Theta-burst Stimulus Research Articles

Pairing of neonatal phencyclidine exposure and acute adolescent stress in male rats as a novel developmental model of schizophrenia

Improved understanding of the neurophysiological and neurochemical mechanisms underlying schizophrenia is essential for the identification of biological markers and developing new therapeutic targets. The development of behaviorally faithful, predictive animal models is crucial to this endeavor. We have developed a novel two-hit paradigm designed to recapitulate in rodents the developmental process leading to appearance of human schizophrenia symptomatology. The model pairs neonatal administration of the NMDA receptor (NMDAR) open-channel blocker phencyclidine (PCP 10 mg/kg) to male rats at 7, 9 and 11 days of age, with later adolescent exposure (34 days of age) to a single prolonged stress paradigm consisting of 2 h restraint, followed by 20 min of forced swimming. Four experimental groups were examined: vehicle and no stress (VEH-NS), vehicle plus stress (VEH-S), PCP and no stress (PCP-NS), and PCP plus stress (PCP-S). Only pairing of neonatal PCP with single prolonged adolescent stress caused deficits in novel object recognition memory and increased anxiety-like behavior in the elevated plus maze task, without altering locomotor activity. In a separate cohort of animals, the PCP-S group showed significant reduction in magnitude of hippocampal long-term potentiation (LTP) at Schaffer collateral-CA1 synapses following a single pair of theta-burst stimuli (TBS), while LTP was diminished in both PCP treated groups when elicited by a second pair of TBS. These results suggest that the combination of neonatal PCP and acute adolescent stress are necessary for lasting cognitive impairment and anxiety-like phenotype, and that these behavioral impairments may be due to deficits in LTP in hippocampus, and perhaps elsewhere in the brain.

Identification of the possible downstream pathways related to alpha7 nicotinic acetylcholine receptor in MK-801 induced schizophrenia model in rats

Improved understanding of the neurophysiological and neurochemical mechanisms underlying schizophrenia is essential for the identification of biological markers and developing new therapeutic targets. The development of behaviorally faithful, predictive animal models is crucial to this endeavor. We have developed a novel two-hit paradigm designed to recapitulate in rodents the developmental process leading to appearance of human schizophrenia symptomatology. The model pairs neonatal administration of the NMDA receptor (NMDAR) open-channel blocker phencyclidine (PCP 10 mg/kg) to male rats at 7, 9 and 11 days of age, with later adolescent exposure (34 days of age) to a single prolonged stress paradigm consisting of 2 h restraint, followed by 20 min of forced swimming. Four experimental groups were examined: vehicle and no stress (VEH-NS), vehicle plus stress (VEH-S), PCP and no stress (PCP-NS), and PCP plus stress (PCP-S). Only pairing of neonatal PCP with single prolonged adolescent stress caused deficits in novel object recognition memory and increased anxiety-like behavior in the elevated plus maze task, without altering locomotor activity. In a separate cohort of animals, the PCP-S group showed significant reduction in magnitude of hippocampal long-term potentiation (LTP) at Schaffer collateral-CA1 synapses following a single pair of theta-burst stimuli (TBS), while LTP was diminished in both PCP treated groups when elicited by a second pair of TBS. These results suggest that the combination of neonatal PCP and acute adolescent stress are necessary for lasting cognitive impairment and anxiety-like phenotype, and that these behavioral impairments may be due to deficits in LTP in hippocampus, and perhaps elsewhere in the brain.

Random Neuronal Networks show homeostatic regulation of global activity while showing persistent changes in specific connectivity paths to theta burst stimuli

Learning in neuronal networks based on Hebbian principle has been shown to lead to destabilizing effects. Mechanisms have been identified that maintain homeostasis in such networks. However, the way in which these two opposing forces operate to support learning while maintaining stability is an active area of research. In this study, using neuronal networks grown on multi electrode arrays, we show that theta burst stimuli lead to persistent changes in functional connectivity along specific paths while the network maintains a global homeostasis. Simultaneous observations of spontaneous activity and stimulus evoked responses over several hours with theta burst training stimuli shows that global activity of the network quantified from spontaneous activity, which is disturbed due to theta burst stimuli is restored by homeostatic mechanisms while stimulus evoked changes in specific connectivity paths retain a memory trace of the training.

Calcium-Permeable AMPA Receptors Mediate the Induction of the Protein Kinase A-Dependent Component of Long-Term Potentiation in the Hippocampus.

