Characterization of an N-methyl-D-aspartate receptor component of synaptic transmission in rat hippocampal slices

Total plasma magnesium in healthy and critically ill foals

Abnormalities in total Mg (tMg) concentration in plasma and/or serum are common in critically ill humans, and the association with increased mortality has been documented in several clinical studies in adults and newborns with hypoxic-ischemic encephalopathy. Abnormalities in tMg were studied in hospitalized dogs, cats, and adult horses. Newborn foals were scarcely studied with regard to Mg concentration. The aims of the present study were: (1) to compare two analytical methods for the determination of tMg in plasma: the automated colorimetric method and the atomic absorption spectrometry; (2) to measure plasma tMg in healthy foals during the first 72 hours after birth and in sick foals during the first 72 hours of hospitalization; (3) to compare total plasma Mg concentration among healthy foals, foals affected by perinatal asphyxia syndrome (PAS), prematurity and/or dismaturity, and sepsis; (4) to evaluate tMg plasma concentration in surviving and non-surviving foals. One hundred seventeen foals were included in the study: 20 healthy and 97 sick foals. The automated method used in clinical practice probably overestimates plasma tMg. Due to its higher sensitivity and specificity, the atomic absorption spectrometry should be considered the method of choice from an analytical point of view, but requires an instrumentation not easily available in any laboratory and specific technical skills and competencies. Plasma tMg in healthy foals were included in the range 0.52 to 1.01 mmol/L and did not show any time-dependent change during the first 72 hours of life. In sick foals, tMg evaluated at T0 was statistically higher than tMg measured at subsequent times. Foals affected by PAS had a tMg at T0 significantly higher (P < 0.01) than healthy, septic, and premature and/or dysmature foals. The t test found significantly higher (P < 0.01) plasma tMg measured at T0 in non-surviving than in surviving foals. Plasma tMg could be a useful parameter for the diagnosis of PAS and the formulation of the prognosis in critically ill foals.

Imaging-based neurochemistry in schizophrenia: a systematic review and implications for dysfunctional long-term potentiation

Cognitive deficits are commonly observed in patients with schizophrenia. Converging lines of evidence suggest that these deficits are associated with impaired long-term potentiation (LTP). In our systematic review, this hypothesis is evaluated using neuroimaging literature focused on proton magnetic resonance spectroscopy, positron emission tomography, and single-photon emission computed tomography. The review provides evidence for abnormal dopaminergic, GABAergic, and glutamatergic neurotransmission in antipsychotic-naive/free patients with schizophrenia compared with healthy controls. The review concludes with a model illustrating how these abnormalities could lead to impaired LTP in patients with schizophrenia and consequently cognitive deficits.

Activation of calcineurin underlies altered trafficking of α2 subunit containing GABAA receptors during prolonged epileptiform activity

Fast inhibitory signalling in the mammalian brain is mediated by gamma-aminobutyric acid type A receptors (GABAARs), which are targets for anti-epileptic therapy such as benzodiazepines. GABAARs undergo tightly regulated trafficking processes that are essential for maintenance and physiological modulation of inhibitory strength. The trafficking of GABAARs to and from the membrane is altered during prolonged seizures such as in Status Epilepticus (SE) and has been suggested to contribute to benzodiazepine pharmacoresistance in patients with SE. However, the intracellular signalling mechanisms that cause this modification in GABAAR trafficking remain poorly understood. In this study, we investigate the surface stability of GABAARs during SE utilising the low Mg(2+) model in hippocampal rat neurons. Live-cell imaging of super ecliptic pHluorin (SEP)-tagged α2 subunit containing GABAARs during low Mg(2+) conditions reveals that the somatic surface receptor pool undergoes down-regulation dependent on N-methyl-d-aspartate receptor (NMDAR) activity. Analysis of the intracellular Ca(2+) signal during low Mg(2+) using the Ca(2+)-indicator Fluo4 shows that this reduction of surface GABAARs correlates well with the timeline of intracellular Ca(2+) changes. Furthermore, we show that the activation of the phosphatase calcineurin was required for the decrease in surface GABAARs in neurons undergoing epileptiform activity. These results indicate that somatic modulation of GABAAR trafficking during epileptiform activity in vitro is mediated by calcineurin activation which is linked to changes in intracellular Ca(2+) concentrations. These mechanisms could account for benzodiazepine pharmacoresistance and the maintenance of recurrent seizure activity, and reveal potential novel targets for the treatment of SE.

Memantine add-on to risperidone for treatment of negative symptoms in patients with stable schizophrenia: randomized, double-blind, placebo-controlled study

We aimed to evaluate the efficacy of memantine add-on in the treatment of primary negative symptoms of patients with stable schizophrenia. In a double-blind placebo-controlled clinical trial, 40 patients with schizophrenia (Diagnostic and Statistical Manual of Mental Disorders, Fourth Edition) who were stabilized on risperidone for a minimum of 8 weeks were randomized to either memantine (20 mg) or placebo in addition to risperidone, 6 mg/d, for eight weeks. Assessment was done using the Positive and Negative Syndrome Scale at baseline, week 4, and week 8. The Hamilton Depression Rating Scale and the Extrapyramidal Symptom Rating Scale at baseline and week 8 were used to assess depression and extrapyramidal symptoms, respectively. All 40 patients had at least one postbaseline measurement, and 38 patients completed the trial. Patients in the memantine group showed a significantly greater improvement on negative subscale than the placebo group at end point (P < 0.001). The same effect was observed for the total score (P < 0.001) and the general psychopathology subscale score (P = 0.002). There was no significant difference in reduction of positive symptoms score between the 2 groups (P = 0.757). Changes in the Hamilton Depression Rating Scale and the Extrapyramidal Symptom Rating Scale scores and frequency of adverse effects did not differ between the 2 groups. Our study showed that memantine is a tolerable and efficacious add-on treatment for primary negative symptoms of schizophrenia.

Human glioma cells induce hyperexcitability in cortical networks

PURPOSE

Patients with gliomas frequently present with seizures, but the factors associated with seizure development are still poorly understood. In this study, we assessed peritumoral synaptic network activity in a glioma animal model and tested the contribution of aberrant glutamate release from gliomas on glioma-associated epileptic network activity.

METHODS

In vitro brain slices were made from glioma-implanted mice. Using extracellular field recordings, we analyzed peritumoral epileptiform activity induced by Mg(2+)-free medium in slices from tumor-bearing animals and sham-operated controls. We assessed the effect of sulfasalazine (SAS), a blocker of system and glutamate release, on spontaneous and evoked activity in tumor-associated slices.

KEY FINDINGS

Tumor-associated cortical networks were hyperexcitable. The onset latency of Mg(2+)-free-induced epileptiform activity was significantly shorter in tumor-bearing slices, and the incidence of Mg(2+)-free-induced ictal-like events was higher. Block of glutamate release from system decreased the response area of evoked activity and completely blocked Mg(2+)-free-induced ictal-like, but not interictal-like events.

