Potentiation of synaptic transmission in the rat hippocampal slice by exogenous L-glutamate and selective L-glutamate receptor subtype agonists

Deficits in ERK and CREB activation in the hippocampus after traumatic brain injury

Traumatic brain injury (TBI) activates several protein kinase signaling pathways in the hippocampus that are critical for hippocampal-dependent memory formation. In particular, extracellular signal-regulated kinase (ERK), a protein kinase activated during and necessary for hippocampal-dependent learning, is transiently activated after TBI. However, TBI patients experience hippocampal-dependent cognitive deficits that occur for several months to years after the initial injury. Although basal activation levels of ERK return to sham levels within hours after TBI, we hypothesized that activation of ERK may be impaired after TBI. Adult male Sprague-Dawley rats received either sham surgery or moderate parasagittal fluid-percussion brain injury. At 2, 8, or 12 weeks after surgery, the ipsilateral hippocampi of sham surgery and TBI animals were sectioned into transverse slices. After 2h of recovery in oxygenated artificial cerebrospinal fluid, the hippocampal slices were stimulated with glutamate or KCl depolarization, then analyzed by western blotting for phosphorylated, activated ERK and one of its downstream effectors, the transcription factor cAMP response element-binding protein (CREB). We found that activation of ERK (p<0.05) and CREB (p<0.05) after 30s of glutamate stimulation or KCl depolarization was decreased in hippocampal slices from animals at 2, 8, or 12 weeks after TBI as compared to sham animals. Basal levels of phosphorylated or total ERK were not significantly altered at 2, 8, or 12 weeks after TBI, although basal levels of phosphorylated CREB were decreased 12 weeks post-trauma. These results suggest that TBI results in chronic signaling deficits through the ERK-CREB pathway in the hippocampus.

Methods of detection of the transcription factor NF-kappa B in rat hippocampal slices

The hippocampus is one of the most studied sites for understanding the cellular and molecular mechanisms underlying long-term potentiation (LTP) and long-term depression (LTD), mechanisms believed to underlie the formation and storage of memories. The early-phases of LTP and LTD have been most intensively studied and have been shown to involve the activation of several kinases and phosphatases, respectively. The factors involved in the later stages have largely yet to be elucidated. We have focused our attention on the transcription factor NF-kappaB as a possible factor involved in such late-phase processes, and have developed both immunocytochemistry and electrophoretic mobility shift assay (EMSA) to measure the activated forms of this factor. This is important as many of the studies in this area are performed in vitro and to our knowledge quantitative assessment has not previously being deemed feasible in slice work. The pro-inflammatory cytokines TNF-alpha and IL-1beta both led to pronounced nuclear activation of NF-kappaB in the dentate granule cells as demonstrated by immunostaining and EMSA, respectively. Electrophysiological measurements taken from slices treated with TNF-alpha showed that it inhibited LTP (field excitatory post-synaptic potentials (fEPSP) 116+/-10%, n = 9, 60 min post-tetanus compared to control fEPSP 185+/-9%, n = 6; P < 0.001). The neurotransmitter L-glutamate also led to activation of NF-kappaB and electrophysiology recordings showed a small but sustained increase in synaptic transmission (fEPSP 106+/-12%, 30 min post-drug). These methods provide valuable tools to forward our understanding of the role of NF-kappaB in plasticity as well as in many neurological disorders being mimicked by in vitro studies.

Developmental regulation of hippocampal excitatory synaptic transmission by metabotropic glutamate receptors

The aims of this study were, to use agonists selective for the 3 mGlu receptor groups to identify developmental changes in their effects, and to assess the usefulness of proposed selective antagonists as pharmacological tools. Hippocampal slices (400 microm) were prepared from neonate (9 - 14 days) and young adult (5 - 7 weeks) Sprague-Dawley rats. Field excitatory postsynaptic potentials (fEPSP) were recorded from CA1. DHPG (100 microM), a group I agonist, produced a slowly developing enhancement of fEPSP slope in slices from adults. In slices from neonates, DHPG (75 microM) depressed fEPSP slope. DCG-IV (500 nM), a group II agonist, did not affect the fEPSP recorded from slices from adults whereas perfusion in neonate slices produced a sustained depression. The group III agonist L-AP4 (50 microM) was ineffective in adult slices but depressed fEPSP slope in slices prepared from neonates. DHPG-induced depression of fEPSP slope was inhibited by 4-CPG (400 microM), a group I antagonist, but was unaffected by MCCG (500 microM) and MAP4 (500 microM), group II and III receptor antagonists respectively. MCCG but not MAP4 antagonized the effects of DCG-IV with 4-CPG producing variable effects. The effect of L-AP4 was unaffected by MCCG, blocked by MAP4, and enhanced by 4-CPG. The results show that the effects of the agonists for all groups of mGlu receptors are developmentally regulated. Furthermore, MCCG and MAP4 behave as effective and selective antagonists for group II and group III mGlu receptors respectively, whereas the usefulness of 4-CPG as a group I antagonist may be limited.

