A novel mechanism of action for hypertension control: Moxonidine as a selective I1-imidazoline agonist

Sympathoadrenal inhibition by a direct action within the central nervous system is an advantageous route to blood pressure control. Stimulation of brain ₂-adrenergic receptors is one mechanism for sympathoadrenal suppression, but comes at the cost of nonspecific depression of CNS function, including sedation and decreased salivary flow. Evidence is accumulating for a second pathway for pharmacological control of sympathoadrenal outflow, mediated by a novel receptor specific for imidazolines. First-generation central antihypertensive agents, which are imidazolines such as clonidine, act primarily to stimulate these I₁-imidazoline receptors in the rostral ventrolateral medulla oblongata (RVLM) to lower blood pressure, but have sufficient agonism at ₂-adrenergic receptors to produce side effects. Second-generation centrally acting antihypertensive agents, such as moxonidine and rilmenidine, are selective for I₁ relative to ₂ receptors. The reduced ₂ potency of these agents correlates with reduced severity of side effects. In this study we further established the selectivity of moxonidine for I₁-imidazoline sites by characterizing the direct interaction of [³H]moxonidine with these receptors in the RVLM and in adrenomedullary chromaffin cells. [³H]Moxonidine preferentially labeled I₁-imidazoline sites relative to ₂-adrenergic sites, only a small portion of which were labeled in the RVLM. [³H]Moxonidine binding to I₁-imidazoline sites was modulated by guanine nucleotides, implying that I₁-imidazoline sites may be membrane receptors coupled to guanine nucleotide binding regulatory proteins (G proteins). Receptor autoradiography with [¹²⁵I]p-iodoclonidine confirmed the presence of I₁-imidazoline sites in the RVLM and other areas of the brainstem reticular formation. In contrast, ₂-adrenergic sites were mainly localized to the nucleus of the solitary tract. Moxonidine selectively displaced [¹²⁵I]p-iodoclonidine binding from reticular areas, including the RVLM. In vivo studies in SHR rats confirmed the ability of moxonidine to normalize hypertension by an action within the RVLM and confirmed the correspondence of I₁ binding affinity and antihypertensive efficacy. We also discuss prior literature on the cardiovascular pharmacology of imidazolines, reinterpreting previous studies that only considered alpha-adrenergic mechanisms.