Chronic morphine administration produces tolerance in vivo and attenuation of μ opioid receptor (MOR)‐mediated G‐protein activation measured in vitro, but the relationship between these adaptations is not clear. The present study examined MOR‐mediated G‐protein activation in the CNS of mice with different levels of morphine tolerance.

Mice were implanted with morphine pellets, with or without supplemental morphine injections, to induce differing levels of tolerance as determined by a range of MOR‐mediated behaviours. MOR function was measured using agonist‐stimulated [³⁵S]guanylyl‐5′‐O‐(γ‐thio)‐triphosphate ([³⁵S]GTPγS) and receptor binding throughout the CNS.

Morphine pellet implantation produced 6‐12‐fold tolerance in antinociceptive assays, hypothermia and Straub tail, as measured by the ratio of morphine ED₅₀ values between morphine‐treated and control groups. Pellet implantation plus supplemental injections produced 25‐50‐fold tolerance in these tests. In morphine pellet‐implanted mice, MOR‐stimulated [³⁵S]GTPγS binding was significantly reduced only in the nucleus tractus solitarius (NTS) and spinal cord dorsal horn in tissue sections from morphine pellet‐implanted mice. In contrast, MOR‐stimulated [³⁵S]GTPγS binding was significantly decreased in most regions examined in morphine pellet+morphine injected mice, including nucleus accumbens, caudate‐putamen, periaqueductal gray, parabrachial nucleus, NTS and spinal cord.

Tolerance and the regional pattern of apparent MOR desensitization were influenced positively by the level of morphine exposure. These results indicate that desensitization of MOR‐mediated G‐protein activity is more regionally widespread upon induction of high levels of tolerance, suggesting that this response contributes more to high than low levels of tolerance to CNS‐mediated effects of morphine.

British Journal of Pharmacology (2007) 151, 1324–1333; doi:10.1038/sj.bjp.0707328