Selective impairment of learning and blockade of long-term potentiation by an N-methyl-D-aspartate receptor antagonist, AP5

Identification and properties of N-methyl-D-aspartate receptors in rat brain synaptic plasma membranes

The excitatory amino acid receptors selectively activated by N-methyl-D-aspartate (N-Me-D-Asp) (also known as NMDA) are a major determinant of central nervous system neuronal excitability. We report here that rat brain synaptic plasma membranes contain a distinct population of L-[3H]glutamate binding sites with pharmacological properties indicative of the N-Me-D-Asp receptor. The N-Me-D-Asp sites are readily distinguished from other L-[3H]glutamate binding and uptake sites by their sharp pH optimum, more rapid association rate, preferential localization in synaptic structures, and lack of dependence on temperature and inorganic ions. As with other receptor systems, ligand binding at the N-Me-D-Asp site is reduced by guanine nucleotides but not by adenosine nucleotides. Binding is insensitive to ketamine and cyclazocine, indicating that sigma opiates inhibit N-Me-D-Asp excitation at a site different from that of the N-Me-D-Asp binding site. The quantitative pharmacological properties of N-Me-D-Asp-sensitive L-[3H]glutamate binding sites determined in a well-defined dendritic field (stratum radiatum of CA1) by quantitative autoradiography closely correlate to those of both the electrophysiologically identified N-Me-D-Asp receptors in the same dendritic field and the N-Me-D-Asp sites studied in membrane preparations. Under conditions that selectively reveal N-Me-D-Asp receptors, these sites are found to exhibit considerable anatomical specificity as evidenced by variations within cortical, striatal, and thalamic regions. Autoradiography also showed that regions in rodent and primate brain that are especially sensitive to anoxic and excitotoxic neuronal damage (e.g., Sommer's sector or CA1) have a high level of N-Me-D-Asp sites. Since N-Me-D-Asp receptors are known to contribute to these causes of neuronal loss, their selective distribution partially accounts for the pattern of selective damage seen in these pathological conditions.

Long-term potentiation of guinea pig mossy fiber responses is not blocked by N-methyl D-aspartate antagonists

Receptors preferentially activated by the excitatory amino acid N-methyl-D-aspartate (NMDA) do not mediate synaptic transmission in the hippocampus but are involved in initiating long-term potentiation (LTP) in hippocampal region CA1. We have examined the role of NMDA receptors in LTP of the commissural/associational and mossy fiber pathways to region CA3 pyramidal neurons. In the commissural/associational pathway, NMDA receptor blockers did not reduce synaptic responses but reversibly blocked the induction of LTP. In contrast, NMDA receptor blockers had no effect on mossy fiber LTP. These results suggest that induction of commissural/associational LTP differs from mossy fiber LTP, although the mechanisms underlying expression of LTP along these pathways could be similar. Kynurenate and L-2-amino-4-phosphonobutyrate, which potently reduce mossy fiber responses, also did not block induction of mossy fiber LTP.

4-Aminopyridine-mediated increase in long-term potentiation in CA1 of the rat hippocampus

The effect of 4-aminopyridine (4-AP) on long-term potentiation (LTP) was studied in the hippocampal slice preparation of the rat. Field excitatory postsynaptic potentials (EPSPs) were recorded and evoked in the stratum radiatum of the CA1. Both the low frequency EPSP and LTP of the EPSP were significantly increased by treatment with 4-AP. These effects were inhibited by increasing the magnesium concentration from 1 to 4 mM. Pretreatment with 20 microM DL-2-amino-5-phosphonovalerate antagonized only the increase in LTP produced by 4-AP. It is suggested that 4-AP enhances Ca influx either pre- or postsynaptically and thereby increases LTP.

Laminar profiles of the changes in extracellular calcium concentration induced by repetitive stimulation and excitatory amino acids in the rat dentate gyrus

Changes in extracellular free calcium concentration ([Ca2+]o) were measured with ion-selective microelectrodes in the dentate gyrus of the rat hippocampal slice preparation. Repetitive stimulation (20 Hz/10 s) of the perforant path or mossy fibers, or alternatively, iontophoresis of the excitatory amino acids N-methyl-D-aspartate (NMDA) or quisqualate (Quis) elicited decreases in [Ca2+]o (delta Ca2+) which were unequally distributed along the axis of the granule cells. Laminar profiles of the [Ca2+]o changes revealed marked differences between stimulus- and amino acid-induced responses. The delta Ca2+ induced by either anti- or orthodromic stimulation were relatively small (less than 0.15 mM) and were found to be maximal at the cell body layer (stratum granulosum). In contrast, the excitatory amino acids NMDA and Quis evoked large delta Ca2+ (greater than 1.2 mM) which were maximal at dendritic sites, 100 microns away from the cell body layer in the inner stratum moleculare. The effect of NMDA was reversibly blocked by 2-amino-5-phosphonovaleric acid without antagonism of the synaptic responses or the stimulus-induced changes in [Ca2+]o. Therefore, under normal conditions, NMDA receptors appear not to participate in synaptically induced delta Ca2+ in the dentate gyrus.

