Impaired Place Learning Research Articles

Hippocampus-sensitive and striatum-sensitive learning one month after morphine or cocaine exposure in male rats

These experiments examined whether morphine and cocaine alter the balance between hippocampal and striatal memory systems measured long after drug exposure. Male rats received injections of morphine (5 mg/kg), cocaine (20 mg/kg), or saline for five consecutive days. One month later, rats were trained to find food on a hippocampus-sensitive place task or a striatum-sensitive response task. Relative to saline controls, morphine-treated rats exhibited impaired place learning but enhanced response learning; prior cocaine exposure did not significantly alter learning on either task. Another set of rats was trained on a dual-solution T-maze that can be solved with either place or response strategies. While a majority (67%) of control rats used place solutions, morphine treatment one month prior resulted in the exclusive use of response solutions (100%). Prior cocaine treatment did not significantly alter strategy selection. Molecular markers related to learning and drug abuse were measured in the hippocampus and striatum one month after drug exposure in behaviorally untested rats. Protein levels of glial-fibrillary acidic protein (GFAP), an intermediate filament specific to astrocytes, increased significantly in the hippocampus after morphine exposure, but not after cocaine exposure. Exposure to morphine or cocaine did not significantly change levels of brain-derived neurotrophic factor (BDNF) or a downstream target of BDNF signaling, glycogen synthase kinase 3β (GSK3β), in the hippocampus or striatum. Thus, exposure to morphine resulted in a long-lasting shift from hippocampal toward striatal dominance during learning, an effect that may be associated with lasting alterations in hippocampal astrocytes. Cocaine produced changes in the same direction, suggesting that use of a higher dose or longer duration of exposure might produce effects comparable to those seen with morphine.

Mitragynine (Kratom)-Induced Cognitive Impairments in Mice Resemble Δ9-THC and Morphine Effects: Reversal by Cannabinoid CB1 Receptor Antagonism.

Kratom is a widely abused plant-based drug preparation with a global interest in recent years, well beyond its native grounds in Southeast Asia. Mitragynine, its major psychoactive constituent is known to exhibit opioid-like behavioral effects with resultant neuroplasticity in the brain reward system. Its chronic administration is associated with cognitive impairments in animal studies. However, the underlying molecular mechanism for such a deficit remains elusive. In this study, the involvement of cannabinoid type-1 (CB1) receptors in cognitive deficits after chronic mitragynine exposures was investigated for 28 days (with incremental dose sensitization from 1 to 25 mg/kg) in adult male Swiss albino mice using the IntelliCage® system. Chronic high-dose mitragynine exposure (5–25 mg/kg, intraperitoneal [i.p.]), but not low-dose exposure (1–4 mg/kg, i.p.), induced hyperlocomotion, potentiated the preference for sucrose reward, increased resistance to punishment, and impaired place learning and its reversal. Comparable deficits were also observed after chronic treatments with Δ-9-tetrahydrocannabinol (THC, 2 mg/kg, i.p.) or morphine (5 mg/kg, subcutaneous). Mitragynine-, morphine-, and THC-induced learning and memory deficits were reversed by co-treatment with the CB1 receptor antagonist, NIDA-41020 (10 mg/kg, i.p.). A significant upregulation of CB1 receptor expression was found in the hippocampal CA1 region and ventral tegmental area after chronic high-dose mitragynine and morphine, whereas a downregulation was observed after chronic THC. In conclusion, the present study suggests a plausible role of the CB1 receptor in mediating the dose-dependent cognitive deficits after chronic high-dose mitragynine exposure. This also highlights the potential of CB1 receptor antagonism in ameliorating the cognitive deficits associated with long-term kratom/mitragynine consumption in humans.

Stress Induces a Shift Towards Striatum-Dependent Stimulus-Response Learning via the Mineralocorticoid Receptor.

