Mu and delta opiate receptors in neuronal and astroglial primary cultures from various regions of the brain—coupling with adenylate cyclase, localisation on the same neurones and association with dopamine (D 1) receptor adenylate cyclase

Abstract

Primary cultures, enriched in neurones or astroglial cells, from three phylogenetically different regions of the brain of the rat, the cerebral cortex, the striatum and the brain stem, were used to investigate the presence of opiate receptors, coupled to adenylate cyclase. Morphine was used as a μ-receptor agonist and [ d-Ala 2, d-Leu 5]-enkephalin (DADLE) was used as a δ -receptor agonist. In the neuronal cultures, both ligands inhibited the prostaglandin (PG)E 1 -stimulated intracellular accumulation of cyclic AMP dose-dependently, with the most prominent effects seen in the cultures of striatum and with DADLE being more potent than morphine. The opiate receptor antagonist, naloxone reversed the effects. Morphine and DADLE, added together, inhibited the PGE 1-stimulated accumulation of cyclic AMP, less than the sum of the effects of each drug. Therefore, it might be that these opioid receptors are localized together on the same neurone. Striatal neurones contained dopamine receptors coupled to cyclic AMP, as second messenger. It was shown that the D 1 (dopamine) receptor-stimulated activity of adenylate cyclase was inhibited by the μ and δ opioid receptor ligands. Thus, interactions at the level of adenylate cyclase seem to exist between D 1, μ and δ opiate receptors. In the astroglial enriched cultures, DADLE inhibited the PGE 1-induced accumulation of cyclic AMP, however, with a less prominent effect in the brain stem cultures. Morphine did not influence the basal or the PGE 1-stimulated intracellular accumulation of cyclic AMP in the cultures used and DADLE was less effective in inhibiting the PGE 1-induced accumulation of cyclic AMP in the astroglial cultures, than in the neuronal enriched cultures, indicating less developed δ receptor-second messenger efficacy in the astroglial cultures, than in the neuronal cultures. These results thus suggest the presence of δ receptors on astroglial cells in culture, while no μ receptors with cyclic AMP as second messenger were identified.