Glutamate Plasticity in the Drunken Amygdala: The Making of an Anxious Synapse
Introduction
The enduring behavioral changes wrought by chronic alcohol abuse and alcoholism are among the most challenging obstacles for the development of effective therapies. These behaviors are currently believed to represent a shift from the initially rewarding effects of ethanol to an avoidance of the negative consequences during alcohol withdrawal following long-term abuse (Koob, 2003). One pervasive manifestation of chronic drug abuse in general and alcohol dependence in particular is the robust and persistent anxiety experienced during withdrawal and throughout abstinence. Long-abstinent alcoholics frequently cite anxiety as a significant contributing factor to relapse (Brown et al., 1990, Noone et al., 1999, Schneider et al., 2001, Thevos et al., 1991). Independent measures of anxiety in recently abstinent alcoholics suggest this negatively reinforcing emotion persists weeks to months after the acute withdrawal (Cohn et al., 2003) and can predict relapse rates in some populations (Roberts et al., 1999). Cause–effect relationships between alcohol dependence, relapse, and anxiety are nonetheless difficult to study in humans (Kushner et al., 2000, Merikangas et al., 1998, Rodgers et al., 2000). However, animal models clearly suggest that withdrawal-associated increases in anxiety-like behavior persist well after the acute withdrawal syndrome (Santucci et al., 2008) and contribute to subsequent increases in ethanol drinking (Valdez et al., 2002). While antecedent anxiety disorders contribute to ethanol drinking and abuse and are likely to make important contributions to the transition from use to dependence, we will focus specifically upon dependence-related anxiety and the excitatory synaptic mechanisms that potentially help regulate this persistent increase in anxiety following chronic ethanol exposure.
Enduring changes in behavior, whether initiated by environmental cues or by chronic drug and alcohol exposure, are most certainly dictated by long-term changes in neurotransmitter signaling and neuronal excitability. Recent evidence in animal models of cocaine (Borgland et al., 2004, Goussakov et al., 2006, Ungless et al., 2001) and opiate (Glass et al., 2005) exposure support the hypothesis that abused drugs can engage many of the cellular mechanisms that are believed to govern long-term changes in synaptic plasticity, particularly within reward- and anxiety-related neural circuits. This neurobiological “allostasis” likely reflects the subversion of synaptic and intrinsic mechanisms that normally serve functions such as learning and memory. This chapter will attempt to consolidate recent findings that suggest chronic ethanol exposure, like cocaine and opiates, can specifically upregulate glutamatergic synaptic function in ways that parallel activity-related changes in synaptic efficacy.
What then are the neuroanatomical substrates that govern anxiety-like behavior in general? There have been many excellent reviews discussing the neuroanatomical contributions to learned fear behaviors as well as innate anxiety-like responses to more naturally aversive environments (Charney and Deutch, 1996, Davidson, 2002, Davis, 2006, Mathew et al., 2008, Phelps and LeDoux, 2005). Critical to both learned fear-like and innate anxiety-like behaviors is the amygdala. As part of the limbic system, the amygdala serves as a central “hub” for information dealing with emotional behaviors and therefore sends/receives information from a number of brain regions. Highly processed sensory and cognitive information flows into the amygdala from the cortex and thalamus as well as the hippocampus (LeDoux, 1996). The amygdala subsequently projects to the lateral hypothalamus to regulate the HPA axis responses to learned fear (Gray et al., 1989) and to the parabrachial nucleus and dorsal motor nucleus of the vagus, which modulate the autonomic and physical components of the conditioned fear (Davis, 1992). In addition, the amygdala modulates innate anxiety-related behaviors via projections to forebrain regions such as the prefrontal cortex, nucleus accumbens, and bed nucleus of the stria terminalis, which have been associated with risk assessment (Jinks and McGregor, 1997, Simpson et al., 2001), reward (Prado-Alcala and Wise, 1984), and unconditioned fear (Walker and Davis, 1997), respectively. Hence the central contributions by the amygdala to both conditioned fear responses and unconditioned anxiety-like behavior make it a likely candidate when looking for neural adaptations related to chronic ethanol exposure/withdrawal-associated increases in anxiety.
The amygdala itself contains several distinct nuclei, which are all associated with specific efferent and afferent projections that serve to modulate conditioned and unconditioned behaviors. The lateral and basolateral nuclei (BLA) are the primary “input” regions of the amygdala. We refer to these regions together as “BLA” to emphasize their cytoarchitectural similarities and their extensively overlapping efferent/afferent arrangements. These nuclei represent the initial “stop” for cognitive and sensory information flowing into the anxiety circuit. The BLA integrates information from sensory and executive cortices, hippocampus, and sensory thalamus and sends glutamatergic efferents to the central nucleus of the amygdala (CeA), the bed nucleus of the stria terminalis, and the nucleus accumbens (De Olmos et al., 1985). Ninety to ninety-five percent of BLA neurons are large, pyramidal-shaped glutamatergic projection neurons and 5–10% small, nonpyramidal GABAergic interneurons (McDonald, 1982). The information processing in the BLA is dictated by local inhibitory GABAergic inputs and excitatory glutamatergic synaptic inputs arising both locally and from distant brain regions. The delicate balance between excitation and inhibition in the BLA ultimately influences the activity of the principal glutamatergic projection neurons.
