Ethanol inhibition of NMDA-induced responses and acute tolerance to the inhibition in rat rostral ventrolateral medulla in vivo: Involvement of cAMP-dependent protein kinases

Our recent study showed that intravenous ethanol selectively inhibited the pressor effects elicited by the microinjection of N-methyl-d-aspartate (NMDA) into rostral ventrolateral medulla (RVLM) and acute tolerance to the inhibition was observed during prolonged application of ethanol in anesthetized Sprague–Dawley rats. In this study, we examined the role of the cAMP-dependent protein kinase (PKA) signaling pathway in acute tolerance to ethanol inhibition of NMDA-induced responses in rat RVLM. A significant increase in the level of PKA-regulated phosphoserine 897 on the NMDA NR1 subunit was found in the rostroventral medulla during acute ethanol tolerance. Reduction of NMDA-induced pressor effects was observed at 10 min but disappeared at 40 min after continuous ethanol infusion. This effect was dose-dependently blocked by microinjection of KT5720 (0.04–4 pmol, a selective PKA inhibitor) or cAMPS-Rp (0.02, 0.2 pmol, a cAMP antagonist) into the RVLM 10 min post-injection of ethanol; KT 5720 or cAMPS-Rp alone at doses tested had no significant effects on NMDA-induced responses. Post-treatment with cAMPS-Sp (10 pmol, a cAMP activator) did not affect acute ethanol tolerance. Interestingly, administration of KT 5720 (0.4, 4 pmol) or cAMPS-Rp (2,10 pmol) into the RVLM 20 min before the injection of ethanol also reduced the inhibitory effects of ethanol on NMDA-induced pressor effects in a dose-dependent manner. Our results provide the first in vivo evidence that PKA signaling pathways participate in acute tolerance to ethanol inhibition of NMDA receptor function. Furthermore, PKA-mediated signaling pathways may also be involved in the interaction between ethanol and NMDA receptors.