[H-3] IDAZOXAN BINDING TO RABBIT CEREBRAL-CORTEX RECOGNIZES MULTIPLE IMIDAZOLINE I(2)-TYPE RECEPTORS - PHARMACOLOGICAL CHARACTERIZATION ANDRELATIONSHIP TO MONOAMINE-OXIDASE

1 In rabbit cerebral cortical homogenates, saturation analysis of [H-3 ]-idazoxan, an alpha2-adrenoceptor antagonist, revealed high affinity binding to a single site with high density. Competition experiments de monstrated that the [H-3]-idazoxan recognition site was insensitive to the catecholamines, adrenaline and noradrenaline and possessed a low affinity for the alpha2- and alpha1-adrenoceptor antagonists, rauwolsc ine, yohimbine and prazosin, suggesting that the site was not an adren oceptor. Mapping [H-3]-idazoxan binding sites in the forebrain of rabb its by autoradiography, showed high densities of I2 sites in the media l preoptic area and in the stria terminalis. Moderate binding was foun d in caudate nucleus, putamen, cerebral cortex and hippocampus. 2 The imidazolines cirazoline, naphazoline, guanabenz and BRL44408 along wit h amiloride, which is structurally related to the imidazolines, all ha d high affinity for the [H-3]-idazoxan site, suggesting that the site was related to the 12 imidazoline-recognition site described by other groups. However, the imidazolines, clonidine and UK-14,304 and the str ucturally related rilmenidine all had a low affinity for the binding s ite, showing that [H-3]-idazoxan was not binding to the I1 imidazoline -recognition site found in rat, bovine and human medulla oblongata. 3 Naphazoline, guanabenz, clonidine and amiloride competition studies ha d Hill slopes which were significantly different from unity (P<0.01) a nd computer analysis showed that the [H-3]-idazoxan binding data could be best fitted to a model which considers binding to two sites (P<0.0 1). One site has a high affinity for idazoxan, cirazoline, naphazoline , guanabenz and amiloride and a moderate affinity for BRL44408 and clo nidine (70% of binding) and the second site (30% of binding) has a hig h affinity for idazoxan and cirazoline, but a lower affinity for napha zoline, guanabenz, amiloride, BRL44408 and clonidine. 4 Experiments us ing [H-3]-RX821002, in contrast to [H-3]-idazoxan, clearly demonstrate d the presence of a single type of alpha2-adrenoceptor in rabbit corte x with a pharmacological profile which is similar to the alpha2A-adren oceptor possessing a high affinity for yohimbine, rauwolscine, BRL4440 8 and oxymetazoline, but a lower affinity for prazosin. 5 The monoamin e oxidase inhibitors, clorgyline, pargyline and deprenyl had at least a ten fold lower affinity at the rabbirt cortex 2 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 5-486 4 both had very low affinities at the I2 site in rabbit cortex suggest ing that the [H-3]-idazoxan binding was not to the peripheral benzodia zepine binding site.