Presynaptic alpha2-autoreceptors in rat and rabbit brain cortex were compared by means of antagonists and agonists. Brain cortex slices were preincubated with [H-3]-noradrenaline and then superfused and stimulated by 3 (rat) or 4 (rabbit) pulses at a frequency of 100 Hz. The alpha2-adrenoceptor agonist bromoxidine (UK 14304) reduced the electrically evoked overflow of tritium with EC50 values of 4.5 nmol/l in the rat and 0.7 nmol/l in the rabbit. The antagonists phentolamine, 2-[2H-(1-methyl-1,3-dihydroisoindole)methyl]-4,5-dihydroimidazole (BRL 44408), rauwolscine, 1,2-dimethyl-2,3,9,13b-tetrahydro-1H-dibenzo(c,f)imidazo(1,5-a)azepine (BRL 41992), 2-(2,6-dimethoxyphenoxyethyl)aminomethyl-1,4-benzodioxane (WB 4101), 6-chloro-9-[(3-methyl-2-bute-nyl)oxy]-3-methyl-1H-2,3,4, 5-tetrahydro-3-benzazepine (SKF 104078), imiloxan, prazosin and corynanthine did not per se increase the evoked overflow of tritium but shifted the concentration-inhibition curve of bromoxidine to the right in a manner compatible with competitive antagonism. Up to 4 concentrations of each antagonist were used to determine its dissociation constant K(D). The K(D) values correlated only weakly between the rat and the rabbit. Dissociation constants K(A) of bromoxidine were calculated from equieffective concentrations in unpretreated brain slices and slices in which part of the alpha2-adrenoceptors had been irreversibly blocked by phenoxybenzamine. The K(A) value was 123 nmol/l in the rat and 7.2 nmol/l in the rabbit. The results confirm the species difference between rat and rabbit brain presynaptic alpha2-autoreceptors. Comparison with data from the literature indicates that the rat brain autoreceptors can be equated with the alpha2D subtype as defined by radioligand binding, whereas the rabbit brain autoreceptors conform to the alpha2A subtype. For example, the antagonist affinities for the rat autoreceptors correlate with their binding affinities for the gene product of alpha2-RG20, the putative rat alpha2D-adrenoceptor gene (r = 0.97; P < 0.01), but not with their binding affinities for the gene product of alpha2-C10, the putative human alpha2A-adrenoceptor gene. Conversely, the rabbit autoreceptors correlate with the alpha2-C10 (r = 0.98; P<0.001) but not with the alpha2-RG20 gene product. Since presynaptic alpha2-autoreceptors are also alpha2D in rat submaxillary gland and perhaps vas deferens and alpha2A in rabbit pulmonary artery, the possibility arises that the majority of alpha2-autoreceptors generally are alpha2D in the rat and alpha2A in the rabbit. Moreover, receptors of the alpha2A/D group generally may be the main mammalian alpha2-autoreceptors.