Long-term potentiation is a family of synaptic mechanisms that are believed to be important for learning and memory. Two of the most extensively studied forms are triggered by the synaptic activation of NMDA receptors and expressed by changes in AMPA receptor function. They can be distinguished on the basis of their requirement for activation of a protein kinase, PKA. We show that the PKA-dependent form also involves the transient insertion of calcium-permeable AMPA receptors. These results have implications for relating synaptic plasticity to learning and memory and suggest a specific linkage between PKA activation and the rapid synaptic insertion of calcium-permeable AMPA receptors during long-term potentiation.

Opposite Neurophysiological Findings Induced by <i>Sideritis scardica</i> and <i>Sideritis euboa</i> Extract in the Rat

Psychophysiological effects of Sideritis herba extracts depend on biologically active ingredients, which might be different for several botanical types of this plant. The present investigation aimed at the characterization of extracts from Sideritis scardica and Sideritis euboa in vivo and in vitro. Construction of electropharmacograms on the base of recording of electrical field potentials from four different brain regions was used to compare the possible pharmacological effects to a database of reference drugs with known clinical indications. Whereas Sideritis scardica produced decreases of spectral power in line with stimulatory frequency patterns as observed in the presence of Ginkgo biloba extract, administration of Sideritis euboa produced opposite effects. Electrical stimulation of the Schaffer Collaterals was used to elicit a pyramidal cell response called population spike in vitro. The amplitude of this spike was determined in the presence of single as well as theta burst stimuli. Direct exposure of brain matter to Sideritis scardica extract led to concentration dependent increases of the population spike amplitude under both stimulation patterns in the range from 12.5 to 100 mg/L. On the opposite, extract from Sideritis euboa did not change the electric response up to 50 mg/L. Higher concentrations of this extract attenuated the signal amplitude. A 1:1 blend of both gave intermediate results. The in vitro results are in line with the in vivo EEG recordings, where both extracts induced opposite changes of the electric power with respect to electric frequency patterns. The results from both models suggest a stimulatory and/or memory-enhancing action for the extract from Sideritis scardica but not for Sideritis euboa extract, where a more tranquillizing effect like that observed in the presence of Humulus cone extract may be expected.

<i>Ex Vivo</i> Characterization of the Action of Sideritis Extract Using Electrical Activity in the Rat Hippocampus Slice Preparation

A hydroethanolic extract (20% V/V) from Herba Sideritis scardica has been recognized to positively influence cognition. The present investigation aimed at the question if this extract would be able to modify intra-hippocampal communication after oral administration of 100 mg/kg daily for one week. The glutamatergic synapse between Schaffer Collaterals and pyramidal cells can be tested by electric stimulation using single pulses or theta burst stimulation. The resulting population spike is modulated by compounds acting at the central nervous system or other preparations directly or as ex vivo approach. In this case the effect of the special extract was tested in vitro the next day after repetitive in vitro administration. Conventional recording technique in the in vitro hippocampus slice revealed an increase of the population spike in the presence of single stimuli and theta burst stimuli resulting in increased long-term potentiation. This effect was tried to modulate by several glutamate receptor antagonists, among them compounds targeting at the ionic NMDA receptor (CGS19755), AMPA receptor (NBQX), Kainate receptor (UBP301) and targeting at three metabotropic glutamate receptors (mGluR I (YM298198), mGluRII ((RS)-APICA)) and mGluRIII (MSOP). Only NBQX was able to prevent the action of the Sideritis scardica extract. Since the AMPA receptor has been related to cognition in several reports in the literature, it is concluded from this result that the positive action of Sideritis scardica extract on brain function involves a modulation of AMPA receptor dependent neurotransmission.

Acute modulation of synaptic plasticity of pyramidal neurons by activin in adult hippocampus.

