Impairment of instrumental learning in rats by glutamic acid diethyl ester

Alcohol and cannabinoid binges and daily exposure to nicotine in adolescent/young adult rats induce sex-dependent long-term appetitive instrumental learning impairment

Adolescence is a critical developmental period, concerning anatomical, neurochemical and behavioral changes. Moreover, adolescents are more sensitive to the long-term deleterious effects of drug abuse. Binge-like consumption of alcohol and marijuana, along with tobacco smoking, is a dangerous pattern often observed in adolescents during weekends. Nevertheless, the long-term effect of their adolescent co-exposure has not been yet experimentally investigated. Long-Evans adolescent male (n = 20) and female (n = 20) rats from postnatal day 30 (P30) until P60 were daily treated with nicotine (0.3 mg/kg, i.p.), and, on two consecutive 'binging days' per week (for a total of eight times), received an intragastric ethanol solution (3 g/kg) and an intraperitoneal (i.p.) dose of cannabinoid 1/2 receptor agonist WIN55,212-2 (1.2 mg/kg). These rats were tested after treatment discontinuation at > P90 for associative food-rewarded operant learning in the two-lever conditioning chambers for six consecutive days on a fixed ratio 1 (FR1) schedule followed by another six days of daily FR2 schedule testing, after 42 days rest. We found the main effects of sex x treatment interactions in FR1 but not in FR2 experiments. Treated females show attenuated operant responses for food pellets during all FR1 and the FR2 schedule, whilst the treated males show an impairment in FR2 but not in the FR1 schedule. Moreover, the treated females' percentage of learners was significantly lower than female controls in FR1 while treated males were lower than controls in FR2. Our findings suggest that intermittent adolescent abuse of common drugs, such as alcohol and marijuana, and chronic tobacco exposure can cause significant long-term effects on motivation for natural reinforcers later in adulthood in both sexes. Females appear to be sensitive earlier to the deleterious effects of adolescent polydrug abuse, with both sexes having an increased likelihood of developing lifelong brain alterations.

From benefit to damage. Glutamate and advanced glycation end products in Alzheimer brain

The glutamatergic system is the most widespread neurotransmitter system in the mammalian brain. It is connected to the acetylcholinergic neurotransmitter system to form the glutamatergic/aspartatergic-acetylcholinergic circuit, which is the morphobiochemical basis of learning, memory and cognition assisted by the glutamatergic N-methyl-D-aspartate receptor, which mediates long-term potentiation as the fundamental molecular mechanisms of these mental capacities. Glutamate and acetylcholine as ligands of the two neurotransmitter systems are products of the neuronal glucose metabolism as holds true also for advanced glycation end products (AGEs), which are markers of damaged and/or aged proteins. During normal aging, both the neurotransmitters glutamate and acetylcholine undergo strong functional variations. Their synthesis was found to be reduced as a common feature. In contrast, basal release of acetylcholine and receptor number decrease, whereas basal release of glutamate and receptor number increase. AGEs increase during aging obviously preferentially in glutamatergic pyramidal neurons in cerebral cortical layers prone to neurodegeneration. In sporadic Alzheimer disease (SAD), glutamate concentration was shown to fall since it may serve as a substitute for lacking glucose in the beginning of the disease. In contrast, glutamate receptor density was found to be much less involved indicating an excessive activation of the glutamatergic neurotransmitter system particularly via the NMDA receptor, mediating endogenous excitotoxicity. The morphological hallmarks of SAD neuritic plaques and neurofibrillary tangles have been demonstrated to crosslink with AGEs causing an increased rate of free radical production. First data from animal studies and investigations on human beings may indicate that the NMDA receptor antagonist memantine may have beneficial effects on the course of SAD and its clinical symptoms.

