Dimorphic myelin in the rat optic nerve as a result of retinal activity blockage by tetrodotoxin during early postnatal period.

The effects of the retinal ganglion cell (RGC) activity blockage on the early myelination of the rat optic nerve (ON) were investigated at the light and ultrastructural levels. The blockage of the RGC action potential was attained by the use of tetrodotoxin (TTX), a blocker of the voltage-sensitive sodium channels. TTX was either infused directly into the left eye (TON) or injected systematically (SON). These two groups of ONs were compared with the untreated paired right nerves (UON) of the eye-infused group. Our observations showed that the general morphology of the ONs in either treated group was similar to that of the UONs. The most noticeable ultrastructural feature of these nerves was the presence of dimorphic myelin sheaths in 4% of the myelinated fibres (MFs) in the TON group at postnatal day twelve, while they were seldom observed in the other groups (0.5%). These abnormal covers were of two types; long flaps of aberrant myelin or redundant myelin profiles. However, at postnatal day seven, the onset of myelination and the percentage of MFs was similar in the three groups. The morphometric results showed that there were no age-group differences in axon size in unmyelinated and MFs. These results suggest that while the bioelectrical activity of the RGCs could not play any role in maintaining axon calibre it may, to some extent, regulate the process of formation of normal myelin sheaths in the rat ON.