Potentiation of mossy fiber EPSCs in the cerebellar nuclei by NMDA receptor activation followed by postinhibitory rebound current

Behavioral and computational studies predict that synaptic plasticity of excitatory mossy fiber inputs to cerebellar nuclear neurons is required for associative learning, but standard tetanization protocols fail to potentiate nuclear cell EPSCs in mouse cerebellar slices. Nuclear neurons fire action potentials spontaneously unless strongly inhibited by Purkinje neurons, raising the possibility that plasticity-triggering signals in these cells differ from those at classical Hebbian synapses. Based on predictions of neuronal activity during delay eyelid conditioning, we developed quasi-physiological induction protocols consisting of high-frequency mossy fiber stimulation and postsynaptic hyperpolarization. Robust, NMDA receptor-dependent potentiation of nuclear cell EPSCs occurred with protocols including a 150-250 ms hyperpolarization in which mossy fiber stimulation preceded a postinhibitory rebound depolarization. Mossy fiber stimulation potentiated EPSCs even when postsynaptic spiking was prevented by voltage-clamp, as long as rebound current was evoked. These data suggest that Purkinje cell inhibition guides the strengthening of excitatory synapses in the cerebellar nuclei.     

A persistent postsynaptic modification mediates long-term potentiation in the hippocampus

Long-term potentiation (LTP) is a long-lasting enhancement of synaptic transmission that can be induced by brief repetitive stimulation of excitatory pathways in the hippocampus. One of the most controversial points is whether the process underlying the enhanced synaptic transmission occurs pre- or postsynaptically. To examine this question, we have taken advantage of the novel physiological properties of excitatory synaptic transmission in the CA1 region of the hippocampus. Synaptically released glutamate activates both NMDA and non-NMDA receptors on pyramidal cells, resulting in an excitatory postsynaptic potential (EPSP) with two distinct components. A selective increase in the non-NMDA component of the EPSP was observed with LTP. This result suggests that the enhancement of synaptic transmission during LTP is caused by an increased sensitivity of the postsynaptic neuron to synaptically released glutamate.     

Selective facilitation of LTP in the ventral hippocampus by calcium stores

The effects of low concentrations of caffeine and ryanodine on field excitatory postsynaptic potentials (EPSPs) and long-term potentiation (LTP) were studied in CA1 region of slices of dorsal and ventral hippocampus (DH and VH, respectively). There was a striking difference between the two regions in the magnitude of effect of both drugs, as well as the ability to interact with a tetanic stimulation to produce LTP. Low concentration of caffeine (1 mM) produced a postsynaptic increase in the slope of population EPSPs in VH, and facilitated LTP in this region. Low concentration of ryanodine (0.2 μM) was able to convert short-term potentiation (STP) to LTP in VH only. These effects are postsynaptic and they reflect a higher concentration of ryanodine receptors (RyRs) in the VH compared to the DH.     

Low intensity tetanic stimulation induces long-term potentiation in hippocampal neurons

A minimal intensity of the stimulation necessary for the induction of long-term potentiation of synaptic transmission (LTP) was investigated by intracellular recording in guinea pig in vitro hippocampal slices. High frequency stimulation of afferent fibres at intensities evoking in CA 1 neurons control excitatory postsynaptic potentials (EPSPs) of amplitudes 1-5 mV, resulted usually in a long-lasting increase in response amplitude. LTP was not observed at lower stimulus strength. The coactivation of a certain, though small number of synaptic contacts is thus necessary for the production of LTP.     

Short-term plasticity of descending synaptic input to phrenic motoneurons in rats.

Respiratory afferent stimulation can elicit increases in respiratory motor output that outlast the period of stimulation by seconds to minutes [short-term potentiation (STP)]. This study examined the potential contribution of spinal mechanisms to STP in anesthetized, vagotomized, paralyzed rats. After C(1) spinal cord transection, stimulus trains (100 Hz, 5-60 s) of the C(1)-C(2) lateral funiculus elicited STP of phrenic nerve activity that peaked several seconds poststimulation. Intracellular recording revealed that individual phrenic motoneurons exhibited one of three different responses to stimulation: 1) depolarization that peaked several seconds poststimulation, 2) depolarization during stimulation and then exponential repolarization after stimulation, and 3) bistable behavior in which motoneurons depolarized to a new, relatively stable level that was maintained after stimulus termination. During the STP, excitatory postsynaptic potentials elicited by single-stimulus pulses were larger and longer. In conclusion, repetitive activation of the descending inputs to phrenic motoneurons causes a short-lasting depolarization of phrenic motoneurons, and augmentation of excitatory postsynaptic potentials, consistent with a contribution to STP.     

