Synapse-specific compartmentalization of signaling cascades for LTP induction in CA3 interneurons

Inhibitory interneurons with somata in strata radiatum and lacunosum-molecular (SR/L-M) of hippocampal area CA3 receive excitatory input from pyramidal cells via the recurrent collaterals (RCs), and the dentate gyrus granule cells via the mossy fibers (MFs). Here we demonstrate that Hebbian long-term potentiation (LTP) at RC synapses on SR/L-M interneurons requires the concomitant activation of calcium-impermeable AMPARs (CI-AMPARs) and N-methyl-d-aspartate receptors (NMDARs). RC LTP was prevented by voltage clamping the postsynaptic cell during high-frequency stimulation (HFS; 3 trains of 100 pulses delivered at 100 Hz every 10s), with intracellular injections of the Ca(2+) chelator BAPTA (20mM), and with the NMDAR antagonist D-AP5. In separate experiments, RC and MF inputs converging onto the same interneuron were sequentially activated. We found that RC LTP induction was blocked by inhibitors of the calcium/calmodulin-dependent protein kinase II (CaMKII; KN-62, 10 μM or KN-93, 10 μM) but MF LTP was CaMKII independent. Conversely, the application of the protein kinase A (PKA) activators forskolin/IBMX (50 μM/25 μM) potentiated MF EPSPs but not RC EPSPs. Together these data indicate that the aspiny dendrites of SR/L-M interneurons compartmentalize synapse-specific Ca(2+) signaling required for LTP induction at RC and MF synapses. We also show that the two signal transduction cascades converge to activate a common effector, protein kinase C (PKC). Specifically, LTP at RC and MF synapses on the same SR/LM interneuron was blocked by postsynaptic injections of chelerythrine (10 μM). These data indicate that both forms of LTP share a common mechanism involving PKC-dependent signaling modulation.

Dopaminergic Modulation of Short-Term Synaptic Plasticity in Fast-Spiking Interneurons of Primate Dorsolateral Prefrontal Cortex

Dopaminergic regulation of primate dorsolateral prefrontal cortex (PFC) activity is essential for cognitive functions such as working memory. However, the cellular mechanisms of dopamine neuromodulation in PFC are not well understood. We have studied the effects of dopamine receptor activation during persistent stimulation of excitatory inputs onto fast-spiking GABAergic interneurons in monkey PFC. Stimulation at 20 Hz induced short-term excitatory postsynaptic potential (EPSP) depression. The D1 receptor agonist SKF81297 (5 μM) significantly reduced the amplitude of the first EPSP but not of subsequent responses in EPSP trains, which still displayed significant depression. Dopamine (DA; 10 μM) effects were similar to those of SKF81297 and were abolished by the D1 antagonist SCH23390 (5 μM), indicating a D1 receptor-mediated effect. DA did not alter miniature excitatory postsynaptic currents, suggesting that its effects were activity dependent and presynaptic action potential dependent. In contrast to previous findings in pyramidal neurons, in fast-spiking cells, contribution of N-methyl-d-aspartate receptors to EPSPs at subthreshold potentials was not significant and fast-spiking cell depolarization decreased EPSP duration. In addition, DA had no significant effects on temporal summation. The selective decrease in the amplitude of the first EPSP in trains delivered every 10 s suggests that in fast-spiking neurons, DA reduces the amplitude of EPSPs evoked at low frequency but not of EPSPs evoked by repetitive stimulation. DA may therefore improve detection of EPSP bursts above background synaptic activity. EPSP bursts displaying short-term depression may transmit spike-timing-dependent temporal codes contained in presynaptic spike trains. Thus DA neuromodulation may increase the signal-to-noise ratio at fast-spiking cell inputs.

Properties of excitatory synaptic responses in fast-spiking interneurons and pyramidal cells from monkey and rat prefrontal cortex

In the prefrontal cortex (PFC) during working memory tasks fast-spiking (FS) interneurons might shape the spatial selectivity of pyramidal cell firing. In order to provide time control of pyramidal cell activity, incoming excitatory inputs should excite FS interneurons more vigorously than pyramidal cells. This can be achieved if subthreshold excitatory responses of interneurons are considerably stronger and faster than those in pyramidal neurons. Here we compared the functional properties of excitatory post-synaptic potentials (EPSPs) between pyramidal cells and FS interneurons in slices from monkey dorsolateral PFC and rat prelimbic cortex. Miniature, unitary (in connected pairs or by minimal stimulation) and compound (evoked by electrical stimulation of the white matter) EPSPs were recorded in whole cell mode. We found that EPSPs were significantly larger and faster in FS interneurons than those recorded from pyramidal cells, consistent with the idea of more efficient recruitment of FS interneurons compared to pyramidal neurons. Similar results were obtained in monkey and rat PFC, suggesting a stable role of FS interneurons in this circuitry across species.

Dopamine modulation of neuronal function in the monkey prefrontal cortex

We developed a brain slice preparation that allowed us to apply whole-cell recordings to examine the electrophysiological properties of identified synapses, neurons, and local circuits in the dorsolateral prefrontal cortex (DLPFC) of macaque monkeys. In this article, we summarize the results from some of our recent and current in vitro studies in the DLPFC with special emphasis on the modulatory effects of dopamine (DA) receptor activation on pyramidal and nonpyramidal cell function in superficial layers in DLPFC areas 46 and 9.

Revisiting the role of the hippocampal mossy fiber synapse

The mossy fiber pathway has long been considered to provide the major source of excitatory input to pyramidal cells of hippocampal area CA3. In this review we describe anatomical and physiological properties of this pathway that challenge this view. We argue that the mossy fiber pathway does not provide the main input to CA3 pyramidal cells, and that the short-term plasticity and amplitude variance of mossy fiber synapses may be more important features than their long-term plasticity or absolute input strength.

Voltage-gated sodium channels shape subthreshold EPSPs in layer 5 pyramidal neurons from rat prefrontal cortex

The role of voltage-dependent channels in shaping subthreshold excitatory postsynaptic potentials (EPSPs) in neocortical layer 5 pyramidal neurons from rat medial prefrontal cortex (PFC) was investigated using patch-clamp recordings from visually identified neurons in brain slices. Small-amplitude EPSPs evoked by stimulation of superficial layers were not affected by the N-methyl-D-aspartate receptor antagonist D-2-amino-5-phosphonopentanoic acid but were abolished by the AMPA receptor antagonist 6-cyano-7-nitroquinoxalene-2,3-dione, suggesting that they were primarily mediated by AMPA receptors. AMPA receptor-mediated EPSPs (AMPA-EPSPs) evoked in the apical dendrites were markedly enhanced, or increased in peak and duration, at depolarized holding potentials. Enhancement of AMPA-EPSPs was reduced by loading the cells with lidocaine N-ethylbromide (QX-314) and by local application of the Na(+) channel blocker tetrodotoxin (TTX) to the soma but not to the middle/proximal apical dendrite. In contrast, blockade of Ca(2+) channels by co-application of Cd(2+) and Ni(2+) to the soma or apical dendrite did not affect the AMPA-EPSPs. Like single EPSPs, EPSP trains were shaped by Na(+) but not Ca(2+) channels. EPSPs simulated by injecting synaptic-like current into proximal/middle apical dendrite (simEPSPs) were enhanced at depolarized holding potentials similarly to AMPA-EPSPs. Extensive blockade of Ca(2+) channels by bath application of the Cd(2+) and Ni(2+) mixture had no effects on simEPSPs, whereas bath-applied TTX removed the depolarization-dependent EPSP amplification. Inhibition of K(+) currents by 4-aminopyridine (4-AP) and TEA increased the TTX-sensitive EPSP amplification. Moreover, strong inhibition of K(+) currents by high concentrations of 4-AP and TEA revealed a contribution of Ca(2+) channels to EPSPs that, however, seemed to be dependent on Na(+) channel activation. Our results indicate that in layer 5 pyramidal neurons from PFC, Na(+), and K(+) voltage-gated channels shape EPSPs within the voltage range that is subthreshold for somatic action potentials.