SIGNIFICANCE

Control of seizures in patients with gliomas is an essential component of clinical management; therefore, understanding the origin of seizures is vital. This work provides evidence that peritumoral synaptic network activity is disrupted by tumor masses resulting in network excitability. We show that blocking glutamate release via system with SAS, a drug already approved by the U.S. Food and Drug Administration (FDA), can inhibit Mg(2+)-free-induced ictal-like epileptiform events similar to other chemicals used to decrease seizure activity. We, therefore, suggest that further studies should consider SAS a promising agent to aid in the treatment of seizures associated with gliomas.

Retinogeniculate EPSPs recorded intracellularly in the ferret lateral geniculate nucleus in vitro: Role of NMDA receptors

We used an in vitro preparation of the ferret lateral geniculate nucleus (LGN) to examine the role of the NMDA class of excitatory amino acid (EAA) receptors in retinogeniculate transmission. Intracellular recordings revealed that blockade of NMDA receptors both shortened the time course and reduced the amplitude of fast and slow components of excitatory postsynaptic potentials (EPSPs) evoked by optic tract stimulation. The amplitude and width of the EPSPs mediated by NMDA receptors increased as membrane potential was depolarized towards spike threshold. Individual LGN cells were influenced to varying extents by blockade of NMDA receptors; NMDA and non-NMDA receptor blockade together attenuated severely the entire retinogeniculate EPSP. The dependence of all components of retinogeniculate EPSPs (and action potentials) on NMDA receptor activation supports the hypothesis that the NMDA receptor participates in fast (<10 ms) synaptic events underlying conventional retinogeniculate transmission. The voltage dependence of the NMDA receptor-gated conductance suggests strongly that the transmission of retinal information through the LGN is subject to modulation by extraretinal inputs that affect the membrane potential of LGN neurons.

Calcium signaling in neurodegeneration

Calcium is a key signaling ion involved in many different intracellular and extracellular processes ranging from synaptic activity to cell-cell communication and adhesion. The exact definition at the molecular level of the versatility of this ion has made overwhelming progress in the past several years and has been extensively reviewed. In the brain, calcium is fundamental in the control of synaptic activity and memory formation, a process that leads to the activation of specific calcium-dependent signal transduction pathways and implicates key protein effectors, such as CaMKs, MAPK/ERKs, and CREB. Properly controlled homeostasis of calcium signaling not only supports normal brain physiology but also maintains neuronal integrity and long-term cell survival. Emerging knowledge indicates that calcium homeostasis is not only critical for cell physiology and health, but also, when deregulated, can lead to neurodegeneration via complex and diverse mechanisms involved in selective neuronal impairments and death. The identification of several modulators of calcium homeostasis, such as presenilins and CALHM1, as potential factors involved in the pathogenesis of Alzheimer's disease, provides strong support for a role of calcium in neurodegeneration. These observations represent an important step towards understanding the molecular mechanisms of calcium signaling disturbances observed in different brain diseases such as Alzheimer's, Parkinson's, and Huntington's diseases.

Extracellular recordings of rodents in vivo: their contribution to integrative neuroscience

The prevalent theory in learning and memory processes is that they are underlain by short and long-term changes in synaptic weight, which continuously modulates neural networks during acquisition and recall. This synaptic plasticity has been revealed by recording extracellular field potentials. The enhancement of synaptic transmission was primarily noted in the hippocampus and was named long-term potentiation (LTP). The opposite mechanism, long-term depression (LTD), a reduction of synaptic transmission, was first discovered in the cerebellum. Since then, the LTP-model has been studied mainly using in vitro and acute anesthetized in vivo preparations. This approach has led to remarkable progress in the comprehension of intracellular molecular processes during LTP and LTD. In this review, we focus mainly on what we can learn about molecular events using extracellular field potential recordings with a more ecological model, i.e., studies using the freely behaving animal, with animals that are genetically modified or not, in several behavioral paradigms aimed at gaining insight into some of the conflicting results obtained with in vitro and in vivo preparations.

Memantine: a NMDA receptor antagonist that improves memory by restoration of homeostasis in the glutamatergic system--too little activation is bad, too much is even worse

The neurotransmitter glutamate activates several classes of metabotropic receptor and three major types of ionotropic receptor--alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA), kainate and N-methyl-D-aspartate (NMDA). The involvement of glutamate mediated neurotoxicity in the pathogenesis of Alzheimer's disease (AD) is finding increasing scientific acceptance. Central to this hypothesis is the assumption that glutamate receptors, in particular of the NMDA type, are overactivated in a tonic rather than a phasic manner. Such continuous, mild, chronic activation ultimately leads to neuronal damage/death. Additionally, impairment of synaptic plasticity (learning) may result not only from neuronal damage per se but may also be a direct consequence of this continuous, non-contingent NMDA receptor activation. Complete NMDA receptor blockade has also been shown to impair neuronal plasticity, thus, both hypo- and hyperactivity of the glutamatergic system leads to dysfunction. Memantine received marketing authorization from the EMEA (European Medicines Agency) for the treatment of moderate to severe AD in Europe and was subsequently also approved by the FDA (Food and Drug Administration) for use in the same indication in the USA. Memantine is a moderate affinity, uncompetitive NMDA receptor antagonist with strong voltage-dependency and fast kinetics. This review summarizes existing hypotheses on the mechanism of action (MOA) of memantine in an attempt to understand how the accepted interaction with NMDA receptors could allow memantine to provide both neuroprotection and reverse deficits in learning/memory by the same MOA.

Involvement of the nitric oxide pathway in synaptic dysfunction following amyloid elevation in Alzheimer's disease

Amyloid-beta (Abeta), a peptide thought to play a crucial role in Alzheimer's disease (AD), has attracted scientific interest with the aim of characterizing the mechanisms by which it is involved in AD pathogenesis. Abeta has been found to markedly impair hippocampal long-term potentiation (LTP), a widely studied cellular model of synaptic plasticity that is thought to underlie learning and memory. The overall purpose of this review is to define the role of the nitric oxide (NO)/cGMP/cAMP-regulatory element binding (CREB) pathway in beta-amyloid-induced changes of basal neurotransmission and synaptic plasticity in the hippocampus, a structure within the temporal lobe of the brain critical for memory storage.

Long-term potentiation and memory

One of the most significant challenges in neuroscience is to identify the cellular and molecular processes that underlie learning and memory formation. The past decade has seen remarkable progress in understanding changes that accompany certain forms of acquisition and recall, particularly those forms which require activation of afferent pathways in the hippocampus. This progress can be attributed to a number of factors including well-characterized animal models, well-defined probes for analysis of cell signaling events and changes in gene transcription, and technology which has allowed gene knockout and overexpression in cells and animals. Of the several animal models used in identifying the changes which accompany plasticity in synaptic connections, long-term potentiation (LTP) has received most attention, and although it is not yet clear whether the changes that underlie maintenance of LTP also underlie memory consolidation, significant advances have been made in understanding cell signaling events that contribute to this form of synaptic plasticity. In this review, emphasis is focused on analysis of changes that occur after learning, especially spatial learning, and LTP and the value of assessing these changes in parallel is discussed. The effect of different stressors on spatial learning/memory and LTP is emphasized, and the review concludes with a brief analysis of the contribution of studies, in which transgenic animals were used, to the literature on memory/learning and LTP.

