Behavioral sensitization to kainic acid and quisqualic acid in mice: comparison to NMDA and substance P responses.

Substance P (SP) and the excitatory amino acid (EAA) agonists NMDA, kainic acid (KA), or quisqualic acid (Quis) each produce a transient, caudally directed biting and scratching response (CBS) in mice after their intrathecal injection. We have previously shown that repeated injections of SP result in a decrease in the intensity of CBS, or desensitization. The goals of the present study were (1) to determine whether desensitization also develops to the CBS behavior produced by EAAs in the spinal cord, (2) to characterize the role of interneurons in desensitization, and (3) to examine possible interactions between EAAs and SP. While injection of NMDA at 2 min intervals resulted in desensitization to its CBS behavioral effect, behavioral responses to repeated injections of KA or Quis increased in intensity, exhibiting sensitization. The NMDA antagonist DL-2-amino-5-phosphonovaleric acid failed to alter sensitization to either KA or Quis but inhibited behaviors produced by SP and NMDA, suggesting an NMDA-mediated component in SP-induced behavior. Concanavalin A, which is reported to block desensitization to the electrophysiologic effect of Quis, blocked sensitization to the behavioral effects of both Quis and KA. Strychnine, bicuculline, and 5-aminovaleric acid each inhibited desensitization to SP and NMDA, supporting the notion of recruitment of inhibitory transmitters in the attenuation of NMDA and SP activity. Pretreatment with capsaicin selectively inhibited the development of behavioral sensitization to KA, suggesting an involvement of small-diameter C-fibers in the enhancement of responsivity to KA. Consistent with this, pretreatment with SP selectively potentiated the CBS response to KA. The potentiation of KA effects by SP and dependence of KA behavioral sensitization on C-fiber activity suggest a possible mechanism by which EAAs and SP may be involved in the mediation of pain.