Substance P release in the cat spinal cord upon afferent C-fibre stimulation is not attenuated by clonidine at analgesic doses.

In anaesthetized cats, antibody microprobes were used to measure the release of immunoreactive substance P (irSP) in the lumbar dorsal horn during electrical stimulation of primary afferent fibres at intensities suprathreshold for unmyelinated fibres. Release of irSP was detected in the region of the superficial dorsal horn. This evoked release was not reduced by clonidine hydrochloride, administered intravenously or by superfusion of the dorsal cord surface. Microprobes inserted during cord superfusion with lignocaine hydrochloride detected less irSP along their entire length, including in the region of evoked release. The results suggest that the analgesic action of clonidine does not involve reduced release of SP from the central terminals of nociceptors in the spinal cord.