Activin A is known as a neuroprotective factor produced upon acute excitotoxic injury of the hippocampus (in pathological states). We attempt to reveal the role of activin as a neuromodulator in the adult male hippocampus under physiological conditions (in healthy states), which remains largely unknown. We showed endogenous/basal expression of activin in the hippocampal neurons. Localization of activin receptors in dendritic spines (=postsynapses) was demonstrated by immunoelectron microscopy. The incubation of hippocampal acute slices with activin A (10 ng/mL, 0.4 nM) for 2 h altered the density and morphology of spines in CA1 pyramidal neurons. The total spine density increased by 1.2-fold upon activin treatments. Activin selectively increased the density of large-head spines, without affecting middle-head and small-head spines. Blocking Erk/MAPK, PKA, or PKC prevented the activin-induced spinogenesis by reducing the density of large-head spines, independent of Smad-induced gene transcription which usually takes more than several hours. Incubation of acute slices with activin for 2 h induced the moderate early long-term potentiation (moderate LTP) upon weak theta burst stimuli. This moderate LTP induction was blocked by follistatin, MAPK inhibitor (PD98059) and inhibitor of NR2B subunit of NMDA receptors (Ro25-6981). It should be noted that the weak theta burst stimuli alone cannot induce moderate LTP. These results suggest that MAPK-induced phosphorylation of NMDA receptors (including NR2B) may play an important role for activin-induced moderate LTP. Taken together, the current results reveal interesting physiological roles of endogenous activin as a rapid synaptic modulator in the adult hippocampus.

Immediate early genes expression in the cerebellar cortex correlates with LTP and LTD induction

The consolidation of changes following activity-dependent neural plasticity are believed to involve specific patterns of gene expression. In the hippocampus, immediate early genes are thought to contribute to long-term synaptic plasticity (LTP and LTD); this phenomenon may occur also in the cerebellum, in which the transcription factors c-Fos and P-CREB have been identified. The cerebellum granular layer (GL) can manifest both LTP and LTD following a Theta Burst Stimulus (TBS) delivered to the mossy fibers. We have employed VSD imaging in rat cerebellar slices (P18-24) in order to map the spatial distribution of LTP and LTD in the cerebellum GL. Fluorescence changes were correlated to LTP or LTD in two different post-TBS time ranges (15 and 120 min). Slices were then fixed and processed for immunohistochemistry in order to identify levels of c-Fos and P-CREB expression. The induction of long-term plasticity increased the average level of P-CREB both at 15 min (+39±4.9, p<0.01%) and 120 min (+24±7.2, p<0.05%) after TBS. The level of c-Fos was unaltered at 15 min, while it significantly increased at 120 min (+37±8.9, p<0.05%). By spatially correlating longterm synaptic plasticity with the corresponding variation of P-CREB and c-Fos, we observed that regions showing LTP well correlated (p<0.05) with positive variations of P-CREB and c-Fos. Conversely, areas showing LTD correlated exclusively (p<0.05) with negative variations of P-CREB. Slices were also evaluated by in situ hybridization and a similar analysis was performed. The levels of fos and CREB mRNA expression and their spatial correlation with the sign of long-term synaptic plasticity corresponded with the immunohistochemical results. As a further test, VSD recordings showed that the induction of granular layer LTP and LTD could be prevented by applying 50 mM D-APV, a selective NMDA receptor blocker. Moreover, in situ hybridization and immunohistochemistry analysis evidenced that in these conditions both mRNA and protein expression levels of c-fos and CREB were unchanged, confirming the involvement of these two transcription factors in cerebellar granular layer plasticity.

The selective phosphodiesterase 9 (PDE9) inhibitor PF-04447943 (6-[(3 S,4 S)-4-methyl-1-(pyrimidin-2-ylmethyl)pyrrolidin-3-yl]-1-(tetrahydro-2 H-pyran-4-yl)-1,5-dihydro-4 H-pyrazolo[3,4- d]pyrimidin-4-one) enhances synaptic plasticity and cognitive function in rodents

