Effect of Spinal Transection on the Metabolism of 5‐Hydroxyindoles Formed in vivo from3H‐Tryptophan in the Rat Spinal Cord

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Abstract
The nerve impulse flow in 5‐hydroxytryptamine (5‐HT) neurons descending to the caudal part of the spinal cord of rats was interrupted by a spinal transection. At various time intervals after the operation3H‐tryptophan was administered i.v. and the accumulations of labelled 5‐HT and 5‐hydroxyindoleacetic acid (5‐HIAA) in the cord were determined. 6–8 days after the section the cord caudal to the lesion was almost depleted of 5‐HT and very little3H‐5‐HT was formed from the labelled precursor. Between 8 and 24 h post‐lesion the level of 5‐HT was increased by 20 % below the lesion, whereas the accumulation of3H‐5‐HT was markedly reduced. Immediately after the transection the endogenous 5‐HT level as well as the3H‐5‐HT accumulation were unchanged in the caudal cord. The accumulation of3H‐5‐HIAA below the section was reduced at all time intervals after the operation. The specific activity of tryptophan seemed to be increased following nerve section. The results indicate that 5‐HIAA synthesis is controlled by nerve impulse activity. The synthesis of 5‐HT was not directly dependent on neuronal activity or tryptophan levels but seemed to be inversely correlated to the endogenous 5‐HT level.