Natural noxious stimulation can induce long-term increase of spinal nociceptive responses

It is conceivable that plasticity in pain control systems and chronic pain may be due to mechanisms similar to learning. Long-term potentiation (LTP) in the hippocampus is often studied as a model of learning and memory. It has recently been shown that long-term excitation may be induced in single wide dynamic range (WDR) neurones in the spinal dorsal horn of rats after tetanic stimulation to the sciatic nerve. The present study shows that similar long-term changes can also be induced by a severe natural stimulus. Single unit extracellular recordings were made in urethane anaesthetized rats and the firing responses of WDR neurones evoked by a single electrical stimulus to the peripheral nerve were recorded every 4 min. After repeated crushing of tissue (including bone) corresponding to the receptive field of the WDR neurones (the conditioning stimulus) followed by a proximal total peripheral nerve block, the C-fibre evoked responses were increased (P < 0.001) for a 3 h observation period compared with baseline responses and control animals. In control animals the nerve block was applied before the conditioning stimulus. We suggest that a long-term increase of the excitability of WDR neurones may be important for the development of long lasting and chronic pain disorders after an acute but severe noxious stimulus.