[Radioreceptor study of [3H]-olanzapine specific binding in the rat frontal cortex and striatum]

The direct radioreceptor assay was used to elucidate the interactions of clinically employed atypical antipsychotic drug olanzapine with the specific binding sites in rat brain membranes. Tritium-labeled olanzapine was obtained by isotope exchange method. HPLC and mass spectrometry assays confirmed the identity of [3H]-olanzapine to the initial compound. The specific [3H]-olanzapine binding sites of the two types were identified in frontal cortex by means of the method of Scatchard's plots. High affinity sites showed steady-state dissociation binding constant (KD) of 1.7 nM, which is indicative of olanzapine affinity to 5-HT-2a and histamine receptors. However, according to available literature data, the density of these receptors is several times smaller than that determined in our study (maximum number of binding sites corresponded to B(max) = 1.4 pmol/mg protein), thus allowing an assumption about the existence of yet unknown binding sites of the antipsychotic drug under consideration. Moderate affinity binding sites of olanzapine in frontal cortex (KD= 68.5 nM, B(max) = 10.8 pmol/mg protein) apparently correspond to dopamine, muscarinic, and alpha-1-adrenoceptors. [3H]olanzapine binding sites of the two types were also determined in striatum. The moderate-affinity sites of the receptor interaction had KD = 18.0 nM and B(max) = 13.0 pmol/mg protein. These values in fact correspond to D2 receptors, whose density prevails in this cortical region. The low-affinity binding sites of olanzapine in striatum had KD = 117.0 nM and B(max) = 32.5 pmol/mg protein.