Abstract
BackgroundMany physiological processes in our body are controlled by the biological clock and show circadian rhythmicity. It is generally accepted that a robust rhythm is a prerequisite for optimal functioning and that a lack of rhythmicity can contribute to the pathogenesis of various diseases. Here, we tested in a heterogeneous laboratory zebrafish population whether and how variation in the rhythmicity of the biological clock is associated with the coping styles of individual animals, as assessed in a behavioural assay to reliably measure this along a continuum between proactive and reactive extremes.ResultsUsing RNA sequencing on brain samples, we demonstrated a prominent difference in the expression level of genes involved in the biological clock between proactive and reactive individuals. Subsequently, we tested whether this correlation between gene expression and coping style was due to a consistent change in the level of clock gene expression or to a phase shift or to altered amplitude of the circadian rhythm of gene expression. Our data show a remarkable individual variation in amplitude of the clock gene expression rhythms, which was also reflected in the fluctuating concentrations of melatonin and cortisol, and locomotor activity. This variation in rhythmicity showed a strong correlation with the coping style of the individual, ranging from robust rhythms with large amplitudes in proactive fish to a complete absence of rhythmicity in reactive fish. The rhythmicity of the proactive fish decreased when challenged with constant light conditions whereas the rhythmicity of reactive individuals was not altered.ConclusionThese results shed new light on the role of the biological clock by demonstrating that large variation in circadian rhythmicity of individuals may occur within populations. The observed correlation between coping style and circadian rhythmicity suggests that the level of rhythmicity forms an integral part of proactive or reactive coping styles.
Highlights
Many physiological processes in our body are controlled by the biological clock and show circadian rhythmicity
In order to further evaluate risk-taking as a proxy for coping style, we tested and confirmed a correlation between emergence rank and frequency of aggressive behaviour during a mirrorbiting test (Fig. 1c; Spearman rank, N = 59, ρ = − 0.77, p < 0.0001)
We measured, by quantitative PCR, the expression patterns of 5 genes, i.e. one member from each of the four core clock genes and one stabilising loop gene that showed diurnal rhythmicity in Diurnal rhythmicity of hormone secretion is associated with coping style Subsequently, we studied the diurnal rhythmicity of levels of cortisol and melatonin, important endocrine regulators of diurnal activity patterns [23]
Summary
Many physiological processes in our body are controlled by the biological clock and show circadian rhythmicity. Personality comprises consistent individual variation of correlated behavioural and habitual traits [1], such as extraversiveness, impulsivity or novelty-seeking [2, 3]. These personality traits are linked to the physiological and genetic makeup of an individual [4,5,6,7,8]. In animals, behavioural traits such as risk-taking or aggressiveness [9] can be consistently correlated with physiological traits such as metabolic rate [10] or endocrine stress response [11].
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