Synaptosomal and vesicular accumulation of L-glutamate, L-aspartate and D-aspartate

We examined the vesicular accumulation of the excitatory amino-acid (EAA) neurotransmitters, L-glutamate and L-aspartate, together with the non-metabolisable EAA analogue D-aspartate. Synaptosomes derived from whole brain were incubated in various concentrations of [3H]-amino acids under conditions to facilitate vesicular turnover. Synaptosomes were then lysed in hypotonic medium and vesicles immunoprecipitated with monoclonal anti-synaptophysin antibodies coupled to sepharose beads. Using this method, saturable vesicular accumulation was observed for [3H]-L-glutamate, [3H]-L-aspartate, and [3H]-D-aspartate but not for the excitatory amino acid receptor ligands [3H]-AMPA or [3H]-kainate. Vesicular accumulation (t(1/2)=7.45 min) was markedly slower than synaptosomal accumulation (t(1/2)=1.03 min) and was substantially reduced at 4 degrees C. Maximal accumulation of [3H]-L-glutamate, [3H]-L-aspartate, and [3H]-D-aspartate was estimated to be 98, 68, and 112 pmol/mg of synaptosomal protein, respectively, and uptake affinities 1.6, 3.4, and 2.1 mM, respectively. Maximal accumulation of [3H]-L-glutamate was non-competitively inhibited by both 100 microM unlabeled L-aspartate and 100 microM D-aspartate, suggesting that all are accumulated into a common vesicular pool by different transporters.

Release of D,L-threo-beta-hydroxyaspartate as a false transmitter from excitatory amino acid-releasing nerve terminals

This study examined whether preaccumulated D,L-threo-beta-hydroxyaspartate (tHA), a competitive substrate for the high-affinity excitatory amino acid (EAA) transporter, is released as a false transmitter from EAA-releasing nerve terminals. Potassium-stimulation (50 mM for 1 min) evoked significant release of the endogenous EAAs (aspartate and glutamate) from superfused neocortical minislices. Endogenous EAA release was largely calcium-dependent and was inhibited by tetanus toxin, a neurotoxin which specifically blocks vesicular exocytosis. In parallel experiments, minislices were pre-incubated with 500 microM tHA. Potassium (50 mM) evoked significant release of tHA and this release was also calcium-dependent and reduced by tetanus toxin. Pre-accumulation of tHA did not affect the release of endogenous glutamate whereas the release of endogenous aspartate was significantly attenuated. These data suggest that tHA selectively accumulates in a vesicular aspartate pool and is released upon depolarization as a false transmitter from EAA nerve terminals.

Synaptic targets of the intrinsic axon collaterals of supragranular pyramidal neurons in monkey prefrontal cortex

The principal axons of supragranular pyramidal neurons in the cerebral cortex travel through the white matter and terminate in other cortical areas, whereas their intrinsic axon collaterals course through the gray matter and form both local and long-distance connections within a cortical region. In the monkey prefrontal cortex (PFC), horizontally oriented, intrinsic axon collaterals from supragranular pyramidal neurons form a series of stripe-like clusters of axon terminals (Levitt et al. [1993] J Comp Neurol 338:360-376; Pucak et al. [1996] J Comp Neurol 376:614-630). The present study examined the synaptic targets of the intrinsic axon collaterals arising from supragranular pyramidal neurons within the same stripe (local projections). Approximately 50% of the within-stripe axon terminals in monkey PFC area 9 targeted dendritic spines. In contrast, for both the intrinsic axon collaterals that travel between stripes (long-range projections), and the axon terminals that project to other PFC areas (associational projections), over 92% of the postsynaptic structures were dendritic spines (Melchitzky et al. [1998] J Comp Neurol 390:211-224). The other 50% of the within-stripe terminals synapsed with dendritic shafts. Dual-labeling studies confirmed that these within-stripe terminals contacted gamma-aminobutyric acid-immunoreactive dendritic shafts, including the subpopulation that contains the calcium-binding protein parvalbumin. The functional significance of the differences in synaptic targets between local and long-range intrinsic axon collaterals was supported by whole-cell, patch clamp recordings in an in vitro slice preparation of monkey PFC. Specifically, the small amplitude responses observed in layer 3 pyramidal neurons during long-range, low-intensity stimulation were exclusively excitatory, whereas local stimulation also evoked di/polysynaptic inhibitory responses. These anatomic and electrophysiological findings suggest that intrinsic connections of the PFC differ from other cortical regions and that within the PFC, feedback (within-stripe) inhibition plays a greater role in regulating the activity of supragranular pyramidal neurons than does feedforward inhibition either between stripes or across regions.

Dopamine increases excitability of pyramidal neurons in primate prefrontal cortex

Dopaminergic modulation of neuronal networks in the dorsolateral prefrontal cortex (PFC) is believed to play an important role in information processing during working memory tasks in both humans and nonhuman primates. To understand the basic cellular mechanisms that underlie these actions of dopamine (DA), we have investigated the influence of DA on the cellular properties of layer 3 pyramidal cells in area 46 of the macaque monkey PFC. Intracellular voltage recordings were obtained with sharp and whole cell patch-clamp electrodes in a PFC brain-slice preparation. All of the recorded neurons in layer 3 (n = 86) exhibited regular spiking firing properties consistent with those of pyramidal neurons. We found that DA had no significant effects on resting membrane potential or input resistance of these cells. However DA, at concentrations as low as 0.5 microM, increased the excitability of PFC cells in response to depolarizing current steps injected at the soma. Enhanced excitability was associated with a hyperpolarizing shift in action potential threshold and a decreased first interspike interval. These effects required activation of D1-like but not D2-like receptors since they were inhibited by the D1 receptor antagonist SCH23390 (3 microM) but not significantly altered by the D2 antagonist sulpiride (2.5 microM). These results show, for the first time, that DA modulates the activity of layer 3 pyramidal neurons in area 46 of monkey dorsolateral PFC in vitro. Furthermore the results suggest that, by means of these effects alone, DA modulation would generally enhance the response of PFC pyramidal neurons to excitatory currents that reach the action potential initiation site.

Impairment of long-term potentiation and associative memory in mice that overexpress extracellular superoxide dismutase

Reactive oxygen species, including superoxide, generally are considered neurotoxic molecules whose effects can be alleviated by antioxidants. Different from this view, we show that scavenging of superoxide with an antioxidant enzyme is associated with deficits in hippocampal long-term potentiation (LTP), a putative neural substrate of memory, and hippocampal-mediated memory function. Using transgenic mice that overexpress extracellular superoxide dismutase (EC-SOD), a superoxide scavenger, we found that LTP was impaired in hippocampal area CA1 despite normal LTP in area CA3. The LTP impairment in area CA1 could be reversed by inhibition of EC-SOD. In addition, we found that EC-SOD transgenic mice exhibited impaired long-term memory of fear conditioning to contextual cues despite exhibiting normal short-term memory of the conditioning experience. These findings strongly suggest that superoxide, rather than being considered exclusively a neurotoxic molecule, should also be considered a signaling molecule necessary for normal neuronal function.

Protein phosphatase-mediated regulation of protein kinase C during long-term depression in the adult hippocampus in vivo

The neural substrates of learning and memory are thought to involve use-dependent long-term changes in synaptic function, including long-term depression (LTD) of synaptic strength. One biochemical event hypothesized to contribute to the maintenance and expression of LTD is decreased protein phosphorylation, caused by a decrease in protein kinase activity and/or an increase in protein phosphatase activity. We tested whether the activity of protein kinase C (PKC) decreases after the induction of LTD in area CA1 of the adult hippocampus in vivo, and then investigated the mechanism responsible for the LTD-associated alteration in PKC activity. We found that LTD was associated with a significant decrease in both autonomous and cofactor-dependent PKC activity. The decrease in PKC activity was prevented by NMDA receptor blockade and was not accompanied by a decrease in the level of either PKCalpha, beta, gamma, or zeta. Western blot analysis with phosphospecific antibodies revealed that phosphorylation of Ser-657 on the catalytic domain of PKCalpha (Ser-660 on PKCbetaII) was decreased significantly after the induction of LTD, and that this dephosphorylation was prevented by the protein phosphatase inhibitor okadaic acid. The decrease in autonomous and cofactor-dependent PKC activity likewise was prevented by okadaic acid. These findings suggest that LTD in the adult hippocampus in vivo involves a decrease in PKC activity that is mediated, at least in part, by dephosphorylation of the catalytic domain of PKC by protein phosphatases activated after LTD-inducing stimulation. Our findings are consistent with the idea that protein dephosphorylation contributes to the expression of LTD.

