The classification of peripheral 5-HT2-like receptors using tryptamine agonist and antagonist analogues

In a previous study, we attempted to verify the classification of 5-hydroxytryptamine2 (5-HT2) receptors in three vascular tissues, by use of the conventional antagonists, ketanserin, spiperone, methysergide and trazodone. However, it was not possible to conclude homogeneity of the receptor type in the three tissues due to the inconsistent behaviour of these antagonists, in particular, their apparently variable affinities between the tissues. These results led to the reliability of the conventional antagonists being questioned as receptor probes. In the present study, a set of tryptamine analogues were investigated in two of the tissues, the rabbit aorta and the rat jugular vein. Unlike the conventional antagonists, these compounds bear a close chemical relation to the natural agonist, 5-HT. In both tissues, alpha, alpha-dimethyltryptamine demonstrated apparently simple competitive antagonism of 5-HT-induced constrictions. Its affinity was estimated to be the same in each case. The affinities and relative efficacies of 5-HT, 5-cyanotryptamine, N,N-dimethyltryptamine and N-benzyl-5-methoxytryptamine were also found to be indistinguishable between the two tissues. Unlike the conventional 5-HT2 receptor antagonists, these tryptamine analogues failed to distinguish between the 5-HT receptors in the rabbit aorta and rat jugular vein implying that they truly belong to the same class. In view of this result, it is suggested that simple tryptamine analogues are more reliable probes for 5-HT receptor classification than ligands which bear little or no chemical relation to the natural agonist.