The genetic architecture of behavioural responses to novelty in mice

Phasic Stimulation of Dopaminergic Neurons of the Lateral Substantia Nigra Increases Open Field Exploratory Behaviour and Reduces Habituation Over Time

Transient nigrostriatal dopaminergic signalling is well known for its role in reinforcement learning and increasingly so for its role in the initiation of voluntary movement. However, how transient bursts of dopamine modulate voluntary movement remains unclear, likely due to the heterogeneity of the nigrostriatal system, the focus of optogenetic studies on locomotion at sub-sec time intervals, and the overlapping roles of phasic dopamine in behaviour and novelty signalling. In this study we investigated how phasic activity in the lateral substantia nigra pars compacta (lateral SNc) over time affects voluntary behaviours during exploration. Using a transgenic mouse model of both sexes expressing channelrhodopsin (ChR2) in dopamine transporter-expressing cells, we stimulated the lateral SNc while mice explored an open field over two consecutive days. We found that phasic activation of the lateral SNc induced an increase in exploratory behaviours including horizontal movement activity, locomotion initiation, and rearing specifically on the first open field exposure, but not on the second day. In addition, stimulated animals did not habituate to the same extent as their ChR2-negative counterparts, as indicated by a lack of decrease in baseline activity. These findings suggest that rather than prompting voluntary movement in general, phasic nigrostriatal dopamine prompts context-appropriate behaviours. In addition, dopamine signalling that modulates movement acts over longer timescales than the transient signal, affecting behaviour even after the signal has ended.

Daily rhythms in the behavioural stress response of the zebrafish Danio rerio

In nature, animals are exposed to stressors that occur with different likelihood throughout the day, such as risk of predation and human disturbance. Hence, the stress response is expected to vary plastically to adaptively match these challenges. Several studies have supported this hypothesis in a wide range of vertebrate species, including some teleost fish, mostly through evidence of circadian variation in physiology. However, in teleost fish, circadian variation in behavioural stress responses is less understood. Here, we investigated the daily rhythm of stress response at the behavioural level in the zebrafish Danio rerio. We exposed individuals and shoals to an open field test every 4h over a 24h cycle, recording three behavioural indicators of stress and anxiety levels in novel environments (thigmotaxis, activity and freezing). Thigmotaxis and activity significantly varied throughout the day with a similar pattern, in line with a stronger stress response in the night phase. The same was suggested by analysis of freezing in shoals, but not in individual fish, in which variation appeared mostly driven by a single peak in the light phase. In a control experiment, we observed a set of subjects after familiarisation with the open-field apparatus. This experiment indicated that activity and freezing might present a daily rhythmicity that is unrelated to environmental novelty, and thus to stress responses. However, the thigmotaxis was constant through the day in the control condition, suggesting that the daily variation of this indicator is mostly attributable to the stress response. Overall, this research indicates that behavioural stress response of zebrafish does follow a daily rhythm, although this may be masked using behavioural indicators other than thigmotaxis. This rhythmicity can be relevant to improve welfare in aquaculture and reliability of behavioural research in fish models.

Genetic background determines behavioral responses during fear conditioning

Freezing behavior is used as a measure of a rodent's ability to learn during fear conditioning. However, it is possible that the expression of other behaviors may compete with freezing, particularly in rodent populations that have not been thoroughly studied in this context. Rearing and grooming are complex behaviors that are frequently exhibited by mice during fear conditioning. Both behaviors have been shown to be stress-sensitive, and the expression of these behaviors is dependent upon strain background. To better understand how genetic background impacts behavioral responses during fear conditioning, we examined freezing, rearing, and grooming frequencies prior to fear conditioning training and across different stages of fear conditioning testing in male mice from eight inbred mouse strains (C57BL/6J, DBA/2J, FVB/NJ, SWR/J, BTBR T + ltpr3Tf/J, SM/J, LP/J, 129S1/SvlmJ) that exhibited diverse freezing responses. We found that genetic background determined rearing and grooming expression throughout fear conditioning, and their patterns of expression across stages of fear conditioning were strain dependent. Using publicly available SNP data, we found that polymorphisms in Dab1, a gene that is implicated in both grooming and learning phenotypes, separated the strains with high contextual grooming from the others using a hierarchical clustering analysis. This suggested a potential genetic mechanism for the observed behavioral differences. These findings demonstrate that genetic background determines behavioral responses during fear conditioning and suggest that shared genetic substrates underlie fear conditioning behaviors.

Aberrant Early in Life Stimulation of the Stress-Response System Affects Emotional Contagion and Oxytocin Regulation in Adult Male Mice

Results over the last decades have provided evidence suggesting that HPA axis dysfunction is a major risk factor predisposing to the development of psychopathological behaviour. This susceptibility can be programmed during developmental windows of marked neuroplasticity, allowing early-life adversity to convey vulnerability to mental illness later in life. Besides genetic predisposition, also environmental factors play a pivotal role in this process, through embodiment of the mother's emotions, or via nutrients and hormones transferred through the placenta and the maternal milk. The aim of the current translational study was to mimic a severe stress condition by exposing female CD-1 mouse dams to abnormal levels of corticosterone (80 µg/mL) in the drinking water either during the last week of pregnancy (PreCORT) or the first one of lactation (PostCORT), compared to an Animal Facility Rearing (AFR) control group. When tested as adults, male mice from PostCORT offspring and somewhat less the PreCORT mice exhibited a markedly increased corticosterone response to acute restraint stress, compared to perinatal AFR controls. Aberrant persistence of adolescence-typical increased interest towards novel social stimuli and somewhat deficient emotional contagion also characterised profiles in both perinatal-CORT groups. Intranasal oxytocin (0 or 20.0 µg/kg) generally managed to reduce the stress response and restore a regular behavioural phenotype. Alterations in density of glucocorticoid and mineralocorticoid receptors, oxytocin and µ- and κ-opioid receptors were found. Changes differed as a function of brain areas and the specific age window of perinatal aberrant stimulation of the HPA axis. Present results provided experimental evidence in a translational mouse model that precocious adversity represents a risk factor predisposing to the development of psychopathological behaviour.

