Milacemide enhances memory storage and alleviates spontaneous forgetting in mice

Forgetting Unwanted Memories: Active Forgetting and Implications for the Development of Psychological Disorders

Intrusive memories are a common feature of many psychopathologies, and suppression-induced forgetting of unwanted memories appears as a critical ability to preserve mental health. In recent years, biological and cognitive studies converged in revealing that forgetting is due to active processes. Recent neurobiological studies provide evidence on the active role of main neurotransmitter systems in forgetting, suggesting that the brain actively works to suppress retrieval of unwanted memories. On the cognitive side, there is evidence that voluntary and involuntary processes (here termed "intentional" and "incidental" forgetting, respectively) contribute to active forgetting. In intentional forgetting, an inhibitory control mechanism suppresses awareness of unwanted memories at encoding or retrieval. In incidental forgetting, retrieval practice of some memories involuntarily suppresses the retrieval of other related memories. In this review we describe recent findings on deficits in active forgetting observed in psychopathologies, like post-traumatic stress disorder, depression, schizophrenia, and obsessive-compulsive disorder. Moreover, we report studies in which the role of neurotransmitter systems, known to be involved in the pathogenesis of mental disorders, has been investigated in active forgetting paradigms. The possibility that biological and cognitive mechanisms of active forgetting could be considered as hallmarks of the early onset of psychopathologies is also discussed.

A NMDA receptor glycine site partial agonist, GLYX-13, simultaneously enhances LTP and reduces LTD at Schaffer collateral-CA1 synapses in hippocampus

N-methyl-D-aspartate glutamate receptors (NMDARs) are a key route for Ca2+ influx into neurons important to both activity-dependent synaptic plasticity and, when uncontrolled, triggering events that cause neuronal degeneration and death. Among regulatory binding sites on the NMDAR complex is a glycine binding site, distinct from the glutamate binding site, which must be co-activated for NMDAR channel opening. We developed a novel glycine site partial agonist, GLYX-13, which is both nootropic and neuroprotective in vivo. Here, we assessed the effects of GLYX-13 on long-term synaptic plasticity and NMDAR transmission at Schaffer collateral-CA1 synapses in hippocampal slices in vitro. GLYX-13 simultaneously enhanced the magnitude of long-term potentiation (LTP) of synaptic transmission, while reducing long-term depression (LTD). GLYX-13 reduced NMDA receptor-mediated synaptic currents in CA1 pyramidal neurons evoked by low frequency Schaffer collateral stimulation, but enhanced NMDAR currents during high frequency bursts of activity, and these actions were occluded by a saturating concentration of the glycine site agonist d-serine. Direct two-photon imaging of Schaffer collateral burst-evoked increases in [Ca2+] in individual dendritic spines revealed that GLYX-13 selectively enhanced burst-induced NMDAR-dependent spine Ca2+ influx. Examining the rate of MK-801 block of synaptic versus extrasynaptic NMDAR-gated channels revealed that GLYX-13 selectively enhanced activation of burst-driven extrasynaptic NMDARs, with an action that was blocked by the NR2B-selective NMDAR antagonist ifenprodil. Our data suggest that GLYX-13 may have unique therapeutic potential as a learning and memory enhancer because of its ability to simultaneously enhance LTP and suppress LTD.

GLYX-13: A monoclonal antibody-derived peptide that acts as an N-methyl-d-aspartate receptor modulator

We previously created a monoclonal antibody (MAb), B6B21, that acts as a partial agonist at the glycine site of the N-methyl-d-aspartate (NMDA) receptor [Moskal, J.R., Schaffner, A.E., 1986. Monoclonal antibodies to the dentate gyrus: immunocytochemical characterization and flow cytometric analysis of hippocampal neurons bearing a unique cell-surface antigen. J. Neurosci. 6, 2045-2053.]. The hypervariable region of the light chain of B6B21 was cloned and sequenced. Peptides were then synthesized based on this sequence information and screened using rat hippocampal membrane preparations to measure [(3)H]MK-801 binding in the presence of 7-chlorokynurenic acid, a glycine site-specific competitive inhibitor of NMDA receptor [Moskal, J.R., Yamamoto, H., Colley, P.A., 2001. The use of antibody engineering to create novel drugs that target N-methyl-d-aspartate receptors. Curr. Drug Targets 2, 331-345.]. Peptides that were able to increase [(3)H]MK-801 binding in a dose-dependent manner under these conditions were named Glyxins. Here we report that GLYX-13, a tetrapeptide (TPPT-amide), was found to readily cross the blood-brain barrier and modulate the NMDA receptor in a glycine-like fashion when examined pharmacologically and electrophysiologically. When GLYX-13 was administered to rats at 0.5-1.0mg/kg i.v., a significant enhancement in learning was observed using a hippocampus-dependent trace eye blink conditioning paradigm. These data indicate that the Glyxins are a new class of NMDA receptor modulators that may have therapeutic potential. Based on the broad agonist range in vitro and the potent cognitive-enhancing properties in a valid in vivo model of learning, GLYX-13 is a new drug candidate with potential for the treatment of cognitive disorders.

