The Return of the Spike
Authors: David J. Linden
Journal: Neuron (1999)

Perhaps the most quoted idea in the history of neuroscience is the synaptic modification postulate proposed by the Canadian psychologist Donald 20Hebb D.O. The Organization of Behavior. Wiley, New York1949Google Scholar book, The Organization of Behavior. Formulated to explain how memories could be stored as alterations in synaptic strength, it reads: When an axon of cell A is near enough to excite a cell B and repeatedly or persistently takes part in firing it, some growth process or metabolic change takes place in one or both cells such that A's efficiency, as one of the cells firing B, is increased. While this proposal generated a certain amount of interest upon its publication, it wasn't until 1973, when Bliss and Lomo first reported the phenomenon of long-term potentiation (LTP) in the hippocampus, that a tractable electrophysiological model system emerged that could be used to study use-dependent alteration of synaptic strength in the mammalian brain and that appeared to embody the Hebb postulate (46Wigström H. Gustafsson B. Huang Y.-Y. Long-term potentiation of synaptic transmission in the hippocampus obeys Hebb's rule for synaptic modification.in: Woody C.D. Alkon D.L. McGaugh J.L. Cellular Mechanisms of Conditioning and Behavioral Plasticity. Plenum Press, New York1988Google Scholar). LTP is a persistent increase in synaptic strength that is typically produced by brief high-frequency stimulation of presynaptic neurons. In its most commonly studied form, it requires activation of postsynaptic NMDA receptors and consequent Ca2+ influx. Phenomenologically, it appears to obey the Hebb postulate: neither presynaptic activation alone nor postsynaptic activation alone is sufficient for LTP induction—there is a requirement for temporal association between presynaptic and postsynaptic activity. The properties of the NMDA receptor seem to explain this requirement, as current flux through the NMDA receptor requires both ligation of glutamate (a consequence of presynaptic activity) and postsynaptic depolarization sufficient to relieve the blockade of the ion channel by external Mg2+. In addition to LTP, several forms of use-dependent synaptic weakening, or long-term depression (LTD), have also been described (reviewed by 26Linden D.J. Connor J.A. Long-term synaptic depression.Annu. Rev. Neurosci. 1995; 18: 319-357Crossref PubMed Scopus (352) Google Scholar). At some synapses, LTP and LTD have been shown to reverse each other, thus creating use-dependent bidirectional modification, a computationally useful property. Whereas the general principle of Hebb's postulate was borne out by studies of LTP, one particular aspect was not. Although there is an absolute requirement for postsynaptic activation in LTP induction, it is not necessary that this activation take the form of spiking. Brief high-frequency (100 Hz) stimulation that does not evoke postsynaptic spiking is sufficient to induce LTP (14Douglas R.M. Goddard G.V. Riives M. Inhibitory modulation of long-term potentiation evidence for a postsynaptic locus of control.Brain Res. 1982; 240: 259-272Crossref PubMed Scopus (91) Google Scholar, 25Lee K.S. Cooperativity among afferents for the induction of long-term potentiation in the CA1 region of the hippocampus.J. Neurosci. 1983; 3: 1369-1372Crossref PubMed Google Scholar), as is high-frequency stimulation together with postsynaptic application of a drug to inhibit spikes via blockade of postsynaptic voltage-gated Na+ channels (19Gustafsson B. Wigstrom H. Abraham W.C. Huang Y.-Y. Long-term potentiation in the hippocampus using depolarizing current pulses as the conditioning stimulus to single volley synaptic potentials.J. Neurosci. 1987; 7: 774-780PubMed Google Scholar, 40Thomas M.J. Watabe A.M. Moody T.D. Makhinson M. O'Dell T.J. Postsynaptic complex spike bursting enables the induction of LTP by theta frequency synaptic stimulation.J. Neurosci. 