Inhibition of phosphodiesterase 9 (PDE9) has been reported to enhance rodent cognitive function and may represent a potential novel approach to improving cognitive dysfunction in Alzheimer’s disease. PF-04447943, (6-[(3 S,4 S)-4-methyl-1-(pyrimidin-2-ylmethyl)pyrrolidin-3-yl]-1-(tetrahydro-2 H-pyran-4-yl)-1,5-dihydro-4 H-pyrazolo[3,4- d]pyrimidin-4-one), a recently described PDE9 inhibitor, was found to have high affinity ( Ki of 2.8, 4.5 and 18 nM) for human, rhesus and rat recombinant PDE9 respectively and high selectivity for PDE9 versus PDEs1–8 and 10–11. PF-04447943 significantly increased neurite outgrowth and synapse formation (as indicated by increased synapsin 1 expression) in cultured hippocampal neurons at low (30–100 nM) but not high (300–1000 nM) concentrations. PF-04447943 significantly facilitated hippocampal slice LTP evoked by a weak tetanic stimulus at a concentration of 100 nM but failed to affect response to the weak tetanus at either 30 or 300 nM, or the LTP produced by a theta burst stimulus. Systemic administration of PF-04447943 (1–30 mg/kg p.o.) dose-dependently increased cGMP in the cerebrospinal fluid 30 min after administration indicating target engagement in the CNS of rats. PF-04447943 (1–3 mg/kg p.o.) significantly improved cognitive performance in three rodent cognition assays (mouse Y maze spatial recognition memory model of natural forgetting, mouse social recognition memory model of natural forgetting and rat novel object recognition with a scopolamine deficit). When administered at a dose of 3 mg/kg p.o., which improved performance in novel object recognition, PF-04447943 significantly increased phosphorylated but not total GluR1 expression in rat hippocampal membranes. Collectively these data indicate that PF-04447943 is a potent, selective brain penetrant PDE9 inhibitor that increased indicators of hippocampal synaptic plasticity and improved cognitive function in a variety of cognition models in both rats and mice. Results with PF-04447943 are consistent with previously published findings using a structurally diverse PDE9 inhibitor, BAY73-6199, and further support the suggestion that PDE9 inhibition may represent a novel approach to the palliative remediation of cognitive dysfunction.

A microfluidic brain slice perfusion chamber for multisite recording using penetrating electrodes

To analyze the spatiotemporal dynamics of network activity in a brain tissue slice, it is useful to record simultaneously from multiple locations. When obtained from laminar structures such as the hippocampus or neocortex, multisite recordings also yield information about subcellular current distributions via current source density analysis. Multisite probes developed for in vivo recordings could serve these purposes in vitro, allowing recordings to be obtained from brain slices at sites deeper within the tissue than currently available surface recording methods permit. However, existing recording chambers do not allow for the insertion of lamina-spanning probes that enter through the edges of brain slices. Here, we present a novel brain slice recording chamber design that accomplishes this goal. The device provides a stable microfluidic perfusion environment in which tissue health is optimized by superfusing both surfaces of the slice. Multichannel electrodes can be inserted parallel to the surface of the slice, at any depth relative to the surface. Access is also provided from above for the insertion of additional recording or stimulating electrodes. We illustrate the utility of this recording configuration by measuring current sources and sinks during theta burst stimuli that lead to the induction of long-term potentiation in hippocampal slices.

Role of hippocampal NMDA receptors in a mouse model for fragile x mental retardation syndrome

