Dual regulation by ethanol of the inhibitory effects of ketamine on spinal NMDA-induced pressor responses in rats

The blood pressure responses induced by intrathecal injection of NMDA were recorded in urethane-anesthetized rats weighing 250-275 g. The levels of several phosphorylated residues on NMDA receptor GluN1 subunits were determined by western blot analysis.Intravenous injection of ethanol or ketamine inhibited spinal NMDA-induced pressor responses in a dose-dependent and reversible manner. Ketamine inhibition of NMDA-induced responses was synergistically potentiated by ethanol when ethanol was applied just before ketamine. However, ketamine inhibition was significantly reduced when applied at 10 min after ethanol administration. Western blot analysis showed that intravenous ethanol increased the levels of phosphoserine 897 on GluN1 subunits (pGluN1-serine 897), selectively phosphorylated by protein kinase A (PKA), in the lateral horn regions of spinal cord at 10 min after administration. Intrathecal administration of cAMPS-Sp, a PKA activator, at doses elevating the levels of pGluN1-serine 897, significantly blocked ketamine inhibition of spinal NMDA-induced responses.The results suggest that ethanol may differentially regulate ketamine inhibition of spinal NMDA receptor function depending on ethanol exposure time and the resulting changes in the levels of pGluN1-serine 897.Ethanol has several effects on the central nervous system, such as intoxication, tolerance, and withdrawal. Although these mechanisms are still not well understood, many evidences suggest an important role of the glutamate neurotransmitter system in ethanol effects [1-3]. It has been repeatedly reported that ethanol antagonizes central effect of glutamate by acting at NMDA (N-methyl-D-asparate) receptors, a subtype of ionotropic glutamate receptors, at pharmacologically relevant concentrations [4,5]. The ability of ethanol to inhibit NMDA-activated current is linearly related to its potency for causing intoxication [6]. NMDA receptors are composed of 7 subunits including a GluN1 subunit, a family of