1998; 18: 7118-7126PubMed Google Scholar; but see 34Scharfman H.E. Sarvey J.M. Postsynaptic firing during repetitive stimulation is required for long-term potentiation in hippocampus.Brain Res. 1985; 331: 267-274Crossref PubMed Scopus (44) Google Scholar). The predominant sentiment in the field was that Hebb had captured the essence of LTP but had been slightly off the mark where postsynaptic spiking was concerned. Recently, an exciting set of experiments have caused renewed interest in postsynaptic spiking and synaptic plasticity and have refocused attention on the literal interpretation of Hebb's postulate. The basic observation that sparked this new era was the conclusive demonstration (with simultaneous whole-cell patch recording at axonal, somatic, and dendritic loci on a single neuron) that Na+ spikes initiated at the axon hillock can backpropagate into the dendritic arbor (reviewed by 39Stuart G. Spruston N. Sakmann B. Hausser M. Action potential initiation and backpropagation in neurons of the mammalian CNS.Trends Neurosci. 1997; 20 (b): 125-131Abstract Full Text Full Text PDF PubMed Scopus (564) Google Scholar). This propagation is active. It is mediated and modulated by the activity of dendritic voltage-gated ion channels. Most importantly, the backpropagating spike provides a signal capable of informing the synapse that the postsynaptic cell has fired. Very recently, this technique has been used to conclusively demonstrate that stimulation of excitatory synapses can result in local Na+ spikes in the dendrite, an idea that had been suggested by classic field potential (11Cragg B.G. Hamlyn L.H. Action potentials of the pyramidal neurones in the hippocampus of the rabbit.J. Physiol. (Lond.). 1955; 129: 608-627Google Scholar, 3Andersen P. Interhippocampal impulses.II. Apical dendritic activation of CA. 1960; 1: 178-208Google Scholar, 17Fujita Y. Sakata H. Electrophysiological properties of CA1 and CA2 apical dendrites of rabbit hippocampus.J. Neurophysiol. 1962; 25: 209-222PubMed Google Scholar) and intracellular (36Spencer W.A. Kandel E.R. Electrophysiology of hippocampal neurons. IV. Fast prepotentials.J. Neurophysiol. 1961; 24: 272-285Google Scholar) recordings. Recordings made from neocortical layer 5 pyramidal neurons (38Stuart G. Schiller J. Sakmann B. Action potential initiation and propagation in rat neocortical pyramidal neurons.J. Physiol. (Lond.). 1997; 505 (a): 617-632Crossref Scopus (425) Google Scholar) and hippocampal CA1 pyramidal neurons (18Golding N.L. Spruston N. Dendritic sodium spikes are variable triggers of axonal action potentials in hippocampal CA1 pyramidal neurons.Neuron. 1998; 21: 1189-1200Abstract Full Text Full Text PDF PubMed Scopus (296) Google Scholar) have shown that sufficiently strong synaptic stimulation can result in dendritic Na+ spikes that precede axosomatic spikes. These dendritically initiated Na+ spikes are attenuated by prior dendritic spiking (through Na+ channel inactivation) and are not reliably propagated to the soma. As such, they have the potential to comprise a local integrative signal. The first demonstration that a backpropagating spike could function as an associative signal for LTP induction came in back-to-back publications in 1997. 30Markram H. Lubke J. Frotscher M. Sakmann B. Regulation of synaptic efficacy by coincidence of postsynaptic APs and EPSPs.Science. 1997; 275: 213-215Crossref PubMed Scopus (2649) Google Scholar utilized cell pair recording in neocortical slices to show that when postsynaptic spikes were paired with small bursts of subthreshold excitatory postsynaptic potentials (EPSPs), LTP of subsequent single EPSPs was produced. This phenomenon was associative in that neither the EPSP bursts nor the postsynaptic spiking alone was sufficient to induce LTP. A parametric study revealed that when the onset of the EPSP either preceded or followed the onset of the spike with an interval of ≥100 ms, no alteration in synaptic strength was produced. When the interval was reduced to 10 ms, LTP was induced when the EPSP preceded the spike, and LTD was induced when this order was reversed (see Figure 1). Concurrently, this same basic finding was made using recordings of hippocampal Schaffer collateral–CA1 synapses by 28Magee J.C. Johnston D. A synaptically controlled, associative signal for Hebbian plasticity in hippocampal neurons.Science. 1997; 275: 209-213Crossref PubMed Scopus (975) Google Scholar, who also showed that LTP induced in this manner could be blocked by bath application of NMDA receptor antagonists. 