[H-3] MK-912 BINDING DELINEATES 2 ALPHA-2-ADRENOCEPTOR SUBTYPES IN RAT CNS ONE OF WHICH IS IDENTICAL WITH THE CLONED PA2D ALPHA-2-ADRENOCEPTOR

1 Simultaneous computer modelling of control and guanfacine-masked [H-3]-MK 912 saturation curves as well as guanfacine competition curves revealed that the drugs bound to two alpha-2-adrenoceptor subtypes in the rat cerebral cortex with very different selectivities. These alpha-2-adrenoceptor subtypes were designated alpha-2A and alpha-2C. The K(d) value of [H-3]-MK 912 for the alpha-2A-subtype was 1.77 nm and for the alpha-2C-subtype 0.075 nm; the receptor sites showing capacities 296 and 33 fmol mg-1 protein, respectively. The K(d)s of guanfacine were 19.9 and 344 nm, respectively. 2 Binding constants of 26 compounds for the two rat cerebral cortex alpha-2-adrenoceptor subtypes were determined by simultaneous computer modelling of control and guanfacine-masked drug competition curves as well as plain guanfacine competition curves using [H-3]-MK912 as labelled ligand (i.e. a '3-curve assay'). Of the tested drugs WB4101, corynanthine, rauwolscine, yohimbine, ARC 239 and prazosin were found to be clearly alpha-2C-selective with selectivities ranging from 16 to 30 fold whereas guanfacine, oxymetazoline, BRL 44408 and BRL 41992 were found to be alpha-2A-selective with selectivities ranging from 9 to 22 fold. 3 The K(d)s of compounds obtained for the cerebral cortex alpha-2C-adrenoceptors showed an almost 1:1 correlation with the corresponding K(d)s for alpha-2-adrenoceptors expressed by the pA2d-gene (the rat 'alpha-2-C4' adrenoceptor) in CHO-cells. The cerebral cortex alpha-2A-adrenoceptors did not correlate well with the pA2d alpha-2-adrenoceptor K(d)s. 4 In the rat spinal cord [H-3]-MK 912 bound to alpha-2A- and alpha-2C-adrenoceptor sites with similar affinities as in the cerebral cortex and with densities 172 a. id 7.4 fmol mg-1 protein, respectively. Drug affinities for some compounds showing major selectivity for alpha-2A- and alpha-2C-adrenoceptors were fully compatible with the notion that the spinal cord sites were alpha-2A- and alpha-2C-adrenoceptors.