High Frequency Synaptic Stimulation Research Articles

Взаимодействие между нервной и секреторной функциями в осуществлении интегративной деятельности нейросекреторной клетки Ретциуса пиявки

Based on the literature review and our own experimental data, it is shown how the two most important functions of the Retzius neuron (RN) of the medical leech — secretory and electrical — are related to each other. During low-frequency activation, RN behaves like a typical nerve cell, releasing serotonin from the presynaptic endings. During high-frequency stimulation, RN demonstrates properties a secretory cell, releasing serotonin by somatic exocytosis. The mechanism of autoinhibition (secretory function), revealed during highfrequency synaptic stimulation of RN, allows to prevent hyperactivation of its impulse activity (nervous function), which, in turn, through autoregulation inhibits hyperactivation of somatic exocytosis (secretory function). These data demonstrate that RN is a unique neurosecretory cell with polyfunctional activity.

Memantine block depends on agonist presentation at the NMDA receptor in substantia nigra pars compacta dopamine neurones

NMDA glutamate receptors (NMDARs) have critical functional roles in the nervous system but NMDAR over-activity can contribute to neuronal damage. The open channel NMDAR blocker, memantine is used to treat certain neurodegenerative diseases, including Parkinson's disease (PD) and is well tolerated clinically. We have investigated memantine block of NMDARs in substantia nigra pars compacta (SNc) dopamine neurones, which show severe pathology in PD. Memantine (10 μM) caused robust inhibition of whole-cell (synaptic and extrasynaptic) NMDARs activated by NMDA at a high concentration or a long duration, low concentration. Less memantine block of NMDAR-EPSCs was seen in response to low frequency synaptic stimulation, while responses to high frequency synaptic stimulation were robustly inhibited by memantine; thus memantine inhibition of NMDAR-EPSCs showed frequency-dependence. By contrast, MK-801 (10 μM) inhibition of NMDAR-EPSCs was not significantly different at low versus high frequencies of synaptic stimulation. Using immunohistochemistry, confocal imaging and stereological analysis, NMDA was found to reduce the density of cells expressing tyrosine hydroxylase, a marker of viable dopamine neurones; memantine prevented the NMDA-evoked decrease. In conclusion, memantine blocked NMDAR populations in different subcellular locations in SNc dopamine neurones but the degree of block depended on the intensity of agonist presentation at the NMDAR. This profile may contribute to the beneficial effects of memantine in PD, as glutamatergic activity is reported to increase, and memantine could preferentially reduce over-activity while leaving some physiological signalling intact.

Mitogen-Activated Protein Kinase Upregulates the Dendritic Translation Machinery in Long-Term Potentiation by Controlling the Mammalian Target of Rapamycin Pathway

Protein synthesis is required for persistent forms of synaptic plasticity, including long-term potentiation (LTP). A key regulator of LTP-related protein synthesis is mammalian target of rapamycin (mTOR), which is thought to modulate translational capacity by facilitating the synthesis of particular components of the protein synthesis machinery. Recently, extracellularly regulated kinase (ERK) also was shown to mediate plasticity-related translation, an effect that may involve regulation of the mTOR pathway. We studied the interaction between the mTOR and ERK pathways in hippocampal LTP induced at CA3-CA1 synapses by high-frequency synaptic stimulation (HFS). Within minutes after HFS, the expression of multiple translational proteins, the synthesis of which is under the control of mTOR, increased in area CA1 stratum radiatum. This upregulation was detected in pyramidal cell dendrites and was blocked by inhibitors of the ERK pathway. In addition, ERK mediated the stimulation of mTOR by HFS. The possibility that ERK regulates mTOR by acting at a component further upstream in the phosphatidylinositide 3-kinase (PI3K)-mTOR pathway was tested by probing the phosphorylation of p90-S6 kinase, phosphoinositide-dependent kinase 1 (PDK1), and Akt. ERK inhibitors blocked HFS-induced phosphorylation of all three proteins at sites implicated in the regulation of mTOR. Moreover, a component of basal and HFS-induced ERK activity depended on PI3K, indicating that mTOR-mediated protein synthesis in LTP requires coincident and mutually dependent activity in the PI3K and ERK pathways. The role of ERK in regulating PDK1 and Akt, with their extensive effects on cellular function, has important implications for the coordinated response of the neuron to LTP-inducing stimulation.

