Effect of TAAR1 knockout on behavioural characteristics of mice in the forced swim test and in the elevated plus maze test

The research was aimed to investigate the functional role of the TAAR1 receptor, one of the representatives of trace amine-associated receptors family (TAARs). Studies describing the behavioural profile of TAAR1-KO animals are few, sometimes contradictory, and insufficiently comprehensive in covering all aspects of behaviour. In recent years, studies using various behavioural tests have increasingly revealed changes in some aspects of behaviour in genetically modified animals. Data on differences between TAAR1-KO and WT in models of depressive-like behaviour in the forced swim test (Porsolt test) are lacking. Therefore, we decided to study the behaviour of TAAR1-KO mice in the elevated plus maze - the level of motor and exploratory behaviour, the anxiety, and depressive-like behaviour (behavioural despair) in the Porsolt test. The study was conducted on male TAAR1-KO mice (n = 10), with male wild type (WT) mice (n = 9) used as controls, with an average weight of 27 ± 0.9 g. The initial lines for TAAR1-KO and WT were 129S1/Sv and C57BL/6 mouse. The animals were obtained from the Vivarium of the Scientific Park of St. Petersburg State University at the age of 3-4 months. All animals were kept under standard conditions with access to food and water ad libitum, and a 12-hour light/dark cycle was maintained in the room. The animals were housed in individual boxes (30 cm x 15 cm x 17 cm). All experiments were conducted in accordance with international standards for conducting animal biomedical research. The experimental protocol was approved by the Ethics Committee of the Biological Faculty of St. Petersburg State University (No. 131-03-2 dated February 17, 2022). To study the anxiety, locomotor and exploratory activity, the elevated plus maze setup was used. The following parameters were recorded: the time the animal spent in the open arms, the total distance traveled, the distance in the open arms, the number of entries into the open arms, the number of rearing and head dipping from the open arms, grooming behaviour (number, latency, and duration), and the number of fecal boluses and urinations. For each animal, the experiment lasted for 5 minutes. The level of depressive-like behaviour in animals was assessed in the Porsolt test, which was conducted 2 days after testing in the elevated plus maze. The testing time was 6 minutes. The following parameters were recorded: the duration of immobility and the number of immobility episodes. A mouse was considered immobile when floating and/or making only necessary small movements with one paw to keep the balance of its body or to keep its head above the water. Additionally, the latency to first immobility was estimated as the time of start to the first bout attempt to immobility lasting longer than 1 s, unless stated otherwise. Each behavioural test was conducted during one day on all animals from 1 p.m. to 6 p.m. The studied behaviour in both experiments was recorded on a video camera and webcam. Since the distributions of the studied variables have a number of features that exclude their normality (pronounced asymmetry of distribution and proximity of mean values to the boundary of the range of definition), non-parametric Mann-Whitney U-test was used for statistical analysis for independent samples. The critical value of the significance level was taken as a = 0.05. Results of the study showed that in the elevated plus maze test, levels of motor and exploratory activity, and also anxiety levels, were identical in both groups of mice (TAAR1-KO and WT) (see Table 1). However, TAAR1-KO mice exhibited increased emotional reactivity, as evidenced by a significant increase in the number of vegetative reactions (urination and fecal boluses) during testing in the elevated plus maze. Analysis of TAAR1-KO mice behaviour in the forced swim test revealed a significant increase in immobility duration and number of immobility episodes during the last 4 minutes of the test compared to the WT group, indicating an enhancement of depressive-like behaviour (see Figure 1). Although the baseline level of motor and exploratory activity, as well as anxiety levels, were identical in both groups of mice under conditions of variable stress, certain changes in animal activity and anxiety were observed in TAAR1-KO mice when testing conditions or animal states (such as aging) were varied. In our study, TAAR1-KO mice exhibited a noticeable delay in their first attempts to escape from the cylinder compared to WT mice. This resulted in WT mice showing longer immobility during the first minute of the test. However, as TAAR1-KO mice began to demonstrate depressive-like passive behaviour, the situation changed and they consistently exceeded WT mice in immobility duration and number of immobility episodes during the rest of the test. Our study is the first to demonstrate the increased emotional reactivity exhibited by TAAR1-KO mice in the elevated plus maze test. Thus, in the elevated plus maze test, no statistically significant differences were found in indicators of anxiety, motor and exploratory activity between TAAR1-KO and WT mice. When testing in the elevated plus maze, significant differences were found in indicators reflecting autonomic reactions - the number of boluses and urinations. In the forced swimming test, TAAR1-KO mice showed a statistically significant increase in the immobility duration and an increase in the number of immobility episodes compared to the WT mice group. The article contains 1 Figures, 1 Tables and 27 References. We thank the Vivarium of the Science Park of St. Petersburg State University for providing mice. The Authors declare no conflict of interest.

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