Control of excitatory synaptic transmission by C-terminal Src kinase

The induction of long-term potentiation at CA3-CA1 synapses is caused by an N-methyl-d-aspartate (NMDA) receptordependent accumulation of intracellular Ca(2+), followed by Src family kinase activation and a positive feedback enhancement of NMDA receptors (NMDARs). Nevertheless, the amplitude of baseline transmission remains remarkably constant even though low frequency stimulation is also associated with an NMDAR-dependent influx of Ca(2+) into dendritic spines. We show here that an interaction between C-terminal Src kinase (Csk) and NMDARs controls the Src-dependent regulation of NMDAR activity. Csk associates with the NMDAR signaling complex in the adult brain, inhibiting the Src-dependent potentiation of NMDARs in CA1 neurons and attenuating the Src-dependent induction of long-term potentiation. Csk associates directly with Src-phosphorylated NR2 subunits in vitro. An inhibitory antibody for Csk disrupts this physical association, potentiates NMDAR mediated excitatory postsynaptic currents, and induces long-term potentiation at CA3-CA1 synapses. Thus, Csk serves to maintain the constancy of baseline excitatory synaptic transmission by inhibiting Src kinase-dependent synaptic plasticity in the hippocampus.     

Ca2+ entry via postsynaptic voltage-sensitive Ca2+ channels can transiently potentiate excitatory synaptic transmission in the hippocampus.

We have studied the role of Ca2+ entry via voltage-sensitive Ca2+ channels in long-term potentiation (LTP) in the CA1 region of the hippocampus. Repeated depolarizing pulses, in the presence of the NMDA receptor antagonist D-APV and without synaptic stimulation, resulted in a potentiation of excitatory postsynaptic potentials (EPSPs) or currents (EPSCs). This depolarization-induced potentiation was augmented in raised extracellular Ca2+ and was blocked by intracellular BAPTA, a Ca2+ chelator, or by nifedipine, a Ca2+ channel antagonist, indicating that the effect was mediated by Ca2+ entry via voltage-sensitive Ca2+ channels. Although the peak potentiation could be as large as 3-fold, the EPSP(C)s decayed back to baseline values within approximately 30 min. However, synaptic activation paired with depolarizing pulses in the presence of D-APV converted the transient potentiation into a sustained form. These results indicate that a rise in postsynaptic Ca2+ via voltage-sensitive Ca2+ channels can transiently potentiate synaptic transmission, but that another factor associated with synaptic transmission may be required for LTP.     

在体大鼠海马Schaffer-CA1条件化双通路与海马组合突触可塑性

在戊巴比妥钠麻醉的Sprague-Dawley大鼠上,运用海马Schaffer-CA1双通路条件化作用(低频配对,600对脉冲,5Hz,配对刺激相应的兴奋性突触后电位峰值时间间隔为10ms)在两条Schaffer-CA1条件化通路上同时诱导出突触可塑性,呈现出海马组合突触可塑性。结果显示:不管海马Schaffer-CA1双通路独立与否,双通路条件化作用均可以同时诱导出长时程增强(long-term potentiation,LTP)和长时程抑制(long-term depression,LTD),呈现出LTP/LTD组合突触可塑性。结果表明:海马Schaffer-CA1双通路技术,可实现海马突触可塑性的双向诱导,可塑性的方向取决于突触的自身状态。由此提示,与传统的高频诱导LTP低频诱导LTD相比,在海马Schaffer-CA1双通路条件化作用诱导出的组合突触可塑性可以更好地编码海马相关的学习记忆,体现了海马突触可塑性的灵活性与稳定性。     

