Morphine Concentration in Brain and Spinal Cord after Subarachnoid Injection in Baboons

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Abstract
Tritium-labeled morphine was injected into the lumbar (L4-5) subarachnoid space of 3 baboons. The animals were sacrificed 3, 6 and 24 h thereafter. Morphine concentrations were measured at 5 predetermined positions within the spinal cord, medulla oblongata and frontal lobes of the brain by scintillation-count assay. Morphine evidently ascends in the subarachnoid space and is absorbed into the spinal cord and medulla oblongata in a time-dependent fashion. Ventilation was most depressed and maximal concentrations of morphine were detected in the medulla 6 h after injection. Delayed respiratory depression, occasionally reported after intraspinal morphine injection, may therefore be caused as a result of the affinity of morphine for binding sites, possibly opiate receptors, situated within the vital respiratory and cardiovascular neuronal complexes of the medulla.

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