Factors underlying bursting behavior in a network of cultured hippocampal neurons exposed to zero magnesium

Factors contributing to reduced magnesium-induced neuronal action potential bursting were investigated in primary hippocampal cell culture at high and low culture density. In nominally zero external magnesium medium, pyramidal neurons from high-density cultures produced recurrent spontaneous action potential bursts superimposed on prolonged depolarizations. These bursts were partially attenuated by the NMDA receptor antagonist d-APV. Pharmacological analysis of miniature excitatory postsynaptic currents (EPSCs) revealed 2 components: one sensitive to d-APV and another to the AMPA receptor antagonist DNQX. The components were kinetically distinct. Participation of NMDA receptors in reduced magnesium-induced synaptic events was supported by the localization of the NR1 subunit of the NMDA receptor with the presynaptic vesicular protein synaptophysin. Presynaptically, zero magnesium induced a significant increase in EPSC frequency likely attributable to increased neuronal hyperexcitability induced by reduced membrane surface charge screening. Mean quantal content was significantly increased in zero magnesium. Cells from low-density cultures did not exhibit action potential bursting in zero magnesium but did show increased EPSC frequency. Low-density neurons had less synaptophysin immunofluorescence and fewer active synapses as determined by FM1-43 analysis. These results demonstrate that multiple factors are involved in network bursting. Increased probability of transmitter release presynaptically, enhanced NMDA receptor-mediated excitability postsynaptically, and extent of neuronal interconnectivity contribute to initiation and maintenance of elevated network excitability.

Intracellularly recorded responses of neurons of the motor cortex of awake cats to presentations of Pavlovian conditioned and unconditioned stimuli

The electrical responses to a conditioned stimulus (click CS) and an unconditioned stimulus (glabella tap US) were studied in single units of the pericruciate cortex of awake cats. There has been no previous direct, in vivo characterization of the intracellular effects in cortical neurons of a CS and US used for associative Pavlovian conditioning. Earlier studies showed that ablation of the pericruciate cortex prevented the development of short latency blink CRs produced by associative pairing of these stimuli. Both the CS and the US produced depolarizing EPSPs and increases in spike discharge in intracellularly recorded units. The magnitude of the EPSPs was greater in response to the tap US than to the click CS, and the degree of unit discharge in response to the tap US was greater than that to the click CS. The onset latencies of the spike responses were distributed in three ranges. For click CS the ranges were 6-15, 20-35 and 40-85 ms. For tap US the ranges were 6-25, 30-45 and 50-85 ms. An additional response was observed in the period 0.5-6.0 ms after tap onset. It was thought to be elicited mechanically by vibration of the electrode tip following tap delivery. Pentobarbital anesthesia abolished the unconditioned motor response to tap, but failed to abolish the neuronal responses in any of the latency ranges, suggesting that the responses were not produced by feedback from the movement. Both the CS and the US are needed for the development of blink conditioning, and are distinguished behaviorally by the US producing an unconditioned motor response whose form resembles that of the conditioned response which develops after the stimuli are paired. Both the click CS and the tap US also produced hyperpolarizing IPSPs. Injection of steady, hyperpolarizing current disclosed two types of IPSPs in response to the tap US. One reversed with hyperpolarization; the other increased in magnitude with hyperpolarization. The magnitude of inhibition, measured by reduced discharge, was greater in response to the click CS than to the tap US. We conclude that the major differences between effects of this CS and US on cells of the motor cortex of cats are that the CS elicits less activity than the US and has a larger inhibiting effect. The US elicits greater depolarization than does the CS, as well as an initial discharge component of greater magnitude and longer duration.

Crocin antagonizes ethanol inhibition of NMDA receptor-mediated responses in rat hippocampal neurons

We have previously found that crocin (crocetin di-gentiobiose ester) antagonizes the inhibitory effect of ethanol on long-term potentiation in the rat hippocampus in vivo and in vitro. To explore mechanisms underlying the antagonism of crocin against ethanol, we investigated the effects of ethanol and crocin on synaptic potentials mediated by N-methyl-d-aspartate (NMDA) receptors in the dentate gyrus of rat hippocampal slices. Synaptic potential mediated by non-NMDA receptors was recorded in normal medium (1.3 mM Mg2+), while NMDA receptor-mediated synaptic potential was isolated in low (0.13 mM) Mg2+ medium containing the non-NMDA receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (10 microM). Crocin (10 microM) alone did not affect synaptic potentials mediated by non-NMDA nor NMDA receptors. Non-NMDA response was slightly inhibited by 100 mM ethanol, while NMDA response was selectively inhibited by lower concentrations (10-50 mM) of ethanol. Crocin (10 microM) did not affect the inhibition of non-NMDA response by 100 mM ethanol, but significantly blocked the inhibition of NMDA response by 10-50 mM ethanol. In addition, we performed whole-cell patch recording with primary cultured rat hippocampal neurons, and confirmed that crocin blocked ethanol inhibition of inward currents evoked by application of NMDA. These results suggest that crocin specifically antagonizes the inhibitory effect of ethanol on NMDA receptor-mediated responses in hippocampal neurons.

Postsynaptic blockade of inhibitory postsynaptic currents by plasmin in CA1 pyramidal cells of rat hippocampus

We have shown previously that plasmin facilitated the generation of long-term potentiation (LTP) in CA1 and dentate region of rat hippocampus. In the present study, we investigated the effects of plasmin on postsynaptic currents in CA1 pyramidal neurons of rat hippocampal slices. Plasmin (100 nM) had no effect on NMDA nor on non-NMDA receptor-mediated excitatory postsynaptic currents. However, plasmin significantly decreased GABA(A) receptor-mediated inhibitory postsynaptic currents. This effect of plasmin disappeared when intracellular Ca2+ was strongly chelated with BAPTA. Furthermore, plasmin attenuated the GABA-induced currents in CA1 pyramidal cells. These results suggest that the STP-enhancing effect of plasmin is due to a blockade of postsynaptic GABA(A) responses and that an increase in intracellular Ca2+ by plasmin may be involved in its mechanism.

Possible involvement of plasmin in long-term potentiation of rat hippocampal slices

Effects of proteases and protease inhibitors on generation of long-term potentiation (LTP) were investigated in the CA1 and dentate regions of rat hippocampus. Plasmin, a serine protease, and its precursor plasminogen significantly enhanced short-term potentiation (STP) induced by a weak tetanic stimulation, without affecting basal responses. The STP-enhancing effect of plasmin disappeared by concomitant perfusion of alpha 2-antiplasmin, an endogenous plasmin inhibitor. Other proteases, such as thrombin, trypsin and cathepsin B, did not affect STP. On the other hand, alpha 2-antiplasmin and leupeptin significantly attenuated LTP induced by a strong tetanus though plasminogen or plasmin itself did not influence LTP. Furthermore, plasminogen and plasmin did not affect NMDA receptor-mediated synaptic responses in the absence of extracellular Mg2+. These results suggest that endogenous plasmin is involved in the mechanism of LTP in CA1 and dentate regions of rat hippocampus and that the STP-enhancing effect of plasmin is independent of NMDA receptors.