Neurogranin enhances synaptic strength through its interaction with calmodulin

Learning‐correlated plasticity at CA1 hippocampal excitatory synapses is dependent on neuronal activity and NMDA receptor (NMDAR) activation. However, the molecular mechanisms that transduce plasticity stimuli to postsynaptic potentiation are poorly understood. Here, we report that neurogranin (Ng), a neuron‐specific and postsynaptic protein, enhances postsynaptic sensitivity and increases synaptic strength in an activity‐ and NMDAR‐dependent manner. In addition, Ng‐mediated potentiation of synaptic transmission mimics and occludes long‐term potentiation (LTP). Expression of Ng mutants that lack the ability to bind to, or dissociate from, calmodulin (CaM) fails to potentiate synaptic transmission, strongly suggesting that regulated Ng–CaM binding is necessary for Ng‐mediated potentiation. Moreover, knocking‐down Ng blocked LTP induction. Thus, Ng–CaM interaction can provide a mechanistic link between induction and expression of postsynaptic potentiation.     

Activity-dependent induction of facilitation,depression, and post-tetanic potentiation at an insect central synapse

Summary In Manduca sexta larvae, sensory neurons innervating planta hairs on the tips of the prolegs make monosynaptic excitatory connections with motoneurons innervating proleg retractor muscles. Tactile stimulation of the hairs evokes reflex retraction of the proleg. In this study we examined activity-dependent changes in the amplitude of the excitatory postsynaptic potentials (EPSPs) evoked in a proleg motoneuron by stimulation of individual planta hair sensory neurons. Deflection of a planta hair caused a phasic-tonic response in the sensory neuron, with a mean peak instantaneous firing frequency of >300 Hz, and a tonic firing rate of 10–20 Hz. Direct electrical stimulation was used to activate individual sensory neurons to fire at a range of frequencies including those observed during natural stimulation of the hair. At relatively low firing rates (e.g., 1 Hz), EPSP amplitude was stable indefinitely. At higher instantaneous firing frequencies (>10 Hz), EPSPs were initially facilitated, but continuous stimulation led rapidly to synaptic depression. High-frequency activation of a sensory neuron could also produce post-tetanic potentiation, in which EPSP amplitude remained elevated for several min following a stimulus train. Facilitation, depression, and post-tetanic potentiation all appeared to be presynaptic phenomena. These activity-dependent changes in sensory transmission may contribute to the behavioral plasticity of the proleg withdrawal reflex observed in intact insects.Abbreviations ACh acetylcholine - AChE acetylcholine esterase - CNS central nervous system - EPSP excitatory postsynaptic potential - I _h injected hyperpolarizing current - LTP long-term potentiation - PPR principal planta retractor motoneuron - PTP post-tetanic potentiation - R _in input resistance - V _h hyperpolarized potential - V _m membrane potential - VN ventral nerve - VNA anterior branch of the ventral nerve - V _r resting potential.     

Role of EGR1 in hippocampal synaptic enhancement induced by tetanic stimulation and amputation

Hippocampal neurons fire spikes when an animal is at a particular location or performs certain behaviors in a particular place, providing a cellular basis for hippocampal involvement in spatial learning and memory. In a natural environment, spatial memory is often associated with potentially dangerous sensory experiences such as noxious or painful stimuli. The central sites for such pain-associated memory or plasticity have not been identified. Here we present evidence that excitatory glutamatergic synapses within the CA1 region of the hippocampus may play a role in storing pain-related information. Peripheral noxious stimulation induced excitatory postsynaptic potentials (EPSPs) in CA1 pyramidal cells in anesthetized animals. Tissue/nerve injury caused a rapid increase in the level of the immediate-early gene product Egr1 (also called NGFI-A, Krox24, or zif/268) in hippocampal CA1 neurons. In parallel, synaptic potentiation induced by a single tetanic stimulation (100 Hz for 1 s) was enhanced after the injury. This enhancement of synaptic potentiation was absent in mice lacking Egr1. Our data suggest that Egr1 may act as an important regulator of pain-related synaptic plasticity within the hippocampus.     