QTL and systems genetics analysis of mouse grooming and behavioral responses to novelty in an open field

The open field is a classic test used to assess exploratory behavior, anxiety and locomotor activity in rodents. Here, we mapped quantitative trait loci (QTLs) underlying behaviors displayed in an open field, using a panel of 53 BXD recombinant inbred mouse strains with deep replication (10 per strain and sex). The use of these strains permits the integration and comparison of data obtained in different laboratories, and also offers the possibility to study trait covariance by exploiting powerful bioinformatics tools and resources. We quantified behavioral traits during 20-min test sessions including (1) percent time spent and distance traveled near the wall (thigmotaxis), (2) leaning against the wall, (3) rearing, (4) jumping, (5) grooming duration, (6) grooming frequency, (7) locomotion and (8) defecation. All traits exhibit moderate heritability making them amenable to genetic analysis. We identified a significant QTL on chromosome M.m. 4 at approximately 104 Mb that modulates grooming duration in both males and females (likelihood ratio statistic values of approximately 18, explaining 25% and 14% of the variance, respectively) and a suggestive QTL modulating locomotion that maps to the same locus. Bioinformatic analysis indicates Disabled 1 (Dab1, a key protein in the reelin signaling pathway) as a particularly strong candidate gene modulating these behaviors. We also found 2 highly suggestive QTLs for a sex by strain interaction for grooming duration on chromosomes 13 and 17. In addition, we identified a pairwise epistatic interaction between loci on chromosomes 12 at 36-37 Mb and 14 at 34-36 Mb that influences rearing frequency in males.

Spatial cognition in mice and rats: similarities and differences in brain and behavior

The increasing use of mice models in cognitive tasks that were originally designed for rats raises crucial questions about cross-species comparison in the study of spatial cognition. The present review focuses on the major neuroethological differences existing between mice and rats, with particular attention given to the neurophysiological basis of space coding. While little difference is found in the basic properties of space representation in these two species, it appears that the stability of this representation changes more drastically over time in mice than in rats. We consider several hypotheses dealing with attentional, perceptual, and genetic aspects and offer some directions for future research that might help in deciphering hippocampal function in learning and memory processes. WIREs Cogn Sci 2016, 7:406-421. doi: 10.1002/wcs.1411 For further resources related to this article, please visit the WIREs website.

The translational study of apathy-an ecological approach

Apathy, a quantitative reduction in goal-directed behavior, is a prevalent symptom dimension with a negative impact on functional outcome in various neuropsychiatric disorders including schizophrenia and depression. The aim of this review is to show that interview-based assessment of apathy in humans and observation of spontaneous rodent behavior in an ecological setting can serve as an important complementary approach to already existing task-based assessment, to study and understand the neurobiological bases of apathy. We first discuss the paucity of current translational approaches regarding animal equivalents of psychopathological assessment of apathy. We then present the existing evaluation scales for the assessment of apathy in humans and propose five sub-domains of apathy, namely self-care, social interaction, exploration, work/education and recreation. Each of the items in apathy evaluation scales can be assigned to one of these sub-domains. We then show that corresponding, well-validated behavioral readouts exist for rodents and that, indeed, three of the five human apathy sub-domains have a rodent equivalent. In conclusion, the translational ecological study of apathy in humans and rodents is possible and will constitute an important approach to increase the understanding of the neurobiological bases of apathy and the development of novel treatments.

Neurogenetic basis of cognition: Facts and hypotheses

In the natural setting, cognitive processes direct behavioural adjustments and sometimes result in behavioural novelties which allow the organism to cope with environmental pressures. The resulting behavioural changes exhibit various forms which are dependent upon different causal factors and cognitive processes. Under long-lasting environmental changes, these behavioural adaptations can become hereditary either through the process of cultural transmission or through genetic mechanisms sensitive to selective forces acting on genotypes. In the last few years, neuroethology and behavioural neurosciences have produced an increasing amount of precise knowledge about brain-behaviour relationships, neurobiological bases of cognitive processes and their development. Unfortunately, the approach to these phenomena is basically normative and does not tell us much about non-pathological determinants of individual variation in cognitive and behavioural competences. In contrast, the differential approach has provided some cases of structural variations in the brain which are under genetic control and thus liable to evolve under selective pressures. Brain size, the ratio of various brain structures to the total brain, the number and density of neurons in various parts of the brain and the variations of neuronal circuitry are potential candidates. This paper reviews them and examines their possible behavioural and cognitive outcomes. The issue here is to examine if and where in the brain potential conditions occur that would allow the genetic evolution of cognitive processes.

Chronic haloperidol-induced spatial memory deficits accompany the upregulation of D(1) and D(2) receptors in the caudate putamen of C57BL/6 mouse

AIMS

Haloperidol (HAL) is an antipsychotic drug that has high affinities to the dopamine D(2), but low affinities to D(1) receptors in the brain. Of brain regions, caudate putamen (CP) has the highest levels of the D(1) and D(2) receptors. In this study we evaluated the spatial memory of C57BL/6 mice following chronic administration of HAL and measured levels of D(1) and D(2) receptors in specific brain regions, with the hypothesis that the D(1) and D(2) receptors in CP are important players in spatial memory function of the brain.

MAIN METHODS

C57BL/6 mice received daily intraperitoneal injections of saline or HAL at 1.0 or 2.0mg/kg/day for 3 or 6 weeks. Two days after the last injection, spontaneous alternation of mice in a Y-maze was evaluated to measure their exploratory behavior and spatial working memory. The Morris water maze test was performed to measure their spatial learning and memory. D(1) and D(2) receptors in specific brain regions were measured by Western-blot analysis.

KEY FINDINGS

HAL treatment for 6 weeks decreased the spontaneous alternation of mice in Y-maze, altered the acquisition process and impaired spatial memory in Morris water maze. The same treatment increased levels of D(1) and D(2) receptors in CP and up-regulated D(2) receptors in the hippocampus, but did not change the receptors in the prefrontal cortex.

