Enhanced binocular interaction in the visual cortex of normal kittens subjected to intracortical norepinephrine perfusion.

It was previously proposed that norepinephrine (NE)-containing nerve terminals in visual cortex are important for the maintenance of cortical plasticity. Observations at that time indicated that local microperfusion of exogenous NE for 1 week directly into kitten visual cortex, with no alteration of the visual environment, resulted in an unexpected bias in ocular dominance toward the contralateral eye. The proportion of binocular cells, however, remained close to normal. In the present study, we examined this contralateral bias in visual cortical neurons addressing the following two issues: the time needed for change in ocular dominance to occur, and its dependence on visually evoked activity. We found no bias in ocular dominance toward the contralateral eye when the continuous local perfusion of 48 microM NE lasted for 3 days. Such change became obvious after 1 week. However, if the animal was placed in the dark during the period of NE perfusion, no change whatsoever in ocular dominance was observed. These results suggest that NE itself does not affect ocular dominance circuitry directly, since both high levels of NE and an extended period of visually evoked activity are necessary for the observed change in ocular dominance to occur. We conclude that the present results are consistent with the previously proposed role for NE in the modulation of visual cortical plasticity.