Do noradrenergic descending tract fibres contribute to the depression of transmission from group II muscle afferents following brainstem stimulation in the cat?

Two alpha 2 noradrenaline antagonists, idazoxan and yohimbine, were injected in midlumbar segments of the spinal cord to test whether they counteract depression of field potentials evoked by group II muscle afferents by conditioning stimuli applied in the brainstem. The tested field potentials were those evoked monosynaptically in the intermediate zone of midlumbar segments. Their depression reflected thus the depression of transmission between group II fibres and their first relay neurones. The conditioning stimuli were applied either within the ipsilateral locus coeruleus/subcoeruleus or outside these nuclei (in the raphe magnus, raphe obscurus, or cuneiform nuclei). The brainstem evoked depression of the tested field potentials (n = 12) was reduced following injection of idazoxan or yohimbine to about two thirds of that which was evoked originally but in three cases to about one half. The study leads thus to the conclusion that noradrenergic descending tract neurones contribute to the depression of transmission from group II afferents to spinal interneurones and that such noradrenergic neurones are activated by stimuli applied within as well as outside their nuclei. However, the relative contribution of monoaminergic and non-monoaminergic descending tract neurones to the control of transmission from group II afferents to these neurones remains to be established.