The multifarious hippocampal mossy fiber pathway: a review

The hippocampal mossy fiber pathway between the granule cells of the dentate gyrus and the pyramidal cells of area CA3 has been the target of numerous scientific studies. Initially, attention was focused on the mossy fiber to CA3 pyramidal cell synapse because it was suggested to be a model synapse for studying the basic properties of synaptic transmission in the CNS. However, the accumulated body of research suggests that the mossy fiber synapse is rather unique in that it has many distinct features not usually observed in cortical synapses. In this review, we have attempted to summarize the many unique features of this hippocampal pathway. We also have attempted to reconcile some discrepancies that exist in the literature concerning the pharmacology, physiology and plasticity of this pathway. In addition we also point out some of the experimental challenges that make electrophysiological study of this pathway so difficult.Finally, we suggest that understanding the functional role of the hippocampal mossy fiber pathway may lie in an appreciation of its variety of unique properties that make it a strong yet broadly modulated synaptic input to postsynaptic targets in the hilus of the dentate gyrus and area CA3 of the hippocampal formation.

The extracellular signal-regulated kinase cascade is required for NMDA receptor-independent LTP in area CA1 but not area CA3 of the hippocampus

Activation of extracellular signal-regulated kinase (ERK) has been shown to be necessary for NMDA receptor-dependent long-term potentiation (LTP). We studied the role of ERK in three forms of NMDA receptor-independent LTP: LTP induced by very high-frequency stimulation (200 Hz-LTP), LTP induced by the K(+) channel blocker tetraethylammonium (TEA) (TEA-LTP), and mossy fiber (MF) LTP (MF-LTP). We found that ERK was activated in area CA1 after the induction of both 200 Hz-LTP and TEA-LTP and that this activation required the influx of Ca(2+) through voltage-gated Ca(2+) channels. Inhibition of the ERK signaling cascade with either PD 098059 or U0126 prevented the induction of both 200 Hz-LTP and TEA-LTP in area CA1. In contrast, neither PD 098059 nor U0126 prevented MF-LTP in area CA3 induced by either brief or long trains of high-frequency stimulation. U0126 also did not prevent forskolin-induced potentiation in area CA3. However, incubation of slices with forskolin, an activator of the cAMP-dependent protein kinase (PKA) cascade, did result in increases in active ERK and cAMP response element-binding protein (CREB) phosphorylation in area CA3. The forskolin-induced increase in active ERK was inhibited by U0126, whereas the increase in CREB phosphorylation was not, which suggests that in area CA3 the PKA cascade is not coupled to CREB phosphorylation via ERK. Overall, our observations indicate that activation of the ERK signaling cascade is necessary for NMDA receptor-independent LTP in area CA1 but not in area CA3 and suggest a divergence in the signaling cascades underlying NMDA receptor-independent LTP in these hippocampal subregions.

Long-term depression in the hippocampus in vivo is associated with protein phosphatase-dependent alterations in extracellular signal-regulated kinase

There is growing evidence that activation of either protein kinases or protein phosphatases determines the type of plasticity observed after different patterns of hippocampal stimulation. Because activation of the extracellular signal-regulated kinase (ERK) has been shown to be necessary for long-term potentiation, we investigated the regulation of ERK in long-term depression (LTD) in the adult hippocampus in vivo. We found that ERK immunoreactivity was decreased following the induction of LTD and that this decrease required NMDA receptor activation. The LTD-associated decrease in ERK immunoreactivity could be simulated in vitro via incubation of either purified ERK2 or hippocampal homogenates with either protein phosphatase 1 or protein phosphatase 2A. The protein phosphatase-dependent decrease in ERK immunoreactivity was inhibited by microcystin. Intrahippocampal administration of the protein phosphatase inhibitor okadaic acid blocked the LTD-associated decrease in ERK2, but not ERK1, immunoreactivity. Collectively, these data demonstrate that protein phosphatases can decrease ERK immunoreactivity and that such a decrease occurs with ERK2 during LTD. These observations provide the first demonstration of a biochemical alteration of ERK in LTD.

Horizontal synaptic connections in monkey prefrontal cortex: an in vitro electrophysiological study

In monkey dorsolateral prefrontal cortex (PFC), long-distance, horizontally oriented intrinsic axon collaterals interconnect clusters of pyramidal neurons in the supragranular layers. In order to study the electrophysiological responses mediated by these long-distance projections, an in vitro slice preparation of monkey PFC was used to obtain whole-cell patch clamp recordings from layer 3 pyramidal neurons. Using in vivo tracer injections, we found that long-distance projections were well preserved in PFC slices cut in the coronal plane. Postsynaptic currents were evoked by low-intensity electrical extracellular stimulation applied successively to 20-30 discrete sites located up to 2200 micron lateral to the recorded cell. Several criteria were applied to discriminate between mono- and polysynaptic responses. Long-distance monosynaptic connections were mediated by fibers with relatively slow conduction velocity (0.14 m/s). Excitatory postsynaptic currents (EPSCs) evoked by stimulation of short- or long-distance horizontal connections did not differ in kinetic properties. The majority (77%) of the 35 layer 3 PFC neurons studied were monosynaptic targets of long-distance connections. EPSCs mediated by long-distance connections had amplitudes that were similar or even larger than short-distance EPSCs, suggesting that excitatory input provided by the former was relatively robust. For most neurons (87.5%) in which a full complement of monosynaptic EPSCs was evoked by multisite stimulation, the EPSC amplitude as a function of stimulation distance from the recorded cells exhibited statistically significant peaks. The spacing between peaks was similar to the spacing between interconnected clusters of neurons observed in previous anatomical studies. The results show that long-distance excitatory connections constitute a significant intrinsic pathway of synaptic communication in layer 3 of monkey PFC.

Transient and persistent increases in protein phosphatase activity during long-term depression in the adult hippocampus in vivo

The neural substrates of learning and memory most likely involve activity-dependent long-term changes in synaptic strength, including long-term potentiation and long-term depression. A critical element in the cascade of events hypothesized to underlie such changes in synaptic function is modification of protein phosphorylation. Long-term depression is thought to involve decreases in protein phosphorylation, which could result from reduction in protein kinase activity and/or enhancement in protein phosphatase activity. We present here direct evidence that long-term depression in the hippocampus in vivo is associated with an increase in the activity of the serine/threonine phosphatases 1 and 2A. The increase in activity of phosphatase 1 was transient, whereas that of phosphatase 2A lasted > 65 min after the induction of long-term depression. Blockade of long-term depression prevented the observed increases in phosphatase activity, as did selective inhibition of phosphatase 1 and 2A. Induction of long-term depression had no effect on the level of either phosphatase, which suggests that our results reflect increases in the intrinsic activity of these two enzymes. Our findings are consistent with a model of synaptic plasticity that implicates protein dephosphorylation by serine/threonine phosphatases in the early maintenance and/or expression of long-term depression of synaptic strength.

Active summation of excitatory postsynaptic potentials in hippocampal CA3 pyramidal neurons

The manner in which the thousands of synaptic inputs received by a pyramidal neuron are summed is critical both to our understanding of the computations that may be performed by single neurons and of the codes used by neurons to transmit information. Recent work on pyramidal cell dendrites has shown that subthreshold synaptic inputs are modulated by voltage-dependent channels, raising the possibility that summation of synaptic responses is influenced by the active properties of dendrites. Here, we use somatic and dendritic whole-cell recordings to show that pyramidal cells in hippocampal area CA3 sum distal and proximal excitatory postsynaptic potentials sublinearly and actively, that the degree of nonlinearity depends on the magnitude and timing of the excitatory postsynaptic potentials, and that blockade of transient potassium channels linearizes summation. Nonlinear summation of synaptic inputs could have important implications for the computations performed by single neurons and also for the role of the mossy fiber and perforant path inputs to hippocampal area CA3.

Amplification of perforant-path EPSPs in CA3 pyramidal cells by LVA calcium and sodium channels

The perforant path forms a monosynaptic connection between the cells of layer II of the entorhinal cortex and the pyramidal cells in hippocampal area CA3. Although this projection is prominent anatomically, very little is known about the physiological properties of this input. The distal location of these synapses suggests that somatically recorded perforant-path excitatory postsynaptic potentials (EPSPs) may be influenced by the activation of voltage-dependent channels in CA3 cells. We observed that perforant-path EPSPs are reduced (by approximately 25%) by blockade of postsynaptic low-voltage-activated calcium and sodium channels, indicating that perforant-path EPSPs are amplified by the activation of these channels. These data suggest that the perforant path may represent an important and highly modifiable direct connection between the entorhinal cortex and area CA3.

