| Abstract: | The relationship between the 24 h rhythm in 5-hydroxy-tryptamine (5HT) levels in rat brain, the availability of precursors of 5HT and the concentration of its major metabolite, 5-hydroxyindole acetic acid (5HIAA) has been investigated. Serum total and "free" tryptophan (TRY) levels and brain TRY levels all show a 24 h rhythm with highest concentrations in the middle of the dark phase i.e. 12 h displaced from that of the 5HT rhythm. No 24 h variation in either tryptophan-5-hydroxylase or monoamine oxidase activity was detected, nor did brain 5-hydroxytryptophan (5HTP) levels vary with clock hour. Changes in 5HIAA concentration paralleled those of 5Ht. The uptake of 14C-5HTP, 14C-TRY and 14C-5HT into homogenates of the septal region of rat brain did not display a circadian rhythm, although there was evidence that uptake of 14C-TRY in an isolated synaptosomal preparation from the same region was greater during the light phase, indicating the possibility that uptake of the precursor into the nerve ending may be, in part, responsible for the 24 h rhythm in brain 5HT. It is concluded that brain 5HT levels are independent of the serum or brain TRY concentrations measured. Since changes in 5HT with clock hour are paralleled by changes in 5HIAA, it also seems unlikely that the increase in brain 5HT during the light phase is caused by a decreased release of 5HT from nerve endings. |
