These results show a requirement for pattern vision in the local refinement and maintenance of topographically appropriate corticocollicular arbors and probably
also in the synapses they establish. To test the dependence of collicular synaptogenesis specifically on the rapidly arborizing corticocollicular projection, we assayed the effect of removing the VC input before EO on spontaneous whole-cell mEPSCs and the locus of any changes in spine and filopodia distribution on DOV neurons. Lesions of ipsilateral VC were made in eGFP transgenic mice between P9-P10 by microaspiration of the cellular layers of VC (Figure S4 and Supplemental Experimental Procedures). find more Animals received either a lesion that eliminated the collicular-projecting Layer V pyramidal cells (VC removed) or surgery with skull-flap incision but without cortical aspiration (sham) (Figure 6A). Consistent with a loss of cortical synapse formation after VC lesion, removal of VC resulted in a significant reduction of mEPSC frequency (Figures 6B and 6C) after EO compared to sham-operated controls. The enhancement in mEPSC amplitude, however, represents a significant potentiation
of the remaining largely retinal synapses compared to EO sham animals (Figure 6D). This increase in strength of remaining inputs after VC removal suggests a competition between retinal and cortical driven Doxorubicin concentration synapses during normal visual synaptogenesis. No significant effect of VC removal was observed on filopodia or spine density on caliber 4 dendrites (p > 0.70, n = 23 lesion, n = 24 sham), consistent with the hypothesis that these dendrites contain primarily retinal inputs, whose strength (rather than number) was adjusted after VC lesion. VC removal prevented the normal appearance of filopodia
on caliber 3 dendrites but had no significant effect on spine density (p > 0.90, n = 9 lesion, n = 9 sham) (Figures 6E and 6F), suggesting Phenibut that filopodia are the sites of new cortical synapse formation. Caliber 3 dendrites are predominantly localized in mid-stratum griseum superficiale (SGS) levels where cortical and retinal terminals overlap, and are the most likely to be contacted by cortical axons. Thus the presence of cortical afferents/growth cones in the neuropil appears necessary for the development of new functional contacts, and also triggers the formation of filopodia on caliber 3 dendrites, on which many of these new contacts form. Hebbian theory suggests that the synaptic elaboration of the late-arriving visual cortical inputs should be at a significant competitive disadvantage compared to the previously established mapped retinal synapses. Nevertheless, cortex successfully establishes a synaptic foothold at proximal sites, in a vision-dependent manner. Such rapid expansion is an apparent violation of Hebb’s postulate, unless the cortical activity does in fact precede and contribute to driving collicular responses.