The objective of this proposal is the characterization of a novel receptor present in the medull oblongata, cultured neuron-like cells, the adrenal medulla, the kidney, and the carotid body. In the medull oblongata, this receptor appears to mediate the vasodepressor action of clonidine and related agents; in the adrenal medulla imidazole receptors might possibly mediate the inhibition of catecholamine release by clonidine Carotid body imidazole receptors appear to facilitate chemosensory discharge. Imidazole receptor sites are specific, high-affinity, and saturable, and have been specifically localized within the VLM,the probable site of clonidine's hypotensive action, but are mostly entirely absent from the frontal cerebral cortex, a possible site of the sedative side-effects of clonidine. Imidazole sites are also absent from platelets. This line of research may culminate in the development of antihypertensive agent which acts selectively on imidazole receptors in the VLM, the probable site of the vasomotor center. Such an agent amy reduce sympathetic outflow without producing the sedative side-effects that have limited the usefulness of centrally-acting antihypertensives. In addition, such an agent may have a facilitory effect on respiration. The putative endogenous ligand at imidazole receptors is clonidine-displacing substance (CDS). The chemical structure of CDS is unknown, and purification of this novel substance is beyond the scope of this proposal. Nonetheless, characterization of receptors for CDS will facilitate efforts by others to identify its structure. Moreover, the existence of an endogenous ligand suggests that imidazole receptors play a psychological role in the regulation of blood pressure. This series of studies will 1) further characterize imidazole receptors in the VLM and in the kidney, and thus establish their unique ligand specificity and identify potential pharmacological tools for functional tests of imidazole receptors, 2) characterize imidazole receptor tissues and cells relevant to cardiopulmonary regulation in order to determine the range of possible physiological roles played by imidazole receptors, 3) examine the regulation of imidazole receptors in vivo by chronic drug treatment, 4) further establish the role of brainstem imidazole receptors in cardiovascular regulation and examine the possible role of these receptors in central respiratory control, and 5) examine the role of imidazole receptors in animal models of human essential hypertension. Preliminary studies conducted have identified agonists and a antagonist with superior selectivity for imidazole binding sites, demonstrated a high density of imidazole site in the renal medulla, and established the usefulness of a novel high-activity radioligand for imidazole sites 125I-p-iodoclonidine (125I-PIC), developed an improved method of membrane preparation for collagenous tissues and demonstrated that imidazole binding sites in the renal medulla are up-regulated early in the development of hypertension induced by chronic infusion of angiotensin II in subpressor doses. Angiotensin II infusion has no effect on alpha2-adrenergic receptor binding in the same tissue. These additional results support the hypothesis that imidazole binding sites are functional receptors which participate in cardiorespiratory regulation and further increase the likelihood that important results will be forthcoming from this line of research.
