Molecular Pharmacology, Vol 16, 154-162, Copyright © 1979 by the American Society for Pharmacology and Experimental Therapeutics

Effect of Opiate Agonists and Antagonists on Lipid Bilayer Fluidity

¹ Langley Porter Neuropsychiatric Institute and Departments of Pharmacology, Biochemistry/Biophysics and Psychiatry, University of California, San Francisco, San Francisco, California 94143

To determine whether opiates increase the fluidity of lipid bilayers that contain opiate-binding lipids, multilamellar bilayers composed of dipalmitoylphosphatidylcholine and 1 mol % cerebroside sulfate, phosphatidylserine, 1-phosphatidylinositol, or 1-phosphatidyl 4,5-bisphosphate were formed. The effect of morphine, naloxone, levorphanol, and dextrophan on the main phase transition temperature was determined by measuring the partition of the spin label, 2,2,6,6-tetramethylpiperidine-l-oxyl, between the aqueous and fluid hydrophobic phases. Although the addition of 1 mol % cerebroside sulfate rendered the bilayers sensitive to the fluidizing effects of drugs, no correlation was observed between the fluidizing and analgesic effects. To show that this lack of correlation was not dependent on the lipid concentration, the experiments with cerebroside sulfate were repeated by measuring the main phase transition temperature with the fluorescence depolarization of 1,6-diphenylhexatriene incorporated into the bilayers. The results of these experiments also showed no correlation between fluidizing and analgesic effects. Hosein et al. (Biochem. Biophys. Res. Comm., 78:194-201, 1977), using differential scanning calorimetry, showed that opiates specifically affected a phase transition in brain lipid bilayers and that this effect required an ether precipitate rich in cerebroside sulfate. Since we found no specificity in the ability of opiates to increase lipid fluidity of bilayers that contained cerebroside sulfate, the work of Hosein et al. was repeated by monitoring fluidity with 1,6-diphenylhexatriene incorporated into the bilayers. Even 1 mM morphine failed to alter the polarization of 1,6-diphenylhexatriene, indicating that there were no alterations in bulk hydrocarbon fluidity. Because the depolarization of 1,6-diphenylhexatriene monitors the bulk hydrocarbon region and differential scanning calorimetry monitors the whole system, our results suggest that the alterations in the phase transition observed with differential scanning calorimetry do not occur because of "melting" of the bulk hydrocarbon regions of lipids.

Note:
ACKNOWLEDGMENTS The authors wish to acknowledge the editorial and typing assistance of Kaye Welch.