SIGNIFICANCE

These results suggest that the D(1) and D(2) receptors in CP are among the main targets of HAL and the receptors in CP play an important role in spatial learning and memory.

Open-field arena boundary is a primary object of exploration for Drosophila

Drosophila adults, when placed into a novel open-field arena, initially exhibit an elevated level of activity followed by a reduced stable level of spontaneous activity and spend a majority of time near the arena edge, executing motions along the walls. In order to determine the environmental features that are responsible for the initial high activity and wall-following behavior exhibited during exploration, we examined wild-type and visually impaired mutants in arenas with different vertical surfaces. These experiments support the conclusion that the wall-following behavior of Drosophila is best characterized by a preference for the arena boundary, and not thigmotaxis or centrophobicity. In circular arenas, Drosophila mostly move in trajectories with low turn angles. Since the boundary preference could derive from highly linear trajectories, we further developed a simulation program to model the effects of turn angle on the boundary preference. In an hourglass-shaped arena with convex-angled walls that forced a straight versus wall-following choice, the simulation with constrained turn angles predicted general movement across a central gap, whereas Drosophila tend to follow the wall. Hence, low turn angled movement does not drive the boundary preference. Lastly, visually impaired Drosophila demonstrate a defect in attenuation of the elevated initial activity. Interestingly, the visually impaired w(1118) activity decay defect can be rescued by increasing the contrast of the arena's edge, suggesting that the activity decay relies on visual detection of the boundary. The arena boundary is, therefore, a primary object of exploration for Drosophila.

Strain dependent gene expression and neurochemical levels in the brain of zebrafish: focus on a few alcohol related targets

The zebrafish is becoming increasingly popular in behavior genetics because it may allow one to conduct large scale mutation and drug screens facilitating the discovery of mechanisms of complex traits. Strain differences in adult zebrafish behavior have already been reported, which may have important implications in neurobehavioral genetics. For example, we have found the AB and SF strains to differ in their behavioral responses to both acute and chronic alcohol exposure. In the current study, we further characterize these strains using semi-quantitative RT-PCR to measure the expression of ten selected genes and HPLC to measure the levels of nine neurochemicals. We chose the target genes and neurochemicals based upon their potential involvement in alcohol and other drugs of abuse related mechanisms. We quantified the expression of the genes encoding D1-R, D2a-R, D4a-R dopamine receptors, GABA(A)-R, GABA(B)-R1, GAD1, MAO, NMDA-R (NR2D subunit), 5HT-R1bd and SLC6 a4a. We found the gene encoding D1 dopamine receptor over-expressed and the genes encoding GABA(B1) receptor and solute family carrier protein 6 (SLC6) 4a under-expressed in SF compared to AB. We also found the level of all (dopamine, DOPAC, Serotonin, GABA, Glutamate, Glycine, Aspartate, Taurine) but one (5HIAA) neurochemicals tested decreased in SF as compared to AB. These results, combined with previously identified behavioral differences between the AB and SF strains, demonstrate the importance of strain characterization in zebrafish. They now also allow formulation of working hypotheses about possible mechanisms underlying the differential effects of acute and chronic alcohol treatment on these two zebrafish strains.

Associative learning in zebrafish (Danio rerio)

The zebrafish has been one of the primary study species utilized in developmental biology. However, it is also gaining increasing amount of interest in other disciplines of biology including behavioral neuroscience; the numerous genetic tools developed and the large amount of genetic information accumulated for this species by now make it an excellent tool for the analysis of the mechanisms of complex central nervous system characteristics. Although several studies have investigated the biological and genetic underpinnings of associative learning (and memory), given the complexity of these phenomena, much remains to be discovered. In the past, the zebrafish has been employed particularly successfully in screening applications where a large number of mutations or drug effects had to be analyzed. Briefly, the practical simplicity and system complexity of the zebrafish may make this species an excellent tool also for the analysis of the mechanisms of associative learning. Screening, however, requires appropriate phenotypical (in this case behavioral) paradigms. A step in this direction is the characterization of learning abilities of zebrafish. The number of studies focused on cognitive and/or mnemonic characteristics of zebrafish is orders of magnitude smaller than those with rats or mice, but recently zebrafish has also started to be utilized in this research. The current chapter reviews these most recent developments. It also discusses certain unique features of zebrafish that must be taken into account when designing an associative learning task and how these tasks may be made high throughput.

Zebrafish antipredatory responses: a future for translational research?

Human neuropsychiatric conditions associated with abnormally exaggerated or misdirected fear (anxiety disorders and phobias) still represent a large unmet medical need because the biological mechanisms underlying these diseases are not well understood. Animal models have been proposed to facilitate this research. Here I review the literature with a focus on zebrafish, an upcoming laboratory organism in behavioral brain research. I argue that abnormal human fear responses are likely the result of the malfunction of neurobiological mechanisms (brain areas, circuits and/or molecular mechanisms) that originally evolved to support avoidance of predators or other harm in nature. I also argue that the understanding of the normal as well as pathological functioning of such mechanisms may be best achieved if one utilizes naturalistic experimental approaches. In case of laboratory model organisms, this may entail presenting stimuli associated with predators and measuring species-specific antipredatory responses. Although zebrafish is a relatively new subject of such inquiry, I review the recently rapidly increasing number of zebrafish studies in this area, and conclude that zebrafish is a promising research tool for the analysis of the neurobiology and genetics of vertebrate fear responses.