Electrophysiological and pharmacological characterization of the direct perforant path input to hippocampal area CA3

Monosynaptic perforant path responses evoked by subicular stimulation were recorded from CA3 pyramidal cells of rat hippocampal slices. These monosynaptic responses were isolated by using low intensities of stimulation and by placing a cut through the mossy fibers. Perforant path-evoked responses consisted of both excitatory and inhibitory components. Excitatory postsynaptic currents (EPSCs) were mediated by both alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acidreceptors (AMPAR) and N-methyl--aspartate receptors (NMDAR). Inhibitory postsynaptic currents consisted of gamma-aminobutyric acid-A (GABAA-) and -B (GABAB)-receptor-mediated components. At membrane potentials more positive than -60 mV and at physiological [Ca2+]/[Mg2+] ratios, >30% of perforant path evoked EPSC was mediated by NMDARs. This value varied as a function of the membrane voltage and external [Mg2+]. Two types of responses were observed after low-intensity stimulation of the perforant path. The first type of response showed paired-pulse facilitation and was reduced by 2-amino-4-phosphonobutyric acid (AP4). The second type of response showed paired-pulse depression and was reduced by baclofen. Electrophysiological and pharmacological characteristics of these two types of responses are similar to the properties of lateral and medial perforant path-evoked EPSPs in the dentate gyrus.

Origin of the apparent asynchronous activity of hippocampal mossy fibers

Fiber volleys (FVs) from the stratum lucidum of rat hippocampal area CA3 were recorded extracellularly from in vitro slices in the presence of 10 mM kynurenic acid. In agreement with previous work, bulk stimulation of the dentate gyrus (DG) near the hilar border leads to an asynchronous FV. Transection of the stratum lucidum between the DG stimulation site and the CA3 recording site reduced or eliminated the early components of the asynchronous FV, indicating that they are of mossy fiber (MF) origin. In contrast, moving the stimulating electrode away from the hilus toward the hippocampal fissure reduced or eliminated the late components of the FV. Subsequently, we found that bulk stimulation on the DG/hilar border induces an antidromic population spike in CA3 pyramidal cells. Finally, the MFs and associational collaterals have different conduction velocities (0.51 and 0.37 m/s, respectively; temperature = 33 degrees C). From these data, we conclude that the late components of the asynchronous FV are due to antidromic activation of CA3 collaterals that have been shown to be present in the DG and hilus. A corollary of these findings is that bulk stimulation on the DG/hilar border can lead to at least two different monosynaptic inputs to CA3 pyramidal cells: the MFs and the antidromically activated associational collaterals. We suggest that when MF synaptic responses are being evoked with the use of bulk stimulation, stimulating electrodes should be placed in the outer molecular layer of the DG to prevent the activation of hilar-projecting associational collaterals. This procedure should be added to the previously proposed criteria for preventing polysynaptic contamination of the intracellularly recorded evoked MF synaptic response.

Large amplitude miniature excitatory postsynaptic currents in hippocampal CA3 pyramidal neurons are of mossy fiber origin

Neonatal (P0) gamma-irradiation was used to lesion selectively the mossy fiber (MF) synaptic input to CA3 pyramidal cells. This lesion caused a > 85% reduction in the MF input as determined by quantitative assessment of the number of dynorphin immunoreactive MF boutons. The gamma-irradiation lesion caused a reduction in the mean number of miniature excitatory postsynaptic currents (mEPSCs) recorded from CA3 pyramidal cells (2,292 vs. 1,429/3-min period; n = 10). The lesion also caused a reduction in the mean mEPSC peak amplitude from 19.1 +/- 0.45 to 14.6 +/- 0.49 pA (mean +/- SE; peak conductance 238.8 +/- 5.6 to 182.0 +/- 6.1 pS). Similarly, there was a reduction in the mean 10-85% rise time from 1.72 +/- 0.02 ms to 1.42 +/- 0.04 ms. The effects of the gamma-irradiation on both mEPSC amplitude and 10-85% rise time were significant at P < 0.002 and P < 0.005 (2-tailed Kolmogorov-Smirnov test). Based on the selectively of the gamma-irradiation, MF and non-MF mEPSC amplitude and 10-85% rise-time distributions were calculated. Both the amplitude and 10-85% rise-time distributions showed extensive overlap between the MF and non-MF mediated mEPSCs. The MF mEPSC distributions had a mean peak amplitude of 24.6 pA (307.5 pS) and a mean 10-85% rise time of 2.16 ms. THe non-MF mEPSC distributions had a mean peak amplitude of 12.2 pA (152.5 pS) and 10-85% rise time of 1.26 ms. The modes of the amplitude distributions were the same at 5 pA (62 pS). The MF and non-MF mEPSC amplitude and 10-85% rise-time distributions were significantly different at P << 0.001 (1-tailed, large sample Kolmogorov-Smirnov test). The data demonstrate that the removal of the MF synaptic input to CA3 pyramidal cells leads to the absence of the large amplitude mEPSCs that are present in control recordings.

Differential expression of short-term potentiation by AMPA and NMDA receptors in dentate gyrus

Both alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate (AMPA) and N-methyl-D-aspartate (NMDA) glutamatergic receptor subtypes in hippocampus have been shown to express long-term potentiation (LTP), a form of synaptic modification believed to be involved in memory formation. Because of their postsynaptic localization, any differential expression of LTP by the two receptor subtypes would strongly support the existence of a postsynaptic mechanism of LTP expression. In this study, electrophysiological recordings from dentate granule cells were used to compare the potentiation of AMPA and NMDA receptor-mediated responses occurring during the initial phase of LTP, typically identified as STP. Results revealed that high-frequency stimulation (HFS) of perforant path afferents induces a robust STP of both AMPA and NMDA receptor-mediated components of granule cell EPSPs (referred to as AMPA STP and NMDA STP, respectively). Although STP for both receptor subtypes decayed to an asymptotic, steady-state level of LTP and could be induced repetitively, there were substantial differences in several aspects of AMPA and NMDA STP dynamics. STP of the AMPA receptor reached its peak magnitude approximately 30 sec after HFS and decayed with a time constant of approximately 6 min. In contrast, peak magnitude of NMDA STP always appeared immediately after HFS and decayed with a time constant of only 1 min. Single-pulse stimulation of perforant path afferents paired with postsynaptic depolarization also induced LTP of both AMPA and NMDA components. When this induction paradigm was used, however, only the AMPA component showed significant STP. Our results demonstrate that AMPA and NMDA receptors exhibit markedly different degrees of activity-dependent, short-term modifiability, with the possibility that STP of the NMDA receptor reflects primarily post-tetanic potentiation (PTP). In addition, our results strongly suggest that the mechanisms underlying STP of the AMPA receptor are postsynaptic in origin.

Properties of LTP induction in the CA3 region of the primate hippocampus

Activity-dependent changes in synaptic strength, such as long-term potentiation (LTP), have been proposed to underlie memory storage in the brains of all mammals, including humans. However, most forms of synaptic plasticity, including LTP, are studied almost exclusively in rodents and related species. Thus, the hypothesis that LTP is important in human memory relies on the assumption that LTP is similar in the primate and rodent brains. We have begun to test this hypothesis by studying the properties and mechanisms of LTP induction in area CA3 of hippocampal slices from cynomolgus monkeys. We have found that LTP can be induced reliably at both mossy fiber-CA3 and collateral/associational-CA3 synapses in the primate brain, and that the properties of LTP induction at these synapses are similar to what we and others have observed in experiments using hippocampal slices from rodents. Also, we have investigated the role of opioids in mossy fiber synaptic transmission and LTP and have found no effect of the opioid antagonist naloxone nor the opioid agonist dynorphin on mossy fiber synaptic transmission or potentiation. These data suggest that LTP in the primate and rat brains has a similar induction mechanism and, thus, that the rodent is a useful animal model in which to study synaptic modification such as LTP.

