SUBCLASSIFICATION OF THE PRESYNAPTIC ALPHA-2-AUTORECEPTORS IN RABBIT BRAIN CORTEX

1 alpha-2-Adrenoceptor binding sites have been subclassified into alpha-2A sites of which a main characteristic is very low affinity for prazosin, and alpha-2B sites with relatively high affinity for prazosin. The presynaptic alpha-2-autoreceptors in rabbit brain cortex were studied in order to classify them in terms of alpha-2A and alpha-2B. Release of [H-3]-noradrenaline in cortical slices was elicited by trains of 4 pulses delivered at 100 Hz. 2 Clonidine caused concentration-dependent inhibition of the stimulation-evoked overflow of tritium, with an EC50 of 7.5 nM and a maximal inhibition by 96%. 3 The following alpha-adrenoceptor antagonists shifted the concentration-response curve of clonidine to the right (antagonist-receptor dissociation constants K(D) in brackets): yohimbine (14 nM), 2-[2H-(1-methyl-1,3-dihydroisoindole)methyl]-4,5-dihydroimidazole (BRL 44408; 15 nM) and 1,2-dimethyl-2,3,9,13b-tetrahydro-1H-dibenzo[c,f]imidazo[1,5-a]azepine (BRL 41992; 630 nM). Prazosin 1-mu-M and 2-[2-[4-(omethoxyphenyl)piperazine-1-yl]-ethyl]-4,4-dimethyl-1,3(2H,4H)-isoquinolinedione (AR-C 239) 1-mu-M failed to antagonize the effect of clonidine. Higher concentrations of prazosin and AR-C 239 greatly accelerated the basal efflux of tritium. 4 The method used permits the functional determination of antagonist affinities undistorted by endogenous alpha-2-autoinhibition. A comparison with affinities derived from radioligand binding experiments indicates that the presynaptic alpha-2-autoreceptors in rabbit brain cortex are markedly different from the alpha-2B-subtype and probably belong to the prazosin-insensitive alpha-2A-subtype.