Prefrontal cortex and hippocampus subserve different components of working memory in rats

NMDA receptors in the medial prefrontal cortex and the dorsal hippocampus regulate methamphetamine-induced hyperactivity and extracellular amino acid release in mice

The medial prefrontal cortex (mPFC) and the dorsal hippocampus (DHC) play significant roles in stimulant-induced neurobehavioral effects. Methamphetamine (MAP)-induced hyperactivity has been reported to be involved in the regulation of the glutamatergic system. The present study examined whether the glutamatergic and GABAergic systems in the mPFC and DHC were involved in MAP-induced hyperactivity in mice. A combined kainic acid (KA) or N-methyl-d-aspartate (NMDA) lesion and microdialysis technique targeting both the mPFC and DHC were used. The results showed that both KA- and NMDA-induced lesions of the mPFC facilitated MAP-induced hyperactivity, while neither KA- nor NMDA-induced lesions of the DHC had a similar effect. MAP increased the extracellular glutamate (Glu) levels in the mPFC and reduced Glu levels in the DHC. GABA levels in both of these regions were reduced. A KA or NMDA lesion of the mPFC inhibited the Glu reduction in the DHC, and the same lesion of the DHC inhibited the Glu increase in the mPFC induced by MAP. A NMDA lesion of the mPFC blocked GABA reduction in the DHC, but a lesion of DHC enhanced the GABA decrease in the mPFC induced by MAP. Furthermore, a NMDA lesion of DHC increased the vesicular glutamate transporter-2 (VGLUT2) expression in the mPFC following MAP-administration. These findings indicate that glutamatergic as well as GABAergic systems in these two regions are involved in MAP-induced hyperactivity. Moreover, there may be an inhibitory role in these two regions, especially mediated by NMDA receptors, in MAP-induced abnormal behavior and neurotransmission responses.

Selective cognitive impairments are related to selective hippocampus and prefrontal cortex deficits after prenatal chlorpyrifos exposure

Prenatal exposure to chlorpyrifos (CPF) leads to cognitive impairments in adulthood. The cytoarchitectural basis is unclear. In the present study, we assessed the effects of prenatal CPF exposure on T-maze delayed alternation task and the win-shift/lose-shift responses associated with the morphology of the dorsal hippocampus (dHPC) and the medial prefrontal cortex (mPFC) in adult animals. Gestational ICR female mice were exposed to 0, 1 or 5mg/kg/d of CPF through gestational days 13-17. Behavioral experiments were performed on postnatal days (PD) 45-60 of the male and female offsprings; morphological samples were collected on PD 60. Our behavioral study results showed a gradual increase in the number of lose-shift errors on increased memory loads in the 5mg/kg/d CPF-treated males. A weak initial increase in the number of lose-shift errors was observed in the females. In all of the groups, no significant differences were observed in the number of win-shift errors and correct of the first choice. The morphological studies showed extensive condensed nucleus and enlarged intercellular spaces in the CA1 and DG sub-regions in the dHPC of the CPF-treated males and the DG sub-region of the CPF-treated females. The cell count was significantly reduced in these sub-regions. The morphological studies showed no obvious abnormalities at PrL and IL of mPFC in the CPF-treated males and females, but the cell count was reduced. Our findings suggest that prenatal CPF exposure at 5mg/kg/d induces selective cognitive impairments, which based on the morphological deficits in the dHPC and the mPFC.

Prefrontal electroencephalographic activity during the working memory processes involved in a sexually motivated task in male rats

The prefrontal cortex is involved in working memory functions, and several studies using food or drink as rewards have demonstrated that the rat is capable of performing tasks that involve working memory. Sexual activity is another highly-rewarding, motivated behaviour that has proven to be an efficient incentive in classical operant tasks. The objective of this study was to determine whether the functional activity of the medial prefrontal cortex (mPFC) changes in relation to the working memory processes involved in a sexually motivated task performed in male rats. Thus, male Wistar rats implanted in the mPFC were subjected to a nonmatching-to-sample task in a T-maze using sexual interaction as a reinforcer during a 4-day training period. On the basis of their performance during training, the rats were classified as 'good-learners' or 'bad-learners'. Only the good-learner rats showed an increase in the absolute power of the 8-13 Hz band during both the sample and test runs; a finding that could be related to learning of the working memory elements entailed in the task. During the maintenance phase only (i.e., once the rule had been learned well), the good-learner rats also showed an increased correlation of the 8-13 Hz band during the sample run, indicating that a high degree of coupling between the prefrontal cortices is necessary for the processing required to allow the rats to make correct decisions in the maintenance phase. Taken together, these data show that mPFC activity changes in relation to the working memory processes involved in a sexually motivated task in male rats.