Effects of memantine and MK-801 on NMDA-induced currents in cultured neurones and on synaptic transmission and LTP in area CA1 of rat hippocampal slices

The effects of the uncompetitive N-methyl-D-aspartate (NMDA) receptor antagonists, memantine (1-amino-3,5-dimethyladamantane) and MK-801 ((+)-5-methyl-10,11-dihydro-5H-dibenzocyclo-hepten-5,10-imin e maleate) were compared on synaptic transmission and long-term potentiation (LTP) in hippocampal slices and on NMDA-induced currents in cultured superior collicular neurones. 2. Memantine (10-100 microM) reversibly reduced, but did not abolish, NMDA receptor-mediated secondary population spikes recorded in area CA1 of hippocampal slices bathed in Mg(2+)-free artificial cerebrospinal fluid. 3. Memantine (100 microM) antagonized NMDA receptor-mediated excitatory postsynaptic currents recorded in area CA1 in a strongly voltage-dependent manner i.e. depressed to 11 +/- 4% of control at -35 mV and 95 +/- 5% of control at +40 mV (n = 9), with no apparent effect on response kinetics. 4. The effects of MK-801 and memantine on the induction of LTP were assessed after prolonged pre-incubations with these antagonists. When present for 6.6 +/- 0.4 h prior to tetanic stimulation, memantine blocked the induction of LTP with an IC50 of 11.6 +/- 0.53 microM. By comparison, similar long pre-incubations with MK-801 (6.4 +/- 0.4 h) blocked the induction of LTP with an IC50 of 0.13 +/- 0.02 microM. 5. Memantine and MK-801 reduced NMDA-induced currents in cultured superior colliculus neurones recorded at -70 mV with IC50s of 2.2 +/- 0.2 microM and 0.14 +/- 0.04 microM respectively. The effects of memantine were highly voltage-dependent and behaved as though the affinity decreased epsilon fold per 50 mV of depolarization (apparent delta = 0.71). In contrast, under the conditions used, MK-801 appeared to be much less voltage-dependent i.e. affinity decreased epsilon fold per 329 mV of depolarization (apparent delta = 0.15). 6. Depolarizing steps from -70 mV to +50 mV in the continuous presence of memantine (10 microM) caused a rapid relief of blockade of NMDA-induced currents from 83.7 +/- 1.9% to 21.8 +/- 1.8% (n = 5). This relief was best fitted by a double exponential function (17.2 +/- 11.7 and 698 +/- 204 ms), the faster component of which was most pronounced. 7. In conclusion, whereas MK-801 is equipotent in blocking NMDA-induced currents (at - 70 mV) and the induction of LTP, memantine is relatively less potent in blocking the induction of LTP. This is due to its rapid relief of blockade upon depolarization; a property which might explain its promising clinical profile in the treatment of chronic neurodegenerative diseases.

Ethanol withdrawal hyperexcitability in vitro is selectively decreased by a competitive NMDA receptor antagonist

Hippocampal slices were prepared immediately after withdrawal from chronic ethanol in vivo. The decreases in thresholds for production of single and multiple population spikes seen after the ethanol treatment were not evident when CGP39551 was included in the perfusion medium at 20 microM. The decrease in paired pulse potentiation seen during ethanol withdrawal, however, was not prevented by CGP39551. For comparison, the effects of CGP39551, at the same concentration, were examined on the changes in field potentials seen in control slices when the magnesium concentration in the bathing medium was lowered to 250 microM. The decreases in thresholds for multiple population spikes produced by the lowered magnesium were prevented, but not other changes including decreases in single spike thresholds. In addition, this 20 microM concentration of CGP39551 did not prevent epileptiform activity, measured by decreases in thresholds for production of single and multiple population spikes caused by addition of the GABAA antagonist, bicuculline, to control hippocampal slices.

The pharmacology of the nicotinic antagonist, chlorisondamine, investigated in rat brain and autonomic ganglion

1. A single administration of the ganglion blocker, chlorisondamine (10 mg kg-1, s.c.) is known to produce a quasi-irreversible blockade of the central actions of nicotine in the rat. The mechanism of this persistent action is not known. It is also unclear whether chlorisondamine can block neuronal responses to excitatory amino acids and whether chronic blockade of nicotinic responses also occurs in the periphery. 2. Acute administration of chlorisondamine (10 mg kg-1, s.c.) to rats resulted in a blockade of central nicotinic effects (ataxia and prostration) when tested 1 to 14 days later, but caused no detectable cell death in tissue sections sampled throughout the rostrocaudal extent of the brain which were stained in order to reveal neuronal degeneration. 3. Long-term blockade of central nicotinic effects by chlorisondamine was not associated with significant alterations in the density (Bmax) of high-affinity [3H]-nicotine binding to forebrain cryostat-cut sections. 4. In cultured dissociated mesencephalic cells of the foetal rat, chlorisondamine and mecamylamine inhibited [3H]-dopamine release evoked by N-methyl-D-aspartate (NMDA, 10(-4) M), but only at high concentrations (IC50 approx. 600 and 70 microM, respectively). A high concentration of chlorisondamine (10(-3) M) had no effect on responses to quisqualate (10(-5) M) and only slightly reduced responses to kainate (10(-4) M). Mecamylamine (10(-3) M) was ineffective against both agonists. 5. In adult rat hippocampal slices, chlorisondamine depressed NMDA receptor-mediated synaptically-evoked field potentials, but again only at high concentrations (10(-4)-10(-3) M). Synaptic responses that were mediated by non-NMDA excitatory amino acid receptors were less affected. 6. In rat isolated superior cervical ganglion, electrically-evoked synaptic transmission was reduced 1 h after acute in vivo administration of chlorisondamine (0.1 mg kg-1, s.c.). However, in vivo administration of a higher dose (10 mg kg-1, s.c.) did not significantly affect ganglionic transmission when tested two weeks later, despite the continued presence of central nicotinic blockade.7. These results indicate that the persistent CNS nicotinic blockade by chlorisondamine is not accompanied by changes in nicotinic [3H]-nicotine binding site density or by neuronal degeneration in the brain; that at doses sufficient to produce nicotinic receptor blockade, chlorisondamine acts in a pharmacologically selective manner; and that chronic central blockade is not accompanied by long-term peripheral ganglionic blockade.

