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.

Environmental enrichment influences novelty reactivity, novelty preference, and anxiety via distinct genetic mechanisms in C57BL/6J and DBA/2J mice.

Environmental factors such as stress drive the development of drug addiction in genetically vulnerable individuals; the genes underlying this vulnerability are unknown. One strategy for uncovering these genes is to study the impact of environmental manipulation on high-throughput phenotypes that predict drug use and addiction-like behaviors. In the present study, we assessed the viability of this approach by evaluating the relative effects of environmental enrichment and isolation housing on three high-throughput phenotypes known to predict variation on distinct aspects of intravenous drug self-administration. Prior to behavioral testing, male and female C57BL/6J and DBA/2J mice (BXD founders) were housed in enrichment or isolation for ten weeks beginning at weaning. Enrichment significantly reduced novelty reactivity; this effect was significantly more robust in C57BL/6J mice relative to DBA/2J mice. Enrichment significantly reduced novelty preference; this effect was significantly dependent on novel environment characteristics and was significantly more robust in DBA/2J mice relative to C57BL/6J mice. Enrichment significantly increased anxiety; this effect was not strain-dependent. Collectively, these data indicate that (1) environmental enrichment influences novelty reactivity, novelty preference, and anxiety via distinct genetic mechanisms in mice, and (2) the BXD panel can be used to discover the genetic and epigenetic mechanisms underlying this phenomenon.

A 1-night operant learning task without food-restriction differentiates among mouse strains in an automated home-cage environment.

Individuals are able to change their behavior based on its consequences, a process involving instrumental learning. Studying instrumental learning in mice can provide new insights in this elementary aspect of cognition. Conventional appetitive operant learning tasks that facilitate the study of this form of learning in mice, as well as more complex operant paradigms, require labor-intensive handling and food deprivation to motivate the animals. Here, we describe a 1-night operant learning protocol that exploits the advantages of automated home-cage testing and circumvents the interfering effects of food restriction. The task builds on behavior that is part of the spontaneous exploratory repertoire during the days before the task. We compared the behavior of C57BL/6J, BALB/cJ and DBA/2J mice and found various differences in behavior during this task, but no differences in learning curves. BALB/cJ mice showed the largest instrumental learning response, providing a superior dynamic range and statistical power to study instrumental learning by using this protocol. Insights gained with this home-cage-based learning protocol without food restriction will be valuable for the development of other, more complex, cognitive tasks in automated home-cages.

Mouse genetic differences in voluntary wheel running, adult hippocampal neurogenesis and learning on the multi-strain-adapted plus water maze.

Moderate levels of aerobic exercise broadly enhance cognition throughout the lifespan. One hypothesized contributing mechanism is increased adult hippocampal neurogenesis. Recently, we measured the effects of voluntary wheel running on adult hippocampal neurogenesis in 12 different mouse strains, and found increased neurogenesis in all strains, ranging from 2- to 5-fold depending on the strain. The purpose of this study was to determine the extent to which increased neurogenesis from wheel running is associated with enhanced performance on the water maze for 5 of the 12 strains, chosen based on their levels of neurogenesis observed in the previous study (C57BL/6 J, 129S1/SvImJ, B6129SF1/J, DBA/2 J, and B6D2F1/J). Mice were housed with or without a running wheels for 30 days then tested for learning and memory on the plus water maze, adapted for multiple strains, and rotarod test of motor performance. The first 10 days, animals were injected with BrdU to label dividing cells. After behavioral testing animals were euthanized to measure adult hippocampal neurogenesis using standard methods. Levels of neurogenesis depended on strain but all mice had a similar increase in neurogenesis in response to exercise. All mice acquired the water maze but performance depended on strain. Exercise improved water maze performance in all strains to a similar degree. Rotarod performance depended on strain. Exercise improved rotarod performance only in DBA/2 J and B6D2F1/J mice. Taken together, results demonstrate that despite different levels of neurogenesis, memory performance and motor coordination in these mouse strains, all strains have the capacity to increase neurogenesis and improve learning on the water maze through voluntary wheel running.

Environmental enrichment does not disrupt standardisation of animal experiments.

Environmental enrichment can prevent abnormal behaviours and improve the well-being of laboratory mice, but concerns have been raised that it might disrupt the standardisation of experiments. Based on a multi-laboratory study, I show that animal welfare can be improved by environmental enrichment without disrupting standardisation.

