Norepinephrine transporter knockout-induced up-regulation of brain alpha sub(2A-C)-adrenergic receptors

The norepinephrine transporter (NET) is responsible for the rapid removal of norepinephrine released from sympathetic neurons; this release is controlled by inhibitory alpha sub(2)-adrenergic receptors ( alpha sub(2)ARs). 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 alpha sub(2A)-adrenergic receptor ( alpha sub(2A)AR) and the alpha sub(2C)-adrenergic receptor ( alpha sub(2C)AR) are up-regulated in the brainstem, and that alpha sub(2C)AR 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 alpha sub(2)AR antagonist [ super(3)H]RX821002 in several brain regions. Most robust increases (20-25%) in alpha sub(2)AR 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 alpha sub(2)AR 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.