Disrupted allocentric but preserved egocentric spatial learning in transgenic mice with impaired glucocorticoid receptor function

Navigation task and action space drive the emergence of egocentric and allocentric spatial representations

In general, strategies for spatial navigation could employ one of two spatial reference frames: egocentric or allocentric. Notwithstanding intuitive explanations, it remains unclear however under what circumstances one strategy is chosen over another, and how neural representations should be related to the chosen strategy. Here, we first use a deep reinforcement learning model to investigate whether a particular type of navigation strategy arises spontaneously during spatial learning without imposing a bias onto the model. We then examine the spatial representations that emerge in the network to support navigation. To this end, we study two tasks that are ethologically valid for mammals—guidance, where the agent has to navigate to a goal location fixed in allocentric space, and aiming, where the agent navigates to a visible cue. We find that when both navigation strategies are available to the agent, the solutions it develops for guidance and aiming are heavily biased towards the allocentric or the egocentric strategy, respectively, as one might expect. Nevertheless, the agent can learn both tasks using either type of strategy. Furthermore, we find that place-cell-like allocentric representations emerge preferentially in guidance when using an allocentric strategy, whereas egocentric vector representations emerge when using an egocentric strategy in aiming. We thus find that alongside the type of navigational strategy, the nature of the task plays a pivotal role in the type of spatial representations that emerge.

Most species rely on navigation in space to find water, food, and mates, as well as to return home. When navigating, humans and animals can use one of two reference frames: one based on stable landmarks in the external environment, such as moving due north and then east, or one centered on oneself, such as moving forward and turning left. However, it remains unclear how these reference frames are chosen and interact in navigation tasks, as well as how they are supported by representations in the brain. We therefore modeled two navigation tasks that would each benefit from using one of these reference frames, and trained an artificial agent to learn to solve them through trial and error. Our results show that when given the choice, the agent leveraged the appropriate reference frame to solve the task, but surprisingly could also use the other reference frame when constrained to do so. We also show that the representations that emerge to enable the agent to solve the tasks exist on a spectrum, and are more complex than commonly thought. These representations reflect both the task and reference frame being used, and provide useful insights for the design of experimental tasks to study the use of navigational strategies.

Influence of Testosterone depletion on Neurotrophin-4 in Hippocampal synaptic plasticity and its effects on learning and memory

Sex steroids are neuromodulators that play a crucial role in learning, memory, and synaptic plasticity, providing circuit flexibility and dynamic functional connectivity in mammals. Previous studies indicate that testosterone is crucial for neuronal functions and required further investigation on various frontiers. However, it is surprising to note that studies on testosterone-induced NT-4 expression and its influence on synaptic plasticity and learning and memory moderation are scanty. The present study is focused on analyzing the localized influence of neurotrophin-4 (NT4) on hippocampal synaptic plasticity and associated moderation in learning and memory under testosterone deprivation. Adult Wistar albino rats were randomly divided into various groups, control (Cont), orchidectomy (ORX), orchidectomy + testosterone supplementation (ORX+T) and control + testosterone (Cont+T). After two weeks, the serum testosterone level was undetectable in ORX rats. The behavioural assessment showed a decline in the learning ability of ORX rats with increased working and reference memory errors in the behavioural assessment in the 8-arm radial maze. The mRNA and protein expressions of NT-4 and androgen receptors were significantly reduced in the ORX group. In addition, there was a decrease in the number of neuronal dendrites in Golgi-Cox staining. These changes were not seen in ORX+T rats with improved learning behaviour. Indicating that testosterone exerts its protective effect on hippocampal synaptic plasticity through androgen receptor-dependent neurotrophin-4 regulation in learning and memory upgrade.

Testosterone Influence on Microtubule-Associated Proteins and Spine Density in Hippocampus: Implications on Learning and Memory

The thorny protrusions or spines increase the neuronal surface area, facilitate synaptic interconnections among neurons, and play an essential role in the hippocampus. Increasing evidence suggests that testosterone, the gonadal hormone, plays an important role in neurogenesis and synaptic plasticity. The role of testosterone on microtubule-associated proteins on dendritic neurite stability in the hippocampus and its impact on learning disability is not elucidated. Adult male Wistar albino rats were randomly selected for the control, castrated, castrated + testosterone, and control + testosterone groups. Bilateral orchidectomy was done, and the testosterone propionate was administered during the entire trial period, i.e., 14 days. The learning assessments were done using working/reference memory versions of the 8-arm radial maze and hippocampal tissues processed for histological and protein expressions. There were reduced expressions of microtubule-associated protein 2 (MAP2), postsynaptic density protein 95 (PSD95), and androgen receptor (AR) and increased expression of pTau in the castrated group. Conversely, the expression of MAP2, PSD95, and AR was increased, and the pTau expression was reduced in the hippocampus of the castrated rat administrated with testosterone. Androgen-depleted rats showed impaired synaptic plasticity in the hippocampus associated with contracted microtubule dynamics. Along with learning disability, there was an increased number of reference memory errors and working memory errors in castrated rats. Observations suggest that androgen regulates expression of neural tissue-specific MAPs and plays a vital role in hippocampus synaptic plasticity and that a similar mechanism may underlie neurological disorders in aging and hypogonadal men.