28Magee J.C. Johnston D. A synaptically controlled, associative signal for Hebbian plasticity in hippocampal neurons.Science. 1997; 275: 209-213Crossref PubMed Scopus (975) Google Scholar also demonstrated that LTP induction could be blocked by local application of tetrodotoxin (TTX) only to that region of dendrite interposed between the soma and the activated synapses. Thus, somatic spikes that fail to propagate to the site of activated synapses cannot contribute to LTP induction in this system.Figure 1Induction of LTP and LTD by Pairing of EPSPs and Postsynaptic SpikesShow full caption(A) LTP and LTD were induced in pyramidal cell pairs in a neocortical slice. Following a baseline recording period, repeated paired stimuli were given in which a postsynaptic spike either preceded (closed squares) or followed (open squares) an EPSP by 10 ms, resulting in LTD or LTP, respectively. However, when a postsynaptic spike either preceded (closed circles) or followed (open circles) an EPSP by 100 ms, no significant alteration of synaptic strength was produced. Reprinted with permission from Markram et al. 1997 (copyright [1997] American Association for the Advancement of Science).(B) LTP and LTD were induced in pyramidal cell pairs in hippocampal culture. This graph illustrates the critical window for induction of LTP and LTD by repeating parings of EPSPs and postsynaptic spikes. Negative numbers on the x axis indicate intervals in which the spike preceded the EPSP. Measurements of change in synaptic strength were made 20–30 min after pairing. Each plot symbol represents a single experiment. Reprinted with permission from Bi and Poo 1998 (copyright [1998] Society for Neuroscience).(C) Two-photon Ca2+ fluorescence imaging was used to measure Ca2+ transients in single spines of the basal dendrites of neocortical layer 5 pyramidal neurons in a slice preparation. These transients were evoked by either a paired stimulus in which a postsynaptic action potential preceded an EPSP by 50 ms (AP-EPSP), a paired stimulus in which this order was reversed (EPSP-AP), or a pair of action potentials given with a 50 ms interval. The y axis represents the peak amplitude of the evoked Ca2+ transient, normalized to that of a single EPSP in the first case and a single AP in each of the second two cases. Each plot symbol represents a single experiment and the horizontal bars represent the means. Reprinted with permission from Koester and Sakmann 1998 (copyright [1998] National Academy of Sciences, USA).View Large Image Figure ViewerDownload Hi-res image Download (PPT)Figure 1Induction of LTP and LTD by Pairing of EPSPs and Postsynaptic SpikesShow full caption(A) LTP and LTD were induced in pyramidal cell pairs in a neocortical slice. Following a baseline recording period, repeated paired stimuli were given in which a postsynaptic spike either preceded (closed squares) or followed (open squares) an EPSP by 10 ms, resulting in LTD or LTP, respectively. However, when a postsynaptic spike either preceded (closed circles) or followed (open circles) an EPSP by 100 ms, no significant alteration of synaptic strength was produced. Reprinted with permission from Markram et al. 1997 (copyright [1997] American Association for the Advancement of Science).(B) LTP and LTD were induced in pyramidal cell pairs in hippocampal culture. This graph illustrates the critical window for induction of LTP and LTD by repeating parings of EPSPs and postsynaptic spikes. Negative numbers on the x axis indicate intervals in which the spike preceded the EPSP. Measurements of change in synaptic strength were made 20–30 min after pairing. Each plot symbol represents a single experiment. Reprinted with permission from Bi and Poo 1998 (copyright [1998] Society for Neuroscience).