[Relative affinities of S 3341 and clonidine for the alpha 2- and alpha 2-adrenoceptors of the rat brain (author's transl)].

1. 3H-prazosin binds to brain alpha 1-adrenoceptors with high affinity (K(D) 25 degrees C approximately or equal to 1 nM). The number of binding sites in different regions of the brain varied from 44-114 fmoles/mg of protein: striatum = medulla oblongata less than hypothalamus less than cortex. 2. 3H-p-aminoclonidine labels brain alpha 1-adrenoceptors. Its binding capacity (66-209 fmoles/mg prot). is higher in the hypothalamus greater than cortex greater than striatum greater than or equal to medulla oblongata. Its affinity varies from one region to another (0.85 mM in cortex, 3.0 nM in hypothalamus). 3. S 3341 in a potent inhibitor of 3H-p-aminoclonidine like clonidine and a weak inhibitor of 3H-prazosin. 4. Comparison of the inhibitory effect of S 3341 and clonidine on the binding of 3H-prazosin and 3H-aminoclonidine indicates that S 3341 is 2-3 times more specific for alpha 2-adrenoceptors than clonidine. 5. Inhibition constants of S 3341 and clonidine (K1) are not identical in various brain regions but clonidine and S 3341 constants vary in parallel.