Neuroadaptations within the BLA due to chronic ethanol and withdrawal would significantly impact the flow of information both within the amygdala and along the amygdala efferent pathways and ultimately influence the expression of anxiety/fear-related behaviors. For example, classic behavioral pharmacology experiments have shown that the BLA plays a central role in the acquisition of learned-fear behaviors (Blair et al., 2001, Fanselow and LeDoux, 1999, Maren, 2005) and can also modulate the expression of innate anxiety-like behaviors (McCool and Chappell, 2007, Sajdyk and Shekhar, 1997). Both behavior measures are dependent upon BLA neurotransmitter systems and can be modulated by direct manipulation of glutamatergic (Fanselow and Kim, 1994, Sajdyk and Shekhar, 1997), GABAergic (Muller et al., 1997, Sanders and Shekhar, 1995), catecholaminergic (Gonzalez et al., 1996, Guarraci et al., 1999), and neuropeptide (Gutman et al., 2008, Sajdyk et al., 1999) systems. Consistent with this, we have also recently shown that microinjection of the AMPA receptor antagonist DNQX into the BLA can alleviate withdrawal-associated increases in anxiety-like behavior as they are represented in the light/dark transition test (Lack et al., 2007). Previous work has also found increased c-fos expression in the BLA following multiple withdrawals from an ethanol liquid diet (Borlikova et al., 2006). Although these findings do not directly implicate glutamatergic dysregulation during ethanol withdrawal, the data do suggest a central role of the lateral/basolateral amygdala in the regulation of withdrawal-associated anxiety. We therefore began our investigations with examining glutamatergic alterations following chronic ethanol and withdrawal. Our rationale centered upon the important role this system plays in both learned and unconditioned amygdala-dependent behaviors.
Section snippets
Use-Dependent Synaptic Plasticity
Synaptic plasticity has been generally defined as a modification in synaptic efficacy resulting from some salient behavioral experience or overt experimental manipulation. This plasticity is currently believed to be the neurobiological mechanism that underlies long-term behavioral changes that manifest as learning and memory (Sigurdsson et al., 2007) or, in the context of the amygdala, Pavlovian fear conditioning (McKernan and Shinnick-Gallagher, 1997, Rogan et al., 1997). The physiological
NMDA Receptors and Chronic Alcohol/Withdrawal
To understand the inhibitory effect of chronic ethanol/withdrawal on LTP initiation/expression in the amygdala, we began by exploring functional alterations of NMDA-type glutamate receptors. NMDA receptors are heteromeric receptors whose subunits include the NR1 as well as NR2A-D subunits; the NR1 subunit is necessary for the formation of a functional channel whereas NR2 subunits confer native pharmacological and biophysical properties to the receptor complex. All NMDA NR1–NR2 subunit
Amygdala AMPA Receptors
AMPA-type ionotropic glutamate receptors mediate the vast majority of excitatory neurotransmission in the nervous system. These receptors are composed of four closely related subunits, GluR1–4, that form tetrameric assemblies to produce functional channels (Rosenmund et al., 1998). GluR1–4 subunits are all expressed within the rat BLA (Farb et al., 1995, McDonald, 1994). The specific subunit composition of the native channel dictates ion permeability. Although all AMPARs are nonselectively
Ethanol-Dependent Plasticity of Additional Amygdala Neurotransmitter Systems
This review has focused primarily upon the plasticity of glutamatergic ionotropic receptors following chronic ethanol and withdrawal. However, we must emphasize that there are many additional signaling pathways which can contribute to increased neuronal output from the BLA. Along with the pre and postsynaptic glutamatergic alterations described here, these additional pathways could also help regulate the enhanced anxiety-related behaviors evident during withdrawal. For example, increased
Conclusions and Future Directions
Our work and that of others have shown that chronic ethanol exposure and withdrawal can enhance function of glutamatergic receptors responsible for both the initiation and expression of BLA synaptic plasticity. The increased NMDA and kainate receptor function both speak to the diversity of LTP initiation-related signaling processes co-opted by a model of chronic alcohol exposure. Similarly, the enhanced AMPA receptor function, neuronal responsiveness, and presynaptic glutamatergic function that
Acknowledgments
This work was supported by NIH/NIAAA grants R01 AA014445 (BAM), P01 AA017056, T32 AA007565 (DTC), F31 AA017576 (MRD), and F31 AA016442 (AKL).
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