Clustering release sites in one location reduces the contribution of individual vesicles of transmitter to membrane depolarization because of the local increase VE-821 in conductance and reduction in driving force (Rall, 1970). To quantify the relative efficacy of synaptic transmission we used computational modeling to predict somatic depolarization with respect to the configuration of thalamic

inputs (Figures 1A–1C). Reconstructions of three interneurons were imported into the NEURON modeling environment (Hines and Carnevale, 1997). Sixteen sites in the dendritic arbor of each neuron were selected as synaptic loci such that the overall distribution of these loci mimicked the distribution of hotspots in our data set (see Experimental Procedures) (Figure 9A). The amplitude of the simulated synaptic conductance at each site was adjusted to produce a 5 mV somatic depolarization (Figure 9B). We then repeated this experiment using only 8, 4, 2, or 1 of the selected loci at a time, to imitate the effect of endowing each hotspot with clusters of 2, 4, 8, or 16 release sites, respectively. As expected, the total conductance required to achieve a 5 mV

somatic depolarization selleck increased with the increasing number of release sites at each locus, representing decreased efficiency of each release site as more sites are concentrated together. Noticeably, however, there was only a modest (∼15%) decrease in efficiency as up to eight release sites were clustered together; a much greater inefficiency (∼60%) occurred when all 16 modeled release sites were concentrated in one location (Figure 9C). These results thus indicate that as long as clusters do not contain more than eight release sites, the contribution of individual release sites to the overall membrane depolarization is not greatly impaired. Interestingly, the highest number

of release sites per hotspot observed here was seven (see above and Figure 4E). Active Metalloexopeptidase sodium and potassium conductances were not incorporated in this model because there are no reliable data on their distribution in cortical FS cells, and thus they would introduce significant free parameters. To evaluate whether these conductances would significantly affect dendritic summation of inputs, we used two-photon glutamate uncaging in recordings from L4 FS neurons. Glutamate was uncaged at two dendritic loci situated either on separate dendritic branches or right next to each other (<5 μm away) on one dendrite (Figure S5). In concordance with our modeling results, summation of both spatially separated and spatially clustered events was linear as long as the total event amplitude (recorded somatically) did not exceed ∼5 mV (Figure S5B).