Effects of intraocular tetrodotoxin on dendritic spines in the developing rat visual cortex: a Golgi analysis

The effect of tetrodotoxin (TTX)-induced monocular impulse blockade on the growth of dendritic spines in the developing rat primary visual cortex was analysed by quantitative Golgi techniques. Between 5 and 21 days postnatal (dpn), rats were injected with TTX every 2 days into the right eye to chronically eliminate optic impulses. Effectiveness of TTX was monitored by loss of the pupillary light reflex. At 21 dpn, the number of spines located on the portion of the apical dendrite within layers III, IV and the superficial region of layer V was reduced by approximately 26%. These decreases were found on the apical dendrites of both large and medium sized pyramidal cells. TTX also reduced the number of spines on the proximal portion of oblique dendrites in layer IV by 16%, yet did not change the number of spines on basilar dendrites. No evidence of transneuronal degeneration was seen following long-term TTX treatment. These data indicate that dendritic spine development in the visual cortex is sensitive to the loss of optic impulses and that the decrease in spine population is principally due to a reduction in spine growth.