Temporal pattern sensitivity of long-term potentiation in hippocampal CA1 neurons

High-frequency electrical stimulation in the hippocampus leads to an increase in synaptic efficacy that lasts for many hours. This long-term potentiation (LTP) of synaptic transmission is presumed to play a crucial role in learning and memory in the brain. However, the frequency of stimulation generally used to obtain LTP is beyond the normal physiological range of activity of hippocampal neurons. We found that LTP can be induced by an electrical stimulation whose frequency is comparable to that of the naturally occurring firing activity of hippocampal neurons if the stimulating pulse-interval train has a special time structure. In the present experiment, we compared the magnitude of LTP induced by the four types of stimuli which have the same pulse number and the same mean frequency but different time structure in interstimulus intervals. One type of stimuli has regular intervals, and this served as a control stimulus. In the other three types of stimuli, the adjacent interstimulus interval had the following statistical properties: in type 1, their correlations are positive; in type 2, negative; and in type 3, independent. The magnitude of LTP induced by these four types of stimuli showed clear order relationships: type 3/type 1 >> control > type 2. Detailed analysis of the evoked potential during a period of temporal pattern stimulation revealed that the amplitude of the population spikes of repetitive firing, especially of the second and third population spikes, had the same order relationship as the LTP.(ABSTRACT TRUNCATED AT 250 WORDS)

Postjunctional alpha 1- and alpha 2-adrenoceptors in cat forepaw vasculature

Preganglionic stimulation of the thoracic sympathetic nerve trunk produced frequency-related decreases of ulnar arterial blood flow in the forepaw of anesthetized cats as measured by ultrasonic flowmetry. Vasoconstrictor responses were not altered by administration of propranolol or atropine. When given alone, both prazosin (3-100 micrograms/kg, i.v.) and rauwolscine (10-300 micrograms/kg, i.v.) produced only a modest reduction of sympathetic evoked vasoconstriction. However, extensive blockade was obtained when both antagonists were administered to the same animal. These results suggest that both alpha 1- and alpha 2-adrenoceptors coexist at postsynaptic sites in the vasculature of the cat paw and that neural activation of either type results in potent vasoconstriction.

Tricyclic antidepressants block NMDA receptor-mediated synaptic responses and induction of long-term potentiation in rat hippocampal slices

The effects of tricyclic antidepressants (imipramine, desipramine and amitriptyline) on the NMDA receptor-mediated synaptic responses and the induction of long-term potentiation were investigated in the CA1 region and the dentate gyrus of rat hippocampal slices. Both in the CA1 region and in the dentate gyrus, all three antidepressants (3 x 10(-5)M) did not affect the evoked potential in normal artificial cerebrospinal fluid (CSF) but inhibited the NMDA receptor-mediated epileptiform responses in Mg(2+)-free artificial CSF. Furthermore, all three agents blocked the induction of long-term potentiation in a concentration-dependent manner, both in the CA1 region and in the dentate gyrus. The concentrations which were effective in blocking the induction of long-term potentiation were consistent with those in inhibiting the NMDA receptor-mediated synaptic responses in Mg(2+)-free artificial CSF. These results suggest that tricyclic antidepressants block the induction of long-term potentiation by inhibitory actions on NMDA receptors.

Effects of glycine and structurally related amino acids on generation of long-term potentiation in rat hippocampal slices

The effects of glycine and structurally related amino acids, serine, alanine and valine, on generation of long-term potentiation (LTP) of evoked potentials were investigated in the CA1, CA3 and dentate regions of rat hippocampal slices. In the Schaffer collateral-CA1 pyramidal cell synapses and in the perforant path-dentate granule cell synapses, glycine (5 x 10(-4) M) significantly enhanced the short-term potentiation (STP) induced by subthreshold tetanic stimulation, without affecting baseline responses. The effects of glycine resulted in generation of LTP in both synapses. On the other hand, glycine did not influence STP induced by subthreshold tetanus in the mossy fiber-CA3 pyramidal cell synapses. These results suggest that exogenous glycine can facilitate the generation of LTP in the CA1 region and in the dentate gyrus but not in the CA3 region. In the CA1 region and the dentate gyrus, D- and L-serine and D-alanine (10(-3) M) also showed the LTP-facilitating effects in a similar manner to glycine, but D- and L-valine had no effect on LTP generation. Furthermore, glycine and D-serine, but not L-valine, enhanced NMDA receptor-mediated synaptic responses in the absence of extracellular Mg2+. Together, these results make it probable that exogenously applied glycine and related amino acids facilitate the generation of LTP in the CA1 and dentate region by activating the glycine modulatory sites associated with NMDA receptors.

Effect of APV and ketamine on epileptiform activity in the CA1 and CA3 regions of the hippocampus

Intracellular recordings were obtained from hippocampal CA1 and CA3 pyramidal neurons maintained in vitro. The ability of the NMDA receptor antagonists ketamine and APV to suppress picrotoxin-induced epileptiform burst activity was examined. Activity was recorded either after a single orthodromic stimulation, which gave rise to paroxysmal depolarization shift (PDS), or during a 500 msec train of 50 Hz stimulation, which produced a sustained depolarization. In the CA1 and CA3 areas, both the PDS and sustained depolarization were reduced by APV (20 microM) and ketamine (100 microM). APV reduced the area under the PDS by 24 +/- 3% and 32 +/- 4% in CA1 and CA3 neurons, respectively. The corresponding reductions in the sustained depolarization were 10 +/- 2% and 22 +/- 4%. Ketamine reduced the PDS by 43 +/- 4% and 31 +/- 4% in CA1 and CA3 and decreased the sustained depolarization by 21 +/- 3% and 12 +/- 3%. In all cases, NMDA receptor antagonists had a significantly greater effect on the PDS than the sustained depolarization. These results indicate that, although not essential for generation of paroxysmal activity, NMDA receptors make significant contributions to epileptiform activity in both CA1 and CA3 regions of the hippocampus.

Electrophysiological characterization of a novel potent and orally active NMDA receptor antagonist: CGP 37849 and its ethylester CGP 39551

The selectivity and potency of the novel competitive N-methyl-D-aspartate (NMDA) receptor antagonists, CGP 37849 and CGP 39551, were investigated in vitro and in vivo using electrophysiological approaches. Like the reference blocker DL-AP5, both compounds acted in vitro (hippocampus, substantia nigra, spinal cord) to antagonize the excitatory actions of exogenously administered NMDA as well as the synaptically elicited, physiological NMDA receptor responses in hippocampus and spinal cord. In all isolated preparations CGP 37849 was more potent than CGP 39551, and 5- to 10-fold more potent than DL-AP5. Neither compound showed any marked effect on responses evoked by quisqualate and kainate. NMDA excited dopaminergic cells in the pars compacta region of the substantia nigra in a concentration-dependent manner. This effect also could be selectively antagonized by CGP 37849 and CGP 39551. In the anaesthetized rat, excitatory responses of hippocampal pyramidal cells evoked by iontophoretic application of NMDA were antagonized by CGP 37849 and CGP 39551 following their oral administration without reducing quisqualate or kainate responses. In contrast to the in vitro situation, CGP 39551 was more potent than CGP 37849 in vivo. Effective doses were 30 mg/kg p.o. for CGP 39551 and 100 mg/kg p.o. for CGP 37849. In conclusion, it is demonstrated that CGP 37849 and CGP 39551 selectively antagonize NMDA evoked neuronal responses in vivo and in vitro and that the drugs are centrally active following their oral administration.

Activation of the glycine site in the NMDA receptor is necessary for the induction of LTP

The effects of 7-chlorokynurenate (7-C1KY) were examined on N-methyl-D-aspartate (NMDA) receptor-mediated synaptic mechanisms in the CA1 region of rat hippocampal slices. 7-C1KY depressed both the NMDA receptor-mediated component of synaptic transmission recorded in Mg2(+)-free medium and the induction of long-term potentiation (LTP) in Mg2(+)-containing medium. Both of these effects were reversed by D-serine, suggesting that the action of 7-C1KY was at the allosteric glycine site on the NMDA receptor.

