Synaptogenesis of electrical and GABAergic synapses of fast-spiking inhibitory neurons in the neocortex

Abstract

Parvalbumin-expressing fast-spiking (FS) cells are interconnected via GAB Aergic and electrical synapses and represent a major class of inhibitory interneurons in the neocortex. Synaptic connections among FS cells are critical for regulating network oscillations in the mature neocortex. However, it is unclear whether synaptic connections among FS inter neurons also play a central role in the generation of patterned neuronal activity in the immature brain, which is thought to underlie the formation of neocortical circuits. Here, we investigated the developmental time course of synaptogenesis of FS cell in mouse visual cortex. In layer 5/6 (L5/6), we recorded from two or three FS and/orpyramidal (PYR)neurons to study the development of electrical and chemical synaptic interactions from postnatal day 3 (P3)toP18.Wedetectednoevidence for functional connectivity for FS-FS or FS-PYR pairsatP3-P4. However,byP5-P6,wefound that 20% of FS pairs were electrically coupled, and 24% of pairs were connected via GAB Aergic synapses; by P15-P18, 42% of FS pairs had established functional electrical synapses, and 47% of FS pairs were connected via GAB Aergic synapses. FS cell GAB Aergic inhibition of pyramidal cells showed a similar developmental time line, but no electrical coupling was detected for FS-PYR pairs. We found that synaptogenesis of electrical and GABAergic connections of FS cells takes place in the same period. Together, our results suggest that chemical and electrical connections among FS cells can contribute to patterned neocortical activity only by the end of the first postnatal week. © 2011 the authors.