Diurnal rhythms in rat brain large neutral amino acids (LNAAs), monoamines and monoamine metabolites

To obtain reliable information on the diurnal rhythms in rat brain large neutral amino acid (LNAA) concentrations, animals were killed, in groups of six, every one h throughout one 24 h period. By using such short sampling intervals, it was possible to detect a statistically significant diurnal variation in the whole brain concentration of each LNAA and to give a more detailed description of these diurnal variations than previously presented. Furthermore, the diurnal variations in whole brain concentrations of the precursor amino acids, tyrosine and tryptophan, were compared with corresponding variations in the respective monoamines and their main metabolites. Similar diurnal patterns were demonstrated for tryptophan, serotonin and 5-hydroxyindoleacetic acid. No such correspondence was detected for tyrosine, dopamine, 3,4-dihydroxyphenylacetic acid and homovanillic acid.