Blockade of spontaneous posttraining performance improvement in mice by NMDA antagonists

We investigated the effects of immediate post-training systemic administration of gamma-L-glutamyl-L-aspartate (gamma-LGLA) and 3-(2-carboxypiperazine-4-yl)-propyl-1-phosphonate (CPP), antagonists at the N-methyl-D-aspartate receptor, in a lever-press task in two inbred strains of mice. When retention performance was tested in control animals 24 h after partial acquisition of the task. BALB/c mice exhibited a spontaneous performance improvement whereas C57BL/6J mice did not gamma-LGLA at doses of 2.5 and 25 mumol/kg and CPP at doses ranging between 0.025 and 2.5 mumol/kg blocked the spontaneous performance improvement found in BALB/c mice but had no apparent effects on the retention performance of C57BL/6J mice. These data suggest that retention impairment induced by CPP and gamma-LGLA in BALB/c mice results from an interference with posttraining memory processes.

Complex roles of glutamate in the Gibbs-Ng model of one-trial aversive learning in the new-born chick

Glutamate is the most widespread excitatory transmitter in the CNS and is probably involved in LTP, a neural phenomenon which may be associated with learning and memory formation. Intracerebral injection of large amounts of glutamate between 5 min and 2.5 min after passive avoidance learning in young chicks inhibits short-term memory, which occurs between 0 and 10 min post-learning in a three-stage model of memory formation first established by Gibbs and Ng(25) [Physiol. Behav. 23:369-375; 1979]. This effect may be attributed to non-specific excitation. Blockade of glutamate uptake by L-aspartic and beta-hydroxamate also abolishes this stage of memory, provided the drug is administered within 2.5 min of learning. Interference with either production of percursors for transmitter glutamate in astrocytes or with glutamate receptors is also detrimental to memory formation, but the effects appear much later. After its release from glutamatergic neurons, glutamate is, to a large extent, accumulated into astrocytes where it is converted to glutamine, which can be returned to glutamatergic neurons and reutilized for synthesis of transmitter glutamate, and partly oxidized as a metabolic substrate. The latter process leads to a net loss of transmitter glutamate which can be compensated for by de novo synthesis of a glutamate precursor alpha-ketoglutarate (alpha KG) in astrocytes, a process which is inhibited by the astrocyte-specific toxin fluoroacetate (R. A. Swanson, personal communication). Intracerebral injection of this toxin abolishes memory during an intermediate stage of memory processing occurring between 20 and 30 min post-training (50) [Cog. Brain Res, 2:93-102; 1994]. Injection of methionine sulfoximine (MSO), a specific inhibitor of glutamine synthetase, which interferes with the re-supply of transmitter glutamate to neurons by inhibition of glutamine synthesis in astrocytes, has a similar effect. This effect of MSO is prevented by intracerebral injection of glutamate, glutamine, or a combination and alpha KG and alanine. MSO must be administered before learning, but does not interfere with acquisition since short-term memory remains intact. Administration of either the NMDA antagonist AP5, the AMPA antagonist DNQX, or the metabotropic receptor antagonist MCPF, also induces amnesia. Memory loss in each case does not occur until after 70 min post-training, during a protein synthesis-dependent long-term memory stage which begins at 60 min following learning. However, to be effective, AP5 must be administered within 60 s following learning, MCPG before 15 min post-learning, and DNQX between 15 and 25 min after learning. Together, these findings suggest that learning results in an immediate release of glutamate, followed by a secondary release of this transmitter at later stages of processing of the memory trace, and that one or both of these increases in extracellular glutamate concentration are essential for the consolidation of long-term memory. Since both fluoroacetate and MSO act exclusively on glial cells, the findings also show that neuronal-glial interactions are necessary during the establishment of memory.

