Role of the guanine nucleotide binding protein, Gα_o, in the development of morphine tolerance and dependence

The use of morphine and other opioids for chronic pain is limited by the development of analgesic tolerance and physical dependence. Morphine produces its effects by activating the μ opioid receptor, which couples to Gα_i/o-containing heterotrimeric G proteins. Evidence suggests that the antinociceptive effects of morphine are mediated by Gα_o. However, the role of Gα_o in the development of morphine tolerance and dependence is unknown.

The objective of the study is to evaluate the contribution of Gα_o to the development of morphine tolerance and dependence in mice.

129S6 mice lacking one copy of the Gα_o gene (Gα_o +/-) were administered morphine acutely or chronically. Mice were examined for tolerance to the antinociceptive action of morphine using the 52 °C hot plate as the nociceptive stimulus and for dependence by evaluating the severity of naltrexone-precipitated withdrawal. Wild-type littermates of the Gα_o +/- mice were used as controls. Changes in μ receptor number and function were determined in midbrain and hindbrain homogenates using radioligand binding and μ agonist-stimulated [³⁵S]GTPγS binding, respectively.

Following either acute or chronic morphine treatment, all mice developed antinociceptive tolerance and physical dependence, regardless of genotype. With chronic morphine treatment, Gα_o +/- mice developed tolerance faster and displayed more severe naltrexone-precipitated withdrawal in some behaviors than did wild-type littermates. Morphine tolerance was not associated with changes in μ receptor number or function in brain homogenates from either wild-type or Gα_o +/- mice.

These data suggest that the guanine nucleotide binding protein Gα_o offers some protection against the development of morphine tolerance and dependence.