Latent learning in zebrafish (Danio rerio)

The zebrafish may represent an excellent compromise between system complexity and practical simplicity for behavioral brain research. It may be particularly appropriate for large scale screening studies whose aim is to identify mutants with altered phenotypes or novel compounds with particular efficacy. For example, the zebrafish may have utility in the analysis of the biological mechanisms of learning and memory. Although learning and memory have been extensively studied and hundreds of underlying molecular mechanisms have been identified, this number may represent only the fraction of genes involved in these complex brain functions. Thus large scale mutagenesis screens may have utility. In order for such screens to succeed, appropriate screening paradigms must be developed. The first step in this research is the characterization of learning and memory capabilities of zebrafish and the development of automatable tasks. Here we show that zebrafish is capable of latent learning, i.e. can acquire memory of their environment after being allowed to explore it. For example, we found experimental zebrafish that experienced an open left tunnel or an open right tunnel of a maze during the unrewarded exploration phase of the test to show the appropriate side bias during a probe trial when they had to swim to a group of conspecifics (the reward). Given that exploration of the maze does not require the presence of the experimenter and the probe trial, during which the subjects are video-recorded and their memory is tested, is short, we argue that the paradigm has utility in high-throughput screening.

Exploratory activity in Drosophila requires the kurtz nonvisual arrestin

When Drosophila adults are placed into an open field arena, they initially exhibit an elevated level of activity followed by a reduced stable level of spontaneous activity. We have found that the initial elevated component arises from the fly's interaction with the novel arena since: (1) the increased activity is independent of handling prior to placement within the arena, (2) the fly's elevated activity is proportional to the size of the arena, and (3) the decay in activity to spontaneous levels requires both visual and olfactory input. These data indicate that active exploration is the major component of elevated initial activity. There is a specific requirement for the kurtz nonvisual arrestin in the nervous system for both the exploration stimulated by the novel arena and the mechanically stimulated activity. kurtz is not required for spontaneous activity; kurtz mutants display normal levels of spontaneous activity and average the same velocities as wild-type controls. Inhibition of dopamine signaling has no effect on the elevated initial activity phase in either wild-type or krz(1) mutants. Therefore, the exploratory phase of open field activity requires kurtz in the nervous system, but is independent of dopamine's stimulation of activity.

Wild mouse open field behavior is embedded within the multidimensional data space spanned by laboratory inbred strains

The vast majority of studies on mouse behavior are performed on laboratory mouse strains (Mus laboratorius), while studies of wild-mouse behavior are relatively rare. An interesting question is the relationship between the phenotypes of M. laboratorius and the phenotypes of their wild ancestors. It is commonly believed, often in the absence of hard evidence, that the behavior of wild mice exceeds by far, in terms of repertoire richness, magnitude of variables and variability of behavioral measures, the behavior of the classical inbred strains. Having phenotyped the open field behavior (OF) of eight of the commonly used laboratory inbred strains, two wild-derived strains and a group of first-generation-in-captivity local wild mice (Mus musculus domesticus), we show that contrary to common belief, wild-mouse OF behavior is moderate, both in terms of end-point values and in terms of their variability, being embedded within the multidimensional data space spanned by laboratory inbred strains. The implication could be that whereas natural selection favors moderate locomotor behavior in wild mice, the inbreeding process tends to generate in mice, in some of the features, extreme and more variable behavior.

Inheritance of Behavioral and Neuroanatomical Phenotypical Variance: Hybrid Mice Are Not Always More Stable Than Inbreds

Many investigators have attempted to confirm the prediction that increased levels of heterozygosity entail greater developmental stability, manifesting itself through decreased phenotypical variation. The evidence presented so far is equivocal. The predicted relationship has been found in some morphological studies, but not in others. I propose that the variability of a character should be seen as different from the character itself. For most morphological characters, natural selection promotes strong canalization of development but, to facilitate responses to environmental changes, the organism needs to retain malleability of physiological and behavioral traits. These different types of selection should lead to distinct genetic architectures for these phenotypes. I report on the results of a diallel cross between four inbred mouse strains. Qualitatively different genetic architectures were in fact revealed for variation in behaviors in the open-field. In a second study, variances of inbred and hybrid populations for hippocampal morphometry were studied. Again, hybrids were not always less variable than inbreds and sometimes even more variable. It follows that there exists no one-to-one relation between heterozygosity and developmental stability.

Behavioural correlates of an altered balance between synaptic and extrasynaptic GABAAergic inhibition in a mouse model

GABAA receptors mediate fast phasic inhibitory postsynaptic potentials and participate in slower tonic extrasynaptic inhibition. Thy1alpha6 mice with ectopic forebrain expression of GABAA receptor alpha6 subunits exhibit increased extrasynaptic GABAA receptor-mediated background conductance and reduced synaptic GABAA receptor currents in hippocampal CA1 neurons [W. Wisden et al. (2002) Neuropharmacology 43, 530-549]. Here we demonstrate that isolated CA1 neurons of these mice showed furosemide-sensitivity of GABA-evoked currents, confirming the functional expression of alpha6 subunit. In addition, receptor autoradiography of the CA1 region of Thy1alpha6 brain sections revealed pharmacological features that are unique for alpha6betagamma2 and alpha6beta receptors. The existence of atypical alpha6beta receptors was confirmed after completely eliminating GABAA receptors containing gamma1, gamma2, gamma3 or delta subunits using serial immunoaffinity chromatography on subunit-specific GABAA receptor antibodies. Behaviourally, the Thy1alpha6 mice showed normal features with slightly enhanced startle reflex and struggle-escape behaviours. However, they were more sensitive to GABAA antagonists DMCM (shorter latency to writhing clonus) and picrotoxinin (shorter latency to generalized convulsions). Tiagabine, an antiepileptic GABA-uptake inhibitor that increases brain GABA levels, delayed picrotoxinin-induced convulsions at a low dose of 3.2 mg/kg in Thy1alpha6 mice, but not in control mice; however, the overall effect of higher tiagabine doses on the convulsion latency remained smaller in the Thy1alpha6 mice. Altered balance between extrasynaptic and synaptic receptors thus affects seizure sensitivity to GABAergic convulsants. Importantly, the increased extrasynaptic inhibition, even when facilitated in the presence of tiagabine, was not able fully to counteract enhanced seizure induction by GABAA antagonists.

