Integrative Transcriptome Profiling of Cognitive Aging and Its Preservation through Ser/Thr Protein Phosphatase Regulation

Environmental enrichment has been reported to delay or restore age-related cognitive deficits, however, a mechanism to account for the cause and progression of normal cognitive decline and its preservation by environmental enrichment is lacking. Using genome-wide SAGE-Seq, we provide a global assessment of differentially expressed genes altered with age and environmental enrichment in the hippocampus. Qualitative and quantitative proteomics in naïve young and aged mice was used to further identify phosphorylated proteins differentially expressed with age. We found that increased expression of endogenous protein phosphatase-1 inhibitors in aged mice may be characteristic of long-term environmental enrichment and improved cognitive status. As such, hippocampus-dependent performances in spatial, recognition, and associative memories, which are sensitive to aging, were preserved by environmental enrichment and accompanied by decreased protein phosphatase activity. Age-associated phosphorylated proteins were also found to correspond to the functional categories of age-associated genes identified through transcriptome analysis. Together, this study provides a comprehensive map of the transcriptome and proteome in the aging brain, and elucidates endogenous protein phosphatase-1 inhibition as a potential means through which environmental enrichment may ameliorate age-related cognitive deficits.

Lack of parvalbumin in mice leads to behavioral deficits relevant to all human autism core symptoms and related neural morphofunctional abnormalities

Gene mutations and gene copy number variants are associated with autism spectrum disorders (ASDs). Affected gene products are often part of signaling networks implicated in synapse formation and/or function leading to alterations in the excitation/inhibition (E/I) balance. Although the network of parvalbumin (PV)-expressing interneurons has gained particular attention in ASD, little is known on PV's putative role with respect to ASD. Genetic mouse models represent powerful translational tools for studying the role of genetic and neurobiological factors underlying ASD. Here, we report that PV knockout mice (PV(-/-)) display behavioral phenotypes with relevance to all three core symptoms present in human ASD patients: abnormal reciprocal social interactions, impairments in communication and repetitive and stereotyped patterns of behavior. PV-depleted mice also showed several signs of ASD-associated comorbidities, such as reduced pain sensitivity and startle responses yet increased seizure susceptibility, whereas no evidence for behavioral phenotypes with relevance to anxiety, depression and schizophrenia was obtained. Reduced social interactions and communication were also observed in heterozygous (PV(+/-)) mice characterized by lower PV expression levels, indicating that merely a decrease in PV levels might be sufficient to elicit core ASD-like deficits. Structural magnetic resonance imaging measurements in PV(-/-) and PV(+/-) mice further revealed ASD-associated developmental neuroanatomical changes, including transient cortical hypertrophy and cerebellar hypoplasia. Electrophysiological experiments finally demonstrated that the E/I balance in these mice is altered by modification of both inhibitory and excitatory synaptic transmission. On the basis of the reported changes in PV expression patterns in several, mostly genetic rodent models of ASD, we propose that in these models downregulation of PV might represent one of the points of convergence, thus providing a common link between apparently unrelated ASD-associated synapse structure/function phenotypes.

The nuclear receptor REV-ERBα regulates Fabp7 and modulates adult hippocampal neurogenesis

The function of the nuclear receptor Rev-erbα (Nr1d1) in the brain is, apart from its role in the circadian clock mechanism, unknown. Therefore, we compared gene expression profiles in the brain between wild-type and Rev-erbα knock-out (KO) animals. We identified fatty acid binding protein 7 (Fabp7, Blbp) as a direct target of repression by REV-ERBα. Loss of Rev-erbα manifested in memory and mood related behavioral phenotypes and led to overexpression of Fabp7 in various brain areas including the subgranular zone (SGZ) of the hippocampus, where neuronal progenitor cells (NPCs) can initiate adult neurogenesis. We found increased proliferation of hippocampal neurons and loss of its diurnal pattern in Rev-erbα KO mice. In vitro, proliferation and migration of glioblastoma cells were affected by manipulating either Fabp7 expression or REV-ERBα activity. These results suggest an important role of Rev-erbα and Fabp7 in adult neurogenesis, which may open new avenues for treatment of gliomas as well as neurological diseases such as depression and Alzheimer.

The Gdap1 knockout mouse mechanistically links redox control to Charcot-Marie-Tooth disease

Mutations in the mitochondrial fission factor GDAP1 are associated with severe peripheral neuropathies, but why the CNS remains unaffected is unclear. Using a Gdap1^−/− mouse, Niemann et al. demonstrate that a CNS-expressed Gdap1 paralogue changes its subcellular localisation under oxidative stress conditions to also act as a mitochondrial fission factor.

The ganglioside-induced differentiation-associated protein 1 (GDAP1) is a mitochondrial fission factor and mutations in GDAP1 cause Charcot–Marie–Tooth disease. We found that Gdap1 knockout mice (Gdap1^−/−), mimicking genetic alterations of patients suffering from severe forms of Charcot–Marie–Tooth disease, develop an age-related, hypomyelinating peripheral neuropathy. Ablation of Gdap1 expression in Schwann cells recapitulates this phenotype. Additionally, intra-axonal mitochondria of peripheral neurons are larger in Gdap1^−/− mice and mitochondrial transport is impaired in cultured sensory neurons of Gdap1^−/− mice compared with controls. These changes in mitochondrial morphology and dynamics also influence mitochondrial biogenesis. We demonstrate that mitochondrial DNA biogenesis and content is increased in the peripheral nervous system but not in the central nervous system of Gdap1^−/− mice compared with control littermates. In search for a molecular mechanism we turned to the paralogue of GDAP1, GDAP1L1, which is mainly expressed in the unaffected central nervous system. GDAP1L1 responds to elevated levels of oxidized glutathione by translocating from the cytosol to mitochondria, where it inserts into the mitochondrial outer membrane. This translocation is necessary to substitute for loss of GDAP1 expression. Accordingly, more GDAP1L1 was associated with mitochondria in the spinal cord of aged Gdap1^−/− mice compared with controls. Our findings demonstrate that Charcot–Marie–Tooth disease caused by mutations in GDAP1 leads to mild, persistent oxidative stress in the peripheral nervous system, which can be compensated by GDAP1L1 in the unaffected central nervous system. We conclude that members of the GDAP1 family are responsive and protective against stress associated with increased levels of oxidized glutathione.

Myelin is dependent on the Charcot-Marie-Tooth Type 4H disease culprit protein FRABIN/FGD4 in Schwann cells

