Neurobiological and endocrine correlates of individual differences in spatial learning ability

Programming effects of peripubertal stress on spatial learning

Exposure to adversity during early life can have profound influences on brain function and behavior later in life. The peripubertal period is emerging as an important time-window of susceptibility to stress, with substantial evidence documenting long-term consequences in the emotional and social domains. However, little is known about how stress during this period impacts subsequent cognitive functioning. Here, we assessed potential long-term effects of peripubertal stress on spatial learning and memory using the water maze task. In addition, we interrogated whether individual differences in stress-induced behavioral and endocrine changes are related to the degree of adaptation of the corticosterone response to repeated stressor exposure during the peripubertal period. We found that, when tested at adulthood, peripubertally stressed animals displayed a slower learning rate. Strikingly, the level of spatial orientation in the water maze completed on the last training day was predicted by the degree of adaptation of the recovery -and not the peak-of the corticosterone response to stressor exposure (i.e., plasma levels at 60 min post-stressor) across the peripubertal stress period. In addition, peripubertal stress led to changes in emotional and glucocorticoid reactivity to novelty exposure, as well as in the expression levels of the plasticity molecule PSA-NCAM in the hippocampus. Importantly, by assessing the same endpoints in another peripubertally stressed cohort tested during adolescence, we show that the observed effects at adulthood are the result of a delayed programming manifested at adulthood and not protracted effects of stress. Altogether, our results support the view that the degree of stress-induced adaptation of the hypothalamus-pituitary-adrenal axis responsiveness at the important transitional period of puberty relates to the long-term programming of cognition, behavior and endocrine reactivity.

Lack of Hypothalamus Polysialylation Inducibility Correlates With Maladaptive Eating Behaviors and Predisposition to Obesity

High variability exists in individual susceptibility to develop overweight in an obesogenic environment and the biological underpinnings of this heterogeneity are poorly understood. In this brief report, we show in mice that the vulnerability to diet-induced obesity is associated with low level of polysialic acid-neural cell adhesion molecule (PSA-NCAM), a factor of neural plasticity, in the hypothalamus. As we previously shown that reduction of hypothalamic PSA-NCAM is sufficient to alter energy homeostasis and promote fat storage under hypercaloric pressure, inter-individual variability in hypothalamic PSA-NCAM might account for the vulnerability to diet-induced obesity. These data support the concept that reduced plasticity in brain circuits that control appetite, metabolism and body weight confers risk for eating disorders and obesity.

Vitamin A status regulates glucocorticoid availability in Wistar rats: consequences on cognitive functions and hippocampal neurogenesis?

A disruption of the vitamin A signaling pathway has been involved in age-related memory decline and hippocampal plasticity alterations. Using vitamin A deficiency (VAD), a nutritional model leading to a hyposignaling of the retinoid pathway, we have recently demonstrated that retinoic acid (RA), the active metabolite of vitamin A, is efficient to reverse VAD-induced spatial memory deficits and adult hippocampal neurogenesis alterations. Besides, excess of glucocorticoids (GCs) occurring with aging is known to strongly inhibit hippocampal plasticity and functions and few studies report on the counteracting effects of RA signaling pathway on GCs action. Here, we have addressed whether the modulation of brain GCs availability could be one of the biological mechanisms involved in the effects of vitamin A status on hippocampal plasticity and functions. Thus, we have studied the effects of a vitamin A-free diet for 14 weeks and a 4-week vitamin A supplementation on plasma and hippocampal corticosterone (CORT) levels in Wistar rats. We have also investigated corticosteroid binding globulin (CBG) binding capacity and 11beta-Hydrosteroid Dehydrogenase type 1 (11β-HSD1) activity, both important modulators of CORT availability at the peripheral and hippocampal levels respectively. Interestingly, we show that the vitamin A status regulates levels of free plasma CORT and hippocampal CORT levels, by acting through a regulation of CBG binding capacity and 11β-HSD1 activity. Moreover, our results suggest that increased CORT levels in VAD rats could have some deleterious consequences on spatial memory, anxiety-like behavior and adult hippocampal neurogenesis whereas these effects could be corrected by a vitamin A supplementation. Thus, the modulation of GCs availability by vitamin A status is an important biological mechanism that should be taken into account in order to prevent age-related cognitive decline and hippocampal plasticity alterations.

