The concentrations of brain and spinal cord beta-endorphin, met-enkephalin, dynorphin and substance P were measured in rats bearing the Freund adjuvant induced arthritis. Beta-endorphin brain concentrations decreased gradually in time with a nadir on day twenty-one, when arthritis was at its maximum, and were back to normal by day thirty-five, when arthritis was no more evident. Met-enkephalin concentrations increased in brain areas and in the lumbar spinal cord and returned to normal with the same time pattern, while dynorphin and substance P concentrations did not change. These data indicate that peripheral lesions can induce important changes in brain concentrations of some opioid peptides involved in the modulation of pain.
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