Effect of chronic administration of morphine on the expression of bovine adrenal medulla 22-like immunoreactivity in the spinal cord of rats

The aim of the present study was to investigate the effects of chronic administration of morphine on the expression of an endogenous opioid peptide in the spinal dorsal horn. Bovine adrenal medulla 22-like immunoreactivity (BAM22-IR) was found in the superficial layers of the spinal cord. Intrathecal (i.t.) administration of morphine (20 μg) for 6 days, but not 2 days, significantly reduced the expression of BAM22-IR whereas i.t. administration of saline for 2 and 6 days did not alter the expression of BAM22-IR. The present study suggests that reduction of BAM22-IR in the spinal cord is involved in the development of morphine tolerance.