Studying the function and malfunction of genes and proteins associated with inherited forms of peripheral neuropathies has provided multiple clues to our understanding of myelinated nerves in health and disease. Here, we have generated a mouse model for the peripheral neuropathy Charcot-Marie-Tooth disease type 4H by constitutively disrupting the mouse orthologue of the suspected culprit gene FGD4 that encodes the small RhoGTPase Cdc42-guanine nucleotide exchange factor Frabin. Lack of Frabin/Fgd4 causes dysmyelination in mice in early peripheral nerve development, followed by profound myelin abnormalities and demyelination at later stages. At the age of 60 weeks, this was accompanied by electrophysiological deficits. By crossing mice carrying alleles of Frabin/Fgd4 flanked by loxP sequences with animals expressing Cre recombinase in a cell type-specific manner, we show that Schwann cell-autonomous Frabin/Fgd4 function is essential for proper myelination without detectable primary contributions from neurons. Deletion of Frabin/Fgd4 in Schwann cells of fully myelinated nerve fibres revealed that this protein is not only required for correct nerve development but also for accurate myelin maintenance. Moreover, we established that correct activation of Cdc42 is dependent on Frabin/Fgd4 function in healthy peripheral nerves. Genetic disruption of Cdc42 in Schwann cells of adult myelinated nerves resulted in myelin alterations similar to those observed in Frabin/Fgd4-deficient mice, indicating that Cdc42 and the Frabin/Fgd4-Cdc42 axis are critical for myelin homeostasis. In line with known regulatory roles of Cdc42, we found that Frabin/Fgd4 regulates Schwann cell endocytosis, a process that is increasingly recognized as a relevant mechanism in peripheral nerve pathophysiology. Taken together, our results indicate that regulation of Cdc42 by Frabin/Fgd4 in Schwann cells is critical for the structure and function of the peripheral nervous system. In particular, this regulatory link is continuously required in adult fully myelinated nerve fibres. Thus, mechanisms regulated by Frabin/Fgd4-Cdc42 are promising targets that can help to identify additional regulators of myelin development and homeostasis, which may crucially contribute also to malfunctions in different types of peripheral neuropathies.

Mouse models of neurodegenerative diseases: criteria and general methodology

The major symptom of Alzheimer's disease is rapidly progressing dementia, coinciding with the formation of amyloid and tau deposits in the central nervous system, and neuronal death. At present familial cases of dementias provide the most promising foundation for modelling neurodegeneration. We describe the mnemonic and other major behavioral symptoms of tauopathies, briefly outline the genetics underlying familiar cases and discuss the arising implications for modelling the disease in mostly transgenic mouse lines. We then depict to what degree the most recent mouse models replicate pathological and cognitive characteristics observed in patients.There is no universally valid behavioral test battery to evaluate mouse models. The selection of individual tests depends on the behavioral and/or memory system in focus, the type of a model and how well it replicates the pathology of a disease and the amount of control over the genetic background of the mouse model. However it is possible to provide guidelines and criteria for modelling the neurodegeneration, setting up the experiments and choosing relevant tests. One should not adopt a "one (trans)gene, one disease" interpretation, but should try to understand how the mouse genome copes with the protein expression of the transgene in question. Further, it is not possible to recommend some mouse models over others since each model is valuable within its own constraints, and the way experiments are performed often reflects the idiosyncratic reality of specific laboratories. Our purpose is to improve bridging molecular and behavioural approaches in translational research.

Lack of neprilysin suffices to generate murine amyloid-like deposits in the brain and behavioral deficit in vivo

Accumulation of the beta-amyloid peptide (Abeta) in the brain is a major pathological hallmark of Alzheimer's disease (AD), leading to synaptic dysfunction, neuronal death, and memory impairment. The levels of neprilysin, a major Abeta-degrading enzyme, are decreased in AD brains and during aging. Because neprilysin cleaves Abeta in vivo, its down-regulation may contribute to the pathophysiology of AD. The aim of this study was to assess the consequences of neprilysin deficiency on accumulation of murine Abeta in brains and associated pathologies in vivo by investigating neprilysin-deficient mice on biochemical, morphological, and behavioral levels. Aged neprilysin-deficient mice expressed physiological amyloid precursor protein (APP) levels and exhibited elevated brain Abeta concentrations and amyloid-like deposits in addition to signs of neuronal degeneration in their brains. Behaviorally, neprilysin-deficient mice acquired a significantly weaker conditioned taste aversion that extinguished faster than the aversion of age-matched controls. Our data establish that, under physiological APP expression levels, neprilysin deficiency is associated with increased Abeta accumulation in the brain and leads to deposition of amyloid-like structures in vivo as well as with signs of AD-like pathology and with behavioral deficits.

Arc/Arg3.1 is essential for the consolidation of synaptic plasticity and memories

Arc/Arg3.1 is robustly induced by plasticity-producing stimulation and specifically targeted to stimulated synaptic areas. To investigate the role of Arc/Arg3.1 in synaptic plasticity and learning and memory, we generated Arc/Arg3.1 knockout mice. These animals fail to form long-lasting memories for implicit and explicit learning tasks, despite intact short-term memory. Moreover, they exhibit a biphasic alteration of hippocampal long-term potentiation in the dentate gyrus and area CA1 with an enhanced early and absent late phase. In addition, long-term depression is significantly impaired. Together, these results demonstrate a critical role for Arc/Arg3.1 in the consolidation of enduring synaptic plasticity and memory storage.

Neuronal neprilysin overexpression is associated with attenuation of Aβ-related spatial memory deficit

Converging evidence links abnormally high brain concentrations of amyloid-beta peptides (Abeta) to the pathology of Alzheimer's disease (AD). Lowering brain Abeta levels, therefore, is a therapeutic strategy for the treatment of AD. Neuronal neprilysin upregulation led to increased degradation of Abeta, reduced the formation of Abeta-plaques and the associated cytopathology, but whether overexpression of neprilysin can improve cognition is unknown. We show that neuronal overexpression of neprilysin improved the Morris water maze memory performance in mice with memory deficits resulting from overexpression of the AD-causing mutated human amyloid precursor protein (APP). This improvement was associated with decreased brain levels of Abeta and with unchanged endoproteolytic processing of APP. These results provide the evidence that lowering of brain Abeta levels by increasing its degradation can improve cognitive functions in vivo, and suggest that increasing the activity of neprilysin in brain may be effective in preventing cognitive decline in AD.

Deletion of the Coffin-Lowry syndrome gene Rsk2 in mice is associated with impaired spatial learning and reduced control of exploratory behavior

Coffin-Lowry Syndrome (CLS) is an X-linked syndromic form of mental retardation associated with skeletal abnormalities. It is caused by mutations of the Rsk2 gene, which encodes a growth factor regulated kinase. Gene deletion studies in mice have shown an essential role for the Rsk2 gene in osteoblast differentiation and function, establishing a causal link between Rsk2 deficiency and skeletal abnormalities of CLS. Although analyses in mice have revealed prominent expression of Rsk2 in brain structures that are essential for learning and memory, evidence at the behavioral level for an involvement of Rsk2 in cognitive function is still lacking. Here, we have examined Rsk2-deficient mice in two extensive batteries of behavioral tests, which were conducted independently in two laboratories in Zurich (Switzerland) and Orsay (France). Despite the known reduction of bone mass, all parameters of motor function were normal, confirming the suitability of Rsk2-deficient mice for behavioral testing. Rsk2-deficient mice showed a mild impairment of spatial working memory, delayed acquisition of a spatial reference memory task and long-term spatial memory deficits. In contrast, associative and recognition memory, as well as the habituation of exploratory activity were normal. Our studies also revealed mild signs of disinhibition in exploratory activity, as well as a difficulty to adapt to new test environments, which likely contributed to the learning impairments displayed by Rsk2-deficient mice. The observed behavioral changes are in line with observations made in other mouse models of human mental retardation and support a role of Rsk2 in cognitive functions.

