Circadian Forced Desynchrony of the Master Clock Leads to Phenotypic Manifestation of Depression in Rats

Abstract

In mammals, a master circadian clock within the suprachiasmatic nucleus (SCN) of the hypothalamus maintains the phase coherence among a wide array of behavioral and physiological circadian rhythms. Affective disorders are typically associated with disruption of this fine-tuned "internal synchronization," but whether this internal misalignment is part of the physiopathology of mood disorders is not clear. To date, depressive-like behavior in animal models has been induced by methods that fail to specifically target the SCN regulation of internal synchronization as the mode to generate depression. In the rat, exposure to a 22-h light-dark cycle (LD22) leads to the uncoupling of two distinct populations of neuronal oscillators within the SCN. This genetically, neurally, and pharmacologically intact animal model represents a unique opportunity to assess the effect of a systematic challenge to the central circadian pacemaker on phenotypic manifestations of mood disorders. We show that LD22 circadian forced desynchrony in rats induces depressive-like phenotypes including anhedonia, sexual dysfunction, and increased immobility in the forced swim test (FST), as well as changes in the levels and turnover rates of monoamines within the prefrontal cortex. Desynchronized rats show increased FST immobility during the dark (active) phase but decreased immobility during the light (rest) phase, suggesting a decrease in the amplitude of the normal daily oscillation in this behavioral manifestation of depression. Our results support the notion that the prolonged internal misalignment of circadian rhythms induced by environmental challenge to the central circadian pacemaker may constitute part of the etiology of depression.

Keywords: behavior; circadian rhythms; depression; emotion; rat.

Graphical abstract

Figure 1.

Forced desynchrony of locomotor activity. Top, Double-plotted actograms for locomotor activity of representative rats maintained on symmetric 24-h (left; 12 h light, 12 h dark) or 22-h (right; 11 h light, 11 h dark) LD cycles. The gray shading indicates lights off. The black line on the LD22 plot indicates the onset of locomotor activity for the LD-dissociated bout of activity. Letters represent phases at which LD22 desynchronized animals were tested. During aligned days, animals were tested in the dark phase (A) or the light phase (B). During misaligned days, animals were also tested in the light phase (C) or the dark phase (D). Although all four phases are shown schematically in the same animal, each animal was tested only once at one of these four phases. Bottom, Periodograms of the time series represented in each actogram. Analysis revealed a single statistically significant peak for the LD24 animal and two statistically significant peaks for the LD22 animal. The numbers above the peaks indicate the period of the significant peaks in hours.

Figure 2.

The effect of forced desynchrony on body mass and locomotor activity. A, Change in mean ± SE body mass in LD24 controls (n = 22) and LD22 desynchronized (n = 47) animals throughout the experiment. B, Mean ± SE locomotor activity [% of total activity during each 24- or 22-h cycle] of LD24 control rats and LD22 desynchronized rats. Shaded gray area denotes lights off. Note that data for desynchronized animals corresponds to 11 h of light and 11 h of darkness. C, Mean ± SE locomotor activity per hour (total IR beam interruptions) averaged over either the light or dark phase for control rats, aligned rats, and misaligned rats. Different letters denote statistically significant differences between groups.

Figure 3.

Effect of circadian forced desynchrony on behavioral manifestations of depression. A, Fraction of 0.1% saccharin consumption averaged over 8 d for LD24 control, LD22 aligned, and LD22 misaligned rats. Boxes outline the lower (25%) and upper (75%) quartiles, and the line denotes the median. Error bars represent the 95% confidence intervals. Different letters denote statistically significant differences (p < 0.05). B, Fraction of time spent in the perimeters of the open field test arena, during the light and dark phases in the same groups. C, Immobility time during the 5-min FST in the light and dark phases. Different letters denote statistically significant differences (p < 0.05). All values represent mean ± SE.

Figure 4.

Effect of circadian forced desynchrony on sexual behavior of male rats during the dark phase. Percentage of LD24 control and LD22 desynchronized male rats that did not mount (A) and did not ejaculate (B) as a function of the time lapsed from the introduction of a receptive female. *p < 0.05, **p < 0.01. Log-rank values were calculated based on a Kaplan–Meier survival analysis. C, Mean ± SE copulatory rate in LD24 control and LD22 desynchronized males. Copulatory rate was calculated as the sum of mounting and intromission events divided by the time lapsed from the first mounting event until the first ejaculation event. Different letters denote statistical differences between groups (p < 0.05).

Figure 5.

The effect of circadian forced desynchrony on behavioral performance. A, A depression score (higher values indicate a depressive phenotype) for LD24 control, LD22 aligned, and LD22 misaligned rats. Boxes outline the lower (25%) and upper (75%) quartiles, and the line denotes the median. Error bars represent the 95% confidence intervals, and the black circles are outliers. Different letters denote statistically significant differences (p < 0.05). The amplitude of circadian locomotor activity predicts the severity of behavioral manifestations of depression. B, Fraction of 0.1% saccharin consumption over a 24-h period in LD24 control and LD22 desynchronized animals as a function of the amplitude of locomotor activity on the day of the test. Latency to mounting (C) and ejaculation (D) during the dark phase as a function of the amplitude of locomotor activity on the day of the test. E, Immobility in the FST during the dark phase as a function of the amplitude of locomotor activity on the day of the test, calculated as the difference between the mean activity per hour during the dark phase and the mean activity per hour during the light phase.

Figure 6.

Forced desynchronized rats show a significant increase of levels of NE and DA and of 5-HT turnover in the prefrontal cortex. A–C, Mean ± SE levels of each monoamine. A′–C′; mean ± SE turnover for each monoamine. *p < 0.05, **p < 0.01 (group effect); #p = 0.05 (phase effect). See Table 1 for statistics.