Behavioral effects of non-NMDA glutamate receptor antagonists

Non-NMDA receptor antagonists decrease motor activity in some situations, alter the sleep-wake cycle, possess anticonvulsant and neuroprotectant actions, and appear to impair some learning tasks but not others. NMDA receptor antagonists affect these same functions but often in different and even opposite ways. NMDA receptor antagonists impair many different spatial learning tasks, including the Morris water maze, the Olton radial maze, and the hole-board task. Non-NMDA receptor antagonists are either ineffective in these spatial tasks or have not yet been evaluated. However, non-NMDA receptor antagonists may impair associative processes required in a bar-press response and in discrimination learning. Further research is needed in the context of comparing NMDA as opposed to non-NMDA receptor antagonists within the same paradigm.

Effects of ketamine and L-glutamic acid diethyl ester on spatial and nonspatial learning tasks in rats

An NMDA antagonist, ketamine, at the highest dose tested (15 mg/kg), impaired the acquisition of a hole-board spatial learning task but not the acquisition of a left-right alternation task. A non-NMDA (quisqualate) antagonist, L-glutamic acid diethyl ester (LGDE), did not impair the acquisition of either task. Both drugs had effects on different aspects of a go-no go discrimination task and a straight runway task, ketamine tending to activate and LGDE tending to slow rats. These results concur with previous research regarding the sensitivity of some spatial tasks to NMDA antagonism. Non-NMDA antagonists affect behavior without causing spatial deficits.

Effects of ketamine and 1-glutamic acid diethyl ester on concept learning in rats

The effects of ketamine, an NMDA receptor antagonist, and 1-glutamic acid diethyl ester (LGDE), a non-NMDA glutamate antagonist, were evaluated in the acquisition of concept learning in a water maze. In concept learning, the rats must locate an invisible platform whose location changes from day to day. In spatial learning (Morris task), the rats must locate an invisible (or visible) platform whose location does not change. Ketamine increased quadrant entries at 5, 10 and 20 mg/kg, and latencies at 10 and 20 mg/kg on the final two days of training on the concept task. At 5 mg/kg ketamine disrupted concept learning but not spatial learning or visuo-motor coordination as assessed by invisible and visible platform conditions of the Morris maze. Progressively higher doses of ketamine affected first the invisible condition and then the visible platform condition. On the other hand, LGDE did not affect the Morris task at any dose. However, there was no decrease in latencies over days in concept learning at the two highest doses (240 and 360 mg/kg) of LGDE. Thus LGDE appeared to slow down decision time in the concept task but not the spatial task in the absence of an effect on quadrant entries in any version. These results indicate that NMDA receptors are involved in spatial and concept learning. Non-NMDA receptors appear to be involved only in concept learning.

Effects of l-glutamic acid diethyl ester on discrimination learning in rats

The acquisition of a visuo-tactile simultaneous discrimination task for food reward was evaluated in male albino rats given 0, 120, 240 and 360 mg/kg of l-glutamic acid diethyl ester, an antagonist of quisqualate and kainate receptors. There was an increase in the number of trials to reach the criterion and in the number of errors at the two highest doses of l-glutamic acid diethyl ester in comparison to the two lowest doses. These results indicate a possible role for non-NMDA amino acid receptors in discrimination learning.

Age-related changes of glutamate dehydrogenase in the rat hippocampus: an enzyme histochemical study

Enzyme histochemical techniques associated with microphotometry were used to evaluate the reactivity of glutamate dehydrogenase (GDH) in the hippocampus of young (3 month) adult (12 month) and old (26 month) male Sprague-Dawley rats. In young rats the highest level of enzymatic reactivity was found in the stratum oriens of the CA-1-CA-4 fields followed in descending order by the molecular layer of fascia dentata, and by the layer of mossy fibers. In all the layers investigated GDH reactivity was higher in adult that in young rats. The older animals exhibited lower GDH reactivity than adult rats in almost all of the layers and higher enzymatic reactivity than in young rats in the stratum oriens of the CA-1-CA-4 fields. The hippocampus is known to be involved in the acquisition of two-way avoidance learning and is thought to utilize glutamate as one of its main neurotransmitters. These data suggest a possible relationship between hippocampal glutamatergic transmission and impairment of acquisition of avoidance learning occurring with age. Moreover, the possibility that a decreased glutamate catabolism occurring in old rats may contribute to the neuronal loss described in the hippocampus of aged rats is discussed.

