Different requirements for action potentials in the induction of different forms of long-term potentiation

Postsynaptic cell firing triggers bidirectional metaplasticity depending on the LTP induction protocol

Cell firing has been reported to variably upregulate or downregulate subsequently induced long-term potentiation (LTP). The aim of this study was to elucidate the parameters critical to driving each direction of the metaplasticity effect. The main focus was on the commonly used θ-burst stimulation (TBS) and high-frequency stimulation (HFS) protocols that are known to trigger distinct intracellular signaling cascades. To study action potential (AP)-induced metaplasticity, we used intracellular recordings from CA1 pyramidal cells of rat hippocampal slices. Somatic current injections were used to induce θ-burst firing (TBF) or high-frequency firing (HFF) for priming purposes, whereas LTP was induced 15 min later via TBS of Schaffer collaterals in stratum radiatum. TBS-LTP was inhibited by both priming protocols. Conversely, HFS-LTP was facilitated by HFF priming but not affected by TBF priming. Interestingly, both priming protocols reduced AP firing during TBS-LTP induction, and this effect correlated with the reduction of TBS-LTP. However, LTP was not rescued by restoring AP firing with somatic current injections during the TBS. Analysis of intrinsic properties revealed few changes, apart from a priming-induced increase in the medium afterhyperpolarization (HFF priming) and a decrease in the EPSP amplitude/slope ratio (TBF priming), which could in principle contribute to the inhibition of TBS-LTP by reducing depolarization and associated Ca²⁺ influx following synaptic activity or AP backpropagation. Overall, these data indicate that the more physiological TBS protocol for inducing LTP is particularly susceptible to homeostatic feedback inhibition by prior bouts of postsynaptic cell firing.NEW & NOTEWORTHY The induction of LTP in the hippocampus was bidirectionally regulated by prior postsynaptic cell firing, with θ-burst stimulation-induced LTP being consistently impaired by prior spiking, whereas high-frequency stimulation-induced LTP was either not changed or facilitated. Reductions in cell firing during LTP induction did not explain the LTP impairment. Overall, different patterns of postsynaptic firing induce distinct intracellular changes that can increase or decrease LTP depending on the induction protocol.

Repetitive transcranial magnetic stimulation: Re-wiring the alcoholic human brain

Alcohol use disorders (AUDs) are one of the leading causes of mortality and morbidity worldwide. In spite of significant advances in understanding the neural underpinnings of AUDs, therapeutic options remain limited. Recent studies have highlighted the potential of repetitive transcranial magnetic stimulation (rTMS) as an innovative, safe, and cost-effective treatment for AUDs. Here, we summarize the fundamental principles of rTMS and its putative mechanisms of action via neurocircuitries related to alcohol addiction. We will also discuss advantages and limitations of rTMS, and argue that Hebbian plasticity and connectivity changes, as well as state-dependency, play a role in shaping some of the long-term effects of rTMS. Visual imaging studies will be linked to recent clinical pilot studies describing the effect of rTMS on alcohol craving and intake, pinpointing new advances, and highlighting conceptual gaps to be filled by future controlled studies.

Rehabilitating the addicted brain with transcranial magnetic stimulation

Substance use disorders (SUDs) are one of the leading causes of morbidity and mortality worldwide. In spite of considerable advances in understanding the neural underpinnings of SUDs, therapeutic options remain limited. Recent studies have highlighted the potential of transcranial magnetic stimulation (TMS) as an innovative, safe and cost-effective treatment for some SUDs. Repetitive TMS (rTMS) influences neural activity in the short and long term by mechanisms involving neuroplasticity both locally, under the stimulating coil, and at the network level, throughout the brain. The long-term neurophysiological changes induced by rTMS have the potential to affect behaviours relating to drug craving, intake and relapse. Here, we review TMS mechanisms and evidence that rTMS is opening new avenues in addiction treatments.

