The postnatal development of the rat primary visual cortex during optic nerve impulse blockade by intraocular tetrodotoxin: a quantitative electron microscopic analysis.

The effect of tetrodotoxin (TTX)-induced monocular impulse blockade on various parameters of synaptogenesis during the first 3 postnatal weeks of the developing rat visual cortex was investigated by quantitative electron microscopy. During the injection period, beyond 14 days postnatal (dpn), the effectiveness of TTX in blocking optic nerve impulses was monitored by loss of the pupillary light reflex. Between 5 and 21 dpn, TTX treatment reduced the number of type I axodendritic synapses by approximately 23%, when compared to sham-injected controls. These reductions were found in layers III, IV, and the superficial region of layer V. Layer IV exhibited the greatest decrease (24%) while layers III and V showed reductions of 20% and 18%, respectively. At 21 dpn, the number of type II axodendritic synapses decreased by 19% in the same layers, but no reductions were found at earlier ages. TTX also reduced the mean number of synaptic vesicles within type I and type II terminals by 27% and 15%, respectively. At 9 dpn, reductions were first found in layers IV and V, but by 21 dpn significant decreases were found in layers II/III, IV and V. TTX had no effect on the length of the postsynaptic density of both synaptic types or on cortical thickness at any age. These data indicate that optic impluses are important mediators of synaptogenesis in the developing visual cortex, the loss of which induces localized and specific synaptic alterations, possibly due to a change in cortical circuitry.