Exposure to Short Photoperiod Regime Restores Spatial Cognition in Ventral Subicular Lesioned Rats: Potential Role of Hippocampal Plasticity, Glucocorticoid Receptors, and Neurogenesis

Ambient light influences our mood, behavior, and cognition. Phototherapy has been considered as an effective non-pharmacological intervention strategy in the restoration of cognitive functions following central nervous system insults. However, the cellular and molecular underpinnings of phototherapy-mediated functional recovery are yet to be studied. The present study examines the effectiveness of short photoperiod regime (SPR; 6:18-h light:dark cycle) in restoring the cognitive functions in ventral subicular lesioned rats. Bilateral ventral subicular lesion (VSL) resulted in significant impairment of spatial navigational abilities when tested in the Morris water maze (MWM) task. Further, VSL resulted in reduced expression of glucocorticoid receptors (GRs) and activity-regulated cytoskeletal (Arc) protein and suppression of neurogenesis in the hippocampus. VSL also suppressed the magnitude of long-term potentiation (LTP) in the hippocampal Schaffer collateral-CA1 synapses. However, exposure to SPR for 21 days showed significant restoration of spatial performance in the MWM task as the ventral subicular lesioned rats could deploy higher cognitive allocentric navigational strategies to reach the hidden platform. Further, SPR resulted in enhanced expression of hippocampal GR and Arc protein and neurogenesis but not hippocampal LTP suggestive of appropriate need-based SPR intervention. In conclusion, the study demonstrates the effectiveness of SPR in establishing functional recovery as well as the possible molecular and cellular basis of cognitive recovery in a rat model of neurodegeneration. Such studies provide a framework in understanding the efficacy of non-pharmacological strategies in establishing functional recovery in neurodegenerative conditions.

Deletion of the Mouse Homolog of CACNA1C Disrupts Discrete Forms of Hippocampal-Dependent Memory and Neurogenesis within the Dentate Gyrus

L-type voltage-gated calcium channels (LVGCCs) have been implicated in various forms of learning, memory, and synaptic plasticity. Within the hippocampus, the LVGCC subtype, Ca_V1.2 is prominently expressed throughout the dentate gyrus. Despite the apparent high levels of Ca_V1.2 expression in the dentate gyrus, the role of Ca_V1.2 in hippocampal- and dentate gyrus-associated forms of learning remain unknown. To address this question, we examined alternate forms of hippocampal-dependent associative and spatial memory in mice lacking the mouse ortholog of CACNA1C (Cacna1c), which encodes Ca_V1.2, with dentate gyrus function implicated in difficult forms of each task. We found that while the deletion of Ca_V1.2 did not impair the acquisition of fear of a conditioned context, mice lacking Ca_V1.2 exhibited deficits in the ability to discriminate between two contexts, one in which the mice were conditioned and one in which they were not. Similarly, Ca_V1.2 knock-out mice exhibited normal acquisition and recall of the location of the hidden platform in a standard Morris water maze, but were unable to form a memory of the platform location when the task was made more difficult by restricting the number of available spatial cues. Within the dentate gyrus, pan-neuronal deletion of Ca_V1.2 resulted in decreased cell proliferation and the numbers of doublecortin-positive adult-born neurons, implicating Ca_V1.2 in adult neurogenesis. These results suggest that Ca_V1.2 is important for dentate gyrus-associated tasks and may mediate these forms of learning via a role in adult neurogenesis and cell proliferation within the dentate gyrus.

Progestogens' effects and mechanisms for object recognition memory across the lifespan

This review explores the effects of female reproductive hormones, estrogens and progestogens, with a focus on progesterone and allopregnanolone, on object memory. Progesterone and its metabolites, in particular allopregnanolone, exert various effects on both cognitive and non-mnemonic functions in females. The well-known object recognition task is a valuable experimental paradigm that can be used to determine the effects and mechanisms of progestogens for mnemonic effects across the lifespan, which will be discussed herein. In this task there is little test-decay when different objects are used as targets and baseline valance for objects is controlled. This allows repeated testing, within-subjects designs, and longitudinal assessments, which aid understanding of changes in hormonal milieu. Objects are not aversive or food-based, which are hormone-sensitive factors. This review focuses on published data from our laboratory, and others, using the object recognition task in rodents to assess the role and mechanisms of progestogens throughout the lifespan. Improvements in object recognition performance of rodents are often associated with higher hormone levels in the hippocampus and prefrontal cortex during natural cycles, with hormone replacement following ovariectomy in young animals, or with aging. The capacity for reversal of age- and reproductive senescence-related decline in cognitive performance, and changes in neural plasticity that may be dissociated from peripheral effects with such decline, are discussed. The focus here will be on the effects of brain-derived factors, such as the neurosteroid, allopregnanolone, and other hormones, for enhancing object recognition across the lifespan.

Regulation of object recognition and object placement by ovarian sex steroid hormones