Stress is assumed to cause a shift from flexible 'cognitive' memory to more rigid 'habit' memory. In the spatial memory domain, stress impairs place learning depending on the hippocampus whereas stimulus-response learning based on the striatum appears to be improved. While the neural basis of this shift is still unclear, previous evidence in rodents points towards cortisol interacting with the mineralocorticoid receptor (MR) to affect amygdala functioning. The amygdala is in turn assumed to orchestrate the stress-induced shift in memory processing. However, an integrative study testing these mechanisms in humans is lacking. Therefore, we combined functional neuroimaging of a spatial memory task, stress-induction, and administration of an MR-antagonist in a full-factorial, randomized, placebo-controlled between-subjects design in 101 healthy males. We demonstrate that stress-induced increases in cortisol lead to enhanced stimulus-response learning, accompanied by increased amygdala activity and connectivity to the striatum. Importantly, this shift was prevented by an acute administration of the MR-antagonist spironolactone. Our findings support a model in which the MR and the amygdala play an important role in the stress-induced shift towards habit memory systems, revealing a fundamental mechanism of adaptively allocating neural resources that may have implications for stress-related mental disorders.

Chronic mitragynine (kratom) enhances punishment resistance in natural reward seeking and impairs place learning in mice.

Kratom (Mitragyna speciosa) is a widely abused herbal drug preparation in Southeast Asia. It is often consumed as a substitute for heroin, but imposing itself unknown harms and addictive burdens. Mitragynine is the major psychostimulant constituent of kratom that has recently been reported to induce morphine-like behavioural and cognitive effects in rodents. The effects of chronic consumption on non-drug related behaviours are still unclear. In the present study, we investigated the effects of chronic mitragynine treatment on spontaneous activity, reward-related behaviour and cognition in mice in an IntelliCage® system, and compared them with those of morphine and Δ-9-tetrahydrocannabinol (THC). We found that chronic mitragynine treatment significantly potentiated horizontal exploratory activity. It enhanced spontaneous sucrose preference and also its persistence when the preference had aversive consequences. Furthermore, mitragynine impaired place learning and its reversal. Thereby, mitragynine effects closely resembled that of morphine and THC sensitisation. These findings suggest that chronic mitragynine exposure enhances spontaneous locomotor activity and the preference for natural rewards, but impairs learning and memory. These findings confirm pleiotropic effects of mitragynine (kratom) on human lifestyle, but may also support the recognition of the drug's harm potential.

Impaired spatial learning and reduced adult hippocampal neurogenesis in histamine H1-receptor knockout mice

The histamine H1-receptor (H1R) is expressed in wide parts of the brain including the hippocampus, which is involved in spatial learning and memory. Previous studies in H1R knockout (H1R-KO) mice revealed deficits in a variety of learning and memory tasks. It was also proposed that H1R activation is crucial for neuronal differentiation of neural progenitors. Therefore, the aim of this study was to investigate negatively reinforced spatial learning in the water-maze and to assess survival and neuronal differentiation of newborn cells in the adult hippocampus of H1R-KO mice. H1R-KO and wild-type (WT) mice were subjected to the following sequence of tests: (a) cued version, (b) place learning, (c) spatial probe, (d) long-term retention and (e) reversal learning. Furthermore hippocampal neurogenesis in terms of survival and differentiation was assessed in H1R-KO and WT mice. H1R-KO mice showed normal cued learning, but impaired place and reversal learning as well as impaired long-term retention performance. In addition, a marked reduction of newborn neurons in the hippocampus but no changes in differentiation of neural progenitors into neuronal and glial lineage was found in H1R-KO mice. Our data suggest that H1R deficiency in mice is associated with pronounced deficits in hippocampus-dependent spatial learning and memory. Furthermore, we herein provide first evidence that H1R deficiency in the mouse leads to a reduced neurogenesis. However, the exact mechanisms for the reduced number of cells in H1R-KO mice remain elusive and might be due to a reduced survival of newborn hippocampal neurons and/or a reduction in cell proliferation.

