Effects of serotonin1 and serotonin2 receptor agonists and antagonists on blood pressure, heart rate and sympathetic nerve activity.

Previous studies indicate that serotonin (5-HT) neurons provide a tonic excitatory input to central sympathetic neurons. The purpose of the present study was to utilize a number of 5-HT agonists in order to provide insights into the general function of the serotonergic system in the regulation of central sympathetic pathways. The 5-HT1A agonists 8-hydroxy-dipropylaminotetralin and p-aminophenyl-ethyl-m-trifluoromethylphenyl piperazine produced a dose-related inhibition of sympathetic nerve discharge (SND) recorded from either the postganglionic inferior cardiac nerve or the preganglionic splanchnic nerve in chloralose-anesthetized cats. The sympatholytic effects of 8-hydroxy-dipropylaminotetralin and p-aminophenyl-ethyl-m-trifluoromethylphenyl piperazine were accompanied by hypotension and bradycardia. The effects of 5-HT1A agonists were reversed by the 5-HT1A antagonist spiperone. In contrast, spiperone alone produced decreases in blood pressure, heart rate and SND. The 5-HT1B agonists 1[3(trifluoromethyl) phenyl]-piperazine, 1-(3-chlorophenyl) piperazine and 1-(2-methoxyphenyl) piperazine all produced variable effects on SND. In some experiments, SND was increased by these agents, whereas it was decreased in others. The 5-HT2 agonist 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane produced a marked increase in SND. A second 5-HT2 agonist, MK212, produced similar effects. The role of 5-HT receptor subtypes in mediating the 5-HT excitation of sympathetic neurons is discussed. It is suggested that 5-HT1A agonists inhibit SND through a process of disfacilitation by inhibiting the firing rate of 5-HT neurons. Possible mechanisms by which 5-HT2 agonists increase SND are proposed.