All drugs were intraperitoneally (i.p.) administered in a total volume of 10.0 mL/kg (body weight). Doses are expressed as milligrams per kilogram of mouse body weight. Melatonin (MEL) (Sigma-Aldrich Corp., St. Louis, MO, USA) was dissolved in the minimum amount of absolute ethanol and then in isotonic (0.9%) saline solution. Ethanol concentration in that solution was 0.006%, so the vehicle (VEH) tested had that same amount of ethanol. Fluoxetine (FLX) and imipramine (IMI) (Sigma-Aldrich Corp., St. Louis, MO, USA) were dissolved in isotonic saline solution. Experiment series comprised independent groups of 8 animals for each specified drug treatment and behavioral test.
Single administration at ZT11 was as follows: Either MEL or its VEH were administered at ZT11 (1 h before the lights were off), and behavioral tests were assessed 7.5 h later (ZT18.5).
Single administration at ZT18 was as follows: Drugs were administered at ZT18 (the middle of the dark phase), 30 min before the behavioral tests.
Triple scheme administration (for the FST only) was as follows: First administration of the drugs was at 24.5 h before the test (ZT18; immediately after the pre-test session); the second injection was applied one hour before lights were off (ZT11; 7.5 h before the test); finally, the third administration was at ZT18, 30 min before the test [39 (link)].
Single administration at ZT11 was as follows: Either MEL or its VEH were administered at ZT11 (1 h before the lights were off), and behavioral tests were assessed 7.5 h later (ZT18.5).
Single administration at ZT18 was as follows: Drugs were administered at ZT18 (the middle of the dark phase), 30 min before the behavioral tests.
Triple scheme administration (for the FST only) was as follows: First administration of the drugs was at 24.5 h before the test (ZT18; immediately after the pre-test session); the second injection was applied one hour before lights were off (ZT11; 7.5 h before the test); finally, the third administration was at ZT18, 30 min before the test [39 (link)].