Role of a neuronal small non-messenger RNA: behavioural alterations in BC1 RNA-deleted mice

BC1 RNA is a small non-messenger RNA common in dendritic microdomains of neurons in rodents. In order to investigate its possible role in learning and behaviour, we compared controls and knockout mice from three independent founder lines established from separate embryonic stem cells. Mutant mice were healthy with normal brain morphology and appeared to have no neurological deficits. A series of tests for exploration and spatial memory was carried out in three different laboratories. The tests were chosen as to ensure that different aspects of spatial memory and exploration could be separated and that possible effects of confounding variables could be minimised. Exploration was studied in a barrier test, in an open-field test, and in an elevated plus-maze test. Spatial memory was investigated in a Barnes maze and in a Morris water maze (memory for a single location), in a multiple T-maze and in a complex alley maze (route learning), and in a radial maze (working memory). In addition to these laboratory tasks, exploratory behaviour and spatial memory were assessed under semi-naturalistic conditions in a large outdoor pen. The combined results indicate that BC1 RNA-deficient animals show behavioural changes best interpreted in terms of reduced exploration and increased anxiety. In contrast, spatial memory was not affected. In the outdoor pen, the survival rates of BC1-depleted mice were lower than in controls. Thus, we conclude that the neuron-specific non-messenger BC1 RNA contributes to the aptive modulation of behaviour.

Genetic animal models of anxiety

The focus of this review is on progress achieved in identifying specific genes conferring risk for anxiety disorders through the use of genetic animal models. We discuss gene-finding studies as well as those manipulating a candidate gene. Both human and animal studies thus far support the genetic complexity of anxiety. Clinical manifestations of these diseases are likely related to multiple genes. While different anxiety disorders and anxiety-related traits all appear to be genetically influenced, it has been difficult to ascertain genetic influences in common. Mouse studies have provisionally mapped several loci harboring genes that affect anxiety-related behavior. The growing array of mutant mice is providing valuable information about how genes and environment interact to affect anxious behavior via multiple neuropharmacological pathways. Classical genetic methods such as artificial selection of rodents for high or low anxiety are being employed. Expression array technologies have as yet not been employed, but can be expected to implicate novel candidates and neurobiological pathways.

Molecular genetic approaches to investigate individual variations in behavioral and neuroendocrine stress responses

A large response range can be observed in both behavioral and neuroendocrine responses to environmental challenges. This variation can arise from central mechanisms such as those involved in the shaping of general response tendencies (temperaments) or involves only one or the other output system (behavioral vs. endocrine response). The participation of genetic factors in this variability is demonstrated by family and twin studies in humans, the comparison of inbred strains and selection experiments in animals. Those inbred strains diverging for specific traits of stress reactivity are invaluable tools for the study of the molecular bases of this genetic variability. Until recently, it was only possible to study biological differences between contrasting strains, such as neurotransmitter pathways in the brain or hormone receptor properties, in order to suggest structural differences in candidate genes. The increase of the power of molecular biology tools allows the systematic screening of significant genes for the search of molecular variants. More recently, it was possible to search for genes without any preliminary functional hypothesis (mRNA differential expression, nucleic acid arrays, QTL search). The approach known as quantitative trait loci (QTL) analysis is based on the association between polymorphic anonymous markers and the phenotypical value of the trait under study in a segregating population (such as F2 or backcross). It allows the location of chromosomal regions involved in trait variability and ultimately the identification of the mutated gene(s). Therefore, in a first step, those studies skip the 'black box' of intermediate mechanisms, but the knowledge of the gene(s) responsible for trait variability will point out to the pathway responsible for the phenotypical differences. Since variations in stress-related responses may be related to numerous pathological conditions such as behavioral and mood disorders, drug abuse, cardiovascular diseases or obesity, and production traits in farm animals, these studies can be expected to bring significant knowledge for new therapeutic approaches in humans and improved efficiency of selection in farm animals.

Behavioral and neuroanatomical characterization of FVB/N inbred mice

The inbred strain FVB/N is becoming increasingly popular to generate transgenic animals. We compared animals from this strain with well-characterized C57BL/6J animals on four different behavioral tests: the elevated plus maze test of anxiety, a standard opponent aggression test, the open-field test, and spatial learning in a radial maze. Our results indicate that FVB/N animals have slightly higher levels of anxiety and aggression, are hyperactive, and have a clear learning deficit. The latter finding seems to be related to an exceptionally small intrapyramidal and infrapyramidal mossy fiber projection. It is recommended that transgenic experiments employing this strain use F1 crosses between FVB/N and C57BL/6J as much as possible for behavioral experiments intended to evaluate spatial learning.

Genetic dissection of mouse exploratory behaviour

A large variety of apparatus and procedures are being employed to measure mouse exploratory behaviour. Definitions of what constitutes exploration also vary widely. The present article reviews two studies whose results permit a genetic dissection of behaviour displayed in an open-field situation. The results agree that factors representing exploration and stress/fear underlie this type of behaviour. Both factors appear to be linked to neuroanatomical variation in the sizes of the hippocampal intra- and infrapyramidal mossy fibre terminal fields. Multivariate analysis of genetic correlations may render important insights into the structure of behaviour and its relations with neuroanatomical and neurophysiological systems.

Analysing hippocampal function in transgenic mice: an ethological perspective

Advances in molecular genetics and technology have led to the dawn of a new era for neuroscience: manipulation of single genes now makes it possible to dissect the complexities of neurobiological phenotypes and to understand many of the intricacies of brain and behaviour, even in mammals. The phenotypical analysis of these mutant animals is complicated because the potential outcome of gene manipulation is difficult to predict. While behavioural analysis should form an integral part of any multidisciplinary research programme investigating the phenotypical effects of single genes on hippocampal function, it is crucial that the behavioural tests are designed and conducted appropriately. Approaches that take species-specific behavioural characteristics into account and use ethological methods could be the most useful for interpreting these behavioural findings and understanding the biological mechanisms of brain function.

