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Spinal Cord Distribution of 3H‐Morphine after Intrathecal Administration: Relationship to Analgesia

The distribution of intrathecally administered 3H-morphine was examined by light microscopic autoradiography in rat spinal cord and temporal changes in silver grain localization were compared with results obtained from simultaneous measurements of analgesia. After tissue processing, radioactivity was found to have penetrated in superficial as well as in deeper layers (Rexed lamina V, VII, and X) of rat spinal cord within minutes after application. Silver grain density reached maximal values at 30 min in every region of cord studied. Radioactivity decreased rapidly between 30 min and 2 hr and then more slowly over the next 24 hr. In rats tested for responses to a thermal stimulus (tail flick test), intrathecal administration of morphine (5 and 15 μg) resulted in significant dose dependent analgesia that peaked at 30 min and lasted up to 5 hr (P < 0.5). There was a close relationship between analgesia and spinal cord silver grain density during the first 4 hr of the study. It is postulated that the onset of spinal morphine analgesia depends on appearance of molecules at sites of action followed by the activation of anti-nociceptive mechanisms.