Effects of endogenous and exogenous prostaglandin in neurotransmission in canine trachea

The effect of PGE2 on neurotransmission in the canine tracheal strip dissected free of epithelium was studied in the single sucrose gap and organ bath. PGE2 was a potent inhibitor of the initiation of excitatory junction potentials (ejps) by just submaximal nerve stimulation. In a concentration of 10(-9) or 10(-8) M PGE2 nearly or completely abolished them. Contractile responses to field stimulation in the sucrose gap at 27 degrees C or in muscle baths at 37 degrees C were also reduced or abolished by PGE2 in the same dose range; reductions were greater at low frequency. Responses to acetylcholine were also depressed but significantly less than to field stimulation. These are consistent with major presynaptic as well as some postsynaptic inhibitory actions of PGE2. No evidence was obtained that endogenous PGE2 affected excitatory junction potentials and contractions; i.e. they were stable for hours and unaffected by indomethacin 10(-6) and 10(-5) M under our conditions. Post-stimulus potentiation of ejps amplitude, maximum at 10 s, was observed and became more marked after the first ejp had been markedly reduced or abolished by PGE2. This potentiation was unaffected by indomethacin. It was suggested that a presynaptic process inhibited by PGE2 might participate in this potentiation. The canine trachea is a useful preparation when studied under the experimental condition used here for study of effects of products of arachidonate on neurotransmission.     

Role of GABAA-Mediated Inhibition and Functional Assortment of Synapses onto Individual Layer 4 Neurons in Regulating Plasticity Expression in Visual Cortex

Layer 4 (L4) of primary visual cortex (V1) is the main recipient of thalamocortical fibers from the dorsal lateral geniculate nucleus (LGN_d). Thus, it is considered the main entry point of visual information into the neocortex and the first anatomical opportunity for intracortical visual processing before information leaves L4 and reaches supra- and infragranular cortical layers. The strength of monosynaptic connections from individual L4 excitatory cells onto adjacent L4 cells (unitary connections) is highly malleable, demonstrating that the initial stage of intracortical synaptic transmission of thalamocortical information can be altered by previous activity. However, the inhibitory network within L4 of V1 may act as an internal gate for induction of excitatory synaptic plasticity, thus providing either high fidelity throughput to supragranular layers or transmittal of a modified signal subject to recent activity-dependent plasticity. To evaluate this possibility, we compared the induction of synaptic plasticity using classical extracellular stimulation protocols that recruit a combination of excitatory and inhibitory synapses with stimulation of a single excitatory neuron onto a L4 cell. In order to induce plasticity, we paired pre- and postsynaptic activity (with the onset of postsynaptic spiking leading the presynaptic activation by 10ms) using extracellular stimulation (ECS) in acute slices of primary visual cortex and comparing the outcomes with our previously published results in which an identical protocol was used to induce synaptic plasticity between individual pre- and postsynaptic L4 excitatory neurons. Our results indicate that pairing of ECS with spiking in a L4 neuron fails to induce plasticity in L4-L4 connections if synaptic inhibition is intact. However, application of a similar pairing protocol under GABA_ARs inhibition by bath application of 2μM bicuculline does induce robust synaptic plasticity, long term potentiation (LTP) or long term depression (LTD), similar to our results with pairing of pre- and postsynaptic activation between individual excitatory L4 neurons in which inhibitory connections are not activated. These results are consistent with the well-established observation that inhibition limits the capacity for induction of plasticity at excitatory synapses and that pre- and postsynaptic activation at a fixed time interval can result in a variable range of plasticity outcomes. However, in the current study by virtue of having two sets of experimental data, we have provided a new insight into these processes. By randomly mixing the assorting of individual L4 neurons according to the frequency distribution of the experimentally determined plasticity outcome distribution based on the calculated convergence of multiple individual L4 neurons onto a single postsynaptic L4 neuron, we were able to compare then actual ECS plasticity outcomes to those predicted by randomly mixing individual pairs of neurons. Interestingly, the observed plasticity profiles with ECS cannot account for the random assortment of plasticity behaviors of synaptic connections between individual cell pairs. These results suggest that connections impinging onto a single postsynaptic cell may be grouped according to plasticity states.     

