Motoneurone Depression by Norepinephrine

Abstract

Dahlström and Fuxe have shown histochemically that norepinephrine (NE) is localized in a pathway from the brain stem to the spinal cord with terminals predominantly in the dorsal and ventral horns¹. Biochemical investigations demonstrating the release of NE from the spinal cord by stimulation of descending tracts² and almost complete disappearance of NE below a spinal transection³ provide additional evidence for a descending NE pathway. A pharmacological investigation of the sensitivity of neurones in the spinal cord had previously reported⁴ that interneurones were not sensitive to the electrophoretic administration of NE in barbiturate anaesthetized cats (the effect of NE on motoneurones was apparently not investigated). Because it is known that barbiturate anaesthesia can reduce or abolish pharmacological responses in the spinal cord^5,6, we reinvestigated this problem in decerebrated cats and a few cats anaesthetized with ether and found that Renshaw cells and a number of interneurones are sensitive to NE (refs. 7 and 8). Similar findings have now been reported by other investigators^9,10. During the course of these investigations, the extracellular action potentials of single motoneurones were only occasionally recorded; the investigation of these cells revealed that NE depressed the firing of some motoneurones.