Individual differences in the effects of chronic stress on memory: behavioral and neurochemical correlates of resiliency

Chronic stress has been shown to impair memory, however, the extent to which memory can be impaired is often variable across individuals. Predisposed differences in particular traits, such as anxiety, may reveal underlying neurobiological mechanisms that could be driving individual differences in sensitivity to stress and, thus, stress resiliency. Such pre-morbid characteristics may serve as early indicators of susceptibility to stress. Neuropeptide Y (NPY) and enkephalin (ENK) are neurochemical messengers of interest implicated in modulating anxiety and motivation circuitry; however, little is known about how these neuropeptides interact with stress resiliency and memory. In this experiment, adult male rats were appetitively trained to locate sugar rewards in a motivation-based spatial memory task before undergoing repeated immobilization stress and then being tested for memory retention. Anxiety-related behaviors, among other characteristics, were monitored longitudinally. Results indicated that stressed animals which showed little to no impairments in memory post-stress (i.e., the more stress-resilient individuals) exhibited lower anxiety levels prior to stress when compared to stressed animals that showed large deficits in memory (i.e., the more stress-susceptible individuals). Interestingly, all stressed animals, regardless of memory change, showed reduced body weight gain as well as thymic involution, suggesting that the effects of stress on metabolism and the immune system were dissociated from the effects of stress on higher cognition, and that stress resiliency seems to be domain-specific rather than a global characteristic within an individual. Neurochemical analyses revealed that NPY in the hypothalamus and amygdala and ENK in the nucleus accumbens were modulated differentially between stress-resilient and stress-susceptible individuals, with elevated expression of these neuropeptides fostering anxiolytic and pro-motivation function, thus driving cognitive resiliency in a domain-specific manner. Findings suggest that such neurochemical markers may be novel targets for pharmacological interventions that can serve to prevent or ameliorate the negative effects of stress on memory.

Pharmacotherapy with fluoxetine restores functional connectivity from the dentate gyrus to field CA3 in the Ts65Dn mouse model of down syndrome

Down syndrome (DS) is a high-incidence genetic pathology characterized by severe impairment of cognitive functions, including declarative memory. Impairment of hippocampus-dependent long-term memory in DS appears to be related to anatomo-functional alterations of the hippocampal trisynaptic circuit formed by the dentate gyrus (DG) granule cells - CA3 pyramidal neurons - CA1 pyramidal neurons. No therapies exist to improve cognitive disability in individuals with DS. In previous studies we demonstrated that pharmacotherapy with fluoxetine restores neurogenesis, granule cell number and dendritic morphology in the DG of the Ts65Dn mouse model of DS. The goal of the current study was to establish whether treatment rescues the impairment of synaptic connectivity between the DG and CA3 that characterizes the trisomic condition. Euploid and Ts65Dn mice were treated with fluoxetine during the first two postnatal weeks and examined 45-60 days after treatment cessation. Untreated Ts65Dn mice had a hypotrophyc mossy fiber bundle, fewer synaptic contacts, fewer glutamatergic contacts, and fewer dendritic spines in the stratum lucidum of CA3, the terminal field of the granule cell projections. Electrophysiological recordings from CA3 pyramidal neurons showed that in Ts65Dn mice the frequency of both mEPSCs and mIPSCs was reduced, indicating an overall impairment of excitatory and inhibitory inputs to CA3 pyramidal neurons. In treated Ts65Dn mice all these aberrant features were fully normalized, indicating that fluoxetine can rescue functional connectivity between the DG and CA3. The positive effects of fluoxetine on the DG-CA3 system suggest that early treatment with this drug could be a suitable therapy, possibly usable in humans, to restore the physiology of the hippocampal networks and, hence, memory functions.

Differential impact of polysialyltransferase ST8SiaII and ST8SiaIV knockout on social interaction and aggression

Previous studies using neuronal cell adhesion molecule (NCAM) -/- knockout (KO) mice provided evidence for a role of NCAMs in social behaviors. However, polysialic acid (PSA), the most important post-translational modification of NCAM, was also absent in these mice, which makes it difficult to distinguish between the specific involvement of either PSA or NCAM in social interactions. To address this issue, we assessed two lines of mice deficient for one of the two sialyltransferase enzymes required for the polysialylation of NCAM, sialyltransferase-X (St8SiaII or STX) and polysialyltransferase (ST8SiaIV or PST), in a series of tests for social behaviors. Results showed that PST KO mice display a decreased motivation in social interaction. This deficit can be partly explained by olfactory deficits and was associated with a clear decrease in PSA-NCAM expression in all brain regions analyzed (amygdala, septum, bed nucleus of the stria terminalis and frontal cortices). STX KO mice displayed both a decreased social motivation and an increased aggressive behavior that cannot be explained by olfactory deficits. This finding might be related to the reduced anxiety-like behavior, increased locomotion and stress-induced corticosterone secretion observed in these mice. Moreover, STX KO mice showed mild increase of PSA-NCAM expression in the lateral septum and the orbitofrontal cortex. Altogether, these findings support a role for PSA-NCAM in the regulation of social behaviors ranging from a lack of social motivation to aggression. They also underscore STX KO mice as an interesting animal model that combines a behavioral profile of violence and hyperactivity with reduced anxiety-like behavior.

