Psycholeine, a natural alkaloid extracted from Psychotria oleoides, acts as a weak antagonist of somatostatin.

The effects of psycholeine, a plant alkaloid, were investigated on binding of radiolabelled somatostatin ([125I]N-Tyr-SRIF) and on somatostatin (SRIF)-induced inhibition of adenylate cyclase activity and growth hormone (GH) secretion by rat anterior pituitary cells. Psycholeine was shown to displace specific binding of [125I]N-Tyr-SRIF to pituitary membrane preparations, with an IC50 of 10(-5) M. At this concentration, psycholeine was also effective in significantly reducing the SRIF-induced inhibition of adenylate cyclase activity previously stimulated by growth hormone releasing factor (GRF). In parallel, it reduced the SRIF-induced inhibition of GH release stimulated by GRF in primary pituitary cell cultures in a dose-dependent manner. At a moderate concentration, the alkaloid affected neither adenylate cyclase activity nor GH release when applied in the absence of SRIF. These data suggest that psycholeine has antagonistic properties at the SRIF receptor. Quadrigemine C, a precursor of psycholeine, has a similar action.