Coping style and stress hormone responses in genetically heterogeneous rats: comparison with the Roman rat strains

QTL mapping in outbred populations: successes and challenges

Quantitative trait locus (QTL) mapping in animal populations has been a successful strategy for identifying genomic regions that play a role in complex diseases and traits. When conducted in an F2 intercross or backcross population, the resulting QTL is frequently large, often encompassing 30 Mb or more and containing hundreds of genes. To narrow the locus and identify candidate genes, additional strategies are needed. Congenic strains have proven useful but work less well when there are multiple tightly linked loci, frequently resulting in loss of phenotype. As an alternative, we discuss the use of highly recombinant outbred models for directly fine-mapping QTL to only a few megabases. We discuss the use of several currently available models such as the advanced intercross (AI), heterogeneous stocks (HS), the diversity outbred (DO), and commercially available outbred stocks (CO). Once a QTL has been fine-mapped, founder sequence and expression QTL mapping can be used to identify candidate genes. In this regard, the large number of alleles found in outbred stocks can be leveraged to identify causative genes and variants. We end this review by discussing some important statistical considerations when analyzing outbred populations. Fine-resolution mapping in outbred models, coupled with full genome sequence, has already led to the identification of several underlying causative genes for many complex traits and diseases. These resources will likely lead to additional successes in the coming years.

Nucleus accumbens core lesions enhance two-way active avoidance

The majority of work examining the nucleus accumbens core (NAc) has focused on functions pertaining to behaviors guided by appetitive outcomes. These studies have pointed to the NAc as being critical for motivating behavior toward desirable outcomes. For example, we have recently shown that lesions of the NAc impaired performance on a reward-guided decision-making task that required rats to choose between differently valued rewards. Unfortunately, much less is known about the role that the NAc plays in motivating behavior when aversive outcomes are predicted. To address this issue we asked if NAc lesions impact performance on a two-way active avoidance task in which rats must learn to shuttle back and forth in a behavioral training box in order to avoid a footshock predicted by an auditory tone. Although bilateral NAc lesions initially impaired reward-guided decision-making, we found that the same lesions improved acquisition and retention of two-way active avoidance.

Gene expression in hippocampus as a function of differential trait anxiety levels in genetically heterogeneous NIH-HS rats

To identify genes involved in the development/expression of anxiety/fear, we analyzed the gene expression profile in the hippocampus of genetically heterogeneous NIH-HS rats. The NIH-HS rat stock is a unique genetic resource for the fine mapping of quantitative trait loci (QTLs) to very small genomic regions, due to the high amount of genetic recombinants accumulated along more than 50 breeding generations, and for the same reason it can be expected that those genetically heterogeneous rats should be especially useful for studying differential gene expression as a function of anxiety, fearfulness or other complex traits. We selected high- and low-anxious NIH-HS rats according to the number of avoidance responses they performed in a single 50-trial session of the two-way active avoidance task. Rats were also tested in unconditioned anxiety/fearfulness tests, i.e. the elevated zero-maze and a "novel-cage activity" test. Three weeks after behavioral testing, the hippocampus was dissected and prepared for the microarray study. There appeared 29 down-regulated and 37 up-regulated SNC-related genes (fold-change>|2.19|, FDR<0.05) in the "Low-anxious" vs. the "High-anxious" group. Regression analyses (stepwise) revealed that differential expression of some genes could be predictive of anxiety/fear responses. Among those genes for which the present results suggest a link with individual differences in trait anxiety, nine relevant genes (Avpr1b, Accn3, Cd74, Ltb, Nrg2, Oprdl1, Slc10a4, Slc5a7 and RT1-EC12), tested for validation through qRT-PCR, have either neuroendocrinological or neuroinmunological/inflammation-related functions, or have been related with the hippocampal cholinergic system, while some of them have also been involved in the modulation of anxiety or stress-related (neurobiological and behavioral) responses (i.e. Avpr1b, Oprdl1). The present work confirms the usefulness of NIH-HS rats as a good animal model for research on the neurogenetic basis or mechanisms involved in anxiety and/or fear, and suggest that some MHC-(neuroinmunological/inflammation)-related pathways, as well as the cholinergic system within the hippocampus, may play a role in shaping individual differences in trait anxiety.

