TTX blocks baclofen-induced phase shifts of the mammalian circadian pacemaker in vitro.

The mammalian circadian pacemaker, located in the suprachiasmatic nucleus (SCN), expresses 24-h rhythms when isolated in vitro. The GABA(A) agonist, muscimol, induces phase advances during the mid-subjective day, while the GABA(B) agonist, baclofen, induces both daytime phase advances and nighttime phase delays. Here, we present evidence that tetrodotoxin (TTX) completely blocks baclofen-induced phase shifts in vitro, but does not block in vitro phase advances induced by muscimol. These results suggest that GABA(A), but not GABA(B), receptors are located on SCN pacemaker cells.