Differential effects of amino acids on the period of the circadian rhythm from the Aplysia eye.

The effect of continuous treatments of single L-amino acids (0.1mM) on the free running rhythm from the isolated Aplysia eye was examined. A variation in the change in free running period produced by different amino acids was observed. Two well-known precursors of neurotransmitters (tyrosine, tryptophan) had the largest effects. These amino acids lengthened the period ca. 1.7 h. Another group of amino acids (alanine, threonine, proline) lengthened the period by about 1 h. Smaller effects were produced by aspartic acid and leucine and no effects were caused by lysine, glycine, valine, and serine. Phenylalanine may shorten the period a small amount. Glucose (5mM) lengthens the period a small amount (0.4 h), decreases the effect of tyrosine somewhat, and has no effect on the lengthening of the period produced by tryptophan. Three amino acids not involved in protein synthesis (ornithine, beta-alanine, citrulline) had at most small effects on the free running period. Also, D-tryptophan lengthened the period by 0.6 h but the effect of D-tryptophan was considerably smaller than the effect of L-tryptophan. A few of the amino acids had small short-term effects on spike rate and longer-term effects on the amplitudes of the rhythms but these effects did not correlate with the effects of the amino acids on the free running period. Though continuous treatments of certain amino acids lengthened the periods, shorter treatments (tryptophan, 6 h) did not phase-shift the rhythm. Since eyes maintained in a commonly used culture medium have longer periods than eyes in a simple seawater medium, the amino acids of the culture medium must be responsible, at least in part, for the lengthening effect of the culture medium. The mechanism of action of the amino acids is unknown. The magnitude of the effects did not correlate with physical-chemical properties of the amino acids nor with whether the amino acids were "essential" or "nonessential." The effects of the amino acids may be mediated by their effects on neurotransmitter and/or protein synthesis.