Facilitation of social learning by treatment with an NMDA receptor antagonist

IDRA-21, a positive AMPA receptor modulator, inhibits synaptic and extrasynaptic NMDA receptor mediated events in cultured cerebellar granule cells

IDRA-21 (7-chloro-3-methyl-3,4-dihydro-2H-1,2,4-benzothiadiazine 1,1-dioxide) reduces alpha-amino-3-hydroxy-5-methylisoxazolepropionic acid (AMPA) receptors desensitisation in vitro and restores learning and cognitive impairment in vivo. In this study, we show that in cerebellar granule cells (CGCs) in culture IDRA-21 reduces N-methyl-d-aspartate receptor (NMDAR) whole-cell currents. The effect is neither competitive nor voltage-dependent. The reduction of NMDA currents is stronger at low glycine concentrations suggesting an interaction with this site. IDRA-21 shortens miniature excitatory postsynaptic currents mediated by NMDARs (NMDA-mEPSCs) in CGCs grown in low potassium with no effect on peak amplitudes. By using fast glutamate application onto CGCs nucleated patches, we found that IDRA-21 decreases both decay time constant and amplitude of the current. Experiments performed on recombinant NMDAR expressed in HEK 293 cells showed that IDRA-21 was more effective on NR1a-NR2B than NR1a-NR2A receptors highlighting a subunit selectivity of the drug. Our findings make light on a novel target for IDRA-21: NMDA receptors function is negatively modulated and the different action at the level of extrasynaptic and synaptic receptors could be ascribed to a partial selectivity for NR2B subunits.

Hippocampal mGluR1 and consolidation of contextual fear conditioning

The effects of post-training intra-hippocampal injections of group I mGluR agonists and antagonists, were examined in the contextual fear test, in rats. It was found that (S)-3,5-dihydroxyphenylglycine (DHPG) (a mGluR1-5 agonist) decreased, and (RS)-1-aminoindan-1,5-dicarboxylic acid (AIDA) (a mGluR1 antagonist) increased fear conditioning (a freezing reaction), examined 24h after conditioning session. (RS)-2-Chloro-5-hydroxyphenylglycine (CHPG) (a mGluR5 agonist), and 2-methyl-6-(phenylethynyl)pyridine hydrochloride (MPEP) (a mGluR5 antagonist) did not cause any effect. In the immunocytochemical study, the post-conditioning administration of AIDA decreased the c-Fos induction in the dentate gyrus and CA1 layer of the hippocampus proper, 2h after exposure of animals to the aversive context, and 24h after conditioning session. It is suggested that overactivation of glutamatergic transmission in the critical for memory trace formation structure and period of time, may result in an attenuation of memory consolidation. On the other hand, reduction of an exaggerated glutamatergic tone can facilitate learning and memory processes. The immunocytochemical study and factor analysis of experimental data revealed that hippocampal mGlu1 receptors significantly influence the memory consolidation in a way dependant on the level of glutamatergic activity. Furthermore, they indicate that changes of glutamatergic activity within brain limbic structures can affect the threshold for the induction of the long-term neuronal plasticity, involved in some forms of learning and memory.

NMDA receptor pathways as drug targets

Since the mid 1980s, there has been a great deal of enthusiasm within both academia and industry about the therapeutic potential of drugs targeting the NMDA subtype of glutamate receptors. That early promise is just beginning to translate into approvable drugs. Here we review the reasons for this slow progress and critically assess the future prospects for drugs that act on NMDA receptor pathways, including potential treatments for some major disorders such as stroke and Alzheimer's disease, for which effective therapies are still lacking.

N-Methyl-d-aspartate improved social recognition potency in rats

Activation of N-methyl-D-aspartate (NMDA) receptors is believed to play an important role in cognitive processes. In the present study we addressed the question of whether systematically administered NMDA can improve social recognition in adult male rats. We utilized the paradigm in which an adult animal is exposed to the same or a novel juvenile during two successive interactions. The adults given NMDA (10, 25 and 50 mg/kg, s.c.) immediately after the initial exposure to a juvenile exhibited significantly reduced social investigation during the second encounter with the same juvenile performed both 120 and 240 min later. Reduction in social investigation was not observed in saline treated animals as well as in those treated with both NMDA and saline but re-exposed to novel juveniles. These results indicate that NMDA improved social recognition potency of adult animals and prolonged retention of previously stored olfactory information.

Synaptic plasticity and memory: an evaluation of the hypothesis

Changing the strength of connections between neurons is widely assumed to be the mechanism by which memory traces are encoded and stored in the central nervous system. In its most general form, the synaptic plasticity and memory hypothesis states that "activity-dependent synaptic plasticity is induced at appropriate synapses during memory formation and is both necessary and sufficient for the information storage underlying the type of memory mediated by the brain area in which that plasticity is observed." We outline a set of criteria by which this hypothesis can be judged and describe a range of experimental strategies used to investigate it. We review both classical and newly discovered properties of synaptic plasticity and stress the importance of the neural architecture and synaptic learning rules of the network in which it is embedded. The greater part of the article focuses on types of memory mediated by the hippocampus, amygdala, and cortex. We conclude that a wealth of data supports the notion that synaptic plasticity is necessary for learning and memory, but that little data currently supports the notion of sufficiency.