Curcumin Suppresses Soluble Tau Dimers and Corrects Molecular Chaperone, Synaptic, and Behavioral Deficits in Aged Human Tau Transgenic Mice

The mechanisms underlying Tau-related synaptic and cognitive deficits and the interrelationships between Tau species, their clearance pathways, and synaptic impairments remain poorly understood. To gain insight into these mechanisms, we examined these interrelationships in aged non-mutant genomic human Tau mice, with established Tau pathology and neuron loss. We also examined how these interrelationships changed with an intervention by feeding mice either a control diet or one containing the brain permeable beta-amyloid and Tau aggregate binding molecule curcumin. Transgene-dependent elevations in soluble and insoluble phospho-Tau monomer and soluble Tau dimers accompanied deficits in behavior, hippocampal excitatory synaptic markers, and molecular chaperones (heat shock proteins (HSPs)) involved in Tau degradation and microtubule stability. In human Tau mice but not control mice, HSP70, HSP70/HSP72, and HSP90 were reduced in membrane-enriched fractions but not in cytosolic fractions. The synaptic proteins PSD95 and NR2B were reduced in dendritic fields and redistributed into perikarya, corresponding to changes observed by immunoblot. Curcumin selectively suppressed levels of soluble Tau dimers, but not of insoluble and monomeric phospho-Tau, while correcting behavioral, synaptic, and HSP deficits. Treatment increased PSD95 co-immunoprecipitating with NR2B and, independent of transgene, increased HSPs implicated in Tau clearance. It elevated HSP90 and HSC70 without increasing HSP mRNAs; that is, without induction of the heat shock response. Instead curcumin differentially impacted HSP90 client kinases, reducing Fyn without reducing Akt. In summary, curcumin reduced soluble Tau and elevated HSPs involved in Tau clearance, showing that even after tangles have formed, Tau-dependent behavioral and synaptic deficits can be corrected.

Lesions of the anterior thalamic nuclei and intralaminar thalamic nuclei: place and visual discrimination learning in the water maze

Medial thalamic damage produces memory deficits in humans (e.g., Korsakoff's syndrome) and experimental animals. Both the anterior thalamic nuclei (ATN) and rostral intralaminar plus adjacent lateral thalamic nuclei (ILN/LT) have been implicated. Based on the differences in their main connections with other neural structures, we tested the prediction that ATN lesions would selectively impair acquisition of spatial location discrimination, reflecting a hippocampal system deficit, whereas ILN/LT lesions would impair acquisition of visual pattern discrimination, reflecting a striatal system deficit. Half the rats were first trained in a spatial task in a water maze before switching to a visual task in the same maze, while the remainder were tested with the reverse order of tasks. Compared with sham-operated controls, (1) rats with ATN lesions showed impaired place learning, but normal visual discrimination learning, (2) rats with ILN/LT lesions showed no deficit on either task. Rats with ATN lesions were also hyperactive when their home cage was placed in a novel room and remained more active than ILN/LT or SHAM rats for the subsequent 21h, especially during the nocturnal phase. These findings confirmed the influence of ATN lesions on spatial learning, but failed to support the view that ILN/LT lesions disrupt striatal-dependent memory.

Estradiol alters Fos-immunoreactivity in the hippocampus and dorsal striatum during place and response learning in middle-aged but not young adult female rats.

Evidence from lesion and inactivation studies suggests that the hippocampus (HPC) and dorsal striatum compete for control over navigation behavior, and there is some evidence in males that the structure with greater relative activation controls behavior. Estradiol has been shown to enhance HPC-dependent place learning and impair dorsal striatum-dependent response learning in female rats, possibly by increasing hippocampal activation and/or decreasing striatal activation. We used Fos-immunoreactivity (Fos-IR) to examine the activation of several subregions of the HPC and striatum in ovariectomized female rats with or without estradiol replacement 30 min after place or response learning. In 4-month-old rats, neither task nor estradiol increased Fos-IR above explore control levels in any subregion analyzed, even though estradiol impaired response learning. In 12-month-old rats, estradiol increased Fos-IR in the dentate gyrus, dorsal medial striatum, and dorsal lateral striatum in place task learners, while the absence of estradiol increased Fos-IR in these regions in response task learners. However, learning rate was not affected by estradiol in either task. We also included a group of long-term ovariectomized 12-month-old rats that displayed impaired place learning and altered Fos-IR in CA1 of the HPC. These results suggest that task-specific effects of estradiol on hippocampal and striatal activation emerge across age but that relative hippocampal and striatal activation are not related to learning rate during spatial navigation learning.

