Retraction of Active Axons Figure 4. Examples of reconstructed arbors of normal-vision animals (A) and BD animals (B). Arrowheads on the right side of each arbor in frontal view indicate the borders between cortical layers. The arbors in frontal view are aligned on the border between layers IV and V. Surface view images were calculated on a computer by rotating frontal-view arbors 90u along the axis indicated by the dotted lines. Only portions above the layer IV and V border are shown as surface view images. The arbors in the inactivated visual cortex of normal-vision animals (A) appear to be smaller than those of BD animals (B). D, dorsal; V, ventral; M, medial; L, lateral; A, anterior; P, posterior. Scale bar, 400 mm. doi:10.1371/journal.pone.0004193.g004 (P40, Fig. 5 and Table 1). Thus, the deprived-eye arbors in the BoNT/E-treated cortex might not be completely rescued from retraction and become smaller than the normal arbors, although the extent of retraction is much smaller than that of the open-eye arbors in the inactivated cortex. The previous findings in the muscimol-treated cortex suggest a mechanism of axon retraction depending on the activity of presynaptic afferents. Only active afferents might express struc- tural changes in response to a repulsive signal, which may be a repulsive substance or a depletion of trophic substances in the inactivated cortex, for example. It is also possible, however, that the signal for retraction might be delivered only to active afferents as a result of local synaptic interaction with cortical neurons, because muscimol treatment does not prevent synaptic transmis- sion itself. The present results obtained in the BoNT/E-treated cortex, in which synaptic transmission was prevented, strongly argue against the latter possibility and support presynaptic mechanisms of open-eye axon retraction. The activity of axons PLoS ONE | www.plosone.org 5 January 2009 | Volume 4 | Issue 1 | e4193