The synaptic response to pairs of stimuli (PPR, see Supplemental Experimental Procedures) can be used as an indirect measure of Pr. We found that PPR is not significantly different between genotypes (Figure 3C). TSA HDAC Taken together, our results demonstrate that reduction in quantal size contributes to only a fraction of the reduced synaptic strength in −/y mice. Because Pr is not altered, we conclude that there must also be a significant decrease in the number of release sites that each RGC makes on a given relay neuron of −/y mice. This mechanism is similar to that described at autaptic hippocampal synapses (Chao et al., 2007), although other studies with densely

cultured hippocampal neurons or hippocampal slices from Mecp2 mutant mice find a disruption in the Pr ( Asaka et al., 2006 and Nelson et al., 2006). Mechanisms underlying synaptic weakening may vary depending on culture conditions and the specific synapse studied. Our physiological selleck chemicals llc data show that the retinogeniculate circuit becomes abnormal in −/y mice after P21. We asked whether these changes are a result of failure to maintain refined axon projections, a process that has been described in mice with disrupted retinal activity ( Demas et al., 2006). Retinal axons organize into eye-specific regions in the LGN in a process that is thought

to be largely complete by P8–P10 in mice ( Godement et al., 1984 and Jaubert-Miazza et al., 2005). To address whether eye-specific segregation is disrupted in the mutant, we injected both eyes with two different β cholera toxin-conjugated

fluorescent dyes to visualize the terminal fields of ipsi- and contralateral retinal projections to the LGN. We quantified segregation by using an unbiased assay that analyzes, for each pixel, Glucocerebrosidase the logarithm of the ratio of fluorescence intensity from each fluorescence channel (R value) ( Torborg and Feller, 2004). The variance of R, defined as the width of the histogram distribution of R values, can be used to compare segregation patterns. High variance indicates a high degree of segregation, whereas low variance indicates a high degree of overlap (see Supplemental Experimental Procedures). By using this analysis, we did not observe a significant difference in the segregation pattern of retinogeniculate projections between −/y and +/y mice at P27–P34. However, by P46–P51, a modest but significant difference in segregation was noted (Figure 4). These results are consistent with our physiological findings that the initial formation and refinement of this synaptic circuit are relatively normal in mutant mice and functional defects arise only during a later, experience-dependent period of development. At the mouse retinogeniculate synapse a vision-dependent sensitive period for synaptic remodeling begins around the age of P20.