The ‘fragile x mental retardation syndrome’ (FRAX) is a prevalent form of inherited mental retardation. It appears developmentally delayed, and it is associated with hyperactivity, attention deficit disorders and autism-like behaviour (de Vries et al. 1997). FRAX is typically caused by a trinucleotide repeat expansion in the X-linked FMR1 gene, which prevents the expression of its encoded protein, named ‘fragile X mental retardation protein’ (FMRP) (O'Donnell & Warren, 2002). FMRP modulates the function of a large number of other proteins, and absence of FMRP will disclose most symptoms of FRAX in humans. So far there is no effective therapeutical treatment. An important step for the understanding of the pathophysiology of FRAX was the creation of an animal model, in which the FMR1 gene was isolated and subsequently silenced through generation of a FMR1 knockout mouse (FMR1 KO1) (The Dutch-Belgian Fragile X Consortium, 1994). These mice revealed learning deficits and anatomical changes at dendrites similar to those in humans suffering from FRAX. Hence, it was suggested that the behavioural and cognitive deficits in humans could be related to dysfunctions at the level of dendritic spines and a potentially altered synaptic development and plasticity (Bear et al. 2004). Several studies examined a potential change of synaptic strength in different brain areas in the FMR1 KO1 mouse, and in summary two major findings were published: (1) an increase in synaptic long-term depression (LTD), mediated by group 1/5 metabotropic glutamate receptors (mGluR) (Huber et al. 2002), and (2) an impaired AMPA receptor (AMPAR) mediated long-term potentiation (LTP) (Hu et al. 2008), or a complete loss of LTP (Desai et al. 2006). Even no change of LTP has been reported (Godfraind et al. 1996), and this variability might be explained by the fact that the studies were performed in various brain areas under different stimulation conditions, and each study used FMR1 KO1 mice at a specific age. The paper by Pilpel et al. (2009) in this issue of The Journal of Physiology presents surprising new data on the developmental changes of glutamatergic neurotransmission and plasticity in the FMRP-silenced hippocampus by use of an alternative mouse model. The ‘FMR1 KO2’ mouse was more recently generated by deleting both the promoter and exon1 of the FMR1 gene (Mientjes et al. 2006). As a result, these mice are deficient for both, the FMR1 RNA and the FMRP protein, so in contrast to the previously used ‘FMR1 KO1’ mouse the remaining level of mRNA is effectively null (Mientjes et al. 2006). Here, Pilpel et al. show for the first time that pyramidal neurons in area CA1 of FMR1 KO2 mice express an enhanced NMDA receptor (NMDAR)-dependent LTP, while the level of evoked LTD is unaltered. Importantly, the LTP effects are age dependent, since they are observed only in animals at P14–16, but not after 6–8 weeks. In addition, the authors show that this increased LTP is associated with a reduced ratio in the amplitudes of evoked AMPA/NMDA receptor mediated EPSCs. A biochemical analysis of synaptoneurosomal fractions in the FMR1 KO2 mice confirmed on the protein level a relatively stronger contribution of NMDARs compared to AMPARs in glutamatergic transmission during early postnatal development (P14). The results are very interesting, since all studies mentioned above used the classical FMR1 KO1 mouse model and none of them observed an increased LTP mediated by NMDARs. Here, Pilpel et al. propose a new role for NMDARs in hippocampus, limited to the first two postnatal weeks in the FMR1 KO2 mice. The narrow window of altered synaptic plasticity in the FMR1 KO2 mice might have an impact for any potential therapy related to FRAX. Yet, the effects of FMRP silencing on LTP/LTD seemed to be critically dependent on the stimulation conditions: Pilpel et al. also tested a theta-burst stimulus (TBS), designed to mimic hippocampal activity during exploratory behaviour. Here the level of LTP was not different, and knock-out animals revealed only a small impairment during spatial memory tasks. Thus, the complete understanding of the pathophysiology of FRAX requires further experiments including a comparison of the behavioural, biochemical and physiological phenotypes between these different mouse lines.

A genetically encoded fluorescent sensor of ERK activity

The activity of the ERK has complex spatial and temporal dynamics that are important for the specificity of downstream effects. However, current biochemical techniques do not allow for the measurement of ERK signaling with fine spatiotemporal resolution. We developed a genetically encoded, FRET-based sensor of ERK activity (the extracellular signal-regulated kinase activity reporter, EKAR), optimized for signal-to-noise ratio and fluorescence lifetime imaging. EKAR selectively and reversibly reported ERK activation in HEK293 cells after epidermal growth factor stimulation. EKAR signals were correlated with ERK phosphorylation, required ERK activity, and did not report the activities of JNK or p38. EKAR reported ERK activation in the dendrites and nucleus of hippocampal pyramidal neurons in brain slices after theta-burst stimuli or trains of back-propagating action potentials. EKAR therefore permits the measurement of spatiotemporal ERK signaling dynamics in living cells, including in neuronal compartments in intact tissues.

A Model of Late Long-Term Potentiation Simulates Aspects of Memory Maintenance

Late long-term potentiation (L-LTP) denotes long-lasting strengthening of synapses between neurons. L-LTP appears essential for the formation of long-term memory, with memories at least partly encoded by patterns of strengthened synapses. How memories are preserved for months or years, despite molecular turnover, is not well understood. Ongoing recurrent neuronal activity, during memory recall or during sleep, has been hypothesized to preferentially potentiate strong synapses, preserving memories. This hypothesis has not been evaluated in the context of a mathematical model representing ongoing activity and biochemical pathways important for L-LTP. In this study, ongoing activity was incorporated into two such models – a reduced model that represents some of the essential biochemical processes, and a more detailed published model. The reduced model represents synaptic tagging and gene induction simply and intuitively, and the detailed model adds activation of essential kinases by Ca2+. Ongoing activity was modeled as continual brief elevations of Ca2+. In each model, two stable states of synaptic strength/weight resulted. Positive feedback between synaptic weight and the amplitude of ongoing Ca2+ transients underlies this bistability. A tetanic or theta-burst stimulus switches a model synapse from a low basal weight to a high weight that is stabilized by ongoing activity. Bistability was robust to parameter variations in both models. Simulations illustrated that prolonged periods of decreased activity reset synaptic strengths to low values, suggesting a plausible forgetting mechanism. However, episodic activity with shorter inactive intervals maintained strong synapses. Both models support experimental predictions. Tests of these predictions are expected to further understanding of how neuronal activity is coupled to maintenance of synaptic strength. Further investigations that examine the dynamics of activity and synaptic maintenance can be expected to help in understanding how memories are preserved for up to a lifetime in animals including humans.

