Adrenal versus nonadrenal sympathetic preganglionic neurones in the lower thoracic intermediolateral nucleus of the cat: physiological properties

Adrenal and nonadrenal sympathetic preganglionic neurones (SPNs) in the intermediolateral nucleus of spinal segments T8–T10 in the cat were compared according to a number of physiological properties. An SPN was classified as "adrenal" (n = 37) if it could be antidromically activated by electrical stimulation of the adrenal medulla. An SPN that could not be activated from the adrenal medulla yet could be antidromically activated by electrical stimulation of the greater splanchnic nerve was classified as "nonadrenal" (n = 123). Approximately 50% of adrenal SPNs (17 out of 37) were activated antidromically by stimulation of both the greater splanchnic nerve and adrenal medulla, suggesting that these neurones projected to the adrenal medulla via the greater splanchnic nerve, with the other adrenal SPNs taking a different route. The mean conduction velocities of adrenal (6.7 ± 1.8 (SD) m/s) and nonadrenal (6.7 ± 1.5 m/s) sympathetic preganglionic axons were similar. Over 80% of adrenal (31 out of 37) and nonadrenal (104 out of 116) SPNs were spontaneously active. The two types of neurone were indistinguishable in terms of the rates and patterns of discharge. Adrenal SPNs discharged with a mean rate of 1.4 ± 1.1 spikes/s, and nonadrenal SPNs discharged with a mean rate of 1.8 ± 1.4 spikes/s. With both types of SPN, the pattern of spontaneous activity was either irregular or phasic. With the latter pattern, periodic bursts of discharge were at the same frequency as oscillations in arterial pressure, frequency of ventilation, or phrenic nerve discharge. These data suggest that adrenal and nonadrenal sympathetic preganglionic neurones in the intermediolateral nucleus in caudal thoracic segments share a number of common physiological properties.Key words: adrenal, sympathetic preganglionic neurone, spinal cord, lateral horn.

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 intermediolaterai 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.Key words: sympathetic preganglionic neurone, spinal cord, lateral horn, iontophoresis, serotonin, substance P, thyrotropin-releasing hormone.