A possible role of AA2 excitatory amino acid receptors in the expression of stimulant drug effects

GDEE, an antagonist of the AA2 or quisqualic acid category of excitatory amino acid receptor, decreases behavioral activity and locomotor stimulation induced by cocaine and amphetamine when locally injected into the nucleus accumbens. The present experiment was intended to examine the effects of systemic GDEE and other excitatory amino acid antagonists on stimulant-induced locomotor activity. GDEE markedly attenuated the stimulant effect of amphetamine, and partially blocked the effects of phencyclidine (PCP). Apomorphine-induced cage climbing behavior was partially decreased by lower dosages of GDEE, but was almost completely blocked by the highest dosage tested. Amphetamine-induced stimulation of locomotor activity was not decreased by any of the other excitatory amino acid antagonists that were tested, including MK-801, 2-amino-7-phosphonoheptanoic acid (APH), or CNQX. APH decreased stereotypy only at a high dosage (250 mg/kg), which also produces ataxia. Several other compounds, including L-glutamic acid gamma ethyl ester (GMEE), L-glutamic acid, glycine, and L-glutamine did not block amphetamine-induced stimulation in molar dosages equivalent to the highest dosage of GDEE (8 mmol/kg). It is concluded that the AA2 excitatory amino acid receptor is important in the expression of activating effects of stimulant drugs.

Glutamate in the mammalian CNS

The excitatory amino acid glutamate plays an important role in the mammalian CNS. Studies conducted from 1940 to 1950 suggested that oral administration of glutamate could have a beneficial effect on normal and retardate intelligence. The neurotoxic nature of glutamate resulting in excitotoxic lesions (neuronal death) is thought possibly to underlie several neurological diseases including Huntington's disease, status epilepticus. Alzheimer's dementia and olivopontocerebellar atrophy. This neurodegenerative effect of glutamate also appears to regulate the formation, modulation and degeneration of brain cytoarchitecture during normal development and adult plasticity, by altering neuronal outgrowth and synaptogenesis. In addition to its function as a neurotransmitter in several regions of the CNS, glutamate seems to be specifically implicated in the memory process. Long-term potentiation (LTP) and long-term depression (LTD), two forms of synaptic plasticity associated with learning and memory, both involve glutamate receptors. Studies with antagonists of glutamate receptors reveal a highly selective dependency of LTP and LTD on the N-methyl-D-aspartate and quisqualate receptors respectively. The therapeutic value of glutamate receptor antagonists is being actively investigated. The most promising results have been obtained in epilepsy and to some extent in ischaemia and stroke. The major drawback remains the inability of antagonists to permeate the blood-brain barrier when administered systemically. Efforts should be directed towards finding antagonists that are lipid soluble and able to cross the blood-brain barrier and to find precursors that would yield the antagonist intracerebrally.

Isolation and structure of a pseudopeptide gamma-L-glutamyl-L-aspartic acid from Datura stramonium that impairs learning retention in mice

Datura stramonium contains a compound that impairs learning retention in mice. It has been purified to homogeneity and its structure has been established as that of a gamma-L-glutamyl-L-aspartate. The biological activity of this pseudodipeptide has been found to be identical with that of the corresponding synthetic one. It has also been compared to those of various synthetic di- and tripeptides containing L- and/or D-enantiomers of the constitutive amino acids. The results show that the activity is associated with a peptidic structure containing only one type of enantiomer.