The ovarian hormones 17β-estradiol (E2) and progesterone (P4) are potent modulators of hippocampal memory formation. Both hormones have been demonstrated to enhance hippocampal memory by regulating the cellular and molecular mechanisms thought to underlie memory formation. Behavioral neuroendocrinologists have increasingly used the object recognition and object placement (object location) tasks to investigate the role of E2 and P4 in regulating hippocampal memory formation in rodents. These one-trial learning tasks are ideal for studying acute effects of hormone treatments on different phases of memory because they can be administered during acquisition (pre-training), consolidation (post-training), or retrieval (pre-testing). This review synthesizes the rodent literature testing the effects of E2 and P4 on object recognition (OR) and object placement (OP), and the molecular mechanisms in the hippocampus supporting memory formation in these tasks. Some general trends emerge from the data. Among gonadally intact females, object memory tends to be best when E2 and P4 levels are elevated during the estrous cycle, pregnancy, and in middle age. In ovariectomized females, E2 given before or immediately after testing generally enhances OR and OP in young and middle-aged rats and mice, although effects are mixed in aged rodents. Effects of E2 treatment on OR and OP memory consolidation can be mediated by both classical estrogen receptors (ERα and ERβ), and depend on glutamate receptors (NMDA, mGluR1) and activation of numerous cell signaling cascades (e.g., ERK, PI3K/Akt, mTOR) and epigenetic processes (e.g., histone acetylation, DNA methylation). Acute P4 treatment given immediately after training also enhances OR and OP in young and middle-aged ovariectomized females by activating similar cell signaling pathways as E2 (e.g., ERK, mTOR). The few studies that have administered both hormones in combination suggest that treatment can enhance OR and OP, but that effects are highly dependent on factors such as dose and timing of administration. In addition to providing more detail on these general conclusions, this review will discuss directions for future avenues of research into the hormonal regulation of object memory.

Object recognition test in mice

The object recognition test is now among the most commonly used behavioral tests for mice. A mouse is presented with two similar objects during the first session, and then one of the two objects is replaced by a new object during a second session. The amount of time taken to explore the new object provides an index of recognition memory. As more groups have used the protocol, the variability of the procedures used in the object recognition test has increased steadily. This protocol provides a necessary standardization of the procedure. This protocol reduces inter-individual variability with the use of a selection criterion based on a minimal time of exploration for both objects during each session. In this protocol, we describe the three most commonly used variants, containing long (3 d), short (1 d) or no habituation phases. Thus, with a short intersession interval (e.g., 6 h), this procedure can be performed in 4, 2 or 1 d, respectively, according to the duration of the habituation phase. This protocol should allow for the comparison of results from different studies, while permitting adaption of the protocol to the constraints of the experimenter.

An enriched environment elevates corticosteroid receptor levels in the hippocampus and restores cognitive function in a rat model of chronic cerebral hypoperfusion

An enriched environment (EE) is beneficial for modifying certain behaviors, particularly those involving complex cognitive functions. In models of chronic cerebral hypoperfusion (CCH), the ability of an EE to restore cognition depends on hippocampal synaptic plasticity and brain-derived neurotrophic factor. The mechanisms for this effect have not, however, been adequately studied. Here we investigated the effects of CCH and an EE on serum corticosteroid concentrations and the levels of the mineralocorticoid receptor (MR) and glucocorticoid receptor (GR) in the hippocampus. Rats were randomly divided into four treatment groups that received either permanent bilateral ligation of the common carotid arteries or sham surgery. Following this procedure, rats were exposed to 4 weeks of either an EE or standard housing. After the environmental intervention, their spatial learning and memory abilities were examined using the Morris water maze. In addition, the levels of MR and GR proteins in the hippocampus were determined. CCH impaired spatial cognitive function in rats, and exposure to an EE diminished these spatial learning and memory deficits. CCH also reduced the levels of MR and GR proteins in the hippocampus, but an EE restored the levels. Our results demonstrate that EE exposure restores cognitive impairments induced by CCH and up-regulates MR and GR expression. As such, MR and GR may contribute to the diminished effects of an EE in rats with CCH.

Forebrain glucocorticoid receptor overexpression increases environmental reactivity and produces a stress-induced spatial discrimination deficit

Reactivity to environmental stressors influences vulnerability to neurological and psychiatric illnesses, but little is known about molecular mechanisms that control this reactivity. Since mice with forebrain-specific glucocorticoid receptor overexpression (GRov mice) display anxiety-like behaviors in novel environments and have difficulty adjusting to change in memory tasks, we hypothesized that these may be facets of a broader phenotype of altered reactivity to environmental demands. Male GRov and wild-type mice were tested in a multiple-trial object interaction test comprising environmental and object habituation and spatial and object novelty trials. Half the mice received restraint stress before testing. GRov mice exhibited more locomotor activity and, without stress, more object interaction than wild-type mice. Following acute stress, GRov mice no longer showed increased object exploration. While stress dampened responses to object novelty in both groups, GRov mice were particularly impaired in discrimination of spatial novelty post-stress. These data demonstrate that GRov leads to increased environmental reactivity, responsiveness to salience, and vulnerability to stress-induced cognitive deficits. They implicate forebrain glucocorticoid receptor (GR) in fine-tuning interactions with the environment and the interplay of emotional salience, coping abilities, and cognitive function.

Learning deficits in C57BL/6J mice following perinatal arsenic exposure: consequence of lower corticosterone receptor levels?

Most studies on arsenic as a drinking water contaminant have focused on its carcinogenic potential but a few suggest that arsenic can adversely affect cognitive development. One parameter of the hypothalamic-pituitary-adrenal axis, the corticosterone receptor (CR) has been shown to be altered by arsenic. These receptors are found throughout the central nervous system, particularly in the hippocampus, an area of the brain of central importance for learning and memory. We examined the impact of perinatal exposure to 50 parts per billion (ppb) sodium arsenate on CRs and learning and memory behavior in the C57BL/6J mouse. Measurements of CRs revealed that arsenic-exposed offspring have significantly lower levels of these receptors in the nucleus than controls. Exposed offspring showed longer latency to approach a novel object than controls in an object recognition task. In the 8-way radial arm maze, arsenic offspring had a significant increase in the number of entry errors compared to controls. Results suggest that moderate exposures to perinatal arsenic can significantly reduce CR levels in the hippocampus and can have adverse effects on learning and memory behavior.

