Inhibitory actions of opioid compounds on calcium fluxes and neurotransmitter release from mammalian cerebral cortical slices.

The effects of opioid agonists on veratrine-stimulated Ca2+ influx and amino acid neurotransmitter release in rat cerebrocortical brain slices were studied. Inhibitory effects were seen on both of these parameters with all of the opioid agonists used. None of the drugs used affected basal 45Ca2+ uptake, basal K+ content or basal amino acid release from the slices. At high concentrations (100 microM) fentanyl, tifluadom, U50,488H, butorphanol and bremazocine greatly inhibited the depolarization of the slices by veratrine as determined by the reduced release of K+. The opioid receptor subtypes at which the drugs were acting were characterized by the antagonistic effects of naloxone and WIN44441-3. The opioid-induced inhibition of stimulated Ca2+ uptake and amino acid release were not antagonized by WIN44441-2, the inactive enantiomer of WIN44441-3. It is concluded that opioid agonists acting through mu- and kappa-receptors and probably through delta- and sigma-receptors, have an inhibitory effect on Ca2+ uptake into cerebrocortical brain slices and the subsequent release of aspartate, glutamate and gamma-aminobutyric acid (GABA).