Indian J Med Res
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The mechanism underlying the development of tolerance to morphine is not clearly understood though a number of factors have been implicated. One of the likely factors may be increased activity of anti-opioid peptides like nociceptin (also known as orphanin FQ or N/OFQ). N/OFQ and morphine bind to opioid receptor-like 1 (ORL1) receptor and muopioid receptor respectively. The present work was undertaken to investigate the density of ORL1 and mu (mu) receptor expression in the spinal cord of mice after inducing morphine tolerance. ⋯ The present study showed that both, ORL1 and mu-opioid receptors were expressed in areas of the spinal cord, concerned with transmission of pain signals. The density of these receptors increased in the superficial laminae (I-II) though not significantly from control after morphine tolerance. The increase in ORL1 receptors could oppose the analgesic action of morphine, contributing to tolerance. Further studies need to be done to elucidate the mechanism of morphine tolerance.