Frequency‐dependent impairment of hippocampal LTP from NMDA receptors with reduced calcium permeability

Changes in postsynaptic Ca2+ levels are essential for alterations in synaptic strength. At hippocampal CA3-to-CA1 synapses, the Ca2+ elevations required for LTP induction are typically mediated by NMDA receptor (NMDAR) channels but a contribution of NMDAR-independent Ca2+ sources has been implicated. Here, we tested the sensitivity of different protocols modifying synaptic strength to reduced NMDAR-mediated Ca2+ influx by employing mice genetically programmed to express in forebrain principal neurons an NR1 form that curtails Ca2+ permeability. Reduced NMDAR-mediated Ca2+ influx did not facilitate synaptic depression in CA1 neurons of these genetically modified mice. However, we observed that LTP could not be induced by pairing low frequency synaptic stimulation (LFS pairing) with postsynaptic depolarization, a protocol that induced robust LTP in wild-type mice. By contrast to LFS pairing, similar LTP levels were generated in both genotypes when postsynaptic depolarization was paired with high frequency synaptic stimulation (HFS). This indicates that the postsynaptic Ca2+ elevation also reached threshold during HFS in the mutant, probably due to summation of NMDAR-mediated Ca2+ influx. However, only in wild-type mice did repeated HFS further enhance LTP. All tested forms of LTP were blocked by the NMDAR antagonist D-AP5. Collectively, our results indicate that only NMDAR-dependent Ca2+ sources (NMDARs and Ca2+-dependent Ca2+ release from intracellular stores) mediate LFS pairing-evoked LTP. Moreover, LTP induced by the first HFS stimulus train required lower Ca2+ levels than the additional LTP obtained by repeated trains.

Activity-Dependent Change in AMPA Receptor Properties in Cerebellar Stellate Cells

High-frequency synaptic stimulation is thought to cause a rapid and lasting change in the expression of GluR2 subunit-containing AMPA receptors (AMPARs) at synapses in cerebellar stellate cells. We examined whether spontaneous synaptic activity affects the expression of GluR2-containing synaptic AMPARs and whether the change in AMPAR subtypes alters their Ca2+permeability and kinetic properties. We used intracellular spermine, which blocks GluR2-lacking receptors at depolarized potentials, to distinguish the presence of GluR2. In most cells, the spontaneous EPSC frequency was low, and evoked EPSCs displayed inwardly rectifyingI–Vrelationships, indicative of the presence of GluR2-lacking AMPARs. However, in cells that displayed a higher rate of spontaneous synaptic activity, EPSCs gave linearI–Vplots, suggesting the presence of GluR2-containingAMPARs. This is consistent with the idea that spontaneous synaptic activity increased the expression of GluR2-containing AMPARs at synapses. The Ca2+permeability of AMPARs that gave inwardly rectifying currents in outside-out patches from TTX-treated cells was six times higher than in control cells that gave linear or outwardly rectifyingI–Vplots. However, increased spontaneous synaptic activity did not significantly alter the EPSC decay time. Furthermore, the decay time course ofEPSCs mediated by GluR2-containing receptors was not different from that mediated by a mixed population of receptors at the same synapse. Our results suggest that the level of spontaneous synaptic activity can determine the subunit composition of postsynaptic receptors at this synapse. The activity-induced expression of GluR2-containing receptors significantly reduced the Ca2+permeability of AMPARs in stellate cells but did not slow the decay time course of synaptic currents.

Activity-dependent change in AMPA receptor properties in cerebellar stellate cells.

High-frequency synaptic stimulation is thought to cause a rapid and lasting change in the expression of GluR2 subunit-containing AMPA receptors (AMPARs) at synapses in cerebellar stellate cells. We examined whether spontaneous synaptic activity affects the expression of GluR2-containing synaptic AMPARs and whether the change in AMPAR subtypes alters their Ca(2+) permeability and kinetic properties. We used intracellular spermine, which blocks GluR2-lacking receptors at depolarized potentials, to distinguish the presence of GluR2. In most cells, the spontaneous EPSC frequency was low, and evoked EPSCs displayed inwardly rectifying I-V relationships, indicative of the presence of GluR2-lacking AMPARs. However, in cells that displayed a higher rate of spontaneous synaptic activity, EPSCs gave linear I-V plots, suggesting the presence of GluR2-containingAMPARs. This is consistent with the idea that spontaneous synaptic activity increased the expression of GluR2-containing AMPARs at synapses. The Ca(2+) permeability of AMPARs that gave inwardly rectifying currents in outside-out patches from TTX-treated cells was six times higher than in control cells that gave linear or outwardly rectifying I-V plots. However, increased spontaneous synaptic activity did not significantly alter the EPSC decay time. Furthermore, the decay time course ofEPSCs mediated by GluR2-containing receptors was not different from that mediated by a mixed population of receptors at the same synapse. Our results suggest that the level of spontaneous synaptic activity can determine the subunit composition of postsynaptic receptors at this synapse. The activity-induced expression of GluR2-containing receptors significantly reduced the Ca(2+) permeability of AMPARs in stellate cells but did not slow the decay time course of synaptic currents.

High-frequency synaptic stimulation induces association of fyn and c-src to distinct phosphorylated components

Signaling via tyrosine kinases appears necessary for regulation of synaptic efficacy. Interactions of the src-family kinases with phosphorylated proteins were studied in area CAI of rat hippocampal slices 10 min after induction of long-term potentiation (LTP) by 100 Hz/l s stimulation (HFS). HFS enhanced association of the src-family kinases fyn and c-src with an approximately 120 kDa tyrosine phosphorylated component containing the focal adhesion kinase (FAK) and its homologue PYK2. Association of fyn with FAK and of c-src with PYK2 was increased following the HFS. Further, increase in tyrosine phosphorylation of PYK2 was detected following the HFS. These results suggest that fyn and c-src are involved in distinct signaling pathways and provide evidence for activation of FAK and PYK2 following synaptic stimulation inducing LTP in vitro.

