The influence of bulbospinal monoaminergic pathways on sympathetic nerve activity.

1. Spontaneous and reflex activity was recorded from renal and splanchnic nerves and thoracic white rami during discrete electrical stimulation within the medulla oblongata of anaesthetized cats.2. Inhibition or excitation of spontaneous sympathetic nerve activity was obtained from several medullary regions.3. The long-circuited reflex elicited in renal nerves and the spinally mediated reflex discharge produced in white rami by single shock stimulation of intercostal nerves were inhibited by stimulation within the sympatho-inhibitory areas of the medulla.4. Activation of spontaneous sympathetic nerve activity or inhibition of spontaneous and reflex sympathetic nerve activity was obtained during electrical stimulation within the lateral funiculi of the cervical spinal cord in unanaesthetized decerebrate cats, spinalized at C1.5. There was a correlation between the position of some sympatho-inhibitory regions of the medulla and spinal cord and the position of the cell bodies and axons of descending monoamine-containing neurones.6. Intravenous administration of the precursor of noradrenaline, L-DOPA, to unanaesthetized decerebrate cats, spinalized at C1, was followed by a depression of spontaneous activity in renal nerves and reflex responses elicited in renal nerves and white rami.7. Similarly the precursor of 5-hydroxytryptamine, 5-HTP, caused a depression of reflex activity elicited in renal nerves and white rami, but had no effect on spontaneous renal nerve activity.8. It is suggested that there exist both noradrenergic and tryptaminergic pathways which descend to the spinal cord from the medulla and which are inhibitory to sympathetic outflow.