Cognitive enhancers: potential strategies and experimental results

N-Methyl D-aspartate receptor subunit signaling in fear extinction

N-Methyl D-aspartate receptors (NMDAR) are central mediators of glutamate actions underlying learning and memory processes including those required for extinction of fear and fear-related behaviors. Consistent with this view, in animal models, antagonists of NMDAR typically impair fear extinction, whereas partial agonists have facilitating effects. Promoting NMDAR function has thus been recognized as a promising strategy towards reduction of fear symptoms in patients suffering from anxiety disorders and post-traumatic disorder (PTSD). Nevertheless, application of these drugs in clinical trials has proved of limited utility. Here we summarize recent advances in our knowledge of NMDAR pharmacology relevant for fear extinction, focusing on molecular, cellular, and circuit aspects of NMDAR function as they relate to fear extinction at the level of behavior and cognition. We also discuss how these advances from animal models might help to understand and overcome the limitations of existing approaches in human anxiety disorders and how novel, more specific, and personalized approaches might help advance future therapeutic strategies.

Modeling Alzheimer's disease: from past to future

Alzheimer’s disease (AD) is emerging as the most prevalent and socially disruptive illness of aging populations, as more people live long enough to become affected. Although AD is placing a considerable and increasing burden on society, it represents the largest unmet medical need in neurology, because current drugs improve symptoms, but do not have profound disease-modifying effects. Although AD pathogenesis is multifaceted and difficult to pinpoint, genetic and cell biological studies led to the amyloid hypothesis, which posits that amyloid β (Aβ) plays a pivotal role in AD pathogenesis. Amyloid precursor protein (APP), as well as β- and γ-secretases are the principal players involved in Aβ production, while α-secretase cleavage on APP prevents Aβ deposition. The association of early onset familial AD with mutations in the APP and γ-secretase components provided a potential tool of generating animal models of the disease. However, a model that recapitulates all the aspects of AD has not yet been produced. Here, we face the problem of modeling AD pathology describing several models, which have played a major role in defining critical disease-related mechanisms and in exploring novel potential therapeutic approaches. In particular, we will provide an extensive overview on the distinct features and pros and contras of different AD models, ranging from invertebrate to rodent models and finally dealing with computational models and induced pluripotent stem cells.

Strain-dependent effects of cognitive enhancers in the mouse

A series of cognitive enhancers (CEs) have been reported to increase spatial memory in rodents, information on behavioral effects, however, is limited. The aim of the study was therefore to examine the behavioral effects of three CEs in two well-documented inbred mouse strains. C57BL/6J and DBA/2 mice were administered intraperitonial. D-cycloserine (DCS; NMDA receptor agonist), 1-(4-Amino-5-chloro-2-methoxyphenyl)-3-[1-butyl-4-piperidinyl]-1-propanone hydrochloride (RS67333; 5HT4-receptor agonist), and (R)-4-{[2-(1-methyl-2-pyrrolidinyl)ethyl]thio}phenol hydrochloride (SIB-1553A; beta-4-nicotinic receptor agonist) and tested in the open field (OF), elevated plus maze (EPM), neurological observational battery and rota-rod. Cognitive performance was tested in the Morris water maze. All compounds modified behavioral performance in the OF, DCS showed an anxiolytic effect in the EPM, and differences in the observational battery were observed i.e. vestibular drop was decreased by SIB-1553A and RS67333 treatment in C57BL/6J and increased with DCS treatment in DBA/2 mice. In the rota rod SIB-1553A improved motor performance. DCS effects on learning and memory was comparable to controls whereas the other compounds impaired performance in the Morris water maze. In conclusion, behavioral testing of CEs in the mouse revealed significant changes that may have to be taken into account for evaluation of CEs, interpretation of cognitive studies and warrant further neurotoxicological studies. Moreover, strain-dependent differences were observed that in turn may confound results obtained from behavioral and cognitive testing.