Induction of hebbian and non-hebbian mossy fiber long-term potentiation by distinct patterns of high-frequency stimulation

The synapse made by hippocampal mossy fibers onto pyramidal neurons of hippocampal area CA3 displays a form of long-term potentiation (LTP) that is independent of the activation of NMDA receptors. Considerable controversy exists as to whether the induction of mossy fiber LTP requires postsynaptic activation and, thus, whether mossy fiber LTP is Hebbian or non-Hebbian. Here we report the induction of both Hebbian and non-Hebbian forms of long-term potentiation at the mossy fiber-CA3 synapse in in vitro slice preparation. These two forms of potentiation can be induced selectively by different induction conditions. Sustained presynaptic activation is sufficient to induce the non-Hebbian form of mossy fiber LTP, whereas brief presynaptic activation coincident with postsynaptic depolarization is required to induce the Hebbian form. We suggest that non-Hebbian forms of plasticity may play an important role in dynamically regulating the thresholds for inducing Hebbian forms of plasticity.

Dendritic morphology and its effects on the amplitude and rise-time of synaptic signals in hippocampal CA3 pyramidal cells

Detailed anatomical analysis and compartmental modeling techniques were used to study the impact of CA3b pyramidal cell dendritic morphology and hippocampal anatomy on the amplitude and time course of dendritic synaptic signals. We have used computer-aided tracing methods to obtain accurate three-dimensional representations of 8 CA3b pyramidal cells. The average total dendritic length was 6,332 +/- 1,029 microns and 5,062 +/- 1,397 microns for the apical and basilar arbors, respectively. These cells also exhibited a rough symmetry in their maximal transverse and septotemporal extents (311 +/- 84 microns and 269 +/- 106 microns). From the calculated volume of influence (the volume of the neuropil from which the dendritic structures can receive input), it was found that these cells show a limited symmetry between their proximal apical and basilar dendrites (2.1 +/- 1.2 x 10(6) microns 3 and 3.5 +/- 1.1 x 10(6) microns 3, respectively). Based upon these data, we propose that the geometry of these cells can be approximated by a combination of two cones for the apical arbor and a single cone for the basilar arbor. The reconstructed cells were used to build compartmental models and investigate the extent to which the cellular anatomy determines the efficiency with which dendritic synaptic signals are transferred to the soma. We found that slow, long lasting signals show only approximately a 50% attenuation when they occur in the most distal apical dendrites. However, synaptic transients similar to those seen in fast glutamatergic transmission are transferred much less efficiently, showing up to a 95% attenuation. The relationship between the distance along the dendrites and the observed attenuation for a transient is described simply by single exponential functions with parameters of 195 and 147 microns for the apical and basilar arbors respectively. In contrast, there is no simple relation that describes how a transient is attenuated with respect to these cells' stratified inputs. This lack of a simple relationship arises from the radial orientation of the proximal apical and basilar dendrites. When combined, the anatomical and modeling data suggest that a CA3b cell can be approximated in three dimensions as the combination of three cones. The amplitude and time-course for a synaptic transient can then be predicted using two simple equations.

NMDA receptor-dependent LTD in different subfields of hippocampus in vivo and in vitro

In simulations with artificial neural networks, efficient information processing and storage has been shown to require that the strength of connections between network elements has the capacity to both increase and decrease in a use-dependent manner. In contrast to long-term potentiation (LTP) of excitatory synaptic transmission, activity-dependent long-term depression (LTD) has been difficult to demonstrate in forebrain in vivo. Theoretical arguments indicate that coincidence of presynaptic excitation and low-magnitude postsynaptic activation are the necessary prerequisites for LTD induction. Here we report that stimulation paradigms which cause 1) sufficient excitation to result in NMDA receptor activation and simultaneously 2) attenuate the level of postsynaptic activation by recruitment of GABAA receptor-mediated inhibition consistently produce LTD of commissural input to area CA1 in the hippocampus of anesthetized adult rats, and of the perforant path input to the dentate gyrus in the hippocampus of anesthetized and unanesthetized adult rabbits. A functionally similar pre- and postsynaptic activation pattern applied to the hippocampal slice preparation by injecting hyperpolarizing current into the postsynaptic cell during NMDA receptor-mediated excitation also was effective in consistently inducing LTD. Results of studies in vitro show that Ca2+ influx through the NMDA channel is necessary for the induction of LTD, and moreover, that NMDA receptors also participate in the expression of LTD. Our findings demonstrate a general mechanism for the implementation of a theoretically derived learning rule in adult forebrain in vivo and in vitro and provide justification for the inclusion of use-dependent decreases of connection weights in formal models of cognitive processing.

Posttetanic potentiation and presynaptically induced long-term potentiation at the mossy fiber synapse in rat hippocampus

A form of long-term potentiation (LTP) is induced at the mossy fiber (MF) synapse in the hippocampus by high-frequency presynaptic stimulation (HFS). It is generally accepted that induction of this form of LTP (MF LTP) does not depend on postsynaptic Ca2+ current gated by N-methyl-D-aspartate receptors, but it has remained controversial whether induction depends on postsynaptic depolarization and voltage-gated entry of Ca2+. There are also contradictory data on the time course of both LTP and post-tetanic potentiation (PTP), a shorter duration form of potentiation observed at MF synapses immediately following HFS. It has been proposed that some of these differences in results may have arisen because of difficulties in isolating monosynaptic responses to MF input. In the present study, whole cell recording was used to observe excitatory postsynaptic currents (EPSCs) elicited in CA3 pyramidal cells by input from MFs. Postsynaptic cells were dialyzed with 1,2-bis(o-aminophenoxy)-ethane-N,N,N',N'-tetraacetic acid (BAPTA) and F- to inhibit postsynaptic mechanisms that required Ca2+, cells were under voltage clamp during HFS, and conditions were selected to minimize the likelihood of polysynaptic contamination. Under these conditions, HFS nevertheless induced robust LTP (mean magnitude, 62%). The possibility that EPSCs were contaminated by polysynaptic components was investigated by exposing the slices to a suppressing medium (one that partially blocked neurotransmission). EPSC waveforms did not change shape during suppression, indicating that contamination was absent. The LTP observed always was accompanied by prominent PTP that lasted through the first 5 to 15 min following HFS (mean decay time constant, 3.2 min). Induction of this LTP was not cooperative; there was no relationship between the size of responses and the magnitude of the LTP induced. LTP magnitude also was unrelated to the extent to which postsynaptic cells depolarized during HFS. These results show that high rates of presynaptic MF activity elicit robust LTP whether or not there is accompanying postsynaptic depolarization or increase in the concentration of postsynaptic Ca2+. High-frequency MF activity also results in a PTP that is unusually large and long.

Major differences between long-term potentiation and ACPD-induced slow onset potentiation in hippocampus

We examined the effects of a long-lasting application of the selective metabotropic glutamate receptor (mGluR) agonist 1S-3R, 1-amino cyclopentane-1,3-dicarboxylic acid (ACPD) on synaptic potentials recorded from the CA1 and CA3 subfields in hippocampal slices maintained in a superfusion slice chamber. In 25% of the slices, ACPD generated an slow onset potentiation (SOP) of population EPSPs (pEPSPs) in CA1. In contrast to long-term potentiation (LTP) induced by a tetanic train, SOP was accompanied by an increase in the magnitude of the presynaptic fiber volley. Potentiation of the isolated afferent volley suggests that the expression of SOP is due to a recruitment of additional presynaptic fibers by the test stimulus caused by the persistent block of K+ channels by ACPD.

