[3H]‐idazoxan binding to rabbit cerebral cortex recognises multiple imidazoline I2‐type receptors: pharmacological characterization and relationship to monoamine oxidase

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Abstract
In rabbit cerebral cortical homogenates, saturation analysis of [3H]‐idazoxan, an α2‐adrenoceptor antagonist, revealed high affinity binding to a single site with high density. Competition experiments demonstrated that the [3H]‐idazoxan recognition site was insensitive to the catecholamines, adrenaline and noradrenaline and possessed a low affinity for the α2‐ and α1‐adrenoceptor antagonists, rauwolscine, yohimbine and prazosin, suggesting that the site was not an adrenoceptor. Mapping [3H]‐idazoxan binding sites in the forebrain of rabbits by autoradiography, showed high densities of I2 sites in the medial preoptic area and in the stria terminalis. Moderate binding was found in caudate nucleus, putamen, cerebral cortex and hippocampus. The imidazolines cirazoline, naphazoline, guanabenz and BRL44408 along with amiloride, which is structurally related to the imidazolines, all had high affinity for the [3H]‐idazoxan site, suggesting that the site was related to the I2 imidazoline‐recognition site described by other groups. However, the imidazolines, clonidine and UK‐14,304 and the structurally related rilmenidine all had a low affinity for the binding site, showing that [3H]‐idazoxan was not binding to the I1 imidazoline‐recognition site found in rat, bovine and human medulla oblongata. Naphazoline, guanabenz, clonidine and amiloride competition studies had Hill slopes which were significantly different from unity (P < 0.01) and computer analysis showed that the [3H]‐idazoxan binding data could be best fitted to a model which considers binding to two sites (P < 0.01). One site has a high affinity for idazoxan, cirazoline, naphazoline, guanabenz and amiloride and a moderate affinity for BRL44408 and clonidine (70% of binding) and the second site (30% of binding) has a high affinity for idazoxan and cirazoline, but a lower affinity for naphazoline, guanabenz, amiloride, BRL44408 and clonidine. Experiments using [3H]‐RX821002, in contrast to [3H]‐idazoxan, clearly demonstrated the presence of a single type of α2‐adrenoceptor in rabbit cortex with a pharmacological profile which is similar to the α2A‐adrenoceptor possessing a high affinity for yohimbine, rauwolscine, BRL44408 and oxymetazoline, but a lower affinity for prazosin. The monoamine oxidase inhibitors, clorgyline, pargyline and deprenyl had at least a ten fold lower affinity at the rabbirt cortex I2 site as compared to their known affinity at monoamine oxidase suggesting that the I2 site is not related to the active site of the enzyme, monoamine oxidase. In addition, the peripheral benzodiazepine ligands, PK‐11195 or Ro 5–4864 both had very low affinities at the I2 site in rabbit cortex suggesting that the [3H]‐idazoxan binding was not to the peripheral benzodiazepine binding site.

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