Subtype classification of presynaptic alpha 2-adrenoceptors.

1. Presynaptic alpha 2-adrenoceptors in a few tissues have recently been pharmacologically classified in functional studies. 2. Autoreceptors are of alpha 2A-subtype in rabbit occipito-parietal cortex, rat cerebral cortex, vas deferens, submandibular gland, kidney, guinea-pig ileum submucosal arterioles and urethra. 3. Heteroreceptors are of alpha 2A-subtype in rat cerebral cortex, vas deferens, guinea-pig ileum submucosal plexus and Auerbach's plexus. 4. In rat atria autoreceptors have been shown to be of alpha 2B-subtype. 5. Classification is done mainly with alpha 2A-adrenoceptor-selective oxymetazoline, WB 4101 and BRL 44408, and the alpha 2B-adrenoceptor-selective prazosin, AR-C 239, chlorpromazine and BRL 41992. 6. With four alpha 2-adrenoceptor subtypes to consider, a larger number of subtype-selective compounds may have to be characterized in the classification in the many tissues, where presynaptic alpha 2-adrenoceptors are found.