity (cLTP). Analyzing the autocorrelation curves from the 2P-FCS measurements and fitting them with two components diffusion model provide us quantitatively results, which show that: (1) actin dynamics within spines are significantly altered upon C-LTP induced morphological changes, (2) the highly dynamic actin filament exhibit a. heterogeneous structural composition, and (3) the regulations of actin filaments in single dendritic spine are precisely controlled individually instead of being a globally homogeneous function.1. Cingolani, L.A. & Goda, Y. Actin in action: the interplay between the actin cytoskeleton and synaptic efficacy. Nat Rev Neurosci 9, 344-56 (2008).2. Bacia, K., Kim, S.A. & Schwille, P. Fluorescence cross-correlation spectroscopy in living cells. Nat Methods 3, 83-9 (2006).

The importance of stochastic signaling processes in the induction of long-term synaptic plasticity

A stochastic model of the signaling network responsible for the induction of long-term depression (LTD) at the parallel fiber to Purkinje cell synapse is described. The model includes a PKC-ERK-cPLA2 positive feedback loop and mechanisms of AMPA receptor trafficking. It was tuned to replicate calcium uncaging experiments that cause LTD. The ultrasensitive activation of ERK makes the signaling network activity bistable, causing either LTD or not. Therefore, in single synapses only two discrete stable states (LTD and non-LTD) can be expressed. The stochastic properties of the signaling network causes threshold dithering and probabilistic expression of LTD, which allows at the macroscopic level for many different and stable mean magnitudes of depression. When the volume of a single spine is simulated no thresholds for the calcium input signal are present. Such thresholds appear only when the volume of simulation is increased by a factor 100 or more and the model output is then bistable. Similarly, deterministic solutions of the same model show only bistable behavior. LTD induction requires activation of the PKC-ERK-cPLA2 positive feedback loop but this activity is not constant: the activities of ERK and of cPLA2 fluctuate strongly. This is much less the case for PKC which is more constantly activated and thereby promotes a stable output of the pathway.

Cocaine Evokes Projection-Specific Synaptic Plasticity of Lateral Habenula Neurons

Addictive drugs share the ability to increase dopamine (DA) levels and trigger synaptic adaptations in the mesocorticolimbic system, two cellular processes engaged in the early stages of drug seeking. Neurons located in the lateral habenula (LHb) modulate the activity of DA neurons and DA release, and adaptively tune goal-directed behaviors. Whether synaptic modifications in LHb neurons occur upon drug exposure remains, however, unknown. Here, we assessed the influence of cocaine experience on excitatory transmission onto subsets of LHb neurons using a combination of retrograde tracing and ex vivo patch-clamp recordings in mice. Recent evidence demonstrates that AMPA receptors lacking the GluA2 subunit mediate glutamatergic transmission in LHb neurons. We find that cocaine selectively potentiates AMPA receptor-mediated EPSCs in LHb neurons that send axons to the rostromedial tegmental nucleus, a GABAergic structure that modulates the activity of midbrain DA neurons. Cocaine induces a postsynaptic accumulation of AMPA receptors without modifying their subunit composition or single-channel conductance. As a consequence, a protocol pairing presynaptic glutamate release with somatic hyperpolarization, to increase the efficiency of GluA2-lacking AMPA receptors, elicited a long-term potentiation in neurons only from cocaine-treated mice. This suggests that cocaine resets the rules for the induction of synaptic long-term plasticity in the LHb. Our study unravels an early, projection-specific, cocaine-evoked synaptic potentiation in the LHb that may represent a permissive step for the functional reorganization of the mesolimbic system after drug exposure.

Light-Induced Plasticity of Synaptic AMPA Receptor Composition in Retinal Ganglion Cells

Light-evoked responses of all three major classes of retinal ganglion cells (RGCs) are mediated by NMDA receptors (NMDARs) and AMPA receptors (AMPARs). Although synaptic activity at RGC synapses is highly dynamic, synaptic plasticity has not been observed in adult RGCs. Here, using patch-clamp recordings in dark-adapted mouse retina, we report a retina-specific form of AMPAR plasticity. Both chemical and light activation of NMDARs caused the selective endocytosis of GluA2-containing, Ca(2+)-impermeable AMPARs on RGCs and replacement with GluA2-lacking, Ca(2+)-permeable AMPARs. The plasticity was expressed in ON but not OFF RGCs and was restricted solely to the ON responses in ON-OFF RGCs. Finally, the plasticity resulted in a shift in the light responsiveness of ON RGCs. Thus, physiologically relevant light stimuli can induce a change in synaptic receptor composition of ON RGCs, providing a mechanism by which the sensitivity of RGC responses may be modified under scotopic conditions.