A new continuous alternation task in T-maze detects hippocampal dysfunction in mice. A strain comparison and lesion study

The mammalian hippocampus has been the focus of several neurobiology studies because of its important behavioral function and because long-term potentiation (LTP) is a prominent feature of this brain region. Converging evidence suggests that hippocampal function is associated with learning multiple relationships of environmental cues. In this paper a novel behavioral test procedure is introduced, a modified T-maze continuous alternation task (T-CAT), that may serve as a simple, automatable, and quick test of hippocampal function in addition to the frequently applied water maze and fear conditioning paradigms. A comparison is made between mice (strain C57BL/6) with ibotenic acid lesioned or vehicle injected hippocampus, two transgenic strains (on CD1 background) overexpressing a calcium binding protein, S100beta, and inbred (C57BL/6, DBA/2, 129/SV and 129/SVEV) and outbred (CD1) strains of mice. This study shows that hippocampal lesioning led to a significant impairment in T-CAT. Furthermore, overexpression of S100beta, which impairs hippocampal LTP, also led to an impairment demonstrating that T-CAT is sensitive to detect hippocampal dysfunction. Analysis of the mouse strains revealed that C57BL/6 and CD1 mice performed well in T-CAT, whereas 129/SV, 129/SVEV and DBA/2 were significantly impaired, a finding that underscores the importance of strain differences in pharmacological or single gene manipulation studies of hippocampal function in mice.

Multiple behavioral anomalies in GluR2 mutant mice exhibiting enhanced LTP

We have previously disrupted the ionotropic glutamate receptor type 2 gene (GluR2) using gene targeting in embryonic stem cells and generated mice which lacked the GluR2 gene product. Neurophysiological analyses of these mice showed a markedly enhanced long-term potentiation (LTP) and a 9-fold increase in kainate induced Ca2+ permeability in the hippocampus. Here, we analyze the behavioral and neuroanatomical consequences of GluR2 deficiency in homozygous null mutant and age-matched littermate control mice. We show that despite unaltered gross brain morphology, several aspects of behavior were abnormal in the mutants. Object exploration, rearing, grooming and locomotion were altered in the novel arena. Eye-closure reflex, motor performance on the rotating rod and spatial and non-spatial learning performance in the water maze were also abnormal in the mutants. These abnormalities together with the widespread expression pattern of GluR2 in most excitatory CNS pathways suggest that the absence of GluR2 leads to neurological phenotypes associated with not only the hippocampus but several other brain regions potentially including the cortex and cerebellum. We speculate that GluR2 mutant mice suffer from an overall non-specifically increased excitability that may alter cognitive functions ranging from stimulus processing to motivation and learning.

Purkinje cell expression of a mutant allele of SCA1 in transgenic mice leads to disparate effects on motor behaviors, followed by a progressive cerebellar dysfunction and histological alterations

Spinocerebellar ataxia type 1 (SCA1) is an autosomal dominant neurological disorder caused by the expansion of a CAG repeat encoding a polyglutamine tract. Work presented here describes the behavioral and neuropathological course seen in mutant SCA1 transgenic mice. Behavioral tests indicate that at 5 weeks of age mutant mice have an impaired performance on the rotating rod in the absence of deficits in balance and coordination. In contrast, these mutant SCA1 mice have an increased initial exploratory behavior. Thus, expression of the mutant SCA1 allele within cerebellar Purkinje cells has divergent effects on the motor behavior of juvenile animals: a compromise of rotating rod performance and a simultaneous enhancement of initial exploratory activity. With age, these animals develop incoordination with concomitant progressive Purkinje neuron dendritic and somatic atrophy but relatively little cell loss. Therefore, the eventual development of ataxia caused by the expression of a mutant SCA1 allele is not the result of cell death per se, but the result of cellular dysfunction and morphological alterations that occur before neuronal demise.

Impaired cerebellar synaptic plasticity and motor performance in mice lacking the mGluR4 subtype of metabotropic glutamate receptor

The application of the glutamate analog L-2-amino-4-phosphonobutyric acid (L-AP4) to neurons produces a suppression of synaptic transmission. Although L-AP4 is a selective ligand at a subset of metabotropic glutamate receptors (mGluRs), the precise physiological role of the L-AP4-activated mGluRs remains primarily unknown. To provide a better understanding of the function of L-AP4 receptors, we have generated and studied knockout (KO) mice lacking the mGluR4 subtype of mGluR that displays high affinity for L-AP4. The mGluR4 mutant mice displayed normal spontaneous motor activity and were unimpaired on the bar cross test, indicating that disruption of the mGluR4 gene did not cause gross motor abnormalities, impairments of novelty-induced exploratory behaviors, or alterations in fine motor coordination. However, the mutant mice were deficient on the rotating rod motor-learning test, suggesting that mGluR4 KO mice may have an impaired ability to learn complex motor tasks. Patch-clamp and extracellular field recordings from Purkinje cells in cerebellar slices demonstrated that L-AP4 had no effect on synaptic responses in the mutant mice, whereas in the wild-type mice 100 microM L-AP4 produced a 23% depression of synaptic responses with an EC50 of 2.5 microM. An analysis of presynaptic short-term synaptic plasticity at the parallel fiber-->Purkinje cell synapse demonstrated that paired-pulse facilitation and post-tetanic potentiation were impaired in the mutant mice. In contrast, long-term depression (LTD) was not impaired. These results indicate that an important function of mGluR4 is to provide a presynaptic mechanism for maintaining synaptic efficacy during repetitive activation. The data also suggest that the presence of mGluR4 at the parallel fiber-->Purkinje cell synapse is required for maintaining normal motor function.

