Long-Term Imaging Reveals a Circadian Rhythm of Intracellular Chloride in Neurons of the Suprachiasmatic Nucleus

Both inhibitory and excitatory GABA transmission exist in the mature suprachiasmatic nucleus (SCN), the master pacemaker of circadian physiology. Whether GABA is inhibitory or excitatory depends on the intracellular chloride concentration ([Cl − ] i ). Here, using the genetically encoded ratiometric probe Cl-Sensor, we investigated [Cl − ] i in AVP and VIP-expressing SCN neurons for several days in culture. The chloride ratio (R Cl ) demonstrated circadian rhythmicity in AVP + neurons and VIP + neurons, but was not detected in GFAP + astrocytes. R Cl peaked between ZT 7 and ZT 8 in both AVP + and VIP + neurons. R Cl rhythmicity was not dependent on the activity of several transmembrane chloride carriers, action potential generation, or the L-type voltage-gated calcium channels, but was sensitive to GABA antagonists. We conclude that [Cl − ] i is under circadian regulation in both AVP + and VIP + neurons.