[3H]-MK 912 binding delineates two 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 [3H]-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 Kd value of [3H]-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 Kds 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 [3H]-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 Kds of compounds obtained for the cerebral cortex alpha 2C-adrenoceptors showed an almost 1:1 correlation with the corresponding Kds 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 Kds. 4. In the rat spinal cord [3H]-MK 912 bound to alpha 2A- and alpha 2C-adrenoceptor sites with similar affinities as in the cerebral cortex and with densities 172 and 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.