Norepinephrine transporter knockout‐induced up‐regulation of brain alpha2A/C‐adrenergic receptors

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Abstract
The norepinephrine transporter (NET) is responsible for the rapid removal of norepinephrine released from sympathetic neurons; this release is controlled by inhibitory α2‐adrenergic receptors (α2ARs). Long‐term inhibition of the NET by antidepressants has been reported to change the density and function of pre‐ and postsynaptic ARs, which may contribute to the antidepressant effects of NET inhibitors such as desipramine. NET‐deficient (NET‐KO) mice have been described to behave like antidepressant‐treated mice. By means of quantitative real‐time PCR we show that mRNAs encoding the α2A‐adrenergic receptor (α2AAR) and the α2C‐adrenergic receptor (α2CAR) are up‐regulated in the brainstem, and that α2CAR mRNA is also elevated in the hippocampus and striatum of NET‐KO mice. These results were confirmed at the protein level by quantitative autoradiography. The NET‐KO mice showed enhanced binding of the selective α2AR antagonist [3H]RX821002 in several brain regions. Most robust increases (20–25%) in α2AR expression were observed in the hippocampus and in the striatum. Significant increases (16%) were also seen in the extended amygdala and thalamic structures. In an ‘in vivo’ test, the α2AR agonist clonidine (0.1 mg/kg) caused a significantly greater reduction of locomotor activity in NET‐KO mice than in wild‐type mice, showing the relevance of our findings at the functional level.

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