Changes in the state of the excitatory synaptic system in the hippocampus on prolonged exposure to excitatory amino acids and antagonists

No effect of glutamate on metabolic disturbances in hippocampal slices of mature fetal guinea pigs after transient in vitro ischemia

The involvement of glutamate in the development of cerebral metabolic disturbances in mature fetuses after transient ischemia was studied using a hippocampal slice model. We investigated the effects of exogenously applied glutamate or glutamate antagonists on the recovery of energy metabolism and protein synthesis rate (PSR) in hippocampal slices of mature guinea pigs after in vitro ischemia. The slices were incubated in a thermostatically controlled flow-through chamber and gassed with carbogen (95% O2/5% CO2). In vitro ischemia was induced by transferring the slices to an aglycemic, artificial cerebrospinal fluid (aCSF) equilibrated with 95% N2/5% CO2. In a first set of experiments slices were exposed to 10 mM glutamate during a 20-40 min period of in vitro ischemia. In a second set slices were incubated in aCSF containing MK-801 (100 microM) or kynurenic acid (0.5 mM) 30 min before, during and 2 h after in vitro ischemia. After a 12 h recovery phase, the concentrations of adenylates in the slices were measured by HPLC after extraction with perchloric acid. PSR was calculated from the rate of incorporation of [14C]leucine into tissue proteins. Neither glutamate nor glutamate antagonists had any effect on the postischemic recovery of energy metabolism and PSR when applied during in vitro ischemia. It is therefore concluded that glutamate does not play a major role in the development of metabolic disturbances in hippocampal slices from mature guinea pig fetuses subjected to transient in vitro ischemia.

Glutamate and theta-rhythm stimulation selectively enhance NMDA component of EPSC in CA1 neurons of young rats

Using in situ whole-cell patch clamp of hippocampal CA1 pyramidal neurons we demonstrate that glutamate initiates processes resulting in an increase in the amplitude of the excitatory post-synaptic current (EPSC). In adult animals both, NMDA and non-NMDA components of the EPSC increase in parallel. In young animals only the NMDA component is increased. A similar enhancement of the EPSC can be achieved by the stimulation of excitatory synaptic inputs to CA1 neurons with the frequency of the theta-rhythm. EPSCs remain enhanced for more than 60 min. The selective enhancement of the NMDA component in young animals is inhibited by preincubation of slices with the NO-synthase blocker, N omega-nitro-L-arginine (NA) or by the NO-scavenger, hemoglobin.

Release of endogenous adenosine and its metabolites by the activation of NMDA receptors in the rat hippocampus in vivo

1. The effects of N-methyl-D-aspartate (NMDA), KCl, and veratridine on the release of endogenous adenosine and its metabolites, inosine and hypoxanthine, from the rat hippocampus have been studied by in vivo microdialysis. 2. In the hippocampus of rats anaesthetized with urethane the adenosine level reached a stable state estimated at 0.93 microM during the first 2 h after the implantation of the dialysis probe. NMDA (50 microM to 25 mM) in the perfusate evoked a concentration-dependent release of adenosine, inosine and hypoxanthine with an EC50 of 180 microM. The release was reduced by 93% by the specific NMDA receptor antagonist 2-amino-5-phosphonopentanoic acid (2-AP5) at 200 microM, indicating an NMDA receptor-mediated process. In addition, the 100 mM KCl-evoked release of adenosine was also substantially reduced by 77% by 2-AP5, suggesting that a large component of the K(+)-evoked release is NMDA-receptor-mediated. 3. Perfusion with zero-Ca2+ artificial cerebrospinal fluid attenuated the NMDA-evoked release of adenosine only by 16% (not significant) but depressed the K(+)-evoked release by 62%, indicating that most of the NMDA-evoked release is directly receptor-mediated, whereas a large component of the K(+)-evoked release could be via the release of an excitatory amino acid acting at the NMDA receptors.

Excitatory action of some aspartate- and glutamate-containing dipeptides after intracerebroventricular injection in mice

The effects of some dipeptides, analogues of N-acetyl-alpha-L-aspartyl-L-glutamate, were studied after i.c.v. administration into mice in acute experiments. N-Acetyl-alpha-L-aspartyl-L-glutamate itself did not induce seizures in animals, but prevented glutamate-induced convulsions. All other dipeptides possessed excitatory glutamate-like actions. Some structural requirements for the excitatory effects of the dipeptides are discussed.

