The imidazoline receptors and the central regulation of the arterial blood pressure: a minireview.
Recently, we proposed the hypothesis according to which the central hypotensive effect of clonidine and related substances could be related to an action upon specific receptors, requiring the imidazoline or imidazoline-like structures, rather than alpha 2-adrenoceptors. Since then, direct evidences have been accumulated to confirm the existence of a population of imidazoline specific binding sites in the brainstem of animals and man, more precisely in the Nucleus Reticularis Lateralis (NRL) region of the ventrolateral medulla (VLM), site of the antihypertensive action of clonidine. The purification of the putative endogenous ligand of the imidazoline receptors--named endazoline--is currently being attempted from human brain extracts. This new concept might at last lead to the expected dissociation of the pharmacological mechanisms involved, on the one hand, in the therapeutic antihypertensive effect, and on the other, in their major side-effect, which is sedation. In fact, it has been recently confirmed that hypotension is mediated by the activation of imidazoline preferring receptors (IPR) within the NRL region, while sedation is attributed to the inhibition of alpha 2-adrenergic mechanisms in the locus coeruleus, which is involved in the control of the sleep-waking cycle. The IPR may constitute an interesting target for new drugs in the treatment of arterial hypertension. Finally, dysfunctions of this modulatory system which could be involved in the pathophysiology of some forms of the hypertensive disease are under investigation.