Age-, region-, and day/night-related variation of the chloride reversal potential in the rat suprachiasmatic nucleus.

The master control of mammalian circadian rhythms is the suprachiasmatic nucleus (SCN), which is formed by the ventral and dorsal regions. In SCN neurons, GABA has an important function and even excitatory actions in adulthood. However, the physiological role of this neurotransmitter in the developing SCN is unknown. Here, we recorded GABAergic postsynaptic currents (in the perforated-patch configuration using gramicidin) to determine the chloride reversal potential (E _Cl ) and also assessed the immunological expression of the Na-K-Cl cotransporter 1 (NKCC1) at early ages of the rat (postnatal days (P) 3 to 25), during the day and night, in the two SCN regions. We detected that E _Cl greatly varied with age and depending on the SCN region and time of day. Broadly speaking, E _Cl was more hyperpolarized with age, except for the oldest age studied (P20-25) in both day and night in the ventral SCN, where it was less negative. Likewise, E _Cl was more hyperpolarized in the dorsal SCN both during the day and at night; while E _Cl was more negative at night both in the ventral and the dorsal SCN. Moreover, the total NKCC1 fluorescent expression was higher during the day than at night. These results imply that NKCC1 regulates the circadian and developmental fluctuations in the [Cl ^- ] _i to fine-tune E _Cl , which is crucial for either excitatory or inhibitory GABAergic actions to occur in the SCN.
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