Investigation of I1-imidazoline receptors using microphysiometry and molecular modelling

The molecular identity and structure of imidazoline receptors is still poorly understood. For example the I1-imidazoline binding site (I1-site) is localised to the plasma membrane, but it is not clear if this represents a conventional receptor. The I1-site reportedly has both high and low affinity binding states. Again it is not clear if these sites represent different states of the same receptor, or distinct molecular entities. The signal transduction mechanisms of I1-imidazoline receptors are beginning to be unravelled. There is clear evidence that ligands with high affinity for I1-sites stimulate phosphatidylcholine-selective phospholipase C in the rat adrenal medullary tumour cell line PC-12, but this may not be the case in all cell types. We investigated the possible role of this novel pathway in bovine adrenal medullary cells. Radioligand binding studies with [3H]clonidine confirmed the presence of I1-sites in membranes from these cells. Using microphysiometry, a recently developed technique for determining cellular activation, the extracellular acidification rates of cultured bovine adrenal medullary cells were unaffected by a number of imidazolines considered to be agonists at the I1-site. This suggests that there is no I1-site mediated stimulation of phosphatidylcholine specific phospholipase C in these cells. However, nicotine-stimulated increases in extracellular acidification were blocked by 100 microM clonidine. Ion channels have been suggested as another possible I1-imidazoline 'receptor' family, and may represent the low affinity I1-site detected in binding studies. I1-Site ligands can be shown to bind to, or block, several members of the ligand-gated ion channel superfamily, including the 5HT3, K+ATP, NMDA and nicotinic acetylcholine receptors. The I1-site ligands appear to be binding to, and acting at, the previously described phencyclidine binding site in these channels. Furthermore, molecular modelling suggests that I1-site selective ligands share a common three-dimensional structure with phencyclidine, and that I2-site selective ligands do not have this structure. This suggests that a phencyclidine-binding site motif may represent a novel site of action for I1-site ligands, and a search for receptors based on this motif may reveal novel imidazoline 'receptors'.