Prolonged Exposure of Mice to a Nest Box Reduces Locomotor Activity in the Plus-Maze Test
Environmental enrichment (EE) has been associated with many effects on the behavior of laboratory animals. The term EE is rather vague, often referring to a variety of item combinations as if what is added to the cage has no significance. EE is indeed housing refinement, and therefore more exact terms should be used to clarify the situation. This study was designed to assess whether access to a nest box (NB) could modify behavior of BALB/c mice in the plus-maze test. Two series of experiments were done with an aspen NB (11 x 11 x 7 cm, wall thickness 1.5 cm, two round holes (d = 3 cm) at opposite sides. Control mice had no added item in the cage. The plus-maze consisted of two open (8 x 17 cm) and two closed arms (8 x 17 x 30 cm) connected by a central platform (8 x 8 cm). Mice were placed on the central platform facing an open arm. During five minutes, the numbers of entries made onto the open and into the closed arms were recorded. From this data, the percentages of entries made onto the open arms, and the percentage of time spent on the open arms, were calculated. Furthermore, the number of fecal boli left by the mice in the plusmaze, as a stress indicator, were counted. In the first series of experiments NB was present for one, two and three weeks but no drugs were administered. NB provided for one or two weeks had no effect on the behavior of mice. However, exposure to NB for three weeks did decrease the locomotor activity of mice in the plus-maze test, as reflected in the decline in the total number of entries made in the test. The presence of NB for one or two weeks resulted in more (p = 0.001) fecal boli voided when compared to the no NB or NB for three weeks groups.
In the second series of experiments we used NB for 10 days and the selective neuronal nitric oxide synthase (nNOS) inhibitor 1-(2-trifluoromethylphenyl)-imidazole (TRIM) as a pharmacological tool (at doses of 25.0, 50.0 and 100.0 mg/kg, i.p.). Depending on the dose, the administration of TRIM induced an anxiolytic (50 mg/kg) or sedative effect (100 mg/kg) as seen in the increase in the percentage of entries made onto the open arms or a decrease in the total number of entries, respectively. NB for 10 days had no effect on the behavior of mice or on the effect of TRIM. In conclusion, NB does not appear to interfere with the anxiolytic effect of TRIM in the plus-maze test but prolonged exposure to NB does reduce the locomotor activity of mice.