The impact of postsynaptic calcium on synaptic transmission--its role in long-term potentiation

Recent studies have gone a long way to explain the steps involved in generating long-term potentiation (LTP). This review focuses on the triggering role of postsynaptic calcium, the sequence of events which might be initiated by calcium, and where the persistent change may ultimately occur during LTP.

Excitatory amino acid-receptor-mediated EPSPs in rat dorsolateral septal nucleus neurones in vitro

1. Intracellular recordings were made from rat dorsolateral septal nucleus (DLSN) neurones in vitro. We investigated depolarizations resulting from pressure application of excitatory amino acids and compared these to synaptically evoked excitatory postsynaptic potentials (EPSPs). 2. EPSPs evoked by focal fimbrial afferent stimulation in saline with 30-50 microM-bicuculline and 1.2 mM-Mg2+ yielded a linear amplitude-voltage relationship: their reversal potential was -3 mV. These EPSPs exhibited little sensitivity to 2-amino-5-phosphonopentanoate (APV), an N-methyl-D-aspartate(NMDA)-receptor-specific antagonist, but were markedly depressed by kynurenic acid, a broad-spectrum excitatory amino acid antagonist. 3. In Mg2(+)-free solution, the amplitude and the duration of EPSPs were increased markedly masking the following inhibitory postsynaptic potential (IPSP) and the late hyperpolarizing potential (LHP). These facilitated and broadened EPSPs were sensitive to APV or Mg2+. The APV or Mg2(+)-sensitive component of the EPSP obtained by digital subtraction suggests a slower time course for the NMDA-receptor-mediated EPSP compared to the non-NMDA-receptor-mediated EPSP. On the other hand, in normal Mg2+ solution an EPSP evoked by either a single strong stimulus or by repetitive stimuli had APV-sensitive components. 4. The depolarizing potentials induced by pressure application of glutamate, kainate, alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA), quisqualate or NMDA were compared. The amplitude-voltage relationship of depolarizations induced by NMDA obtained in a normal Mg2+ solution was non-linear, but approached linearity when the same responses were recorded in a Mg2(+)-free solution. Depolarizations induced by kainate, AMPA and quisqualate were linear in their amplitude-voltage relationship in the presence or absence of Mg2+. APV blocked NMDA-induced depolarizations specifically, while kynurenic acid blocked all the depolarizations induced by NMDA, quisqualate, or kainate. 5. Our data demonstrate the existence of NMDA-receptor-mediated synaptic potentials in the rat DLSN, the characteristics of which are similar to those in other central nervous system regions.

Low-frequency activation of the NMDA receptor system can prevent the induction of LTP

In rat hippocampal slices bathed in Mg2(+)-free medium tetanic stimulation generally failed to elicit long-term potentiation (LTP) in the Schaffer collateral-commissural pathway. However, LTP could be induced in the same slices following the re-introduction of 1 mM Mg2+. In Mg2(+)-free medium, LTP could be induced in the presence of 20 microM D-2-amino-5-phosphonovalerate (APV) but was blocked by increasing the APV concentration to 200 microM. The lower concentration of APV is sufficient to prevent N-methyl-D-aspartate (NMDA) receptor activation during low-frequency transmission but not during the tetanus. We suggest therefore, that certain types of activation of the NMDA receptor system can disable the LTP induction mechanism.

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

A large, extracellular negative DC shift, termed epileptic depolarization, could be elicited during zero magnesium-induced epileptic activity in the rat hippocampal slice. In 10 mM glucose medium, epileptic depolarization was elicited by high-frequency synaptic stimulation. During epileptic depolarization synaptic responses were abolished, but recovered in 10.4 +/- 2.1 min. In low glucose (2 mM) medium, epileptic depolarization either occurred spontaneously or could be elicited by high frequency synaptic stimulation, and no recovery of synaptic responses was observed for at least 30 min. This long-term synaptic failure was blocked by the competitive NMDA antagonists, 3-[+/-)-2-carboxypiperazin-4-yl)-propyl-1-phosphonate (CPP, 100 microM) and D-2-amino-7-phosphonoheptanoate (D-AP7, 100 microM) when added at the peak of epileptic depolarization, but not 5 min afterwards. Intracellular analysis showed that this extracellular DC shift was correlated with a membrane depolarization which approached 0 mV. With 10 mM glucose medium, the membrane potential returned to resting level in 6.3 +/- 1.9 min. In 2 mM glucose medium, neurons remained depolarized and no recovery was observed. This persistent depolarization could account for the loss of synaptic function recorded extracellularly. Application of 100 microM CPP blocked persistent depolarization and allowed for the recovery of the membrane potential. Epileptic depolarization was also observed during picrotoxin-induced epileptic activity. Both anoxic depolarization during experimental ischemia and epileptic depolarization can trigger long-term synaptic failure and persistent depolarization. Epileptic depolarization and anoxic depolarization may be triggers which can lead to neuronal failure in diseases associated with neuronal degeneration.

The Influence of Magnesium Ions on the NMDA Mediated Responses of Ventral Rhythmic Neurons in the Spinal Cord of Xenopus Embryos

Xenopus embryos immobilized in tubocurarine respond to natural skin stimulation with fictive swimming. This can also occur in saline without Mg2+ and is blocked by NMDA antagonists. Ventral spinal cord neurons which are rhythmically active during swimming are depolarized by bath applied N-methyl-D-aspartate (NMDA) (in 1 microM tetrodotoxin (TTX) to block indirect effects). By using current clamp techniques this depolarization is shown to be partially blocked by 0.5 and 1 mM Mg2+ in a voltage-dependent manner similar to that described in cultured neurons. Mg2+ partially and reversibly reduces the slow NMDA-mediated component of excitatory post-synaptic potentials (EPSPs) in ventral neurons. However, in 1 mM Mg2+ fictive swimming can still be evoked by natural stimulation. The frequency of swimming is slightly lower than in nominally 0 mM Mg2+, but the pattern of ventral root activity and synaptic drive to ventral neurons seems little affected. Fictive swimming can also be induced by applying NMDA to spinal preparations. In 0 mM Mg2+, such rhythmic activity is unstable and transient over a narrow NMDA concentration range. In 0.5 mM Mg2+, continuous rhythmic activity is induced over a wide range of NMDA concentrations. Lower spinal preparations need higher NMDA concentrations to induce activity. We conclude that the neurons rhythmically active in swimming have NMDA receptor channels which show a voltage dependent block in the presence of Mg2+. However, while Mg2+ exerts a powerful stabilizing influence on rhythmic activity induced in spinal embryos by exogenous NMDA, its influence on 'naturally' evoked fictive swimming is less clear. The fictive swimming machinery in the brain and spinal cord can produce stable swimming with or without Mg2+ induced voltage dependency of the NMDA channels.

