INFRADIAN RHYTHMS OF DEPRESSIVE-LIKE BEHAVIOR IN RATS
Authors: Chajka A. V. 
Published: April 2022

It is customary to check the effectiveness of antidepressant substances in preclinical experimental tests on key days, often weekly [1, 2], or according to the “before-after” scheme [3], without taking into account the daily dynamics of behavior. Such studies are typical, to a greater extent, for the clinic and practically have no analogues when working with laboratory animals. Understanding how specific treatments for depression affect not only quantitatively on certain days, but also qualitatively on the dynamics of symptoms of the disease in general, will help to see the big picture and better select antidepressants and predict their effectiveness.
The study of the fundamental rhythmological features of the course of depressive disorders is also an important issue because the endogenous part of the biological clock system makes it possible to predict future stress effects and to react homeostatically to them proactively, and not only reactively [4]. It has been established that the organism of animals (including humans) is characterized by rhythmic fluctuations of functional parameters, rhythmic disturbances of which can lead to a decrease in adaptive capabilities, maladjustment and the development of various diseases [5]. In the modern literature there are a number of works proving the participation of biorhythms in the pathogenesis of depression [6–8] and the presence of rhythmic changes in the manifestation of symptoms of this disease [9–11].
Much is known and written about circadian rhythms, which has contributed to a better understanding of how the physiology of living organisms’ changes over the course of twenty-four hours. It is less clear whether and how physical and mental parameters in animals change on a larger time scale [12]. It is only recently that attention has been paid to the study of not only circadian (≤ 28 h), but also longer infradian biorhythms, which is promising for the development of chronobiological approaches to the diagnosis and correction of somatic and mental diseases [13]. Moreover, most biorhythmological studies are devoted to assessing the body’s reactions to the impact of various factors of meteorological and geomagnetic nature [14], but not factors of a mental and chemical nature.
The experiments were carried out on 60 male Wistar rats weighing 200-250 g, divided into 6 equal groups (n = 10): “control 3”, “stress 2”, “haloperidol 2”, “stress + amitriptyline”, “haloperidol + amitriptyline” and “stress + haloperidol”. Also, the final biorhythmological analysis included data from previous studies, which had already been partially published earlier [15, 16]: groups (n = 10) “control 1”, “control 2”, “stress 1” and “haloperidol 1” — in order to check the reproducibility of the data and reduce the likelihood of the influence of random (atypical) fluctuations on the final conclusions.
The “stress” groups were subjected to chronic, unpredictable mild stress for 21 days, according to the previously described method [16].
Selective D2-receptor blocker haloperidol (“Haloperidol”, N. A. Semashko Moskhimpharmpreparaty, Russia) was administered for 24 days (3 days of preliminary administration to achieve a high % of blocked receptors) at a dose of 2.5 mg/kg.
The tricyclic antidepressant amitriptyline (Amitriptyline Hydrochloride, Sigma-Aldrich, USA) was administered for 21 days at a dose of 15 mg/kg.
All substances were diluted in saline and injected intraperitoneally in a volume of 0.2 ml/animal 30 minutes before testing. The control group received saline solution in the same volume. With the combined administration of several substances, a break of 30 minutes was made between injections.
Starting from the 4th day of the experiment, the behavior of the animals was examined daily in the Forsed swim test (FST), 30 minutes after the last injection. Testing duration – 3 minutes (shortened procedure) [17, 18]; the time of immobility was recorded in seconds.
The significance of differences between groups was determined using the Mann-Whitney test or Student’s t test, according to the nature of the distribution of the data. The calculations and visualization of the results were carried out in the GraphPad Prism 8. The rhythmic component in the data dynamics was determined using Fourier spectral analysis in the STATISTICA 10.
Studying the graphs of immobility dynamics in the FST caused by chronic stress and blockade of D2-like receptors, it becomes obvious that the level of depression in animals has a pronounced periodicity.
From these data, it follows that the dynamics of the immobility time in FST:
It is largely characterized by semi-weekly (2.5–3.5 days) and weekly (7 ± 3 days) rhythmic changes.
In the groups “haloperidol” and “stress + haloperidol”, in contrast to the others, there is no rhythm of 4.40 days.
In the groups “stress”, “haloperidol”, and partly “stress + haloperidol” and “stress + amitriptyline”, longer rhythms are pronounced – 5.50, 7.33 and 11.00 days.
In the groups with stress and haloperidol, there are smaller rhythms with a duration of 2.00 and 2.20 days, which are not in the control.
Stress factors can also cause phase shifts in the studied parameters [19]. Similar changes were observed in this study, both in the stress group and in the haloperidol group.
Chronic blockade of D2-like receptors changes the adaptive mechanisms of the rat organism: in the temporal dynamics of the immobility level in FST, the 4-day rhythm typical for the physiological systems of many animals disappears, but a stress-induced 2-day rhythm occurs, and a phase shift in the level of immobility compared with control animals. This characterizes the chronic blockade of D2-like receptors as a powerful stress factor causing depression-like behavior in rats, and indicates a significant contribution of the central dopaminergic system to the infradian rhythm of this behavior.