NMDA receptor agonists selectively block N-type calcium channels in hippocampal neurons

The modulation of voltage-dependent calcium channels by various neurotransmitters has been demonstrated in many neurons. Because of the critical role of Ca2+ in transmitter release and, more generally, in transmembrane signalling, this modulation has important functional implications. Hippocampal neurons possess low-threshold (T-type) Ca2+ channels and both L- and N-type high voltage-activated Ca2+ channels. N-type Ca2+ channels are blocked selectively by omega-conotoxin and adenosine. These substances both block excitatory synaptic transmission in the hippocampus, whereas dihydropyridines, which selectively block L-type channels, are ineffective. Excitatory synaptic transmission in the hippocampus displays a number of plasticity phenomena that are initiated by Ca2+ entry through ionic channels operated by N-methyl-D-aspartate (NMDA) receptors. Here we report that NMDA receptor agonists selectively and effectively depress N-type Ca2+ channels which are involved in neurotransmitter release from presynaptic sites. The inhibitory effect is eliminated by the competitive NMDA antagonist D-2-amino-5-phosphonovalerate, does not require Ca2+ entry into the cell, and is probably receptor-mediated. This phenomenon may provide a negative feedback between the liberation of excitatory transmitter and entry of Ca2+ into the cell, and could be important in presynaptic inhibition and in the regulation of synaptic plasticity.

Properties of kainate receptor/channels on cultured Bergmann glia

Following the localization, at the electron microscope level, of the immunoreactivity towards a putative kainate receptor on Bergmann glial cells in the chick cerebellar cortex, cultures of Bergmann glia were used to establish the presence of functional kainate receptor/channels and study their properties. Bergmann glia were identified by their fusiform morphology and their ability to bind an anti-kainate binding protein monoclonal antibody, a kainate receptor high affinity ligand--kainyl-bovine serum albumin--and a glial marker--anti-vimentin monoclonal antibody. Membranes prepared from the culture cells displayed, using 25 nM [3H]kainate, the binding of 4.1 pmol of [3H]kainate/mg protein and showed the presence in Western blots of the two polypeptides of 49 and 93 kDa attributed to the kainate binding protein. Kainate, at concentrations above 0.1 mM, was found to increase the influx into cultured Bergmann glia of 22Na+, 86Rb+, 45Ca2+ and 36Cl- ions. The traffic of 22Na+, induced by kainate and glutamate, observed only in the presence of 1 mM ouabain, was blocked by kainate receptor antagonists and by 0.01 mM quisqualate. Analysis of the kinetics of incorporation of 22Na+ and 45Ca2+ ions showed an initial accumulation of 22Na+ and 45Ca2+ ions followed by their total dissipation. The results indicate that the kainate-induced influx of Na+ ions through the kainate receptor/channel causes the reverse transport of Na+ ions, by activation of the Na+/Ca2+ and Na+/H+ exchangers which remove intracellular Na+ ions. Pre-exposure of the cells to 0.5 mM dibutyryl cAMP was found to greatly enhance the kainate-induced 22Na+ ion influx. We propose that the Bergmann glia kainate receptors modulate the efficacy of the glutamatergic synapses between the parallel fibers and Purkinje cell spines and form part of a glial machinery responsible for plastic changes in synaptic transmission.

Role of excitatory amino acid receptors in K+- and glutamate-evoked release of endogenous adenosine from rat cortical slices

K+ and glutamate released endogenous adenosine from superfused slices of rat parietal cortex. The absence of Ca2+ markedly diminished K+- but not glutamate-evoked adenosine release. Tetrodotoxin decreased K+- and glutamate-evoked adenosine release by 40 and 20%, respectively, indicating that release was mediated in part by propagated action potentials in the slices. Inhibition of ecto-5'-nucleotidase by alpha,beta-methylene ADP and GMP decreased basal release of adenosine by 40%, indicating that part of the adenosine was derived from the extracellular metabolism of released nucleotide. In contrast, inhibition of ecto-5'-nucleotidase did not affect release evoked by K+ or glutamate, suggesting that adenosine was released as such. Inhibition of glutamate uptake by dihydrokainate potentiated glutamate-evoked release of adenosine. Glutamate-evoked adenosine release was diminished 50 and 55% by the N-methyl-D-aspartate (NMDA) receptor antagonists, DL-2-amino-5-phosphonovaleric acid and (+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imine hydrogen maleate (MK-801), respectively. The remaining release in the presence of MK-801 was diminished a further 66% by the non-NMDA receptor antagonist, 6,7-dinitroquinoxaline-2,3-dione, suggesting that both NMDA and non-NMDA receptors were involved in glutamate-evoked adenosine release. Surprisingly, K+-evoked adenosine release was also diminished about 30% by NMDA antagonists, suggesting that K+-evoked adenosine release may be partly mediated indirectly through the release of an excitatory amino acid acting at NMDA receptors.