Correlations between radial-maze learning and structural variations of septum and hippocampus in rodents

Large, but non-pathological, individual differences in neuroanatomy of the brain exist in rodents, which have been shown to covary with behavioral traits. In the present review, we explore the relationship between variations in the extent of the intra- and infrapyramidal mossy fiber projection of the hippocampus and spatial and non-spatial learning capacities in mice and rats. Preliminary data concerning anatomical variation in the septo-hippocampal cholinergic system and its consequences for individual behavior are also presented. We conclude that the hippocampal intra- and infrapyramidal mossy fiber projection is intimately involved in the regulation of spatial, but not of non-spatial learning capabilities. Although lesion studies have shown that a well-functioning cholinergic system is a prerequisite for performance in spatial learning tasks, our preliminary data suggest that individual differences in the cholinergic system do not explain individual differences in learning.

Association of autosomal loci with the grooming activity in mice observed in open-field

We studied the reactions of mice when placed in an open-field environment and counted the grooming score in response to novelty. We used animals from Mendelian F2s and backcrosses, obtained from the parental strains ABP/Le and C57BL/6By, to test the hypothesis that the differences in this behavior were due to genetic variation at loci associated with visible recessive markers. Furthermore, the analysis of the segregating populations by means of non parametric Collins' method was used to test the one segregating unit hypothesis. We provide evidence for genes involved in the variation of grooming activity and situated close to the locus se on chromosome 9. Although the one-locus hypothesis was ruled out in the general analysis, a possible major gene effect in some crosses suggests a maternal environment effect.

Female specific hyperactivity in S100 beta transgenic mice does not habituate in open-field

S100 beta, a calcium-binding brain specific protein, may affect brain development and long-term potentiation. Its gene maps to a region of chromosome 21 duplicated in Down's Syndrome (DS), and its levels are elevated in DS. To test the hypothesis that elevated S100 beta levels cause brain dysfunction in a mammalian system, transgenic mice carrying multiple copies of the human S100 beta gene have been generated and their locomotory patterns are analyzed in open field situations. Female-specific hyperactivity was observed in 2-month-old and in 12-month-old transgenic mice, which rules out the previous speculation that postmenopausal hormonal changes constitute a necessary factor in this behavioral abnormality. Analysis of temporal patterns of activity showed a profound abnormality in transgenic females: the initially elevated activity quickly habituated in males and in normal females, however, its level remained high in the transgenic females throughout the 9-min recording session. These observations are compatible with the suggestion that hippocampal function is abnormal in the females of S100 beta transgenic mice.

Female transgenic mice carrying multiple copies of the human gene for S100 beta are hyperactive

Down syndrome (DS) (trisomy 21) is the most frequent genetic cause of mental retardation in man. The gene coding for the beta subunit of human S100 protein (S100 beta) has been mapped to chromosome 21. The dimeric form of S100 beta may function as a neurotrophic factor in the CNS and may also influence the establishment of hippocampal long-term potentiation (LTP). To study the behavioral consequences of overexpression of S100 beta in an animal model, we derived four lines of transgenic mice carrying multiple copies of the human S100 beta gene. The human S100 beta gene was expressed in the brain of these mice in a cell-specific and gene-dose-dependent manner. The motor and posture patterns of 16-month-old transgenic mice and their control (non-transgenic) littermates were studied in two tests, open field and bar-crossing, in order to examine novelty induced exploratory activities. Transgenic female mice were significantly hyperactive in both tests in comparison with their female control littermates. These differences were independent of the line of origin of the mice suggesting a causal relationship between the observed hyperactivity and the presence of multiple copies of the integrated human S100 beta gene. In contrast, transgenic males were not hyperactive in comparison with controls. Neither male nor female transgenic mice displayed any coordination defects. We speculate about how an interaction between the effects of elevated S100 beta levels and female specific hormonal changes could have resulted in the observed female restricted hyperactivity.(ABSTRACT TRUNCATED AT 250 WORDS)

Bi- and multivariate analyses of diallel crosses: a tool for the genetic dissection of neurobehavioral phenotypes

The genetic-correlational approach provides a very powerful tool for the analysis of causal relationships between phenotypes. It appears to be particularly appropriate for investigating the functional organization of behavior and/or causal relationships between brain and behavior. A method for the bivariate analysis of diallel crosses that permits the estimation of correlations due to environmental effects, additive-genetic effects, and/or dominance deviations is described, together with a worked-out example stemming from a five times replicated 4 x 4 diallel cross between inbred mouse strains. The phenotypes chosen to illustrate the analysis were locomotor activity and rearing frequency in an open field. Large, positive additive-genetic and dominance correlations between these two phenotypes were obtained. This finding was replicated in another, independently executed, diallel cross.

A genetic-correlational study of hippocampal neurochemical variation and variation in exploratory activities of mice

Previously, we have demonstrated that hippocampal mossy fibers, containing the opioid peptide dynorphin B, are functionally connected with the gene-dependent exploratory behavior of mice. In order to seek further evidence of causal relationships between dynorphin B action and exploration, a quantitative-genetic method was chosen. For this purpose, mice from the inbred strains C57BL/6, DBA/2, BLN, and CPB-K were used. By means of radioimmunoassay, the hippocampal level of dynorphin B was monitored in mice that had been exposed to environmental novelty, as compared to naive animals. Clear evidence was obtained that novelty induces the release of hippocampal dynorphin B. Furthermore, low tissue content was found to be causally connected with high exploratory scores.

A genetic-correlational study of hippocampal structural variation and variation in exploratory activities of mice

Our previous work provided evidence that hippocampal opioid peptides form an important neurochemical substrate underlying the gene-dependent exploratory behavior of mice. A prominent hippocampal opioid is dynorphin B, which resides in the mossy fibers exclusively. In order to seek support for causal relationships between dynorphinergic hippocampal mechanisms and exploration, a quantitative-genetic method was chosen. For this purpose, mice from the inbred strains C57BL/6, DBA/2, BLN, and CPB-K were used. Their hippocampal mossy fiber projections were visualized by means of immunohistochemistry, using a highly specific anti-dynorphin B antiserum. The additive-genetic correlations that were estimated suggest pleiotropic gene effects on locomotion, rearing-up, wall-leaning, and several intra- and infrapyramidal mossy fiber (iipMF) variables. Long iipMF, in particular, were found to be associated with high exploratory activity.