Experience-Dependent Switch in Sign and Mechanisms for Plasticity in Layer 4 of Primary Visual Cortex

Neural circuits are extensively refined by sensory experience during postnatal development. How the maturation of recurrent cortical synapses may contribute to events regulating the postnatal refinement of neocortical microcircuits remains controversial. Here we show that, in the main input layer of rat primary visual cortex, layer 4 (L4), recurrent excitatory synapses are endowed with multiple, developmentally regulated mechanisms for induction and expression of excitatory synaptic plasticity. Maturation of L4 synapses and visual experience lead to a sharp switch in sign and mechanisms for plasticity at recurrent excitatory synapses in L4 at the onset of the critical period for visual cortical plasticity. The state of maturation of excitatory pyramidal neurons allows neurons to engage different mechanisms for plasticity in response to the same induction paradigm. Experience is determinant for the maturation of L4 synapses, as well as for the transition between forms of plasticity and the mechanisms they may engage. These results indicate a tight correlation between the effects of sensory drive and maturation on cortical neurons and provide a new set of cellular mechanisms engaged in the postnatal refinement of cortical circuits.

5 Properties of real phototherapy laser and LED devices

Much of our understanding of dendritic and synaptic physiology comes from in vitro experimentation, where the afforded mechanical stability and convenience of applying drugs allowed patch-clamping based recording techniques to investigate ion channel distributions, their gating kinetics, and to uncover dendritic integrative and synaptic plasticity rules. However, with current efforts to study these questions in vivo, there is a great need to translate existing knowledge between in vitro and in vivo experimental conditions. In this review, we identify discrepancies between in vitro and in vivo ionic composition of extracellular media and discuss how changes in ionic composition alter dendritic excitability and plasticity induction. Here, we argue that under physiological in vivo ionic conditions, dendrites are expected to be more excitable and the threshold for synaptic plasticity induction to be lowered. Consequently, the plasticity rules described in vitro vary significantly from those implemented in vivo.

Studying Signal Transduction in Single Dendritic Spines

Many forms of synaptic plasticity are triggered by biochemical signaling that occurs in small postsynaptic compartments called dendritic spines, each of which typically houses the postsynaptic terminal associated with a single glutamatergic synapse. Recent advances in optical techniques allow investigators to monitor biochemical signaling in single dendritic spines and thus reveal the signaling mechanisms that link synaptic activity and the induction of synaptic plasticity. This is mostly in the study of Ca2+-dependent forms of synaptic plasticity for which many of the steps between Ca2+ influx and changes to the synapse are now known. This article introduces the new techniques used to investigate signaling in single dendritic spines and the neurobiological insights that they have produced.

Calcium Signaling in Induction of Synaptic Plasticity in Cortical Pyramidal Neurons

Pairing of pre‐ and postsynaptic activity in layer 2/3 pyramidal neurons in visual cortex at a fixed temporal delay results in variable plasticity outcomes (long term synaptic potentiation ‐ LTP, long term synaptic depression ‐ LTD or no significant change ‐ NC). In order to understand the role that intracellular calcium signaling may play in the different outcomes, we measured [Ca2+]i transients during pairing while pharmacologically inhibiting different calcium sources. In control experiments, pre‐postsynaptic pairing resulted in a net LTP but individual cells in the sample underwent either LTD or LTP or NC. Selective inhibition of individual intracellular calcium sources such as IP3 receptors or ryanodine receptors changed the plasticity outcome to a net LTD when RyRs were blocked with 200μM ryanodine while inhibition of IP3 receptors with 1.0 uM xestospongin resulted in a net LTP. Individual [Ca2+]i. Controls had peak transients =430±87 nM and an average decay time constant = 323±44 msec. However, ryanodine treatment reduced the calcium transient peaks to 310±56 nM and reduced the decay time constant to 239±38 msec. Inhibition of IP3 receptors with xestospongin c reduced the peak transient level to 380±62 nM and the decay time constant to 417±55). Thus, the specific and selective mobilization of different intracellular calcium signaling pathways may be one source of the variable plasticity outcomes between cells.Supported by NIH grant EY‐12782 to MJF