Excitatory stimulation during postsynaptic inhibition induces long-term depression in hippocampus in vivo

1. As part of an effort to evaluate the biological plausibility of theoretically derived principles of synaptic modification, we studied activity-dependent long-term depression (LTD) of glutamatergic transmission in the hippocampus of anesthetized adult rats. Field potentials of CA1 pyramidal cells evoked by single-pulse stimulation (0.1 Hz) of the commissural afferents were recorded before and after paired-pulse stimulation (0.5 Hz) of the same pathway. A train of 150 or 200 paired pulses produced robust LTD of the commissural input to the CA1 pyramidal neurons when the interstimulus interval (ISI) of the pairs was short (25 ms) but not when the ISI was long (1,000 ms). 2. Paired-pulse stimulation with the short but not with the long ISI also was associated with pronounced inhibition of pyramidal cell firing upon the second pulse of a pair, despite the fact that the excitatory input was facilitated with the short-ISI paradigm. The inhibition of pyramidal cell activity was mediated by input to the pyramidal cells from local gamma-aminobutyric acid (GABA)-releasing interneurons activated by commissural fibers and/or CA1 recurrent collaterals, because the inhibition was eliminated by local administration of the selective GABAA receptor antagonist, bicuculline (50 microM), near the recording site. 3. Postsynaptic input from GABAergic interneurons was necessary for the induction of LTD, because short-ISI paired-pulse stimulation failed to produce LTD in the presence of bicuculline. 4. N-methyl-D-aspartate (NMDA) receptor-mediated excitation also was necessary for the induction of LTD, because administration of the selective NMDA receptor antagonist, D-2-amino-5-phosphonvaleric acid (100 microM), near the recording site prevented the development of LTD.(ABSTRACT TRUNCATED AT 250 WORDS)

Morphology of developing rat genioglossal motoneurons studied in vitro: changes in length, branching pattern, and spatial distribution of dendrites

The aim of this study is to describe the postnatal change in dendritic morphology of those motoneurons in the hypoglossal nucleus that innervate the genioglossus muscle. Forty genioglossal (GG) motoneurons from four age groups (1-2, 5-6, 13-15, and 19-30 postnatal days) were labeled by intracellular injection of neurobiotin in an in vitro slice preparation of the rat brainstem and were reconstructed in three-dimensional space. The number of primary dendrites per GG motoneuron was approximately 6 and remained unchanged with age. The development of these motoneurons from birth to 13-15 days was characterized by a simplification of the dendritic tree involving a decrease in the number of terminal endings and dendritic branches. Motoneurons lost their 6th-8th order branches, in parallel with an elongation of their terminal dendritic branches maintaining the same combined dendritic length. The elongation of terminal branches was attributed to both longitudinal growth and the apparent lengthening caused by resorption of distal branches. The elimination of dendritic branches tended to increase the symmetry of the tree, as revealed by topological analysis. Later, between 13-15 days and 19-30 days, there was a reelaboration of the dendritic arborization returning to a configuration similar to that found in the newborn. The length of terminal branches was shorter at 19-30 days, while the length of preterminal branches did not change, suggesting that the proliferation of branches at 19-30 days takes place in the intermediate parts of terminal branches. The three-dimensional distribution of dendrites was analyzed by dividing space into six equal volumes (hexants). This analysis revealed that GG motoneurons have major components of their dendritic tree oriented in the lateral, medial, and dorsal hexants. Further two-dimensional polar analysis (consisting of eight sectors) revealed a reconfiguration of the tree from birth up to 5-6 days involving resorption of dendrites in the dorsal, dorsomedial, and medial sectors and growth in the lateral sector. Later in development (between 13-15 days and 19-30 days), there was growth in all sectors, but of a greater magnitude in the dorsomedial, medial, and dorsolateral sectors.

Asynchrony of mossy fibre inputs and excitatory postsynaptic currents in rat hippocampus

1. Excitatory postsynaptic currents (EPSCs) were studied by whole-cell voltage-clamp recording (WCR) from pyramidal cells in the CA3 field of rat hippocampal slices. Input from mossy fibres was evoked by stimuli applied to stratum granulosum ('dentate gyrus stimulation'). This often resulted in complex, multi-component EPSCs with rise times as long as 5.0 ms (mean = 2.5 ms). In contrast, individual EPSC components typically had rise times between 0.3 and 1.0 ms. 2. To isolate monosynaptic, mossy fibre-driven EPSC components, slices were exposed to 'suppressing' media that reduced response amplitudes by 64-88%. In five out of six cases, long EPSC rising phases (> 3 ms) retained the same shape during suppression. This implied that EPSCs were driven by asynchronously active mossy fibre inputs. 3. From latencies of antidromically driven granule cell population spikes (GCPSs) a mean conduction velocity of 0.67 m/s was inferred. Conduction distance had practically no correlation with GCPS duration, implying that velocity dispersion was small and did not desynchronize mossy fibre impulses. EPSC components exhibited 'surplus' latency; they occurred 0.9-4.8 ms after latencies expected on the basis of direct conduction distances. 4. Mossy fibre volleys (MFVs) were evoked by dentate gyrus stimulation and studied with neurotransmission disabled. MFV negative phases lasted from 2.5 to 4.5 ms and had multiple components. By comparison, negative phases of Schaffer collateral fibre volleys (SCFVs) were always simple in shape and lasted 1.5 ms or less. MFV components had surplus latencies similar to those of EPSC components. Late MFV components did not require high stimulus intensities. 5. Widespread activation of granule cells occurred when stimuli were applied to single loci in the stratum granulosum. This implies that such stimuli elicit antidromic impulses in hilar collaterals of mossy fibres, which could result in activation of orthodromic impulses in mossy fibre trunks that had not been stimulated directly. After anti-, then orthodromic conduction, impulses would arrive in the CA3 subfield with 'surplus' latency. 6. When cuts were made in the hilus to prevent anti-/orthodromic conduction, MFV durations were reduced, but only to a small extent. This implies that surplus latency and asynchrony arise in part by anti-/orthodromic conduction, and partly by a mechanism that is intrinsic to mossy fibres or their 'giant' boutons. 7. Because of desynchronization of mossy fibre inputs, there probably are significant differences between kinetic properties of averaged, compound mossy fibre EPSCs and those of unitary mossy fibre EPSCs (i.e. currents driven by input from single presynaptic axons).(ABSTRACT TRUNCATED AT 400 WORDS)

In vitro electrophysiology of developing genioglossal motoneurons in the rat

1. Experiments were performed to determine the change in membrane properties of genioglossal (GG) motoneurons during development. Intracellular recordings were made in 127 GG motoneurons from rats postnatal ages 1-30 days. 2. The input resistance (R(in)) and the membrane time constant (t(aum)) decreased between 5-6 and 13-15 days from 84.8 +/- 25.4 (SD) to 47.0 +/- 18.9 M omega (P < 0.01) and from 10.0 +/- 4.2 to 7.3 +/- 3.3 ms (P < 0.05), respectively. During this period, the rheobase (Irh) increased (P < 0.01) from 0.13 +/- 0.07 to 0.27 +/- 0.14 nA, and the percentage of cells exhibiting inward rectification increased from 5 to 40%. Voltage threshold (Vthr) of the action potential remained unchanged postnatally. 3. There was also a postnatal change in the shape of the action potential. Specifically, between 1-2 and 5-6 days, there was a decrease (P < 0.05) in the spike half-width from 2.23 +/- 0.53 to 1.45 +/- 0.44 ms, resulting, in part, from a steepening (P < 0.05) of the slope of the falling phase of the action potential from 21.6 +/- 10.1 to 32.9 +/- 13.1 mV/ms. The slope of the rising phase also increased significantly (P < 0.01) between 1-2 and 13-15 days from 68.4 +/- 31.0 to 91.4 +/- 44.3 mV/ms. 4. The average duration of the medium afterhyperpolarization (mAHPdur) decreased (P < 0.05) between 1-2 (193 +/- 53 ms) and 5-6 days (159 +/- 43 ms). Whereas the mAHPdur was found to be independent of membrane potential, there was a linear relationship between the membrane potential and the amplitude of the medium AHP (mAHPamp). From this latter relationship, a reversal potential for the mAHPamp was extrapolated to be -87 mV. No evidence for the existence of a slow AHP was found in these developing motoneurons. 5. All cells analyzed (n = 74) displayed adaptation during the first three spikes. The subsequent firing pattern was classified into two groups, adapting and nonadapting. Cells at birth were all adapting, whereas all cells but two from animals 13 days and older were nonadapting. At the intermediate age (5-6 days), the minority (27%) was adapting and the majority (73%) was nonadapting. 6. The mean slope of primary range for the first interspike interval (1st ISI) was approximately 90 Hz/nA. This value was similar for both adapting and nonadapting cells and did not change postnatally.(ABSTRACT TRUNCATED AT 400 WORDS)

Aspartate and glutamate mediate excitatory synaptic transmission in area CA1 of the hippocampus

