Circadian tryptophan hydroxylase levels and serotonin release in the suprachiasmatic nucleus of the rat

Abstract Serotonin (5‐HT) plays an important role in the regulation of the time‐keeping system in rodents. In the present study, we have investigated the interplay between the rhythms of 5‐HT synthesis and release in the suprachiasmatic nuclei (SCN) of the rat. The quantitative distribution of tryptophan hydroxylase (TpH) protein was used as an index of 5‐HT synthesis, in perikarya and terminals areas. In the raphe medianus, the maximal levels of TpH was reached in the early daytime period, followed by a decrease before the onset of darkness. Conversely, in the axon terminals of the SCN the highest levels of TpH were found before the onset of the dark‐period. Furthermore, TpH amount in SCN displays variations depending on the anatomical area of the SCN. Extracellular 5‐HT peaked at the beginning of the night, as evidenced by in vivo microdialysis in the SCN. The 5‐HT metabolite, 5‐HIAA, presented a similar pattern, but the acrophase occurred in the middle of the dark period. These results suggest that TpH is transported from the soma to the nerve terminals in which 5‐HT is synthesized during daytime. This would fill the intracellular stores of 5‐HT to provide for its nocturnal release.