N-METHYL-D-ASPARTATE (NMDA) AND NON-NMDA RECEPTOR ANTAGONISTS BLOCK THE HYPEREXCITABILITY OF DORSAL HORN NEURONS DURING DEVELOPMENT OF ACUTE ARTHRITIS IN RATS KNEE-JOINT

1. In 22 anesthetized rats we studied the involvement of N-methyl-D-aspartate (NMDA) and non-NMDA receptors in the generation and maintenance of hyperexcitability in spinal cord neurons with knee input that develops in the course of an acute inflammation in the knee. In all experiments one neuron with knee input was identified, and the responses to mechanical stimuli and the receptive fields were monitored before and after induction of inflammation by the intra-articular injections of kaolin and carrageenan into the joint cavity. In most experiments multibarrel electrodes were used to administer specific NMDA and non-NMDA antagonists ionophoretically close to the neuron to test their effects on the inflammation-evoked changes. 2. Six neurons in the deep dorsal horn in six rats were used to establish the time course of the development of hyperexcitability in the untreated animal. In control periods of up to 3 h, the responses to mechanical stimuli and the receptive fields were stable. After induction of inflammation, the neurons developed increased responsiveness to mechanical stimuli applied to the injected knee but also to mechanical stimuli applied to the ipsilateral ankle and paw (including a reduction in the mechanical threshold in nociceptive specific neurons). The receptive fields expanded in five out of six neurons. The changes of responsiveness occurred mainly in the 2nd to 3rd h after the injection of kaolin. 3. In four rats three to four intravenous injections of the NMDA antagonist ketamine (2 mg/kg) were given during the injections of kaolin and carrageenan and in the following periods (up to 101 min postkaolin). During this treatment none of the four neurons exhibited the changes of responsiveness that were usually seen in control animals, although swelling of the knee developed in the same fashion as in control rats. Similarly, the generation of hyperexcitability was prevented when the NMDA antagonists ketamine and DL-2-amino-5-phosphonovalerate (AP5) were administered ionophoretically (ketamine in 4, AP5 in 2 rats) during the injections of kaolin and carrageenan and up to 100 min postkaolin. The doses of ketamine and AP5 were sufficient to reduce the responses to NMDA, whereas the responses to the non-NMDA agonist alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) were not influenced. 4. The ionophoretic application of the non-NMDA antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) during the injections of the kaolin and carrageenan and up to 103 min postkaolin also prevented the generation of hyperexcitability in six neurons in six rats. During CNQX the responses to AMPA were reduced, whereas those to NMDA were not changed. 5. Seventy to 95 min after the last application of the NMDA antagonists and 50-90 min after the last application of CNQX the neurons started to show phenomena of hyperexcitability qualitatively similar to those observed in the control rats. Thus the development of hyperexcitability was not permanently prevented by the application of the antagonists in the initial period of inflammation. 6. Once the hyperexcitability was established, both the NMDA antagonists and CNQX reduced the responses to innocuous and noxious pressure in all neurons tested. By contrast, in the control period the NMDA antagonists reduced only the responses to noxious pressure. Both the NMDA antagonists and CNQX reduced the size of the receptive fields of the majority of the hyperexcitable neurons but only exceptionally in the control period. 7. These findings show that NMDA and non-NMDA receptors are involved in the inflammation-evoked activity of spinal cord neurons with joint input at all stages of acute inflammation. The development of hyperexcitability requires the activation of both NMDA and non-NMDA receptors because either antagonist was able to prevent changes of excitability. The state of inflammation-evoked hyperexcitability is characterized by continuous activation of non-NMDA and NMDA receptors.