The pharmacology, neuroanatomy and neurogenetics of one-trial object recognition in rodents

Rats and mice are attracted by novel objects. They readily approach novel objects and explore them with their vibrissae, nose and forepaws. It is assumed that such a single explorative episode leaves a lasting and complex memory trace, which includes information about the features of the object explored, as well as where and even when the object was encountered. Indeed, it has been shown that rodents are able to discriminate a novel from a familiar object (one-trial object recognition), can detect a mismatch between the past and present location of a familiar object (one-trial object-place recognition), and can discriminate different objects in terms of their relative recency (temporal order memory), i.e., which one of two objects has been encountered earlier. Since the novelty-preference paradigm is very versatile and has some advantages compared to several other memory tasks, such as the water maze, it has become a powerful tool in current neurophamacological, neuroanatomical and neurogenetical memory research using both rats and mice. This review is intended to provide a comprehensive summary on key findings delineating the brain structures, neurotransmitters, molecular mechanisms and genes involved in encoding, consolidation, storage and retrieval of different forms of one-trial object memory in rats and mice.

Role of hippocampal Cav1.2 Ca2+ channels in NMDA receptor-independent synaptic plasticity and spatial memory

Current knowledge about the molecular mechanisms of NMDA receptor (NMDAR)-independent long-term potentiation (LTP) in the hippocampus and its function for memory formation in the behaving animal is limited. NMDAR-independent LTP in the CA1 region is thought to require activity of postsynaptic L-type voltage-dependent Ca2+ channels (Cav1.x), but the underlying channel isoform remains unknown. We evaluated the function of the Cav1.2 L-type Ca2+ channel for spatial learning, synaptic plasticity, and triggering of learning-associated biochemical processes using a mouse line with an inactivation of the CACNA1C (Cav1.2) gene in the hippocampus and neocortex (Cav1.2(HCKO)). This model shows (1) a selective loss of protein synthesis-dependent NMDAR-independent Schaffer collateral/CA1 late-phase LTP (L-LTP), (2) a severe impairment of hippocampus-dependent spatial memory, and (3) decreased activation of the mitogen-activated protein kinase (MAPK) pathway and reduced cAMP response element (CRE)-dependent transcription in CA1 pyramidal neurons. Our results provide strong evidence for a role of L-type Ca2+ channel-dependent, NMDAR-independent hippocampal L-LTP in the formation of spatial memory in the behaving animal and for a function of the MAPK/CREB (CRE-binding protein) signaling cascade in linking Cav1.2 channel-mediated Ca2+ influx to either process.

THE RELATIONSHIP BETWEEN ACUTE GLUCOCORTICOID LEVELS AND HIPPOCAMPAL FUNCTION DEPENDS UPON TASK AVERSIVENESS AND MEMORY PROCESSING STAGE

This review evaluates the effects of glucocorticoids (GCs), the adrenal steroids released in response to stress, on memory functions requiring the hippocampus in animals and humans. The data support the hypothesis that the learning function between GCs and hippocampal-dependent memory is modulated by 1) the aversive nature of the learning paradigm and 2) stage of memory processing (acquisition, consolidation, retrieval). When tasks are minimally aversive, the glucocorticoid receptor (GR) mediates an inverted U-shaped relationship between GC levels and hippocampal function, while the mineralocorticoid receptor (MR) mediates attentional processes and/or reaction to novelty. This inverted U-shaped relationship during minimally aversive training paradigms describes GC-mediated memory processing at both acquisition and consolidation. In contrast, highly aversive paradigms activate the amygdala and elevate GCs as part of the training procedure, revealing a nonlinear inverted U-shaped relationship during acquisition and a positive linear function during consolidation. Thus, highly aversive tasks that activate the amygdala shift the memory function from an inverted U-shaped curve to a linear representation between GC levels and memory consolidation.

Differential learning abilities of 129T2/Sv and C57BL/6J mice as assessed in three water maze protocols

Knockout mice are generated by using ES cells from 129 mouse strains and are frequently backcrossed with other strains, like C57BL/6. It is important to characterise the physiological and, in particular, the behavioural profile of each strain in order to correctly analyse the functional contribution of a single gene mutation on the 'cognitive' phenotype. The present study compared 129T2/Sv (129) and C57BL/6J (C57) mice in three different spatial learning protocols in the water maze, using a hidden platform. In the 'standard' reference memory protocol, 129 and C57 attained an equivalent level of performance as assessed by accuracy in reaching the platform (path length), despite a faster swim speed exhibited by C57 mice. In a stepwise learning task, C57 mice showed poorer performances over all stages of learning. However they performed better than 129 in a massed learning protocol which taxes short-term memory, and in which they exhibited lower levels of perseveration. The results emphasize the importance of using various tasks differing in cognitive demand, but using the same experimental environment and motivation, in order to 1) evaluate strain- or mutation-dependent learning abilities, and 2) dissociate the roles played by cognitive and non-cognitive factors in the behavioural requirements of the tasks.

