Conversion of 3H-nitrendipine binding to the low affinity binding state following the treatment of hippocampal slices with morphine

Effect of morphine on aluminium fluoride and dibutyryl-cyclic AMP induced reduction of field potentials in hippocampal slices

Effects of morphine and aluminum fluoride on field potentials evoked in hippocampal pyramidal cells were investigated revealing the physiological significance of adenylate cyclase in morphine action. Dibutyryl-cyclic AMP (db-cAMP) reduces the amplitude of potentials, while morphine enhances it. Morphine was without effects on db-cAMP induced reduction of potentials. Aluminum fluoride, known to activate GTP binding proteins, also reduced potentials and this was antagonized by morphine. Furthermore, N-[2-(methylamino)ethyl]-5-isoquinolinesulphonamide dihydrochloride (H-8), a protein kinase A inhibitor, enhanced potentials. When GABA synthesis was inhibited by 3-mercaptopropinoic acid, both morphine and db-cAMP was without effect. These results suggested the inhibition of adenylate cyclase by morphine which might be related with the reduction of GABA release in hippocampal slices.

Antagonism by glibenclamide of the effect of morphine in hippocampal preparations

We previously showed that morphine lowered the affinity of Ca2+ antagonist binding and subsequently enhanced field potentials in hippocampal preparations. In the present study, the effect of various K+ channel antagonists on these actions of morphine was studied. Higher Kd value of [3H]nitrendipine binding was obtained for membranes prepared from slices treated with morphine. Concomitant treatment of slices with morphine and tetramethylammonium (TMA) or glibenclamide attenuated the effect of morphine. Apamin and mast cell-degranulating (MCD) peptide were without effect on morphine-induced change in [3H]nitrendipine binding. In those experiments, no change in concentration of binding sites was observed. Glibenclamide reduced the morphine enhancement of field potentials. These results suggested the regulation of Ca2+ channels by morphine through K+ channel opening.

(D-Ala, D-Leu) enkephalin reduces the binding of GTP in hippocampal membranes

The effect of (D-Ala, D-Leu) enkephalin (DADLE) on the binding of GTP in hippocampal preparations was studied. It was observed that treatment of hippocampal slices with 10(-5) -5 x 10(-5) M DADLE followed by the preparation of membrane fractions reduced the binding of 35S-GTP-gamma-S. There was no change in the affinity of the binding. This decrease of 35S-GTP-gamma-S binding was reversed when 5 x 10(-5) M naltrindole was included. The effect was not observed when the membrane fractions were incubated with DADLE. Photoaffinity labeling with the use of 32P P3-(4-azidoanilido)-P1 5'-GTP followed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and autoradiography revealed the incorporation of radioactivity into molecular mass of the 43 kDa and 33-34 kDa proteins. 32P Photolabeling of both the 43 kDa and 33-34 kDa bands decreased following treatment of hippocampal slices with 10(-4) M DADLE. These results suggested that DADLE reduces the GDP-GTP exchange in hippocampal membranes.