Increased ability to induce long-term potentiation of spinal dorsal horn neurones in monoarthritic rats.

Long-term potentiation (LTP) of transmission of impulses in unmyelinated (C-fibre) primary afferents by prior tetanic conditioning stimulation has been demonstrated in the dorsal horn of the spinal cord. Since this potentiation has been proposed to be relevant to the increased responsiveness of spinal neurones associated with peripheral inflammation (central sensitisation), the present experiments compared the induction of LTP in normal rats and rats with monoarthritis. Monoarthritis was induced by injection of complete Freund's adjuvant (CFA) into the left ankle joint of 12 rats. All animals showed behavioural signs of thermal hyperalgesia and were used for electrophysiological experiments after 4-8 days. In each animal, extracellular recordings were obtained from a single, wide dynamic range (WDR) dorsal horn neurone. High frequency tetanic conditioning stimulation of the sciatic nerve gave varying effects on the C-fibre-evoked responses of neurones in the normal rats, with potentiation in two, no change in five and a depression in five. By contrast, conditioning stimulation in rats with inflammation produced a long-lasting potentiation of C-fibre-evoked responses in 11 out of 12 neurones, with no effect in one. The ease with which LTP was induced in animals with inflammation supports the proposal that the underlying mechanisms of LTP are similar to those of the central sensitisation associated with peripheral inflammation.