Doses of 6-hydroxydopamine sufficient to deplete norepinephrine are not sufficient to decrease plasticity in the visual cortex.

These experiments were designed to test Kasamatsu and Pettigrew's (1979, 1983, and see below) hypothesis that plasticity in the visual cortex requires cortical norepinephrine (NE). Kittens were treated with various doses of intraventricular 6-hydroxydopamine (6-OHDA) or vehicle solution. Cortical NE content was measured with high-performance liquid chromatography with electrochemical detection. We sutured the right eyes of some kittens approximately 6 weeks of age for 1 week and recorded from the left visual cortex of these kittens at the end of the week of suture. We measured the ability of the deprived eye to drive cortical cells in animals that received either 0.2 or 4.8 mg of 6-OHDA, and also in control animals that received only vehicle solution. We concluded that a particular dose of 6-OHDA decreased plasticity if it increased (relative to controls) the ability of the deprived eye to drive cortical cells. Doses of 6-OHDA as small as 0.2 mg were sufficient to produce approximately maximal depletion of NE but did not decrease cortical plasticity. Doses of 4.8 mg or more did decrease cortical plasticity, although not as much as was reported by Kasamatsu and Pettigrew. We conclude that 6-OHDA can alter cortical plasticity but the decrease in plasticity does not result from NE depletion.