Serotonin induces four pharmacologically separable contractile responses in the pharynx of the leech Hirudo medicinalis

Stimulation of the serotoninergic innervation of the leech pharynx or appli cation of serotonin to the isolated pharynx induced four distinct types of contractile activity: an increase in basal tonus, large phasic contractions of 10-15 s in duration, smaller phasic contractions occurring at approxima tely 1 Hz, and a relaxation after washout of serotonin. Application to the isolated pharynx of the selective serotonin agonists (+/-)-8-hydroxy-2-(di- n-propylamino)tetralin N-(3-trifluoromethylphenyl)piperazine, 1-(m-chloroph enyl)-piperazine, (+/-)-2,5-dimethoxy-4-iodoamphetamine, 2-methyl-5-hydroxy trypamine, alpha-methyl-5-hydroxytryptamine, and 5-methoxytryptamine induce d distinct types of pharyngeal contractile activity. The results of this st udy suggest that the leech pharynx possesses more than one type of serotoni n receptor. (C) 1999 Elsevier Science Inc. All rights reserved.