Inhibition of sympathetic activity by stimulating in the raphe nuclei and the role of 5-hydroxytryptamine in this effect

The possibility that the putative transmitter 5-hydroxytryptamine (5-HT) is involved in the mediation of long latency to onset raphe-spinal inhibition of sympathetic preganglionic neurones was investigated in anaesthetized cats by stimulating sites located in nucleus raphe pallidus and obscurus and recording sympathetic discharge in T₃ or T₁₀ white rami evoked either reflexively or by intraspinal stimulation at cervical level. Several putative 5-HT anttagonists were administered intravenously (i.V.) or topically to the spinal cord. In 7 cats lysergic acid diethylamide (LSD) in a dose range 25–50 μg/kg i.v. or 0.6 μg topically, reversibly reduced the raphe spinal inhibition by 40–100%. Topical application was more effective than i.v. administration. In 5 cats stimulating within the ventromedial reticular formation at sites unlikely to involved 5-HT neurons produced a short latency to onset inhibition which was unaffected by LSD. Methysergide, cinanserin and cyproheptadine depressed sympathetic discharge in the absence of brain stimulation in cats with CNS intact and in unanaesthetized decerebrate spinal cats. The results are discussed in the light of the known actions of the putative 5-HT antagonists.