α 2 -Adrenoceptor Blockade Accelerates the Neurogenic, Neurotrophic, and Behavioral Effects of Chronic Antidepressant Treatment

Slow-onset adaptive changes that arise from sustained antidepressant treatment, such as enhanced adult hippocampal neurogenesis and increased trophic factor expression, play a key role in the behavioral effects of antidepressants. α 2 -Adrenoceptors contribute to the modulation of mood and are potential targets for the development of faster acting antidepressants. We investigated the influence of α 2 -adrenoceptors on adult hippocampal neurogenesis. Our results indicate that α 2 -adrenoceptor agonists, clonidine and guanabenz, decrease adult hippocampal neurogenesis through a selective effect on the proliferation, but not the survival or differentiation, of progenitors. These effects persist in dopamine β-hydroxylase knock-out ( Dbh −/− ) mice lacking norepinephrine, supporting a role for α 2 -heteroceptors on progenitor cells, rather than α 2 -autoreceptors on noradrenergic neurons that inhibit norepinephrine release. Adult hippocampal progenitors in vitro express all the α 2 -adrenoceptor subtypes, and decreased neurosphere frequency and BrdU incorporation indicate direct effects of α 2 -adrenoceptor stimulation on progenitors. Furthermore, coadministration of the α 2 -adrenoceptor antagonist yohimbine with the antidepressant imipramine significantly accelerates effects on hippocampal progenitor proliferation, the morphological maturation of newborn neurons, and the increase in expression of brain derived neurotrophic factor and vascular endothelial growth factor implicated in the neurogenic and behavioral effects of antidepressants. Finally, short-duration (7 d) yohimbine and imipramine treatment results in robust behavioral responses in the novelty suppressed feeding test, which normally requires 3 weeks of treatment with classical antidepressants. Our results demonstrate that α 2 -adrenoceptors, expressed by progenitor cells, decrease adult hippocampal neurogenesis, while their blockade speeds up antidepressant action, highlighting their importance as targets for faster acting antidepressants.