The BMP2 nuclear variant, nBMP2, is expressed in mouse hippocampus and impacts memory

The novel nuclear protein nBMP2 is synthesized from the BMP2 gene by translational initiation at an alternative start codon. We generated a targeted mutant mouse, nBmp2NLStm, in which the nuclear localization signal (NLS) was inactivated to prevent nuclear translocation of nBMP2 while still allowing the normal synthesis and secretion of the BMP2 growth factor. These mice exhibit abnormal muscle function due to defective Ca2+ transport in skeletal muscle. We hypothesized that neurological function, which also depends on intracellular Ca2+ transport, could be affected by the loss of nBMP2. Age-matched nBmp2NLStm and wild type mice were analyzed by immunohistochemistry, behavioral tests, and electrophysiology to assess nBMP2 expression and neurological function. Immunohistochemical staining of the hippocampus detected nBMP2 in the nuclei of CA1 neurons in wild type but not mutant mice, consistent with nBMP2 playing a role in the hippocampus. Mutant mice showed deficits in the novel object recognition task, suggesting hippocampal dysfunction. Electrophysiology experiments showed that long-term potentiation (LTP) in the hippocampus, which is dependent on intracellular Ca2+ transport and is thought to be the cellular equivalent of learning and memory, was impaired. Together, these results suggest that nBMP2 in the hippocampus impacts memory formation.

Nitric Oxide Is Required for L-Type Ca(2+) Channel-Dependent Long-Term Potentiation in the Hippocampus

Nitric oxide (NO) has long been implicated in the generation of long-term potentiation (LTP) and other types of synaptic plasticity, a role for which the intimate coupling between NMDA receptors (NMDARs) and the neuronal isoform of NO synthase (nNOS) is likely to be instrumental in many instances. While several types of synaptic plasticity depend on NMDARs, others do not, an example of which is LTP triggered by opening of L-type voltage-gated Ca²⁺ channels (L-VGCCs) in postsynaptic neurons. In CA3-CA1 synapses in the hippocampus, NMDAR-dependent LTP (LTP_NMDAR) appears to be primarily expressed postsynaptically whereas L-VGCC-dependent LTP (LTP_L−VGCC), which often coexists with LTP_NMDAR, appears mainly to reflect enhanced presynaptic transmitter release. Since NO is an excellent candidate as a retrograde messenger mediating post-to-presynaptic signaling, we sought to determine if NO functions in LTP_L−VGCC in mouse CA3-CA1 synapses. When elicited by a burst type of stimulation with NMDARs and the associated NO release blocked, LTP_L−VGCC was curtailed by inhibition of NO synthase or of the NO-receptor guanylyl cyclase to the same extent as occurred with inhibition of L-VGCCs. Unlike LTP_NMDAR at these synapses, LTP_L−VGCC was unaffected in mice lacking endothelial NO synthase, implying that the major source of the NO is neuronal. Transient delivery of exogenous NO paired with tetanic synaptic stimulation under conditions of NMDAR blockade resulted in a long-lasting potentiation that was sensitive to inhibition of NO-receptor guanylyl cyclase but was unaffected by inhibition of L-VGCCs. The results indicate that NO, acting through its second messenger cGMP, plays an unexpectedly important role in L-VGCC-dependent, NMDAR-independent LTP, possibly as a retrograde messenger generated in response to opening of postsynaptic L-VGCCs and/or as a signal acting postsynaptically, perhaps to facilitate changes in gene expression.

Calcium signaling, excitability, and synaptic plasticity defects in a mouse model of Alzheimer's disease

Alzheimer's disease (AD) and aging result in impaired ability to store memories, but the cellular mechanisms responsible for these defects are poorly understood. Presenilin 1 (PS1) mutations are responsible for many early-onset familial AD (FAD) cases. The phenomenon of hippocampal long-term potentiation (LTP) is widely used in studies of memory formation and storage. Recent data revealed long-term LTP maintenance (L-LTP) is impaired in PS1-M146V knock-in (KI) FAD mice. To understand the basis for this phenomenon, in the present study we analyzed structural synaptic plasticity in hippocampal cultures from wild type (WT) and KI mice. We discovered that exposure to picrotoxin induces formation of mushroom spines in both WT and KI cultures, but the maintenance of mushroom spines is impaired in KI neurons. This maintenance defect can be explained by an abnormal firing pattern during the consolidation phase of structural plasticity in KI neurons. Reduced frequency of neuronal firing in KI neurons is caused by enhanced calcium-induced calcium release (CICR), enhanced activity of calcium-activated potassium channels, and increased afterhyperpolarization. As a result, "consolidation" pattern of neuronal activity converted to "depotentiation" pattern of neuronal activity in KI neurons. Consistent with this model, we demonstrated that pharmacological inhibitors of CICR (dantrolene), of calcium-activated potassium channels (apamin), and of calcium-dependent phosphatase calcineurin (FK506) are able to rescue structural plasticity defects in KI neurons. Furthermore, we demonstrate that incubation with dantrolene or apamin also rescued L-LTP defects in KI hippocampal slices, suggesting a role for a similar mechanism. This proposed mechanism may be responsible for memory defects in AD but also for age-related memory decline.

