Memantine and recognition memory: Possible facilitation of its behavioral effects by the nitric oxide (NO) donor molsidomine

The Nitric Oxide (NO) Donor Molsidomine Counteract Social Withdrawal and Cognition Deficits Induced by Blockade of the NMDA Receptor in the Rat

The deficiency of the gaseous molecule nitric oxide (NO) seems to be critically involved in the pathogenesis of schizophrenia. Thus, molecules that can normalize NO levels, as are NO donors, might be of utility for the medication of this psychiatric disease. The aim of the present study was to detect the ability of the NO donor molsidomine to reduce schizophrenia-like impairments produced by the blockade of the N-methyl-D-aspartate (NMDA) receptor in rats. Molsidomine's ability to attenuate social withdrawal and spatial recognition memory deficits induced by the NMDA receptor antagonist ketamine were assessed using the social interaction and the object location test, respectively. Further, the efficacy of the combination of sub-effective doses of molsidomine with sub-effective doses of the atypical antipsychotic clozapine in alleviating non-spatial recognition memory deficits was evaluated utilizing the object recognition task. Molsidomine (2 and 4 mg/kg) attenuated social withdrawal and spatial recognition memory deficits induced by ketamine. Co-administration of inactive doses of molsidomine (1 mg/kg) and clozapine (0.1 mg/kg) counteracted delay-dependent and ketamine-induced non-spatial recognition memory deficits. The current findings suggest that molsidomine is sensitive to glutamate hypofunction since it attenuated behavioral impairments in animal models mimicking the negative symptoms and cognitive deficits of schizophrenia. Additionally, the present results support the potential of molsidomine as an adjunctive drug for the therapy of schizophrenia.

Effects of the active constituents of Crocus sativus L. crocins and their combination with memantine on recognition memory in rats

Crocus sativus L., is a plant cultivated in many countries of the world. Crocins are among the active constituents of C. sativus and their implication in cognition has been proposed. The present study was designed to investigate in the rat the effects of crocins on distinct recognition memory components (encoding, storage and retrieval). Subsequently, the potential use of crocins as adjunctive agents for the treatment of memory disorders was examined. Thus, the effects exerted by a combination of subthreshold doses of crocins and memantine on recognition memory were evaluated. To assess the effects of compounds on memory, the novel object-recognition task (NORT) was used. In a preliminary study, the influence of the retention time (the delay between the two trials) on the performance of rats was assessed. Rats' recognition memory abilities remained intact up to 6 h, but were extinguished when a delay of 24 h was utilized. Crocins, at any dose tested (5, 15, and 30 mg/kg), did not affect rats' performance, whereas administration of higher doses (15 and 30 mg/kg) reversed delay-dependent deficits in the NORT. The combination of subthreshold doses of crocins (5 mg/kg) and memantine (3 mg/kg) did not influence the performance, but counteracted delay-dependent deficits in the NORT. These findings suggest that crocins counteract natural forgetting and may modulate different aspects of recognition memory, and that the combined use of crocins and memantine might represent a novel strategy to treat memory disorders.

Animal models of cognitive aging and circuit-specific vulnerability

Medial temporal lobe and prefrontal cortical structures are particularly vulnerable to dysfunction in advanced age and neurodegenerative diseases. This review focuses on cognitive aging studies in animals to illustrate the important aspects of the animal model paradigm for investigation of age-related memory and executive function loss. Particular attention is paid to the discussion of the face, construct, and predictive validity of animal models for determining the possible mechanisms of regional vulnerability in aging and for identifying novel therapeutic strategies. Aging is associated with a host of regionally specific neurobiologic alterations. Thus, targeted interventions that restore normal activity in one brain region may exacerbate aberrant activity in another, hindering the restoration of function at the behavioral level. As such, interventions that target the optimization of "cognitive networks" rather than discrete brain regions may be more effective for improving functional outcomes in the elderly.

Age-related impairment in a complex object discrimination task that engages perirhinal cortex

Previous lesion studies have shown compromised complex object discrimination in rats, monkeys, and human patients with damage to the perirhinal cortical region (PRC) of the medial temporal lobe. These findings support the notion that the PRC is involved in object discrimination when pairs of objects have a high degree of overlapping features but not when object discrimination can be resolved on the basis of a single feature (e.g., size or color). Recent studies have demonstrated age-related functional changes to the PRC in animals (rats and monkeys) resulting in impaired complex object discrimination and object recognition. To date, no studies have compared younger and older humans using paradigms previously shown to engage the PRC. To investigate the influence of age on complex object discrimination in humans, the present study used an object matching paradigm for blob-like objects that have previously been shown to recruit the PRC. Difficulty was manipulated by varying the number of overlapping features between objects. Functional MRI data was acquired to determine the involvement of the PRC in the two groups during complex object discrimination. Results indicated that while young and older adults performed similarly on the easy version of the task, most older adults were impaired relative to young participants when the number of overlapping features increased. fMRI results suggest that older adults do not engage bilateral anterior PRC to the same extent as young adults. Specifically, complex object matching performance in older adults was predicted by the degree to which they engage left anterior PRC. These results provide evidence for human age-related changes in PRC function that impact complex object discrimination.

Characterizing cognitive aging of recognition memory and related processes in animal models and in humans

Analyses of complex behaviors across the lifespan of animals can reveal the brain regions that are impacted by the normal aging process, thereby, elucidating potential therapeutic targets. Recent data from rats, monkeys, and humans converge, all indicating that recognition memory and complex visual perception are impaired in advanced age. These cognitive processes are also disrupted in animals with lesions of the perirhinal cortex, indicating that the the functional integrity of this structure is disrupted in old age. This current review summarizes these data, and highlights current methodologies for assessing perirhinal cortex-dependent behaviors across the lifespan.

