PHARMACOLOGICAL CHARACTERIZATION OF 5-HT RECEPTORS IN PARASYMPATHETICINNERVATION OF RAT-HEART

A study was made of the effects of 5-hydroxytryptamine (5-HT) on brady cardia induced in vivo by electrical stimulation of the vagus nerves i n pithed rats pretreated with atenolol. 5-HT significantly decreased v agally induced, but not acetylcholine-induced, bradycardia. The first effect was blocked by methiothepin, ketanserin or methiothepin with ke tanserin. When 5-HT1 and 5-HT2 receptors were blocked, 5-HT produced a n increase in vagally induced bradycardia. Both the inhibition and the potentiation were blocked by simultaneous pretreatment with methiothe pin, ketanserin and MDL-72222. The 5-HT2 receptor agonist m-CPP (1-(3- chlorophenyl) piperazine dihydrochloride) caused an inhibition of vaga lly induced bradycardia whereas the 5-HT3 receptor agonist m-CPBG (1-( m-chlorophenyl)biguanide hydrochloride) produced a significant increas e. The data suggest the presence of presynaptic and/or ganglionic 5-HT 2 receptors in parasympathetic innervation of the rat heart, stimulati on of which inhibits the release of acetylcholine. The presence of 5-H T3 receptors is also suggested, stimulation of which induces the relea se of acetylcholine.