(C) Two-photon Ca2+ fluorescence imaging was used to measure Ca2+ transients in single spines of the basal dendrites of neocortical layer 5 pyramidal neurons in a slice preparation. These transients were evoked by either a paired stimulus in which a postsynaptic action potential preceded an EPSP by 50 ms (AP-EPSP), a paired stimulus in which this order was reversed (EPSP-AP), or a pair of action potentials given with a 50 ms interval. The y axis represents the peak amplitude of the evoked Ca2+ transient, normalized to that of a single EPSP in the first case and a single AP in each of the second two cases. Each plot symbol represents a single experiment and the horizontal bars represent the means. Reprinted with permission from Koester and Sakmann 1998 (copyright [1998] National Academy of Sciences, USA).View Large Image Figure ViewerDownload Hi-res image Download (PPT) (A) LTP and LTD were induced in pyramidal cell pairs in a neocortical slice. Following a baseline recording period, repeated paired stimuli were given in which a postsynaptic spike either preceded (closed squares) or followed (open squares) an EPSP by 10 ms, resulting in LTD or LTP, respectively. However, when a postsynaptic spike either preceded (closed circles) or followed (open circles) an EPSP by 100 ms, no significant alteration of synaptic strength was produced. Reprinted with permission from Markram et al. 1997 (copyright [1997] American Association for the Advancement of Science). (B) LTP and LTD were induced in pyramidal cell pairs in hippocampal culture. This graph illustrates the critical window for induction of LTP and LTD by repeating parings of EPSPs and postsynaptic spikes. Negative numbers on the x axis indicate intervals in which the spike preceded the EPSP. Measurements of change in synaptic strength were made 20–30 min after pairing. Each plot symbol represents a single experiment. Reprinted with permission from Bi and Poo 1998 (copyright [1998] Society for Neuroscience). (C) Two-photon Ca2+ fluorescence imaging was used to measure Ca2+ transients in single spines of the basal dendrites of neocortical layer 5 pyramidal neurons in a slice preparation. These transients were evoked by either a paired stimulus in which a postsynaptic action potential preceded an EPSP by 50 ms (AP-EPSP), a paired stimulus in which this order was reversed (EPSP-AP), or a pair of action potentials given with a 50 ms interval. The y axis represents the peak amplitude of the evoked Ca2+ transient, normalized to that of a single EPSP in the first case and a single AP in each of the second two cases. Each plot symbol represents a single experiment and the horizontal bars represent the means. Reprinted with permission from Koester and Sakmann 1998 (copyright [1998] National Academy of Sciences, USA). (A) LTP and LTD were induced in pyramidal cell pairs in a neocortical slice. Following a baseline recording period, repeated paired stimuli were given in which a postsynaptic spike either preceded (closed squares) or followed (open squares) an EPSP by 10 ms, resulting in LTD or LTP, respectively. However, when a postsynaptic spike either preceded (closed circles) or followed (open circles) an EPSP by 100 ms, no significant alteration of synaptic strength was produced. Reprinted with permission from Markram et al. 1997 (copyright [1997] American Association for the Advancement of Science). (B) LTP and LTD were induced in pyramidal cell pairs in hippocampal culture. This graph illustrates the critical window for induction of LTP and LTD by repeating parings of EPSPs and postsynaptic spikes. Negative numbers on the x axis indicate intervals in which the spike preceded the EPSP. Measurements of change in synaptic strength were made 20–30 min after pairing. Each plot symbol represents a single experiment. Reprinted with permission from Bi and Poo 1998 (copyright [1998] Society for Neuroscience). (C) Two-photon Ca2+ fluorescence imaging was used to measure Ca2+ transients in single spines of the basal dendrites of neocortical layer 5 pyramidal neurons in a slice preparation. These transients were evoked by either a paired stimulus in which a postsynaptic action potential preceded an EPSP by 50 ms (AP-EPSP), a paired stimulus in which this order was reversed (EPSP-AP), or a pair of action potentials given with a 50 ms interval. The y axis represents the peak amplitude of the evoked Ca2+ transient, normalized to that of a single EPSP in the first case and a single AP in each of the second two cases. Each plot symbol represents a single experiment and the horizontal bars represent the means. Reprinted with permission from Koester and Sakmann 1998 (copyright [1998] National Academy of Sciences, USA). These initial findings have since been confirmed and extended using several different preparations, including hippocampal CA3–CA3 cell pairs in cultured slices (13Debanne D. Gahwiler B.H. Thompson S.M. Long-term synaptic plasticity between pairs of individual CA3 pyramidal cells in rat hippocampal slice cultures.J. Physiol. 