Mechanisms of heterosynaptic metaplasticity

Synaptic plasticity is fundamental to the neural processes underlying learning and memory. Interestingly, synaptic plasticity itself can be dynamically regulated by prior activity, in a process termed 'metaplasticity', which can be expressed both homosynaptically and heterosynaptically. Here, we focus on heterosynaptic metaplasticity, particularly long-range interactions between synapses spread across dendritic compartments, and review evidence for intracellular versus intercellular signalling pathways leading to this effect. Of particular interest is our previously reported finding that priming stimulation in stratum oriens of area CA1 in the hippocampal slice heterosynaptically inhibits subsequent long-term potentiation and facilitates long-term depression in stratum radiatum. As we have excluded the most likely intracellular signalling pathways that might mediate this long-range heterosynaptic effect, we consider the hypothesis that intercellular communication may be critically involved. This hypothesis is supported by the finding that extracellular ATP hydrolysis, and activation of adenosine A2 receptors are required to induce the metaplastic state. Moreover, delivery of the priming stimulation in stratum oriens elicited astrocytic calcium responses in stratum radiatum. Both the astrocytic responses and the metaplasticity were blocked by gap junction inhibitors. Taken together, these findings support a novel intercellular communication system, possibly involving astrocytes, being required for this type of heterosynaptic metaplasticity.

Postsynaptic protein synthesis is required for presynaptic enhancement in persistent forms of long-term potentiation

Long-term potentiation (LTP) in the hippocampus is a fundamental process underlying learning and memory in the brain. At CA3-CA1 synapses, three discrete forms of LTP (LTP1, 2, and 3) have been differentiated on the basis of their persistence, maintenance mechanisms, Ca²⁺ signaling pathways, expression loci, and electrophysiological requirements. We previously showed that LTP2 and LTP3 involve a presynaptic expression component that is established in a translation-dependent manner. Here we investigate the locus of translation required for presynaptic expression. Neurotransmitter release rate was estimated via FM 1-43 destaining from CA3 terminals in hippocampal slices from male Wistar rats (6–8 weeks). Destaining was measured at sites making putative contact with CA1 dendritic processes in stratum radiatum that had been filled with a membrane impermeable translation inhibitor and a fluorescent indicator. Our results suggest that inhibition of postsynaptic translation eliminates the enhanced release ordinarily observed at 160 min post-LTP induction, and that this effect is limited to sites closely apposed to the filled postsynaptic cell. We conclude that postsynaptic translation is required for the presynaptic component of LTP2 and LTP3 expression. These data considerably strengthen the mechanistic separation of LTP1, 2, and 3 and provide evidence for an expanded repertoire of communication between synaptic elements.

Postnatal systemic inflammation exacerbates impairment of hippocampal synaptic plasticity in an animal seizure model

OBJECTIVE

To investigate the effects of systemic inflammation in the critical postnatal stages on neurophysiological actions of immune processes and neural plasticity in adult rats after kainic acid (KA)-induced seizures.

METHODS

To determine changes in hippocampal synaptic plasticity after postnatal central nervous system inflammatory responses and seizure attacks, we performed intraperitoneal injections of lipopolysaccharide (LPS) in postnatal Sprague Dawley rats on day 14 (P14) to induce central nervous system inflammation. We then used a KA tail vein injection on P35 to induce seizure attacks. We compared the variability in synaptic plasticity in the hippocampal Schaffer collateral-CA1 region of seizure animals with or without LPS-induced inflammation preconditioning.

RESULTS

P14 injection of LPS increased susceptibility to seizures, while treatment with KA on P35 induced seizures. Long-term potentiation (LTP) of the Schaffer collateral-CA1 region was impaired in seizure animals, and this effect was more pronounced in the P14 LPS injection group. Fluoro-Jade staining revealed an increase in degenerated hippocampal CA1 pyramidal cells in the P14 LPS injection group. Cytokine expression in the hippocampus in the pre-, peri- and postictus periods was greater in P14 LPS rats than in saline-treated rats.

