Enhanced nociception by exogenous and endogenous substance P given into the spinal cord in mice lacking NR(2)A/epsilon(1), an NMDA receptor subunit

Abstract

In capsaicin-pretreated mice, the nociceptive responses induced by intrathecally (i.t.) administered substance P (SP) were enhanced by N-methyl-D-aspartate (NMDA)-type receptor antagonists, dizocilpine (MK801) and D-2-amino-5-phosphonopentanoate (D-AP5) in a dose-dependent manner. Similar enhancement of SP-induced nociception was also observed in mice lacking the NMDA-type glutamate receptor NR2A/epsilon(1) subunit gene (GluRepsilon(1)(-/-) mice). On the other hand, GluRepsilon(1)(-/-) mice showed a marked enhancement of the peripheral nociceptive responses induced by intraplantar (i.pl.) injection of SP and bradykinin (BK). As the nociceptive responses to SP and BK (i.pl.) were both antagonized by CP-99994, an neurokinin(1) (NK(1)) antagonist (i.t.), these results suggest that GluRepsilon(1) receptor may play an inhibitory role in the downstream mechanisms of primary nociceptive SP neurones, possibly through activation of unidentified inhibitory neurones.

Figure 1

Enhancement of intrathecal application of SP-responses by glutamate receptor antagonist in capsaicin-pretreated mice, and in GluRε₁^−/− mice. (a) Enhancement of SP (i.t.)-induced SBL responses by MK-801, a non-competitive NMDA receptor antagonist or D-AP5, a competitive NMDA receptor antagonist in capsaicin-pretreatment mice. (b) Enhancement of SP (i.t.)-induced SBL responses in GluRε₁^−/− mice. Nociceptive activity was expressed as total period (s) showing nociceptive SBL behaviour for 20 min following i.t. injection of SP. The data were the mean±s.e.mean from five or six separate experiments. ^*P<0.05, compared to vehicle-treatment.

Figure 2

Enhancement of local application of BK- or SP-responses in GluRε₁^−/− mice. (a, b) Signal transduction in spinal cord with SP (or BK)-induced nociception. Blockade of SP (or BK)-induced nociceptive responses by the specific NK₁ receptor antagonist CP-99994 (100 pmol, i.t.), but not CP-100263 (100 pmol, i.t.), its inactive isomer. ^*P<0.05, compared to vehicle-treated mice. The data were analysed using Student's t-test following multiple comparisons of the analysis of variance (ANOVA). (c, d) Dose-responses curve for SP or BK (i.pl.)-induced nociceptive responses in GluRε₁^−/−. SP- or BK-induced nociceptive activity was expressed as the ratio of maximal reflex in each mouse. Results represent the mean±s.e.mean from five or six separate experiments.