Pharmacology of the allodynia in rats evoked by high dose intrathecal morphine.

Morphine sulfate in doses of 90 to 150 micrograms/3 microliters evoke a prominent behavioral syndrome characterized by 1) periodic bouts of spontaneous agitation during which the rat scratches and bites at the skin of the caudal dermatomes and 2) vigorous agitation, vocalization and coordinated efforts to bite and escape evoked by a light tactile stimulus applied to the flank, suggestive of a pain state (allodynia). The phenomenon is not reversed by naltrexone or is it subject to tolerance. The ordering of activity of an opioid alkaloid related agent in producing this touch-evoked agitation is: noroxymorphone-3-glucuronide, morphine-3-glucuronide, morphine-3-ethereal sulfate, dihydromorphine, noroxymorphone dihydrate, hydromorphone, dihydrocodeine tartrate, morphine sulfate, dihydroisomorphine, morphine-HCl, 6-acetylmorphine, N-normorphine-HCl and (+)-morphine. The following agents were essentially without effect at the highest doses examined: 3,6-diacetylmorphine, N-normeperidine-HCl, nalorphine-HCl, alfentanil, sufentanil, naloxone, naltrexone, methadone, dextrorphan tartrate, meperidine-HCl, oxycodone, levorphanol, oxymorphone, codeine phosphate, thebaine, nalbuphine and naltrexone-3-glucuronide. The observations that the sulfated and conjugated metabolites are 10 to 50 times more potent than their unmetabolized precursor suggest the possibility that, in high concentrations certain phenanthrene opioid alkaloids with a free 3-OH position, an ether bridge and no N-methyl extension will be subject to conjugation and this metabolite will alter the processing of otherwise innocuous tactile stimuli. The fact that the phenomenon appeared at least partially stereospecific may reflect upon the fact that other laboratories have shown that glucuronyl transferase may preferentially convert (-)-morphine to the 3-glucuronide and (+)-morphine to the 6-glucuronide which may be less active.(ABSTRACT TRUNCATED AT 250 WORDS)