Genetic experiments with animal learning: a critical review

Neonatal handling decreases unconditioned anxiety, conditioned fear, and improves two-way avoidance acquisition: a study with the inbred Roman high (RHA-I)- and low-avoidance (RLA-I) rats of both sexes

The present study evaluated the long-lasting effects of neonatal handling (NH; administered during the first 21 days of life) on unlearned and learned anxiety-related responses in inbred Roman High- (RHA-I) and Low-avoidance (RLA-I) rats. To this aim, untreated and neonatally-handled RHA-I and RLA-I rats of both sexes were tested in the following tests/tasks: a novel object exploration (NOE) test, the elevated zero maze (ZM) test, a “baseline acoustic startle” (BAS) test, a “context-conditioned fear” (CCF) test and the acquisition of two-way active—shuttle box—avoidance (SHAV). RLA-I rats showed higher unconditioned (novel object exploration test -“NOE”-, elevated zero maze test -“ZM”-, BAS), and conditioned (CCF, SHAV) anxiety. NH increased exploration of the novel object in the NOE test as well as exploration of the open sections of the ZM test in both rat strains and sexes, although the effects were relatively more marked in the (high anxious) RLA-I strain and in females. NH did not affect BAS, but reduced CCF in both strains and sexes, and improved shuttle box avoidance acquisition especially in RLA-I (and particularly in females) and in female RHA-I rats. These are completely novel findings, which indicate that even some genetically-based anxiety/fear-related phenotypes can be significantly modulated by previous environmental experiences such as the NH manipulation.

Studying the evolutionary ecology of cognition in the wild: a review of practical and conceptual challenges

Cognition is defined as the processes by which animals collect, retain and use information from their environment to guide their behaviour. Thus cognition is essential in a wide range of behaviours, including foraging, avoiding predators and mating. Despite this pivotal role, the evolutionary processes shaping variation in cognitive performance among individuals in wild populations remain very poorly understood. Selection experiments in captivity suggest that cognitive traits can have substantial heritability and can undergo rapid evolution. However only a handful of studies have attempted to explore how cognition influences life-history variation and fitness in the wild, and direct evidence for the action of natural or sexual selection on cognition is still lacking, reasons for which are diverse. Here we review the current literature with a view to: (i) highlighting the key practical and conceptual challenges faced by the field; (ii) describing how to define and measure cognitive traits in natural populations, and suggesting which species, populations and cognitive traits might be examined to greatest effect; emphasis is placed on selecting traits that are linked to functional behaviour; (iii) discussing how to deal with confounding factors such as personality and motivation in field as well as captive studies; (iv) describing how to measure and interpret relationships between cognitive performance, functional behaviour and fitness, offering some suggestions as to when and what kind of selection might be predicted; and (v) showing how an evolutionary ecological framework, more generally, along with innovative technologies has the potential to revolutionise the study of cognition in the wild. We conclude that the evolutionary ecology of cognition in wild populations is a rapidly expanding interdisciplinary field providing many opportunities for advancing the understanding of how cognitive abilities have evolved.

Predictability and heritability of individual differences in fear learning

Our objective was to characterize individual differences in fear conditioning and extinction in an outbred rat strain, to test behavioral predictors of these individual differences, and to assess their heritability. We fear-conditioned 100 Long-Evans rats, attempted to extinguish fear the next day, and tested extinction recall on the third day. The distribution of freezing scores after fear conditioning was skewed, with most rats showing substantial freezing; after fear extinction, the distribution was bimodal with most rats showing minimal freezing, but a substantial portion showing maximal freezing. Longer rearing episodes measured prior to conditioning predicted less freezing at the beginning of extinction, but differences in extinction learning were not predicted by any baseline exploratory behaviors. We tested the heritability of extinction differences by breeding rats from the top and bottom 20 % of freezing scores during extinction recall. We then ran the offspring through the same conditioning/extinction procedure, with the addition of recording ultrasonic vocalizations throughout training and testing. Only a minority of rats emitted distress vocalizations during fear acquisition, but the incidence was less frequent in the offspring of good extinguishers than in poor extinguishers or randomly bred controls. The occurrence of distress vocalizations during acquisition predicted higher levels of freezing during fear recall regardless of breeding line, but the relationship between vocalization and freezing was no longer evident following extinction training, at which point freezing levels were influenced only by breeding and not by vocalization. The heritability (h²) of extinction recall was estimated at 0.36, consistent with human estimates.