Spatial and nonspatial Morris maze learning: impaired behavioral flexibility in mice with ectopias located in the prefrontal cortex

About half of BXSB/MpJ-Yaa (BXSB) mice have neocortical ectopias (misplaced clusters of neurons located in layer I of cortex). Previous behavioral studies have suggested that ectopic mice have superior spatial, but equivalent nonspatial, reference memory learning. However, since spatial and nonspatial learning were not assessed in the same apparatus and with the same testing procedure, it is unclear if this conclusion is accurate. We have created a new nonspatial Morris maze for mice that differs from the spatial task only in the type of cues that must be utilized to efficiently locate the platform (intra-maze black/white patterns vs. extra-maze room cues) and does not differ in the level of task complexity or the presence of objects within the maze. Ectopic mice were very good in utilizing extra-maze cues when learning the spatial version and in utilizing intra-maze cues when learning the nonspatial version of the Morris maze, while non-ectopics were not, suggesting that ectopics have superior spatial and nonspatial reference memory. Ectopias in BXSB mice are usually located in prefrontal and/or motor cortex. The prefrontal cortex is involved in behavioral flexibility (e.g. being able to easily switch from using spatial to nonspatial cues). Only ectopic mice with ectopias specifically located in the prefrontal region of cortex demonstrated difficulty switching from using extra-maze to intra-maze cues and vice versa. Thus, the presence of one or more ectopias in the prefrontal region of cortex disrupted one of the normal functions of the prefrontal cortex.

Genetic modification of corticosteroid receptor signalling: novel insights into pathophysiology and treatment strategies of human affective disorders

Every disturbance of the body, either real or imagined, evokes a stress response. Essential to this stress response is the activation of the hypothalamic-pituitary-adrenocortical (HPA) system, finally resulting in the release of glucocorticoid hormones from the adrenal cortex. Glucocorticoid hormones, in turn, feed back to this system by central activation of two types of corticosteroid receptors: the glucocorticoid receptor (GR) and the mineralocorticoid receptor (MR) which markedly differ in their neuroanatomical distribution and ligand affinity. Whereas a brief period of controllable stress, experienced with general arousal and excitement, can be a challenge and might thus be beneficial, chronically elevated levels of circulating corticosteroids are believed to enhance vulnerability to a variety of diseases, including affective disorders. Corticosteroids are known to influence emotions and cognitive processes, such as learning and memory. In addition, corticosteroids play extremely important roles in modulating fear and anxiety-related behaviour. The mechanisms by which corticosteroids exert their effects on behaviour are often indirect, by modulating particular sets of neurons or neurotransmitter systems. In addition, the timing of corticosteroid increase (before, during or after exposure to a stressor) determines whether and how behaviour is affected. The cumulative evidence makes a strong case implicating corticosteroid receptor dysfunction in the pathogenesis of affective disorders. Although definitive controlled trials remain to be conducted, there is evidence indicating that cortisol-lowering or corticosteroid receptor antagonist treatments may be of clinical benefit in selected individuals with major depression. A more detailed knowledge of the GR signalling pathways therefore opens up the possibility to specifically target GR function. In recent years, refined molecular technologies and the generation of genetically engineered mice (e.g. "conventional" and "conditional" knock-outs) have allowed to specifically target individual genes involved in corticosteroid receptor signalling and stress hormone regulation. Given the fundamental role of corticosteroid receptors in hippocampal integrity and mental performance during aging and psychiatric disorders, the identification and detailed characterization of these molecular pathways will ultimately lead to the development of novel neuropharmacological intervention strategies.

Genetically engineered mice for studies of stress-related clinical conditions

Genetically engineered mice with a specific deletion of targeted genes provide a novel and useful tool to study the endogenous mechanisms underlying aberrant behaviour. In this review we take the stress hormone (hypothalamic-pituitary-adrenocortical) system as an example to demonstrate how refined molecular technologies have allowed to target individual genes involved in stress hormone regulation. We describe different gene targeting methods: the generation of "conventional" knock-out mice enables us to delete a gene of interest in every cell of the body. Equally important for the studies of gene function in the mouse is the use of tissue-specific regulatory systems that allow gene inactivation to be restricted to specific tissues and, in some cases, to specific time points during development, such as the "conditional" knock-out, or the application of antisense techniques. Importantly, deletion of individual genes is not providing animal models for certain psychiatric disorders as these are caused by a manifold of minor changes in a series of so-called susceptibility genes. However, these gene targeting methods have become valuable tools to dissect the functions of individual components of complex biological systems in behavioural neuroscience: genetically engineered animals help to unravel the complex interactions and correlations between individual genes, hormonal regulation and behaviour, the most complex form of biological organization.

Using signal detection methods for analysis of operant performance in mice

Several factors account for murine cognitive abilities, and manipulation of genes which would act at the effector molecules involved in stimulus processing, reward-related properties and/or motor output can easily confound behavioural data obtained from mouse mutants responding on cognitive tasks. Therefore, tests may be needed which allow a better dissociation between true cognitive processes (accuracy) and other factors that may alter performance (motor or motivational bias). Part of this can be achieved by using methods which enable parametric variation of task difficulty. Part of it can also be achieved by using data analysis that allows a distinction between accuracy and bias, such as the mathematical methods of signal detection theory (SDT). SDT formally addresses the possibility that a given gene product or lack thereof affects performance by affecting motivation rather than cognition. It proposes that performance in a task depends on two factors, that is the sensitivity (or accuracy) of the neural systems mediating a cognitive process and the subject's motivational state, the latter of which can be represented as bias. SDT analysis can be easily applied to murine data. This overview will discuss the advances and limitations of the various SDT measures and illustrate the value of this type of analysis for understanding cognitive performance of mice.

