Jiun Youn

Rodent models in neuroscience research: is it a rat race?

ABSTRACT Rodents (especially Mus musculus and Rattus norvegicus) have been the most widely used models in biomedical research for many years. A notable shift has taken place over the last two decades, with mice taking a more and more prominent role in biomedical science compared to rats. This shift was primarily instigated by the availability of a much larger genetic toolbox for mice, particularly embryonic-stem-cell-based targeting technology for gene disruption. With the recent emergence of tools for altering the rat genome, notably genome-editing technologies, the technological gap between the two organisms is closing, and it is becoming more important to consider the physiological, anatomical, biochemical and pharmacological differences between rats and mice when choosing the right model system for a specific biological question. The aim of this short review and accompanying poster is to highlight some of the most important differences, and to discuss their impact on studies of human diseases, with a special focus on neuropsychiatric disorders. Summary: In this At a Glance poster review, Ellenbroek and Youn describe important differences between rats and mice that impact on their use as model organisms for brain disorders.

Bidirectional modulation of classical fear conditioning in mice by 5-HT(1A) receptor ligands with contrasting intrinsic activities

5-HT(1A) receptors are implicated in the modulation of cognitive processes including encoding of fear learning. However, their exact role has remained unclear due to contrasting contributions of pre- vs. postsynaptic 5-HT(1A) receptors. Therefore, their role in fear conditioning was studied in mice using the selective ligand S15535, which fully activates 5-HT(1A) autoreceptors, yet only weakly stimulates their postsynaptic counterparts. The effects of S15535 were compared to those of the full agonist 8-OH-DPAT and the selective antagonist NAD-299. 8-OH-DPAT dose-dependently (0.01-0.5 mg/kg) and markedly impaired both context- and tone-dependent fear conditioning, as determined by complementary measures of inactivity and freezing. 8-OH-DPAT-mediated impairments were blocked by pre-injection of the selective 5-HT(1A) antagonist WAY100635. S15535 (0.01-5.0 mg/kg) mimicked 8-OH-DPAT in predominantly impairing conditioned contextual fear, though with smaller effect size than 8-OH-DPAT, consistent with lower efficacy at postsynaptic 5-HT(1A) receptors. Furthermore, S15535 (1.0 mg/kg) tended to attenuate the impairment of fear conditioning by 8-OH-DPAT (0.3 mg/kg). In contrast, NAD-299 (0.3 and 1 mg/kg) facilitated contextual freezing. WAY100635 (0.3 mg/kg) prevented the impairment of contextual fear by S15535 (1 and 5 mg/kg), underpinning the role of 5-HT(1A) receptors in the actions of S15535. Collectively, these data indicate that 5-HT(1A) receptor ligands modulate fear conditioning in a bidirectional manner: activation of postsynaptic 5-HT(1A) sites exerts an inhibitory influence, whereas their blockade promote facilitation of fear conditioning. The results with S15535 underscore the importance of ligand efficacy in determining the actions of 5-HT(1A) receptor ligands in fear conditioning and other models of cognitive function.

Finding the right motivation: Genotype-dependent differences in effective reinforcements for spatial learning

Memory impairments of DBA/2J mice have been frequently reported in spatial and emotional behavior tests. However, in some memory tests involving food reward, DBA/2J mice perform equally well to C57BL/6J mice or even outperform them. Thus, it is conceivable that motivational factors differentially affect cognitive performance of different mouse strains. Therefore, spatial memory of DBA/2J and C57BL/6J mice was investigated in a modified version of the Barnes maze (mBM) test with increased complexity. The modified Barnes maze test allowed using either aversive or appetitive reinforcement, but with identical spatial cues and motor requirements. Both mouse strains acquired spatial learning in mBM tests with either reinforcement. However, DBA/2J mice learned slower than C57BL/6J mice when aversive reinforcement was used. In contrast, the two strains performed equally well when appetitive reinforcement was used. The superior performance in C57BL/6J mice in the aversive version of the mBM test was accompanied by a more frequent use of the spatial strategy. In the appetitive version of the mBM test, both strains used the spatial strategy to a similar extent. The present results demonstrate that the cognitive performance of mice depends heavily on motivational factors. Our findings underscore the importance of an effective experimental design when assessing spatial memory and challenges interpretations of impaired hippocampal function in DBA/2J mice drawn on the basis of behavior tests depending on aversive reinforcement.

The role of the dopamine D1 receptor in social cognition: studies using a novel genetic rat model­

ABSTRACT Social cognition is an endophenotype that is impaired in schizophrenia and several other (comorbid) psychiatric disorders. One of the modulators of social cognition is dopamine, but its role is not clear. The effects of dopamine are mediated through dopamine receptors, including the dopamine D1 receptor (Drd1). Because current Drd1 receptor agonists are not Drd1 selective, pharmacological tools are not sufficient to delineate the role of the Drd1. Here, we describe a novel rat model with a genetic mutation in Drd1 in which we measured basic behavioural phenotypes and social cognition. The I116S mutation was predicted to render the receptor less stable. In line with this computational prediction, this Drd1 mutation led to a decreased transmembrane insertion of Drd1, whereas Drd1 expression, as measured by Drd1 mRNA levels, remained unaffected. Owing to decreased transmembrane Drd1 insertion, the mutant rats displayed normal basic motoric and neurological parameters, as well as locomotor activity and anxiety-like behaviour. However, measures of social cognition like social interaction, scent marking, pup ultrasonic vocalizations and sociability, were strongly reduced in the mutant rats. This profile of the Drd1 mutant rat offers the field of neuroscience a novel genetic rat model to study a series of psychiatric disorders including schizophrenia, autism, depression, bipolar disorder and drug addiction. Summary: Rats with a novel dopamine D1 receptor mutation, which causes reduced membrane Drd1 expression, display impaired social cognition, although their exploratory and anxiety-like behaviours are unaffected.

