Optic fibers and synapses labeled with HRP were counted in the primary optic innervation layer of tectum after continuously blocking visual impulse activity with TTX during regeneration. Normal numbers of optic and nonoptic fibers and synapses were found at both 30 and 60 d, and key ultrastructural features of optic afferents such as fiber fasciculation, myelination, terminal clustering, synaptogenesis onto different classes of postsynaptic targets and general morphology were not notably

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... d by impulse blockade. These findings indicate that during regeneration the normal proliferation and elimination of optic fibers and the formation of normal numbers of optic synapses are not regulated by activity and are consistent with a pattern formation role for impulse activity rather than a trophic one. Several recent studies have shown that the elimination of impulse activity by intraocular injection of the sodium channel blocker TTX disrupts the patterned distribution of nerve connections in the visual system of fish, frogs and mammals. In cats, TTX impulse blockade during postnatal development prevents the segregation of dorsal lateral geniculate (dLGN) fibers into ocular dominance columns in visual cortex (Chapman et al., 1986; Stryker and Harris, 1986) and interferes with the formation of On and Off center input, X and Y input, and binocularity in the dorsal lateral geniculate nucleus (dLGN) . TTX also has been reported to block or delay the development of the refined retinotopy in the retinocollicular projection in rats (Fawcett et al., 1984; O'Leary et al., 1986) . For the regenerating retinotectal projection of goldfish, impulse blockade with TTX inhibits the refinement of the retinotopic map in fish (Meyer, 1983 (Meyer, ,1987 as well as the formation of ocular dominance columns after 2 eyes are made to innervate the same tectum (Meyer, 1982; Boss and Schmidt, 1984 ; see also Reh and Constantine-