Dantrolene modulates the influence of steady magnetic fields on hippocampal evoked potentials in vitro

Direct current-generated magnetic fields (2-3 mT, 20-min exposure) exerted biphasic effects on the population spike recorded from hippocampal slices. The initial decrease in the potential, observed during exposure of the slices to magnetic fields was followed by a recovery/amplification phase, which began after terminating the magnetic field action. During that phase the population spike exceeded the amplitude observed before application of the magnetic fields. The pattern of magnetic fields influence was not affected either by (+)-5-methyl-10,11-dihydro-5H-dibenzo (a,d) cyclohepten-5, 10-imine maleate (MK801), or by D,L,-2amino-5phosphonovalerate (APV), a noncompetitive and competitive NMDA receptor antagonist, respectively. The rising phase of the potential, however, was eliminated by dantrolene, an inhibitor of intracellular Ca(2 +) channels. This suggests that intracellular calcium channels participate in the mechanism of the influence of the direct current magnetic fields on the function of the hippocampal tissue.

Diminution of N-methyl-D-aspartate-induced perturbation of neurotransmission by dexmedetomidine in the CA1 field of rat hippocampus in vitro

The effects of alpha(2)-adrenoceptor activation on N-methyl-D-aspartic acid (NMDA)-induced perturbation of neurotransmission and normal NMDA-receptor dependent function (long-term potentiation, (LTP)) were investigated in the hippocampal CA1 field in vitro. Bath perfusion of dexmedetomidine hydrochloride (50 nM), which was initiated before NMDA (100 microM) exposure, enhanced the extent of recovery of extracellular field excitatory postsynaptic potentials after NMDA infusion. On the other hand, the induction and early maintenance of LTP was normal in the presence of dexmedetomidine. Thus, dexmedetomidine can diminish acute NMDA-induced perturbation of neurotransmission while the same dose of this drug does not influence the normal activation of NMDA receptors.

An in vitro blood-brain barrier model: cocultures between endothelial cells and organotypic brain slice cultures

This communication describes a novel in vitro blood-brain barrier (BBB) model: organotypic slice cultures from the central nervous system were overlaid on endothelial cell monolayers grown on permeable membranes. Morphological, electrophysiological, and microdialysis approaches were carried out to characterize and validate this model. After 10 days in coculture, morphological studies reveal the presence of tight junctions. Electrophysiological recordings of neuronal activity performed on organotypic cultures with or without an endothelial cell monolayer show that amplitude of evoked responses were comparable, indicating good viability of cocultures after 2 weeks. Perfusion of known BBB permeable or nonpermeable molecules was used to test the coculture tightness in conjunction with electrophysiological or microdialysis approaches: application of glutamate (Glu), which doesn't easily cross the BBB, triggers off rhythmic activity only in control cultures, whereas epileptogenic activity was observed in both control cultures and cocultures during perfusions with picrotoxin, a molecule that can diffuse through the BBB. Finally, the microdialysis technique was used to determine the permeability of molecules coming from the perfusion chamber: L-dopa, dopamine, and Glu were employed to assess the selective permeability of the coculture model. Thus, these results indicate that the in vitro model described possesses characteristics similar to those of the BBB in situ and that cocultures of organotypic slices and endothelial cell monolayers have potential as a powerful tool for studying biochemical mechanisms regulating BBB function and drug delivery to the central nervous system.

Arachidonic acid metabolites and the synaptic potentiation evoked by activation of metabotropic glutamate receptors

We have previously shown that coapplication of arachidonic acid (10 microM) and (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid (ACPD, 50 microM) evokes an enhancement of synaptic transmission in the CA1 region of the rat hippocampal slice. Here we have investigated whether the metabolites of arachidonic acid are implicated in this potentiation. Inclusion of the cyclo-oxygenase inhibitor indomethacin (10 microM) did not block the potentiation induced by coapplication of arachidonic acid and ACPD. However, the presence of either the cyclo-, lipo- and epoxygenase inhibitor 5,8,11,14-eicosatetraynoic acid (ETYA, 20 microM), or the lipoxygenase inhibitor nordihydroguaiaretic acid (10 microM), prevented the long-lasting enhancement. The results suggest that the lipoxygenase and epoxygenase metabolites of arachidonic acid may be involved in the induction of this form of synaptic potentiation.

