Morphine does not reduce the intraspinal release of calcitonin gene-related peptide in the cat

In anaesthetised cats, antibody microprobes were used to measure the release of immunoreactive calcitonin gene-related peptide (irCGRP) in the lumbar dorsal horn during stimulation of non-nociceptive or nociceptive primary afferent fibres. Release of irCGRP was detected in the substantia gelatinosa, where immunostaining for CGRP was subsequently observed. Intravenous administration of morphine did not affect release of irCGRP detected during either non-nociceptive or nociceptive afferent stimulation. The results suggest that the analgesic action of morphine does not involve reduced release of CGRP from the central terminals of nociceptive primary afferents.