The relation between cerebral serotonin levels and conditioned behaviour in the rat following the administration of LSD-25 and UML.

Successive daily injections of LSD-25 and UML (1-methyl-d-lysergic acid butanolamide) caused progressive depression of brain 5-HT levels in the rat. On the fourth day, the decrease was significant with respect to the highly significant fall observed after a single administration, whereas it had been shown earlier that conditioned behaviour is no longer affected by LSD-25 after 3 days and that simultaneous administration of a single dose of LSD-25 and UML is equally ineffective in this respect. Its depression of 5-HT levels, however, has now been shown to be equal to that of LSD-25 alone at doses that influence conditioned behaviour. The findings indicate that changes in such behaviour are not dependent on brain 5-HT levels and that no link exists between such levels and the psychotomimetic effect of LSD-25 in man.