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The Role of Spinal Neurokinin‐2 Receptors in the Processing of Nociceptive Information from the Joint and in the Generation and Maintenance of Inflammation‐evoked Hyperexcitability of Dorsal Horn Neurons in the Rat

In spinal cord neurons in anesthetized rats, the role of neurokinin A and neurokinin‐2 receptors in the processing of nociceptive information from the knee joint was studied. The specific non‐peptide antagonist at the neurokinin‐2 receptor, SR48968, its inactive R‐enantiomer, SR48965, neurokinin A, substance P and (R, S)‐α‐amino‐3‐hydroxy‐5‐methylisoxazole‐4‐propionic acid (AMPA), were administered ionophoretically close to neurons with input from the knee joint. SR48968 reduced the effects of exogenous neurokinin A, but not those of exogenous substance P and AMPA, indicating selective blockade of neurokinin‐2 receptors. In most neurons with input from the normal knee joint, SR48968 reduced dose‐dependently the responses to noxious pressure applied to the knee, and in ˜50% of the neurons the responses to innocuous pressure. The administration of SR48968 during the induction of an experimental joint inflammation markedly attenuated the development of inflammation‐evoked hyperexcitability. In hyperexcitable neurons with input from the inflamed joint, SR48968 reduced the responses to noxious and innocuous pressure. The relative reduction of the responses was more pronounced than in neurons with input from the normal joint. None of the effects of SR48968 was mimicked by SR48965. These data show that neurokinin‐2 receptors are involved in the spinal processing of nociceptive information from the normal joint. Furthermore, neurokinin‐2 receptors must be coactivated at an early stage of inflammation, to allow the generation of hyperexcitability. Finally, neurokinin‐2 receptors are involved in the maintenance of hyperexcitability during inflammation. In summary, spinal neurokinin‐2 receptors are important in the generation of pain in the normal and inflamed joint.