The molecular neurobiology of stress--evidence from genetic and epigenetic models

Knowledge of the genetic and molecular events underlying the neuroendocrine and behavioural sequelae of the response to stress has advanced rapidly over recent years. The response of an individual to a stressful experience is a polygenic trait, but also involves non-genetic sources of variance. Using a combination of top-down (quantitative trait locus [QTL] and microarray analysis) and bottom-up (gene targeting, transgenesis, antisense technology and random mutagenesis) strategies, we are beginning to dissect the molecular players in the mediation of the stress response. Given the wealth of the data obtained from mouse mutants, this review will primarily focus on the contributions made by transgenesis and knockout studies, but the relative contribution of QTL studies and microarray studies will also be briefly addressed. From these studies it is evident that several neuroendocrine and behavioural alterations induced by stress can be modelled in mouse mutants with alterations in hypothalamic-pituitary-adrenal axis activity or other, extrahypothalamic, neurotransmitter systems known to be involved in the stress response. The relative contribution of these models to understanding the stress response and their limitations will be discussed.

Point mutation in the mouse glucocorticoid receptor preventing DNA binding impairs spatial memory

Activation of central glucocorticoid receptors caused by the stress that is associated with a learning task facilitates storage of the acquired information. The molecular mechanism underlying this phenomenon is entirely unknown. Glucocorticoid receptors can influence transcription both through DNA binding-dependent and -independent mechanisms. To assess the importance of these two modes of action for spatial memory, we here used male mutant mice in which homodimerization and DNA binding of the glucocorticoid receptor is largely prevented (GR(dim/dim)) while protein-protein interactions still can take place. These mice showed a selective impairment of spatial memory in the water maze. Locomotion and anxiety-related parameters measured in an open field and a light/dark preference task were comparable for mutant and control mice. Mutant mice released more corticosterone than control mice under basal resting conditions and in response to swimming, which could have influenced memory processes of the mice. However, mimicking the task-related increase in corticosterone by supplementary injection of corticosterone (250 microg/kg, i.p.) in adrenalectomized mice, resulting in equal plasma corticosterone concentrations in both genotypes, improved spatial memory of control mice but had no effect on mutant mice. These findings suggest that task-related facilitating effects of corticosterone on spatial memory indeed depend on DNA binding of the glucocorticoid receptor rather than on protein-protein interactions of the receptor with other transcription factors. Although it cannot be excluded that both processes are involved in a coordinated way, interrupting the DNA-binding capacity of the receptor is sufficient to induce impairment.

Interaction between the cholinergic system and CRH in the modulation of spatial discrimination learning in mice

Both cholinergic and CRH systems have been linked to cognitive processes such as learning and memory, and neuroanatomical as well as neurochemical evidence suggests important interactions between these two systems. Moreover, recent reports of pro-mnestic effects of CRH open the possibility that CRH could have beneficial effects in animals with cholinergic dysfunction. In a first experiment, spatial discrimination of C57BL/6 mice treated with various doses of scopolamine (0.5--2.0 mg/kg IP) was tested in a two-choice water maze task. Scopolamine, but not methylscopolamine, impaired accuracy and decreased responsivity. In contrast, similar doses of the nicotinic antagonist mecamylamine had no effect on choice accuracy but altered responsivity, as indicated by increased errors of omission and a reduction in swim speed during early experimental stages. ICV CRH (0.5--1.0 microg) also failed to significantly affect accuracy, but a strong tendency was observed to impair percentage correct responses. Measures of responsivity, such as errors of omission, choice latency and distance traveled, and of thigmotaxis were not significantly affected by CRH. However, initial swim speed was reduced by the peptide. Combined treatment with scopolamine (0.5 mg/kg IP) and CRH (0.5 microg ICV) had only mild, and primarily independent, effects, but overall suggested that concomitant blockade of muscarinic receptors and activation of the CRH system would rather act synergistically to disrupt spatial discrimination learning. Synergistic effects were also observed when animals receiving a combination of mecamylamine (2.0 mg/kg IP) and CRH (0.5 microg ICV) were tested, both in terms of responsivity and thigmotaxis, and there was limited evidence that part of these effects were potentiating. Thus, the cholinergic and CRH systems interact in the modulation of learning, but CRH, contrary to prediction, worsens the impairment caused by cholinergic blockade.

Effects of cholinergic manipulation on operant delayed non-matching to position performance in two inbred strains of mice

With the increasing demand on phenotyping of mouse mutants there is a clear need to develop novel paradigms for testing mice. Mice are able to learn a non-matching to position rule to high accuracy in a variety of maze paradigms, but an operant version of this task is desirable. In the present study, mice of the C57BL/6 and DBA/2 strains were trained and tested on an operant delayed non-matching to position (DNMTP) paradigm. Data were analysed according to the methods of signal detection theory (SDT), which allows conclusions as to whether strain differences in DNMTP performance are more related to changes in accuracy or in motivational factors. Mice can learn to respond on an operant DNMTP paradigm with high accuracy, and accurate performance depends on the duration of the delay-period, i.e. forgetting curves can be generated. Comparison between the two strains of mice revealed that DBA/2 mice learned faster than C57BL/6 mice to associate the lever press with food during initial shaping, but no further strain differences were observed in accurate responding during later stages of the experiment. However, differences in biased responding and, in particular, responsivity were observed between the two strains. Muscarinic blockade with scopolamine (0.1--1.0 mg/kg) failed to affect accuracy in the two strains, but altered responsivity. This task should be of great value for a more in-depth analysis of cognitive function in mutant mice as it allows a better dissociation between mnemonic and non-mnemonic factors. In particular, such paradigm may be of interest for testing conditional mutants, which allow time-sensitive induction or inhibition of gene expression, i.e. where animals can be trained while non-impaired to stable baseline and then tested once the gene is activated or inhibited.