Comprehensive neurocognitive endophenotyping strategies for mouse models of genetic disorders

There is a need for refinement of the current behavioral phenotyping methods for mouse models of genetic disorders. The current approach is to perform a behavioral screen using standardized tasks to define a broad phenotype of the model. This phenotype is then compared to what is known concerning the disorder being modeled. The weakness inherent in this approach is twofold: First, the tasks that make up these standard behavioral screens do not model specific behaviors associated with a given genetic mutation but rather phenotypes affected in various genetic disorders; secondly, these behavioral tasks are insufficiently sensitive to identify subtle phenotypes. An alternate phenotyping strategy is to determine the core behavioral phenotypes of the genetic disorder being studied and develop behavioral tasks to evaluate specific hypotheses concerning the behavioral consequences of the genetic mutation. This approach emphasizes direct comparisons between the mouse and human that facilitate the development of neurobehavioral biomarkers or quantitative outcome measures for studies of genetic disorders across species.

How heritable is innate behaviour?

The relations between innateness and heritability of behaviour, environmental heterogeneity, genetic diversity of populations, and rates of evolution of behaviour are discussed on the basis of a simple quantitative model. An index I of innateness is proposed which can be related to the heritability index h2. I and h2 are positively related but the degree of correlation depends on the genetic variability of the population, the heterogeneity of the conditioning environment, and the level of spontaneous behavioural variability. The influence of innateness on rates of evolution is moderate. The frequency distributions of I and h2 are unimodal, there is no evidence for an innate-acquired dichotomy.

Choice, optimal foraging, and the delay-reduction hypothesis

Behaving organisms are continually choosing. Recently the theoretical and empirical study of decision making by behavioral ecologists and experimental psychologists have converged in the area of foraging, particularly food acquisition. This convergence has raised the interdisciplinary question of whether principles that have emerged from the study of decision making in the operant conditioning laboratory are consistent with decision making in naturally occurring foraging. One such principle, the “parameter-free delay-reduction hypothesis, ” developed in studies of choice in the operant conditioning laboratory, states that the effectiveness of a stimulus as a reinforcer may be predicted most accurately by calculating the decrease in time to food presentation correlated with the onset of the stimulus, relative to the length of time to food presentation measured from the onset of the preceding stimulus. Since foraging involves choice, the delay-reduction hypothesis may be extended to predict aspects of foraging. We discuss the strategy of assessing parameters of foraging with operant laboratory analogues to foraging. We then compare the predictions of the delay-reduction hypothesis with those of optimal foraging theory, developed by behavioral ecologists, showing that, with two exceptions, the two positions make comparable predictions. The delay-reduction hypothesis is also compared to several contemporary pscyhological accounts of choice. Results from several of our experiments with pigeons, designed as operant conditioning simulations of foraging, have shown the following: The more time subjects spend searching for or traveling between potential food sources, the less selective they become, that is, the more likely they are to accept the less preferred outcome; increasing time spent procuring (“handling”) food increases selectivity; how often the preferred outcome is available has a greater effect on choice then how often the less preferred outcome is available; subjects maximize reinforcement whether it is the rate, amount, or probability of reinforcement that is varied; there are no significant differences between subjects performing under different types of deprivation (open vs. closed economies). These results are all consistent with the delay-reduction hypothesis. Moreover, they suggest that the technology of the operant conditioning laboratory may have fruitful application in the study of foraging, and, in doing so, they underscore the importance of an interdisciplinary approach to behavior.