Loss of the limbic mineralocorticoid receptor impairs behavioral plasticity

Corticosteroid action in the brain is mediated by the mineralocorticoid (MR) and the glucocorticoid (GR) receptor. Disturbances in MR- and GR-mediated effects are thought to impair cognition, behavior, and endocrine control. To assess the function of the limbic MR in these processes, we inactivated the MR gene in the forebrain of the mouse using the Cre/loxP-recombination system. We screened the mice with a limbic MR deficiency in various learning and exploration tests. The mutant mice show impaired learning of the water-maze task and deficits in measures of working memory on the radial maze due to behavioral perseverance and stereotypy. They exhibit a hyperreactivity toward a novel object but normal anxiety-like behavior. The behavioral changes are associated with abnormalities of the mossy fiber projection and an up-regulation of GR expression in the hippocampus. Adult mutant mice show normal corticosterone levels at circadian trough and peak. This genetic model provides important information about the consequences of a permanently altered balance between limbic MR and GR, with implications for stress-related neuroendocrine and neuropsychiatric diseases.

Enhanced episodic-like memory and kindling epilepsy in a rat model of tuberous sclerosis

Tuberous sclerosis complex (TSC) is a common neurological autosomal-dominant syndrome caused by mutations in the TSC1 or TSC2 genes. TSC starts in early childhood and is characterized by cerebral hamartomas (benign tumours), severe epilepsy and cognitive deficits such as mental retardation and autism. The hamartomas are characterized by loss of the remaining wild-type TSC allele, and clinical data implicate cerebral hamartomas in the generation of epileptic seizures, which may play a significant role in the development of mental retardation. The TSC2 mutation predicts alterations in mitogen-associated protein kinase (MAPK) and, together with the TSC1 mutation, in mammalian target of rapamycin (mTOR) signalling pathways. Both pathways are involved in neuronal plasticity. We therefore hypothesized that the heterozygous mutation itself, besides cerebral hamartomas, contributes to the pathogenesis of cognitive deficits and possibly also epilepsy. Here, we show that young adult TSC2+/- rats, which are virtually free of cerebral hamartomas, exhibit enhanced episodic-like memory and enhanced responses to chemically-induced kindling. The activation of cyclic adenosine monophosphate (cAMP) in the hippocampus results in stronger induction of phospho-p42-MAPK in TSC2+/- rats than in wild-type animals. Thus, the cognitive phenotype and, possibly, epilepsy in TSC patients may result not only from the focal hamartomatous lesions but also, from altered neuronal plasticity in the heterozygous tissue.

Impaired conditioned taste aversion learning in APP transgenic mice

Cognition in transgenic mouse models of Alzheimer's disease (AD) has been predominantly characterized in explicit spatial orientation tasks. However, dementia in AD encompasses also implicit memory systems. In the present study a line of transgenic mice (TgCRND8) encoding a double mutated allele of the human amyloid precursor protein (APP) genes was evaluated in an implicit associative learning task of conditioned taste aversion (CTA). CTA is a form of Pavlovian classical conditioning, in which a mouse learns to avoid a novel taste of saccharine (conditioned stimulus) paired with an experimentally induced (systemic injection of lithium chloride) nausea (unconditioned stimulus). In contrast to conditioned non-Tg mice, TgCRND8 APP mice developed weaker aversion against saccharine and quickly increased its consumption in repeated tests. These results indicate that TgCRND8 mice show a significant impairment not only in explicit spatial memory, as has been previously shown [Nature 408 (2000) 979], but also in implicit memory. Control experiments confirmed that TgCRND8 and non-Tg mice had comparable taste sensitivities in response to appetitive as well as aversive tastes. The study suggests that the CTA paradigm can be a sensitive tool to evaluate deficits in implicit associative learning in APP transgenic mouse models of AD.

Accelerated extinction of conditioned taste aversion in P301L tau transgenic mice

Neurofibrillary tangles, insoluble protein deposits composed of filamentous tau aggregates, are neuropathological hallmarks of Alzheimer's disease and familial frontotemporal dementia (FTDP-17). Transgenic mice expressing the FTDP-17 mutation P301L of tau recapitulate key features of the human pathology, that is, tau proteins aggregate and neurofibrillary tangles begin to appear in the amygdala at 6 months of age. To detect early signs of tau aggregate-associated changes, we investigated behavioral alterations and cognitive deficits in such mice using an amygdala-specific test battery for anxiety-related and cognitive behavior. P301L mice had anxiety levels not different from wild-types, but their exploratory behavior was significantly increased. Acquisition of a fear response to tone and context as well as taste aversion was comparable to wild-types. However, extinction of a conditioned taste aversion was significantly accelerated. We conclude that already aggregation of tau proteins not yet accompanied by massive formation of neurofibrillary tangles causes selective behavioral deficits.

GlyR 3: An Essential Target for Spinal PGE2-Mediated Inflammatory Pain Sensitization

Prostaglandin E2 (PGE2) is a crucial mediator of inflammatory pain sensitization. Here, we demonstrate that inhibition of a specific glycine receptor subtype (GlyR alpha3) by PGE2-induced receptor phosphorylation underlies central inflammatory pain sensitization. We show that GlyR alpha3 is distinctly expressed in superficial layers of the spinal cord dorsal horn. Mice deficient in GlyR alpha3 not only lack the inhibition of glycinergic neurotransmission by PGE2 seen in wild-type mice but also show a reduction in pain sensitization induced by spinal PGE2 injection or peripheral inflammation. Thus, GlyR alpha3 may provide a previously unrecognized molecular target in pain therapy.

Cell adhesion molecules: key players in memory consolidation?

Experimental evidence implies that L1 and neural cell adhesion molecule (NCAM) are involved in long-term memory formation. Changes in their expression and glycosylation appear to modify the synaptic strength underlying memory consolidation. Interference with L1 and NCAM function in a variety of learning tasks in different species severely attenuates memory consolidation, indicating their involvement in an evolutionary conserved mechanism of neural plasticity.

Does cAMP response element-binding protein have a pivotal role in hippocampal synaptic plasticity and hippocampus-dependent memory?

Previous studies addressing the role of the transcription factor cAMP response element-binding protein (CREB) in mammalian long-term synaptic plasticity and memory by gene targeting were compromised by incomplete deletion of the CREB isoforms. Therefore, we generated conditional knock-out strains with a marked reduction or complete deletion of all CREB isoforms in the hippocampus. In these strains, no deficits could be detected in lasting forms of hippocampal long-term potentiation (LTP) and long-term depression (LTD). When tested for hippocampus-dependent learning, mutants showed normal context-dependent fear conditioning. Water maze learning was impaired during the early stages, but many mutants showed satisfactory scores in probe trials thought to measure hippocampus-dependent spatial memory. However, conditioned taste aversion learning, a putatively hippocampus-independent memory test, was markedly impaired. Our data indicate that in the adult mouse brain, loss of CREB neither prevents learning nor substantially affects performance in some hippocampus-dependent tasks. Furthermore, it spares LTP and LTD in paradigms that are sensitive enough to detect deficits in other mutants. This implies either a species-specific or regionally restricted role of CREB in the brain and/or a compensatory upregulation of the cAMP response element modulator (CREM) and other as yet unidentified transcription factors.

Deletion of the mental retardation gene Gdi1 impairs associative memory and alters social behavior in mice

Non-specific mental retardation (NSMR) is a common human disorder characterized by mental handicap as the only clinical symptom. Among the recently identified MR genes is GDI1, which encodes alpha Gdi, one of the proteins controlling the activity of the small GTPases of the Rab family in vesicle fusion and intracellular trafficking. We report the cognitive and behavioral characterization of mice carrying a deletion of Gdi1. The Gdi1-deficient mice are fertile and anatomically normal. They appear normal also in many tasks to assess spatial and episodic memory and emotional behavior. Gdi1-deficient mice are impaired in tasks requiring formation of short-term temporal associations, suggesting a defect in short-term memory. In addition, they show lowered aggression and altered social behavior. In mice, as in humans, lack of Gdi1 spares most central nervous system functions and preferentially impairs only a few forebrain functions required to form temporal associations. The general similarity to human mental retardation is striking, and suggests that the Gdi1 mutants may provide insights into the human defect and into the molecular mechanisms important for development of cognitive functions.

