Central sensitization is an essential factor in the development of neuropathic pain (Baron 2001, Johnson et al. 2001) and in animal experiments the NMDA receptor has been shown to be involved in this process. Calcium plays a central role, as an increased influx of calcium into the neuron is observed when the NMDA receptor is activated. The enhanced concentration of intracellular calcium functions as a second messenger for a number of neurotransmitters and calcium thus contributes to the maintenance of the central sensitization (Nicholson 2000).