The effect of 5-hydroxytryptamine on the chick biventer cervicis muscle.

The actions of 5-hydroxytryptamine have been studied in the chick biventer cervicis nerve--muscle preparation. At concentrations between 2.5 X 10(-3) and 3.7 X 10(-3) M, 5HT produced a transient increase in responses to indirect stimulation, but not to direct stimulation. At higher concentrations, 5HT produced a reversible depression of twitches, which was much more marked in indirectly than in directly stimulated preparations. In a concentration range of 2.5-3.7 X 10(-3) M, 5HT increased the response to exogenous acetylcholine but reduced the response to carbachol. After pretreatment with neostigmine, 5HT inhibited responses to both acetylcholine and carbachol. It is suggested that the facilitatory effects of 5HT are due to its anticholinesterase activity. The blocking actions of 5HT are thought to be curare-like. In addition, irreversible toxic effects on the muscle preparations were found after repeated exposures to high concentrations of 5HT.