CHARACTERIZATION OF BINDING-SITES OF A NEW NEUROTENSIN RECEPTOR ANTAGONIST, [H-3]SR 142948A, IN THE RAT-BRAIN

The present study describes the characterization of the binding proper ties and autoradiographic distribution of a new nonpeptide antagonist of neurotensin receptors, [H-3]SR 142948A razole-3-carbonyl]-amiono)-a damantane-2-carboxylic acid, hydrochloride), in the rat brain. The bin ding of [H-3]SR 142948A in brain membrane homogenates was specific, ti me-dependent, reversible and saturable. [H-3]SR 142948A bound to an ap parently homogeneous population of sites, with a K-d of 3.5 nM and a B -max value of 508 fmol/mg of protein, which was 80% higher than that o bserved in saturation experiments with [H-3]neurotensin. [H-3]SR 14294 8A binding was inhibited by SR 142948A, the related nonpeptide recepto r antagonist, SR 48692 pyrazole-3-carbonyl]amino)-adamantane-2-carboxy lic acid) and neurotensin. Saturation and competition studies in the p resence or absence of the histamine H-1 receptor antagonist, levocabas tine, revealed that [H-3]SR 142948A bound with similar affinities to b oth the levocabastine-insensitive neurotensin NT1 receptors (20% of th e total binding population) and the rec entry cloned levocabastine-sen sitive neurotensin NT2 receptors (80% Of the receptors) (K-d = 6.8 and 4.8 nM, respectively). The regional distribution of [H-3]SR 142948A b inding in the rat brain closely matched the distribution of [I-125]neu rotensin binding. In conclusion, these findings indicate that [H-3]SR 142948A is a new potent antagonist radioligand which recognizes with h igh affinity both neurotensin NT1 and NT2 receptors and represents thu s an excellent tool to study neurotensin receptors in the rat brain. ( C) 1998 Elsevier Science B.V.