The Naples High- and Low-Excitability rats: selective breeding, behavioral profile, morphometry, and molecular biology of the mesocortical dopamine system

The Naples High- (NHE) and Low-Excitability (NLE) rat lines have been selected since 1976 on the basis of behavioral arousal to novelty (Làt-maze). Selective breeding has been conducted under continuous genetic pressure, with no brother-sister mating. The behavioral analyses presented here deal with (1) activity in environments of different complexity, i.e., holeboard and Làt maze; (2) maze learning in hexagonal tunnel, Olton, and Morris water mazes and; (3) two-way active avoidance and conditioned taste aversion tests. Morphometric analyses deal with central dopaminergic systems at their origin and target sites, as well as the density of dopamine transporter immunoreactivity. Molecular biology analyses are also presented, dealing with recent experiments on the prefrontal cortex (PFc), cloning and identifying differentially expressed genes using subtractive libraries and RNAase protection. The divergence between NLE and NHE rats varies as a function of the complexity level of the environment, with an impaired working and reference memory in both lines compared to random bred (NRB) controls. Moreover, data from the PFc of NHE rats show a hyperdopaminergic innervation, with overexpression of mRNA species involved in basal metabolism, and down-regulation of dopamine D1 receptors. Altogether, the evidence gathered so far supports a hyperfunctioning mesocorticolimbic system that makes NHE rats a useful tool for the study of hyperactivity and attention deficit, learning and memory disabilities, and drug abuse.

Knockout of ERK1 MAP kinase enhances synaptic plasticity in the striatum and facilitates striatal-mediated learning and memory

Extracellular signal-regulated kinases (ERK1 and 2) are synaptic signaling components necessary for several forms of learning. In mice lacking ERK1, we observe a dramatic enhancement of striatum-dependent long-term memory, which correlates with a facilitation of long-term potentiation in the nucleus accumbens. At the cellular level, we find that ablation of ERK1 results in a stimulus-dependent increase of ERK2 signaling, likely due to its enhanced interaction with the upstream kinase MEK. Consistently, such activity change is responsible for the hypersensitivity of ERK1 mutant mice to the rewarding properties of morphine. Our results reveal an unexpected complexity of ERK-dependent signaling in the brain and a critical regulatory role for ERK1 in the long-term adaptive changes underlying striatum-dependent behavioral plasticity and drug addiction.

Attentional set-shifting in mice: modification of a rat paradigm, and evidence for strain-dependent variation

Increasingly precise molecular genetic tools are available to study in mice the cellular mechanisms underlying complex brain functions, but the behavioural paradigms to assess these functions often lack the required specificity. In this study, an attentional set-shifting paradigm to assess medial frontal cortex functions in rats was modified for use in mice and variation between two relevant mouse strains assessed. Male 129/SvEv and C57BL/6J mice and their F1 intercross (n=8 per genotype) were trained to dig in bowls for a food reward. On four consecutive days, mice performed a series of discriminations to criterion (six consecutive correct choices) between pairs of food bowls that differed along two dimensions (odour, digging medium), including a reversal, an intra-dimensional shift, and an extra-dimensional shift. Mice from the 129 strain performed significantly better than C57 mice in the initial acquisition of a simple discrimination and in the final extra-dimensional shift test, with no difference in the reversal and intra-dimensional shift. Performance of the F1 mice was intermediate or similar to that of the 129 mice. These results indicate a selective difference between these two strains in attentional selection processes that have been shown in humans, monkeys and rats to be mediated by prefrontal cortex.

Conditioned taste aversion as a learning and memory paradigm

Conditioned taste aversion (CTA) is a well established learning and memory paradigm in rats and mice that is considered to be a special form of classical conditioning. Rodents--as well as many other species including man--learn to associate a novel taste (CS) with nausea (US), and as a consequence avoid drinking fluid with this specific taste. In contrast to other types of classical conditioning, even CS-US intervals lasting several hours lead to an aversion to the gustatory CS. With increasing CS-US delay duration, however, the aversion against the CS gradually decreases. Mice differ from rats in their reaction to the CS as well as the US. They tolerate a much higher concentration of saccharin and they do not show any clear signs of nausea when injected with the US. Advantages of this task are its relative independence of motor behavior, well described pathways for the CS and partly the US, and the wealth of available anatomical and pharmacological data implying several brain structures (e.g. parabrachial nucleus, amygdala, insular cortex), neurotransmitters and their receptors (e.g. cholinergic system, NMDA-receptors), and cellular processes (e.g. expression of immediate early genes, Ras-MAP kinase signaling pathway, CREB phosphorilation, protein tyrosine phosphorilation, protein synthesis) in CTA. The CTA paradigm has also been successfully used to phenotype mouse mutants.

The effects of acute and chronic stress on motor and sensory performance in male Lewis rats

Any behavioral testing induces stress to some degree. A meaningful interpretation of behavioral results can be difficult if stress, caused by handling or the testing situation, modifies the experimental outcome. Especially for neurological animal models, it is important to know how stress affects motor and sensory performance. Therefore, we investigated the effects of varying degrees of stress on several motor and sensory tasks that are frequently used to assess functional recovery after lesion-induced impairments in adult rats. Acute, subchronic, and chronic stress impaired ladder walking and prolonged the duration of grasping a bar. Stress also altered walking patterns by increasing the base of support and foot rotation and reducing stride length. Furthermore, chronic stress induced hypersensitivity to painful stimuli, but did not significantly influence the latency to remove sticky papers from the hindpaws (sticky paper test). In the light--dark (L/D) test, stress reduced the latency to enter the dark compartment and enhanced the number of transitions supporting that cold swim stress modifies the animal's level of anxiety. These data point towards a critical influence of acute or chronic stress on motor control and sensory performance of rats, suggesting that stress might be a critical intervening variable of the outcome of behavioral tests.

Lewis/Fischer rat strain differences in endocrine and behavioural responses to environmental challenge

The Lewis (LEW) and Fischer (F344) rat strains provide a comparative model of hypothalamic-pituitary-adrenal (HPA) function in which LEW is relatively hypoactive at homeostasis and hyporeactive to environmental challenge. The present study describes a comparison of LEW and F344 rats, males and females, in terms of their corticosterone (CORT) or behavioural responses to a range of behavioural tasks, where each of the tasks used contains a stressor component and has been demonstrated to be sensitive to corticotropin releasing factor (CRF) and/or CORT manipulation: acoustic startle response (ASR), elevated plus maze, schedule-induced polydipsia, and fear-conditioned suppression of drinking. Our aim was to determine to what extent the LEW trait of HPA axis hyporesponsiveness is associated with strain differences in behavioural responsiveness to environmental challenge. As expected, young (2-3 months)-mature (5-10 months) LEW males and females exhibited a lesser CORT response to restraint and novel confinement than did F344 males and females, although in old adulthood (18 months) the CORT stress response was equable in LEW/F344 males and actually higher in LEW than in F344 females. In young-mature adults, the ASR was greater in LEW males than in the other groups; all groups spent a low proportion of time on the open arms of the elevated plus maze; polydipsia was greater in F344 females than in the other groups; and fear-conditioned suppression of drinking was greater in F344 males and females than in LEW males and females. Therefore, relative hyporeactivity of the HPA axis in LEW rats is clearly not associated with uniform behavioural hyporeactivity, including CRF-dependent behaviours. Rather, this study suggests further evidence that environmental reactivity reflects a number of distinct emotional states and underlying neural circuits.

