The effects of several narcotic analgesics on brain levels of 3-methoxy-4-hydroxyphenylethylene glycol sulfate in the rat

The acute administration of levorphanol, morphine, anileridine, methadone, cyclazocine and pentazocine was found to increase brain levels of 3-methoxy-4-hydroxyphenylethylene glycol sulfate (MOPEG-SO₄) in rats. Drug-induced increases in brain levels of this norepinephrine metabolite were dose-dependent and peak drug effects generally occurred 1 hr after intraperitoneal injection. Six to 8 hr after opiate agonist or partial agonist administration, MOPEG-SO₄ levels returned to control values or below. The narcotic effect appeared to be stereospecific, since high doses of d-methadone and dextrorphan produced either no change or only minimal increases in brain MOPEG-SO₄ levels when compared with saline-treated controls. The relative potency of the analgesics tested in producing increases in brain MOPEG-SO₄ levels appeared to correlate reasonably well with the ability of these drugs to produce other characteristic pharmacological effects. The drug-induced increases in brain MOPEG-SO₄ levels were antagonized by naloxone. The rate of disappearance of MOPEG-SO₄ from the brains of animals treated with pargyline was not decreased by the opiate agonists or partial agonists tested, indicating that these drugs did not inhibit the acid transport process. These results indicate that the production of an increase in brain norepinephrine turnover is a specific component of the pharmacological actions of narcotic analgesics.