THE ACTION OF 5‐HYDROXYTRYPTAMINE AND RELATED COMPOUNDS ON THE ACTIVITY OF RETZIUS CELLS OF THE LEECH Hirudo medicinalis

1 The equipotent molar ratios of a range of tryptamine analogues, as compared with 5‐hydroxytryptamine (5‐HT), have been determined on the basis of their ability to hyperpolarize the membrane potential of the Retzius cell of the leech, Hirudo medicinalis. 2 The substitution of methyl, fluoro, chloro, methoxy or acetyl groups onto the 5‐HT molecule progressively reduced the potency. 3 5‐Methoxylation or terminal N‐methylation of tryptamine considerably increased the potency of tryptamine but these compounds tended to depolarize cells rather than cause hyperpolarization. In some experiments they were ineffective on preparations pretreated with 5‐HT. 4 It is suggested that these compounds may act by a different mechanism from the 5‐hydroxylated indoles, perhaps involving a different receptor.