Neocortical-hippocampal dynamics of working memory in healthy and diseased brain states based on functional connectivity

Working memory (WM) is the ability to transiently maintain and manipulate internal representations beyond its external availability to the senses. This process is thought to support high level cognitive abilities and been shown to be strongly predictive of individual intelligence and reasoning abilities. While early models of WM have relied on a modular perspective of brain functioning, more recent evidence suggests that cognitive functions emerge from the interactions of multiple brain regions to generate large-scale networks. Here we will review the current research on functional connectivity of WM processes to highlight the critical role played by neural interactions in healthy and pathological brain states. Recent findings demonstrate that WM abilities are not determined solely by local brain activity, but also rely on the functional coupling of neocortical-hippocampal regions to support WM processes. Although the hippocampus has long been held to be important for long-term declarative memory, recent evidence suggests that the hippocampus may also be necessary to coordinate disparate cortical regions supporting the periodic reactivation of internal representations in WM. Furthermore, recent brain imaging studies using connectivity measures, have shown that changes in cortico-limbic interactions can be useful to characterize WM impairments observed in different neuropathological conditions. Recent advances in electrophysiological and neuroimaging techniques to model network activity has led to important insights into how neocortical and hippocampal regions support WM processes and how disruptions along this network can lead to the memory impairments commonly reported in many neuropathological populations.

Dysfunctional prefrontal cortical network activity and interactions following cannabinoid receptor activation

Coordinated activity spanning anatomically distributed neuronal networks underpins cognition and mediates limbic-cortical interactions during learning, memory, and decision-making. We used CP55940, a potent agonist of brain cannabinoid receptors known to disrupt coordinated activity in hippocampus, to investigate the roles of network oscillations during hippocampal and medial prefrontal cortical (mPFC) interactions in rats. During quiet wakefulness and rest, CP55940 dose-dependently reduced 0.1-30 Hz local field potential power in CA1 of the hippocampus while concurrently decreasing 30-100 Hz power in mPFC; these contrasting population-level effects were paralleled by differential effects on underlying single-unit activity in the two structures. During decision-making phases of a spatial working memory task, CP5540-induced deficits in hippocampal theta and prefrontal gamma oscillations were observed alongside disrupted theta-frequency coherence between the two structures. These changes in coordinated limbic-cortical network activities correlated with (1) reduced accuracy of task performance, (2) impaired phase-locking of prefrontal single-unit spiking to the local gamma and hippocampal theta rhythms, and (3) impaired task-dependent activity in a subset of mPFC units. In addition to highlighting the importance of CA1-mPFC network oscillations for cognition, these results implicate disrupted theta-frequency coordination of CA1-mPFC activity in the cognitive deficits caused by exogenous activation of brain cannabinoid receptors.

Nicotine improves working memory span capacity in rats following sub-chronic ketamine exposure

Ketamine, an NMDA-receptor antagonist, produces cognitive deficits in humans in a battery of tasks involving attention and memory. Nicotine can enhance various indices of cognitive performance, including working memory span capacity measured using the odor span task (OST). This study examined the effects of a sub-chronic ketamine treatment to model cognitive deficits associated with schizophrenia, and to evaluate the effectiveness of nicotine, antipsychotic clozapine, and the novel mGlu2/3 agonist, LY404039, in restoring OST performance. Male hooded Lister rats were trained in the OST, a working memory task involving detection of a novel odor from an increasing number of presented odors until they exhibited asymptotic levels of stable performance. Sub-chronic ketamine exposure (10 and 30 mg/kg i.p. for 5 consecutive days) produced a dose-dependent impairment that was stable beyond 14 days following exposure. In one cohort, administration of graded doses of nicotine (0.025-0.1 mg/kg) acutely restored the performance in ketamine-treated animals, while significant improvements in odor span were observed in control subjects. In a second cohort of rats, acute tests with clozapine (1-10 mg/kg) and LY404039 (0.3-10 mg/kg) failed to reverse ketamine-induced deficits in doses that were observed to impair performance in the control groups. These data suggest that sub-chronic ketamine exposure in the OST presents a valuable method to examine novel treatments to restore cognitive impairments associated with neuropsychiatric disorders such as schizophrenia. Moreover, it highlights a central role for neuronal nicotinic receptors as viable targets for intervention that may be useful adjuncts to the currently prescribed anti-psychotics.

