Lower tryptophan:phenylalanine ratios in culture media increase medium: pineal melatonin ratios in early dark but not late light phase.

Pineals from male Long-Evans rats (60-65 days old; adapted to a 0700-1900 photoperiod) were cultured for 6 h either in light (1200-1800) or in dark (1800-2400). The objective was to ascertain the effects of tryptophan (trp) and phenylalanine (phe) levels and ratios in the culture medium on melatonin levels in the pineals and their respective media. Total culture (pineal + medium) melatonin levels, determined by RIA, were similar under all conditions. However, in cultures during the early dark phase (1800-2400) lower trp:phe ratios in the medium led to lower pineal:medium ratios of melatonin content. In cultures during the late light phase (1200-1800) the trp:phe ratio had little impact on the pineal:medium distribution of melatonin. Trp:phe ratio rather than absolute level of either amino acid appeared responsible for this effect. Functionally this means that during early dark phase, but not late light phase, movement of melatonin from cultured pineal to medium is progressively facilitated by lower trp:phe ratios. It remains to be determined to what extent darkness per se and/or endogenous pineal rhythmic mechanisms have a permissive role in the action of trp:phe ratio on pineal melatonin release. A melatonin compartmentalization/release effect of these or other amino acids, or their ratios, has not been reported previously and may possibly contribute to mechanisms for melatonin's transport or release at night.