Effects of serotonin and some other neurohumoral agents on adrenergic neurotransmission in spontaneously hypertensive rat vasculature.

The effects of serotonin (5HT), acetylcholine (ACh), histamine and dopamine on the pressor responses of the mesenteric vasculature were examined in view of their potential role in neuromodulation. The responses to periarterial sympathetic nerve stimulation (NS, 8 Hz, 2 msec, 30 sec) and to exogenous norepinephrine (NE, 0.2 nmol) were compared between spontaneously hypertensive rats (SHR) and the control Wistar Kyoto rats (WKY). In both WKY and SHR, ACh (3-30 nM), histamine (0.3-3 microM) and dopamine (0.3 microM) attenuated the NS-induced vasoconstrictor response as much as the NE-induced response, indicative of predominance of postsynaptic inhibition. 5HT (10-100 nM) potentiated the vasoconstrictor responses to NS significantly less than that to NE in WKY, suggestive of presynaptic inhibition. Such difference was absent in SHR. These results suggest that the presynaptic inhibition of vascular adrenergic neurotransmission by 5HT is diminished in SHR, and this may contribute to the elevated blood pressure.