A comparison of the effects of intrathecally administered 5-hydroxytryptamine and thyrotropin-releasing hormone on renal and muscle sympathetic nerve activity.

Thyrotropin-releasing hormone (TRH) has been shown to coexist with 5-hydroxytryptamine (5-HT) in spinally projecting raphe neurones, some of which terminate in the sympathetic nuclei of the spinal cord. In an attempt to mimic the actions of these neurones, the effects of intrathecal administration of 5-HT was compared to that of TRH on activity in two sympathetic postganglionic nerves, the renal nerve and sympathetic fibres to skeletal muscle of the hind limb. In chloralose-urethane anaesthetised rats intrathecal infusion of TRH at the T9 level in doses of 5-30 micrograms increased activity in renal nerve as did low doses (20-100 micrograms) of 5-HT. In contrast, intrathecal infusion of TRH in doses of 5-30 micrograms and 5-HT in doses of 20-800 micrograms at L4 level decreased activity in sympathetic fibres to muscle. The results show that TRH, unlike 5-HT, has an excitatory but not an inhibitory action on renal sympathetic activity whereas both substances have only an inhibitory action on muscle sympathetic activity.