CONCLUSIONS

Intraperitoneal LPS injection on P14 induces higher cytokine secretion after KA-induced seizures, enhancing neuronal excitability, shortening seizure onset time and exacerbating neuronal degeneration and impairment of LTP formation in the hippocampal Schaffer collateral-CA1 region. Central nervous system inflammation during critical stages of childhood development could disrupt the balance needed for neurophysiological actions of immune processes, producing direct, pernicious effects on memory, neural plasticity and neurogenesis into adulthood.

Making synapses strong: metaplasticity prolongs associativity of long-term memory by switching synaptic tag mechanisms

One conceptual mechanism for the induction of associative long-term memory is that a synaptic tag, set by a weak event, can capture plasticity-related proteins from a nearby strong input, thus enabling associativity between the 2 (synaptic tagging and capture, STC). So far, STC has been observed for only a limited time of 60 min. Nevertheless, association of weak memory forms can occur beyond this period and its mechanism is not well understood. Here we report that metaplasticity induced by ryanodine receptor activation or synaptic activation of metabotropic glutamate receptors prolongs the durability of the synaptic tag, thus extending the time window for associative interactions mediating storage of long-term memory. We provide evidence that such metaplasticity alters the mechanisms of STC from a CaMKII-mediated (in non-primed STC) to a protein kinase Mzeta (PKMζ)-mediated process (in primed STC). Thus the association of weak synapses with strong synapses in the "late" stage of associative memory formation occurs only through metaplasticity. The results also reveal that the short-lived, CaMKII-mediated tag may contribute to a mechanism for a fragile form of memory while metaplasticity enables a PKMζ-mediated synaptic tag capable of prolonged interactions that induce a more stable form of memory that is resistant to reversal.

Developmental regulation of the late phase of long-term potentiation (L-LTP) and metaplasticity in hippocampal area CA1 of the rat

Long-term potentiation (LTP) is a form of synaptic plasticity thought to underlie memory; thus knowing its developmental profile is fundamental to understanding function. Like memory, LTP has multiple phases with distinct timing and mechanisms. The late phase of LTP (L-LTP), lasting longer than 3 h, is protein synthesis dependent and involves changes in the structure and content of dendritic spines, the major sites of excitatory synapses. In previous work, tetanic stimulation first produced L-LTP at postnatal day 15 (P15) in area CA1 of rat hippocampus. Here we used a more robust induction paradigm involving theta-burst stimulation (TBS) in acute slices and found the developmental onset of L-LTP to be 3 days earlier at P12. In contrast, at P8-11, TBS only reversed the synaptic depression that occurs from test-pulse stimulation in developing (P8-15) hippocampus. A second bout of TBS delivered 30-180 min later produced L-LTP at P10-11 but not at P8-9 and enhanced L-LTP at P12-15. Both the developmental onset and the enhanced L-LTP produced by repeated bouts of TBS were blocked by the N-methyl-d-aspartate receptor antagonist dl-2-amino-5-phosphonovaleric acid. Thus the developmental onset age is P12 for L-LTP induced by the more robust and perhaps more naturalistic TBS induction paradigm. Metaplasticity produced by repeated bouts of TBS is developmentally regulated, advancing the capacity for L-LTP from P12 to P10, but not to younger ages. Together these findings provide a new basis from which to investigate mechanisms that regulate the developmental onset of this important form of synaptic plasticity.

Coordination of size and number of excitatory and inhibitory synapses results in a balanced structural plasticity along mature hippocampal CA1 dendrites during LTP

Enlargement of dendritic spines and synapses correlates with enhanced synaptic strength during long-term potentiation (LTP), especially in immature hippocampal neurons. Less clear is the nature of this structural synaptic plasticity on mature hippocampal neurons, and nothing is known about the structural plasticity of inhibitory synapses during LTP. Here the timing and extent of structural synaptic plasticity and changes in local protein synthesis evidenced by polyribosomes were systematically evaluated at both excitatory and inhibitory synapses on CA1 dendrites from mature rats following induction of LTP with theta-burst stimulation (TBS). Recent work suggests dendritic segments can act as functional units of plasticity. To test whether structural synaptic plasticity is similarly coordinated, we reconstructed from serial section transmission electron microscopy all of the spines and synapses along representative dendritic segments receiving control stimulation or TBS-LTP. At 5 min after TBS, polyribosomes were elevated in large spines suggesting an initial burst of local protein synthesis, and by 2 h only those spines with further enlarged synapses contained polyribosomes. Rapid induction of synaptogenesis was evidenced by an elevation in asymmetric shaft synapses and stubby spines at 5 min and more nonsynaptic filopodia at 30 min. By 2 h, the smallest synaptic spines were markedly reduced in number. This synapse loss was perfectly counterbalanced by enlargement of the remaining excitatory synapses such that the summed synaptic surface area per length of dendritic segment was constant across time and conditions. Remarkably, the inhibitory synapses showed a parallel synaptic plasticity, also demonstrating a decrease in number perfectly counterbalanced by an increase in synaptic surface area. Thus, TBS-LTP triggered spinogenesis followed by loss of small excitatory and inhibitory synapses and a subsequent enlargement of the remaining synapses by 2 h. These data suggest that dendritic segments coordinate structural plasticity across multiple synapses and maintain a homeostatic balance of excitatory and inhibitory inputs through local protein-synthesis and selective capture or redistribution of dendritic resources.