A grease-gap method for studying the excitatory amino acid pharmacology of CA1 hippocampal pyramidal cells

A grease-gap method for studying the pharmacology of CA1 hippocampal pyramidal cells was developed with use of rat hippocampal slices that included only area CA1 and the retrohippocampal area. These slices were transferred to a two-compartment superfusion chamber and the pyramidal cell bodies in area CA1 were separated from their axons in the subiculum with a grease barrier. The CA1 pyramidal cells were depolarized relative to their axons by superfusion with N-methyl-D-aspartate (NMDA), (RS)-alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA), kainate and L-glutamate. NMDA was unusually potent in the CA1-subiculum slice compared to other preparations. The NMDA receptor antagonists D(-)-2-amino-5-phosphonovalerate (D-AP5), phencyclidine and Mg2+ shifted the NMDA dose-response curve to the right in a parallel manner. Similarly, the quisqualate receptor antagonist pentobarbitone shifted the AMPA dose-response curve to the right. Schild plots for these antagonists had slopes insignificantly different from 1. These results are consistent with the presence of a substantial NMDA receptor reserve on CA1 pyramidal cells. They are also in line with the high density of excitatory amino acid receptors on CA1 hippocampal pyramidal cells and with the known pharmacological properties of these receptors. Grease-gap studies on the CA1-subiculum slice fill the need for a means of obtaining quantitative pharmacological data on CA1 pyramidal cells.

Effects of new non-N-methyl-D-aspartate antagonists on synaptic transmission in the in vitro rat hippocampus

1. The effects of new, potent non-N-methyl-D-aspartate (NMDA) receptor antagonists, 6,7-dinitroquinoxaline-2,3-dione (DNQX) and 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX), have been examined using intra- and extracellular recordings in the hippocampal slice preparation. In terms of potency and selectivity, the action of the two blockers was similar and CNQX was used in most experiments. 2. CNQX reduced the responses to ionophoretic applications of the non-NMDA agonists kainate (KAI) and quisqualate (QUIS) with IC50 values of 1.2 and 4.8 microM, respectively. In Mg2+-free solutions responses to NMDA were generally not affected by concentrations of CNQX up to 25 microM. 3. The action of CNQX was only slowly and poorly reversible on washing. Responses to QUIS and KAI were also reversibly reduced by ionophoretic application of CNQX. 4. CNQX blocked the evoked EPSP in CA1 and CA3 neurones with an IC50 of around 2 microM, which is similar to the IC50 for responses to KAI. CNQX was without effect on the passive membrane properties, the afferent volley and paired pulse potentiation. 5. In the presence of CNQX (greater than 5 microM) a small EPSP remained which was largest in CA1 neurones. It was blocked by low concentrations of the NMDA receptor antagonist (+/-)-2-amino-5-phosphonovaleric acid (APV), was markedly enhanced on removing Mg2+ ions from the bathing medium and, in voltage-clamp experiments, showed a potential dependence which is characteristic of the NMDA ionophore. 6. The latency of the APV-sensitive EPSP in CA1 was the same as the CNQX-sensitive EPSP, indicating that NMDA receptors participate in monosynaptic excitation. 7. Feedback and feed-forward inhibition in both area CA1 and CA3 were sensitive to CNQX. There seemed to be two components of the inhibition, both of which appear to be GABAergic since they could be blocked by picrotoxin (PTX), but only one of which was blocked by CNQX. The CNQX-resistant IPSP was not affected by APV. 8. In conclusion, quinoxalinediones have been used to demonstrate that non-NMDA receptors mediate the majority of the EPSP. Additionally, a component of the EPSP in CA1 is mediated by NMDA receptors and is manifested at resting membrane potentials and in the presence of Mg2+.

Effect of changing extracellular levels of magnesium on spontaneous activity and glutamate release in the mouse neocortical slice

1. The mouse neocortical slice preparation, maintained in a two compartment, grease gap bath, exhibits spontaneous depolarizing activity (with or without rhythmic after potentials) after perfusion with magnesium-free artificial cerebrospinal fluid. 2. If the magnesium concentration is decrementally lowered over an extended time period, then incrementally raised following a similar time course, the spontaneous depolarizing shift activity shows a hysteresis (with regard to both frequency and amplitude), the depolarizing shifts being more resistant to magnesium during the incremental period. 3. The amino acid content of the perfusing fluid was analysed by high performance liquid chromatography (h.p.l.c.). Although a basal efflux of 6 amino acids was quantifiable, only glutamate levels increased following superfusion of the preparation with magnesium-free, artificial cerebrospinal fluid. 4. Glutamate release increased to 266% of the resting release in the presence of magnesium within the first 12 min of the change into magnesium-free artificial cerebrospinal fluid. This increase in release preceded the onset of spontaneous depolarising activity. The release of glutamate remained elevated at 182% of control up to 60 min after perfusion with magnesium-free buffer, when depolarizing activity was well established. 5. A model is presented and discussed for the genesis and maintenance of the spontaneous depolarizing shifts. It is suggested that the maintenance of this spontaneous activity reflects a long term enhancement of neocortical neurone excitability which may be related to long term potentiation in the hippocampus.

NMDA receptor antagonists block norepinephrine-induced long-lasting potentiation and long-term potentiation in rat dentate gyrus

In the in vitro rat dentate gyrus, norepinephrine-induced long-lasting potentiation (NELLP) and long-term potentiation (LTP) of responses to perforant path stimulation were blocked by the N-methyl-D-aspartate (NMDA) receptor antagonists, D(-)-2-amino-5-phosphonovaleric acid (D(-)APV) and 3-[(+/-)-2-carboxypiperazin-4-yl]propyl-1-phosphonic acid (CPP). CPP and D(-)APV, but not L(+)APV, also depressed the orthodromic population spike but not the antidromic spike, which suggests that these receptors may function in low-frequency evoked activity of granule cells. We conclude that NELLP, like LTP in the dentate gyrus, requires NMDA receptor activation.

Excitatory amino acid blockers differentially affect bursting of in vitro hippocampal neurons in two pharmacological models of epilepsy

The role of excitatory amino acid neurotransmitters in generating two distinct types of epileptiform discharge in the CA3 region of hippocampal slices was examined in the guinea-pig using a variety of different excitatory amino acid blockers. In magnesium-free medium the N-methyl-D-aspartate receptor antagonist, DL-aminophosphonovaleric acid, and the putative N-methyl-D-aspartate channel blockers phencyclidine and ketamine, reduced the amplitude and duration of spontaneous bursts and blocked their afterdischarges. In marked contrast none of these compounds significantly depressed spontaneous bursts recorded in the presence of 2 mM Mg2+ and penicillin. This was confirmed for individual slices where Mg2+-free bursts had previously been suppressed by the same concentration of blocker. Therefore, the N-methyl-D-aspartate receptor mechanism does not contribute significantly to the generation of this type of epileptiform discharge. The channel blockers lost their effect on Mg2+-free bursts in the presence of penicillin; however, the receptor blockers did not. This is probably due to the unique mechanisms of action of the channel blockers. This implies that different types of N-methyl-D-aspartate blockers might be effective anticonvulsants only in specific conditions. While the non-specific excitatory amino acid blocker, kynurenic acid, was effective against both burst types it appeared to suppress them by different mechanisms. Kynurenic acid depressed the amplitude and duration of Mg2+-free bursts but its only effect on penicillin bursts was to reduce their frequency. This suggests that neither N-methyl-D-aspartate nor non-N-methyl-D-aspartate receptors play a major role in the production of the paroxysmal depolarizing shift resulting from the block of GABA-mediated inhibition by penicillin. However, these receptors may be involved in generating the spontaneous activity which triggers the bursts.