We examined whether L-aspartate (ASP) and L-glutamate (GLU) both function as endogenous neurotransmitters in area CA1 of the rat hippocampus. Radioligand displacement experiments using 3H-DL-alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (3H-AMPA) to label AMPA/kainate receptors and 3H-cis-4-phosphonomethyl-2-piperidine carboxylic acid (3H-CGS-19755) to label NMDA receptors confirmed that GLU (Ki approximately 500 nM) but not ASP (Ki > 1 mM) has high affinity for AMPA/kainate receptors whereas GLU (Ki approximately 250 nM) and ASP (Ki approximately 1.3 microM) both have high affinity for NMDA receptors. Elevating extracellular potassium concentration (50 mM, 1 min) evoked the calcium-dependent release of both ASP (approximately 50% increase) and GLU (approximately 200% increase) from hippocampal slices and from minislices of area CA1. Reducing extracellular glucose concentration (0.2 mM) reduced GLU release, enhanced ASP release, and reduced AMPA/kainate receptor-mediated responses more than NMDA receptor-mediated responses (to 7% and 34% of control, respectively). Fiber volleys, antidromic population spikes, membrane potential, input resistance, and ATP content all were not affected by glucose reduction. Unlike low glucose, the inhibitory neuromodulator adenosine (5 microM), which reduces ASP and GLU release to a similar extent, reduced AMPA/kainate and NMDA receptor-mediated population EPSPs similarly (to 11% and 12% of control, respectively). AMPA/kainate and NMDA receptor-mediated population EPSPs were also similarly reduced by 0.4 microM TTX (to 32% and 22% of control, respectively) and similarly enhanced by 10 microM 4-aminopyridine (to 206% and 248% of control, respectively). Finally, NMDA receptor-mediated EPSCs measured by whole-cell recording decayed faster in low glucose (73 msec vs 54 msec) but not in adenosine (73 msec vs 78 msec). Together, these results confirm that ASP and GLU are both involved in excitatory synaptic transmission at the Schaffer collateral-commissural terminals in area CA1 of the rat hippocampus.

Potassium-induced long-term potentiation in rat hippocampal slices

We observed that a transient increase in extracellular potassium concentration (50 mM for 40 s) was sufficient to induce long-term potentiation (LTP) of synaptic transmission in area CA1 of the hippocampal slice. Potassium-induced potentiation of the Schaffer collateral/commissural synapses demonstrated several features characteristic of tetanus-induced LTP: (1) population excitatory post-synaptic potential (EPSP) amplitudes were enhanced to a similar magnitude (on average 70% above baseline) which (2) lasted for more than 20 min; (3) induction was blocked by bath application of the specific N-methyl-D-aspartate (NMDA) receptor antagonist D-2-amino-5-phosphonovalerate (D-APV), and (4) was attenuated by reduction of the concentration of calcium in the extracellular medium. Induction of either potassium-induced LTP or tetanus-induced LTP occluded the subsequent expression of the other. Finally, exposure to high potassium in the absence of electrical stimulation was sufficient to induce LTP. Taken together, these data indicate that brief depolarizing stimuli other than tetanus can induce LTP. Because potassium-induced LTP is not restricted to the subset of afferents examined electrophysiologically, such a method could facilitate analyses of the biochemical events underlying both the induction and expression of LTP.

Isolated NMDA receptor-mediated synaptic responses express both LTP and LTD

1. The possibility of use-dependent, long-lasting modifications of pharmacologically isolated N-methyl-D-aspartate (NMDA) receptor-mediated synaptic transmission was examined by intracellular recordings from granule cells of the hippocampal dentate gyrus in vitro. In the presence of the non-NMDA receptor antagonist 6-cyano-7-nitroquinaxaline-2,3-dione (CNQX, 10 microM) robust, long-term potentiation (LTP) of NMDA receptor-mediated synaptic potentials was induced by brief, high (50 Hz) and lower (10 Hz) frequency tetanic stimuli of glutamatergic afferents (60 +/- 6%, n = 8, P less than 0.001 and 43 +/- 12%, n = 3, P less than 0.05, respectively). 2. Hyperpolarization of granule cell membrane potential to -100 mV during 50-Hz tetanic stimuli reversibly blocked the induction of LTP (-6 +/- 2%, n = 6, P greater than 0.05) indicating that simultaneous activation of pre- and postsynaptic elements is a prerequisite for potentiation of NMDA receptor-mediated synaptic transmission. In contrast, hyperpolarization of the granule cell membrane potential to -100 mV during 10-Hz tetanic stimuli resulted in long-term depression (LTD) of NMDA receptor-mediated synaptic potentials (-34 +/- 8%, n = 8, P less than 0.01). 3. We also studied the role of [Ca2+]i in the induction of LTP and LTD of NMDA receptor-mediated synaptic responses. Before tetanization, [Ca2+]i was buffered by iontophoretic injections of bis-(o-aminophenoxy)-N,N,N',N'-tetraacetic acid (BAPTA). BAPTA completely blocked the induction of LTP (3 +/- 5%, n = 13) and partially blocked LTD (-14.8 +/- 6%, n = 10).(ABSTRACT TRUNCATED AT 250 WORDS)

Actions of phosphomonoesters on CA1 hippocampal neurons as revealed by a combined electrophysiological and nuclear magnetic resonance study

Phosphomonoesters (PMEs), precursors of membrane phospholipids, are found in high levels in the developing brain and Alzheimer's disease brain. The present study details the neurophysiological and metabolic effects of acute PME elevation on the Fisher 344 rat in vitro hippocampal slice. Two abundant PMEs, phosphoethanolamine (PE) and L-phosphoserine (PS), reliably altered properties of synaptic transmission at the Schaffer collateral/commissural-CA1 cell synapse. Specifically, PE reversibly depressed the amplitude of population EPSPs at millimolar concentrations but had no effect at micromolar concentrations. PS had biphasic effects on population EPSPs, inducing first a reduction followed by an enhancement of response amplitude. In contrast to PE, the effects of PS were not reversible; population EPSPs were augmented during the wash of PS, and the CA3 region generated evoked (but not spontaneous) epileptiform discharges. 31P nuclear magnetic resonance spectroscopy revealed enhanced slice uptake of PS compared to PE. There was no significant effect of PE on slice high-energy phosphates but incubation with PS significantly lowered slice phosphocreatine (PCr) and ATP concentrations. These observations indicate that the slice uptake of PS could be energy requiring and the enhanced response amplitude observed at 5 mM PS also could produce a drain on high-energy phosphates. Possible modes of PME action on hippocampal physiology are discussed.

Heterosynaptic correlates of long-term potentiation induction in hippocampal CA3 neurons

Previous studies have demonstrated that tetanization of hippocampal mossy fibers induces a long-term potentiation of non-tetanized (heterosynaptic) non-mossy fiber afferents (Schaffer collateral/commissural and fimbrial fibers). Tetanization of these non-mossy fiber afferents, in contrast, does not induce mossy fiber long-term potentiation, but induces a long-term depression of mossy fiber responses (Bradler and Barrionuevo, Synapse 4, 132-142, 1989). The synaptic activity necessary to evoke these heterosynaptic alterations of efficacy is not known. Specifically, the dependence of heterosynaptic efficacy on the activation of N-methyl-D-aspartate receptors has not been assessed. In addition, the capability of different afferents to CA3 neurons to support alterations in heterosynaptic efficacy remains largely unknown. In the present study, heterosynaptic alterations of efficacy in the rat did not require the activation of N-methyl-D-aspartate receptors. Mossy fibers supported N-methyl-D-aspartate receptor-independent heterosynaptic long-term depression, and N-methyl-D-aspartate receptor-independent long-term potentiation. In contrast, non-mossy fiber afferents expressed N-methyl-D-aspartate receptor-independent heterosynaptic long-term potentiation induced by a mossy fiber tetanus, and an N-methyl-D-aspartate receptor-independent long-term depression, in addition to N-methyl-D-aspartate receptor-dependent homosynaptic long-term potentiation. The possibility that non-N-methyl-D-aspartate receptor activity in non-tetanized afferents is necessary for heterosynaptic long-term potentiation induction is discussed. Heterosynaptic long-term depression was induced in the absence of homosynaptic long-term potentiation, suggesting that these concomitant forms of synaptic plasticity rely on different mechanisms.