Conditioned activity to amphetamine in transgenic mice expressing an antisense RNA against the glucocorticoid receptor

Glucocorticoids enhance the locomotion-stimulating and the rewarding properties of stimulant drugs. Amphetamine-induced conditioned activity was investigated in B6C3F1 (controls) and antisense transgenic mice. The latter expresses a neurofilament-promotor-driven antisense RNA complementary to a fragment of cDNA that codes for the mouse glucocorticoid receptor. This gene expression leads to approximately a 50% reduction in glucocorticoid receptor mRNA in the brain. Transgenic mice showed an increased novelty response when tested in an open field, in terms of both distance traveled and number of rearings. Moreover, they displayed enhanced amphetamine-induced conditioned activity. Behavioral sensitization was observed in controls, whereas behavioral tolerance developed in transgenic mice. These data support the concept of an enhanced stress response in these transgenic mice, rather than a general downregulation of the stress response because of impaired glucocorticoid receptor function.

Effects of the monoamine oxidase A inhibitor moclobemide on hippocampal plasticity in GR-impaired transgenic mice

A reduction in glucocorticoid receptor (GR) function leads to hippocampus-dependent allocentric spatial learning deficits, altered novelty exploration and disrupted hippocampal long-term potentiation (LTP) in transgenic mice expressing a GR antisense construct. After continuous long-term treatment of these mice with moclobemide (a reversible inhibitor of monoamine oxidase A), spatial navigation performance but not accuracy improved during initial acquisition. These changes were associated with a shift of the threshold for the induction of hippocampal LTP at low stimulation frequencies. Moreover, novel object exploration increased in both control and transgenic animals following long-term treatment with moclobemide. These findings open the possibility that antidepressants might improve hippocampal function under conditions of impaired stress hormone regulation, and that these drugs might in part act through this mechanism to attenuate cognitive deficiency in disorders such as depression.

Dehydroepiandrosterone--a neurosteroid

Dehydroepiandrosteone (DHEA) and its sulfate ester (DHEAS) are the major secretory products of the human adrenal glands and serve as precursors for both androgenic and estrogenic steroids. DHEA/S concentrations are particularly high in the brain, and DHEA/S and related steroids can be synthesized de novo in brain glial cells. Therefore, the term 'neurosteroids' has been coined for these compounds. This review summarizes findings in neurosteroid physiology on a cellular and molecular level, and outlines current concepts of how these compounds modulate physiological functions of the brain. Today, despite promising preclinical and human data the present clinical studies provide only weak evidence, if any, in favour of a DHEA replacement therapy.

Detection of protein with anticonsolidation properties in the rat brain

Isolation and identification of rat brain protein regulated by serotonin-modulated protein SMP-69 is described. Intracerebral administration of anti-SMP-69 antibodies activates cerebral cortex cell genome and increases the content of electropheretic fraction 28. Intracerebral administration of isolated and purified protein to rats disturbed memory consolidation. It is concluded that nerve cells have a molecular switch regulating consolidation of memory traces.

Glucocorticoid receptor impairment enhances impulsive responding in transgenic mice performing on a simultaneous visual discrimination task

Transgenic mice with impaired glucocorticoid receptor (GR) function were tested for their ability to learn and perform a series of simultaneous visual discriminations which allowed a dissociation between accuracy of discrimination from those of motivation and behavioural disinhibition. Animals were first trained on an operant five-choice simultaneous discrimination autoshaping procedure, followed by a continuous reinforcement schedule on that task. Subsequently, the number of choices was limited to two and data were analysed according to the mathematical methods of signal detection theory (SDT). The effects of GR-antisense expression on accuracy when different rates of responding were required were studied under different fixed ratio response requirements (FR1-FR10). Autoshaping was retarded in transgenic animals and accuracy was impaired in both the five-choice and the two-choice discrimination tasks, although transgenic mice showed clear evidence for learning. Under conditions of low response requirements, transgenic mice showed increased response and cognitive biases, but reduced perceptual bias, and a behavioural disinhibition, characterized by a reduction in errors of omission, decreased response latencies and increased number of responses during the inter-trial interval. Increasing the response requirement improved performance in transgenic animals as reflected by enhanced accuracy. Moreover, transgenics were less susceptible to the deleterious effects of higher response requirements, as indicated by relatively unaffected bias measures in this group, while bias increased in controls. These results indicate that altered performance in GR-antisense transgenic animals cannot simply be interpreted as a mnemonic deficit, but that altered motivation and enhanced impulsive responding may account for some of these impairments.

Evolved mechanisms underlying wayfinding. further studies on the hunter-gatherer theory of spatial sex differences

Based on Silverman and Eals' hunter-gatherer theory of the origin of sex-specific spatial attributes, the present research sought to identify the evolved mechanisms involved in hunting that contribute to the dimorphism. The focus of these studies was the relationship between three-dimensional mental rotations, the spatial test showing the largest and most reliable sex difference favoring males, and wayfinding in the woods. Space constancy was presumed to be the evolved mechanism fundamental to both of these abilities. Measures of wayfinding were derived by leading subjects individually on a circuitous route through a wooded area, during which they were stopped at prescribed places and required to set an arrow pointing in the direction the walk began. As well, subjects were eventually required to lead the experimenters back to the starting point by the most direct route. In support of the hypotheses, males excelled on the various measures of wayfinding, and wayfinding was significantly related across sexes to mental rotations scores but not to nonrotational spatial abilities or general intelligence.