Synaptic activation of N-methyl-D-aspartate and non-N-methyl-D-aspartate receptors in the mossy fibre pathway in adult and immature rat cerebellar slices

The participation of excitatory amino acid receptors in mossy fibre-granule cell synapses in lobule VIa of adult and immature rat cerebellar slices was investigated using an extracellular grease-gap technique. For the immature slices, the age selected (14 days after birth) was one at which the sensitivity of granule cells to exogenous N-methyl-D-aspartate is much higher than in the adult. The principal synaptic potentials observed after low-frequency electrical stimulation of the white matter resembled closely those found to be centred in the granule cell layer in field potential studies in the cat in vivo. They comprised a short latency negative potential, a slow negative wave and, in the adult, a further late negative wave. In the adult, with 1.2 mM Mg2+ in the perfusing solution, none of these potentials was significantly affected by the N-methyl-D-aspartate antagonist, 2-amino-5-phosphonovalerate, but they were all markedly inhibited by the broad spectrum antagonist, kynurenate, and, more potently, by the selective non-N-methyl-D-aspartate receptor blocker, 6-cyano-2,3-dihydroxy-7-nitro-quinoxaline. After removal of Mg2+, which has a blocking action on the ion channels associated with N-methyl-D-aspartate receptors, the size of all the potentials increased. The increase in the short latency potential was insensitive to 2-amino-5-phosphonovalerate but a component of the slow negative wave (and of the late negative wave) was reduced back to control levels by the antagonist. Application of 6-cyano-2,3-dihydroxy-7-nitro-quinoxaline (10 microM) in Mg2+-free solution revealed, in near isolation, a slow wave (latency to peak, 28 ms) which could be abolished by 2-amino-5-phosphonovalerate. In the immature slices, bathed in normal (Mg2+-containing) medium, 2-amino-5-phosphonovalerate caused a small reduction in the short latency potential and inhibited a component of the slow negative wave which could, again, be observed in relative isolation after perfusion of 6-cyano-2,3-dihydroxy-7-nitro-quinoxaline. Removal of Mg2+ increased the amplitudes of the short latency potential and the slow negative wave in a manner which was sensitive to 2-amino-5-phosphonovalerate and increased the size of the slow, 6-cyano-2,3-dihydroxy-7-nitro-quinoxaline-resistant wave. It is concluded that glutamate is likely to be the transmitter released by mossy fibres, at least those innervating lobule VIa.(ABSTRACT TRUNCATED AT 400 WORDS)

The role of excitatory amino acids in synaptic transmission in the hippocampus

We have highlighted some aspects of the action of excitatory amino acid transmission in the hippocampus. Fast epsps can be blocked by CNQX to reveal a component of synaptic transmission which is mediated by NMDA receptors. Extracellular recordings of ionic activities show that NMDA and non-NMDA ionophores are permeable to the major monovalent cations, while NMDA ionophores also appear to be permeable to Ca2+. Interactions of agonists applied by iontophoresis may be correlates of phenomena such as LTP, which can be evoked by appropriate synaptic stimulation.

NMDA receptor dependent paroxysmal discharges in the dentate gyrus

The N-methyl-D-aspartic acid subclass of glutamate receptors is thought to be involved in events that depend on repetitive activation of neurons. The present study shows that repetitive stimulation of CA3 can produce epileptiform discharges in the contralateral dentate gyrus. This distinctive paroxysmal response was blocked by the NMDA antagonists, ketamine and MK-801. The MK-801 blockade required previous activation of the dentate gyrus in the presence of the drug, thus demonstrating, in vivo, a use-dependent effect. These studies show that repetitive, low intensity stimulation of hippocampal circuits can produce NMDA-mediated epileptiform discharges in the dentate gyrus.

GABAergic inhibition and epileptiform discharges in the turtle hippocampus in vitro

The effects of excitatory amino acid (EAA) receptor antagonists were examined on intracellularly recorded epileptiform discharges in turtle hippocampal (ventromedial cortical) pyramidal neurons in vitro. Afferent synaptic activation of turtle hippocampal neurons evoked monophasic or biphasic GABAergic inhibitory postsynaptic potentials (IPSPs). In the presence of bicuculline (5 microM) or picrotoxin (100 microM) IPSPs were reduced, and long-lasting ictal-like discharges were transiently observed prior to the establishment of a regular rhythm of discharge of spontaneous paroxysmal depolarization shifts (PDSs). Bicuculline-induced PDSs were reversibly reduced in amplitude and duration, but not abolished by the EAA receptor antagonists kynurenic acid (1 mM), cis-2,3-piperidine dicarboxylic acid (cis-2,3-PDA) (1 mM), or DL-2-amino-5-phosphonovalerate (DL-AP-5) (100 microM), revealing a long-lasting hyperpolarizing afterpotential. These results indicate that the blockade of GABAergic inhibition leads to the genesis of epileptiform discharges, and EAA receptor antagonists (particularly those of the N-methyl-D-aspartate (NMDA) receptor subtype) block the maintained depolarization underlying PDSs, but do not prevent their spontaneous discharge in turtle hippocampus.

CNQX blocks acidic amino acid induced depolarizations and synaptic components mediated by non-NMDA receptors in rat hippocampal slices

6-Cyano-2,3-dihydroxy-7-nitro-quinoxaline (CNQX; FG 9065) is a new excitatory amino acid antagonist. In the spinal cord it has been reported to selectively block responses to acidic amino acids acting at receptors of the non-N-methyl-D-aspartate (non-NMDA) type. Here we report that in rat hippocampal slices bathed in Mg2+-free medium 10 microM CNQX reversibly blocks responses to alpha-amino-3-hydroxy-5-methyl-4-isoxazole-propionic acid (AMPA), quisqualate and kainate but not NMDA. The synaptic response evoked by low frequency stimulation of Schaffer collateral-commissural fibres in 1 mM Mg2+-containing medium is completely blocked by this concentration of CNQX. In contrast the synaptic response evoked in Mg2+-free medium is not fully blocked by CNQX. The CNQX-insensitive component is, however, abolished by addition of a selective NMDA antagonist. The use of CNQX has allowed for the first time selective synaptic activation of NMDA receptors in the hippocampus.

Effects of the NMDA antagonist 2AP5 on complex spike discharge by hippocampal pyramidal cells

The N-methyl-D-aspartate receptor antagonist D,L-2-amino-5-phosphonopentanoate (2AP5) was administered intraventricularly to determine its effect on the complex spike firing pattern of spontaneously active hippocampal pyramidal cells recorded in urethane anaesthetized rats. Following 2AP5 delivery, complex spike firing decreased by a mean 36%, while only a 5% decrease was observed after saline injection. This effect could not be explained by changes in firing rate per se but appeared to be related to the degree of blockade of commissurally induced long-term potentiation. Thus 2AP5 not only disrupts synaptic plasticity in the hippocampus but can also alter the pattern of ongoing activity of the pyramidal cells.