Melatonin is the most important hormone regulating the rhythm of sleep-wake cycles; it has the essential amino acid tryptophan as precursor (Dajk & Lockley 2002). The aim of this study was to determine the possible correlation of tryptophan adminstration with circadian activity-inactivity rhythms as well as with body temperature and weight.
The birds, ring dove (Streptopelia roseogrisea; n = 10 per experimental group) with diurnal habit, were maintained under a 12 h (08.00-20.00 h) light (3000 lux) and 12 h dark cycle and handled according to the quidelines of European Community Council Directives 86/6091 EEC. Tryptophan was administered orally at doses of 125 mg or 300 mg L-tryptophan (kg b.w.)-1 in single adminstration at 09.00 or 19.00 h. The anal temperature and the weignt of animals were measured at 10.00 h. The activity was recorded in pulses with one actometer per animal (two perpendicular infrared transmitters with a frequency of 4866 Hz). These activity pulses were logged every 5 min by a computer program (Das 16) throughout the experiment. The result are expressed as a comparison of the mean ± standard deviation (S.D.) activity between 10 animals admimistered tryptophan versus 10 control animals. Data were compared with Student’s unpaired t test. Values of P ²le³ 0.05 were considered significant.
The result showed that the animals administered tryptophan at 19.00 h presented a decline in their activity during the period of light at both the 125 mg dose (20.01 ± 5.10 vs. 23.71 ± 6.18 control values) and the 300 mg dose (15.14 ± 1.62 vs. 20.37 ± 3.02). When the adminstration was at 09.00 h, the 300 mg dose group of animals presented a decline in their mesor (mean activity of the circadian rhythm; 8.37 ± 4.18 vs. 11.30 ± 6.64 control values). Tryptophan administration caused no significant changes in either the weight or the temperature relative to the controls. We can therefore conclude that the circadian activity-inactivity rhythms can be modified by tryptophan in the diet, and that this amino acid (the precursor of melatonin) could affect sleep by diminishing activity, if the administration is made at the precise moment.
The authors thank Elena Circujano for technical assistance. This research was supported by the Ministerio de Ciencia y Tecnología AGL 2000-0182-P4-03.