Minimal impairment in a rat model of duration discrimination following excitotoxic lesions of primary auditory and prefrontal cortices

We present a behavioral paradigm for the study of duration perception in the rat, and report the result of neurotoxic lesions that have the goal of identifying sites that mediate duration perception. Using a two-alternative forced-choice paradigm, rats were either trained to discriminate durations of pure tones (range = [200,500] ms; boundary = 316 ms; Weber fraction after training = 0.24 ± 0.04), or were trained to discriminate frequencies of pure tones (range = [8,16] kHz; boundary = 11.3 kHz; Weber = 0.16 ± 0.11); the latter task is a control for non-timing-specific aspects of the former. Both groups discriminate the same class of sensory stimuli, use the same motions to indicate decisions, have identical trial structures, and are trained to psychophysical threshold; the tasks are thus matched in a number of sensorimotor and cognitive demands. We made neurotoxic lesions of candidate timing-perception areas in the cerebral cortex of both groups. Following extensive bilateral lesions of the auditory cortex, the performance of the frequency discrimination group was significantly more impaired than that of the duration discrimination group. We also found that extensive bilateral lesions of the medial prefrontal cortex resulted in little to no impairment of both groups. The behavioral framework presented here provides an audition-based approach to study the neural mechanisms of time estimation and memory for durations.

Functional correlation between GABAergic and dopaminergic systems of dorsal hippocampus and ventral tegmental area in passive avoidance learning in rats

The aim of the present study was to investigate the existence of possible functional correlation between GABA-A and dopamine (DA) receptors of the dorsal hippocampus and the ventral tegmental area (VTA) in passive avoidance learning. Two guide cannulas were stereotaxically implanted in the CA1 region of the dorsal hippocampus and the VTA of male Wistar rats. In order to measure memory retrieval, the animals were trained in a step-through type passive avoidance task and tested 24 h after training. Post-training intra-CA1 administration of a GABA-A receptor agonist, muscimol (0.01-0.02 μg/rat) dose-dependently impaired memory retrieval. Post-training intra-VTA administration of SCH23390 (a dopamine D1 receptor antagonist; 0.1-0.8 μg/rat) or sulpiride (a D2 receptor antagonist; 0.5-1.5 μg/rat) decreased the inhibitory effect of muscimol (0.02 μg/rat, intra-CA1) on memory retrieval. Intra-VTA administration of the same doses of SCH23390, but not sulpiride, decreased the step-through latencies. On the other hand, post-training administration of muscimol (0.02 μg/rat) into the VTA inhibited memory retrieval. The administration of SCH23390 (0.01-0.2 μg/rat) or sulpiride (0.1-1 μg/rat) into the CA1 region, immediately after training, had no effect on memory retrieval. Furthermore, the amnesic effect of intra-VTA administration of muscimol was significantly decreased by intra-CA1 administration of sulpiride (0.5 and 1 μg/rat, intra-CA1), but not SCH23390. The practical conclusion is that the relationship between the hippocampus and the VTA may regulate memory formation in passive avoidance learning. Also, the correlation between the hippocampus and VTA by a dopaminergic system may be involved in mediating muscimol-induced amnesia.