gamma-L-glutamyl-L-aspartate induces specific deficits in long-term memory and inhibits [3H]glutamate binding on hippocampal membranes

gamma-L-Glutamyl-L-aspartic acid (gamma-LGLA) has been isolated from Datura stramonium; its structure has been determined and it was then synthesized. In male Swiss mice intraperitoneal administration of the natural peptide (125 nmol/kg) or of the synthetic peptide (25-2500 nmol/kg) 24 h after acquisition of a Y-maze avoidance task induced a dose-dependent deficit in retention performance 48 h later. gamma-LGLA had no effect on locomotor activity or emotional reactivity at the doses used. Separate or simultaneous administration of aspartate or glutamate (each at 250 nmol/kg) had no effect on learning retention, indicating that deficit induced by gamma-LGLA was specific to the peptide. gamma-LGLA impaired learning acquisition in a time-dependent manner when administered from 3 min to 24 h post-training, but had no effect when administered 3 days following acquisition. gamma-LGLA administered just after the training session did not affect retention performance during the first 3 h, but suppressed the retention improvement observed in control animals from 6 to 24 h after acquisition; this deficit was still evident 7 days after the treatment. gamma-LGLA partially inhibited L-[3H]glutamate binding on crude hippocampal or striatal membrane preparations; this inhibition was not observed on cerebellar membrane preparations. These results suggest a specific action of gamma-LGLA on excitatory amino acid systems which may be responsible for its effects on learning retention.

2-Amino-4-phosphonobutyric acid selectively blocks two-way avoidance learning in the mouse

There seems to be ample evidence supporting the view that glutamate plays a significant role in the mammalian brain as a neurotransmitter. It is considered to be a likely transmitter candidate in one or more hippocampal pathways. Recently it has been visualized in excitatory, possibly glutamatergic, neurons in the hippocampus. Glutamate has been proposed to mediate memory formation. We wanted to see if blocking glutamate action by a specific glutamate antagonist could result in reduction of learning ability. 2-Amino-4-phosphonobutyric acid (APB) is an analogue of glutamic acid and has been used as a glutamate antagonist in electrophysiological studies on invertebrate neuromuscular junction, retina and hippocampus. We tested the influence of APB on the acquisition of two way avoidance learning in the shuttle box and on learning in the water maze. Our results show that intraperitoneal injection of APB led to a reduction in avoidance learning, whereas learning in the water maze was unaffected.

Selective inhibition of homocysteine-induced seizures by glutamic acid diethyl ester and other glutamate esters

Homocysteine thiolactone causes convulsions when administered to animals, and has recently been reported to have excitatory effects on neurons in the central nervous system. Glutamic acid diethyl ester (GDEE) has previously been found to be an effective antagonist of the central excitation induced by homocysteine and is thought to be a selective antagonist of the quisqualate-sensitive excitatory amino-acid-receptor site. If an interaction of homocysteine with the quisqualate-sensitive receptor site is responsible for its convulsive properties, GDEE might also block the induction of seizures by homocysteine. GDEE in a dosage of 4 mmol/kg almost completely blocked homocysteine-induced seizures in mice; smaller dosages had no effect or only slight inhibitory effects. Glutamic acid dimethyl ester (GDME) and glutamic acid gamma-methyl ester (GMME) also partially blocked homocysteine-induced seizures, but monosodium glutamate and glutamic acid gamma-monoethyl ester (GMEE) had only a slight effect. None of the glutamate esters inhibited seizures induced by pentylenetetrazole. It is therefore suggested that certain types of seizures involve the quisqualic acid excitatory amino-acid-receptor site. Homocysteine-induced seizures may serve as a model of seizures of this type, and GDEE, GDME, and GMME may be effective antagonists of such seizures.

Glutamic acid diethyl ester induces catalepsy in rats. A new model for schizophrenia?

The glutamate antagonist glutamic acid diethyl ester is found to produce catalepsy in rats, when administered into the lateral ventricle. Since the cerebrospinal fluid content of glutamate is reduced in patients with schizophrenia, the central effects of glutamate antagonists are a possible experimental model for schizophrenia.