Restoration of visual cortical plasticity by local microperfusion of norepinephrine

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Abstract
Using a newly developed technique of continuous microperfusion, we obtained further evidence in support of our hypothesis that the neocortical catecholamines (CAs), particularly norepinephrine (NE), are responsible for a high level of cortical plasticity. We used the visual cortical changes in ocular dominance which follow a brief monocular deprivation as a simple and reliable index of cortical plasticity. Local perfusion of kitten visual cortex with 1 mg/ml (4.0 mM) 6-hydroxydopamine (6-OHDA) prevented the effects of monocular deprivation in kittens, thus replicating the results we had obtained using intraventricular injections (Kasamatsu and Pettigrew, '76b, '79). Locally perfused NE at a concentration of 10−2 mg/ml (48.6 μM) restored visual cortical plasticity in animals which were no longer susceptible to brief monocular lid-suture. These numbers refer to the concentration of solutions in the cannula/minipump system. The effective concentrations at the site of recording (about 2 mm away) are probably much lower than these. This effect of NE perfusion was seen both in kittens which had received prior 6-OHDA treatment as well as in older animals which had outgrown the susceptible period. In the kittens we obtained a nearly complete shift in ocular dominance toward the open eye and in the older animals a decrease in binocularity was obtained. The changes were found only in the local region of visual cortex perfused with either NE or 6-OHDA, while nearby cortical regions in the same animals were unaffected. There were no obvious changes in receptive field properties of individual neurons other than ocularity, and externally perfused NE did not itself reduce binocularity in normal animals: the effects of NE described above only occurred when the animal's visual experience was simultaneously altered. These results support the view that NE plays an important role in cortical plasticity.