Inheritance of species-specific behaviors in the paradise fish (Macropodus opercularis): a diallel study

Species-specific elements of the paradise fish's ethogram were recorded in one familiar and three different unfamiliar environments, which were designed to model certain features of this species' natural habitat: (1) a densely vegetated home range, (2) a novel open field, (3) a small novel place, and (4) a small novel place with a predator. The inheritance of the behavioral elements was investigated employing a five-times-replicated diallel cross among three inbred strains. A detailed Hayman analysis of variance and a variance-covariance analysis were performed to uncover the genetic architectures of these phenotypes. Additive genetic effects and/or ambidirectional dominance was found to be characteristic of most species-specific behavioral elements studied, suggesting an evolutionary history of stabilizing selection.

Genotype-environment interaction and the correlation structure of behavioral elements in paradise fish (Macropodus opercularis)

Three inbred strains and all their possible F1 crosses were monitored in one familiar and three unfamiliar situations. Their behavior was described by species-specific elements of the ethogram. Genetic variability was demonstrated both for behavioral elements and for factors extracted by Principal Components Analyses (PCA). We studied how the behavior of genetically different fish changed across situations and examined the interrelations both among variables measured in one situation and between those measured in different ones. Behavioral changes across situations proved to be different for certain strains and crosses, that is, genotype-environment interaction was found. The PCA's carried out for the 4 situations separately yielded unlike factor structures. Another PCA, in which all the variables were included, proved that there was correlation among certain variables measured in different situations. In general, it seems that the corresponding behavioral elements do not always represent the same phene in different situations. We discuss how the genotype-environment interactions can be interpreted, try to define behavioral strategies using the extracted factor structures, and construct a model for the organization of Macropodus behavior.

Genetic control of hippocampal cholinergic and dynorphinergic mechanisms regulating novelty-induced exploratory behavior in house mice

Neurobehavioral genetics endeavors to trace the pathways from genetic and environmental determinants to neuroanatomical and neurophysiological systems and, thence, to behavior. Exploiting genetic variation as a tool, the behavioral sequelae of manipulating these neuronal systems by drugs and antisera are analyzed. Apart from research in rats, this paper deals mainly with the genetically-influenced regulation in mice of exploratory behaviors that are adaptive in novel surroundings and are hippocampally-mediated. Special attention is paid to neuropeptidergic, GABAergic, and cholinergic synaptic functions in the mouse hippocampus. The behaviorally different inbred mouse strains C57BL/6 and DBA/2 show opposite reactions (reductions and increases, respectively, in exploration rates) to peripheral and intrahippocampal injections with agents that interfere with peptidergic, cholinergic, and GABAergic neurotransmission. These findings can be explained by an interdependent over-release of opioids, arrested GABA release, and excess acetylcholine in the hippocampal neuronal network of DBA/2 mice, as compared to C57BL/6 mice where these systems are functionally well balanced. Very similar results have been obtained with the lines SRH and SRL, derived from C57BL/6 and DBA/2, and genetically selected for rearing behavior. Most probably, the opioids act to disinhibit exploratory responses. An additional genetic approach is mentioned, in which four inbred mouse strains and one derived heterogeneous stock are used for estimating genetic correlations between structural properties of the hippocampal mossy fibers and levels of hippocampal dynorphin B, on the one hand, and frequencies of exploratory responses to environmental novelty, on the other.

Behavioral responses to novelty and structural variation of the hippocampus in mice. I. Quantitative-genetic analysis of behavior in the open-field

As a first step towards a multivariate quantitative-genetic analysis of covariations between heritable variation in hippocampal structure and mouse behavior, a univariate analysis of the genetic architecture of behavioral responses to novelty is presented. For several components of exploratory behavior considerable amounts of genetic variation were found and an evolutionary history of stabilizing selection for intermediate levels of exploration was inferred. Comparison of these results with those from a previous study indicated that even a relatively small diallel cross, involving 4-5 inbred strains, may provide useful genetic information on a specific sample of animals. Larger numbers of strains are needed to provide precise estimates of genetic parameters in a population.

Genetic selection for novelty-induced rearing behavior in mice produces changes in hippocampal mossy fiber distributions

Previous investigations in mice revealed the existence of a set of genes that influence variations in hippocampal anatomy as well as variations in behavioral responses to novelty. In particular, a positive genetic correlation was found between the size of the intra- and infrapyramidal mossy fiber (iip-MF) projection and rearing frequency in an open-field. On the basis of these findings, we hypothesized that genetic selection for rearing would entail correlated changes in hippocampal morphology. This was tested in the inbred selection lines SRH (selection for rearing: high) and SRL (selection for rearing: low). As expected, the SRH mice appeared to possess iip-MF terminal fields that were larger than those of the SRL mice. Because the behavioral difference between the two lines is most probably caused by a single genetic unit, these animals represent valuable material for molecular-genetic investigations into the mechanisms that control behavioral and neuroanatomical variation.

Zinc-induced peripheral anosmia and behavioral responses to novelty in mice: a quantitative-genetic analysis

Adult male mice were made anosmic by intranasal flushing with a 5% zinc sulfate solution. Twelve behavioral variables were measured in treated as well as saline-irrigated control animals placed in a novel environment. The genetic underpinnings and the genotype-treatment interactions with regard to these behaviors were analyzed in a classical Mendelian cross between the inbred strains C57BL/6 and DBA/2 and in a full 4 X 4 diallel cross, replicated five times, between these strains and strains C3H/St an CPB-K. Based on the hypothesis of an evolutionary history of directional selection for a well-balanced information-processing system, one might expect directional dominance for decrease in exploration after anosmization. Although decreases were found for several behavioral phenotypes, only few and relatively unimportant genotype-treatment interactions were present. This absence of any kind of genetic variation for behavioral change after anosmization points to an extremely strong directional selection which has eliminated all less favorable alleles. The findings support the hypothesis of directional selection for an efficient olfactory information-processing system.