Rubrospinal synaptic influences on the lumbar motoneurons of the rat

Intracellular recording was employed in experiments on rats with the nervous system intact and after acute pyramidotomy to study the postsynaptic effects produced in the lumbar motoneurons on stimulation of the nucleus ruber. Stimulation of this nucleus with single stimuli and with a short series of stimuli caused excitatory and inhibitory postsynaptic potentials (EPSP and IPSP) to develop in the motoneurons. Most of the EPSP recorded were disynaptic, but response development involved a monosynaptic segmental delay in five of the 124 cells that exhibited EPSP. A capacity for high-frequency potentiation was a characteristic feature of the disynaptic excitatory and inhibitory effects. Transmembrane polarization of the motoneurons had a marked influence on the amplitude of the disynaptic EPSP and IPSP. The properties of the disynaptic rubrospinal influences were similar to those described for the cat.I. M. Sechenov Institute of Evolutionary Physiology and Biochemistry, Academy of Sciences of the USSR, Leningrad. Translated from Neirofiziologiya, Vol. 3, No. 3, pp. 266–273, May–June, 1971.     

Synaptic plasticity at archicortical and neocortical levels

Research carried out by the author and his collaborators, devoted to analysis of the properties and neurophysiological mechanisms of long-term (for several hours) potentiation, is surveyed. Long-term potentiation of focal potentials and unitary responses of strictly hippocampal structures (areas CA₁ and CA₃) in the unanesthetized rabbit is described. Enhancement of excitatory (EPSPs) and inhibitory (IPSPs) postsynaptic potentials was found after tetanization. No corresponding changes of sensitivity to acetylcholine or acetylcholinesterase activity were found by microiontophoretic and histochemical methods during long-term potentiation. Statistical analysis of EPSPs evoked by microstimulation, based on the quantal hypothesis of synaptic transmission, showed an increase in the number of quanta of transmitter release during potentiation. Long-term potentiation of focal potentials during stimulation of the subcortical white matter in surviving neocortical slices and also long-term potentiation of focal and unitary responses of the sensomotor cortex of the unanesthetized rabbit are described. Potentiation of the "indirect" component of the global response of the pyramidal tract was found. The data suggest the presence of long-term potentiation of monosynaptic neocortical responses. It is concluded that the main mechanism of both hippocampal and neocortical long-term potentiation is increased efficiency of excitatory synapses. It is postulated that synapses modified in this way are used in the formation of memory traces.Brain Institute, All-Union Mental Health Research Center, Academy of Medical Sciences of the USSR, Moscow. Translated from Neirofiziologiya, Vol. 16, No. 5, pp. 651–665, September–October, 1984.     

Long-term potentiation of neuronal glutamate transporters

Persistent, use-dependent modulation of synaptic strength has been demonstrated for fast synaptic transmission mediated by glutamate and has been hypothesized to underlie persistent behavioral changes ranging from memory to addiction. Glutamate released at synapses is sequestered by the action of excitatory amino acid transporters (EAATs) in glia and postsynaptic neurons. So, the efficacy of glutamate transporter function is crucial for regulating glutamate spillover to adjacent presynaptic and postsynaptic receptors and the consequent induction of plastic or excitotoxic processes. Here, we report that tetanic stimulation of cerebellar climbing fiber-Purkinje cell synapses results in long-term potentiation (LTP) of a climbing fiber-evoked glutamate transporter current recorded in Purkinje cells. This LTP is postsynaptically expressed and requires activation of an mGluR1/PKC cascade. Together with a simultaneously induced long-term depression (LTD) of postsynaptic AMPA receptors, this might reflect an integrated antiexcitotoxic cellular response to strong climbing fiber synaptic activation, as occurs following an ischemic episode.     

