Comparison of two alpha-noradrenergic agonists (clonidine and guanfacine) on norepinephrine turnover in the cortex of rats during morphine abstinence.

The effects of the alpha-agonists clonidine and guanfacine on rat cortical norepinephrine turnover during morphine withdrawal were assessed. Cortical norepinephrine levels were measured, after administration of alpha-methyl-p-tyrosine, by fluorometric assay. Chronic treatment with morphine did not affect norepinephrine levels or norepinephrine turnover. In control animals, doses up to 200 micrograms/kg clonidine and 1060 micrograms/kg guanfacine did not affect norepinephrine steady state levels, but did reduce alpha-methyl-p-tyrosine-induced norepinephrine depletion. As little as 5 micrograms/kg clonidine reversed the accelerated turnover observed during naloxone precipitated morphine withdrawal; while for guanfacine, the minimum effective dose was 212 micrograms/kg. the ED50 for guanfacine's reversal of the withdrawal-induced acceleration of turnover was approximately 60 times that for clonidine.