Conductance mechanism responsible for long-term potentiation in monosynaptic and isolated excitatory synaptic inputs to hippocampus

The biophysical mechanisms underlying long-term potentiation (LTP) were investigated in identifiable and monosynaptic excitatory inputs to hippocampal neurons. The results provide the first insights into the conductance changes that are responsible for the expression of LTP. Both current- and voltage-clamp measurements of the mossy fiber synaptic response in pyramidal neurons of region CA3 were made with a single-electrode-clamp system. The excitatory postsynaptic response was pharmacologically isolated by bathing hippocampal slices in saline containing 10 microM picrotoxin, which blocks the synaptic inhibition that normally accompanies the experimentally evoked mossy fiber response. LTP was induced by tetanically stimulating the mossy fiber input for 1 s at 100 Hz. Before and 20 min to 1 h after inducing LTP, we attempted to measure the mean excitatory postsynaptic potential (EPSP) amplitude, intrasomatic current-voltage relationship to a step (RN) or alpha function (AN) current waveform, membrane time constant (tau m), spike threshold (T50), peak excitatory postsynaptic current amplitude (IP), synaptic conductance increase (delta G), and synaptic reversal potential (VR); but adequate assessments of all eight of these were not always obtained for every cell that was studied. The induction of LTP increased the mean (+/- SE) EPSP amplitude form 10.5 +/- 1.4 mV during the control period to 16.8 +/- 2.4 mV after the induction of LTP (n = 14; P less than 0.05). This change was not accompanied by increases in the mean value of RN (63 +/- 11 M omega before and 61 +/- 11 M omega after induction; n = 8; P greater than 0.05); AN, which approximates the effective synaptic input resistance at the soma (10.0 +/- 1.50 M omega before and 10.5 +/- 1.60 M omega after; n = 10; P greater than 0.05); or tau m (22 +/- 2 ms before and 20 +/- 2 ms after; n = 8; P greater than 0.05). There was no significant change in T50, which was also assessed with an alpha function current waveform (1.48 +/- 0.11 nA before and 1.49 +/- 0.10 nA after; n = 6; P greater than 0.05). The mean value of IP increased from 1.1 +/- 0.2 nA during the control period to 1.8 +/- 0.3 nA after inducing LTP (n = 15; P less than 0.05). Similarly, delta G increased from 30 +/- 4 nS before to 47 +/- 4 nS after induction (n = 10; P less than 0.05).(ABSTRACT TRUNCATED AT 250 WORDS)

Neurophysiological changes in the in vitro rat hippocampus following chronic lithium administration

The effect of chronic lithium exposure on the electrophysiological responses of the Schaffer collateral-commissural (SCC) input to the CA1 pyramidal neurons in the in vitro hippocampus was investigated. Experimental animals were intubated intragastrically with lithium carbonate (150 mg/kg) for 3-4 weeks. This treatment produced lithium levels in serum and hippocampus of 0.3-1.3 mM. During the recording period, the hippocampal slice retained a stable lithium concentration of 53% of the initial value. Chronic lithium exposure had a depressive effect on input/output relationships, paired-pulse facilitation and strength of orthodromic inhibition. The antidromic inhibition was virtually intact. No obvious differences were found between control and lithium slices in amplitude, latency or waveforms, of synaptic and antidromic extracellular potentials. These findings are compatible with a major action of lithium on the excitability of SCC axons and synaptic terminals.

Associative long-term potentiation in hippocampal slices

Interactions between two excitatory monosynaptic inputs to hippocampal neurons of the CA1 region were examined in the in vitro slice. By adjusting the strengths of the electrical stimuli delivered to the two input pathways, one was made to generate a weak and the other a strong synaptic response. Simultaneous tetanic stimulation of both input pathways resulted in a subsequent long-term enhanced synaptic efficacy in the weak input under conditions in which the same tetanic stimulation of either input alone failed to have this effect. This form of long-term synaptic potentiation (LTP), known as associative LTP, was shown in some cases to last hours without decrement. The plastic changes were localized within the CA1 region and appear to reside in the pre- or postsynaptic elements of the monosynaptic excitatory input to the pyramidal neurons. The increased synaptic efficacy could not be accounted for by any of several measured postsynaptic passive membrane properties.

Intracellular injections of EGTA block induction of hippocampal long-term potentiation

Hippocampal long-term potentiation (LTP) is a remarkably stable facilitation of synaptic responses resulting from very brief trains of high-frequency stimulation. Because of its persistence and modest induction conditions, LTP represents a promising candidate for a substrate of memory. Some progress has been made in localizing the changes responsible for the effect; for example, it has been shown that LTP is not accompanied by changes in the fibre volleys of the test afferents or by generalized alterations of the dendrites of their target cells. However, it is unknown whether the potentiation is due to pre- or postsynaptic changes and there is evidence in favour of each (for example, see refs 5, 6). We now report that intracellular injections of the calcium chelator EGTA block the development of LTP. These results strongly suggest that LTP is caused by a modification of the postsynaptic neurone and that its induction depends on the level of free calcium.

Lateral geniculate nucleus unitary discharge in sleep and waking: state- and rate-specific aspects

The relationship between behavioral state, discharge pattern, and discharge rate was investigated in 26 lateral geniculate nucleus (LGN) units recorded in cats in the dark during waking (W), synchronized sleep (S), and desynchronized sleep (D). A distinctive state-dependent discharge pattern was the presence of stereotyped bursts of 2-7 spikes that occurred in 63% of the units. These bursts were most frequent in S, much less frequent in D, and rarely occurred in W. Lack of association with discharge rate changes between states showed the bursting to be a true state-dependent phenomenon. A burst consisted of 2-7 spikes, with each successive interspike interval being longer than the preceding one; in the 200 ms prior to burst occurrence, discharge probability decreased markedly. This structure of burst organization suggested a model of generation wherein each burst was caused by a unitary event of varying intensity, perhaps a rebound following a hyperpolarization. Spectral and autocorrelational analyses showed bursts occurred rhythmically in three cells at a frequency of 3-4 Hz and in two cells at a frequency of 10-12 Hz, indicating a possible linkage with slow-wave generators. While the number of bursts in the various behavioral states was a state-dependent phenomena, other aspects of discharge pattern were shown to be rate dependent. To evaluate discharge pattern apart from the occurrence of bursts, a "primary event spike train" was formed; this consisted of individual spikes and the first spike of each burst. This analysis showed that, within S, the probability of burst occurrence was highest when the primary spike rate was low. Quantitative analyses showed that first-order pattern measures (the form of the interspike interval histogram, IH) were dependent on the mean interspike interval (ISI, the inverse of mean rate). This association explained 83-89% of the variance in a power series approximation of IH form. Joint interval histograms (JIH) were used to evaluate the signature of bursts and of the form of the primary spike train. As with interval histograms, the main features of the form of the primary spike JIH were dependent on the primary spike rate. Thus, we concluded that first- and second-order discharge patterns of primary events were rate dependent and not state dependent. Our data are compatible with a model where in the absence of retinal input, the frequency of LGN primary spikes over behavioral state changes is largely determined by brain stem reticular formation input.(ABSTRACT TRUNCATED AT 400 WORDS)

Vincamine: a psychogeriatric agent blocking synaptic potentiation in hippocampus

The action of vincamine on the physiology of the CA1 region of the in vitro hippocampal slice preparation was investigated. At concentrations of 1, 10 and 100 microM, a five-minute perfusion with vincamine did not affect the synaptically-mediated activation of pyramidal neurons evoked by stimulation of the Schaffer-commissural fiber system. The effect of vincamine on the excitability of the pyramidal neurons was investigated by studying its effect on the antidromically-elicited field potential and the input-output relation of Schaffer-commissural fiber input. No effect on either of the two parameters was seen at a concentration of 100 microM of vincamine. Vincamine did, however, attenuate both the post-tetanic (PTP) and long-term potentiation (LTP) evoked by repetitive stimulation of the Schaffer-commissural fiber system. At a concentration of 100 microM of vincamine, PTP was significantly reduced and LTP was almost completely suppressed.

Negative effects of chronic hemicerebellectomy of epileptiform after-discharges elicited by focal cortical stimulation in baboons (Papio papio)

The influence of chronic hemicerebellectomy on cortical epileptiform after-discharge (AD) induced by focal electrical stimulation was studied in the baboon. These preliminary results include 22 ADs elicited from motor cortex and 22 ADs elicited from premotor cortex before and after hemicerebellectomy. Only full-developed, generalized seizures with postictal silence were considered. EEG morphology, average duration and average current threshold were compared for each set of ictal events. No significant differences were found before and after hemicerebellectomy.