Differential regional distribution of phosphorylated tau and synapse loss in the nucleus accumbens in tauopathy model mice

Tauopathies differ in terms of the brain regions that are affected. In Alzheimer's disease, basal forebrain and hippocampus are mainly involved, while frontotemporal lobar degeneration affects the frontal and temporal lobes and subcortical nuclei including striatum. Over 90% of human cases of tauopathies are sporadic, although the majority of established tau-transgenic mice have had mutations. This prompted us to establish transgenic mice expressing wild-type human tau (Tg601). Old (>14months old) Tg601 mice displayed decreased anxiety in the elevated plus maze test and impaired place learning in the Morris water maze test. Immunoblotting of brain tissue identified that soluble tau multimer was increased with aging even though insoluble tau was not observed. In the striatum of old Tg601, the level of AT8- or AT180-positive tau was decreased compared with that of other regions, while PHF-1-positive tau levels remained equal. Phosphorylated tau-positive axonal dilations were present mainly in layers V and VI of the prefrontal cortex. Loss of synaptic dendritic spine and decreased immunohistochemical level of synaptic markers were observed in the nucleus accumbens. In vivo 2-[18F]fluoro-2-deoxy-d-glucose positron emission tomography analysis also showed decreased activity exclusively in the nucleus accumbens of living Tg601 mice. In Tg601 mice, the axonal transport defect in the prefrontal cortex–nucleus accumbens pathway may lead to decreased anxiety behavior. Differential distribution of hyperphosphorylated tau may cause region-specific neurodegeneration.

Decreased cytogenesis in the granule cell layer of the hippocampus and impaired place learning after irradiation of the young mouse brain evaluated using the IntelliCage platform

Radiation therapy is used to treat malignant tumors in the brain and central nervous system involvement of leukemia and lymphomas in children. However, ionizing radiation causes a number of adverse long-term side effects in the brain, including cognitive impairment. Hippocampal neurogenesis is important for place learning and has been shown to be decreased by irradiation (IR) in rats and mice. In the present study, 10-day-old male mice received 6-Gy IR to the brain on postnatal day 10. We used BrdU labeling of the granule cell layer (GCL) of the hippocampus to evaluate cell proliferation and survival. An unbiased, automated platform for monitoring of behavior in a group housing environment (IntelliCage) was used to evaluate place learning 2 months after IR. We show that cranial IR impaired place learning and reduced BrdU labeling by 50% in the GCL. Cranial IR also reduced whole body weight gain 5%. We conclude that this experimental paradigm provides a novel and time-saving model to detect differences in place learning in mice subjected to IR. This method of detecting behavioral differences can be used for further studies of adverse effects of IR on hippocampal neurogenesis and possible new strategies to ameliorate the negative effects of IR on cognition.

Peripheral anxiogenic drug injections differentially affect cognitive and habit memory: role of basolateral amygdala

Findings from single-solution plus-maze tasks that require the use of either place or response learning indicate that post-training intra-basolateral amygdala (BLA) administration of the anxiogenic α-2 adrenoreceptor antagonist RS 79948 can both enhance dorsal striatal-dependent response learning and impair hippocampus-dependent place learning. Whether post-training peripheral administration of RS 79948 can also enhance and impair response and place learning respectively, is not known. If peripheral drug administration can also produce this “dual” effect on cognitive and habit memory, it would be of interest to know whether the functional integrity of the BLA is critical. In order to examine these questions, the present experiments combined peripheral administration of RS 79948 with concurrent neural inactivation of the BLA. Adult male Long–Evans rats were trained in place or response learning tasks in a water plus-maze. On days 1–3 of training, rats received post-training peripheral injections of saline or RS 79948 (0.1 mg/kg) combined with intra-BLA injections of saline or the sodium channel blocker bupivacaine (1.0% solution, 0.5 μl). Post-training peripheral injections of RS 79948 enhanced acquisition of response learning, and impaired acquisition of place learning. Bupivacaine infusions into the BLA had no effect on acquisition of either task. However, intra-BLA infusions of bupivacaine blocked both the enhancement of response learning and the impairment of place learning produced by RS 79948. Taken together, the findings indicate that although the functional integrity of BLA is not necessary for acquisition of place and response learning, BLA activity is critical in order for peripheral injections of an anxiogenic drug to differentially modulate hippocampus-dependent and dorsal striatal-dependent memory.

