GABA acts as inhibitory neurotransmitter in the adult central nervous system but as excitatory neurotransmitter during early postnatal development. This shift in GABA's action from excitation to inhibition is caused by a decrease in intracellular chloride concentration ([Cl-](i)), which in turn is caused by changes in the relative expression levels of the K+-Cl- co-transporter (KCC2) and the Na+, K+-Cl- co-transporter (NKCC1) proteins. Previous studies have used slices containing the medullary pre-Botzinger complex (pre-BotC) to record respiration-related rhythmic activity (RRA) from the hypoglossal nucleus (12 N). The role of GABAergic transmission in the regulation of medullary BRA neonatally, however, is yet to be determined. Here, we examined how GABA and chloride co-transporters contribute to BRA during development in the 12 N where inspiratory neurons reside. We recorded extracellular BRA in medullary slices obtained from postnatal day (P) 0-7 mice. BRA was induced by soaking slices in artificial cerebrospinal fluid (aCSF) containing 8 mM-K+. Application of GABA significantly increased the frequency of BRA after P3, whereas application of a KCC2 blocker (R (+)-[(2-n-butyl-6,7-dichloro-2-cydopentyl-2,3-dihydro-1-oxo-1H-indenyl-5-yl)oxy]acetic acid (DIOA)) significantly decreased the frequency of BRA after P1. In addition, dense KCC2 immunolabeling was seen in the superior longitudinalis (SL) of the 12 N, which is responsible for retraction of the tongue, from PO and P7. These results indicate that GABA administration can increase BRA frequency during the first week following birth. This in turn suggests that decreasing [Cl-](i) levels caused by increasing KCC2 levels in the 12 N could play important roles in regulating the frequency of BRA during development. (C) 2015 Elsevier B.V. All rights reserved.