A lack of μ-opioid receptors modulates the expressions of neuropeptide Y and substance P mRNA

The present study was undertaken to investigate changes in the expressions of neuropeptide Y (NPY) and substance P (SP) in mice lacking μ-opioid receptors. In an in situ hybridization study, in which we compared wild type and μ-opioid receptor knockout mice, NPY mRNA levels were found to be lower in the caudate-putamen and nucleus accumbens of μ-opioid receptor knockout mice. In addition, SP mRNA levels were lower in the ventromedial hypothalamic nucleus of μ-opioid receptor knockout mice. Our findings suggest that a lack of μ-opioid receptors modulates basal NPY mRNA levels in striatal regions and SP mRNA levels in the ventromedial hypothalamic nucleus of the mouse, and that these changes are due to compensatory modulation in the brain.