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.

Reversion of beta 25-35-amyloid peptide-induced amnesia by NMDA receptor-associated glycine site agonists

The effects of D-cycloserine (DCS), a N-methyl-D-aspartate receptor-associated glycine site agonist, and milacemide (MIL), a glycine prodrug, were examined on learning impairments induced by administration of beta 25-35-amyloid peptide (3 nmol i.c.v.). Mice were examined for spontaneous alternation and step-down passive avoidance, 7 and 14 days after beta 25-35, respectively. The beta 25-35-induced deficits were reversed by DCS, 1-30 mg/kg i.p., or MIL, 3-100 mg/kg i.p., each drug being ineffective on control mice behaviours. These observations strengthen the therapeutic potential of glycine site agonists against the memory impairments induced by beta-amyloid peptides.

The co-agonist concept: is the NMDA-associated glycine receptor saturated in vivo?

Our current knowledge of the structure and function of NMDA receptors is expanding at a rapid pace; however, advances regarding regulation of the supply of glutamate and its co-agonist, glycine, have been slower. While the anatomical sources and metabolic compartmentation of glutamate have been studied, limited efforts have been dedicated to defining the dynamics and compartmentation of the co-agonist, glycine. In fact, most investigators have made the assumption that glycine is freely available, via diffusion, for synaptic transmission at NMDA-type synaptic clefts. This assumption ignores the intricate inactivation mechanisms potentially involved in regulating synaptic levels of this amino acid and the recent descriptions of high levels of endogenous D-serine, another potential agonist of the NMDA-associated glycine receptor, in the brain. In this review, the relevance of these data and pharmacological experiments pertinent to the question of whether the NMDA-associated glycine receptor is saturated in vivo or not, is presented.

Blockade of metabotropic glutamate receptors prevents long-term memory consolidation

Metabotropic glutamate receptor activation has been shown to be essential for establishment of long-term potentiation, a phenomenon increasingly thought to be associated with the laying down of permanent memory. However, these receptors may also play a part in the initiation of protein kinase C activity, which has been demonstrated to be involved in prelong-term memory processes. Blockade of the metabotropic glutamate receptors by the specific antagonist, (RS)-alpha-Methyl-4-carboxyphenylglycine (500 microM) is shown to induce amnesia during a long-term memory stage in day-old chicks trained on a passive avoidance task, and to have no effect on prelong-term stages. The results suggest a specific role for these receptors in a possibly LTP associated mechanism of memory processing.

Milacemide treatment in mice enhances acquisition of a Morris-type water maze task

The N-methyl-D-aspartate (NMDA) subtype of the glutamate receptor appears to be involved with processes of learning and memory. A neutral amino acid binding site is known to exist on the NMDA complex. Glycine binds with high affinity to this site and has been found to potentiate NMDA activity. 2-N-Pentylaminoacetamide HCl (milacemide) is a glycine agonist that has been found to enhance performance of rodents in passive and active avoidance tasks and has improved the performance of humans in several word retrieval tasks. We evaluated the effects of milacemide on the performance of male C57BL/6J mice in a complex spatial task, the Morris water maze. Because NMDA receptor activation appears involved in induction of long-term potentiation, it was hypothesized that milacemide administration would be involved in task acquisition. Therefore, mice were treated with either milacemide (10 mg/kg) or vehicle 1 h prior to training on each of 4 consecutive days. Results indicated that mice treated with milacemide learned the task significantly faster than controls over 4 days of training, as measured by mean distance (cm) to reach the goal platform. Therefore, agonism of the glycine site on the NMDA receptor appears to facilitate performance of learning in a spatial memory task.

