In the hippocampus,
the labeling of all O-LM cell axons revealed their striking subcellular specificity. These axons form a prominent band in stratum lacunosum molecular, which contains the apical tufts of pyramidal neurons with a razor-sharp boarder with stratum radiatum ( Figure 5A). The SST-ires-Cre driver provides the first robust in vivo system for manipulating SST interneurons to discover their function and the mechanisms underlying the subcellular GSK1210151A ic50 specificity of their axons. Using the Ai9 reporter ( Madisen et al., 2010), we imaged cortical SST neurons in live mice with synaptic resolution using 2-photon microscopy ( Figure S3, Movie S1). This experimental system allows “online” identification of SST interneurons during in vivo physiology and imaging experiments, and longitudinal studies of their development and plasticity. SST is highly expressed in developing cortical GABA neurons beginning by mid-gestation (Batista-Brito et al., 2008). Since reporter expression remains restricted to SST neurons in the mature cortex, this indicates that Cre activity in SST-ires-Cre driver is specific to the SST population
throughout development. SST neurons can be labeled as early as E13, soon after they exit the SVZ of MGE ( Figures 5E and 5F). They reach the developing cortex by E14 and mainly migrate in the marginal zone and subventricular zone ( Figures 5E–5G). By P0, migrating SST neurons in layer 1 appear to have Selleckchem Ku 0059436 extended axons and some have entered the cortex trailed by vertically oriental neurites ( Figures 5H and 5I). By P5, most SST cells are in the cortex, and layer 1 axons are already prominent with conspicuous synaptic boutons ( Figures 5J and 5K). Therefore, the SST-ires-Cre
driver provides an experimental system to examine the developmental history of SST neurons, including their PAK6 migration, subcellular synapse targeting, and maturation. In the SST-CreER line, tamoxifen-induced recombination is also restricted to SST neurons but the efficiency is very low as assayed with both the RCE ( Figures 5C and 5D) and Ai9 reporters (see Discussion). The SST-CreER driver allows imaging and reconstruction of single cortical SST interneurons ( Figures 5C and 5D). Both SST-ires-Cre and SST-CreER drivers are also active in many other brain regions including: olfactory bulb, striatum, reticular nucleus of the thalamus, superior colliculus, brainstem, as well as in stripes of cerebellar Purkinje cells ( Figure S3; Table 2). Inhibition directed toward the soma and proximal dendrites of pyramidal neurons controls the gain of summed inputs and thereby the spike discharge. It also regulates the phasing and synchronization of neural ensembles (Freund and Katona, 2007). Perisomatic inhibition is mediated by two broad classes of interneurons that express either the calcium-binding protein parvalbumin (PV) or the neuropeptide cholecystokinin (CCK) (Figure 6A).