Moderate prenatal alcohol exposure reduces plasticity and alters NMDA receptor subunit composition in the dentate gyrus

Abstract

Although it is well documented that heavy consumption of alcohol during pregnancy impairs brain development, it remains controversial whether moderate consumption causes significant damage. Using a limited access, voluntary consumption paradigm, we recently demonstrated that moderate prenatal alcohol exposure (MPAE) is associated with dentate gyrus-dependent learning and memory deficits that are manifested in adulthood. Here, we identified a novel mechanism that may underlie this effect of MPAE. We found that MPAE mice exhibit deficits in NMDA receptor (NMDAR)-dependent long-term potentiation (LTP) in the dentate gyrus. Further, using semiquantitative immunoblotting techniques, we found that the levels of GluN2B subunits were decreased in the synaptic membrane, while levels of C2'-containing GluN1 and GluN3A subunits were increased, in the dentate gyrus of MPAE mice. These data suggest that MPAE alters the subunit composition of synaptic NMDARs, leading to impaired NMDAR-dependent LTP in the dentate gyrus.

Figure 1.

MPAE impaired NMDAR-dependent LTP and reduced GluN2B-containing NMDAR contribution to fEPSPs in the dentate gyrus. A, Ionotropic glutamate receptor-mediated I/O curves from SAC and MPAE slices. B, TBS-induced LTP in DG was blocked by ifenprodil (10 μm) and by MPAE. C, HFS-induced LTP was not significantly affected by MPAE or by ifenprodil. D1, D2, Ifenprodil-induced inhibition of NMDAR-dependent fEPSP area was significantly decreased in MPAE mice compared with SAC. D3, NMDAR-mediated I/O curves from SAC and MPAE slices. E1, E2, Ifenprodil-induced inhibition of NMDAR-EPSCs did not differ between the SAC and MPAE groups. F1, F2, AMPA/NMDA was not different between the SAC and MPAE groups.

Figure 2.

MPAE altered NMDAR subunit levels in the synaptic fraction of the dentate gyrus. A, Characterization of synaptic and nonsynaptic fractions. Data were normalized to the amount present in total homogenate (TH). In control tissue, PSD-95 was enriched approximately threefold in the synaptic (TxP) fraction compared with the nonsynaptic (TxS) fraction (n = 4; A1) and GABA_AR α4 subunit was enriched approximately twofold in the TxS fraction compared with the TxP fraction (n = 4; A2). B, GluN1 (B1) subunit levels, specifically C2′-containing GluN1 subunit levels (B4), were significantly increased in MPAE mice compared with SAC (GluN1: n = 7, *p < 0.05; C2′: n = 9, **p < 0.01), with no changes in C1-containing (B2) or C2-containing (B3) GluN1 subunits. C, GluN2A subunit levels were unchanged by MPAE. D, GluN2B subunit levels were significantly decreased by MPAE mice (n = 7, *p < 0.05). E, GluN3A subunit levels were significantly increased in MPAE mice compared with SAC (n = 8, *p < 0.05). F, Levels of GluN1 (F1), GluN2A (F2), and GluN2B (F3) did not differ in the nonsynaptic fraction of SAC and MPAE mice.

Figure 3.

Surface expression of GluN1 C2′-containing, GluN2B and GluN3A subunits, measured indirectly using the membrane-impermeable cross-linker BS³, did not differ in SAC and MPAE dentate gyrus tissue slices. Tissue slices were treated with (+) or without (−) BS³ membrane impermeable cross-linker and then analyzed for NMDAR subunit levels, as described in Materials and Methods.