Prenatal stress in rats: effects on plasma corticosterone, hippocampal glucocorticoid receptors, and maze performance

The present experiments were designed to investigate the effects of maternal stress on cognitive and endocrine parameters in the adult offspring. Pregnant rats were stressed daily during the last week of pregnancy (days 15-19) by restraint, and the performance of their offspring in the Morris water maze was recorded. Plasma corticosterone levels after swimming and the status of hippocampal glucocorticoid receptors (GRs) were determined. During acquisition of the task, prenatally stressed (PS) males - but not females - showed longer escape latencies than non-stressed controls when swimming in cold (10 degrees C) but not in warm (20 degrees C) water. This sex- and prenatal stress-specific difference was even more pronounced during reversal learning of the task. In contrast, PS females - but not males - had higher basal corticosterone levels and a lower density of hippocampal corticosteroid receptors than non-stressed controls. In all animals irrespective of treatment, swimming in the water maze causes an increase of corticosterone that was smaller on day 8 of swimming than on day 1. After swimming in cold water, the rise in corticosterone levels in females was steeper and returned faster to baseline values than after swimming in warm water. A similar pattern could be seen in PS females when compared to their non-stressed controls. The data suggest that prenatal stress impairs spatial learning in males but not in females. Basal and stress-induced increases in corticosterone levels, however, were altered in PS females and not in PS males; i.e., prenatal stress-induced changes in corticosterone secretion were not paralleled by prenatal stress-induced deficits in spatial learning.

Recovery of emotional behaviour in neural cell adhesion molecule (NCAM) null mutant mice through transgenic expression of NCAM180

In the present study we further investigate functions of the neural cell adhesion molecule (NCAM) in the mature central nervous system and its implications for animal behaviour. To this end we generated transgenic mice expressing the major NCAM isoform with the largest cytoplasmic domain, NCAM180, under control of a promoter for the small form neurofilament gene. Transgenic mice were also bred with mice deficient in endogenous NCAM (Ncam-/- mice) so that effects of NCAM180 could be analysed in the presence and absence of endogenous NCAM. While overexpression of transgenic NCAM180 was without apparent behavioural or morphological effect, its expression in Ncam-/- mice counteracted NCAM ablation-induced aggressive, anxiety-like and antidepressant-like behaviour. It furthermore prevented a hypersensitivity of Ncam-/- mice to the anxiolytic serotonin1A (5-HT1A) receptor agonist buspirone. Such recovery of emotional behaviour and behavioural 5-HT1A response occurred in spite of misdevelopment of the olfactory bulb and hippocampus that is characteristic of Ncam-/- mice, and without an apparent change in the expression of 5-HT1A binding sites in the brain. Hippocampus- and amygdala-dependent learning, though disturbed in Ncam-/- mice, remained unaffected by the transgenic NCAM180. We suggest an involvement of NCAM180-mediated cell recognition processes in the serotonergic modulation of emotional behaviour in adult mice.

Anxiety and increased 5-HT1A receptor response in NCAM null mutant mice

Mice deficient in the neural cell adhesion molecule (NCAM) show behavioral abnormalities as adults, including altered exploratory behavior, deficits in spatial learning, and increased intermale aggression. Here, we report increased anxiety-like behavior of homozygous (NCAM-/-) and heterozygous (NCAM/-) mutant mice in a light/dark avoidance test, independent of genetic background and gender. Anxiety-like behavior was reduced in both NCAM+/+ and NCAM-/- mice by systemic administration of the benzodiazepine agonist diazepam and the 5-HT1A receptor agonists buspirone and 8-OH-DPAT. However, NCAM-/- mice showed anxiolytic-like effects at lower doses of buspirone and 8-OH-DPAT than NCAM+/+ mice. Such increased response to 5-HT1A receptor stimulation suggests a functional change in the serotonergic system of NCAM-/- mice, likely involved in the control of anxiety and aggression. However, 5-HT1A receptor binding and tissue content of serotonin and its metabolite 5-hydroxyindolacetic acid were found unaltered in every brain area of NCAM-/- mice investigated, indicating that expression of 5-HT1A receptors as well as synthesis and release of serotonin are largely unchanged in NCAM-/- mice. We hypothesize a critical involvement of endogenous NCAM in serotonergic transmission via 5-HT1A receptors and inwardly rectifying K+ channels as the respective effector systems.

Memory formation and the regulation of gene expression

On a cellular level, formation of memory is based on a selective change in synaptic efficacy that is both fast and, in case of important information, long-lasting. Rapidity of cellular changes is achieved by modifying preexisting synaptic molecules (receptors, ion channels), which instantaneously alters the efficacy of synaptic transmission. Endurance, that is the formation of long-term memory (LTM), is based on transient and perhaps also long-lasting changes in protein synthesis. A number of different methods exist to interfere with the synthesis of specific proteins or proteins in general. Other methods, in turn, help to identify proteins whose synthesis is changed following learning. These mostly molecular methods are briefly described in the present review. Their successful application in a variety of memory paradigms in invertebrates and vertebrates is illustrated. The data support the importance of selective changes in gene expression for LTM. Proteins newly synthesized during memory consolidation are likely to contribute to restructuring processes at the synapse. altering the efficiency of transmission beyond the scope of STM. Increased or, less often, decreased synthesis of proteins appears during specific time windows following learning. Recent evidence supports older data suggesting that two or even more waves of protein synthesis exist during the consolidation period. It is expected that the new molecular methods will help to identify and characterize molecules whose expression changes during LTM formation even in complex vertebrate learning paradigms.

Novelty-induced increased expression of immediate-early genes c-fos and arg 3.1 in the mouse brain

The detection of novel stimuli is a memory-dependent process. The presented stimulus has to be compared with memory contents to judge its novelty. In addition, the novelty of stimuli activates attention-related processes that facilitate memory formation. To determine the involvement of limbic and neocortical brain structures in novelty detection, we exposed mice to a novel gustatory stimulus (0.5% saccharin) added to their drinking fluid. We then compared the novelty-induced expression of the two immediate-early genes (IEGs) c-fos and arg 3.1, with their expression in mice familiarized with the same stimulus or mice not exposed to that stimulus. Exposure to taste novelty increased expression of c-fos and arg 3.1 mRNA in the cingulate cortex and deep layers of the parietal cortex. In addition, c-fos mRNA expression was increased in the amygdala and arg 3.1 mRNA was increased in the dentate gyrus. Expression of c-fos and arg 3.1 was elevated 30 min after the exposure to novelty. For arg 3.1, a second peak of expression was found 4.5 h after presentation of the novel stimulus. Our results indicate that the amygdala, the dentate gyrus, and the cingulate and parietal cortices may be involved in novelty detection and associated cognitive events, and suggest that c-fos and arg 3.1 play distinct roles in these processes.

