Thus whisker-driven sensory BIBF-1120 experience is required for the rapid increase in stellate cell functional connectivity at P9. We analyzed the synaptic properties of the connections detected by photostimulation, calculating three parameters:

unitary amplitude (average amplitude of the synaptic response across all trials), success rate (the fraction of presynaptic action potentials producing an EPSC), and potency (amplitude of evoked EPSCs ignoring failures) (see Supplemental Experimental Procedures). Unitary amplitude of the connections was on average small; however, the distribution showed a long tail of connections with larger amplitudes (Figure 4A). This is consistent with previous work on neocortex, including that from barrel cortex, showing that there is a small proportion of connections that are strong, whereas the majority are weak (Lefort et al., 2009, Feldmeyer et al., 1999 and Song et al., 2005). The mean unitary amplitude of the connections was on average small but did show a trend Vismodegib cell line to a gradual increase in unitary amplitude during development (Figure 4B), which was associated with an increase in the reliability of transmission (success rate; Figure S6A). In contrast, the absolute size of EPSCs remained relatively constant

during this developmental period (potency; Figure S6B). In contrast to connectivity, no rapid change in any synaptic properties was observed at any one developmental time point. In whisker-trimmed animals, unitary EPSC amplitude, success rate, and potency were very similar to stellate cells in undeprived barrels at the same age (Figure 4B and Figures S6A and S6B). Thus, unlike functional connectivity, synaptic function between stellate cells appears to change Resminostat only

gradually during development and is regulated independent of sensory experience. An inverse relationship between connectivity probability and distance between neurons has been noted at different scales within the cortex, including local microcircuits (Braitenberg and Schüz, 1998 and Holmgren et al., 2003; but see Song et al., 2005 and Lefort et al., 2009). To assess this characteristic during development of the layer 4 local circuit, we analyzed the relationship between intersoma distance and connectivity or synaptic strength. Pconnection at P4–8 exhibited a weak inverse dependence on distance, such that cells closer together had a slightly higher probability of being connected (Figure 3E). Pconnection at P9–12, however, was strongly dependent on distance with ∼5-fold increased chance of cells 20 μm apart being connected compared to those 100 μm apart. Thus, even though the axonal and dendritic arbors of the cells are rapidly growing to span larger volumes at this developmental stage (Figures 3A and 6A), the rapid increase in connectivity at P9–12 is due to a preferential increase in connections between close neighbors.