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 bin ding of radiolabelled somatostatin ([I-125]N-Tyr-SRIF) and on somatost atin (SRIF)-induced inhibition of adenylate cyclase activity and growt h hormone (GH) secretion by rat anterior pituitary cells. Psycholeine was shown to displace specific binding of [I-125]N-Tyr-SRIF to pituita ry membrane preparations, with an IC50 of 10(-5) M. At this concentrat ion, psycholeine was also effective in significantly reducing the SRIF -induced inhibition of adenylate cyclase activity previously stimulate d by growth hormone releasing factor (GRF). In parallel, it reduced th e SRIF-induced inhibition of GH release stimulated by GRF in primary p ituitary cell cultures in a dose-dependent manner. At a moderate conce ntration, the alkaloid affected neither adenylate cyclase activity nor GH release when applied in the absence of SRIF. These data suggest th at psycholeine has antagonistic properties at the SRIF receptor. Quadr igemine C, a precursor of psycholeine, has a similar action.