Aims: 

Melatonin supplementation in drinking water is a common method to administer exogenous melatonin. However, caution must be taken so that the possible unpleasantness of the melatonin solution will not interfere with normal feeding behaviour, especially water intake.

Methods: 

Standard two-bottle tests were conducted on eight 200 g male Wistar rats during eight weeks for 0 (group W), 0.1 (group M01), 1 (group M1), and 5 (group M5) mg/kgbw melatonin solutions including 0.01% ethanol. After one week acclimation, each animal was offered ad libitum water intake from two identical bottles A and B including 0.01% ethanol each. Each bottle position in the cage was exchanged daily. Two weeks later, bottle B was replaced by 0.1 mg/kgbw melatonin, to be followed the next two weeks by 1 mg/kg BW solution. During the last two weeks period melatonin concentration in bottle B was close to 5 mg/kgbw. Rat feeding behaviour was daily monitored by a drinking/feeding monitor (TSE systems GmbH, Bad Homburg, Germany).

Results: 

Preference scores for bottle A were 0.4960.028 (W), 0.5180.024 (M01), 0.4870.023 (M1), and 0.5690.021 (M5). Daily food intake was significantly increased in M5 (0.3210.009 vs. 0.2350.012 g/24h/bw^0.75). Average meal size was significantly depressed in M5 (0.0170.001 vs. 0.0230.001 g/bw^0.75), as it was for total fluid intake (0.3640.011 vs. 0.5430.014 ml/24h/BW^0.75).

Conclusions: 

We conclude that melatonin solutions up to 1 mg/kg BW are behaviourally safe to be used in rat supplementation studies.