Hippocampal activation during a cognitive task is associated with subsequent neuroendocrine and cognitive responses to psychological stress

Increased activation of the hypothalamus pituitary adrenal (HPA) axis, marked by increased secretion of cortisol, is a biological marker of psychological stress. It is well established that the hippocampus plays an important role in the regulation of HPA axis activity. The relationship between cortisol (stress-related elevation or exogenous administration) and the hippocampal-related cognitive function is often examined. However, few human studies to date have examined the effect of stress on hippocampal activity and the interactions between stress-induced activation of the HPA axis and hippocampal function during different phases of cognitive function. On the basis of our previous work, we hypothesized that group differences in stress-sensitivity relate to differences in hippocampal-related stress-integration. To test this hypothesis, we conducted a functional MRI study using tasks known to involve the hippocampal formation: novel-picture encoding, psychological stress, and paired-picture recognition. On the basis of their cortisol responses to stress, we divided subjects into stress-responders (increase in cortisol, n = 9) and nonresponders (decrease in cortisol, n = 10). Responders showed higher hippocampal deactivation during the stress task and lower recognition scores due to a larger number of misses. Intriguingly, stress-responders showed significant differences in hippocampal activation already prior to stress, with higher levels of hippocampal activity during the picture encoding. Although effects of both cortisol and hippocampal activation on recognition were present in responders, similar effects were absent in the nonresponder group. Our results indicate that hippocampus plays an important role in adaptive behavioral responses. We hypothesize that states of hippocampal activation prior to stress might reflect states of vigilance or anxiety, which might be important for determining interindividual differences in subsequent stress response and cognitive performance.

H3 receptor antagonism enhances NCAM PSA-mediated plasticity and improves memory consolidation in odor discrimination and delayed match-to-position paradigms

To further understand the procognitive actions of GSK189254, a histamine H(3) receptor antagonist, we determined its influence on the modulation of hippocampal neural cell adhesion molecule (NCAM) polysialylation (PSA) state, a necessary neuroplastic mechanism for learning and memory consolidation. A 4-day treatment with GSK189254 significantly increased basal expression of dentate polysialylated cells in rats with the maximal effect being observed at 0.03-0.3 mg/kg. At the optimal dose (0.3 mg/kg), GSK189254 enhanced water maze learning and the associated transient increase in NCAM-polysialylated cells. The increase in dentate polysialylated cell frequency induced by GSK189254 was not attributable to enhanced neurogenesis, although it did induce a small, but significant, increase in the survival of these newborn cells. GSK189254 (0.3 mg/kg) was without effect on polysialylated cell frequency in the entorhinal and perirhinal cortex, but significantly increased the diffuse PSA staining observed in the anterior, ventromedial, and dorsomedial aspects of the hypothalamus. Consistent with its ability to enhance the learning-associated, post-training increases in NCAM PSA state, GSK189254 (0.3 mg/kg) reversed the amnesia induced by scopolamine given in the 6-h post-training period after training in an odor discrimination paradigm. Moreover, GSK189254 significantly improved the performance accuracy of a delayed match-to-position paradigm, a task dependent on the prefrontal cortex and degree of cortical arousal, the latter may be related to enhanced NCAM PSA-associated plasticity in the hypothalamus. The procognitive actions of H3 antagonism combined with increased NCAM PSA expression may exert a disease-modifying action in conditions harboring fundamental deficits in NCAM-mediated neuroplasticity, such as schizophrenia and Alzheimer's disease.

