Pharmacological differences between the alpha-presynaptic adrenoceptors in the peripheral and the central nervous systems

The relatives affinities of agonists and antagonists for the presynaptic alpha-adrenoceptors in the cat spleen, rat vas deferens and occipital cortex were compared. The order of potencies of antagonists was as follows: a) for increasing ³H-transmitter overflow in the perfused cat spleen: phentolamine = WB 4101 ≫ prazosin; b) in the rat vas deferens for antagonizing the inhibition of the twitch responses induced by clonidine: phentolamine > WB 4101 > prazosin and c) in the rat occipital cortex: prazosin (0.3 to 10 μM) and WB 4101 (0.1 to 10 μM) did not modify while phentolamine increased significantly the potassium evoked ³H-transmitter overflow. In addition the alpha presynaptic adrenoceptors of these three tissues differ in their affinity for the alpha adrenoceptor agonists: clonidine, oxymetazoline and methoxamine. These results would suggest that the presynaptic alpha adrenoceptors from peripheral tissues and central structures might differ in their affinity for agonists and antagonists when tissues from the same or different species are compared.