The facilitatory actions of 5-hydroxytryptamine and bradykinin in the superior cervical ganglion of the rabbit.

1 The effects of 5-hydroxytryptamine (5-HT) on ganglionic transmission and on intrinsic modulation of transmission have been re-examined and compared with the effects of bradykinin by means of electrophysiological techniques.2 Early facilitation, which is maximal 40-75 ms after a conditioning stimulus, was considerably enhanced by 5-HT. This enhancement was concentration-dependent, the threshold concentration lying between 0.1 and 1 muM. With concentrations of 5-HT 10 muM or greater, there was some depression of the Sa response to the conditioning stimulus.3 5-HT reduced or abolished the inhibition of a test response induced by a conditioning response 100-300 ms earlier. Facilitation was observed at these intervals at concentrations of 5-HT of 25 muM or greater.4 Late facilitation, which is maximal 700-2000 ms after a conditioning stimulus, was increased by 5-HT, but the effect was not as great as on early facilitation and was not always seen with a concentration of 1 muM.5 Bradykinin reduced early facilitation but increased the amplitude of the transmitted action potential in response to a single stimulus. The threshold concentration producing these effects was between 1 and 2 muM.6 5-HT produced a rapid depolarization of the ganglion cell membrane which was followed by an after-hyperpolarization.7 Bradykinin either produced no measurable change in ganglion cell resting potential or only very small, transient depolarizations.8 The depression of transmission, enhancement of intrinsic facilitation and the depolarization of the ganglion cell membrane induced by 5-HT may indicate more than one mode of action of this amine at the ganglionic synapse.