Feedforward inhibitory GABAergic transmission is critical for mature cortical circuit function; in the neonate, however, GABA is depolarizing and believed to have a different role. Here we show that the GABAA receptor–mediated conductance is depolarizing in excitatory (stellate) cells in neonatal (postnatal day [P]3–5) layer IV barrel cortex, but GABAergic transmission at this age is not engaged by thalamocortical input in the feedforward circuit and has no detectable circuit function. However, recruitment occurs at P6–7 as a result of coordinated increases in thalamic drive to fast-spiking interneurons, fast-spiking interneuron–stellate cell connectivity and hyperpolarization of the GABAA receptor–mediated response. Thus, GABAergic circuits are not engaged by thalamocortical input in the neonate, but are poised for a remarkably coordinated development of feedforward inhibition at the end of the first postnatal week, which has profound effects on circuit function at this critical time in development.
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We are grateful to K. Roche, K. Pelkey and J. Diamond for comments on the manuscript, and to C. McBain for comments on the work and providing access to the GIN mice. We thank Z. Molnar for the use of his drawing of the thalamocortical slice. This work was supported by the Wellcome Trust and the National Institute of Neurological Disorders and Stroke Intramural Program.
The authors declare no competing financial interests.
Non-FS interneurons are distinct from fast-spiking interneurons and are inhibitory. (PDF 97 kb)
Non-FS interneurons receive a weak thalamic input compared with simultaneously recorded stellate cells or with fast-spiking interneurons at the same developmental stage (all at P7–9). (PDF 81 kb)
Electrophysiological properties of fast-spiking interneurons and stellate cells at different ages between P3 and P9. (PDF 108 kb)
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