Hippocampal N‐Methyl‐d‐Aspartate Receptor Subunit Expression Profiles in a Mouse Model of Prenatal Alcohol Exposure

Alcohol ingestion during pregnancy, even in moderate amounts, can affect normal brain development, resulting in changes in brain functioning that can persist throughout life. Fetal alcohol spectrum disorder (FASD) is a term used to describe the wide range of clinical disorders, ranging from moderate to severe, that can occur in an individual whose mother drank alcohol during pregnancy. Investigations on the mechanisms that underlie the learning and memory impairments associated with FASD have focused on numerous processes, including glutamatergic neurotransmission. One class of glutamate receptors, the N-methyl-D-aspartate (NMDA) receptors, have received considerable attention because the levels and subcellular localization of these receptors impact the cellular processes underlying learning and memory, as well as various forms of synaptic plasticity (Gardoni et al., 2009; Tang et al., 1999; Zhang et al., 2008; Zhao et al., 2005). Several studies have found that NMDA receptors are significantly affected by prenatal alcohol exposure (see Table 1 and Costa et al., 2000a, for review). In particular, researchers have focused on NMDA receptors in the hippocampal formation, as this area of the brain has been shown to be particularly vulnerable to the effects of fetal alcohol exposure and is known to be densely populated with NMDA receptors (Berman and Hannigan, 2000). For example, prenatal alcohol exposure has been shown to decrease NMDA-sensitive glutamate binding sites in the hippocampus of 45-day-old rats (Savage et al., 1991) and alter [3H]MK-801 binding in the hippocampus of rats and guinea pigs (Abdollah and Brien, 1995; Diaz-Granados et al., 1997; Naassila and Daoust, 2002). Table 1 Summary of Literature Determining the Effects of Prenatal Alcohol Exposure on NMDA Receptor Subunit Expression NMDA receptors are ion channels that exist as heteromeric complexes composed of mandatory NR1 and modulatory NR2 (A-D) or NR3 (A, B) subunits. The NMDA receptor NR1 subunit is essential to the formation of a functional receptor (Paoletti and Neyton, 2007). Of the modulatory subunits, NR2A and NR2B are the most highly expressed forms in the adult rodent hippocampus (Al-Hallaq et al., 2007; Wenzel et al., 1997); NR3A is also present in the rodent hippocampus, but at much lower levels than NR2A and NR2B in the adult (Al-Hallaq et al., 2002; Wong et al., 2002). NMDA receptors are localized both within the synapse and extrasynaptically (Harris and Pettit, 2007; Lau and Zukin, 2007). While the overall function of an individual NMDA receptor is dependent on its subunit composition (Yashiro and Philpot, 2008), the location of the receptor governs its coupling to specific intracellular signaling pathways (Hardingham, 2006); the balance between synaptic and extrasynaptic receptor activation shapes the overall cellular response. Even though both synaptic and extrasynaptic NMDA receptors mediate Ca2+ influx, studies show that extrasynaptic receptors are mainly activated under conditions of high concentrations of NMDA (or glutamate) and mediate opposing effects on intracellular signaling pathways compared to those produced by synaptic NMDA receptors (Chandler et al., 2001; Ivanov et al., 2006; Mulholland et al., 2008). Synaptic NMDA receptors are organized and spatially restricted in large macromolecular signaling complexes called postsynaptic densities (PSDs) within the synaptic membrane (Lau and Zukin, 2007). PSD-95, a member of the membrane-associated guanylate kinase (MAGUK) family, is an anchoring and essential synaptic adapter protein, not found extrasynaptically, that targets the NMDA receptor NR2A and NR2B subunits to the PSD (Al-Hallaq et al., 2007; Chen et al., 2006; Hughes et al., 2001). Although there is evidence that the NR1 C2′ cassette contains a motif allowing it to associate with PSD-95, NR1 is generally not considered to bind to PSD-95 (Honse et al., 2003; Lim et al., 2002). This complex of PSD-95 and NMDA receptors has been implicated in the regulation of several cellular processes, including synaptic plasticity and learning and memory (Chen et al., 2006; Gardoni et al., 2009). Studies employing a variety of prenatal and neonatal alcohol exposure paradigms have yielded conflicting results regarding the effects of the exposure on NMDA receptor subunit composition (Table 1). It is also of note that most of these studies have employed relatively young animals and, in general, have not assessed the subcellular localization of the NMDA receptor subunits. It is important to determine whether such alterations in NMDA receptor subunit levels or localization exist in adult animals that were prenatally exposed to alcohol in order to determine if there is an association between these alterations and the life-long cognitive impairments observed in FASD. The current study aimed to test the hypothesis that moderate prenatal alcohol exposure is associated with differential NMDA receptor subunit expression and/or subcellular localization in adult animals.