Plasticity of opioid adaptations to chronic morphine.

Abstract

Prolonged administration of opioids in the clinical setting leads to eventual resistance to its effects. Many mechanisms have been proposed to explain the phenomenon of opioid tolerance with the common theme that tolerance reestablishes initial steady state conditions and neutralizes the perturbing consequences of continued opioid presence. The Gintzler laboratory has previously shown that tolerance results, in part, from two parallel pathways that intersect to negate acute inhibitory opioid signaling: (1) change in G-protein signaling, from Giα inhibitory to Gi-derived Gβγ stimulatory, through multiple changes in signaling molecules which converge to the shift to Gβγ stimulation of adenylyl cyclase (AC) and (2) increased association between the μ-opioid receptor (MOR) and the stimulatory G protein (Gs). In the current study, we demonstrate that these adaptations to chronic morphine are not hard-wired, but can exhibit plasticity based on the internal cellular milieu. In Chinese Hamster Ovary (CHO) cells stably transfected with the rat μ-opioid receptor (MOR-CHO) and overexpressing ACII (ACII MOR-CHO), a Gβγ stimulatory isoform, we have engineered a cell that both exhibits default opioid stimulation and manifests chronic opioid tolerance. In ACII MOR-CHO, the tolerance adaptations that previously caused a shift to Gβγ stimulation of AC are either negated or reversed, decreasing the stimulatory interactions between Gβγ and AC, manifest by the following observations: chronic morphine fails to augment Gβ phosphorylation, decreases AC phosphorylation, membrane translocation of PKCγ and MOR-Gs coupling. The importance of isoform-specific Gβγ regulation of AC in the plasticity exhibited by chronic morphine is underscored by its ability to elicit adaptations in cells overexpressing ACI, a Gβγ-inhibitory isoform, which are identical to those in MOR-CHO. These observations suggest pre-existing levels of isoform-specific AC is a critical determinant of adaptations to the persistent presence of morphine. This could explain the lack of ubiquity and uniformity of the degree and mechanistic underpinnings of opioid tolerance throughout the nervous system.