Blockade of autoreceptor-mediated inhibition of norepinephrine release by atipamezole is maintained after chronic reuptake inhibition

The role of α₂-adrenergic autoreceptor desensitization in the delayed onset of antidepressant efficacy of selective norepinephrine (NE) reuptake inhibitors is unclear. Using the α₂-antagonist yohimbine, we showed previously that chronic treatment with desipramine (DMI) did not alter autoreceptor-mediated inhibition of NE release in the cortex. However, yohimbine may have non-selective effects that could confound this interpretation. Thus, using microdialysis, we measured acute effects of the highly selective α₂-antagonist atipamezole on NE release in the prefrontal cortex following chronic DMI treatment, after 0-8 d washout. Atipamezole induced a similar elevation of extracellular NE in all treatment groups, indicating no change in autoreceptor function. Further, the effect was most rapid in DMI-treated rats with 0- and 2-d washout, suggesting that autoreceptor-mediated inhibition was most prominent when NE levels were highest. This provides further evidence that autoreceptor-mediated inhibition of NE neurotransmission remains functional after chronic DMI treatment, arguing against the hypothesis that desensitization of α₂-autoreceptors accounts for the delayed onset of action of selective NE reuptake inhibitors.