Alterations in hippocampal GAP-43 phosphorylation and protein level following contextual fear conditioning.

C57BL/6 (B6) mice display better contextual learning than the DBA/2 (D2) mice. The possibility that GAP-43, is differentially affected as a function of strain and learning was investigated in the present study. No basal difference between C57BL/6J (B6) and DBA/2J (D2) mice in the amount of hippocampal GAP-43 was observed, but naive D2 mice have slightly lower basal levels of GAP-43 phosphorylation than do B6 mice. Interestingly, alterations in hippocampal GAP-43 protein levels and phosphorylation state in response to training for contextual learning were observed only in B6 mice. Immediate-shocked mice, serving as nonlearning controls, showed no GAP-43 alterations, nor did D2 mice subjected to either training condition. These results suggest that modulation of hippocampal GAP-43 may be important for contextual learning and that strain-specific alterations in GAP-43 may be part of a disrupted pathway in D2 mice that is essential for learning.

Strain and sex differences on olfactory discrimination learning in C57BL/6J and DBA/2J inbred mice (Mus musculus).

In this study, the authors explored potential strain and sex differences in nonspatial cognitive ability. Beginning around 90 days of age, male and female C57BL/6J (C57) and DBA/2J (DBA) inbred mice (Mus musculus) were tested on a task of simple odor discrimination learning with 3 repeated reversals. Males learned the task more readily than females, and DBA mice learned the task more readily than C57 mice. All differences became evident after repeated testing. Similarity of perseveration measures indicated the differences were not due to inhibitory deficits. Instead, a phase analysis localized differences to a transitional period of reversal learning. Females increased transitional errors that more likely indicated adaptive sampling strategies than memory failures. C57 females used this strategy indiscriminately, but DBA females sampled as a function of environmental uncertainty.

Nesting material and number of females per cage: effects on mouse productivity in BALB/c, C57BL/6J, DBA/2 and NIH/S mice.

Two different materials-aspen wood-wool and paper towel-were compared as nesting material for three inbred mouse strains (BALB/c, C57BL/6J and DBA/2) housed in barrier conditions. In addition, the effect of varying the number of females per cage (one to three per cage) of these three strains and with NIH/S outbred mouse stock was studied. The number of litters, litter size and neonatal mortality were determined, as well as age, sex and weight of weanlings. The type of nesting material did not affect the characteristics monitored. In all strains, the number of weanlings per female was greatest in singly-housed females. In terms of the number of weanlings per cage, two females per cage gave the best result. In DBA/2 mice, neonatal mortality increased when several females were caged together.

The cage design affects intermale aggression in small groups of male laboratory mice: strain specific consequences on social organization, and endocrine activations in two inbred strains (DBA/2J and CBA/J).

Effects of the enrichment of conventional laboratory housing cages with an additional labyrinth on intermale aggression, social organization, and activations of the pituitary-adrenocortical and sympathetic-adrenomedullary neuroendocrine subsystems are compared between adult male DBA/2J and CBA/J mice, differing genetically in their intermale aggression. Mice of both strains were kept three per cage for six weeks either in standard laboratory cages (SC) or in enriched cages (EC). Intermale aggression against a strange intruder and between cagemates was monitored once a week within each group. Dominance relations were derived from the distribution of attacks within a group. Plasma corticosterone titers (PCT) and activities of the tyrosinehydroxylase (TH) and phenylethanolamine N-methyltransferase (PNMT) were determined for each mouse at the end of the study. The main findings were: 1. Intermale aggression increased in both strains in groups kept in EC. 2. In the more aggressive DBA/2J the pattern of social organization shifted from groups with a single permanent dominant mouse in SC to groups with a frequently changing dominant mouse in EC. 3. In CBA interchanges of the dominant mouse was prevailing and did not differ between the two housing conditions. 4. In DBA/2J mice PCT were significantly elevated in EC. 5. In CBA/J mice activities of TH and PNMT were significantly elevated in EC. 6. Body weight gain was significantly delayed in mice kept in EC in both strains. Findings revealed strainspecific environmental effects on both social organization and endocrine states. The PCT increase obtained only in DBA/2J indicate that alterations of the social relations between cagemates are more likely to induce states of stress than an increase in intermale aggression alone.