Regulation of g-protein mRNA abundancy and cAMP accumulation after long-term opioid incubation in primary cultures of astroglia from the rat cerebral cortex

Abstract

Primary astroglial cultures were incubated with δ (10 −6 M DPDPE) or κ (10 −5 M U-50,488H) receptor agonists for 5 days. Thereafter, the acute inhibitory actions of δ or κ receptor agonists on forskolin stimulated cAMP accumulation were assayed. The G αs, G αi−1 and G αi−2 mRNA levels were quantified after 5 days of either δ or κ receptor agonist treatment using a solution hybridization. RNase protection assay. Pronounced effects were observed after 5 days of κ receptor agonist [10 −5 M U-50,488H] incubation. This treatment resulted in an attenuation in the acute inhibitory action of δ and κ receptor agonists. Furthermore, a decreased stimulatory action of forskolin was seen. Similar effects were also seen after δ receptor stimulation. We also investigated the effects after 24 h and 3 days of incubation with the κ receptor agonist (10 −5 M) U-50,488H. The 24 h incubation resulted in a decreased sensitivity to the acute inhibitory action of δ and κ receptor agonists in the astroglial cultures. This effect was further accentuated after the 3 days of incubation with 10 −5 M U-50,488H. No significant change was seen in the basal accumulation of cAMP after incubation with the κ agonist U-50,488H. However, after 5 days of incubation with the δ agonist DPDPE, a significantly increased basal accumulation of cAMP was seen in the astroglial cultures. After 5 days of δ or κ agonist incubation, an increase in G αs mRNA level and a decrease in G αi−2 mRNA level was seen compared with controls. No statistically significant alterations in the amount of G αi−1 mRNA were seen. The data obtained in the present study indicate that the effects of long-term opioid treatment alters the sensitivity of glial cell opioid receptors. Furthermore, long term opioid treatment induces alterations in glial G-protein mRNA levels.