Curcumin-induced degradation of PKC delta is associated with enhanced dentate NCAM PSA expression and spatial learning in adult and aged Wistar rats

Polysialylation of the neural cell adhesion molecule (NCAM PSA) is necessary for the consolidation processes of hippocampus-based learning. Previously, we have found inhibition of protein kinase C delta (PKCdelta) to be associated with increased polysialyltransferase (PST) activity, suggesting inhibitors of this kinase might ameliorate cognitive deficits. Using a rottlerin template, a drug previously considered an inhibitor of PKCdelta, we searched the Compounds Available for Purchase (CAP) database with the Accelrys((R)) Catalyst programme for structurally similar molecules and, using the available crystal structure of the phorbol-binding domain of PKCdelta, found that diferuloylmethane (curcumin) docked effectively into the phorbol site. Curcumin increased NCAM PSA expression in cultured neuro-2A neuroblastoma cells and this was inversely related to PKCdelta protein expression. Curcumin did not directly inhibit PKCdelta activity but formed a tight complex with the enzyme. With increasing doses of curcumin, the Tyr(131) residue of PKCdelta, which is known to direct its degradation, became progressively phosphorylated and this was associated with numerous Tyr(131)-phospho-PKCdelta fragments. Chronic administration of curcumin in vivo also increased the frequency of polysialylated cells in the dentate infragranular zone and significantly improved the acquisition and consolidation of a water maze spatial learning paradigm in both adult and aged cohorts of Wistar rats. These results further confirm the role of PKCdelta in regulating PST and NCAM PSA expression and provide evidence that drug modulation of this system enhances the process of memory consolidation.

Olanzapine, but not haloperidol, enhances PSA-NCAM immunoreactivity in rat prefrontal cortex

Repeated antipsychotic treatment may produce adaptive changes ranging from cytoarchitectural rearrangements to synaptic modifications that might contribute to clinical improvement. We performed a prolonged treatment (2 wk) with the first-generation antipsychotic (FGA) haloperidol (1 mg/kg) and the second-generation antipsychotic (SGA) olanzapine (2 mg/kg twice daily) and analysed the expression of the polysialylated form of neural cell adhesion molecule (PSA-NCAM) in rat hippocampus and prefrontal cortex via immunohistochemistry. We found a regional- and drug-selective increase of PSA-NCAM expression in prefrontal cortex of olanzapine-treated rats with no effects in hippocampus; conversely, haloperidol did not produce a change in either brain region. Our findings reveal a possible role for PSA-NCAM in the mechanism of action of the SGA olanzapine adding complexity as well as specificity to the molecular changes set in motion by this drug.

Exposure to stressors during juvenility disrupts development-related alterations in the PSA-NCAM to NCAM expression ratio: potential relevance for mood and anxiety disorders

Childhood trauma is associated with higher rates of both mood and anxiety disorders in adulthood. The exposure of rats to stressors during juvenility has comparable effects, and was suggested as a model of induced predisposition for these disorders. The neural cell adhesion molecule (NCAM) and its polysialylated form PSA-NCAM are critically involved in neural development, activity-dependent synaptic plasticity, and learning processes. We examined the effects of exposure to stressors during juvenility on coping with stressors in adulthood and on NCAM and PSA-NCAM expression within the rat limbic system both soon after the exposure and in adulthood. Exposure to stressors during juvenility reduced novel-setting exploration and impaired two-way shuttle avoidance learning in adulthood. Among naive rats, a development-related decrease of about 50% was evident in the PSA-NCAM to NCAM expression ratio in the basolateral amygdala, in the CA1 and dentate gyrus regions of the hippocampus, and in the entorhinal cortex. In juvenile-stressed rats, we found no such decrease, but rather an increase in the polysialylation of NCAM ( approximately 50%), evident soon after the exposure to juvenile stress and also in adulthood. Our results suggest that exposure to stressors during juvenility alters the maturation of the limbic system, and potentially underlies the predisposition to exhibit stress-related symptoms in adulthood.

Selective learning and memory impairments in mice deficient for polysialylated NCAM in adulthood

The neural cell adhesion molecule (NCAM) has been implicated in regulating synaptic plasticity mechanisms as well as memory consolidation processes. Attachment of polysialic acid to NCAM (PSA-NCAM) has been reported to down-regulate its adhesive forces, a process hypothesized to be implicated in synapse selection after learning experiences. PSA-NCAM has been critically implicated in hippocampus-related synaptic plasticity and memory storage, but information about its functional role in other brain areas remains scarce. Here, we studied mice deficient for polysialyltransferase-1 (ST8SialV/PST-1), an enzyme which attaches PSA to NCAM during postnatal development and adulthood, and whose deficiency results in a drastic reduction of PSA-NCAM expression throughout the brain in adulthood. Mice were tested for their performance in the water maze and auditory fear conditioning (AFC). We report that ST8SiaIV knockout mice were impaired in spatial as well as reversal learning in the water maze. On the other hand, AFC was intact and ST8SiaIV mice exhibited no impairments in the acquisition or retention of cued fear memories. Spatial orientation learning and reversal learning require complex integration of spatial information and response selection involving the hippocampus and prefrontal cortex, whereas cued fear conditioning is an associative type of emotional memory that highly depends on amygdala function. Therefore, our results indicate that PSA-NCAM contributes differentially to learning processes that differ in the nature of the neural computations involved, which probably reflects a differential role of this molecule in different brain regions.