COMPLEX MAGNETIC FIELDS ENABLE STATIC MAGNETIC FIELD CUE USE FOR RATS IN RADIAL MAZE TASKS

Male Wistar rats were trained in an eight-arm radial maze task (two sessions per day, delayed-non-matching-to-sample) that included an intramaze static magnetic field “cue” (185 μT) specific to the entrance point of one of the arms. Rats were exposed daily for 60 min to a complex magnetic field waveform (theta-burst pattern, 200–500 nT), presented with several different interstimulus intervals (ISIs), either immediately following training sessions or immediately preceding testing sessions. Application of the theta-burst stimulus with a 4000 ms ISI significantly improved the rats' memory for the arm of the radial maze whose position was indicated by the presence of a static magnetic field cue. Reference memory errors were homogenously distributed among all eight arms of the maze for sham-exposed rats, and among the other seven arms of the maze for complex magnetic field-treated rats. These results suggest that static magnetic field cues may be salient orienting cues even in a microenvironment such as a radial maze, but their use as a cue during maze learning in rats is dependent on whole-body application of a specific time-varying complex magnetic field.

Endocannabinoids Lower the LTP Threshold

Endocannabinoids have been implicated in various forms of synaptic depression (see Related Resources and Fitzjohn and Collingridge). Chevaleyre and Castillo demonstrate that endocannabinoid-mediated long-term depression of inhibitory synapses (I-LTD) in the hippocampus lowers the threshold for the induction of long-term potentiation (LTP) of nearby excitatory synapses. Only theta-burst stimuli (TBS) that were close (10 μm) to the site of the induced inhibitory postsynaptic potentials (IPSPs) produced I-LTD. (TBS is a well-established repetitive stimulation protocol that can produce long-term synaptic changes.) Using a stimulation frequency that produced I-LTD, but not LTP in the excitatory Schaffer collateral to CA1 synapses, the authors triggered I-LTD and then investigated the magnitude of the LTP produced in response to subsequent stimuli. LTP magnitude was larger following priming by I-LTD, and this effect required GABA A ion channels and the CB1 endocannabinoid receptor. The authors suggest that repetitive stimulation produces LTP directly in a restricted region and I-LTD in a wider region, because lower stimulation frequencies are required, and this I-LTD facilitates the induction of LTP in synapses adjacent to those directly receiving the repetitive stimuli. S. M. Fitzjohn, G. L. Collingridge, Endocannabinoids: Losing inhibition to increase learning capacity? Neuron 43 , 762-764 (2004). [Online Journal] V. Chevaleyre, P. E. Castillo, Endocannabinoid-mediated metaplasticity in the hippocampus. Neuron 43 , 871-881 (2004). [Online Journal]

Changes in intracellular calcium transients and LTP in the surround of visual cortex lesions in rats

Injury and loss of neurons are observed in the center of a cerebral cortical lesion. Mechanisms of early functional reorganization post-lesion involve changes in the strength of synaptic coupling as measured in long-term potentiation (LTP). Since these changes in LTP may depend on the intraneuronal calcium concentration ([Ca 2+] I), the present study analyzed the strength of synaptic LTP combined with measurements of the stimulus-induced peak calcium influx in slices from rat visual cortex in vitro. Slices were analyzed 1–7 days post-lesion by use of electrophysiological and calcium fluorescence imaging techniques. A theta-burst stimulus (TBS) was electrically applied to cortical layer IV, while changes in extracellular field potentials (FPs) and in the corresponding peak calcium influx were recorded in layers II/III. Both the strength of LTP and of the FP mediated peak calcium influx were significantly enhanced 1–6 days post-lesion at a distance of 4 mm from the lesion border. Pharmacological experiments revealed that the expression of LTP was dependent on the activation of NMDA receptors. The area of increased stimulus-evoked peak calcium influx correlated with the enhanced LTP, suggesting that changes in [Ca 2+] I mediate the strength of long-term synaptic plasticity following a cortical lesion. This mechanism may support synaptic reorganization in the surround of the deafferented region in rat visual cortex.