A reinforcement learning approach to instrumental contingency degradation in rats

Goal-directed action involves a representation of action consequences. Adapting to changes in action-outcome contingency requires the prefrontal region. Indeed, rats with lesions of the medial prefrontal cortex do not adapt their free operant response when food delivery becomes unrelated to lever-pressing. The present study explores the bases of this deficit through a combined behavioural and computational approach. We show that lesioned rats retain some behavioural flexibility and stop pressing if this action prevents food delivery. We attempt to model this phenomenon in a reinforcement learning framework. The model assumes that distinct action values are learned in an incremental manner in distinct states. The model represents states as n-uplets of events, emphasizing sequences rather than the continuous passage of time. Probabilities of lever-pressing and visits to the food magazine observed in the behavioural experiments are first analyzed as a function of these states, to identify sequences of events that influence action choice. Observed action probabilities appear to be essentially function of the last event that occurred, with reward delivery and waiting significantly facilitating magazine visits and lever-pressing respectively. Behavioural sequences of normal and lesioned rats are then fed into the model, action values are updated at each event transition according to the SARSA algorithm, and predicted action probabilities are derived through a softmax policy. The model captures the time course of learning, as well as the differential adaptation of normal and prefrontal lesioned rats to contingency degradation with the same parameters for both groups. The results suggest that simple temporal difference algorithms with low learning rates can largely account for instrumental learning and performance. Prefrontal lesioned rats appear to mainly differ from control rats in their low rates of visits to the magazine after a lever press, and their inability to initially detect weak contingency changes.

Rapid task acquisition of spatial-delayed alternation in an automated T-maze by mice

The spatial-delayed alternation task using a T-maze is the standard method for testing working memory in rodents and is widely used. Until now, however, there has been a gap in the understanding of the underlying brain mechanisms. The development of new manganese-enhanced brain imaging methods now permit a more specific examination of these mechanisms by allowing behavioural brain stimulation to take place outside the MRI scanner and the scan identifying the activation of specific brain regions to take place subsequently. The requirements for this method are a frequent repetition of the behaviour of interest, a control group that differs in only one task parameter and the minimization of unspecific environmental factors to avoid irrelevant stimulation. To meet these requirements, a fully automated spatial-delayed alternation task in a T-maze was developed that used identity detectors and automated gates to route mice individually from their social home cage to the T-maze. An experimental and a control group of mice were trained in procedures that differed only in the parameter "working-memory based alternation". Our data demonstrate that both groups can be trained concurrently with a rapid procedure using the automated T-maze. With its high level of stimulation, the minimization of unspecific stimulation through environmental factors and the simultaneous training of a control group that differs in only one task parameter our set-up and procedure met the requirements of new imaging techniques for the study of the influence of a specific cognitive component of spatial-delayed alternation on activity in specific brain regions.

S 18986 reverses spatial working memory impairments in aged mice: comparison with memantine

RATIONALE

Normal or pathological ageing is characterized by working-memory dysfunction paired with a marked reduction in several neurotransmitters activity. The development of therapeutic strategy centered on the glutamatergic system known to bear a critical role in cognitive functions, is therefore of major importance in the treatment of mild forms of AD or age-related memory dysfunctions.

OBJECTIVES

In Experiment 1, we investigated the effects of ageing on spatial working memory measured by sequential alternation (SA). Thus, the decay of alternation rates over a series of trials separated by varying intertrial temporal intervals (ITI, from 5 sec to 180 sec) was studied in mice of different age groups. In Experiment 2, we investigated the memory-enhancing potential of S 18986--a modulator of AMPA receptors--on age-related SA impairments, in comparison with memantine--an antagonist of NMDA receptors--.

RESULTS

In Experiment 1, aged mice responded at chance with shorter ITI's and exhibited greater levels of interference in the SA task as compared to young adult mice. In Experiment 2, (1) S 18986 at 0.03 and 0.1 mg/kg reversed the memory deficit in aged mice but did not modify performance in young adult mice; (2) memantine at 10 mg/kg also increased SA rates in aged mice but did not improve performance in young adult mice.

CONCLUSION

The SA task is a useful tool to reveal age-induced time-dependent working memory impairments. As compared to memantine, S 18986--a compound targeting AMPA receptors--contributes a valuable therapy in the treatment of age-related cognitive dysfunctions or mild forms of AD.

