Loss of microRNAs in pyramidal neurons leads to specific changes in inhibitory synaptic transmission in the prefrontal cortex.

MicroRNAs (miRNAs) are critical regulators of nervous system function, and in vivo knockout studies have demonstrated that miRNAs are necessary for multiple aspects of neuronal development and survival. However, the role of miRNA biogenesis in the formation and function of synapses in the cerebral cortex is only minimally understood. Here, we have generated and characterized a mouse line with a conditional neuronal deletion of Dgcr8, a miRNA biogenesis protein predicted to process miRNAs exclusively. Loss of Dgcr8 in pyramidal neurons of the cortex results in a non-cell-autonomous reduction in parvalbumin interneurons in the prefrontal cortex, accompanied by a severe deficit in inhibitory synaptic transmission and a corresponding reduction of inhibitory synapses. Together, these results suggest a vital role for miRNAs in governing essential aspects of inhibitory transmission and interneuron development in the mammalian nervous system. These results may be relevant to human diseases such as schizophrenia, where both altered Dgcr8 levels as well as aberrant inhibitory transmission in the prefrontal cortex have been postulated to contribute to the pathophysiology of the disease.

Pathway-specific genetic attenuation of glutamate release alters select features of competition-based visual circuit refinement.

A hallmark of mammalian neural circuit development is the refinement of initially imprecise connections by competitive activity-dependent processes. In the developing visual system retinal ganglion cell (RGC) axons from the two eyes undergo activity-dependent competition for territory in the dorsal lateral geniculate nucleus (dLGN). The direct contributions of synaptic transmission to this process, however, remain unclear. We used a genetic approach to reduce glutamate release selectively from ipsilateral-projecting RGCs and found that their release-deficient axons failed to exclude competing axons from the ipsilateral eye territory in the dLGN. Nevertheless, the release-deficient axons consolidated and maintained their normal amount of dLGN territory, even in the face of fully active competing axons. These results show that during visual circuit refinement glutamatergic transmission plays a direct role in excluding competing axons from inappropriate target regions, but they argue that consolidation and maintenance of axonal territory are largely insensitive to alterations in synaptic activity levels.