Probable pre- and postsynaptic modifications by 5-hydroxytryptamine of contractile responses to electrical...

Two kinds of electrical stimulation, low frequency stimulation (5 Hz, 1 msec, 5 pulses, every 20 sec) and high frequency stimulation (30 Hz, 0.05-0.1 msec, 10 pulses, every 20 sec), produced contractions in the isolated guinea-pig vas deferens. These responses were blocked by tetrodotoxin but not hexamethonium. Phentolamine potentiated the contractions produced by low frequency stimulation, while it reduced the contractions produced by high frequency stimulation. Diametrically, 5-hydroxytryptamine reduced the contractions produced by low frequency stimulation, while it potentiated the contractions produced by high frequency stimulation. These inhibitory and potentiating actions of 5-hydroxytryptamine were reversed by cyproheptadine and 2-bromolysergic acid diethylamide. Moreover, that 5-hydroxytryptamine produced a depolarization of the smooth muscle membrane was shown by the sucrose gap technique. The results suggest that a 5-hydroxytryptamine receptor exists pre- and postsynaptically in the neuroeffector transmission of the guinea-pig vas deferens, that the stimulation of the presynaptic receptor by 5-hydroxytryptamine inhibits the release of a transmitter from noradrenergic nerves, and that the stimulation of the postsynaptic receptor by a high concentration of 5-hydroxytryptamine produces a depolarization of the smooth muscle membrane, and this relates to the potentiation of contractile responses.