1998; 507: 237-247Crossref PubMed Scopus (434) Google Scholar), developing Xenopus retinotectal synapses in vivo (49Zhang L.I. Tao H.W. Holt C.E. Harris W.A. Poo M.-M. A critical window for cooperation and competition among developing retinotectal synapses.Nature. 1998; 395: 37-44Crossref PubMed Scopus (634) Google Scholar), and cultured hippocampal pyramidal neurons (8Bi G.-Q. Poo M.-M. Activity-induced synaptic modifications in hippocampal culture dependence on spike timing, synaptic strength and cell type.J. Neurosci. 1998; 18: 10464-10472Crossref PubMed Google Scholar). In each case, LTP was induced by pairing in which spikes followed EPSPs by ∼10 ms and LTD was induced when this order was reversed (see Figure 1b ). While Hebb did not envision use-dependent synaptic weakening, his postulate does state the requirement for temporal order seen in these experiments (EPSP must precede the spike to strengthen the synapse). This strong dependence on temporal order may be understood in terms of the voltage-dependent blockade of the NMDA receptor by Mg2+. Because the forms of LTP and LTD described above depend upon the activation of the NMDA receptor and subsequent Ca2+ entry, arrival of the backpropagating spike during the EPSP functions to depolarize the postsynaptic membrane strongly and thereby facilitate Ca2+ flux mediated by the NMDA receptor. In contrast, when the spike precedes the EPSP, the EPSP coincides with the afterhyperpolarization, which may dampen NMDA receptor-mediated Ca2+ influx. However, the role of the afterhyperpolarization in this process has not been systematically studied. At present, it is not clear if the time course of the afterhyperpolarization as measured in the dendrites is a good predictor of the timing requirements for LTD induction. A number of Ca2+ imaging studies that have examined the interaction between backpropagating spikes and EPSPs in dendrites appear to support this model. 48Yuste R. Denk W. Dendritic spines as basic functional units of neuronal integration.Nature. 1995; 375: 682-684Crossref PubMed Scopus (695) Google Scholar used two-photon microscopy to show that while both bursts of subthreshold EPSPs and backpropagating spikes could produce Ca2+ transients in dendritic spine heads of hippocampal CA1 pyramidal cells, simultaneous spikes and EPSPs produced a supralinear Ca2+ transient (a Ca2+ transient larger than that predicted by adding the spike and EPSP transients together). Several subsequent studies have confirmed this basic finding using both two-photon (24Koester H.J. Sakmann B. Calcium dynamics in single spines during coincident pre- and postsynaptic activity depend on relative timing of back-propagating action potentials and subthreshold excitatory postsynaptic potentials.Proc. Natl. Acad. Sci. USA. 1998; 95: 9596-9601Crossref PubMed Scopus (278) Google Scholar, 35Schiller J. Schiller Y. Clapham D.E. NMDA receptors amplify calcium influx into dendritic spines during associative pre- and postsynaptic activation.Nat. Neurosci. 1998; 1: 114-118Crossref PubMed Scopus (106) Google Scholar) and conventional (28Magee J.C. Johnston D. A synaptically controlled, associative signal for Hebbian plasticity in hippocampal neurons.Science. 