Central 5‐HT1A receptor‐mediated modulation of heart rate dynamics and its adjustment by conditioned and unconditioned fear in mice

The beat‐by‐beat fluctuation (dynamics) of heart rate (HR) depends on centrally mediated control of the autonomic nervous system (ANS) reflecting the physiological state of an organism. 5‐HT1A receptors are implicated in affective disorders,associated with ANS dysregulation which increases cardiac risk but their role in autonomic HR regulation under physiological conditions is insufficiently characterized.

Cardiovascular Conditioning: Neural Substrates

The detailed knowledge of the neural substrates underlying cardiovascular adjustments, in particular to learned (conditioned) and innate (unconditioned) threatening conditions, is important for the understanding of the brain-heart interaction. Commonly, various aversive stimuli are used to investigate fear-induced autonomic adjustments. In general, the observed strong autonomic response to any stressor results in parasympathetic withdrawal and concomitant sympathetic activation, resulting in profound heart rate and blood pressure increase in many species. An improved knowledge of the functional role of specific molecular signals in defined brain areas of the central autonomic network will contribute to a better understanding of mechanisms contributing to central autonomic dysregulation. This is particularly important for various affective disorders that show high comorbidity with cardiovascular disease potentially due to autonomic dysregulation. Thereby, emotionally challenging conditions can induce pathological states increasing the risk for cardiac mortality including sudden cardiac death.

Does Prenatal Valproate Interact with a Genetic Reduction in the Serotonin Transporter? A Rat Study on Anxiety and Cognition

There is ample evidence that prenatal exposure to valproate (or valproic acid, VPA) enhances the risk of developing Autism Spectrum Disorders (ASD). In line with this, a single injection of VPA induces a multitude of ASD-like symptoms in animals, such as rats and mice. However, there is equally strong evidence that genetic factors contribute significantly to the risk of ASD and indeed, like most other psychiatric disorders, ASD is now generally thought to results from an interaction between genetic and environmental factors. Given that VPA significantly impacts on the serotonergic system, and serotonin has strong biochemical and genetic links to ASD, we aimed to investigate the interaction between genetic reduction in the serotonin transporter and prenatal valproate administration. More specifically, we exposed both wildtype (SERT+/+) rats and rats heterozygous for the serotonin transporter deletion (SERT+/−) to a single injection of 400 mg/kg VPA at gestational day (GD) 12. The offspring, in adulthood, was assessed in four different tests: Elevated Plus Maze and Novelty Suppressed Feeding as measures for anxiety and prepulse inhibition (PPI) and latent inhibition as measures for cognition and information processing. The results show that prenatal VPA significantly increased anxiety in both paradigm, reduced PPI and reduced conditioning in the latent inhibition paradigm. However, we failed to find a significant gene–environment interaction. We propose that this may be related to the timing of the VPA injection and suggest that whereas GD12 might be optimal for affecting normal rat, rats with a genetically compromised serotonergic system may be more sensitive to VPA at earlier time points during gestation. Overall our data are the first to investigate gene * environmental interactions in a genetic rat model for ASD and suggest that timing may be of crucial importance to the long-term outcome.

The Genetic Basis of Behavior

In this chapter we will describe the general genetic principles. Starting with the description of the molecular structure of DNA, we will explain how DNA can self-replicate and thus transfer the genetic information from one cell to the next. The genetic information coded in “genes” is transferred from DNA via RNA to proteins, in a process known as the “Central Dogma”. We will see that the traditional idea that each gene codes for one protein is no longer valid. Especially in vertebrates including humans, most genes consist of exons (protein coding) and introns (nonprotein coding) elements, and through the process of alternative splicing, a gene can lead to many different proteins (so-called isoforms). Although the process of DNA duplication is very stable and through many different repair processes it is in general faithfully replicated, alterations can occur. In the remainder of this chapter we will describe the most important genetic alterations ranging from very subtle single nucleotide variants (in which a single nucleotide is changed), to duplications of very large parts of a chromosome, to even duplications of entire chromosomes (such as in trisomies).

Conclusions and the Road Ahead

In this chapter, we summarize the major findings of the previous chapters. We will discuss why so far studies have failed to identify genes with a major impact on one specific psychiatric disorder. In addition, we will provide a framework of how the field can move forward. Realizing that psychiatric disorders on the one hand, are notoriously heterogeneous and, on the other hand, show significant comorbidity and overlap in symptomatology, it seems important to focus our attention on symptoms or endophenotypes. Several of those have already been identified and more have been suggested. Thus we propose to focus on studying genetic and environmental factors underlying such endophenotypes and developing procedures that can be used both in humans and rodents with similar technologies. Together it is hoped that this will move the field forward more rapidly and will ultimately improve the prognosis of patients with a psychiatric disorder.

The Environmental Basis of Behavior

In the chapter we specifically look at the effect that environmental changes can have on behavior. We start off by describing how studies of family (preferably from several generations) can help us determine whether genetic factors play a role in the trait investigated. As we shall see, as soon as more than one single gene is involved, it becomes more difficult to deduce the exact mode of inheritance. Nonetheless, family studies have been and continue to be very important, especially twin and adoption studies which allow us to separate (to a certain degree at least) the genetic and environmental factors.