Excitatory synaptic currents in Purkinje cells

The N-methyl-D-aspartate (NMDA) and non-NMDA classes of glutamate receptor combine in many regions of the central nervous system to form a dual-component excitatory postsynaptic current. Non-NMDA receptors mediate synaptic transmission at the resting potential, whereas NMDA receptors contribute during periods of postsynaptic depolarization and play a role in the generation of long-term synaptic potentiation. To investigate the receptor types underlying excitatory synaptic transmission in the cerebellum, we have recorded excitatory postsynaptic currents (EPSCS), by using whole-cell techniques, from Purkinje cells in adult rat cerebellar slices. Stimulation in the white matter or granule-cell layer resulted in an all-or-none synaptic current as a result of climbing-fibre activation. Stimulation in the molecular layer caused a graded synaptic current, as expected for activation of parallel fibres. When the parallel fibres were stimulated twice at an interval of 40 ms, the second EPSC was facilitated; similar paired-pulse stimulation of the climbing fibre resulted in a depression of the second EPSC. Both parallel-fibre and climbing-fibre responses exhibited linear current-voltage relations. At a holding potential of -40 mV or in the nominal absence of Mg2+ these synaptic responses were unaffected by the NMDA receptor antagonist 2-amino-5-phosphonovaleric acid (APV), but were blocked by the non-NMDA receptor antagonist 6-cyano-2,3-dihydro-7-nitroquinoxalinedione (CNQX). NMDA applied to the bath failed to evoke an inward current, whereas aspartate or glutamate induced a substantial current; this current was, however, largely reduced by CNQX, indicating that non-NMDA receptors mediate this response. These results indicate that both types of excitatory input to adult Purkinje cells are mediated exclusively by glutamate receptors of the non-NMDA type, and that these cells entirely lack NMDA receptors.     

The involvement of nonspiking cells in long-term potentiation of synaptic transmission in the hippocampus

In guinea pig hippocampal slices, stimulation of stratum radiatum during depolarization (with intracellular current injections) of nonspiking cells (presumed to be glia) in the apical dendritic area of CA1 pyramidal neurons resulted in a subsequent long-term potential of intracellularly recorded excitatory postsynaptic potentials as well as extracellularly recorded population spikes in the CA1 area. Tetanic stimulation of stratum radiatum resulted in a subsequent prolonged depolarization of the presumed glial cells, and this depolarization was smaller when the tetanus was given during the presence of 2-amino-5-phosphonovalerate or when the slices were exposed to Ca2+-free medium containing Mn2+ and Mg2+. These results suggest that glial depolarization is involved as one of the steps in generating long-term potentiation.     

Generalized posttetanic changes in the excitatory postsynaptic and acetylcholine-evoked currents in snail neurons

Heteroreceptor posttetanic changes in excitatory postsynaptic currents (EPSC) and inward currents evoked by the local iontophoretic application of acetylcholine (ACh) on the dorsal surface of PLa3 and PRa3 Helix lucorum neurons were studied. The following changes in the currents were revealed over the course of 1-1.5 h after tetanization. The rhythmical ACh application (0.5-1.0 cps, 10-40 s) evokes potentiation of the orthodromic EPSC. The tetanic orthodromic stimulation of one of the nerves (n. intestinalis, n. pallialis dexter, or n. pallialis sinister; 1-5 cps, 1-2 min) causes the potentiation of the ACh current and also heterosynaptic depression of the EPSC. It is concluded that activation of subsynaptic and nonsynaptic neurotransmitter chemoreceptors evokes the development of generalized posttetanic changes in neuronal responses.     

Long-term potentiation in hippocampal oriens interneurons: postsynaptic induction,presynaptic expression and evaluation of candidate retrograde factors

Several types of hippocampal interneurons exhibit a form of long-term potentiation (LTP) that depends on Ca²⁺-permeable AMPA receptors and group I metabotropic glutamate receptors. Several sources of evidence point to a presynaptic locus of LTP maintenance. The retrograde factor that triggers the expression of LTP remains unidentified. Here, we show that trains of action potentials in putative oriens-lacunosum-moleculare interneurons of the mouse CA1 region can induce long-lasting potentiation of stimulus-evoked excitatory postsynaptic currents that mimics LTP elicited by high-frequency afferent stimulation. We further report that blockers of nitric oxide production or TRPV1 receptors failed to prevent LTP induction. The present results add to the evidence that retrograde signalling underlies N-methyl-d-aspartate (NMDA) receptor-independent LTP in oriens interneurons, mediated by an unidentified factor.