Glucocorticoid receptor impairment alters CNS responses to a psychological stressor: an in vivo microdialysis study in transgenic mice

To study the consequences of impaired functioning of the glucocorticoid receptor (GR) for behavioural, neuroendocrine and neurochemical responses to a psychological stressor, a transgenic mouse expressing antisense RNA against GR was used. Previous studies on these transgenic mice have shown that impairment of GR evolves in disturbed neuroendocrine regulation and certain behavioural responses to stress. Here we investigated putative disturbances on the level of brain neurotransmission in GR-impaired (GR-i) mice using an in vivo microdialysis method. Through a microdialysis probe in the hippocampus, serotonin (5-HT), 5-hydroxyindoleacetic acid (5-HIAA) and free corticosterone [as an index of hypothalamic-pituitary-adrenocortical (HPA) axis activity] were monitored. Moreover, specific behaviours (e.g. grooming, eating/drinking, sniffing, nest building and locomotion) displayed by the mice during collection of the dialysates were scored. Measurement of dialysate concentrations of corticosterone on days 1 and 3 after insertion of the microdialysis probe showed that the free levels of this glucocorticoid were significantly lower in GR-i mice toward the evening. On day 2 after insertion of the microdialysis probe, baseline values of dialysate corticosterone, 5-HT and 5-HIAA were assessed, after which mice were exposed to a rat placed into their home cage. The rat and mouse were separated by a Plexiglas wall. A positive correlation between baseline hippocampal extracellular levels of 5-HT and 5-HIAA and the time spent performing active behaviours was observed in both genotypes. The main active behaviour performed at the baseline was grooming behaviour. During the rat exposure period, control mice remained mostly sitting and/or lying with their eyes fixed on the rat. Moreover, they showed a profound rise in free corticosterone levels. In contrast, GR-i mice displayed significantly more activities along the separation wall and a trend toward more grooming behaviour, but no increase of free corticosterone. In both mouse lines, exposure to a rat increased hippocampal extracellular levels of 5-HT and 5-HIAA. The rise in 5-HT was, however, more pronounced in the GR-i mice. From these data it may be concluded that life-long GR impairment has profound consequences for behavioural and neuroendocrine responses to a psychological stressor. Moreover, long-term impaired functioning of GR evolves in hyper-responsiveness of the raphe-hippocampal serotonergic system.

Glucocorticoids and depression

Depression has been associated with impaired mineralocorticoid receptor function, restrained glucocorticoid receptor feedback at the level of the hypothalamic-pituitary-adrenal (HPA) axis, raised cortisol level and increased corticotropin-releasing factor activity, which may act in concert to induce the signs and symptoms of the disorder. Pre-clinical and clinical evidence suggests that both genetic and environmental factors contribute to the development of these HPA axis abnormalities in depressed patients. Support for this view derives from models using genetically modified animals and/or chronic stress exposure at different developmental stages, although all of the current approaches have to be viewed within their limitations to model the disease. However, both animal and human studies challenging the HPA system show at least some neuroendocrine and behavioural changes comparable to those seen in depression, suggesting that some of the depressive symptoms can be attributed to HPA axis hyperactivity. Moreover, normalization of the neuroendocrine function following chronic antidepressant drug treatment seems to be a prerequisite for stable remission of depressive psychopathology, i.e. that normalization of HPA function is critical for relief of the clinical symptomatology of this disorder.

Excitotoxic hippocampal lesions disrupt allocentric spatial learning in mice: effects of strain and task demands

Spatial discrimination of ibotenic acid-lesioned C57BL/6 (B6) and DBA/2 (D2) mice was tested in two-choice water maze and plus maze tasks. B6 but not D2 mice learned the spatial discrimination in the water maze, but strains did not differ in learning a spatial discrimination in the plus maze paradigm. Ibotenic acid lesions of the hippocampus impaired percentage correct choices in the water maze spatial discrimination task in B6 but not in D2 mice, the latter of which may have been due to a floor effect. Furthermore, lesioned mice were more thigmotaxic, the distance travelled until a choice was made was longer and animals made more errors of omission. Despite the poor performance during water maze acquisition, lesioned animals, as well as sham-lesioned D2 mice, eventually acquired some place response in the water maze, as was evident when the location of the platform was reversed. However, hippocampus-lesioned mice of both strains were impaired when tested in the plus maze spatial discrimination task. Thus, ibotenic acid-induced lesions of the hippocampus impair acquisition of spatial discrimination in mice. These deficits were strain-dependent and likely comprise impaired accuracy as well as changes in non-mnemonic types of behaviour. Importantly, lesions in both strains impaired spatial learning, and whether a deficit was seen in mice of the D2 strain seemed to depend on the demands of the task.

Enhanced conditioned approach responses in transgenic mice with impaired glucocorticoid receptor function

The long-term consequences of impaired glucocorticoid receptor (GR) function on reward-related learning were studied in transgenic mice with impaired GR function in a series of experiments taxing conditioned and unconditioned approach responses to stimuli predictive of food. There was a double-dissociation in that transgenic mice with impaired GR activity showed enhanced conditioned exploration in situations when stimuli predicted reward, while free-feeding food consumption over 24 h was reduced. Previous experiments have shown altered accumbens dopaminergic activity in these animals. In line with these findings, we observed an enhanced behavioural stimulation of transgenic mice following administration of d-amphetamine (2 mg/kg). This suggests that the increase in preparatory responses in transgenic mice may be mediated via an enhanced accumbens dopaminergic activity, possibly secondary to alterations in other brain systems.