Correlation between long-term potentiation and release of endogenous amino acids from dentate gyrus of anaesthetized rats

The relationship between long-term potentiation (l.t.p.) and the release of endogenous amino acid transmitters has been investigated in the dentate gyrus of rats anaesthetized with urethane. The molecular layer was perfused with artificial cerebrospinal fluid using a push-pull cannula. The perfusate was collected and analysed for glutamate, aspartate, glycine, glutamine and gamma-aminobutyric acid (GABA) using high-performance liquid chromatography (h.p.l.c.) with fluorometric detection. Recording electrodes were attached to the cannula to enable responses evoked by test stimuli to the perforant path to be monitored in the molecular and cell body layers. Perfusion was continued for 3 h while test stimuli were delivered to the perforant path at 30 s intervals. In the control group (n = 8), no further stimulation was given. In a second group (n = 8), a single high-frequency train (250 Hz for 200 ms) was delivered at the end of the first hour to induce l.t.p. The average potentiation of the slope of the excitatory post-synaptic potential (e.p.s.p.) 2 h later was 15%. In a third group (n = 8), the train to the perforant path was paired with a train to the commissural input to the dentate gyrus, a procedure which blocks the induction of l.t.p. In the potentiated group, there was an increase in the concentrations of glutamate and aspartate following the induction of l.t.p., relative to the decline seen in corresponding periods of the control group. This increase remained statistically significant for 1.5 h in the case of glutamate and for 45 min in the case of aspartate. There were no l.t.p.-associated changes in the release of glutamine or glycine; there was an indication that l.t.p. may be associated with a decrease in the release of GABA. Increasing the frequency and intensity of perforant path activation resulted in enhanced concentrations of glutamate and aspartate in the perfusate; no such changes occurred when granule cells were activated antidromically. We discuss the origin of the relative increases in the concentration of glutamate and aspartate which are found in the perfusate following the induction of l.t.p. and conclude that the most likely source is a sustained increase in activity-dependent release of these amino acids from perforant path terminals.(ABSTRACT TRUNCATED AT 400 WORDS)

Aniracetam augments, and midazolam inhibits, the long-term potentiation in guinea-pig hippocampal slices

The effects of aniracetam, a nootropic drug, and midazolam, which produces amnesia, on the long-term potentiation (LTP) of population spikes was investigated using hippocampal slices (CA3 area) from the guinea pig. Aniracetam at concentrations of 10(-7) and 10(-8) M, but not at 10(-6) M, significantly augmented LTP. On the other hand, midazolam (10(-9) M) significantly suppressed LTP. The suppressive effect was antagonized by Ro 15-1788 (10(-8) M). Both drugs did not affect the population spikes in the absence of tetanic stimulation at those concentrations. It was suggested that in vitro application of LTP is a feasible model system for evaluating the nootropic activity of drugs.

Frequency-dependent involvement of NMDA receptors in the hippocampus: a novel synaptic mechanism

Acidic amino acids, such as l-glutamate, are believed to be excitatory neurotransmitters in the mammalian brain and exert effects on several different receptors named after the selective agonists kainate, quisqualate and N-methyl-D-aspartate (NMDA). The first two receptors collectively termed non-NMDA receptors, have been implicated in the mediation of synaptic transmission in many excitatory pathways in the central nervous system (CNS), whereas NMDA receptors, with few exceptions do not appear to be involved; this is typified in the hippocampus where there is a high density of NMDA receptors yet selective NMDA receptor antagonists, such as D-2-amino-5-phosphonovalerate (APV), do not affect synaptic potentials. NMDA receptors have, however, been shown to be involved in long-term potentiation (LTP) in the hippocampus, a form of synaptic plasticity which may be involved in learning and memory. NMDA receptors have also been found to contribute to epileptiform activity in this region. We now describe how NMDA receptors can participate during high-frequency synaptic transmission in the hippocampus, their involvement during low-frequency transmission being greatly suppressed by Mg2+. A frequency dependent alleviation of this blockade provides a novel synaptic mechanism whereby a single neurotransmitter can transmit very different information depending on the temporal nature of the input. This mechanism could account for the involvement of NMDA receptors in the initiation of LPT and their contribution, in part, to epileptic activity.