New pharmacological strategies for cognitive enhancement using a rat model of age-related memory impairment

We have developed the Stone maze paradigm for use as a rat model of memory impairment observed in normal aging and in Alzheimer's disease. Evidence produced thus far clearly implicates both the cholinergic and glutamatergic systems in acquisition performance in this complex maze task. Although results have been very inconsistent regarding the cognitive enhancing abilities of cholinomimetics for use in Alzheimer's disease, new classes of cholinesterase inhibitors may offer greater therapeutic efficacy. The use of glycine and polyamine agonists appears to be a viable strategy for positive modulation of the NMDA receptor. In addition, an approach that combines stimulation both of cholinergic and glutamatergic systems may have greater potential than agonism of either separately. Manipulation of signal transduction events might also have potential for cognitive enhancement. The influx of Ca2+ through the NMDA receptor stimulates production of NO via the action of NOS. By using NARG to block NOS activity, we have demonstrated in rats that NO production appears to influence learning in the Stone maze. We are currently exploring the age-related changes in NOS activity in specific brain regions of rats to determine if loss in the NO generating system is related to age-related memory impairment observed in the Stone maze. In addition, we are exploring pharmacological strategies for inducing NO production; however, because of the potential neurotoxicity for NO overstimulation, this strategy will present some obstacles. The identification of NO as a simple molecule serving vital physiological functions but representing potential for neurotoxicity presents an important unifying area for neurobiological investigations searching for mechanisms of normal brain aging and of age-related neuropathology, as observed in Alzheimer's disease.

Cognitive effects of milacemide and methylphenidate in healthy young adults

Cognitive effects of the novel glycine prodrug milacemide (400 mg), the catecholaminergic agonist methylphenidate (20 mg), and placebo were evaluated in 48 healthy young adults. Throughout a 6-h drug treatment day, subjects repeatedly performed tests of target-detection vigilance, immediate and delayed verbal free recall, and Buschke Selective Reminding; total free recall and forced-choice recognition tests were administered at the end of the day. Significant improvement in both vigilance reaction time and Selective Reminding Sum Recall was observed in the methylphenidate group. Contrary to expectations, the milacemide group evidenced significant declines in both vigilance perceptual sensitivity and free-recall difference scores (delayed-immediate). Vigilance reaction times significantly decreased over repeat testing in all groups, but only the methylphenidate group differed from placebo. The reaction-time functions for milacemide and placebo were similar, suggesting arousal was not diminished under milacemide and could not account for the cognitive decrements. No significant drug effects obtained for total free recall or recognition performance. Although the glycine prodrug milacemide was ineffective as a cognitive enhancer, the involvement of the NMDA receptor in memory function reported in the literature supports continued exploration of other approaches for manipulating NMDA receptor activity.

Rodent models of memory dysfunction in Alzheimer's disease and normal aging: moving beyond the cholinergic hypothesis

The Stone maze paradigm has been developed for use as a rat model of memory impairment observed in normal aging and in Alzheimer's disease. Results from several studies have demonstrated the involvement of both cholinergic and glutamatergic systems in acquisition performance in this complex maze task. Although results of clinical studies on the cognitive enhancing abilities of cholinomimetics for treatment of memory impairment in Alzheimer's disease have been inconsistent, new classes of cholinesterase inhibitors offer greater potential for therapeutic efficacy. The physostigimine derivative, phenserine, appears to have marked efficacy for improving learning performance of aged rats or of young rats treated with scopolamine in the Stone maze. Declines in markers of glutamatergic neurotransmission in Alzheimer's disease and in normal aging suggest that pharmacological manipulation of this system might also prove beneficial for cognitive enhancement. Treatment with glycine and/or polyamine agonists is suggested as a strategy for activating the N-methyl-D-aspartate (NMDA) subtype of glutamate receptor. In addition, the use of combined pharmacological activation of cholinergic and glutamatergic systems is suggested. Manipulation of signal transduction events should also be considered as a strategy for cognitive enhancement. The influx of Ca2+ through the channel formed by the NMDA receptor stimulates the production of the oxyradical, nitric oxide (NO*), via the action of nitric oxide synthase (NOS). Compounds that inhibit NOS activity impair acquisition in the Stone maze, suggesting an involvement of NO*. Thus, strategies for inducing NO* production to enhance cognitive performance may be beneficial. Because of the potential neurotoxicity for NO*, this strategy is not straightforward. Although many new directions beyond the cholinergic hypothesis can be suggested, each has its potential benefits which must be weighed against its risks. Nonetheless, an important unifying area for neurobiological research examining mechanisms of normal brain aging and of age-related neuropathology, as observed in Alzheimer's disease, might emerge from the identification of NO* as a simple molecule serving vital physiological functions but representing potential for neurotoxicity.

