Respiratory-related activity of lower thoracic and upper lumbar sympathetic preganglionic neurones in the rat.

1. The segmental distribution of cell bodies of sympathetic preganglionic neurones projecting into the left lumbar sympathetic chain beyond the L4 ganglion was determined in rats using retrograde transport of horseradish peroxidase. Labelled perikarya were found mainly in segments T13-L2 in the ipsilateral intermediolateral cell column. 2. Neurophysiological studies were conducted on this cell population in anaesthetized, vagotomized and artificially ventilated rats. 3. One hundred and thirty sympathetic preganglionic neurones were studied in spinal segments T13-L2, inclusive, after their antidromic identification following electrical stimulation of the left lumbar sympathetic chain between the 4th and 5th lumbar ganglia; sixty-six had on-going activity and thirty of the remainder were activated by the ionophoretic application of glutamate. Their axonal conduction velocities were 0.5-12.5 m/s which is in the B and C fibre range. 4. The discharge patterns of these sympathetic preganglionic neurones were analysed in relation to phrenic nerve discharge. Of forty-eight neurones analysed in this manner five had peak firing during inspiration (inspiratory related), five during early expiration (post-inspiratory related), four during expiration (expiratory related) with the remainder having a firing pattern unrelated to phrenic nerve discharge (non-modulated). 5. The ECG-related patterns of discharge of the same forty-eight neurones were analysed. Twenty-one had an ECG-related pattern of discharge. Glutamate-activated quiescent neurones and those with on-going activity had a similar incidence of such modulation. Sympathetic preganglionic neurones with each type of respiratory modulation were found to have an ECG-related firing pattern. 6. The patterns of discharge of these neurones have been compared to those recorded, in previous studies, from sympathetic preganglionic neurones located at T2 which had axons projecting into the cervical sympathetic nerve and 'sympathoexcitatory' neurones of the rostral ventrolateral medulla. There appear to be some differences in the respiratory modulations of baroreceptor-sensitive neurones found within each of the three populations. Consequently, it is suggested that supraspinal pathways, other than those arising in the rostral ventrolateral medulla, may be important in influencing the activity of baroreceptor-sensitive sympathetic preganglionic neurones.