Taurine-induced potentiation is partially reversed by low-frequency synaptic stimulation.

Neuroinhibitory actions of taurine in the brain are well known to be mediated by the activation of GABAA receptors11. We have recently reported a new neuromodulatory taurine action in the hippocampus independent of this GABA receptor type8,9. The new taurine action consists of a long-lasting potentiation of synaptic transmission, that is induced when taurine is applied at a concentration of 5–10 mM over a 10–30 min period. Excitatory postsynaptic potentials (EPSP) increase during taurine application, remaining elevated long after taurine withdrawal (at least three hours). This new taurine action could have important relevance to brain function, since persistent changes in the synaptic strength are envisaged as the cellular substrate of learning and memory4. Taurine-induced potentiation, unlike long-term potentiation (LTP) induced by high frequency synaptic stimulation of afferent fibers, is independent of NMDA receptor activation8. Nevertheless, taurine-induced potentiation has some points in common with LTP, primarily because its induction requires a rise in intracellular calcium, and both potentiation phenomena occlude mutually (Del Olmo et al., in preparation).

Enhancement of transmission at the developing retinogeniculate synapse

Axon terminals from retinal ganglion cells in the left and right eyes initially overlap with each other in the lateral geniculate nucleus of the neonatal ferret, then segregate into eye-specific layers via an activity-dependent process. Brain slices were used to show that, during this period of reorganization, retinal terminals within the lateral geniculate nucleus evoke excitatory postsynaptic currents composed of both NMDA and non-NMDA receptor-mediated currents. The amplitude of these currents could be enhanced for several tens of minutes to more than an hour by several bursts of high frequency synaptic stimulation, and the induction of enhancement appears to depend on NMDA receptor activation. Synaptic enhancement such as this could provide one of the physiological mechanisms by which retinal terminals segregate into eye-specific layers during development.

Epileptiform activity in vitro can produce long-term synaptic failure and persistent neuronal depolarization

A large, extracellular negative DC shift, termed epileptic depolarization, could be elicited during zero magnesium-induced epileptic activity in the rat hippocampal slice. In 10 mM glucose medium, epileptic depolarization was elicited by high-frequency synaptic stimulation. During epileptic depolarization synaptic responses were abolished, but recovered in 10.4 ± 2.1min. In low glucose (2 mM) medium, epileptic depolarization either occurred spontaneously or could be elicited by high frequency synaptic stimulation, and no recovery of synaptic responses was observed for at least 30 min. This long-term synaptic failure was blocked by the competitive NMDA antagonists,3-((±)-2-car☐ypiperazin-4-yl)-propyl-1-phosphonate (CPP, 100 μM) and D-2-amino-7-phosphonoheptanoate ( D-AP7, 100 μM) when added at the peak of epileptic depolarization, but not 5 min afterwards. Intracellular analysis showed that this extracellular DC shift was correlated with a membrane depolarization which approached 0 mV. With 10 mM glucose medium, the membrane potential returned to resting level in6.3 ± 1.9min. In 2 mM glucose medium, neurons remained depolarized and no recovery was observed. This persistent depolarization could account for the loss of synaptic function recorded extracellularly. Application of 100 μM CPP blocked persistent depolarization and allowed for the recovery of the membrane potential. Epileptic depolarization was also observed during picrotoxin-induced epileptic activity. Both anoxic depolarization during experimental ischemia and epileptic depolarization can trigger long-term synaptic failure and persistent depolarization. Epileptic depolarization and anoxic depolarization may be triggers which can lead to neuronal failure in diseases associated with neuronal degeneration,

Evidence that the 40,000 Mr phosphoprotein influenced by high frequency synaptic stimulation is the alpha subunit of pyruvate dehydrogenase

We have previously shown that brief periods of high frequency synaptic stimulation of the rat hippocampus influence the endogenous phosphorylation of a 40,000 Mr brain protein (Browning et al. 6). The results of the present study demonstrate that this brain phosphoprotein is enriched in a purified mitochondrial fraction and co-migrates with the α-subunit of pyruvate dehydrogenase in sodium dodecyl sulfate polyacrylamide gels. Comparisons of total and partial proteolytic fingerprints indicate that the two proteins are essentially identical. In addition, the phosphorylation of the 40,000 Mr brain protein is sensitive to both dichloroacetate and magnesium as has been reported for pyruvate dehydrogenase 35. Taken together these data provide persuasive evidence that the brain protein is the a-subunit of pyruvate dehydrogenase and thereby raise the possibility that even very short periods of synaptic activity influence an enzyme of particular importance to mitochondrial metabolism in brain.