Developmental and sensory-dependent changes of phosphoinositide-linked metabotropic glutamate receptors

Metabotropic glutamate receptors (mGluRs) can modulate synaptic transmission, and there is evidence that phosphoinositide (PI)-linked mGluRs may be involved in sensory-dependent plasticity during the development of cat visual cortex. Consequently, we asked the questions: Where are the PI-linked mGluRs (mGluR1alpha and mGluR5) in the visual cortex? Does the quantity and distribution of these receptors change in the cat visual cortex during postnatal development, and are these features sensory-dependent? We found that the quantity of mGluR1alpha decreases with age, whereas the laminar distribution of mGluR1alpha remains the same. Quantity of mGluR5 also decreases, but the laminar distribution of mGluR5 changes during development. The pattern and timing of the mGluR5 change in distribution follow the development of geniculocortical afferents. Immunostaining indicates that reduction of receptor occurs mainly in layers V-VI for mGluR1alpha and outside layer IV for mGluR5. Dark-rearing postpones the laminar change of mGluR5 and produces an increased level of mGluR5 between postnatal 1.5-6 weeks of age but has no significant effect on the mGluR1alpha distribution or the mGluR1alpha quantity. These results suggest that mGluR1alpha and mGluR5 are involved in different aspects of cortical development. The mGluR5 is more likely to be involved in sensory-dependent events than mGluR1alpha. The lack of developmental correlation between mGluR quantities and the critical period for ocular dominance plasticity also suggests that other factors besides mGluR quantities are important for ocular dominance plasticity.

Proline-induced potentiation of glutamate transmission

The amino acid proline has long been suspected to serve as a modulator of synaptic transmission in the mammalian brain, but no such function has been identified. The selective expression of high affinity proline transport by a subset of glutamate pathways suggested that proline might play a role in synaptic transmission at these sites. This idea was tested with use of one such pathway, the Schaffer collateral-commissural projection to CA1 pyramidal cells of the rat hippocampus. Proline enhanced the initial slope of the field EPSP without affecting axonal excitability or the magnitude of paired-pulse facilitation. Proline-induced potentiation far outlasted the period of proline application and required the activation of NMDA receptors. Proline enhanced Schaffer collateral-commissural synaptic transmission even when the connections between areas CA1 and CA3 had been interrupted. Potentiation was observed with a proline concentration normally present in human CSF (3 microM). A concentration typical of CSF in persons with the genetic disorder hyperprolinemia type II (30 microM) produced a somewhat greater effect. Occlusion experiments suggested that proline-induced potentiation and tetanus-induced long-term potentiation utilize largely distinct transduction mechanisms. Proline-induced potentiation could be blocked by a prior high frequency stimulus, whether or not the stimulus evoked long-term potentiation. These results suggest that endogenous extracellular proline regulates the basal function of some glutamate synapses by maintaining them in a partially potentiated state. They may also facilitate understanding of the seizures and/or mental retardation associated with genetic disorders of proline metabolism.

Long-term changes in glial fibrillary acidic protein and glutamine synthetase immunoreactivities in the supraoptic nucleus of portacaval shunted rats

The present study was undertaken to ascertain whether, and to what extent, glial fibrillary acidic protein (GFAP) and glutamine synthetase (GS) expressions in the supraoptic nucleus (SON) could be modulated after one month and six months of portacaval shunting (PCS) in rats. GFAP and GS immunoreactivities were significantly higher in PCS rats than in control rats at one and six months. The increased GFAP and GS immunoreactivities observed in the SON astrocytes were directly related to the duration of PCS. In PCS rats, the number and length of both GFAP and GS immunopositive astroglial processes increased not only in the hypothalamic nucleus but in the perinuclear zone, where glutamatergic pathways have been described, whereas GFAP and GS expressions decreased in the ventral glial lamina. Since GS is one of the glutamate metabolizing enzymes and the SON is one of the areas of glutamatergic activity, our results show that astrocytes respond differentially to glutamate toxicity. This suggests that overexpression of GFAP and GS immunoreactivities could be associated with glutamatergic neurotransmission disorders.

