Glutamate is a key transmitter for neuronal plasticity and learning. My lab and others have shown that behavioral changes in animal models of addiction require glutamate-dependent forms of plasticity, and that learning and addiction involve common brain signaling pathways and cellular changes. Thus, addiction may be viewed as a form of maladaptive but extremely strong learning. An important question is how drugs like cocaine and amphetamine, which initially target dopamine (DA) systems in the brain, ultimately produce adaptations in glutamate neurotransmission. A better understanding of plasticity mechanisms engaged by drugs of abuse may lead to the development of pharmacological treatments for drug dependency and craving.
Glutamate receptor trafficking is critical for controlling the strength of glutamate synapses in well-studied forms of plasticity such as LTP. My lab focuses on the role of glutamate receptor trafficking in addiction-related plasticity. Some members of the lab use primary neuronal cultures to study mechanisms by which psychomotor stimulants influence glutamate receptor trafficking, while others use biochemical methods to investigate the functional significance of receptor trafficking for in vivo models of addiction. This work is funded by a Merit Award (R37), R01 and K05 Award from the National Institute on Drug Abuse.
Our in vitro studies have focused on determining if DA receptors regulate glutamate receptor trafficking in primary cultures of neurons from addiction-related brain regions such as the nucleus accumbens (NAc), prefrontal cortex, hippocampus, and ventral tegmental area (for review, see Wolf, 2010b). Principal neurons in the first three regions receive convergent DA and glutamate inputs. We have shown that DA receptors regulate AMPA receptor trafficking to extrasynaptic sites on the cell surface and thereby modulate synaptic plasticity (Chao et al., 2002a,b; Mangiavacchi & Wolf, 2004a.b; Sun et al., 2005; Gao et al., 2006; Sun et al., 2008). We hypothesize that DA-releasing psychomotor stimulants such as cocaine and amphetamine usurp this mechanism, leading to abnormal plasticity that contributes to addiction. More recently, we have demonstrated that synaptic scaling occurs in the nucleus accumbens (Sun a&Wolf, 2009). This and other types of homeostatic plasticity may be important during drug withdrawal. Current projects focus on the roles of BDNF and metabotropic glutamate receptors in regulating AMPA receptor trafficking in the NAc. Major techniques involved are cell culture, fluorescence microscopy and immunocytochemistry. Protocols for preparation of NAc cultures are provided under Protocols.
Our in vivo studies have focused on the role of glutamate receptor trafficking in rodent models of cocaine addiction (for review, see Wolf & Ferrario, 2010; Wolf, 2010a). Several years ago, we developed a protein crosslinking assay that enables us to study glutamate receptor redistribution after in vivo treatments (Boudreau & Wolf, 2005). Brain slices are rapidly prepared from control or drug-treated rats and incubated with BS3, a bifunctional protein crosslinking reagent. BS3 does not cross cell membranes, so it selectively crosslinks cell surface receptors, forming high molecular weight aggregates. Intracellular receptors are not modified. Surface and intracellular receptor pools can therefore be distinguished based on molecular weight using SDS-PAGE and Western blotting. Using this assay, we found that behavioral sensitization to cocaine, produced by repeated cocaine injections, is associated with increased cell surface expression of GluR1/2-containing AMPA receptors in the NAc (Boudreau & Wolf, 2005; Boudreau et al., 2007, 2009; Ferrario et al., 2010a). The NAc serves as the interface between corticolimbic glutamate inputs that initiate cocaine-seeking and motor regions responsible for its execution. Enhanced AMPA receptor transmission in the NAc may therefore explain stronger addiction-related behaviors in sensitized rats.
More recently, we have focused on the role of GluR2-lacking, calcium-permeable AMPA receptors (CP-AMPARs) in the incubation model of cocaine addiction, in which cue-induced cocaine craving progressively intensifies ("incubates") during withdrawal from extended access cocaine self-administration. In drug-naïve rats or rats with limited cocaine exposure, CP-AMPARs are normally expressed at very low levels by NAc neurons (Boudreau et al., 2009; Reimers et al., 2010). However, CP-AMPARs are added to NAc synapses after withdrawal from extended access cocaine self-administration and mediate the expression of "incubated" cue-induced cocaine craving (Conrad et al., 2008). This work, published in Nature in 2008, was conducted in collaboration with the laboratories of Dr. Michela Marinelli and Dr. Kuei Tseng (both at Rosalind Franklin University of Medicine and Science) and Dr. Yavin Shaham (NIDA). Current goals include understanding mechanisms that enable CP-AMPARs to accumulate in NAc synapses during "incubation" (Ferrario et al., 2011b) and strategies for normalizing synaptic transmission by removing CP-AMPARs. Important regulators of CP-AMPAR levels may include BDNF and group I metabotropic glutamate receptors.