Do specific NMDA receptor subunits act as gateways for addictive behaviors?

Abstract

Addiction to alcohol and drugs is a major social and economic problem, and there is considerable interest in understanding the molecular mechanisms that promote addictive drives. A number of proteins have been identified that contribute to expression of addictive behaviors. NMDA receptors (NMDARs), a subclass of ionotropic glutamate receptors, have been of particular interest because their physiological properties make them an attractive candidate for gating induction of synaptic plasticity, a molecular change thought to mediate learning and memory. NMDARs are generally inactive at the hyperpolarized resting potentials of many neurons. However, given sufficient depolarization, NMDARs are activated and exhibit long-lasting currents with significant calcium permeability. Also, in addition to stimulating neurons by direct depolarization, NMDARs and their calcium signaling can allow strong and/or synchronized inputs to produce long-term changes in other molecules (such as AMPA-type glutamate receptors) which can last from days to years, binding internal and external stimuli in a long-term memory trace. Such memories could allow salient drug-related stimuli to exert strong control over future behaviors and thus promote addictive drives. Finally, NMDARs may themselves undergo plasticity, which can alter subsequent neuronal stimulation and/or the ability to induce plasticity. This review will address recent and past findings suggesting that NMDAR activity promotes drug- and alcohol-related behaviors, with a particular focus on GluN2B subunits as possible central regulators of many addictive behaviors, as well as newer studies examining the importance of non-canonical NMDAR subunits and endogenous NMDAR cofactors.

Keywords: Activity; NMDA; addiction; alcohol; intake; intoxicant; mechanism; plasticity; receptor; subunit.

Figure 1

NMDAR adaptations after exposure to addictive intoxicants. Summary of NMDAR adaptations observed after passive or active intoxicant exposure. Adaptations are grouped near the brain region they have been reported within. List of abbreviations: 1: GluN1; 2A: GluN2A; 2B: GluN2B; ADE: alcohol deprivation effect; Alc: alcohol; bottle: alcohol intake from a bottle; brief: brief daily access; CIE: chronic intermittent ethanol exposure; Coc: cocaine; Compul: compulsion-like; direct: direct-pathway (DA1R–containing) cells; Dser: d-serine; extend: extended daily access; indirect: indirect-pathway (DA2R–or adenosine-2-receptor-containing) cells; KO: knockout; l: lateral; m: medial; Meth: methamphetamine; Morph: morphine; n.c. no change; Nicot: nicotine; nonCan: non-canonical NMDARs; NR: NMDAR; operant: operant intake; Reinst: reinstatement; Reward: acute reward; Rpt: repeated; SAdm: self-administration; Sensit: sensitization; Single: single exposure.

Figure 2

Behavioral impact of inhibiting NMDARs in different brain regions. Summary of studies where the impacts of NMDAR function within a specific brain region was directly addressed with intracranial NMDAR manipulation. Results are grouped near the brain region they have been reported within. Also, only studies examining expression (not development) of addictive behaviors are included, except for KO studies. See Fig. 1 for list of abbreviations.