Repeated electroconvulsive stimulation impairs synaptic plasticity in the dentate gyrus in vivo but has no effect in CA1 in vitro

Repeated electroconvulsive stimulation (ECS) spaced at 48 h intervals significantly increased the synaptic response in the dentate gyrus in vivo, as measured by input/output curves, and reduced the degree of long-term potentiation (LTP) obtained following high frequency stimulation. An identical course of ECS had no effect on synaptic responses recorded in the stratum radiatum of CA1 in vitro and did not impair high frequency-induced LTP. These results suggest that either ECS has a selective effect on the sub-fields of the hippocampus or that in vitro recording techniques are unsuitable for detecting the increase in synaptic efficacy produced by the treatments.

Concurrent blockade of hippocampal metabotropic glutamate and N-methyl-D-aspartate receptors disrupts working memory in the rat

In order to clarify the roles of hippocampal metabotropic glutamate and N-methyl-D-aspartate receptors in working and reference memory performance of rats, the effects of intrahippocampal administration of selective antagonists for both receptors on these behaviours were examined with a three-panel runway task. In the working memory task, the potent and competitive N-methyl-D-aspartate receptor antagonist, (+/-)-3-(2-carboxypiperazin-4-yl)propyl-1-phosphonic acid (CPP), significantly increased the number of errors (attempts to pass through two incorrect panels of the three panel-gates at four choice points), when injected bilaterally at 10 and 32 ng/side into the dorsal hippocampus. Intrahippocampal injection of CPP at a dose of 3.2 ng/side had no effect on the number of working memory errors. The metabotropic glutamate receptor antagonist, (+)-alpha-methyl-4-carboxyphenylglycine (+)-(MCPG), injected into the hippocampus at doses up to 3.2 micrograms/side, did not significantly affect the number of working memory errors. Combined administration of (+)-MCPG (3.2 micrograms/side) and CPP (3.2 ng/side) into the hippocampus, neither of which had an individual effect on errors, significantly increased the number of working memory errors. However, intrahippocampal administration of the relatively inactive isomer, (-)-MCPG, at 3.2 micrograms/side did not affect working memory errors, whether given independently or concurrently with the behaviourally ineffective dose of CPP (3.2 ng/side). In the reference memory task, intrahippocampal injection of CPP at doses up to 32 ng/side had no effect on the number of errors. Intrahippocampal (+)-MCPG at doses up to 3.2 micrograms/side did not affect the number of reference memory errors, whether administered alone or together with 3.2 ng/side of CPP. These results indicate that blockade of hippocampal metabotropic glutamate receptors aggravates impairment of working memory resulting from deficiency of N-methyl-D-aspartate receptor-mediated glutamatergic neurotransmission, suggesting that mechanisms regulated by co-activation of hippocampal metabotropic glutamate and N-methyl-D-aspartate receptors are involved in working memory performance of rats.

Class I metabotropic glutamate receptor agonists do not facilitate the induction of long-term potentiation in the dentate gyrus of the rat in vitro

The possibility that activation of class I (phospholipase C-coupled) metabotropic glutamate receptors (mGluRs) can facilitate the induction of long-term potentiation (LTP) was investigated in the dentate gyrus of rat hippocampal slices. In the presence of picrotoxin, a weak tetanus led to a short-term potentiation (STP) lasting 10-15 min. Application of the class I mGluR agonists trans-azetidine-2,4-dicarboxylic acid (tADA, 100 microM) or 3,5-dihydroxyphenylglycine (DHPG, 100 microM) for 15 or 30 min before the weak tetanus did not affect baseline synaptic transmission or the magnitude of the subsequent potentiation. DHPG (70 microM) did, however, reduce accommodation of neuronal firing in response to depolarizing current injection. These results suggest that at the medial perforant path-granule cell synapse, class I mGluR activation by exogenous agonist application does not facilitate the induction of LTP.

