Thyrotropin-releasing hormone in spinal cord: coexistence with serotonin and with substance P in fibers and terminals apposing identified preganglionic sympathetic neurons.

In this study we utilized the technique of simultaneous immunofluorescent double-labeling to investigate possible coexistence of the putative neurotransmitter thyrotropin-releasing hormone (TRH) with serotonin (5-HT) and with substance P (SP) in the intermediolateral cell column (IML) of rat spinal cord. We observed fibers and terminals immunoreactive for both TRH and 5-HT or TRH and SP in IML. In addition, this technique was used in animals in which we retrogradely labeled, with fluorescent tracer dyes, preganglionic sympathetic neurons within IML from either the adrenal medulla or the proximal cut end of the cervical sympathetic trunk. In these animals, fibers and terminals containing these combinations of neurotransmitters appeared to oppose identified preganglionic sympathetic neurons in IML. These data represent the first direct immunohistochemical demonstration of fibers and terminals in spinal cord which display coexistence of TRH- with either 5-HT- or SP-immunoreactivity. In addition, the proximity of TRH-immunoreactive fibers and terminals to sympathetic preganglionic neurons in IML support a role for TRH in the regulation of central sympathetic outflow.