26 pA, n = 14 and ET33-Cre::VGLUT2flox/flox = 1136.36 ± 126.19 pA, n = 12; p > 0.05 by Student’s t test), whereas ipsilateral responses were significantly reduced (Figures 2H and 2J; VGLUT2flox/flox = 256.08 ± 49.90pA, n = 17 and ET33-Cre::VGLUT2flox/flox = 7.54 ± 3.60 pA, n = 22; p < 0.0001 by Mann-Whitney U test). In P10 ET33-Cre::VGLUT2flox/flox slices, only 18% of dLGN neurons responded to ipsilateral axon stimulation (4 of 22 compared to 17 of 19 in controls) and selleck chemical their average response sizes were reduced by 97%. AMPAR-mediated ipsilateral responses were also further reduced

between P5 and P10 (Figures S2H–S2M). Collectively, our electrophysiological findings demonstrate that glutamatergic

Src inhibitor synaptic transmission is selectively and progressively reduced in the ipsilateral retinogeniculate pathway of early postnatal ET33-Cre::VGLUT2flox/flox mice. What role does synaptic competition play in eye-specific retinogeniculate refinement? To address this question, we analyzed ipsilateral and contralateral projections at different developmental stages in ET33-Cre::VGLUT2flox/flox animals by labeling axons from each eye with CTb-488 or CTb-594. In wild-type mice, ipsilateral and contralateral axon territories overlap in the dLGN at P4 (Godement et al., 1984 and Jaubert-Miazza et al., 2005) and we found that on P4 both Cre-negative and Cre-expressing VGLUT2flox/flox littermates exhibited overlapping axonal projection patterns typical for this age (Figures 3A–3C). In wild-type mice, Parvulin eye-specific territories are clearly visible by P10 (Godement et al., 1984, Jaubert-Miazza et al., 2005 and Muir-Robinson et al., 2002) (Figure 3A). Based on previous studies (Penn et al., 1998 and Stellwagen and Shatz, 2002), we predicted that the synaptically weakened ipsilateral axons would fail to outcompete and eject contralateral axons from their territory

and that the ipsilateral eye territory would be reduced. Indeed, we found that in the ET33-Cre::VGLUT2flox/flox mice, contralateral eye axons failed to retract from the ipsilateral region of the dLGN (Figure 3A), resulting in a greater than normal degree of overlap between ipsilateral and contralateral axons (Figure 3D; n = 8 mice for each genotype). The increased overlap was significant over a wide range of signal-to-noise thresholds (Figure 3D) (see Experimental Procedures). The abnormal degree of overlap did not occur in animals expressing ET33-Cre alone or ET33-Cre and one floxed VGLUT2 allele (Figure S3D). These data provide evidence that effective glutamatergic transmission is crucial for mediating axon-axon competition during CNS refinement. Surprisingly, however, reducing ipsilateral synaptic transmission did not alter the overall pattern of the ispilateral terminal field (Figures 3A and 3E and Figure S3).