Effects of stress, corticosterone, and epinephrine administration on learning in place and response tasks

These experiments examined the effects of prior stress, corticosterone, or epinephrine on learning in mazes that can be solved efficiently using either place or response strategies. In a repeated stress condition, rats received restraint stress for 6 h/day for 21 days, ending 24 h before food-motivated maze training. In two single stress conditions, rats received a 1-h episode of restraint stress ending 30 min or 24 h prior to training. Single stress ending 30 min prior to training resulted in a significant interaction of stress and learning on the two tasks, with significant enhancement of learning in the response task and non-significant impairment in the place task. Neither acute nor chronic stress significantly altered learning in either task when the stress ended 24 h before training. Thus, the anterograde effects of stress on maze learning ended within a single day. Two stress-related hormones, corticosterone and epinephrine, were tested for effects on learning parallel to those of acute stress. When administered 30 min prior to training, a corticosterone dose (40 mg/kg) that enhanced memory on a spontaneous alternation task did not significantly enhance or impair learning in either task. Two doses of epinephrine that modulate memory in other settings were used to test the effects of epinephrine on learning. Pre-training injections of 0.03 mg/kg epinephrine impaired place learning, while 0.1 mg/kg epinephrine impaired response learning. The epinephrine results mimicked those seen with acute stress on the place task, but were opposite those seen after acute stress on the response task. Thus, corticosterone does not appear to be a major factor mediating the effects of acute stress on place and response learning and epinephrine is, at most, a partial contributor to these effects.

Hyperphosphorylated tau in parahippocampal cortex impairs place learning in aged mice expressing wild-type human tau

To investigate how tau affects neuronal function during neurofibrillary tangle (NFT) formation, we examined the behavior, neural activity, and neuropathology of mice expressing wild-type human tau. Here, we demonstrate that aged (>20 months old) mice display impaired place learning and memory, even though they do not form NFTs or display neuronal loss. However, soluble hyperphosphorylated tau and synapse loss were found in the same regions. Mn-enhanced MRI showed that the activity of the parahippocampal area is strongly correlated with the decline of memory as assessed by the Morris water maze. Taken together, the accumulation of hyperphosphorylated tau and synapse loss in aged mice, leading to inhibition of neural activity in parahippocampal areas, including the entorhinal cortex, may underlie place learning impairment. Thus, the accumulation of hyperphosphorylated tau that occurs before NFT formation in entorhinal cortex may contribute to the memory problems seen in Alzheimer's disease (AD).

Long-term study of dendritic spines from hippocampal CA1 pyramidal cells, after neuroprotective melatonin treatment following global cerebral ischemia in rats

Melatonin reduces pyramidal neuronal death in the hippocampus and prevents the impairment of place learning and memory in the Morris water maze, otherwise occurring following global cerebral ischemia. The cytoarchitectonic characteristics of the hippocampal CA1 remaining pyramidal neurons in brains of rats submitted 120 days earlier to acute global cerebral ischemia (15-min four vessel occlusion, and melatonin 10mg/(kg h 6h), i.v. or vehicle administration) were compared to those of intact control rats in order to gain information concerning the neural substrate underlying preservation of hippocampal functioning. Hippocampi were processed according to a modification of the Golgi method. Dendritic bifurcations from pyramidal neurons in both the oriens-alveus and the striatum radiatum; as well as spine density and proportions of thin, stubby, mushroom-shaped, wide, ramified, and double spines in a 50 microm length segment of an oblique dendrite branching from the apical dendrite of the hippocampal CA1 remaining pyramidal neurons were evaluated. No impregnated CA1 pyramidal neurons were found in the ischemic-vehicle-treated rats. CA1 pyramidal neurons from ischemic-melatonin-treated rats showed stick-like and less ramified dendrites than those seen in intact control neurons. In addition, lesser density of spines, lower proportional density of thin spines, and higher proportional density of mushroom spines were counted in ischemic-melatonin-treated animals than those in the sinuously branched dendrites of the intact control group. These cytoarchitectural arrangements seem to be compatible with place learning and memory functions long after ischemia and melatonin neuroprotection.

