Differential effect of μ, δ, and κ ligands on G protein α subunits in cultured brain cells

Rat and guinea pig fetal brain cell cultures and immunoblotting techniques were used to study the effect of receptor selective opioids on the level of the membrane‐bound α₁ and α₀ GTP binding protein subunits. Incubation of rat hindbrain cultures with the μ selective peptide DAGO decreased the amount of both α proteins. The reduction observed was equivalent to 36% in α₀ and 41% in α₁. On the other hand, incubation of rat forebrain cultures with this peptide had an opposite effect, increasing the α₀ and α₁ levels by 66% and 68%, respectively. This differential effect of the peptide on the G proteins at the two brain areas may reflect the selective interaction at the receptor level; DAGO induced a fast and effective receptor down‐regulation (50% decrease in B_max) in hindbrain but not in forebrain cultures. Moreover, δ and μ selective ligands differed in their effect, as indicated by the finding that the δ selective peptide DPDPE increased the amount of both α proteins in hindbrain cultures by 40%. Similar experiments conducted with guinea pig brain aggregate cultures indicated that the κ selective agonist U50, 488 decreased the amount of the membrane bound α₁ protein subunit by 56%. The results thus indicate that opioid agonists, interacting selectively with the three types of opioid receptors, induce a complex repertoire of changes in the immunoreactive levels of the membrane‐bound α GTP binding protein subunits in various CNS structures.