Abstract

A growing body of literature indicates that chronic morphine exposure alters the expression and function of cytoskeletal proteins in addition to the well established interactions between μ opioid receptors and G proteins. In the present study, we hypothesized that chronic morphine alters the expression and functional effects of Gα12, a G protein that regulates downstream cytoskeletal proteins via its control of RhoA. Our results showed that chronic morphine treatment decreased the expression of Gαi2 (64%) and Gαi3 (60%), had no effect of Gαo, and increased Gα12 (66%) expression in Chinese hamster ovary (CHO) cells expressing the cloned human μ opioid receptors (hMOR-CHO cells) but not in cells expressing a mutant μ opioid receptor that do not develop morphine tolerance and dependence (T394A-CHO cells). Morphine treatment had no significant effect on PAR-1 thrombin receptor-activated G protein activity, as measured by thrombin-stimulated guanosine 5′-O-(3-[³⁵S]thio)triphosphate binding. Chronic morphine treatment significantly enhanced thrombin-stimulated RhoA activity and thrombin-stimulated expression of α-actinin, a cytoskeletal anchoring protein, in hMOR-CHO cells. Proteomic analysis of two-dimensional gel spots prepared from hMOR-CHO cells showed that morphine treatment affected the expression of a number of proteins associated with morphological changes. Up-regulation of Gα12 and α-actinin by chronic morphine was also observed in mouse brain. Viewed collectively, these findings indicate, for the first time, that chronic morphine enhances the Gα12-associated signaling system, which is involved in regulating cellular morphology and growth, supporting other findings that chronic morphine may alter cellular morphology, in addition to cellular function.