Genetic approaches to molecular and cellular cognition: a focus on LTP and learning and memory

Long-term potentiation (LTP) is the predominant experimental model for the synaptic plasticity mechanisms thought to underlie learning and memory. This review is focused on the contributions of genetics to the understanding of the role of LTP in learning and memory. These studies have used a combination of genetics, molecular biology, neurophysiology, and psychology to demonstrate that molecular mechanisms of synaptic plasticity are critical for learning and memory. Because of the large scope of this literature, we focus primarily on genetic studies of hippocampal-dependent learning. Altogether, these findings not only demonstrate a role for plasticity in learning, they also lay down the foundations for the new field of molecular and cellular cognition.

Behavioural and physiological characterization of inbred mouse strains: prospects for elucidating the molecular mechanisms of mammalian learning and memory

With the advent of recombinant DNA methodology, it has become possible to dissect the molecular mechanisms of complex traits, including brain function and behaviour. The increasing amount of available information on the genomes of mammalian organisms, including our own, has facilitated this research. The present review focuses on a somewhat neglected area of genetics, one that involves the study of inbred mouse strains. It is argued that the use of inbred mice is complementary to transgenic approaches in the analysis of molecular mechanisms of complex traits. Whereas transgenic technology allows one to manipulate a single gene and investigate the in vivo effects of highly specific, artificially induced mutations, the study of inbred mouse strains should shed light on the roles of naturally occurring allelic variants in brain function and behaviour. Systematic characterization of the behavioural, electrophysiological, neurochemical, and neuroanatomical properties of a large number of inbred strains is required to elucidate mechanisms of mammalian brain function and behaviour. In essence, a 'mouse phenome' project is needed, entailing the construction of databases to investigate possible causal relationships amongst the phenotypical characteristics. This review focuses on electrophysiological and behavioural characterization of mouse strains. Nevertheless, it is emphasized that the full potential of the analysis of inbred mouse strains may be attained if techniques of numerous disciplines, including gene expression profiling, biochemical analysis, and quantitative trait loci (QTL) mapping, to name but a few, are also included.

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.

Central monoamine activity in genetically distinct strains of mice following a psychogenic stressor: effects of predator exposure

The effects of psychogenic stressors, rat exposure and fox urine odor, on central monoamine functioning was assessed in two inbred strains of mice, BALB/cByJ and C57BL/6ByJ, thought to be differentially reactive to stressors. These stressors markedly increased NE utilization, as reflected by MHPG accumulation, in the locus coeruleus, hippocampus, prefrontal cortex and central amygdala. Likewise, the 5-HT metabolite, 5-HIAA, was elevated in hippocampus, prefrontal cortex and central amygdala, and to some extent DOPAC accumulation was increased in the prefrontal cortex. In most brain regions, the neurochemical effects of the stressors were comparable in the two mouse strains. However, central amygdala 5-HIAA elevations as well as DOPAC increases in the prefrontal cortex elicited by fox odor were greater in C57BL/6ByJ than in BALB/cByJ mice. Although BALB/cByJ mice are more behaviorally reactive than C57BL/6ByJ mice, and also show greater corticosterone elevations in response to neurogenic and systemic stressors, it was previously shown that differential corticosterone changes were not elicited by a predator exposure. Taken together with earlier findings, it appears that despite greater behavioral reactivity/anxiety, the strain-specific neurochemical changes elicited may be situation-specific such that the profile apparent in response to neurogenic and systemic stressors may not be evident in response to predator-related threats.