The effects of NMDA-induced retrohippocampal lesions on performance of four spatial memory tasks known to be sensitive to hippocampal damage in the rat

Four separate cohorts of rats were employed to examine the effects of cytotoxic retrohippocampal lesions in four spatial memory tasks which are known to be sensitive to direct hippocampal damage and/or fornix-fimbria lesions in the rat. Selective retrohippocampal lesions were made by means of multiple intracerebral infusions of NMDA centred on the entorhinal cortex bilaterally. Cell damage typically extended from the lateral entorhinal area to the distal ventral subiculum. Experiment 1 demonstrated that retrohippocampal lesions spared the acquisition of a reference memory task in the Morris water maze, in which the animals learned to escape from the water by swimming to a submerged platform in a fixed location. In the subsequent transfer test, when the escape platform was removed, rats with retrohippocampal lesions tended to spend less time searching in the appropriate quadrant compared to controls. Experiment 2 demonstrated that the lesions also spared the acquisition of a working memory version of the water maze task in which the location of the escape platform was varied between days. In experiment 3, both reference and working memory were assessed using an eight-arm radial maze in which the same four arms were constantly baited between trials. In the initial acquisition, reference memory but not working memory was affected by the lesions. During subsequent reversal learning in which previously baited arms were now no longer baited and vice versa, lesioned animals made significantly more reference memory errors as well as working memory errors. In experiment 4, spatial working memory was assessed in a delayed matching-to-position task conducted in a two-lever operant chamber. There was no evidence for any impairment in rats with retrohippocampal lesions in this task. The present study demonstrated that unlike direct hippocampal damage, retrohippocampal cell loss did not lead to a general impairment in spatial learning, implying that the integrity of the retrohippocampus and/or its interconnection with the hippocampal formation is not critical for normal hippocampal-dependent spatial learning and memory. This outcome is surprising for a number of current hippocampal theories, and suggests that other cortical as well as subcortical inputs to the hippocampus might be of more importance, and further raises the question regarding the functional significance of the retrohippocampal region.

Differential effects of prenatal stress in two inbred strains of rats

The long-term effects of prenatal stress (three times daily restraint stress during the last week of gestation) on the behavioral response to stress, as assessed by novelty-induced locomotion, performance in the forced swim test, and the acquisition of a two-way active avoidance, were investigated in two inbred strains of rats, Fischer 344 (F344/NHsd/Zur) and Lewis (LEW/SsNHsd/Zur). Additional measures included birth weights, pain threshold on the hot plate, and basal and stress-induced corticosterone secretion. In all of the behavioral paradigms strain differences were found: LEW rats showed poorer acquisition of avoidance conditioning, displayed higher levels of activity on the open plate, less immobility time in the forced swim test, and lower pain thresholds in the hot-plate test compared with F344 rats. LEW rats had higher birth weights after prenatal stress, whereas F344 rats were lighter. Following prenatal stress the pattern of behavioral effects obtained in LEW rats in stress-related tests could be interpreted as improved coping abilities with stress, i.e., improved acquisition of active avoidance, less immobility in the forced swim test, and reduced novelty-induced locomotion. Prenatal stress was much less effective in inducing long-term behavioral changes in F344 rats, yielding only one effect, namely, enhanced novelty-induced locomotion in female F344 rats. Pain thresholds were increased as a consequence of prenatal stress, irrespective of strain and gender. Basal and stress-induced corticosterone release differed in the two strains, with LEW rats showing less stress-induced corticosterone release. Prenatal stress did not, however, affect basal or stress-induced corticosterone release. The results suggest that prenatal stress exerts long-term effects on behavior, which depend on the genetic background.

Increased intermale aggression and neuroendocrine response in mice deficient for the neural cell adhesion molecule (NCAM)

Mice deficient for the neural cell adhesion molecule (NCAM) show morphological and behavioural abnormalities in the adult form, including a reduced size of the olfactory bulb, reduced exploratory behaviour, and deficits in spatial learning. Here we report increased aggressive behaviour of both homozygous (NCAM -/-) and heterozygous (NCAM +/-) male mutant mice towards an unfamiliar male intruding into their home cage. While plasma testosterone concentrations did not differ between genotypes before or after behavioural testing, corticosterone levels were higher in mutant residents than in wild-type (NCAM +/+) residents 30 min after encountering the intruder. Levels of c-fos mRNA, analysed to monitor neuronal activation, were similar in primary output structures of the olfactory bulb in NCAM-deficient and NCAM +/+ mice, but were increased in brain areas of the limbic system in both NCAM -/- and NCAM +/- mutant mice after the behavioural test. These results indicate that abnormalities in social behaviour correlate with enhanced neuronal activity in limbic brain areas and result in increased social stress in NCAM-deficient mice.

The effect of continuous intraventricular infusion of L1 and NCAM antibodies on spatial learning in rats

Recent studies suggest a role of the neural cell adhesion molecules L1 and NCAM in mechanisms of memory storage. In the present study we analyzed the effect of continuous intraventricular infusion of polyclonal antibodies directed against L1 (antiL1) or NCAM (antiNCAM) on the performance of male Wistar rats during the acquisition and retention of a spatial learning task (Morris water-maze). In this task animals have to learn the spatial position of a hidden escape platform in a water tank to escape onto it. During acquisition of the task animals with continuous infusion of antiNCAM - but not those infused with antiL1 - showed day-dependent attenuated learning in comparison to controls (P = 0.001). Control animals were either injected with vehicle (PBS) or with polyclonal antibodies raised against liver cell membrane. When the escape platform was removed during the retention test (transfer test), the performance of animals continuously infused with antiL1 as well as those continuously infused with antiNCAM showed an impaired search pattern when compared with the performance of control animals (P = 0.001 and 0.04, respectively). Whereas control animals spent up to 46% of their time searching for the platform in the correct quadrant, the time antiL1- and antiNCAM-infused animals spent in this quadrant was closer to chance level (30.5% and 36.5%), respectively). The present data provide additional support for an involvement of the two adhesion molecules L1 and NCAM in synaptic plasticity underlying memory storage.

The polysialic acid modification of the neural cell adhesion molecule is involved in spatial learning and hippocampal long-term potentiation

The alpha-2,8-linked polysialic acid (PSA) modification of the neural cell adhesion molecule (NCAM) modulates morphogenetic cell interactions. PSA is strongly expressed during neural development and generally down-regulated in the adult. However, it remains prominent in some areas of the brain, e.g., the hippocampus. We assayed the functional role(s) of PSA in synaptic plasticity in the hippocampus in two experimental paradigms by removing PSA with endo-neuraminidase NE (endo-N) an enzyme which specifically cleaves alpha-2,8-linked polysialic acid. (1) The acquisition and retention of spatial memory of rats in the Morris water maze, critically dependent on the hippocampus, was significantly impaired after a localized injection of endo-N into the hippocampus, whereas visual and motor capacities were unaffected. (2) Tetanic stimulation of the Schaffer collaterals in endo-N-treated hippocampal slices in vitro failed to elicit LTP and yielded only a short post-tetanic potentiation, but the response returned to control levels within 2 minutes, whereas basal synaptic activity and short-term potentiation were not affected. Our findings suggest that the carbohydrate epitope PSA plays an important role in synaptic plasticity.

