In the manuscript by Osuna-Lopez et al., the study focused on GABAergic currents in developing SCN, recording chloride reversal potential (ECl), and examining the Na-K-Cl cotransporter 1 (NKCC1) expression in rat pups (postnatal days 3-25) during day and night in ventral and dorsal SCN. The authors observed that ECl greatly varied depending on age, SCN region, and circadian time (day vs. night). They also found that ECl displayed more hyperpolarization with age, in the dorsal SCN, and during the night, and an increased NKCC1 expression was perceived during the daytime and at an early age (P10). The authors argue that NKCC1 regulates the circadian and developmental fluctuations in the [Cl-]i to fine-tune ECl, which is crucial for either excitatory or inhibitory GABAergic actions that occur in the SCN.
The article concept is interesting, and the manuscript is well-written. However, there are a few issues that the authors should address for publication. Therefore, the authors are encouraged to address the following points when revising their manuscript.

1) The authors used sodium pentobarbital to anesthetize rats when they made slice preparations. However, it is well-known that sodium pentobarbital affects the sensitivity of GABA in the brain. The authors need to state the justification for using sodium pentobarbital anesthesia for the experiments. Indeed, the authors may need to show the effects of sodium pentobarbital on their slice preparations.

2) The authors used male and female pups in their experiments. How did the authors determine the sex? Did they measure the anogenital distance? Is it completely reliable? The author should state the details about the determination of pups’ sex.

3)Abbreviations: ZT and CT are abbreviated words. Please state those originals in the first appearance.

4)Figure 2 is tough to see. It needs to be improved. Primarily, it is tough to find where statistical significance is.

5) In Figure 3B, the graphs (Figure 3C) show data for the ventral and dorsal SCN during the day and night at P10 and P25. However, there are only representative micrographs for day and night at P10 and P25, and we also do not know well which region is shown. The authors should display representative micrographs for all groups.
Also, how did the authors divide the ventral and dorsal regions? The authors should state it somewhere in the manuscript.

6) In Figure 3C, any mark (asterisk or anything) to show statistical significance (day vs. night) might be needed.