1. The neural messenger molecule nitric oxide (NO) has been shown to be involved in several forms of plasticity including hippocampal long-term potentiation. We examined the effects of chronic intracortical infusion of inhibitors of NO synthase (NOX) activity on the plasticity of visual cortical responses following monocular lid suture during the critical period. 2. Single unit recordings (618 cells) made in both the NOS inhibitor-treated (30mM N(G)-methyl-L-arginine (L-NMMA), or 22 or 2 mM nitro-L-arginine (L-NOArg)) and saline-treated control hemispheres of barbiturate-anaesthetized, critical-period kittens (n = 8) revealed a profound shift in favour of the non-deprived eye. Shifts were of similar magnitude in hemispheres in which NOS was inhibited and in saline control hemispheres. 3. Subsequent analysis of NOX activity in the same cortical tissue in which recordings had been made showed a pronounced decrease in NOX activity in inhibitor-treated hemispheres. In the region in which all the single unit recordings mere made (< 3 mm from the infusion cannula), 22 mM L-NOArg resulted in a reduction of NOX activity to 5.55 ± 5.33% of control hemisphere NOX activity levels. L-NOArg (2 mar) and L-NMMA (30 mM) also produced clear, but smaller, inhibition of NOX activity. 4. These findings demonstrate that NOS activity is not essential for ocular dominance plasticity in visual cortex.
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