Elsevier

Behavioural Brain Research

Volume 222, Issue 1, 12 September 2011, Pages 10-14
Behavioural Brain Research

Research report
Neonatal exposure to constant light prevents anhedonia-like behavior induced by constant light exposure in adulthood

https://doi.org/10.1016/j.bbr.2011.03.022Get rights and content

Abstract

Depressive episodes are associated with disturbances in circadian rhythms, and constant illumination has been reported to induce depressive-like behavior in rodents. Rats kept in constant darkness express the endogenous circadian rhythm, and most animals under constant light conditions lose circadian locomotor rhythmicity. Exposure to constant light in rats during lactation was reported to prevent this loss of circadian rhythm in adulthood. Thus, the aim of the present study was to verify whether exposure to constant light during lactation prevents anhedonia-like behavior induced by constant light in adult rats. In experiment 1, we replicated the anhedonia-like effects of constant light in adult male rats. We showed that this effect is reversed by imipramine treatment in the drinking water. In experiment 2, we subjected rats to constant darkness (neonatal-DD), constant light (neonatal-LL) or to normal light/dark cycle (neonatal-LD) during the neonatal phase and evaluated them after constant light exposure in adulthood. The group exposed to constant light during the neonatal phase did not reduce their sucrose preference and exhibited greater locomotor activity than the other groups. The neonatal-DD group exhibited decreased sucrose preference earlier than controls and had higher serum corticosterone concentrations. Prevention of arrhythymicity might protect neonatal-LL rats from anhedonia-like behavior induced by constant light, whereas constant darkness during the neonatal phase rendered the neonatal-DD group more susceptible to depressive-like behavior. These results corroborate with the literature data indicating that circadian disruption may contribute in mood disorders and that early life stress can influence stress responsivity in adulthood.

Highlights

► Constant light promoted depressive-like behavior, which was reversed by imipramine. Neonatal exposure of constant light prevented the constant light-induced anedonia. Neonatal exposure to constant darkness facilitated constant light-induced anedonia. These data reinforce the relation between mood disorders and circadian rhythms.

Introduction

Disturbances in circadian rhythm have been associated with mood disorders. Changes in mood throughout the day are a common trait in depressed patients [1] and a worse pattern in the morning is incorporated in the Diagnostic and Statistical Manual of Mental Disorders, 4th edition, text revision, criteria for the melancholic subtype of major depression [2]. Alterations in circadian rhythms, including the sleep/wake cycle, are heterogeneous in major depressive disorder but generally include high core body temperature at night, early cortisol secretion, reduced slow wave sleep, and increased and phase-advanced rapid eye movement sleep (reviewed in [3]). Core body temperature may even lack 24 h rhythmicity in some depressed patients [4]. Patients may show internal desynchronization or can be considered free-running [5], [6], [7].

In humans, depression is also associated with a reduction of light during the winter. Seasonal affective disorder occurs more frequently in countries located in high latitudes [8]. Most patients have phase delays in their circadian rhythm, but a small subgroup is phase-advanced in winter depression [9]. Accordingly, diurnal rats kept under short photoperiods or that received melatonin injections during the light phase of the light/dark cycle show depressive-like behavior in the forced swim and sucrose preference tests [10], [11]. Exposure to 1 h of bright light was shown to reverse depressive-like behavior in the forced swim test [12]. Not only diurnal rodents were shown to be affected by short periods, but also nocturnal animals, such as the Siberian hamsters [13], [14] and Wistar rats [15]. It was also observed in Wistar rats that total light deprivation for 6 weeks promotes depressive-like behavior in the forced swim test in adulthood and damages monoaminergic neurons [16].

By contrast, mice exposed for 3 weeks to constant light exhibited depressive-like behavior in the sucrose preference and forced swim tests, which was partially prevented by enriching the environment with an opaque tube that allowed the mice to escape from the light [17]. Prolonged exposure to constant light is also known to promote arrhythmicity of locomotor activity [18]. However, neonatal exposure to constant illumination was reported to prevent the loss of rhythmicity of locomotor activity in rats [19], [20].

Thus, the main objective of the present study was to investigate whether neonatal constant light exposure prevents anhedonia-like behavior induced by constant illumination in adulthood.

Section snippets

Animals and study design

Adult male and female Wistar rats were obtained from the Federal University of Paraná and kept under controlled temperature (22 ± 3 °C) and a 12 h/12 h light/dark (LD) cycle (lights on 07:00–19:00 h). Food and water were available ad libitum. All animal procedures were approved by the Ethical Committee of Animal Experiment (protocol no. 389) and in accordance with the Guide for the Care and Use of Laboratory Animals adopted by the Department of Pharmacology, Federal University of Paraná.

In experiment

LL in adulthood induces anhedonia and it is prevented by neonatal exposure to LL

In experiment 1, after 3 weeks of constant illumination, the LL group exhibited a reduction in sucrose preference compared with the LD group (U = 79, p < 0.05) and its own baseline (T = 29, p < 0.05, Fig. 1). The data were further replicated in the fourth week of constant light (U = 96, p < 0.05; T = 4, p < 0.05). Imipramine treatment reversed LL-induced anhedonia-like behavior (Fig. 2). In the LL group, imipramine-treated animals had higher sucrose preference compared with LL controls (U = 9, p < 0.05).

In

Discussion

In accordance to our initial hypothesis, animals exposed to constant light during lactation did not reduced sucrose preference when submitted to three weeks of constant light exposure in adulthood. As described previously for mice [17], we found that constant light induces depressive-like behavior in adulthood of neonatally naïve rats. Moreover, to our knowledge, we showed for the first time that imipramine treatment can reverse anhedonia-like behavior in this animal model. We also found that

Conclusion

In conclusion, our data corroborate the evidence and hypothesis regarding an important role of circadian rhythm disruption and neonatal stress in the development of mood disorders. In addition to already existing chronobiological therapeutics for depression, such as one night of sleep deprivation and bright light in the morning [35], we propose that the prevention of circadian rhythm disturbances may lower the risk of developing major depressive disorder.

Acknowledgments

D.C. and B.J.M. are recipients of student graduate fellowships from CAPES; L.H.M. is recipient of a student undergraduate fellowship from CNPq; M.L.A., F.M.L. and R.A. are recipients of research fellowships from CNPq.

References (35)

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