Effect of the calcium channel blocker amlodipine on memory in mice

Five experiments were performed to investigate the effects of amlodipine, a calcium channel antagonist of the 1,4-dihydropyridine class, on consolidation and retrieval of memory in mice. In a single-trial passive avoidance task, amlodipine was administered pretraining, posttraining, or pretesting. Results of temporal and dose-response studies showed that memory enhancement (significant increase in step-through latency) occurred when amlodipine (5, 7, 9, 15, and 30 mg/kg) was given either immediately post-training or (15 mg/kg) 15 min pretesting. Using a conditioned emotional response task, tone was paired with shock using Pavlovian conditioning procedures. Strength of conditioning was assessed by measuring suppression of drinking in the presence of a tone. Amlodipine (7 mg/kg) given immediately following both high- and low-intensity shock significantly enhanced conditioned suppression. In the third experiment thirsty mice were trained on a spatial discrimination task in a linear maze. Correct choices were reinforced with liquid reinforcement. Amlodipine (10 mg/kg) injected immediately after the training session produced a significant enhancement of discrimination performance on a 24-h retention test. In the fourth experiment mice were given 25 training trials in a two-way active avoidance task and were treated with either amlodipine (10 mg/kg) or saline after training. Amlodipine-treated mice made significantly more avoidances on the test session than control animals. The final experiment demonstrated that the deficit in approach-avoidance behavior seen in 18-month-old mice could be reversed by amlodipine treatment after the training session. These studies suggest that amlodipine can facilitate memory consolidation and retrieval.

Evaluation of multiple doses of milacemide in the treatment of senile dementia of the Alzheimer's type

A multicenter, double-blind, placebo-controlled, parallel group study was conducted to assess the safety and efficacy of three doses of milacemide in the treatment of patients with senile dementia of the Alzheimer type of mild to moderate severity. Patients were randomly assigned to receive one of three dosages of milacemide (400, 800, or 1200 mg/day) or placebo for 4 weeks followed by a single-blind 4-week placebo period. One hundred forty-eight men and women older than 50 years of age were enrolled, and 129 patients completed the study. The differences among treatment groups were not statistically different with respect to total scores on the Alzheimer's Disease Assessment Scale or any items and subscales that were examined, nor were significant differences on the Clinical Global Impression Scale found. Clinically significant increases in liver function tests, specifically aspartate aminotransferase and alanine aminotransferase (AST and ALT), were reported for five of the patients receiving milacemide, requiring their withdrawal from the study.

D-cycloserine, a modulator of the N-methyl-D-aspartate receptor, improves spatial learning in rats treated with muscarinic antagonist

The results of the present study indicate that D-cycloserine, a partial agonist at the glycine binding site, which is a positive modulation site of the N-methyl-D-aspartate receptor, could improve dose dependently the acquisition of a water maze task in rats treated with scopolamine, a muscarinic antagonist. The low to moderate doses of D-cycloserine which improved learning did not affect swimming speed in scopolamine-treated rats. The higher dose did not improve learning, but it increased swimming speed in scopolamine-treated rats. These results show a dissociation between impaired acquisition and abnormal behavioral activity in scopolamine-treated rats in the water maze task. Furthermore, the present results suggest that D-cycloserine can act as a cognitive enhancer at the appropriate doses.

Milacemide: a placebo-controlled study in senile dementia of the Alzheimer type

OBJECTIVE

Milacemide, a MAO-B inhibitor that is also a prodrug for glycine, was tested as a treatment for senile dementia of the Alzheimer type (SDAT) because of its potential for enhancing cognition in animal models of impaired learning and memory.

DESIGN

Double-blind, placebo-controlled, randomized clinical trial.

SETTING

Sixteen study sites, both university-affiliated and private.

PATIENTS

A total of 228 outpatients (116 men and 112 women) with SDAT, ranging in age from 49-93 years.

INTERVENTION

1200 mg/day milacemide treatment for 1 month (113 patients received milacemide, and 115 patients received placebo).

MAIN OUTCOME MEASURES

Alzheimer's Disease Assessment Scale and the Mini-Mental State Examination.

RESULTS

Milacemide-treated SDAT patients did not show significant improvement in any of the outcome measures used. Significant elevations in liver enzymes in four subjects were of sufficient magnitude to necessitate withdrawal from the study.

CONCLUSIONS

Milacemide does not appear to be an effective treatment in enhancing cognition in SDAT patients.

Milacemide and other drugs active at glutamate NMDA receptors as potential treatment for dementia

Interest in the NMDA receptor as a key mechanism in long-term potentiation and possibly improved learning and memory has led to the development of milacemide as a prodrug for glycine, a modulator of the NMDA receptor. Trials of milacemide in doses ranging from 400-1,200 mg once daily in Alzheimer's disease (AD) for 4 weeks have been disappointing, but milacemide has increased the accuracy and speed of word retrieval in human volunteers. The failure of milacemide in AD has led to an interest in partial agonists of the glycine-B site in the hope of obtaining a better clinical response.