Neuronal protein kinase C gamma-dependent proliferation and hypertrophy of spinal cord astrocytes following repeated in vivo administration of morphine

Repeated administration of morphine induced a time-dependent inhibition of the morphine-induced antinociceptive action, indicating the development of tolerance to morphine. We demonstrated that mice tolerant to morphine exhibited a significant increase in the level of protein kinase Cgamma-like immunoreactivity (PKCgamma-IR) in the dorsal horn of the spinal cord. The PKCgamma-IR was exclusively colocalized with the neuron-specific markers neuronal nuclei (NeuN) and microtubule associated protein 2ab (MAP2ab). Here we found a dramatic increase in reactive astrocytes in the dorsal horn of the spinal cord following repeated treatment with morphine, as characterized by the increase and morphological changes in glial fibrillary acidic protein (GFAP)-positive cells. Furthermore, transgenic mice that express enhanced green fluorescent protein (EGFP) under the control of the mouse GFAP promoter displayed enhanced levels of EGFP expression after repeated treatment with morphine. Under these conditions, mice lacking the PKCgamma gene failed to show any changes in astroglial hypertrophy or proliferation after repeated treatment with morphine. These findings strongly support the idea that the sustained activation of neuronal PKCgamma is implicated in the increased levels of reactive astrocytes in the dorsal horn of the spinal cord following repeated treatment with morphine. This neuron-glia communication may lead to the development of tolerance to morphine-induced antinociception.