Immediate early genes and brain DNA remodeling in the Naples high- and low-excitability rat lines following exposure to a spatial novelty

The aim of these studies was to map the neural consequences of exposure to a spatial novelty on the expression of immediate gene (IEG) and on unscheduled brain DNA synthesis (UBDS) in two genetic models of altered activity and hippocampal functions, i.e., the Naples High- (NHE) and Low-excitability (NLE) rats. Adult male rats of NLE and NHE lines, and of a random-bred stock (NRB) were tested in a Làt-maze, and corner crossings, rearings, and fecal boli were counted during two 10-min tests 24 h apart. For IEG expression, rats were exposed to a Làt-maze with nonexposed or repeatedly exposed rats used as controls, and were sacrificed at different time intervals thereafter. For UBDS, rats were sacrificed immediately after the first or the second exposure o a Làt-maze. IEG expression was measured by immunocytochemistry for the FOS and JUN proteins. NRB rats exposed for the first time to the maze showed extensive FOS and JUN positive cells in the reticular formation, the granular and pyramidal neurons of hippocampus, the amygdaloid nuclei, all layers of somatosensory cortex, and the granule cells of the cerebellar cortex. The positivity, stronger in rats exposed for the first time, was present between 2 and 6 h and was prevented by the NMDA receptor antagonist CPP (5 mg/kg). The positivity was very low in NHE rats, and it was stronger in NLE compared to NRB rats. UBDS was measured in ex vivo homogenates of brain areas by the incorporation into DNA of 3H-[methyl]-thymidine given intraventricularly 15 min before test trial 1 or 2 (pulse of 0.5 h).(ABSTRACT TRUNCATED AT 250 WORDS)

Water maze performance and hippocampal weight of prenatally stressed rats

Prenatally stressed rats were tested for water maze performance with the water temperature kept at 18 degrees C (low stress) or cooled down to 12 degrees C (high stress). When the platform had been removed from the pool and the water was kept at 12 degrees C, prenatally stressed males--but not females--spent more time searching for the platform in the correct quadrant of the pool than their controls. Prenatal stress reduced hippocampal weight in both sexes.

Circadian activity, nociceptive thresholds, nigrostriatal and mesolimbic dopaminergic activity in the Naples High- and Low-Excitability rat lines

These experiments were designed to further characterize the differential phenotypic constellation of the Naples High- (NHE) and Naples Low-Excitability (NLE) lines. In order to determine possible differences between NHE and NLE rats in activity and circadian rhythms, besides reactivity to novelty (selection trait), adult male rats of both strains were tested during two 10-min exposures to a Làt-maze. They were then kept in activity cages continuously for 3 days. Moreover, nociceptive thresholds were measured with the hot-plate and the tail-flick test, to probe the possibility that these rats could be differentially sensitive to nociceptive stimuli. Further, the integrity of the nigro-striatal and mesolimbic system was investigated by measuring tyrosine-hydroxylase activity in the striatum and 3,4-dihydroxyphenylacetic acid (DOPAC) levels in the striatum as well as in the nucleus accumbens. In addition, TH activity was measured in the adrenals to probe the sympathetic section of the neurovegetative system. The results indicate that NHE and NLE rats differ by a factor of two in their phasic activity in a Làt-maze. In contrast, no differences in 24-h activity during the dark or light phase could be observed in the activity cages. However, NHE rats anticipated the light-on stimulus in the morning by reducing their activity 1 h earlier than NLE rats. Further, no difference could be found with the hot-plate and the tail-flick test. Finally, biochemical analyses revealed no difference in the NHE and NLE rats in the main terminal zone of mesolimbic system (n. accumbens) nor of nigrostriatal system (striatum) nor in the adrenal glands. In conclusion, since the only consistent difference between NHE and NLE rats appears to be reactivity to spatial novelty, an hippocampus-dependent behavioral trait (selection trait), independent of altered activity in the sympathetic system or dopaminergic activity in the major dopaminergic brain systems, the usefulness of these strains as genetic model to test current hypotheses of spatial processor device(s) in the mammalian brain is supported.

Facilitation of tunnel maze performance by systemic injection of the neurokinin substance P

The effect of peripheral injections of substance P (SP) on performance in two different configurations of an automated tunnel maze was examined in three experiments. In two experiments, the effect of pretrial SP injections (10-1000 micrograms/kg) on performance in the hexagonal and radial maze configuration of an automated tunnel maze was investigated. In the hexagonal maze, which measures activity, exploratory efficiency, habituation, and perimeter walking, injection of SP affected perimeter walking only. In the radial maze, SP produced a facilitation of measures of efficiency and long-term and short-term memory without affecting activity. In the third experiment, the effect of pre- and posttrial injections of SP (50 or 500 micrograms/kg) on performance in the radial maze configuration was tested. Again, pretrial injections of 500 micrograms of SP facilitated performance with respect to measures of efficiency and short- and long-term memory; 50 micrograms produced a weaker effect. Virtually no effect was seen with posttrial injections.

Distributed changes in c-Fos and c-Jun immunoreactivity in the rat brain associated with arousal and habituation to novelty

The effects of exposure to spatial novelty on expression of the immediate early gene (IEG) products c-Fos and c-Jun were mapped in the rat brain by immunohistochemistry. Adult male Sprague-Dawley rats were tested for 10 min in a Làt-maze, and corner-crossings, rearings, and fecal boli were recorded. Rats were sacrificed at different time intervals after exposure to the maze (0.5, 2, 6, or 24 h). Unexposed rats or rats repeatedly exposed for 3 days at 24 h interval served as controls. Nonperfused brains were processed for immunocytochemistry for c-Fos and c-Jun on adjacent slices using the avidin-biotin method and diaminobenzidine as chromogen. In unexposed control rats the constitutive expression of the two IEGs products was low and scattered. In contrast, rats that had been exposed for the first time to the maze (spatial novelty) showed an extensive c-Fos- and c-Jun-like immunoreactivity in the reticular formation, the caudate-putamen complex, the hippocampus (granular and pyramidal neurons), the cerebellum (granular neurons), and all layers of somatosensory cortex. The positivity was stronger in rats exposed for the first time to the box than in repeatedly exposed or unexposed control rats. A maximal IEG expression was found in animals with postexposure survival times of 2 and 6 h. IEG expression in repeatedly exposed rats was still above baseline expression of unexposed rats but still lower than that of rats having been exposed only once to the maze.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of CGS 19755 and dizocilpine (MK 801) on delayed time discrimination performance

The effects of the competitive NMDA (N-methyl-D-aspartate) receptor antagonist CGS 19755 and the non-competitive NMDA receptor antagonist dizocilpine (MK 801) on time discrimination and short-term memory were investigated in rats trained on a delayed time discrimination task. In a two-lever operant chamber, pressing one lever was correct and reinforced with a food pellet after presentation of a stimulus light for 2 s (SD(short)); pressing the opposite lever was correct after presentation of a stimulus light for 8 s (SD(long)). CGS 19755 (3.0 mg/kg) attenuated performance, decreased nose-pokes (an activity necessary to trigger the presentation of the discriminative stimulus and the presentation of the response levers), and increased response latencies (time from 'opportunity to leverpress' to 'actual leverpress'). The highest dose of dizocilpine (0.2 mg/kg) tested also attenuated performance. Further, the number of nose-pokes and response latencies were not altered by any dose of dizocilpine. With increasing delays, saline-injected animals developed a bias towards reporting an occurrence of an SD(long), independent of the actual stimulus presented. This bias was attenuated or even reversed by CGS 19755 (3.0 mg/kg) and (0.2 mg/kg). Our results suggest that NMDA receptors are directly or indirectly involved in time discrimination performance.

