EFFECTS OF SEROTONIN ON SYMPATHETIC NORADRENERGIC TRANSMISSION IN RABBIT ISOLATED EAR ARTERY

The amplifying effect of serotonin in subconstrictor concentrations on steady-state vasoconstrictor responses to norepinephrine (NE; 0.001-3 .mu.M) and to stimulation of periarterial sympathetic nerves (SNS; 0.3-ms pulses, 0.1-3 Hz) has been studied in rabbit isolated ear artery. Ketanserin (30 nM) by itself had no effect on responses to NE or SNS; however, the amplifying effect of serotonin (10 nM) on responses to NE was abolished; ketanserin also markedly reduced the amplifying effect of serotonin on responses to SNS. Idazoxan (100 nM) had no effect on responses to NE or SNS; furthermore, idazoxan had no effect on the amplifying effect of serotonin. Prazosin (10 nM) reduced responses to NE and SNS; however, prazosin had no effect on the amplifying effect of serotonin on responses to NE and SNS. Intraluminally applied serotonin increased responses to intraluminal and extraluminal NE and to SNS. Extraluminally applied serotonin increased responses to extraluminal NE and to SNS. Extraluminally applied serotonin increased responses to extraluminal NE but not to intraluminal NE or SNS; however, in the presence of cocaine (1 .mu.M), an amplifying effect of extraluminal serotonin on responses to intraluminal NE and to SNS was also observed. It thus appears that the amplification by serotonin of the .alpha.1-adrenoceptor-mediated responses to NE and SNS in rabbit ear artery involves postjunctional 5-HT2-receptor activation. Moreover, the neuronal uptake process appears to modify the amplifying effect of serotonin.