Effects of memantine, a non-competitive N-methyl-D-aspartate receptor antagonist, on genomic stability

Memantine is an aminoadamantane drug useful in neurodegenerative diseases, with beneficial effects on cognitive functions. Some studies have shown that memantine protects brain cells, thereby decreasing glutamate excitotoxicity. This study evaluated the genotoxic/antigenotoxic and mutagenic effects of memantine in CF-1 mice, following standardized protocols. Memantine was administered i.p. at 7.5, 15 or 30 mg/kg for three consecutive days. Blood and brain samples were collected to assess DNA damage using the alkaline comet assay. The mutagenic effect was assessed using the bone marrow micronucleus test. In addition, possible antioxidant effects were evaluated measuring the survival of Saccharomyces cerevisiae yeast strains [wild-type (WT) and isogenic mutants lacking superoxide dismutase] to cotreatment of memantine plus hydrogen peroxide. Memantine decreased DNA oxidative damage mainly in brain tissue. This antigenotoxic effect corroborated an increase observed in the survival of S. cerevisiae WT strain against hydrogen peroxide-induced damage. Furthermore, memantine did not increase the micronucleus frequency. The overall results indicate that memantine showed no mutagenic activity, did not cause DNA damage in the blood and brain tissues and showed antigenotoxic effects in brain tissue.

Pattern separation deficits may contribute to age-associated recognition impairments

Normal aging is associated with impairments in stimulus recognition. In the current investigation, object recognition was tested in adult and aged rats with the standard spontaneous object recognition (SOR) task or two variants of this task. On the standard SOR task, adult rats showed an exploratory preference for the novel object over delays up to 24 h, whereas the aged rats only showed significant novelty discrimination at the 2-min delay. This age difference appeared to be because of the old rats behaving as if the novel object was familiar. To test this hypothesis directly, rats participated in a variant of the SOR task that allowed the exploration times between the object familiarization and the test phases to be compared, and this experiment confirmed that aged rats falsely "recognize" the novel object. A final control examined whether or not aged rats exhibited reduced motivation to explore objects. In this experiment, when the environmental context changed between familiarization and test, young and old rats failed to show an exploratory preference because both age groups spent more time exploring the familiar object. Together these findings support the view that age-related impairments in object recognition arise from old animals behaving as if novel objects are familiar, which is reminiscent of behavioral impairments in young rats with perirhinal cortical lesions. The current experiments thus suggest that alterations in the perirhinal cortex may be responsible for reducing aged animals' ability to distinguish new stimuli from ones that have been encountered previously.

Effects of long-term memantine on memory and neuropathology in Ts65Dn mice, a model for Down syndrome

Memantine is a partial NMDA receptor antagonist that has been shown to improve learning and memory in several animal models, and is approved for the treatment of Alzheimer's disease (AD). Chronic treatments using memantine in animal models of Alzheimer's disease show disease-modifying effects and suggest a potential neuroprotective function. The present study assessed the effects of both short- and long-term memantine treatment in a mouse model of Down syndrome (DS), the Ts65Dn mouse. The Ts65Dn mouse contains a partial trisomy of murine chromosome 16, and exhibits hippocampal-dependent memory deficits, as well as progressive degeneration of basal forebrain cholinergic neurons (BCFNs). Ts65Dn mice were treated with memantine for a period of 6 months, beginning at 4 months of age. At the end of treatment the mice underwent memory testing using novel object recognition and water radial arm maze tasks, and then histologically analyzed for markers of neurodegeneration. Memantine treatment improved spatial and recognition memory performance in the Ts65Dn mice, though not to the level of normosomic littermate controls. Despite these memory improvements, histological analysis found no morphological signs of neuroprotection of basal forebrain cholinergic or locus coeruleus neurons in memantine-treated Ts65Dn mice. However, memantine treatment of Ts65Dn mice gave rise to elevated brain-derived neurotrophic factor expression in the hippocampus and frontal cortex, suggesting a mechanism of behavioral modification. Thus, our findings provide further evidence for memory facilitation of memantine, but suggest pharmacological rather than neuroprotective effects of memantine both after acute and chronic treatment in this mouse model.

Glutamate receptor-mediated restoration of experience-dependent place field expansion plasticity in aged rats

Place fields of hippocampal pyramidal cells expand asymmetrically when adult rats repeatedly follow the same route. This behaviorally induced expression of neuronal plasticity uses an NMDAR-dependent, LTP-like mechanism and could be used by hippocampal networks to store information. Aged spatial memory-impaired rats exhibit defective experience-dependent place field expansion plasticity. One possible explanation for this aged-associated deficit is alterations in glutamatergic function. In fact, both NMDAR- and AMPAR-mediated field excitatory postsynaptic potentials in CA1 decrease with aging. The current study investigated whether modulation of either AMPA or NDMA receptor activity could restore this experience-dependent plasticity by prolonging AMPAR activity with the ampakine CX516 and modulating the NMDAR with the noncompetitive antagonist memantine. The spatial firing characteristics of multiple CA1 pyramidal cells were monitored under both treatment conditions as aged rats repeatedly traversed a circular track. Compared to the saline baseline condition, acute administration of memantine, but not CX516, reinstated experience-dependent place field expansion. Taken together, these data suggest that pharmacological manipulation of the NMDAR can improve the function of hippocampal networks critical to optimal cognition in aging.