Windup of Nociceptive Flexion Reflex Depends on Synaptic and Intrinsic Properties of Dorsal Horn Neurons in Adult Rat

Windup, a progressive increase in spinal response to repetitive stimulations of nociceptive peripheral fibres, is a useful model to study central sensitization to pain. Windup is expressed by neurons in of both dorsal and ventral horn of the spinal cord. In juvenile rats, it has been demonstrated both in vivo and in vitro that windup depends on calcium-dependent intrinsic properties and their modulation by synaptic components. However, the involvement of these two components in the adult remain controversial. In the present study, by means of electromyographic and extracellular recordings, we show that windup in adult, in vivo, depends on a synaptic balance between excitatory NMDA receptors and inhibitory glycinergic receptors. We also demonstrate the involvement of L-type calcium channels in both the dorsal and ventral horn of the spinal cord. These results indicate that windup in adults is similar to juveniles rats and that windup properties are the same regardless spinal network, i.e. sensory or motor.