An excitatory action of adrenaline, noradrenaline, and 5‐hydroxytryptamine on the spinal cord

Abstract

SUMMARY1. Fasciculation was produced in the trapezius and sternomastoid muscles of anaesthetized cats by adrenaline, noradrenaline and 5-hydroxytryptamine (5-HT) put into the cisterna magna. Isoprenaline, similarly applied, was ineffective.2. Each of the three active amines was without effect when applied for a second time after the fasciculation in response to the first application had passed off, but 5-HT was effective after adrenaline and, similarly, adrenaline after 5-HT.3. Fasciculation produced by adrenaline, but not by 5-HT, was inhibited by ergotamine or phenoxybenzamine, given intravenously.4. In producing fasciculation, the adrenaline appeared to be acting on or through the lateral aspect of the upper cervical cord, about the line of emergence of the roots of the spinal accessory nerve, mainly in C 1 and C 2.5. The electromyograms of the fasciculation due to adrenaline and 5-HT showed intermittent bursts of activity. After adrenaline, the bursts consisted of fewer spikes than after 5-HT. The intervals between consecutive spikes were 3'75-5 msec after either amine, and the bursts occurred irregularly, at frequencies between 7 and 12/sec. 6. It is suggested that adrenaline and 5-HT have excitatory actions on the dendrites or somata of the spinal accessory motor neurones, and the possible role of these amines as synaptic transmitters in the spinal nucleus of the accessory nerve is discussed.