Rats are the smart choice: Rationale for a renewed focus on rats in behavioral genetics

Due in part to their rich behavioral repertoire rats have been widely used in behavioral studies of drug abuse-related traits for decades. However, the mouse became the model of choice for researchers exploring the genetic underpinnings of addiction after the first mouse study was published demonstrating the capability of engineering the mouse genome through embryonic stem cell technology. The sequencing of the mouse genome and more recent re-sequencing of numerous inbred mouse strains have further cemented the status of mice as the premier mammalian organism for genetic studies. As a result, many of the behavioral paradigms initially developed and optimized for rats have been adapted to mice. However, numerous complex and interesting drug abuse-related behaviors that can be studied in rats are very difficult or impossible to adapt for use in mice, impeding the genetic dissection of those traits. Now, technological advances have removed many of the historical limitations of genetic studies in rats. For instance, the rat genome has been sequenced and many inbred rat strains are now being re-sequenced and outbred rat stocks are being used to fine-map QTLs. In addition, it is now possible to create "knockout" rats using zinc finger nucleases (ZFN), transcription activator-like effector nucleases (TALENs) and related techniques. Thus, rats can now be used to perform quantitative genetic studies of sophisticated behaviors that have been difficult or impossible to study in mice. This article is part of a Special Issue entitled 'NIDA 40th Anniversary Issue'.

Gene expression in amygdala as a function of differential trait anxiety levels in genetically heterogeneous NIH-HS rats

To identify genes involved in anxiety/fear traits, we analyzed the gene expression profile in the amygdala of genetically heterogeneous NIH-HS rats. The NIH-HS rat stock has revealed to be a unique genetic resource for the fine mapping of Quantitative Trait Loci (QTLs) to very small genomic regions, due to the high amount of genetic recombinants accumulated along more than 50 breeding generations, and for the same reason it can be expected that those genetically heterogeneous rats should be especially useful for studying differential gene expression as a function of anxiety-(or other)-related traits. We selected high- and low-anxious NIH-HS rats differing in their number of avoidances in a single 50-trial session of the two-way active avoidance task. Rats were also tested in unconditioned anxiety tests (e.g., elevated zero-maze). Three weeks after behavioural testing, the amygdala was dissected and prepared for the microarray study. There appeared 6 significantly down-regulated and 28 up-regulated genes (fold-change >|2|, FDR<0.05) between the low- and high-anxious groups, with central nervous system-related functions. Regression analyses (stepwise) revealed that differential expression of some genes could be predictive of anxiety/fear responses. Among those genes for which the present results suggest a link with individual differences in trait anxiety, six relevant genes were examined with qRT-PCR, four of which (Ucn3, Tacr3, H2-M9 and Arr3) were validated. Remarkably, some of them are characterized by sharing known functions related with hormonal HPA-axis responses to (and/or modulation of) stress, anxiety or fear, and putative involvement in related neurobehavioural functions. The results confirm the usefulness of NIH-HS rats as a good animal model for research on the neurogenetic basis of anxiety and fear, while suggesting the involvement of some neuropeptide/neuroendocrine pathways on the development of differential anxiety profiles.

Behavioral and neurogenomic transcriptome changes in wild-derived zebrafish with fluoxetine treatment

Background

Stress and anxiety-related behaviors are seen in many organisms. Studies have shown that in humans and other animals, treatment with selective serotonin reuptake inhibitors (e.g. fluoxetine) can reduce anxiety and anxiety-related behaviors. The efficacies and side effects, however, can vary between individuals. Fluoxetine can modulate anxiety in a stereospecific manner or with equal efficacy regardless of stereoisomer depending on the mechanism of action (e.g. serotonergic or GABAergic effects). Zebrafish are an emerging and valuable translational model for understanding human health related issues such as anxiety. In this study we present data showing the behavioral and whole brain transcriptome changes with fluoxetine treatment in wild-derived zebrafish and suggest additional molecular mechanisms of this widely-prescribed drug.

Results

We used automated behavioral analyses to assess the effects of racemic and stereoisomeric fluoxetine on male wild-derived zebrafish. Both racemic and the individual isomers of fluoxetine reduced anxiety-related behaviors relative to controls and we did not observe stereospecific fluoxetine effects. Using RNA-sequencing of the whole brain, we identified 411 genes showing differential expression with racemic fluoxetine treatment. Several neuropeptides (neuropeptide Y, isotocin, urocortin 3, prolactin) showed consistent expression patterns with the alleviation of stress and anxiety when anxiety-related behavior was reduced with fluoxetine treatment. With gene ontology and KEGG pathway analyses, we identified lipid and amino acid metabolic processes, and steroid biosynthesis among other terms to be over-enriched.

Conclusion

Our results demonstrate that fluoxetine reduces anxiety-related behaviors in wild-derived zebrafish and alters their neurogenomic state. We identify two biological processes, lipid and amino acid metabolic synthesis that characterize differences in the fluoxetine treated fish. Fluoxetine may be acting on several different molecular pathways to reduce anxiety-related behaviors in wild-derived zebrafish. This study provides data that could help identify common molecular mechanisms of fluoxetine action across animal taxa.