The effect of tryptamine on serotonin release from hypothalamic slices is mediated by a cholinergic interneurone.

Tryptamine produced a concentration-related inhibition of potassium-evoked release of tritium from slices of rat hypothalamus preloaded with 3H-serotonin. This effect of tryptamine was blocked by a series of serotonin antagonists with a relative order of potency which suggested that tryptamine was acting on a post-junctional serotonin receptor. However, the response to tryptamine was also blocked by tetrodotoxin, indicating that tryptamine may be acting indirectly via the release of a second neurotransmitter. The finding that physostigmine enhanced, whilst atropine antagonised the effect of tryptamine suggests that the second neurotransmitter may be acetylcholine. This possibility is discussed.