Tryptophan is the precursor in the synthesis of serotonin (5-HT) and melatonin. It has been shown that the increase in brain tryptophan and/or in 5-hydroxytryptophan (5-HTP) causes a parallel increase in slow wave sleeping time. It is also well know that melatonin synthesis occurs only during night time and causes marked increases in motor activity in rats. Thus, two opposite effects could be expected after a meal with high tryptophan contents.

To elucidate this question, fourteen 200-240 g male Wistar rats were kept individually housed in cages equipped for activity recording and submitted to a 1 h-23 h L-D photoperiod (lights on at 08.00 h) in an isolated, thermostated (22 ± 2 °C) chamber. After acclimatizing, the animals received three different diet schedules, each during three consecutive days: (1) commercial chow (0.2 % tryptophan) for the whole day; (2) chow enriched with 2 % tryptophan from 01.00 to 13.00 h, replaced by standard chow during the following period 13.00 to 01.00 h; and (3) standard chow from 01.00 to 13.00 h and tryptophan enriched one from 13.00 to 01.00 h. The experiments were performed under approval of the Ethical Committee of the University of Balearic Islands.

When the tryptophan was administered during night time a mild increase (17 ± 3 %, mean ± S.E.M.) in activity counts was observed, while no effect was found during day time. In another experiment, rats received either L-tryptophan (300 mg kg^-1) or vehicle (methylcellulose) by oesophageal cannula either at 08.30 or at 20.30 h. After nocturnal tryptophan administration, rats showed also increased activity (45 ± 5 %, 4 h after injection, P < 0.05, ANOVA), but again no effects after diurnal one. After these experiments, tryptophan (300 mg kg^-1) was administered through an oral cannula and we used the accumulation of 5-HTP (after decarboxylase inhibition with NDS 1015, 100 mg kg^-1, I.P.) as a measure of tryptophan hydroxylation rate in the rat brain in vivo. Animals were humanely decapitated 4 h later and their brain was homogenized for 5-HTP, 5-HT and 5-hydroxyindolacetic acid (5-HIAA) electrochemical HPLC detection. A diurnal cycle in tryptophan hydroxylase activity was observed. Tryptophan induced an increase in daytime 5-HTP (103 ± 16 % P < 0.05, ANOVA-Scheffé’s test), 5-HT (78 ± 11 %, P < 0, 05) and 5HIAA (188 ± 31, P < 0.01), but this increase was much lower when administered at 20.30 h, in 5-HTP accumulation (36 %, n.s.), 5-HT content (19 %, n.s.) and 5-HAA (5 %, n.s.).

In summary, the results indicate that nocturnal tryptophan administration produces a short-term activation of motor activity in rats and this action could be mediated by melatonin.

This work was supported by a DGICYT grant AGL2000-0182-P4-03.