The effects of anticholinergic drugs on delayed time discrimination performance in rats

To investigate the effects of the muscarinic antagonist scopolamine and the nicotinic antagonist mecamylamine on time discrimination and short-term memory, rats were trained on a delayed conditional time discrimination task until performance stabilized. In a two-lever operant chamber, pressing one lever was correct after the presentation of a stimulus light for 2 seconds (SD short); and pressing the other lever was correct after presentation of a stimulus light for 8 seconds (SD long). Scopolamine (0.06 mg, 0.25 mg, and 1.0 mg/kg) attenuated performance in a dose-dependent manner. Furthermore, the drug decreased nose-pokes (an activity necessary to trigger the presentation of the discriminative stimuli and the presentation of the response levers), and increased response delay (time from opportunity to lever press to actual lever press). Performance attenuating effects of mecamylamine in the time discrimination task did not appear unless high doses (8 mg/kg) of the drug were given. With increasing delays between 0 and 8 seconds, animals injected with saline developed a bias towards reporting the occurrence of the SD long, independent of the actual stimulus presented. A reversal of this bias was seen in animals injected with scopolamine; they more often reported the occurrence of the SD short. Our results support a role for muscarinic receptors in discrimination learning, attention, and time estimation.

Effects of nicotine, caffeine, and their combination on locomotor activity in rats

The interactive effect of caffeine and nicotine on spontaneous locomotor activity in a tunnel maze was determined in nicotine-naive and nicotine-tolerant rats. Rats were daily injected subcutaneously for 12 days with nicotine (0.4 mg/kg) to induce nicotine tolerance. Nicotine-naive rats were injected with saline. During the next two days, they were exposed to a tunnel maze for two 6-min trials. On the third day, locomotor activity was measured (30-min trial) in the tunnel maze 15 minutes after subcutaneous injection of saline, nicotine (0.2 mg/kg), caffeine (8 mg/kg), or nicotine (0.2 mg/kg) and caffeine (8 mg/kg) in combination. Acute exposure to nicotine decreased locomotor activity in nicotine-naive rats. This decrease was antagonized by simultaneous injection of caffeine. Chronic nicotine exposure induced the development of tolerance to the acute behavioral depressive effects of nicotine. In nicotine-tolerant rats, caffeine and nicotine in combination significantly increased locomotor activity above saline level, whereas given alone they had no significant stimulant effect. Neither chronic nicotine treatment nor acute drug treatments affected exploratory efficiency of rats.

Prenatal stress effects on exploratory activity and stress-induced analgesia in rats

Pregnant rats were exposed three times daily to immobilization stress during gestational Days 15-19. The behavior of their offspring was compared with the behavior of offspring from unstressed control mothers. Although the stress procedure decreased the weight gain of mothers during pregnancy, it slightly but significantly increased the weight of their offspring at birth and at weaning. On postnatal Day 10, prenatally stressed pups returned to their home cage more quickly than did prenatally unstressed control pups during a nest odor discrimination task, but no differences between groups in the number of correct responses were found. On postnatal Days 70-72, prenatally stressed offspring showed increased exploratory activity in a complex tunnel maze compared with control offspring. On postnatal Day 80, analgesia induced by stress (swimming for 3 min in cold water) was determined (tail flick latency). The degree of stress-induced analgesia was smaller in prenatally stressed rats than in control rats. These data suggest that the effects of prenatal stress on behavior are most clearly discernable when such animals are confronted with a novel or stressful situation.

Acute effects of nicotine injection into the nucleus accumbens on locomotor activity in nicotine-naive and nicotine-tolerant rats

To assess the role of nicotine receptors in the nucleus accumbens on locomotor activity we bilaterally implanted guide cannulae for later injection of (-)-nicotine into the nucleus accumbens of Wistar rats. Motor activity was tested in a complex tunnel maze equipped with photocells for automatic recording. This system of dark tunnels elicits spontaneous exploration even after repeated exposure. Half of the rats were made nicotine-tolerant by daily systemic injections of (-)-nicotine for 15 days (nicotine pretreatment); the other half remained nicotine-naive (saline pretreatment). Whereas (-)-nicotine (40 nmol/0.3 microliter) bilaterally injected into the nucleus accumbens of nicotine-naive animals suppressed locomotor activity, the same amount injected into the nucleus accumbens of nicotine-tolerant rats had no effect on locomotor activity. Systemic injections of nicotine (0.4 mg/kg) induced a depression and stimulation of locomotor activity in saline-pretreated and nicotine-pretreated rats, respectively. Our results support a dual role for nicotine in locomotor activity with the initial depressant effect in nicotine-naive animals due to stimulation of the nucleus accumbens and perhaps other structures.

Self-administration of small amounts of morphine through glass micropipettes into the ventral tegmental area of the rat

A micropipette-system was used to investigate intracranial self-injection of morphine in the rat. The system consisted of a glass micropipette (tip dia 5 microns) connected by flexible tubing and a swivel to a pressure source. Using nose-poking through a hole in one wall of the cage as the operant, rats self-injected quantities as small as 50 ng/5 nl of morphine into the ventral tegmental area. In contrast, rats in the saline-treated group and yoked-control group did not increase their rates of nose-poking behaviour above baseline levels. Intermittent reinforcement in the morphine-treated group resulted in a further increase in the rate of nose-poking. Histological inspection of the site of injection revealed minimal damage to brain tissue and likely confinement of the injected solution to the area around the tip of the pipette.

Effects of ketamine on tunnel maze and water maze performance in the rat

The NMDA receptor, which has been implicated in memory formation, is noncompetitively blocked by ketamine. The present study examines the effect of ketamine (0, 3, 6, 12, and 25 mg/kg body wt; ip) on tunnel maze and water maze performance in Wistar rats. In the hexagonal tunnel maze (HTM) high doses of ketamine (12 and 25 mg/kg) decreased locomotor activity. Moreover, ketamine induced perimeter walking (6, 12, and 25 mg/kg) and attenuated exploratory efficiency (25 mg/kg). When the HTM was converted into a modified six-arm radial maze, ketamine impaired short-term but not long-term memory. In the Morris water maze, rats injected with ketamine (12 and 25 mg/kg) acquired a spatial navigation task more slowly than controls. When the escape platform was removed, the drug-treated rats did not preferentially search for it in the area where the platform had been during the acquisition phase. However, when the escape platform was visible, no differences in the performance of ketamine-treated and control rats could be found. In summary, ketamine seems to attenuate some but not all forms of learning in the tunnel maze and it impairs the acquisition of a spatial navigation task.

The effects of long-term nicotine treatment on locomotion, exploration and memory in young and old rats

To assess the effects of long-term treatment with nicotine on several behavioral measures (locomotor activity, exploratory efficiency, habituation, short-term and long-term memory) of young (5 months) and old (22 months) rats in a hexagonal tunnel maze, nicotine was added to the drinking water (0, 20 or 50 mg/l) for up to 131 experimental days. With the exception of effects on exploratory efficiency, young and old rats did not differ in their response to the drug. Nicotine decreased body weight throughout the experiment. Nicotine treatment reduced water intake during the first 30 min of the daily 4.5 h access to drinking water. Nicotine increased locomotor activity throughout the experiment. When nicotine treatment was discontinued during a 7-day withdrawal period, locomotor activity immediately dropped to control values. Intertrial habituation was not affected by nicotine. Long-term nicotine treatment had an attenuating effect on exploratory efficiency in young rats; however, the drug did not influence performance in tasks measuring spatial memory. Finally, age increased weight, decreased locomotor activity and impaired exploratory efficiency and short-term memory. Age, however, did not affect the performance of the long-term memory task.