Opposite effects of tryptophan intake on motor activity in ring doves (diurnal) and rats (nocturnal)

The role Of L-tryptophan as precursor of serotonin and melatonin synthesis on activity-rest rhythm was studied in ring doves, Streptopelia risoria, as a representative of diurnal animals and rats, Rattus norvegicus, as a typical nocturnal one. The animals were housed in cages equipped for horizontal activity recording in a thermostatized chamber and submitted to a 12/12h light/dark photoperiod (lights on at 08:00h). After acclimatization, the animals received vehicle (methylcellulose) and L-tryptophan (240mg/kg) by esophagic cannula 2h before the onset of either light or dark phase. Also, oral melatonin (2.5mg/kg) was tested for comparative purposes. After nocturnal L-tryptophan administration, rats showed increased activity (149%), while the opposite occurred in ring doves (39% decrease). No significant changes were found after diurnal L-tryptophan intake in either species. Melatonin produced effects similar to those Of L-tryptophan. These results suggest that the effects Of L-tryptophan administration are dependent on the nocturnal/diurnal habits of the studied species and, most probably, are mediated by increased melatonin synthesis. (c) 2006 Elsevier Inc. All rights reserved.