1997; 275: 209-213Crossref PubMed Scopus (975) Google Scholar) microscopy. An important extension of this finding was that pairing in which postsynaptic spikes preceded EPSPs resulted in a sublinear Ca2+ transient (24Koester H.J. Sakmann B. Calcium dynamics in single spines during coincident pre- and postsynaptic activity depend on relative timing of back-propagating action potentials and subthreshold excitatory postsynaptic potentials.Proc. Natl. Acad. Sci. USA. 1998; 95: 9596-9601Crossref PubMed Scopus (278) Google Scholar), consistent with the LTD induced by this stimulation protocol (see Figure 1c). A supralinear Ca2+ transient could result from voltage-dependent unblock of NMDA receptors, nonlinear activation of voltage-gated Ca2+ channels or Ca2+-induced Ca2+ release. Dissociating these contributions is not a simple matter of adding the appropriate drugs to the bath, because some of these mechanisms, particularly opening of voltage-gated Ca2+ channels, may be operative in both transmitter release and postsynaptic processes. To circumvent this problem, 35Schiller J. Schiller Y. Clapham D.E. NMDA receptors amplify calcium influx into dendritic spines during associative pre- and postsynaptic activation.Nat. Neurosci. 1998; 1: 114-118Crossref PubMed Scopus (106) Google Scholar repeated the two-photon imaging experiments of dendritic spines described above but utilized uncaging of glutamate by a UV laser in place of synaptic stimulation. By calibrating the uncaging stimulus, they produced postsynaptic depolarizations that approximated the size and shape of an EPSP. These artificial EPSPs produced spine Ca2+ transients that were largely mediated by voltage-gated Ca2+ channels, with only 18% of the Ca2+ signal contributed by the NMDA receptor. EPSP–spike pairing produced a supralinear Ca2+ signal in which 59% of the total signal was mediated by NMDA receptors, this increase being sufficient to fully account for the supralinearity. While this result is exciting and could explain LTP, several caveats need to be considered. First, while local uncaging of glutamate produces a depolarization that looks like an EPSP, it is possible that the former activates a higher proportion of extrasynaptic receptors. Because extrasynaptic glutamate receptors have a higher proportion of AMPA relative to NMDA receptors as compared with synaptic glutamate receptors, this could result in a fractional NMDA receptor contribution calculated from local uncaging that is lower than for true synaptic stimulation. Second, increases in intracellular Na+, as would be expected to occur following spikes or spike bursts, can transiently increase the open probability of NMDA receptors an a manner that requires the presence of an NMDA receptor–associated Src kinase (47Yu X.-M. Salter M.W. Gain control of NMDA-receptor currents by intracellular sodium.Nature. 1998; 396: 469-474Crossref PubMed Scopus (163) Google Scholar). Thus, depolarization is not the only way in which postsynaptic spikes might potentiate NMDA receptor function. It will be informative to couple Na+ imaging and whole-cell patch recording with EPSP–spike pairs to assess the time course of Na+ transients in relation to the timing requirements for the induction of LTP and LTD. Third, spine Ca2+ signals evoked by single EPSPs in hippocampal CA1 pyramidal cells can be blocked nearly completely by either an NMDA receptor antagonist or by blockers of Ca2+-induced Ca2+ release, suggesting a model in which sequential activation of these two processes gives rise to spine Ca2+ signals (15Emptage N. Bliss T.V.P. Fine A. Single synaptic events evoke NMDA receptor–mediated release of calcium from internal stores in hippocampal dendritic spines.Neuron. 1999; 22: 115-124Abstract Full Text Full Text PDF PubMed Scopus (362) Google Scholar). Importantly, neither NMDA receptor antagonists nor blockers of Ca2+-induced Ca2+ release altered the amplitude or kinetics of the somatically recorded EPSP, suggesting that they did not alter presynaptic glutamate release. While not examined in this study, Ca2+-induced Ca2+ release could potentially contribute to the supralinear Ca2+ signals induced by EPSP–spike pairing. In contrast to reports in mammalian brain, 7Bell C.C. Han V.Z. Sugawara Y. Grant K. Synaptic plasticity in a cerebellum-like structure depends on temporal order.Nature. 