Effects of neonatal intraocular colchicine on synaptogenesis and on the retention of the ipsilateral retinofugal projection within the superior colliculus

Summary A normally transient ipsilateral retinofugal projection exists in the rat but is retained following eye removal because of the loss of competitive interaction between crossed and uncrossed fibers. To further explore this phenomenon, colchicine (10^–3M) was injected into the right eye of newborn albino rats to partially suppress axonal transport in optic fibers, alter the developmental time course of retinofugal synaptic terminals and determine if this would in turn extend the period of survival of the ipsilateral projection. Measurements of the number of fibers in the nerve were also made to insure that colchicine was not lethal to the retinofugal projection. Projections into the superior colliculus were demonstrated by anterograde movement of HRP from the left eye. TMB histochemistry revealed dense labeling of the contralateral retino-recipient layers at 5 dpn in untreated or saline-injected controls. The ipsilateral projection was seen as a lighter band of activity across the colliculus which was most concentrated in the antero-medial quadrant. This pathway was transient and degenerated by 10 dpn, except for a few antero-medial fibers. Animals treated with colchicine demonstrated a retention of this pathway through 20 dpn. A concomitant quantitative analysis of synaptic development within the superior colliculus revealed populations of boutons with round (R) and flattened (F) vesicles, as well as multiple junctional (MJ) and serial (S) complexes, most of which were specialized R boutons. The various synaptic categories displayed specific ratios unique to the different stages of maturation. Intraocular colchicine reduced the ratio of R, MJ and S boutons to F terminals between 5–15 dpn (P < 0.01). By 20 dpn, the proportions of MJ and S boutons remained depressed but the normal ratio of R to F boutons was restored. Areal determinations of each synaptic profile included in the counts revealed a significant reduction in the size of MJ synaptic profiles examined in colchicine-treated animals and this may have been reflected in the slight loss of tectal volume (6–9%). Removal of the left eye and assessment of degenerating boutons showed that the expanded ipsilateral projection was not sufficiently dense to produce such a restoration. It thus appears that colchicine delayed the growth of the R population, but the effect was reversible and this category of boutons continued to develop, albeit on a later time course. Continued depression of MJ and S boutons suggests that suppression of the rate and quantity of axonally-transported substances retards the final stages of tectal synaptic differentiation, reduces their competitive advantage and allows the retention of the ipsilateral optic projection.