N-methyl-D-aspartate, kainate and quisqualate release endogenous adenosine from rat cortical slices

N-Methyl-D-aspartate, kainate, and quisqualate released endogenous adenosine from superfused slices of rat parietal cortex. N-Methyl-D-aspartate-evoked adenosine release was blocked by D,L-2-amino-5-phosphono-valeric acid and (+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imine hydrogen maleate (MK-801), indicating that it was receptor-mediated, although it did not show the expected potentiation in the absence of Mg2+. In contrast, N-methyl-D-aspartate-evoked release of [3H]noradrenaline from the same slices was markedly potentiated in Mg2(+)-free medium. Therefore, the lack of Mg2+ modulation of N-methyl-D-aspartate-evoked adenosine release was not due to depolarization-induced alleviation of the Mg2+ block in the slices. Kainate-evoked adenosine release was diminished by the non-specific excitatory amino acid antagonist, gamma-D-glutamyl-glycine, and kainate- and quisqualate-evoked adenosine release was diminished by 6,7-dinitroquinoxaline-2,3-dione, indicating that these agonists release adenosine by acting at non-N-methyl-D-aspartate receptors. Tetrodotoxin decreased N-methyl-D-aspartate- and kainate-evoked adenosine release by 40% and 19% respectively, indicating that release was mediated in part by propagated action potentials in the slices. Total release of adenosine by N-methyl-D-aspartate, kainate or quisqualate was not diminished in the absence of Ca2+. A second exposure to kainate following restoration of Ca2+ to slices previously depolarized in the absence of Ca2+ resulted in an amount of adenosine release equal to an initial release by slices in the presence of Ca2+, a result suggesting the presence of separate Ca2(+)-dependent and Ca2(+)-independent pools of adenosine. The present experiments demonstrate that activation of all three major subtypes of excitatory amino acid receptors in the cortex releases adenosine, possibly from separate Ca2(+)-dependent and -independent pools. Adenosine released from the cortex following excitatory amino acid stimulation may, by acting at inhibitory P1 purinoceptors, diminish excitatory neurotransmission and protect against excitotoxicity.

Hippocampal synaptic plasticity induced by excitatory amino acids includes changes in sensitivity to the calcium channel blocker, omega-conotoxin

The hippocampus is widely used in investigations of different forms of synaptic plasticity, including long-term potentiation and kindling. Receptors for excitatory amino acids (EAAs) play a prominent role in these phenomena. Recently, is has been demonstrated that exposure of hippocampal slices to EAAs and related agonists produces biphasic effects on excitatory synaptic transmission: initial blockade of synaptic responses is followed by a delayed recovery. The recovered responses demonstrate altered pharmacological properties: they acquire sensitivity to N-methyl-D-aspartate (NMDA) antagonists during L-glutamate (Glu) exposure and lose sensitivity to both NMDA and non-NMDA antagonists under L-aspartate (Asp). These changes persist for many hours. It was suggested that this form of hippocampal plasticity may involve transitions between distinct states of synaptic functioning. To explore this possibility, we investigated several properties of synaptic transmission in the initial and EAA-modified states. Here we report that hippocampal postsynaptic potentials (PSPs) evoked under Glu or Asp exposure completely lose sensitivity to omega-conotoxin GVIA (omega-CgTX), a potent, specific, and irreversible blocker of certain types of neuronal calcium channels. After washout of the EAA, sensitivity to the toxin is regained. These results indicate that prolonged EAA exposure induces profound changes in the machinery of synaptic transmission, which include, but are not limited to, changes in calcium channel functioning.

Fade of the response to prolonged glutamate application in the rat hippocampal slice

The effect of prolonged glutamate (GLU) application was examined on 60 CA1 pyramidal neurons in the in vitro rat hippocampal slice preparation. Continuous application of L-GLU, either by bath perfusion (0.5-2 mM) of the slices or iontophoresis (200 mM) into the dendritic region of the neurons, elicited a transient depolarization which faded to a mean of 53% of the initial peak amplitude despite continued exposure to the agonist. Membrane depolarization to aspartate (ASP) and the d-isomer of GLU also faded with time. In contrast, the depolarizing response to the excitatory amino acid agonists N-methyl-D,L-aspartate (NMA), quisqualate (QUIS), and kainate (KA) did not fade significantly during continuous application. The fade of the GLU depolarization was not affected by the NMDA antagonist D-2-amino-5-phosphonovalerate (APV) or by blocking synaptic transmission with tetrodotoxin. At the time of maximum fade of the GLU depolarization, there was no change in input resistance or GLU reversal potential. In addition, fade of the response was not a consequence of changes in extracellular potassium concentration, GLU uptake mechanisms, or the electrogenic pump. The most likely explanation for fade is postsynaptic receptor desensitization.