A protein synthesis and nitric oxide-dependent presynaptic enhancement in persistent forms of long-term potentiation

Long-term potentiation (LTP) is an important process underlying learning and memory in the brain. At CA3-CA1 synapses in the hippocampus, three discrete forms of LTP (LTP1, 2, and 3) can be differentiated on the basis of maintenance and induction mechanisms. However, the relative roles of pre- and post-synaptic expression mechanisms in LTP1, 2, and 3 are unknown. Neurotransmitter release in the expression of LTP1, 2, and 3 was measured via FM 1-43 destaining from CA3 terminals in hippocampal slices from male Wistar rats (7-8 wk). No difference in vesicle turnover rate was observed for LTP1 up to 160 min following induction by one train of theta-burst stimulation (1TBS). A presynaptic enhancement was found for LTP2 at 160 min after induction by 4TBS, and for LTP3 at both 80 and 160 min after induction by 8TBS. Inhibition of nitric oxide (NO) signaling blocked both LTP2 and LTP3 maintenance and the associated enhanced release. LTP2 maintenance and its presynaptic expression were dependent on protein synthesis, but not gene transcription. LTP3 maintenance was dependent on both translation and transcription, but like LTP2, the enhanced release only required translation. These data considerably strengthen the mechanistic separation of LTP1, 2, and 3, supporting a model of multiple, discrete forms of LTP at CA3-CA1 synapses rather than different temporal phases.

The current functional state of local neuronal circuits controls the magnitude of a BOLD response to incoming stimuli

The purpose of this study was to determine how the history-dependent activation state of neuronal networks controls fMRI signals to incoming stimuli. Simultaneous electrophysiological and blood oxygen level-dependent (BOLD) responses were monitored during stimulation of the perforant pathway with low, high, and again low intensity but, otherwise identical pulse trains. Under three different anesthetics (alpha-chloralose, medetomidine, isoflurane) consecutive low intensity stimulation trains, set just below the threshold for population spike generation to single pulses, yielded a stable BOLD response, although at different magnitudes. The first high intensity train increased the BOLD response under all anesthetics and generated population spikes, with varying amplitudes and latencies (alpha-chloralose, metedomidine) or in a regular pattern (isoflurane). Concurrent to the second high intensity train, the BOLD response became minimal, then slowly increasing with subsequent trains (alpha-chloralose, metedomidine), or immediately rising to a stable level (isoflurane). Second train population spikes became regularized, but at low amplitudes and long latencies that were slowly reversed across trains (alpha-chloralose, medetomidine); while under isoflurane, amplitude and latencies became stabilized with the second train. In comparison to initial stimulation, the final low intensity stimulation trains failed to produce BOLD responses (alpha-chloralose, medetomidine), or left the response unchanged (isoflurane), only reaching stable potentiation of population spikes when under isoflurane. Therefore, the fate of BOLD responses depends on whether a new stable functional state of the intrinsic network can be reached after high intensity stimulation.