Glycine(B) receptor antagonists and partial agonists prevent memory deficits in inhibitory avoidance learning

Activation of N-methyl-d-aspartate (NMDA) receptors has been hypothesized to mediate certain forms of learning and memory. This hypothesis is based on the ability of competitive and uncompetitive NMDA receptor antagonists to disrupt learning. We investigated the effects of glycine site antagonists and partial agonists on deficits of acquisition (learning) and consolidation (memory) in a single trial inhibitory avoidance learning paradigm. Posttraining administration of either hypoxia (exposure to 7% oxygen) or the convulsant drug pentylenetetrazole (PTZ) (45 mg/kg) to mice impaired consolidation without producing neuronal cell death. Pretreatment with the competitive glycine antagonist 7-chlorokynurenic acid (7KYN) and the glycine partial agonists 1-aminocyclopropanecarboxylic acid (ACPC) and (+)HA-966 prevented memory deficits induced by hypoxia and PTZ, but did not affect scopolamine-induced learning impairment. In addition, ACPC prevented consolidation deficits evoked by a nonexcitotoxic concentration of l-trans-pyrrolidine-2, 4-dicarboxylate, a competitive inhibitor of glutamate transport that increases extracellular levels of glutamate. Moreover, (+)HA-966, 7KYN, and ACPC facilitated both acquisition and consolidation of inhibitory avoidance training, an effect that was dose-dependent and reversed by glycine. These results indicate that memory deficits induced by both hypoxia and PTZ involve NMDA receptor activation. Furthermore, the present findings demonstrate that glycine site antagonists and partial agonists prevent memory deficits of inhibitory avoidance learning by affecting consolidation, but not acquisition processes.

Neurohypophyseal peptides and social recognition in rats

An encounter between rats results in bouts of social investigation consisting mainly of sniffing, nosing, following and grooming. The assessment of social recognition is based on the tendency of rodents to investigate unfamiliar conspecifics more intensely, than familiar ones. In the laboratory an immature conspecific is normally used as the social stimulus because the use of juveniles eliminates possible sexual and/or aggressive behaviors of the rat whose memory is assessed. When a juvenile is presented for the first time, it is intensely investigated. A second presentation shortly after the first one elicits less attention. This is not due to satiation or fatigue, since the presentation of a novel juvenile triggers the full sequence of investigation. Social recognition is defined as a specific decrease in social investigation during the second encounter of the same individual. This form of memory is short lasting (< 40 min) and based on the olfactory characteristics of the stimulus animal. Social memory is prolonged by repeated exposure to the stimulus juvenile rat and is impaired by retroactively interfering stimuli. It can be facilitated by vasopressin and derivatives as well as by several other memory facilitating compounds, and, depending on the dose, attenuated or facilitated by oxytocin and derivatives. Ethologically oriented memory tests, that are based on olfactory characteristics of the information to-be-remembered, have an advantage over 'classical' ones: they estimate behavioral patterns which are important to an animal and not only to the investigator. Social memory paradigms can reveal information about memory processes in animals that is relevant for memory deficits in humans.

Effects of oral administration of the competitive N-methyl-D-aspartate antagonist, CGP 40116, on passive avoidance, spatial learning, and neuromotor abilities in mice

The effects were investigated of the potent competitive N-methyl-D-aspartate (NMDA) receptor antagonist CGP 40116[D-(E)-2-amino-4-methyl-5-phosphono-3-pentenoic acid] on the performance of mice in water maze and passive avoidance tasks, and in wire suspension, rotarod, and cage activity tests. The drug was administered per os (p.o.) in its anticonvulsant dose range. CGP 40116 dose-dependently impaired passive avoidance learning when given before, but not when given after training. The antagonist (5, 10, and 20 mg/kg, administered 4 h before each training session) dose-dependently affected water maze acquisition, and impaired retention test performance in both hidden- and visible-platform water maze tasks. In addition, the drug dose-dependently decreased swimming speed during water maze acquisition. Repeated administration of CGP 40116 (20 mg/kg, p.o.) persistently decreased cage activity and wire suspension test performance, whereas motor coordination and equilibrium on the rotarod apparatus remained unimpaired. In our administration protocol, no tolerance was found to the effects of the drug on passive avoidance learning and neuromotor abilities. The parallel effects of CGP 40116 on memory and motor performance are discussed, and it was concluded that the antagonist impairs neuromotor abilities and also induces memory impairments which cannot be entirely reduced to motor interference.