Interactions between arachidonic acid and metabotropic glutamate receptors in the induction of synaptic potentiation in the rat hippocampal slice

Perfusion of neither the metabotropic glutamate receptor agonist (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid (ACPD), nor arachidonic acid caused any long-term enhancement of synaptic transmission in the CA1 region of the rat hippocampal slice. However, co-perfusion of ACPD (50 microM) and arachidonic acid (10 microM) for 5 min induced a rapidly evoked and long-lasting enhancement of synaptic transmission. This enhancement persisted in the presence of D(-)-2-amino-5-phosphonopentanoic acid (40 microM) and is therefore independent of NMDA receptor activation. The potentiation was mimicked by perfusion of the phospholipase A2 activator melittin (10 micrograms/ml) for 5 or 10 min, or exogenous phospholipase A2 (1 microgram/ml) for 5 min, immediately before ACPD application. We propose a role for arachidonic acid in the induction of synaptic potentiation, possibly as a retrograde transmitter substance.

On the mechanism of long-term potentiation induced by (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid (ACPD) in rat hippocampal slices

We have reported previously that transient application of a specific metabotropic glutamate receptor (mGluR) agonist (1S,3R)-1-aminocyclopentane-1,3-dicarboxylate (ACPD) can induce a slow-onset form of long-term potentiation (LTP) of synaptic transmission in the CA1 region of rat hippocampal slices [Bortolotto Z. A. and Collingridge G. L. (1993) Neuropharmacology 32, 1-9]. Here we have investigated further the mechanisms involved in the induction and expression of ACPD-induced LTP. Unless otherwise stated, field excitatory postsynaptic potentials (EPSPs) were recorded in stratum radiatum in response to low frequency (0.033 Hz stimulation) of the Schaffer collateral-commissural pathway and 10 microM ACPD was added for 20 min to the perfusate. ACPD-induced LTP was still observed following blockade of GABAA receptor-mediated synaptic inhibition using picrotoxin (50 microM) and was not the result of a change in the presynaptic fibre volley. Intracellular recording from area CA1 revealed an increase in the size of the EPSP but no associated change in membrane potential or input resistance. However, ACPD-induced potentiation was never seen when intracellular electrodes contained the Ca(2+)-chelating agent 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (BAPTA; 0.5 M). In area CA3, ACPD elicited a slow-onset LTP of the intracellularly recorded EPSP, evoked by stimulation of associational fibres. In contrast to area CA1, 10 microM ACPD depolarized CA3 neurones. Unlike certain other forms of tetanus- and chemically-induced potentiation, ACPD-induced LTP was not affected by the L-type Ca2+ channel antagonist nimodipine (50 microM). It was, however, prevented by delivering low frequency stimulation (900 shocks at 1 Hz) immediately following termination of the application of ACPD; an effect which was inhibited by the specific N-methyl-D-aspartate (NMDA) receptor antagonist D-2-amino-5-phosphonopentanoate (AP5; 50 microM). ACPD failed to induce LTP of pharmacologically-isolated NMDA receptor-mediated EPSPs. The induction of ACPD-induced LTP was blocked by the alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX), in a reversible manner. In slices in which area CA3 had been removed ACPD failed to induce LTP when applied alone or together with AMPA. However, a slow-onset form of LTP was induced, in slices lacking area CA3, when a tetanus (100 Hz, 1 sec) was delivered in the presence of ACPD and 50 microM AP5 (the latter applied to prevent conventional tetanus-induced LTP). ACPD-induced LTP was associated with a parallel increase in the sensitivity of CA1 neurones to AMPA. Considered together, these data suggest that ACPD-induced LTP is due to a direct increase in the AMPA receptor-mediated synaptic conductance and involves postsynaptic induction and expression mechanisms.

Indirect potentiation of synaptic transmission by metabotropic glutamate receptors in the rat hippocampal slice

The role that the metabotropic glutamate receptor plays in synaptic transmission is complex due to the multiple subtypes involved, which initiate a number of intracellular mechanisms. Here we have investigated the role of the metabotropic glutamate receptor in the induction of long-term potentiation (LTP). We have shown that, providing the CA3 region remains attached to the slice, it is possible to induce potentiation by bath perfusion of the metabotropic receptor agonist (1S,3R) 1-aminocyclopentane-1,3-dicarboxylic acid (ACPD) alone. The extent of the potentiation observed showed a strong negative correlation with the age of the animal from which the slices were prepared. Perfusion of ACPD was associated with an increase in the excitability of antidromically activated CA3 neurones, the appearance of spontaneous burst firing within the CA3 region, and an increased fibre volley recorded in the CA1 region. Blockade of N-methyl-D-aspartate (NMDA) receptors prevented all these effects. We suggest that the ACPD-induced potentiation of CA1 fEPSPs is an indirect effect caused by spontaneous burst firing and/or increased excitatory drive from CA3 neurones.