1997; 387: 278-281Crossref PubMed Scopus (434) Google Scholar have shown that in the electrosensory lobe of a mormyrid electric fish (which is a cerebellum-like structure), pairing in which a parallel fiber–evoked glutamatergic EPSP is followed 10 ms later by a postsynaptic Na+ spike produces LTD rather than LTP. Pairings at other delays lead to enhancement or no change in EPSP size. The LTD induced in this fashion is blocked by an NMDA receptor antagonist or a postsynaptic Ca2+ chelator and is not induced when postsynaptic depolarization that is just subthreshold for spike generation is used in place of a spike (C. C. Bell, personal communication). At present, it is not known if the Ca2+ transients produced in the postsynaptic (Purkinje-like) cell by paired stimuli resemble those in previously studied mammalian neurons. While EPSP–spike pairing causes large spine Ca2+ transients that presumably result in LTP induction, it is clear that postsynaptic spikes alone can result in smaller Ca2+ transients in dendritic shafts (21Jaffe D.B. Johnston D. Lasser-Ross N. Lisman J.E. Miyakawa H. Ross W.N. The spread of Na+ spikes determines the pattern of dendritic Ca2+ entry into hippocampal neurons.Nature. 1992; 357: 244-246Crossref PubMed Scopus (345) Google Scholar, 28Magee J.C. Johnston D. A synaptically controlled, associative signal for Hebbian plasticity in hippocampal neurons.Science. 1997; 275: 209-213Crossref PubMed Scopus (975) Google Scholar) and spines (48Yuste R. Denk W. Dendritic spines as basic functional units of neuronal integration.Nature. 1995; 375: 682-684Crossref PubMed Scopus (695) Google Scholar, 24Koester H.J. Sakmann B. Calcium dynamics in single spines during coincident pre- and postsynaptic activity depend on relative timing of back-propagating action potentials and subthreshold excitatory postsynaptic potentials.Proc. Natl. Acad. Sci. USA. 1998; 95: 9596-9601Crossref PubMed Scopus (278) Google Scholar, 35Schiller J. Schiller Y. Clapham D.E. NMDA receptors amplify calcium influx into dendritic spines during associative pre- and postsynaptic activation.Nat. Neurosci. 1998; 1: 114-118Crossref PubMed Scopus (106) Google Scholar). Indeed, it was seen that the spine Ca2+ transients produced by LTD-inducing EPSP–spike pairing were similar in amplitude to those produced by pairs of spikes (24Koester H.J. Sakmann B. Calcium dynamics in single spines during coincident pre- and postsynaptic activity depend on relative timing of back-propagating action potentials and subthreshold excitatory postsynaptic potentials.Proc. Natl. Acad. Sci. USA. 1998; 95: 9596-9601Crossref PubMed Scopus (278) Google Scholar). This observation is consistent with reports that LTD may be induced by prolonged high-frequency postsynaptic spiking alone, in a manner that requires voltage-gated Ca2+ channels but that does not require NMDA receptor activation (32Pockett S. Brookes N.H. Bindman L.J. Long-term depression at synapses in slices of rat hippocampus can be induced by bursts of postsynaptic activity.Exp. Brain Res. 1990; 80: 196-200Crossref PubMed Scopus (56) Google Scholar, 9Christofi G. Nowicky A.V. Bolsover S.R. Bindman L.J. The postsynaptic induction of nonassociative long-term depression of excitatory synaptic transmission in rat hippocampal slices.J. Neurophysiol. 1993; 69: 219-229PubMed Google Scholar, 12Cummings J.A. Mulkey R.M. Nicoll R.A. Malenka R.C. Ca2+ signaling requirements for long-term depression in the hippocampus.Neuron. 1996; 16: 825-833Abstract Full Text Full Text PDF PubMed Scopus (343) Google Scholar). In fact, in some cases repeated high-frequency bursts of postsynaptic spiking alone have been reported to produce LTP as well (44Volgushev M. Voronin L.L. Chistiakova M. Singer W. Induction of LTP and LTD in visual cortex neurones by intracellular tetanization.Neuroreport. 1994; 5: 2069-2072Crossref PubMed Scopus (28) Google Scholar). However, while LTP/LTD induction by postsynaptic spiking alone is an instructive experiment, it is unlikely to represent a situation that is common in vivo. Recently, it has been shown that postsynaptic spiking is important for LTP and LTD of an inhibitory GABAergic synapse (2Aizenman C. Manis P.B. Linden D.J. Polarity of long-term synaptic gain change is related to postsynaptic spike firing at a cerebellar inhibitory synapse.Neuron. 1998; 21: 827-835Abstract Full Text Full Text PDF PubMed Scopus (181) Google Scholar). Inhibitory postsynaptic potentials (IPSPs) recorded in cerebellar deep nuclear neurons, in response to stimulation of the axons of cerebellar Purkinje cells in a slice preparation, undergo LTP following a series of brief high-frequency IPSP bursts. The LTP produced by this stimulation is al