Uncompetitive NMDA receptor antagonists attenuate NMDA-induced impairment of passive avoidance learning and LTP

In general, N-methyl-D-aspartate (NMDA) receptor antagonists inhibit learning and long term potentiation (LTP). However, it has been suggested that direct tonic, i.e. non-temporal, activation of NMDA receptors, in contrast to learning, may lead to an increase in synaptic "noise" and, in turn, to a loss of association detection. In the present study, a two-choice passive avoidance task and LTP in vitro (CA1 hippocampal region) were used to address this issue. Dark avoidance learning was impaired by systemic NMDA administration (starting at 25 mg/kg) that was not related to either toxic effects or state-dependent learning. NMDA-induced amnesia was antagonized by ((+)-5-methyl-10,11-dihydro-5H-dibenzocyclohepten-5,10-imine maleate (MK-801) and 1-amino-3,5-dimethyladamantane (memantine), starting at low doses of 0.05 and 2.5 mg/kg, respectively, in a bell-shaped dose-response relationship. A competitive NMDA receptor antagonist CGP-39551 failed to reverse NMDA-induced amnesia. In hippocampal slices, NMDA (10 microM) depressed (S)-alpha-amino-3-hydroxy-5-methyl-4-isoxazolproprionic acid (AMPA) receptor-mediated field potentials in CA1 and also caused a moderate reduction of LTP induction/expression. It was this latter effect that was antagonized by memantine (1 microM). Thus, under conditions of tonic activation of NMDA receptors, uncompetitive NMDA receptor antagonists can paradoxically reverse deficits in learning and synaptic plasticity.

CGP 36,742, an orally active GABAB receptor antagonist, facilitates memory in a social recognition test in rats

CGP 36,742, an orally active GABAB receptor antagonist, improves the retention performance of rats in a social recognition test. This effect is detectable over a very wide range of doses (0.03 to 300 mg/kg, p.o.). Considering its binding (32 mumol affinity for the GABAB site) the surprisingly potent activity of CGP 36,742 makes it appear quite possible that the effect is mediated by an as yet unknown receptor subtype.

Effects of the uncompetitive NMDA receptor antagonist memantine on hippocampal long-term potentiation, short-term exploratory modulation and spatial memory in awake, freely moving rats

Chronic treatment of adult male F-344 rats (9-12 months old) with therapeutically relevant doses of memantine (30 mg/kg/day in chow for > 8 weeks) increased the maintenance of long-term potentiation of field excitatory postsynaptic potentials from perforant path-granule cell hippocampal synapses recorded in the fascia dentata in vivo. In contrast, there was no effect of memantine on baseline synaptic responses or population spikes. Likewise, short-term exploratory modulation of these hippocampal evoked responses was not different between memantine-treated and control rats. Both groups of rats were able to learn the spatial version of the Morris water task equally well, but the memantine-treated group showed a strong tendency to show more selective spatial search patterns in the training quadrant of the water pool during a final probe trial. As such, these studies provide the first electrophysiological evidence that memantine can increase the durability of synaptic plasticity and provide preclinical confirmation of the cognitive improvement seen with memantine in the treatment of demented patients.

Effects of the competitive NMDA receptor antagonist CGP 37849 on performance of reference and working memory tasks by rats

The effects of the competitive NMDA receptor antagonist CGP 37849 (DL-(E)-2-amino-4-methyl-5-phosphono-3-pentenoic acid) were tested in rats performing (1) a nonspatial working memory task, and (2) a reference memory task that was either partly or fully learnt. CGP 37849 attenuated accuracy in all three tests, suggesting that sensorimotor effects may obscure any drug effects on memory itself.

Effects of post-ethanol administration of NMDA and non-NMDA receptor antagonists on the development of ethanol tolerance in C57B1 mice

The effect of post-ethanol administration of NMDA and non-NMDA receptor antagonists on the development of environment-dependent ethanol tolerance was studied in C57B1 mice. Ethanol tolerance was produced by daily injections of ethanol (3.5 g/kg, IP) in the same experimental environment and measured as ethanol-produced "sleep-time" during 5 consecutive days. The non-competitive NMDA receptor antagonist, dizocilpine (MK-801; 0.1 mg/kg, IP), and the competitive NMDA receptor antagonist, CGP 39551 (5 mg/kg, IP), both given 120 min after the administration of ethanol, inhibited the development of tolerance to the hypnotic actions of ethanol. In contrast, the development of ethanol tolerance was not altered by administration of the specific AMPA/KA receptor blocking agents, NBQX (10 mg/kg, IP), and LY326325 (2.5 mg/kg, IP), respectively. Modulation of NMDA receptor activity by drugs like NMDA, d-cycloserine, and milacemide, which are known to enhance learning and memory in rodents, had no significant effect on the development of ethanol tolerance. Our present data confirm and extend previous findings which indicate that NMDA, but not non-NMDA, glutamate receptors may play an important role in the neuroadaptive processes associated with the development of ethanol tolerance.