Natural drug extracts for a nutritive-tonic drink, promotes the induction of long-term potentiation in rat hippocampal dentate gyrus in vivo

We have shown previously that oral administration of a nutritive-tonic drink (NTD) improves scopolamine-induced memory impairment in the passive avoidance task and Morris water-maze in mice and that this action is attributable to the natural drug extracts, rather than synthetic drugs such as taurine and caffeine, in the NTD. In order to investigate the mechanism underlying the antiamnesic effects of the natural drug extracts, the effects of the extracts on the induction of long-term potentiation (LTP) was investigated in the dentate gyrus (DG) of normal and scopolamine-treated rats. Oral administration of natural drug extracts enhanced the induction of population spike amplitude induced by weak tetanic stimulation (30 pulses at 60 Hz). Scopolamine (0.3 mg/kg, i.p.) completely inhibited the induction of LTP induced by both weak and strong tetanic stimulation (100 pulses at 100 Hz). Natural drug extracts enhanced partially but significantly the induction of LTP by strong tetanus, but had a very weak effect on that induced by weak tetanus. These results suggest that LTP induced by strong tetanus is sensitive to natural drug extracts, and that the antiamnesic effect of the NTD is at least partly attributable to the LTP-improving effect of the natural drug extracts in the DG.

Post-training intrahippocampal injection of synthetic poly-alpha-2,8-sialic acid-neural cell adhesion molecule mimetic peptide improves spatial long-term performance in mice

Several data have shown that the neural cell adhesion molecule (NCAM) is necessary for long-term memory formation and might play a role in the structural reorganization of synapses. The NCAM, encoded by a single gene, is represented by several isoforms that differ with regard to their content of alpha-2,8-linked sialic acid residues (PSA) on their extracellular domain. The carbohydrate PSA is known to promote plasticity, and PSA-NCAM isoforms remain expressed in the CA3 region of the adult hippocampus. In the present study, we investigated the effect on spatial memory consolidation of a PSA gain of function by injecting a PSA mimetic peptide (termed pr2) into the dorsal hippocampus. Mice were subjected to massed training in the spatial version of the water maze. Five hours after the last training session, experimental mice received an injection of pr2, whereas control mice received PBS or reverse peptide injections in the hippocampal CA3 region. Memory retention was tested at different time intervals: 24 h, 1 wk, and 4 wk. The results showed that the post-training infusion of pr2 peptide significantly increases spatial performance whenever it was assessed after the training phase. By contrast, administration of the control reverse peptide did not affect retention performance. These findings provide evidence that (1) PSA-NCAM is involved in memory consolidation processes in the CA3 hippocampal region, and (2) PSA mimetic peptides can facilitate the formation of long-term spatial memory when injected during the memory consolidation phase.

Reversible changes in hippocampal CA1 synapses associated with water maze training in rats

Long-term memories seem to require protein synthesis to be established. This process can be related with synaptogenesis resulting in changes in the form or even in the number or proportion of synaptic contacts. Results from behavioral studies assessing quantitative changes associated with different learning tasks are controversial. The aim of our work was to assess whether the number of CA1 hippocampal synaptic contacts can be modified after training in different tasks in the Morris water maze (MWM). We found transient changes in the synaptic density of the symmetric synapses associated with place learning. A reduced synaptic density of the symmetric synapses in the stratum radiatum of CA1 was found at 48 h posttraining, returning to control levels 72 h posttraining. The same effect was observed 1 h after training in a nonspatial task. Synaptic changes found in the CA1 shortly after water maze training suggest a possible participation of the hippocampus in the acquisition of nonspatial tasks together with a role in the short-term consolidation of spatial memory. As no changes were found in the total number of synapses counted, it is likely that subtle changes in synaptic efficacy than new synapse generation may be sufficient to support the acquisition and maintenance of new memories.

