Distribution of 3H-Morphine Following Lumbar Subarachnoid Injection in Unanesthetized Rabbits

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Abstract
Morphine sulfate (40-100 .mu.g) and 3H-morphine (125-200 pmol) were injected into the lumbar subarachnoid space of 18 unanesthetized rabbits through a surgically implanted catheter. Radioactivity remaining in the spinal cord 2, 4, 6, and 12 h later revealed recovery (mean .+-. SEM) of 45 .+-. 5.6% (n = 3), 30.5 .+-. 14.1% (n = 4), 11.23 .+-. 4.4% (n = 3), and 3.7 .+-. 1.1% (n = 3), respectively, of the injected radioactivity. Triated morphine was found to be predominantly centered around the injection site, with limited rostral and caudal spread in the cord. Nonsignificant radioactivity was detected in plasma or cerebrospinal fluid (CSF) samples from the cisterna magna taken at 5, 10, 30, min and 1, 2, 4, 6, 12, and 24 h after receiving radioactive labeled drug (with the exception of that in one rabbit). Of the injected radioactivity, 75% was recovered of morphine in the spinal cord could account for its prolonged analgesic effect following intrathecal administration.