Astrocytic energy metabolism consolidates memory in young chicks

In a single trial discrimination avoidance learning task, chicks learn to distinguish between beads of two colors, which are dipped in either a strong or weak tasting aversant (methyl anthranilate) to induce strongly-reinforced and weakly-reinforced learning, respectively. Consolidation of strongly-reinforced learning can be prevented by inhibitors of glycolysis, such as 2-deoxyglucose and iodoacetate and by inhibitors of oxidative metabolism and the consolidation of weakly-reinforced learning can be promoted by administration of glucose. In the present study we show that bilateral, intracerebral injection of 30 nmol acetate can act like glucose to consolidate labile memory and to restore memory impaired by 2-deoxyglucose administration. Acetate is a metabolic substrate that feeds into the tricarboxylic acid cycle, it is oxidized in astrocytes, but not in neurones. Our data suggest that effects of glucose administered 15-25 min post-training on memory consolidation are mediated via astrocytes not neurons.

Cholinergic grafts, memory and ageing

Neural grafts rich in cholinergic neurones can survive transplantation to the neocortex or hippocampus in rats. Such grafts have the capacity to ameliorate a variety of functional deficits associated both with explicit lesions that deafferent the neocortex or hippocampus and with natural ageing. The transplantation technique enhances our understanding of the involvement of forebrain cholinergic systems in normal cognitive functions (including memory) and of the role of cholinergic degeneration in the dysfunctions associated with ageing. It is unlikely, however, that these observations will extend to a therapeutic strategy for dementia using neural transplantation, because the human diseases (at least in the case of Alzheimer's disease and multi-infarct dementia) involve widespread degeneration of other populations of cortical neurones that are not so amenable to functional transplantation as the diffuse forebrain cholinergic systems.

Improvement of cognitive function by MAO-B inhibitor L-deprenyl in aged rats

This study evaluated the ability of the selective MAO-B inhibitor, L-deprenyl, to reverse cognitive impairments appearing in aged rats, using the reference memory, Morris Water Maze paradigm. L-Deprenyl significantly improved learning and memory deficits associated with old age in doses of 1.25 and 5 mg/kg PO (escape latency measure) and doses of 1.25, 2.5 and 5 mg/kg PO (path length measure). L-Deprenyl also improved reversal learning impairments in doses of 1.25, 2.5 and 5 mg/kg PO, as expressed by the escape latency measure. The data suggest that L-deprenyl possesses potential cognitive enhancement abilities probably due to an increase in dopaminergic activity.

BMY 21502 and piracetam facilitate performance of two-choice win-stay water-escape in normal rats

Sprague-Dawley rats given either 5 or 10 mg/kg of a new compound, BMY 21502, 150 mg/kg of piracetam or a dose of methylcellulose vehicle (p.o.) daily for 38 days beginning two days before training were compared on performance of a win-stay water-escape task in a circular water maze requiring the use of working memory. The task involved giving the rats pairs of trials in which the location of a submerged escape platform remained the same within a pair of trials but changed semirandomly across pairs. Rats receiving either 5 mg/kg BMY 21502 or piracetam made more correct choices than did rats receiving only the vehicle (p less than 0.05 in each case). The facilitated performance was associated with making fewer perseverative responses that resulted in errors.

An hypothesis on the role of glucose in the mechanism of action of cognitive enhancers

This review presents evidence that some cognition enhancing drugs produce their beneficial effects on learning and memory by increasing the availability of glucose for uptake and utilization into the brain. The hypothesis further suggests that many cognition enhancing drugs act through a peripheral mechanism rather than directly on the brain. The general hypothesis is supported by four independent and converging pieces of evidence: 1) Some cognition enhancing drugs may not cross the blood-brain barrier, but can still facilitate memory; 2) Some cognition enhancing drugs are effective only when injected peripherally, but not when injected directly into the brain; 3) Many cognition enhancing drugs are not effective after adrenalectomy; 4) Cognitive function is correlated with glucose regulation in aged animals and humans. These four lines of research have implications for the role of glucose in the action of specific cognitive enhancers.