Adrenal versus nonadrenal sympathetic preganglionic neurones in the lower thoracic intermediolateral nucleus of the cat: effects of serotonin, substance P, and thyrotropin-releasing hormone.

Adrenal and nonadrenal sympathetic preganglionic neurones (SPNs) in the intermediolateral nucleus of spinal segments T8-T10 in the cat were compared according to their responses to iontophoretic application of serotonin, substance P, and thyrotropin-releasing hormone (TRH). Responses of both types of SPN to iontophoretic application of serotonin were characterized by an increase in the rate of discharge that was slow in onset (mean +/- SD = 36 +/- 21 s) and prolonged in afterdischarge (115 +/- 70 s) following termination of application. Depression was never observed and responses were similar whether using serotonin at a pH of 3.3 or 4.5, suggesting that the absence of a depressant effect cannot be accounted for by pH, as has been reported with cortical neurones. Iontophoretic application of methysergide resulted in a decrease in the rate of discharge of both types of SPN and blocked the excitatory responses to serotonin. Adrenal and nonadrenal SPNs were excited by iontophoretic application of substance P. Responses of both types of SPN were similar and were characterized by a gradual increase in the rate of discharge that was slow in onset (42 +/- 27 s) and prolonged in afterdischarge (96 +/- 42 s). Finally, adrenal and nonadrenal SPNs were also weakly excited by iontophoretic application of TRH. These responses were slow in onset (48 +/- 27 s) and prolonged in afterdischarge (78 +/- 35 s). These data indicate that serotonin, substance P, and TRH exert excitatory effects on functionally dissimilar sympathetic preganglionic neurones and support the possibility that they may be chemical mediators of synaptic transmission in the intermediolateral nucleus. In addition, these data may be interpreted to support the notion that serotonin, substance P, and TRH are involved in global activation of the sympathetic nervous system.