Water maze impairments after combined depletion of somatostatin and serotonin in the rat

This study examined the effect of both separate and combined depletion of brain somatostatin and serotonin on acquisition of the water maze (WM) task. Naïve male Long-Evans rats received injections of p-chlorophenylalanine (PCPA; 500 mg/kg × 2) to deplete serotonin or cysteamine (90 or 200 mg/kg) to deplete somatostatin, or both treatments prior to spatial and reversal training in the water maze. Some rats first received Morris’ nonspatial pretraining to train them in the behavioral strategies that are required for successful spatial place learning in this task, prior to drug treatment and spatial training. A detailed behavioral analysis indicated that somatostatin or serotonin depletion alone caused little or no impairment in naïve animals. Depletion of both somatostatin and serotonin in naïve rats impaired performance, with differences in the impairments that depended on the dose of cysteamine. Nonspatially pretrained rats were not impaired. Thus, neither somatostatin nor serotonin alone is crucial for the water maze task, but impairments occur if both somatostatin and serotonin are depleted in naïve rats. The results indicate that some of the performance impairment was due to strategies impairment rather than a spatial place learning impairment. Depletion of both somatostatin and serotonin in naïve rats produces results comparable to the spatial navigation deficits seen in some Alzheimer patients and suggests that combinations of antagonist treatments may better model this disorder than single antagonist treatments do.

Long‐term morphological and functional evaluation of the neuroprotective effects of post‐ischemic treatment with melatonin in rats

Consensus on neuroprotection has pointed out the relevance of the long-term morphological and functional evaluation of the effectiveness of putative neuroprotective procedures. In the present study, place learning (Morris water maze) and working memory (eight-arm Olton radial maze) were evaluated in adult male rats 90 days after 15 min of global cerebral ischemia (four-vessel occlusion) followed by continuous i.v. infusion (10 mg/kg/hr) of melatonin (Isch + Mel) or vehicle (Isch + Veh) for 6 hr, and the pyramidal neuron population of the cornus Ammoni (CA) of the hippocampus and layers III and V of the medial prefrontal cortex was assessed at the end of the behavioral testing period (120 days after ischemia). Impairment of place learning, a significant delay in working memory acquisition, and a significant loss of pyramidal neurons in the Ammon's horn (CA1: 23%, CA2: 52% CA3: 73%, hilus: 64% remaining neurons), were observed in the Isch + Veh group. By contrast, a similar performance of the Isch + Mel group to that in the Intact and Sham groups and better than that of the Isch + Veh group, besides a significant reduction of pyramidal neuron loss in the CA subfields (CA1: 79%, CA2: 88% CA3: 86%, hilus: 72% remaining neurons), documented that melatonin treatment led to a long-term preservation of both the neural substrate, and the capability for integration of spatial learning and memory, mainly dependent on a normal hippocampal functioning. Overall the results emphasize the efficacy of melatonin in counteracting the pathophysiological processes induced by ischemia, by exerting its actions during a short but critical period early after the ischemic episode.

Effects of injections of glucose into the dorsal striatum on learning of place and response mazes

The present experiment tested the hypothesis that facilitation of striatal function with intra-striatal glucose injections would facilitate learning a striatum-dependent response maze and impair learning a hippocampus-dependent place maze. Food-deprived Sprague–Dawley male rats were trained to find food in a Y-maze. In the place version of the maze, rats were rewarded for learning to go to an arm located in a fixed location while in the response task rats were rewarded for consistently turning in the same direction at the choice point. Artificial cerebrospinal fluid (1 μL) containing either 0.7 nmol of glucose (control) or 20 nmol of glucose was injected bilaterally into the dorsal striatum immediately before training. The animals were trained to a criterion of 9/10 correct choices. In the place maze, glucose injections impaired learning, as measured by number of trials required to reach 9/10 correct. However, in the response task, glucose injections did not enhance learning. A subsequent experiment examined the effects of intra-striatal glucose injections on acquisition of the response task under two different visual cue conditions, addition of an intra-maze light cue that predicted the correct arm or with removal of most visual extramaze cues. Glucose again failed to facilitate acquisition of the response task under these conditions. These findings suggest that facilitation of striatal function via intra-striatal glucose injections is sufficient to impair place learning but not to enhance response learning, perhaps separating the neurochemical mechanisms for striatal involvement in impairment of place and enhancement of response learning.

