Summary: Optogenetic stimulation of the amygdala increases the motivation to consume cocaine in rats, producing behavior that resembles aspects of addiction.
Source: University of Michigan.
Activation of an Amygdala Circuit Intensifies Cocaine Seeking
Researchers at the University of Michigan have identified a region within the amygdala that can dramatically amplify an animal’s motivation to obtain a reward—in this study, intravenous cocaine. Using optogenetic methods to selectively stimulate neurons in the central amygdala, the team observed that rats developed intense, focused pursuit of cocaine well beyond normal consumption levels, similar to earlier findings showing amplified motivation for sweet foods such as sugar.
“This suggests that the amygdala plays a central role in the narrowing and intensification of motivation seen in addiction,” said Shelley Warlow, a University of Michigan psychology graduate student and the study’s lead author. While many individuals may try drugs experimentally without becoming addicted, addiction is characterized by an overwhelming drive to obtain the drug at the expense of other normal rewards. Identifying the brain regions and mechanisms that produce that focused motivation is a key step toward developing better treatments for addiction and related compulsive disorders.
Experimental design
In the experiment, researchers implanted intravenous catheters in rats and trained them to self-administer cocaine by nose-poking through small holes in the wall of the test chamber. One nose-poke port delivered an intravenous dose of cocaine paired with painless optogenetic activation of neurons in the central nucleus of the amygdala (CeA) via a laser. A second, identical port delivered the same dose of cocaine but without any amygdala stimulation. The rats could freely choose between the two options.
Findings
When given a choice, rats overwhelmingly chose the port that delivered cocaine paired with CeA stimulation. Those rats consumed considerably more cocaine than control rats and concentrated their behavior on the stimulation-paired port, often nibbling around that hole as if trying to access more of the cue associated with the laser. In progressive-ratio tests designed to measure motivational effort, rats worked nearly three times harder for the cocaine option paired with CeA activation, indicating a large increase in incentive motivation.
Importantly, optogenetic activation of the central amygdala by itself did not act as a reinforcer: rats did not nose-poke to receive amygdala stimulation alone. The intensification of motivation occurred only when amygdala stimulation was paired with the cocaine reward. Conversely, temporarily inactivating the central amygdala using a local infusion of inhibitory drugs prevented the acquisition of cocaine self-administration, and optogenetic inhibition also suppressed cocaine intake. These results indicate that normal CeA circuitry is necessary for ordinary cocaine motivation and that artificial excitation of this circuit can selectively amplify and focus that motivation toward a single option.
Implications
The study shows that excitation of central amygdala circuitry can make a particular reward option and its associated cues become dramatically more attractive, producing intense, narrowly focused pursuit that resembles some features of addiction. Because the laser stimulation did not have intrinsic reinforcing properties, the effect appears to be one of amplifying the incentive value of a paired reward and its cues rather than acting as a standalone reward.
The study, published in the Journal of Neuroscience, was conducted by Shelley M. Warlow, Mike J.F. Robinson and Kent C. Berridge. Funding for the work was provided by the National Institute on Drug Abuse. The research demonstrates a specific role for central amygdala circuitry in intensifying and narrowing motivation for intravenous cocaine in a rat model.
The researchers paired optogenetic stimulation of channelrhodopsin (ChR2) in either the central nucleus of the amygdala (CeA) or basolateral amygdala (BLA) with one of two nose-poke options for earning identical intravenous cocaine infusions. Rats with CeA ChR2 stimulation quickly and selectively pursued the laser-paired cocaine option, increasing intake while largely ignoring the alternative. BLA stimulation did not enhance cocaine motivation. CeA-paired rats emitted consummatory bites toward the laser-paired port, suggesting that associated cues acquired greater incentive salience. Progressive-ratio testing showed CeA stimulation substantially increased the effort rats would expend for cocaine. However, CeA laser alone had no reinforcing value, and CeA inhibition blocked cocaine self-administration, indicating CeA circuitry is required for ordinary cocaine motivation. The authors conclude that CeA excitation paired with a cocaine option specifically focuses and amplifies motivation, producing intense pursuit and consumption directed at that single target.
This work demonstrates how specific amygdala circuitry can transform a reward option into a dominant focus of intense motivation, mirroring a crucial aspect of addictive behavior: narrow, overpowering pursuit of a drug at the expense of other rewards. Understanding how the central amygdala amplifies incentive motivation could help guide new approaches for treating addiction and other compulsive conditions.