| Abstract: | The study was designed to classify in terms of  2A, 2B, 2C and 2D the presynaptic 2-autoreceptors, as well as the 2-receptors modulating the release of acetylcholine, in the myenteric plexus-longitudinal muscle (MPLM) preparation of the guinea-pig ileum. A set of antagonists was chosen that was able to discriminate between the four subtypes. Small pieces of the MPLM preparation were preincubated with 3H-noradrenaline or 3H-choline and then superfused and stimulated electrically.The stimulation periods used (3H-noradrenaline: 3 trains of 20 pulses, 50 Hz, train interval 60 s; 3H-choline: single trains of 30 pulses, 0.2 Hz) did not lead to 2-autoinhibition or inhibition of 3H-acetylcholine release by endogenous noradrenaline. The 2-selective agonist 5-bromo6-(2-imidazolin-2-ylamino)-quinoxaline (UK 14,304) reduced the evoked overflow of tritium in both 3H-noradrenaline and 3H-choline experiments. Most (3H-noradrenaline) or all (3H-choline) of the 10 antagonists shifted the concentration-inhibition curves of UK 14,304 to the right. pKd values of the antagonists were calculated from the shifts. pKd values from 3H-noradrenaline experiments correlated with pKd values from 3H-choline experiments (r = 0.981).It is concluded that 2-autoreceptors and 2-heteroreceptors modulating the release of acetylcholine in the MPLM preparation are of the same subtype. Comparison with antagonist affinities for prototypic native 2 binding sites, binding sites in cells transfected with 2 subtype genes, and previously classified presynaptic 2-adrenoceptors — all taken from the literature — indicates that both are 2D. The results are consonant with the hypothesis that at least the majority of 2-autoreceptors belong to the 2A/D branch of the 2-adrenoceptor tree, across mammalian or at least across rodent and lagomorph species. The same may hold true for 2-adrenoceptors on non-noradrenergic neurones. |
