Serotonergic Phase Shifts of the Mammalian Circadian clock: Effects of Tetrodotoxin and High Mg²⁺

The mammalian circadian clock in the suprachiasmatic nuclei (SCN) can be phase-shifted in vitro by the serotonin agonist quipazine. Here we show that quipazine resets the SCN clock in the presence of tetrodotoxin or 10 mM Mg²⁺, treatments that block Na⁺ action potentials and Ca²⁺ channels, respectively. These results support the hypothesis that quipazine resets the clock by stimulating receptors located on clock elements rather than on cells afferent to the clock.