Genetic differences in response to novelty and spatial memory using a two-trial recognition task in mice

A two-trial memory task, based on a free-choice exploration paradigm in a Y-maze, was previously developed to study recognition processes in Sprague-Dawley rats. Because this paradigm avoids the use of electric shock or deprivation that may have nonspecific effects and does not require learning of a rule, it may be particularly useful for studying memory in mice. Four inbred strains (Balb/cByJ, DBA/2J, C57BL/6J, and SJL/J), an F1 hybrid (C57BL/6 x SJL/J), and one outbred strain (CD1) were used to validate this task in mice and to characterize a strain distribution in response to novelty and working memory. Exploration was measured with a short (2 min) intertrial interval (ITI) between acquisition and retrieval, while memory was examined with longer intervals (30 min, 1 h, and 2 h). A study of the time course of the response to novelty revealed varying degrees of preference and/or habituation to novelty among the different strains, with CD1 exhibiting a very high response to novelty and others showing lower (C57 x SJL hybrids) to complete absence (SJL) of exploration of novelty. Memory span, assessed with increasing ITIs, varied widely among strains from 30 min (C57 x SJL hybrids) to at least 2 h (C57 and BALB). Such demonstrated sensitivity to a wide range of behavioral phenotypes supports the use of this spatial memory task as an effective tool for the study of genetic influences on the response to novelty and recognition processes in mice.

A behavioral and neuroanatomical assessment of an inbred substrain of 129 mice with behavioral comparisons to C57BL/6J mice

The inbred 129 substrains have been characterized as poor learners that display hypoplasia of the corpus callosum. However, they are used extensively as a source of embryonic stem (ES) cells for creating mice carrying altered copies of a targeted gene ('knockout mice'). The present research investigated callosal agenesis and behavior in the 129/SvEvTac substrain and compared their behavior to that of C57BL/6J mice. In addition, the degree to which callosal agenesis affected behavior was assessed. Nearly 80% of 129/SvEvTac mice in the current sample exhibited callosal hypoplasia, although this was not subsequently found to be associated with any measure of cognition. They learned the Morris maze and a non-spatial pattern discrimination task, though at a level inferior to C57BL/6J mice. They were unable to learn shuttlebox avoidance or the Lashley III maze. The only measure on which they performed better than C57BL/6J mice was a simple water escape task. Thus, 129/SvEvTac mice, in addition to displaying aberrant neuroanatomy, perform poorly on many behavioral tasks, resulting in potential interpretational difficulties.

Assessment of learning by the Morris water task and fear conditioning in inbred mouse strains and F1 hybrids: implications of genetic background for single gene mutations and quantitative trait loci analyses

Genetic methods including the creation of transgenic or null mutant models and mapping studies using quantitative trait loci strategies can be used to identify candidate genes in mice that regulate learning processes. Interpretations as to the impact of single gene mutations for polygenic behaviours like learning will depend in part on the genetic background of the animals used for these manipulations. To address the issue of genetic variability, 12 inbred strains and seven different F1 hybrids were tested on multiple behavioural tasks, including two complex learning paradigms: the Morris water task and fear conditioning. Strain differences were found for all variables measured. In the hidden platform version of the Morris task, the albino animals performed poorly while overall the F1 hybrids showed the best selectivity for the trained quadrant as measured in a probe trial. In contrast, almost all genotypes performed well on the contextual fear conditioning task and learned to associate the test context with the pairing of a foot shock and auditory stimulus as demonstrated 24 h after training by increased freezing in the test environment compared to an altered context. Significant genetic correlations were obtained for behavioural measures suggesting that the same genes regulate various aspects of performance on behavioural tasks. Scores from these multiple inbred strains and F1 hybrids provide a baseline level of learning ability for fear conditioning and the Morris water task. The results of the present study confirm the importance of genetic background in the performance of various learning tasks. This variability should be considered when developing new transgenic or null mutant animal models.