Protein phosphatases mediate depotentiation induced by high-intensity theta-burst stimulation.

We have previously reported that varying stimulus intensity produces qualitatively different types of synaptic plasticity in area CA1 of hippocampal slices: brief low-intensity (LI) theta-burst (TB) stimuli induce long-term potentiation (LTP), but if the stimulus intensity is increased (to mimic conditions that may exist during seizures), LTP is not induced; instead, high-intensity (HI) TB stimuli erase previously induced LTP ("TB depotentiation"). We now have explored the mechanisms underlying TB depotentiation using extracellular field recordings with pharmacological manipulations. We found that TB depotentiation was blocked by okadaic acid and calyculin A (inhibitors of serine/threonine protein phosphatases PP1 and PP2A), FK506 (a specific blocker of calcineurin, a Ca(2+)/calmodulin (CaM) protein phosphatase), and 8-Br-cAMP (an activator of protein kinase A) with 3-isobutyl-1-methylxanthine (IBMX, a phosphodiesterase inhibitor). These results suggest that protein phosphatase pathways are involved in the TB depotentiation similar to other type of down-regulating synaptic plasticity such as low-frequency stimulation (LFS)-induced long-term depression (LTD) and depotentiation in the rat hippocampus. However, TB depotentiation and LFS depotentiation could have differential functional significance.

Optically detected long-lasting depolarization of the post-synaptic dendrite membrane upon high frequency and theta burst stimulus in rat hippocampus CA1 region

Optically detected long-lasting depolarization of the post-synaptic dendrite membrane upon high frequency and theta burst stimulus in rat hippocampus CA1 region

Age and Dose-Dependent Effects of Ethanol on the Induction of Hippocampal Long-Term Potentiation

Hippocampal long-term potentiation (LTP) is strongly associated with the acquisition of spatial memory and is attenuated by ethanol. Recent studies have shown that the inhibitory potency of ethanol against n-methyl-d-aspartate (NMDA) receptor-mediated synaptic activity is enhanced in hippocampal slices taken from juvenile rats compared to those taken from adults. In addition, ethanol has been reported to impair spatial memory acquisition at lower doses in adolescent rats compared to adults. We therefore hypothesized that the suppression of hippocampal LTP by ethanol would be more potent in hippocampal slices taken from adolescent rats compared to those taken from adults. The potency of ethanol against NMDA receptor-mediated LTP was assessed in area CA1 of hippocampal slices taken from adolescent (30 days old) and adult (90 days old) rats. In slices from adolescent rats, theta-burst stimulus trains reliably induced robust LTP in the absence of ethanol, but when the stimulus trains were presented in the presence of either 10 mM or 30 mM ethanol, LTP induction was significantly suppressed relative to controls. In contrast, there was no effect of these ethanol concentrations on the induction of LTP in hippocampal slices from adult rats. These observations indicate that ethanol suppresses LTP in the adolescent hippocampus at concentrations that do not affect than it suppresses in the adult slices, suggesting a much greater sensitivity to ethanol in adolescence.

Prenatal dietary choline supplementation decreases the threshold for induction of long-term potentiation in young adult rats.

Choline supplementation during gestation in rats leads to augmentation of spatial memory in adulthood. We hypothesized that prenatal (E12-E17) choline supplementation in the rat would lead to an enhancement of hippocampal synaptic plasticity as assessed by long-term potentiation (LTP) at 3-4 mo of age. LTP was assessed blindly in area CA1 of hippocampal slices with first suprathreshold (above threshold for LTP generation in control slices) theta-burst stimulus trains. The magnitude of potentiation after these stimuli was not different between slices from control and prenatally choline supplemented animals. Next, threshold (reliably leading to LTP generation in control slices) or subthreshold theta-burst stimulus trains were applied to slices from control, prenatally choline-supplemented, and prenatally choline-deprived rats. Threshold level stimulus trains induced LTP in slices from both the control and choline-supplemented rats but not in those from the choline-deficient rats. Subthreshold stimulus trains led to LTP induction in slices from prenatally choline-supplemented rats only. These observations indicate that prenatal dietary manipulation of the amino acid, choline, leads to subsequent significant alterations of LTP induction threshold in adult animals.