Forebrain NR2B overexpression facilitating the prefrontal cortex long-term potentiation and enhancing working memory function in mice

Prefrontal cortex plays an important role in working memory, attention regulation and behavioral inhibition. Its functions are associated with NMDA receptors. However, there is little information regarding the roles of NMDA receptor NR2B subunit in prefrontal cortical synaptic plasticity and prefrontal cortex-related working memory. Whether the up-regulation of NR2B subunit influences prefrontal cortical synaptic plasticity and working memory is not yet clear. In the present study, we measured prefrontal cortical synaptic plasticity and working memory function in NR2B overexpressing transgenic mice. In vitro electrophysiological data showed that overexpression of NR2B specifically in the forebrain region resulted in enhancement of prefrontal cortical long-term potentiation (LTP) but did not alter long-term depression (LTD). The enhanced LTP was completely abolished by a NR2B subunit selective antagonist, Ro25-6981, indicating that overexpression of NR2B subunit is responsible for enhanced LTP. In addition, NR2B transgenic mice exhibited better performance in a set of working memory paradigms including delay no-match-to-place T-maze, working memory version of water maze and odor span task. Our study provides evidence that NR2B subunit of NMDA receptor in prefrontal cortex is critical for prefrontal cortex LTP and prefrontal cortex-related working memory.

Diffusion tensor imaging differences relate to memory deficits in diffuse traumatic brain injury

Background

Memory is one of the most impaired functions after traumatic brain injury (TBI). We used diffusion tensor imaging (DTI) to determine the structural basis of memory deficit. We correlated fractional anisotropy (FA) of the fasciculi connecting the main cerebral regions that are involved in declarative and working memory functions.

Methods

Fifteen patients with severe and diffuse TBI and sixteen healthy controls matched by age and years of education were scanned. The neuropsychological assessment included: Letter-number sequencing test (LNS), 2-back task, digit span (forwards and backwards) and the Rivermead profilet. DTI was analyzed by a tract-based spatial statics (TBSS) approach.

Results

Whole brain DTI analysis showed a global decrease in FA values that correlated with the 2-back d-prime index, but not with the Rivermead profile. ROI analysis revealed positive correlations between working memory performance assessed by 2-back d-prime and superior longitudinal fasciculi, corpus callosum, arcuate fasciculi and fornix. Declarative memory assessed by the Rivermead profile scores correlated with the fornix and the corpus callosum.

Conclusions

Diffuse TBI is associated with a general decrease of white matter integrity. Nevertheless deficits in specific memory domains are related to different patterns of white matter damage.

The H3 antagonist, ciproxifan, alleviates the memory impairment but enhances the motor effects of MK-801 (dizocilpine) in rats

Antagonists of H(3)-type histamine receptors exhibit cognitive-enhancing properties in various memory paradigms as well as evidence of antipsychotic activity in normal animals. The present study determined if a prototypical H(3) antagonist, ciproxifan, could reverse the behavioral effects of MK-801, a drug used in animals to mimic the hypoglutamatergic state suspected to exist in schizophrenia. Four behaviors were chosen for study, locomotor activity, ataxia, prepulse inhibition (PPI), and delayed spatial alternation, since their modification by dizocilpine (MK-801) has been well characterized. Adult male Long-Evans rats were tested after receiving a subcutaneous injection of ciproxifan or vehicle followed 20 min later by a subcutaneous injection of MK-801 or vehicle. Three doses of MK-801 (0.05, 0.1, & 0.3 mg/kg) increased locomotor activity. Each dose of ciproxifan (1.0 & 3.0 mg/kg) enhanced the effect of the moderate dose of MK-801, but suppressed the effect of the high dose. Ciproxifan (3.0 mg/kg) enhanced the effects of MK-801 (0.1 & 0.3 mg/kg) on fine movements and ataxia. Deficits in PPI were observed after treatment with MK-801 (0.05 & 0.1 mg/kg), but ciproxifan did not alter these effects. Delayed spatial alternation was significantly impaired by MK-801 (0.1 mg/kg) at a longer delay, and ciproxifan (3.0 mg/kg) alleviated this impairment. These results indicate that some H(3) antagonists can alleviate the impact of NMDA receptor hypofunction on some forms of memory, but may exacerbate its effect on other behaviors.