Behavioural performance in three substrains of mouse strain 129

Recently, the possibility has been raised that the behavioural abnormalities seen in null-mutant mice might be determined by their genetic background rather than by loss of gene function, especially when the 129 mouse strain is used as supplier for embryonic stem (ES) cells. To examine this issue we tested three 129 mouse substrains (129/J, 129/Ola, 129/Sv-ter/+) and C57BL/6 (B6) in the Morris water maze, the open field, the plus maze and two tests assessing motor co-ordination. We identified only for the 129/J substrain substantial behavioural deficits. These mice are albinos and carry the pink-eyed dilution allele and differed in their basal anxiety level as assessed in the open-field test. They were severely impaired in spatial learning and memory (Morris water maze test), in the Porsolt swim test, which also measures learning and in motor co-ordination. However, the 129/J substrain has not been used as ES cell donor in null-mutant mice where behavioural abnormalities were observed. Instead, mice from 129/Ola and 129/Sv-ter/+ substrains have been commonly used as suppliers for ES cells. These performed normally in most of the tests, including Morris water maze test.

A proposed test battery and constellations of specific behavioral paradigms to investigate the behavioral phenotypes of transgenic and knockout mice

Behavioral phenotyping of transgenic and knockout mice requires rigorous, formal analyses. Well-characterized paradigms can be chosen from the established behavioral neuroscience literature. This review describes (1) a series of neurological and neuropsychological tests which are effectively used as a first screen for behavioral abnormalities in mutant mice, and (2) a series of specific behavioral paradigms, clustered by category. Included are multiple paradigms for each category, including learning and memory, feeding, analgesia, aggression, anxiety, depression, schizophrenia, and drug abuse models. Examples are given from the experiences of the authors, in applying these experimental designs to transgenic and knockout mice. Extensive references for each behavioral paradigm are provided, to allow new investigators to access the relevant literature on behavioral methodology.

Tyrosine hydroxylase expression within Balb/C and C57black/6 mouse locus coeruleus. II. Quantitative study of the enzyme level

The tyrosine hydroxylase phenotype expression was further investigated in the perikarya and pericoerulean areas of the locus coeruleus of two pure inbred mouse strains, Balb/C and C57Black/6, which the topological organization and phenotypic plasticity of the enzyme-containing cell population were previously studied. The tyrosine hydroxylase level and the mean protein quantity provided by each cell were significantly higher within the spaces delimited by the enzyme containing perikarya in the C57Black/6 strain as compared to the Balb/C strain. Three days after RU24722 administration, tyrosine hydroxylase tissue concentration and quantity were significantly increased in both strains. Two strain-dependant mechanisms of this pharmacologically induced protein modulation were demonstrated: the mean tyrosine hydroxylase quantity provided by each cell was increased in the C57Black/6 strain whereas the increase was obviously explained by the subset of additional tyrosine hydroxylase expressing cells previously reported in Balb/C strain. A comparable volume of pericoerulean immunolabeled neuropile, which contains a similar tyrosine hydroxylase level, was measured between the two strains. Within this tissue compartment, an undissociated RU24722 responsiveness was observed between the two strains: a significant increase in the protein level was measured principally resulting from a significant increase in the volume. These results revealed a strain-dependent difference in the response to the RU24722 treatment which may result from a genetic separation of two kinds of tyrosine hydroxylase phenotypic regulations within the perikarya area of the locus coeruleus; whereas the surrounding neuropile seemed to have a different mechanism of the phenotypic protein expression and modulation.

The use of null mutant mice to study complex learning and memory processes

A number of neural substrates have been proposed to mediate complex learning and memory processes in mammalian organisms. One strategy for testing the involvement of a particular gene in learning and memory is to create a mouse line with a null mutation in that gene. Recently, embryonic stem cell-based gene-targeted homologous recombination techniques have been employed to create a number of such mutant mouse lines that do not express interesting candidate genes. These animals have been examined for impairments in several complex learning paradigms which are known to depend on the integrity of the hippocampus. In this review several complex learning and memory paradigms are described, the techniques to create null mutants are reviewed, and the results of recent studies with null mutants are described. Finally, the limitations for interpretation of behavioral data using null mutants are discussed.