Hippocampal up-regulation of NCAM expression and polysialylation plays a key role on spatial memory

Memory formation has been associated with structural and functional modifications of synapses. Cell adhesion molecules are prominent modulators of synaptic plasticity. Here, we investigated the involvement of the cell adhesion molecules, NCAM, its polysialylated state (PSA-NCAM) and L1 in spatial learning-induced synaptic remodeling and memory storage. A differential regulation of these adhesion molecules was found in the hippocampus of rats submitted to one training session in the spatial, but not cued, version of the Morris water maze. Twenty-four hours after training, synaptic expression of NCAM and PSA-NCAM was increased, whereas L1 appeared markedly decreased. The regulation of these molecules was spatial learning-specific, except for L1 reduction, which could be attributed to swimming under stressful conditions rather than to learning. Subsequent psychopharmacological experiments were performed to address the functional role of NCAM and PSA-NCAM in the formation of spatial memories. Rats received an intracerebroventricular injection of either a synthetic peptide (C3d) aimed to interfere with NCAM function, or endoneuraminidase, an enzyme that cleaves polysialic acid from NCAM. Both treatments affected acquisition of spatial information and lead to impaired spatial memory abilities, supporting a critical role of the observed learning-induced up-regulation of synaptic NCAM expression and polysialylation on spatial learning and memory. Therefore, our findings highlight NCAM as a learning-modulated molecule critically involved in the hippocampal remodeling processes underlying spatial memory formation.

Trimethyltin-induced alterations in behavior are linked to changes in PSA-NCAM expression

The neurotoxic heavy metal trimethyltin (TMT) primarily damages neurons of the hippocampus and limbic areas of the temporal lobe, and causes a dose-dependent decrease in the polysialated form of the neural cell adhesion molecule (PSA-NCAM) in the mouse hippocampus. In the current study, we attempted to associate deficits in spatial learning following TMT exposure at various stages in learning with changes in levels of NCAM-180 and PSA-NCAM in both the hippocampus and frontal cortex. Mice were treated with TMT either before or after training on a spatial learning paradigm and examined for changes in NCAM and PSA-NCAM 12h later. In the first set of experiments, male BALB/c mice were injected with TMT (2.25 mg/kg) or saline i.p. and tested 24-168 h later using hidden and visible versions of the water maze, as well as light avoidance and motor activity. Mice in both treated and control groups which demonstrated a significant improvement in water maze performance also showed an elevation in hippocampal PSA-NCAM at all time points examined. TMT exposure impaired spatial learning and blocked learning-induced elevations in PSA-NCAM expression 24-96 h post-treatment, but these deficits disappeared by 168 h post-treatment. Mice exposed to TMT during reconsolidation of spatial learning (after repeated water maze training) demonstrated a mild and transient difference in escape latency compared to saline exposed mice. TMT administration during this period did not result in the attenuation of PSA-NCAM expression observed when animals were exposed before training. These results confirm a specific role for PSA-NCAM in acquisition and consolidation of spatial memory.

Chronic exposure of rats to cognition enhancing drugs produces a neuroplastic response identical to that obtained by complex environment rearing

Recent data suggest that Alzheimer's patients who discontinue treatment with cholinesterase inhibitors have a significantly delayed cognitive decline as compared to patients receiving placebo. Such observations suggest cholinesterase inhibitors to provide a disease-modifying effect as well as symptomatic relief and, moreover, that this benefit remains after drug withdrawal. Consistent with this suggestion, we now demonstrate that chronic administration of tacrine, nefiracetam, and deprenyl, drugs that augment cholinergic function, increases the basal frequency of dentate polysialylated neurons in a manner similar to the enhanced neuroplasticity achieved through complex environment rearing. While both drug-treated and complex environment reared animals continue to exhibit memory-associated activation of hippocampal polysialylated neurons, the magnitude is significantly reduced suggesting that such interventions induce a more robust memory pathway that can acquire and consolidate new information more efficiently. This hypothesis is supported by our findings of improved learning behavior and enhanced resistance to cholinergic deficits seen following either intervention. Furthermore, the level of enhancement of basal neuroplastic status achieved by either drug or environmental intervention correlates directly with improved spatial learning ability. As a combination of both interventions failed to further increase basal polysialylated cell frequency, complex environment rearing and chronic drug regimens most likely enhanced cognitive performance by the same mechanism(s). These findings suggest that improved memory-associated synaptic plasticity may be the fundamental mechanism underlying the disease modifying action of drugs such as cholinesterase inhibitors. Moreover, the molecular and cellular events underpinning neuroplastic responses are identified as novel targets in the search for interventive drug strategies for the treatment of neurodegenerative and neuropsychiatric disorders.