Intra-LPGi microinjection of glutamate receptors antagonists abolish 17�-estradiol-induced analgesic effect in the ovariectomized rats

Abstract

This study was designed to investigate a possible interaction between 17�-estradiol and glutamate receptors of the paragigantocellularis lateralis (LPGi) nucleus on pain coping behavior using the formalin test in ovariectomized (OVX) rats. The results showed that intra-LPGi injection of 17�-estradiol declined flexing behavior in both phases of the formalin test. Still, it only diminished the late phase of licking behavior in the OVX rats. NMDA receptor antagonist, AP5, reversed the analgesic effect of 17�-estradiol on flexing behavior in both phases of the formalin test in the OVX rats. The 17�-estradiol-induced anti-nociceptive effect on the flexing duration was prevented by CNQX (AMPA receptor antagonist) only in the early phase of the formalin test in the OVX rats. AP5 and CNQX reduced the anti-nociceptive effect of 17�-estradiol in the late phase, but not the early phase of licking response in the OVX rats. These results suggested: (i) The intra-LPGi injection of 17�-estradiol is satisfactory in producing modest analgesia on the formalin-induced inflammatory pain in the OVX rats; (ii) Co-treatment of glutamate receptors (NMDA and AMPA) antagonists and 17�-estradiol in the LPGi nucleus decrease the analgesic effect of 17�-estradiol in the OVX rats; (iii) There is a possible association between 17�-estradiol and glutamate receptors of the LPGi nucleus on pain coping behavior in the OVX rats.