The nociception induced by glutamate in mice is potentiated by protons released into the solution

In this study we compare the effect of a glutamate solution with pH adjusted to 7 (3-30 micromol/paw), a non-pH-adjusted glutamate solution (.3-30 micromol/paw, pH range 2.24-1.14), and an acid solution (2% acetic acid, pH 1.4-7) in terms of causing licking behavior in mice. The sum of licking seconds was recorded in the first 15 minutes following the intraplantar (i.pl.) injection of the solutions. Protons potentiated the nociception induced by glutamate. The ED(50) values were 2.5 (1.5-4.2) and 15.1 (11.5-19.9) micromol/paw for the non-pH-adjusted and pH-adjusted glutamate solutions, respectively. The acid solutions at pH 1.4, 2 and 4 induced a similar nociception. The blocking of the acid-sensitive ion channels (ASICs) by amiloride and the antagonism of the transient receptor potential vanilloid subtype-1 (TRPV1) by capsazepine, injected via i.pl., significantly decreased the nociception mediated by acid and by non-pH-adjusted glutamate solutions, but did not affect the nociception caused by the pH-adjusted glutamate solution. The pretreatment with the NMDA-receptor antagonist (MK-801, i.pl.), with the cyclooxygenase inhibitor (indomethacin, i.pl.) or the disruption of the sensorial C fibers by capsaicin, decreased the nociceptive effect of the 3 algogen tested. In summary, the protons present in aqueous solution of glutamate can cause nociception per se or can potentiate the nociception caused by glutamate. These effects are related to the activation of ASICs, TRPV1 and NMDA receptors, inhibition of the synthesis of prostanoids, and disruption of the C fibers.

The nociception induced by glutamate is a useful method for investigation of the mechanisms of nociception and the effects of new analgesic drugs. Our findings showed that the protons released from glutamic acid have to be removed from the solution to avoid misinterpretation of results in the search for new analgesic drugs.