Margaret Walsh's Blog

Addiction is derived from a Latin term for “enslaved by” or “bound to”.  Drug addiction exerts a long and powerful influence in the brain that manifests in three distinct ways: craving for the object of addiction, loss of control over its use, and continuing involvement with it despite adverse consequences. The brain registers all pleasure in the same way by releasing the neurotransmitter dopamine the the nucleus assumbens, a cluster of nerve cells lying underneath the cerebral cortex (brain’s pleasure center). Drugs of abuse cause addiction by acting on circuits of the brain and interacting with neurotransmitters secreted from area os the brain associated with pleasure and addictive behavior. For example, cocaine acts on the re-uptake transporters for dopamine and other monoamine neurotransmitters.

In this paper, biologist studied the two main regions of the brain: the prefrontal cortex (FFC) and the nucleus accumbens core (NAcore). These two area are directly associated with the ventral tegmental area (VTA) by receiving dopaminergic afferents including glutamatergic projections from the PFC into the NAcore, which are necessary for the resistance of seeking behavior in several models of drug relapse.

The family of G-proteins are responsible for sensing molecules outside of the cell and activating inside the cell signal transduction pathways that determine cellular response. The two main pathways that the G-proteins are involved in are the cAMP signal pathway and the phosphatidylinositol signal pathway. Protein expression and signaling through the non-receptor Activator of heterotrimeric G-protein Signaling 3 (AGS3) is altered by commonly abused substances. AGS3 is a highly conserved G-protein dissociation inhibitor found in tissues of neurons, astroglia, and microglia. It also has a homolog, LGN and GPSM2, that is found in some higher organisms. AGS3 binds and stabilizes all three Gα-protein isoforms in the GDP bound inactive state through GPR domains that inhibit guanine nucleotide exchange. Exchange of GDP for GTP is necessary for G-protien activation. This allows for AGS3 to take over the now displaced Gβγ-protein complex and alter signaling through the G-protein receptors. AGS3 stabilized inactive Gα, which augments signaling through Gβγ-mediated effectors, but inhibits signaling through Gα-coupled receptors onto effector, while also prohibiting G-protein heterotrimer reformation and receptor re-asssociation. 

This review article uses rat models with cocaine addiction to test expression of AGS3. The AGS3 protein was found to be unregulated and a subsequent super activation of downstream signaling that occurred only after drug was not administered. The unregulation of AGS3 in regions of the brain controlling motivation, reinforcement, and reactivity of drug-associated cues during recovery show that AGS3 controls aspects of drug and alcohol relapse. Thus, during cocaine withdrawal, AGS3 expression increased, signaling through Gα-coupled receptors decreased, and behavioral sensitivity was expressed. Increased AGS3 expression appears to be a deleterious gatekeeper on the path towards addiction. This data can be used to design a protein that acts as an AGS3-targeted inhibitor, and can selectively disrupt AGS3 activity. Selectively disrupting binding would simultaneously augment signaling through Gα-coupled receptors and focus signaling through Gβγ-mediated effectors. This data can also be used as information for tailoring treatments to individuals dealing with addiction and withdrawal.

The figure above shows an experiment to test the effects of a cell-permeable AGS3-like mimetic peptide. The peptide which acts in similarity to the unregualtion of AGS3, bind to inactive Gα and uncouples the receptor. Injection of the AGS3 mimetic peptide into the PFC, which had no prior introduction to cocaine, was then injected with cocaine. This caused a sensitized-like behavioral and neurochemical response similar to that observed after abstinence from repeated cocaine exposures. Neither the sensitized behavioral nor neurochemical response was observed following acute systemic cocaine if the mutant peptide was injected into the PFC instead of the AGS3 mimetic peptide. Neither AGS3 mimetic nor the mutant peptide altered changes in the saline samples. This data shows that AGS3 expression levels have behavioral and neurochemical plasticity that is commonly observed during abstinence from repeated cocaine exposure.

Animals were trained to press a lever in order to receive a cocaine infusion or food reinforcement. Extinction 1 includes the removal of the lever pressing ability by the animals (a). During this time of relapse, AGS3 expression increased. Food reinforcement was not effected during this time period (b), but cocaine-primed seeking of cocaine was blocked (failure to evoke drug-seeking behavior when PFC was treated with scrambled oglionucleotide) (c). Olgionucleotides were taken out so the AGS3 protein could be expressed again. Graph d shows that after the second extinction of the cocaine reinforcement and retest, rats expressed high levels of cocaine relapse. You can also see in graph e that after injection of the mutant AGS3-like mimetic peptide into the PFC the number of lever presses decreases unlike the number of lever presses for the AGS3-mimetic peptide. This graph confirmed conclusions made from the graph above. Expression levels of AGS3 are directly associated with behavioral and neurological pathways that deal with drug relapse.