Exploratory, anxiety and spatial memory impairments are dissociated in mice lacking the LPA1 receptor

Lysophosphatidic acid (LPA) is a new, intercellular signalling molecule in the brain that has an important role in adult hippocampal plasticity. Mice lacking the LPA(1) receptor exhibit motor, emotional and cognitive alterations. However, the potential relationship among these concomitant impairments was unclear. Wild-type and maLPA(1)-null mice were tested on the hole-board for habituation and spatial learning. MaLPA(1)-null mice exhibited reduced exploration in a novel context and a defective intersession habituation that also revealed increased anxiety-like behaviour throughout the hole-board testing. In regard to spatial memory, maLPA(1) nulls failed to reach the controls' performance at the end of the reference memory task. Moreover, their defective working memory on the first training day suggested a delayed acquisition of the task's working memory rule, which is also a long term memory component. The temporal interval between trials and the task's difficulty may explain some of the deficits found in these mice. Principal components analysis revealed that alterations found in each behavioural dimension were independent. Therefore, exploratory and emotional impairments did not account for the cognitive deficits that may be attributed to maLPA(1) nulls' hippocampal malfunction.

Stress impairs optimal behavior in a water foraging choice task in rats

Stress is a biologically significant social-environmental factor that plays a pervasive role in influencing human and animal behaviors. While stress effects on various types of memory are well characterized, its effects on other cognitive functions are relatively unknown. Here, we investigated the effects of acute, uncontrollable stress on subsequent decision-making performance in rats, using a computer vision-based water foraging choice task. Experiencing stress significantly impaired the animals' ability to progressively bias (but not maintain) their responses toward the larger reward when transitioning from equal to unequal reward quantities. Temporary inactivation of the amygdala during stress, however, blocked impairing effects on decision making.

Kalirin loss results in cortical morphological alterations

Morphogenesis of pyramidal neuronal dendrites and spines is crucial for the formation and refinement of forebrain neuronal circuits underlying cognition. Aberrant dendrite and spine morphology is associated with neuropathological disorders. However, the molecular mechanisms controlling pyramidal neuronal dendrite and spine morphogenesis in vivo remain largely unknown. Kalirin is a brain-specific guanine-nucleotide exchange factor for Rho-like small GTPases, and an important regulator of spine morphogenesis in cultured neurons. Here we show that RNAi-dependent knockdown of kalirin in cultured neurons affected dendrite morphology. Cortical pyramidal neurons from KALRN-null mice showed reduced spine density and impaired activity-dependent spine plasticity; and they exhibited reduced complexity of dendritic trees. KALRN-null mice also displayed smaller neuronal cell bodies and reductions in the size of the cortex and cortical layers. These data demonstrate important roles for kalirin in the regulation of cortical structure, ultrastructure, and spine structural plasticity.

Perseveration related to frontal lesion in mice using the olfactory H-maze

The delayed reaction paradigm, consisting to discover two different rules consecutively (delayed alternation and non-alternation task) followed by a delayed reversal task, is a specific marker for the functioning of primate prefrontal cortex. Although several works in rodents report the use of operant delayed alternation tasks, in none of the studies mice with lesion of the prefrontal cortex were used in this paradigm. In the current study, mouse experiments were conducted using a new, totally automated device, the olfactory H-maze. Here, we show that unilateral lesion of the dorsomedial prefrontal cortex in mice induced similar deficits to those observed after frontal lesions in monkeys and humans. These pronounced learning deficits seem to come from difficulty elaborating a new rule and the inability to inhibit the previous rule, characterized by perseveration after prefrontal cortex lesion. The present results demonstrate that this very simple experimental paradigm using the olfactory H-maze presents the advantage to be fast (one training session) and well suited to assess the frontal functions in mice. It should be useful for testing pharmacological or stem cell approaches in order to reduce organic damages or gain insight into the cognitive functions of the frontal cortex using transgenic or gene-targeting mice.

The role of rat dorsomedial prefrontal cortex in spatial working memory

We used an operant delayed spatial alternation task to examine the role of rat dorsomedial prefrontal cortex (dmPFC) in spatial working memory. The task was designed to restrict movements during the delay period to minimize use of motor-mediating strategies. Inactivation of dmPFC (muscimol) resulted in increased errors and increased the temporal variability of responding. Animals did not show perseveration after errors (i.e., responding again at the erroneous location). Under control conditions, the time between spatial responses was greater and more variable before errors as compared to correct responses. These effects were eliminated when muscimol was infused into dmPFC. Trial outcome also affected movement and delay times in the next trial. This effect was diminished with muscimol in dmPFC. By contrast, when muscimol was infused in dorsal agranular insular cortex (AId)-a region that is strongly interconnected with dorsomedial prefrontal regions-there was no effect on delayed spatial alternation performance. These experiments confirm that dmPFC is necessary for successful delayed spatial alternation and establish that there is a relationship between response time variability and trial outcome that depends on dorsomedial prefrontal function.

