Stabilization of morphine tolerance with long-term dosing: association with selective upregulation of mu-opioid receptor splice variant mRNAs

Abstract

Chronic morphine administration is associated with the development of tolerance, both clinically and in animal models. Many assume that tolerance is a continually progressive response to chronic opioid dosing. However, clinicians have long appreciated the ability to manage cancer pain in patients for months on stable opioid doses, implying that extended dosing may eventually result in a steady state in which the degree of tolerance remains constant despite the continued administration of a fixed morphine dose. Preclinical animal studies have used short-term paradigms, typically a week or less, whereas the clinical experience is based upon months of treatment. Chronic administration of different fixed morphine doses produced a progressive increase in the ED50 that peaked at 3 wk in mice, consistent with prior results at shorter times. Continued morphine dosing beyond 3 wk revealed stabilization of the level of tolerance for up to 6 wk with no further increase in the ED50. The degree of tolerance at all time points was dependent upon the dose of morphine. The mRNA levels for the various mu opioid receptor splice variants were assessed to determine whether stabilization of morphine tolerance was associated with changes in their levels. After 6 wk of treatment, mRNA levels of the variants increased as much as 300-fold for selected variants in specific brain regions. These findings reconcile preclinical and clinical observations regarding the development of morphine tolerance.

Keywords: MOR-1; analgesia; opiate receptor; opioid; splice variant.

Fig. 1.

Effect of extended morphine dosing on the development of tolerance. Groups of CD-1 mice (n = 60) were s.c. injected with saline (control group, n = 20) or the indicated morphine dose (5, 10, 20, or 40 mg/kg) twice daily (0830 hours and 2030 hours). Each morphine group started with 60 mice. Ten mice were removed from each group on days 7, 15, 21, 28, 39, and 43 for determination of their respective morphine ED₅₀ values as described in Methods and were then removed from the study. bid, twice daily.

Fig. 2.

Heat map of the changes in mRNA levels of MOR-1 splice variants by region. The relative abundance of the different variants in saline and both morphine treatment groups is presented in various brain regions. The heat map cluster is based upon the values [Log₂ (E^−∆C(t)] plotting from the highest level (left, red) to the lowest level (right, green). BS, brainstem; Cb, cerebellum; Hip, hippocampus; Hyp, hypothalamus; PAF, periaqueductal gray; PFC, prefrontal cortex; SpC, spinal cord; Str, striatum; Thal, thalamus; WB, whole brain.

Fig. 3.

Changes in mRNA levels of mMOR-1 splice variants after 6 wk of morphine administration in whole brain and brain regions. The levels of the full-length variants following morphine administration were determined as described in Methods, and the changes were normalized to saline levels. The changes on the y axis are presented on a log scale to accommodate the wide range of differences among the variants and among the regions. Values of 1 correspond to no change from saline controls. Values less than 1 (within the gray zone) correspond to decreases, whereas values greater than 1 represent increases. Significance was determined by ANOVA with Bonferroni’s multiple comparison test.