The behavior of the homozygous and heterozygous sub-types of rats which are genetically-selected for diabetes insipidus: a comparison with Long Evans and Wistar stocks

Several aspects of spontaneous and conditioned behavior (food and water intake, locomotion and emotionality, passive and active avoidance acquisition and retention) of standard (albino and pigmented) rats, and rats heterozygous (HEDI) and homozygous (HODI) for diabetes insipidus, are reviewed. As would be expected, HODI rats have been repeatedly found to consume far more fluid than either HEDI or control rats. Pigmented rats appear to be more active than albinos. HODI rats exhibit less marked emotional responses than do control rats, among which the pigmented ones exhibit the highest emotionality. Light aversion is more evident in albino than in pigmented rats. No differences are found among HEDI, HODI and normal Long Evans rats. It is quite difficult to provide a clear-cut statement concerning inter-strain differences in passive avoidance behavior, possibly because of the variety of techniques employed. In any case, HODI rats do not perform worse than normal controls do. In one-way active avoidance paradigms, pigmented rats perform better than albinos, and the performance of HODI rats does not differ from that of controls. In two-way avoidance paradigms, albinos appear to outperform pigmented rats. Once again, there are no obvious differences between HODI and control animals. In addition to indicating that HODI rats may actually be less emotional than the other groups of rats reviewed here, the studies described once again fail to confirm the previously alleged functions of vasopressin in memory consolidation.

Aversive conditioning of homozygous and heterozygous D.I. Brattleboro rats in the light-dark box

Active and passive avoidance, and conditioned freezing acquisition and retention were studied in HODI and HEDI Brattleboro rats. All animals were from the same source and of the same age and sex. The light-dark box test was employed. 0.6 and 2.0 mA footshocks were administered for the same number (7) of daily trials. Extinction time-course was followed for seven consecutive daily trials. Passive avoidance: the conditioned response was acquired and retained equally well by all Ss and for both shock intensities. Active avoidance: for 0.6 mA shocks HODI Ss acquired and retained the response significantly better than HEDI Ss; for 2.0 mA shocks the response was acquired equally by both groups of Ss, and retained significantly better by HODI Ss. Freezing: in general, HODI Ss exhibited less freezing then HEDI Ss. The diverse conditioned behavior of HODI and HEDI Ss in this paradigm, which allows the contemporaneous investigation of several aversive responses, does not support the hypothesis that vasopressin deficiency impairs learning and memory in the rat.

Differences in acquisition of discrete lever-press and shuttle avoidance responses in 6 strains of mice

Characteristics of the acquisition processes of discrete lever-press and shuttle avoidance responses (intertrial interval = 25 sec, warning duration = 5 sec with an escape contingency, and 1 session = 1 hr training/day) as well as those of the ambulatory activity were investigated in 6 strains of mice (dd, ICR, BALB/c, C57BL/6, C3H/He and DBA/2). In the lever-press avoidance situation, the dd, BALB/c and DBA/2 strains demonstrated a good avoidance response, showing average avoidance rates of higher than 80% within 10-15 sessions of the training. The C3H/He also demonstrated a good avoidance response, though 20 sessions or more of the training were required to attain to the level. The ICR and C57BL/6 strains demonstrated a poorer avoidance response than the other 4 strains, and achieved average avoidance rates of 30-40%. The dd, BALB/c, C3H/He and DBA/2 strains also demonstrated a good shuttle avoidance response when they were trained in an experimental box of 50 cm in width. In contrast, the ICR and C57BL/6 strains demonstrated an extremely poor avoidance response in this situation, showing an average avoidance rate of less than 10%. However, when the dd, ICR and C57BL/6 strains were trained in a shuttle box of 30 cm in width, they rapidly acquired the avoidance response, and the average avoidance rates finally achieved were much higher than those of the mice trained in the shuttle box of 50 cm in width. In particular, the C57BL/6 strain exhibited the best avoidance response among the 3 strains. The ambulatory activity of the C57BL/6 strain was less than those of the other 5 strains.