Heterozygous alpha 2A-adrenergic receptor mice unveil unique therapeutic benefits of partial agonists

Genetic manipulation of the alpha(2A)-adrenergic receptor (alpha(2A)-AR) in mice has revealed the role of this subtype in numerous responses, including agonist-induced hypotension and sedation. Unexpectedly, alpha(2)-agonist treatment of mice heterozygous for the alpha(2A)-AR (alpha(2A)-AR(+/-)) lowers blood pressure without sedation, indicating that more than 50% of alpha(2A)-AR must be activated to evoke sedation. We postulated that partial activation of alpha(2A)-AR in wild-type alpha(2A)-AR(+/+) animals could be achieved with partial agonists, agents with variable ability to couple receptor occupancy to effector activation, and might elicit one versus another pharmacological response. In vitro assays reveal that moxonidine is a partial agonist at alpha(2A)-AR. Although moxonidine was developed to preferentially interact with imidazoline binding sites, it requires the alpha(2A)-AR to lower blood pressure because we observe no hypotensive response to moxonidine in alpha(2A)-AR-null (alpha(2A)-AR(-/-)) mice. Moreover, we observe that moxonidine lowers blood pressure without sedation in wild-type mice, consistent with the above hypothesis regarding partial agonists. Our findings suggest that weak partial agonists can evoke response-selective pathways and might be exploited successfully to achieve alpha(2A)-AR pharmacotherapy where concomitant sedation is undesirable, i.e., in treatment of depression or attention deficit hyperactivity disorder, in suppression of epileptogenesis, or enhancement of cognition. Furthermore, rigorous physiological and behavioral assessment of mice heterozygous for particular receptors provides a general strategy for elucidation of pathways that might be selectively activated by partial agonists, thus achieving response-specific therapy.