Pharmacological analysis of the variation in behavioural responses to tryptamine in five strains of mice

The effects of tryptamine on behaviour were studied in five strains of mice. There were significant strain differences in the intensity of the 5-HT syndrome (head weaving and hindlimb abduction) and head twitch responses. The intensity of the 5-HT syndrome correlated significantly with the brain tryptamine content, although the occurrence of head twitch was unrelated. The 5-HT₂ receptor antagonist, ketanserin, antagonized the head twitch responses elicited by tryptamine without affecting the head weaving or hindlimb abduction. Metergoline, an antagonist of both the 5-HT₁ and 5-HT₂ receptors, strongly inhibited both the 5-HT syndrome and the head twitch responses in all five strains. ³H]5-HT and ³H]ketanserin binding in the frontal cortex of the five strains of mice was also studied. The number of ³H]5-HT binding sites did not differ among the five strains of mice although there was a noticeable difference in the number of ³H]ketanserin binding sites. These results suggest that the 5-HT syndrome and the head twitch responses induced by tryptamine are linked separately with the 5-HT₁ and 5-HT₂ receptors, respectively. The strain differences in the tryptamine-induced 5-HT syndrome can be explained by the different levels of brain tryptamine. The differences in the head twitch responses in the mice are probably derived from the differences in the 5-HT₂ receptor binding sites in the brain and the content of brain tryptamine.