Kalirin regulates cortical spine morphogenesis and disease-related behavioral phenotypes

Dendritic spine morphogenesis contributes to brain function, cognition, and behavior, and is altered in psychiatric disorders. Kalirin is a brain-specific guanine-nucleotide exchange factor (GEF) for Rac-like GTPases and is a key regulator of spine morphogenesis. Here, we show that KALRN-knockout mice have specific reductions in cortical, but not hippocampal, Rac1 signaling and spine density, and exhibit reduced cortical glutamatergic transmission. These mice exhibit robust deficits in working memory, sociability, and prepulse inhibition, paralleled by locomotor hyperactivity reversible by clozapine in a kalirin-dependent manner. Several of these deficits are delayed and age-dependent. Our study thus links spine morphogenic signaling with age-dependent, delayed, disease-related phenotypes, including cognitive dysfunction.

Nicotine withdrawal-induced deficits in trace fear conditioning in C57BL/6 mice--a role for high-affinity beta2 subunit-containing nicotinic acetylcholine receptors

Nicotine alters cognitive processes that include working memory and long-term memory. Trace fear conditioning may involve working memory during acquisition while also allowing the assessment of long-term memory. The present study used trace fear conditioning in C57BL/6 mice to investigate the effects of acute nicotine, chronic nicotine and withdrawal of chronic nicotine on processes active during acquisition and recall 24 h later and to examine the nicotinic acetylcholine receptor subtypes (nAChRs) involved in withdrawal deficits in trace fear conditioning. During training, acute nicotine (0.09 mg/kg) enhanced, but chronic nicotine (6.3 mg/kg/day, 13 days) and withdrawal of chronic nicotine (6.3 mg/kg/day, 12 days) had no significant effect on, acquisition of trace conditioning. At recall, acute treatment enhanced conditioning while chronic nicotine had no effect and withdrawal of chronic nicotine resulted in deficits. Antagonist-precipitated withdrawal was used to characterize the nAChRs involved in the withdrawal deficits. The low-affinity nAChR antagonist MLA (1.5, 3 or 9 mg/kg) had no effect on trace fear conditioning, but the high-affinity nAChR antagonist DHbetaE (3 mg/kg) precipitated deficits in trace fear conditioning if administered at training or training and testing, but not if administered at testing alone. The beta2 nAChR subunit is involved in the withdrawal effects as withdrawal of chronic nicotine produced deficits in trace fear conditioning in wildtype but not in beta2-knockout mice. Thus, nicotine alters processes involved in both acquisition and long-term memory of trace fear conditioning, and high-affinity beta2 subunit-containing nAChRs are critically involved in the effects of nicotine withdrawal on trace fear conditioning.

Prefrontal cortex lesions and scopolamine impair attention performance of C57BL/6 mice in a novel 2-choice visual discrimination task

Sustained attention is defined as the ability or capacity to remain focused on the occurrence of rare events over long periods of time. We describe here the development of a novel, operant-based attention task that can be learned by mice in 8-10 days. Mice were trained on a 2-choice visual discrimination task in an operant chamber, wherein the correct response on any given trial was a lever-press cued by a stimulus light. Upon reaching a criterion of greater than 80% correct responses, all subjects were tested in a mixed-trial attention paradigm combining four different stimulus durations within a single session (0.5, 1, 2, or 10 s). During attention testing, the percentage of correct responses decreased as a function of stimulus duration, indicating a performance decrement which parallels increasing attentional demand within the task. Pretreatment with the muscarinic-receptor antagonist scopolamine yielded a reliable, dose-dependent performance deficit whereas nicotine treatment improved the percentage of correct responses during trials with the greatest attentional demand. Moreover, medial prefrontal cortex lesions impaired attention performance without affecting acquisition or retention of the discrimination rule. These results underscore the utility of this task as a novel means of assessing attentional processes in mice in a relatively high-throughput manner.