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.

Electrical stimulation of primary afferent A fibres does not reduce substance P release in the dorsal horn of the cat.

Antibody microprobes were used to measure the release of immunoreactive substance P in the dorsal horn of anaesthetised cats during noxious mechanical or thermal stimulation of the hind limb. Concomitant electrical stimulation of the ipsilateral tibial or sural nerve at intensities sufficient to excite only A alpha beta or additionally A delta primary afferent fibres did not reduce the release of substance P evoked by noxious stimuli. The results suggest that segmental inhibition produced in the dorsal horn by electrical stimulation of peripheral nerves is not mediated by presynaptic inhibition of substance P release from nociceptive primary afferent fibres.