Oxysterols Modulate the Acute Effects of Ethanol on Hippocampal N-Methyl-d-Aspartate Receptors, Long-Term Potentiation, and Learning

Abstract

Ethanol is a noncompetitive inhibitor of N-methyl-d-aspartate receptors (NMDARs) and acutely disrupts hippocampal synaptic plasticity and learning. In the present study, we examined the effects of oxysterol positive allosteric modulators (PAMs) of NMDARs on ethanol-mediated inhibition of NMDARs, block of long-term potentiation (LTP) and long-term depression (LTD) in rat hippocampal slices, and defects in one-trial learning in vivo. We found that 24S-hydroxycholesterol and a synthetic oxysterol analog, SGE-301, overcame effects of ethanol on NMDAR-mediated synaptic responses in the CA1 region but did not alter acute effects of ethanol on LTD; the synthetic oxysterol, however, overcame acute inhibition of LTP. In addition, both oxysterols overcame persistent effects of ethanol on LTP in vitro, and the synthetic analog reversed defects in one-trial inhibitory avoidance learning in vivo. These results indicate that effects of ethanol on both LTP and LTD arise by complex mechanisms beyond NMDAR antagonism and that oxysterol NMDAR PAMS may represent a novel approach for preventing and reversing acute ethanol-mediated changes in cognition. SIGNIFICANCE STATEMENT: Ethanol acutely inhibits hippocampal NMDARs, LTP, and learning. This study found that certain oxysterols that are NMDAR-positive allosteric modulators can overcome the acute effects of ethanol on NMDARs, LTP, and learning. Oxysterols differ in their effects from agents that inhibit integrated cellular stress responses.

Fig. 1.

24S-HC and a synthetic oxysterol analog prevent effects of ethanol on NMDA EPSPs. (A) The graph shows the time course of change of isolated NMDAR-mediated fEPSPs in the presence of 1 μM 24S-HC (blue bar, denoted as 24S in the graph) followed by 60 mM ethanol (denoted as EtOH in the graph) normalized to baseline (100%). In the absence of 24S-HC, ethanol depressed NMDAR-mediated fEPSPs. (B) Even when administered after 60 mM ethanol, which acutely inhibits NMDAR responses, 24S-HC dampened the block of NMDA fEPSPs. (C) SGE-301 (red bar), a synthetic mimic of 24S-HC, also prevents effects of ethanol on NMDA fEPSPs. (D) Akin to 24S-HC, SGE-301 reverses the effects of ethanol on NMDA fEPSPs when administered after block is established. Traces to the right of the graphs show representative NMDA fEPSPs at the times denoted, with baseline responses shown as dashed lines. Calibration: 1 mV, 5 milliseconds.

Fig. 2.

Disparate interaction of 24S-HC and SGE-301 with the acute effects of ethanol on long-term synaptic plasticity. (A) Despite altering effects of ethanol on NMDA EPSPs, 24S-HC (blue bar) did not alter the block of homosynaptic LTD of AMPAR fEPSPs by 60 mM ethanol. Administration of 1-Hz LFS (hatched bar) induces LTD in control naïve slices and in slices treated with 24S-HC. Gray circles show the effects of ethanol alone, whereas blue squares depict the effects of ethanol plus 24S-HC. (B) Similarly, SGE-301 failed to alter acute LTD inhibition by ethanol. LTD is induced in the presence of SGE-301 alone. (C) Akin to effects on LTD, 1 µM 24S-HC did not alter the acute effects of ethanol on LTP. The 100-Hz HFS (arrow) induces LTP in control naïve slices and in slices treated with 1 µM 24S-HC. Ethanol inhibits LTP induction in the presence or absence of 24S-HC. (D) Unlike 24S-HC, SGE-301 prevented acute LTP inhibition by ethanol. Administration of 24S-HC alone did not alter LTP induction. Traces to the right of the graphs show representative fEPSPs 60 minutes after the conditioning stimulation (solid lines), with dashed lines as control baseline responses. Delivery of 100 Hz × 1 second HFS is denoted by the arrows in (C and D). Calibration: 1 mV, 5 milliseconds.

Fig. 3.

24S-HC and SGE-301 both reverse the persisting effects of ethanol on LTP. (A) The graph shows the block of LTP in slices treated with 60 mM ethanol for 15 minutes and then washed out for 30 minutes prior to delivery of HFS (arrow) (blue squares). 24S-HC (1 μM) overcame the effects of ethanol and allowed robust LTP (gray circles). (B) 24S-HC also overcame the persisting effects of ethanol on LTP when administered after ethanol. (C) A synthetic mimic of 24S-HC, SGE-301 (1 μM), also overcame the persisting effects of ethanol on LTP. Traces show representative fEPSPs as in Fig. 2 with solid traces taken 60 minutes after HFS. In (A), the upper set of traces is with 24S-HC. Calibration: 1 mV, 5 milliseconds.

Fig. 4.

SGE-301 overcomes the effects of ethanol on one-trial inhibitory avoidance learning. The top bar depicts the latency for rats to enter the dark chamber in the absence of drug treatment (saline controls) 24 hours after receiving a shock upon entry into the dark compartment during conditioning. All of these animals learned the task and failed to enter the dark chamber during the 300-second test. Rats treated with 2 g/kg ethanol 15 minutes prior to the conditioning trial showed a marked decrement in learning and readily entered the dark chamber. Pretreatment with SGE-301 prevented the acute effects of ethanol on learning compared with controls pretreated with vehicle alone (CDX) prior to ethanol. SGE-301 alone had no effect on learning compared with vehicle controls. ****P ≤ 0.0001 by one-way ANOVA, followed by Dunnett’s multiple comparisons test vs. ethanol (F = 17.60).