Analysis of the 5-HT receptor in rabbit saphenous vein examplifies the problems of using exclusion criteria for receptor classification

Publisher Website
Google Scholar
Abstract
5-Hydroxytryptamine (5-HT) contracts ring preparations of rabbit saphenous vein via direct and indirect components, the latter being- compatible with a “tyramine-like” action at sympathetic nerve terminals. Here an attempt was made to establish the identity of the receptor mediating contraction directly, in terms of the currently accepted proposals (Bradley et al. 1986). Results with agonists suggested 5-HT1-like receptor activation: methysergide behaved as a partial agonist with microcolar affinity and 5-HT effects were mimicked by 5-carboxamidotryptamine (5-CT) and GR43175. The agonist potency order was 5-CT > 5-HT > methysergide ≥ GR43175, the same as that reported at the 5-HT1-like receptor in dog saphenous vein (Feniuk et al. 1985; Humphrey et al. 1988). Consistent with this, 5-HT effects were resistant to blockade by the selective 5-HT3 receptor antagonist MDL72222 (1.0 μmol/l). In contrast, methiothepin (0.01–0.3 μmol/l), ketanserin (0.3–30.0 μmol/l) and spiperone (0.3–30.0 μmol/l) each produced surmountable antagonism which, although competitivv in nature only for methiothepin (pKB = 9.45 ± 0.09, 17 d. f.), implied 5-HT2 receptor involvement. The possibility that these discrepancies resulted from mixed populations of 5-HT1-like and 5-HT2 receptors can be excluded because; 1). Ketanserin and spiperone blocked the actions of 5-HT and the selective 5-HT1-like receptor agonist GR43175 with equal facility and 2). Responses to all of the agonists studied were similarly antagonised by flesinoxan (pKB∼ 6.4), a simple competitive antagonist at the receptor in rabbit saphenous vein. This novel result with flesinoxan demonstrates that the ligand displays affinity at 5-HT receptors other than the 5-HT1A subtype. These data show that the 5-HT receptor in rabbit saphenous vein shares features in common with, and may be identical to, the 5-HT1-like receptor in dog saphenous vein. However, unlike the latter it demonstrates qualities evident in both 5-HT1-like and 5-HT2 receptors and for this reason fails to meet the currently accepted criteria for admission into any of the recognised classes. It is suggested that this sort of problem reflects the generally unreliable behaviour of the available receptor antagonists and the emphasis which the Bradley et al. (1986) scheme places upon them for classification by exclusion. A complementary approach which provides a rigorous, quantitative basis for receptor differentiation uses “finger-prints” comprising affinity and relative efficacy estimates for a set of tryptamines. This study illustrates the power and economy of this approach by showing how affinity and relative efficacy “finger-prints” obtained using 5-HT, 5-CT, (±) α-methyl-5-HT, 5-methyltryptamine and N,N-dimethyltryptamine establish a positive identity for the 5-HT receptor in rabbit saphenous vein and at the same time enable it to be distinguished from other 5-HT receptor types presently allocated to the 5-HT1-like, 5-HT2 and so-called “orphan” receptor classes.