Simultaneous recording of the field-EPSP as well as the population spike in the CA1 region in freely moving rats by using a fixed "double"-recording electrode

The recording of field potentials in freely moving rats is a very appropriate and commonly used method to describe changes in cellular mechanisms underlying synaptic plasticity. Recently, we introduced a method for the simultaneous recording of both the field-EPSP as well as the population spike in the dentate gyrus of freely moving rats. We used self-made "double"-recording electrodes, consisting of two wires straighten together with a constant distance between both tips. This method was now further developed to obtain stable long-term recordings of CA1 field potentials. Rats were chronically implanted with a bipolar recording electrode; one tip of which reached the stratum radiatum to record the field-EPSP, the other tip was lowered into the stratum pyramidale of the same neuron population to record the population spike by stimulation of the contralateral CA3 (cCA3). In such prepared rats, simultaneously recorded field-EPSP as well as the population spike where thus obtained from their places of generation in a very reliable manner. This kind of preparation allowed a better standardization of stimulation intensities between different animals and stable electrophysiological recordings of both CA1-potentials over a time period of at least 24h in freely behaving animals. Furthermore, primed burst stimulation of the cCA3 (a single biphasic priming pulse was followed by a burst of 10 pulses (frequency of 100 Hz) 190 ms later; pulse duration per half-wave: 0.1 ms) resulted in an early-LTP of both measured parameters, the field-EPSP and the population spike in the CA1 region of freely moving rats.

Synaptic plasticity and the analysis of the field-EPSP as well as the population spike using separate recording electrodes in the dentate gyrus in freely moving rats

Commonly, synaptic plasticity events such as long-term potentiation (LTP) are investigated by using a stimulation electrode and a single, monopolar field recording electrode in the dentate gyrus in intact, freely moving rats. The recording electrode is mostly positioned in the granular cell layer, or the hilar region of the dentate gyrus, i.e. far away from the place of generation of monosynaptic postsynaptic excitatory potentials (EPSP). Since LTP is a synaptic phenomenon and field recordings far away from the activated synapses do not guarantee a specific interpretation of the overlaid, mixture of complex potentials of several different electrical fields it is often difficult or even impossible to interpret the data obtained by such a single recording electrode. Therefore, at least a separate or two recording electrodes should be used to record the EPSP as well as the spike, respectively, ideally at their places of generation. Here, we describe a method by implanting a chronic bipolar recording electrode which fulfils the above requirements by recording the field-EPSP as well as the population spike at their places of generation and describe the time course of LTP measured using this "double-recording" electrode. We show that different tetanization protocols resulted in EPSP- or population spike-LTP but only if the potentials were recorded by electrodes positioned within adequate places of potential generation. Interestingly, the commonly used recording in the hilus of a distinct part of a potential, mistakenly analyzed as an "EPSP" did not reveal any LTP.

Synaptic depolarization is more effective than back-propagating action potentials during induction of associative long-term potentiation in hippocampal pyramidal neurons

Long-term potentiation (LTP) requires postsynaptic depolarization that can result from EPSPs paired with action potentials or larger EPSPs that trigger dendritic spikes. We explored the relative contribution of these sources of depolarization to LTP induction during synaptically driven action potential firing in hippocampal CA1 pyramidal neurons. Pairing of a weak test input with a strong input resulted in large LTP (approximately 75% increase) when the weak and strong inputs were both located in the apical dendrites. This form of LTP did not require somatic action potentials. When the strong input was located in the basal dendrites, the resulting LTP was smaller (< or =25% increase). Pairing the test input with somatically evoked action potentials mimicked this form of LTP. Thus, back-propagating action potentials may contribute to modest LTP, but local synaptic depolarization and/or dendritic spikes mediate a stronger form of LTP that requires spatial proximity of the associated synaptic inputs.

Postsynaptic action potentials are required for nitric-oxide-dependent long-term potentiation in CA1 neurons of adult GluR1 knock-out and wild-type mice

Neocortical long-term potentiation (LTP) consists of both presynaptic and postsynaptic components that rely on nitric oxide (NO) and the GluR1 subunit of the AMPA receptor, respectively. In this study, we found that hippocampal LTP, induced by theta-burst stimulation in mature (>8-week-old) GluR1 knock-out mice was almost entirely NO dependent and involved both the alpha splice variant of NO synthase-1 and the NO synthase-3 isoforms of NO synthase. Theta-burst induced LTP was also partly NO-dependent in wild-type mice and made up approximately 50% of the potentiation 2 h after tetanus. Theta-burst stimulation reliably produced postsynaptic spikes, including a high probability of complex spikes. Inhibition of postsynaptic somatic spikes with intracellular QX314 or local TTX application prevented LTP in the GluR1 knock-out mice and also blocked the NO component of LTP in wild types. We conclude that theta-burst stimulation is particularly well suited to producing the postsynaptic somatic spikes required for NO-dependent LTP.