In vivo Inhibition of Hippocampal Ca2+/Calmodulin-Dependent Protein Kinase II by RNA Interference

Hippocampal alpha-Ca2+/calmodulin-dependent protein kinase II (alpha-CaMKII) has been implicated in spatial learning, neuronal plasticity, epilepsy, and cerebral ischemia. In the present study, an adeno-associated virus (AAV) vector was designed to express green fluorescent protein (GFP) from the CBA promoter and a small hairpin RNA targeting alpha-CaMKII (AAV-shCAM) driven from the U6 promoter. The AAV-shCAM or control vector was microinfused into the rat hippocampus and behavioral testing conducted 19-26 days following surgery. Expression of the marker gene and alpha-CaMKII was evaluated 31 days following AAV infusion. GFP expression was localized to the hippocampus and extended +/-2 mm rostral and caudal from the injection site. Hippocampal alpha-CaMKII was significantly reduced following AAV-shCAM treatment as demonstrated using immunohistochemical and Western analysis. This suppression of alpha-CaMKII was associated with changes in exploratory behavior (open field task) and impaired place learning (water maze task). These results demonstrate the efficacy of a viral-based delivered shRNA to produce gene suppression in a specific circuit of the brain.

Corticohippocampal contributions to spatial and contextual learning.

Spatial and contextual learning are considered to be dependent on the hippocampus, but the extent to which other structures in the medial temporal lobe memory system support these functions is not well understood. This study examined the effects of individual and combined lesions of the perirhinal, postrhinal, and entorhinal cortices on spatial and contextual learning. Lesioned subjects were consistently impaired on measures of contextual fear learning and consistently unimpaired on spatial learning in the Morris water maze. Neurotoxic lesions of perirhinal or postrhinal cortex that were previously shown to impair contextual fear conditioning (Bucci et al., 2000) or contextual discrimination (Bucci et al., 2002) caused little or no impairment in place learning and incidental learning in the water maze. Combined lesions of perirhinal plus lateral entorhinal or postrhinal plus medial entorhinal cortices resulted in deficits in acquisition of contextual discrimination but had no effect on place learning in the water maze. Finally, a parahippocampal lesion comprising combined neurotoxic damage to perirhinal, postrhinal, and entorhinal cortices resulted in profound impairment in acquisition of a standard passive avoidance task but failed to impair place learning. In the same experiment, rats with hippocampal lesions were impaired in spatial navigation. These results indicate that tasks requiring the association between context and an aversive stimulus depend on corticohippocampal circuitry, whereas place learning in the water maze can be accomplished without the full complement of highly processed information from the cortical regions surrounding the hippocampus. The evidence that different brain systems underlie spatial navigation and contextual learning has implications for research on memory when parahippocampal regions are involved.

Effects of the synthetic cannabinoid nabilone on spatial learning and hippocampal neurotransmission

Cannabinoids, the active components of marijuana, affect memory and hippocampal neurotransmission. It has been claimed that nabilone, a synthetic cannabinoid endowed with antiemetic properties, has a peculiar profile of actions. We studied the effects of the drug on spatial learning and in vitro hippocampal CA1 electrophysiology in the rat. Nabilone (0.1, 0.5, and 1.0 mg/kg ip) does not impair place learning in a water maze task, whereas Δ 8-tetrahydrocannabinol (Δ 8-THC) disrupts this function. At concentrations ranging from 1 nM to 10 μM nabilone does not influence basal glutamatergic neurotransmission, which is decreased by Δ 8-THC. Although cannabinoids have been consistently reported to affect synaptic plasticity, nabilone 1 μM does not change paired-pulse facilitation, long-term potentiation and the magnitude of long-term depression. However, the time course of the latter phenomenon is significantly changed by the drug, the depression being lower than in control experiments from 7 to 35 min postinduction. Altogether, our data indicate that there might be differences in the effects of agonists for central cannabinoid receptors, which could help to understand the pharmacology of this class of molecules. The results also suggest that amnesia induced by cannabinoids be possibly related to their effects on hippocampal neurotransmission. The study supports the use of nabilone in conditions the course of which is complicated by cognitive impairment.