A stressed rat will seek a dose of cocaine that is too weak to motivate an unstressed rat. The reason, NIDA researchers report, is that the stress hormone corticosterone increases dopamine activity in the brain’s reward center. When an animal is stressed, the cocaine-induced dopamine surge that drives drug seeking rises higher because it occurs on top of the stress-related elevation.
Graduate student Evan N. Graf, Dr. Paul J. Gasser, and colleagues at Marquette University in Milwaukee, Wisconsin, traced the physiological pathway that links stress and corticosterone to increased dopamine activity and heightened responsiveness to cocaine. Their findings provide new insight into cocaine use and relapse, and point to possible new medication strategies for helping people stay drug free.
Stress Increases Sensitivity to Relapse Triggers
Former drug users who relapse often cite stress as a contributing factor. The Marquette researchers observed that when stress figures in relapse, other relapse promoters are almost always present as well. Dr. Gasser explains, “It’s never one single event that triggers relapse. It’s the convergence of many events and conditions, such as the availability of the drug, cues that remind people of their former drug use, and also stress.” On the basis of this observation, the researchers hypothesized that stress promotes relapse by making a person more sensitive to other relapse triggers.
To test their hypothesis, the researchers put stressed and unstressed rats through an experimental protocol that simulates regular drug use in people followed by abstinence and exposure to a relapse trigger. As the stressor, they used a mild electric foot shock; as the relapse trigger, they administered a low dose of cocaine (2.5 milligrams per kilogram).
The results confirmed the hypothesis. The stressed rats, but not the stress-free animals, responded to the small cocaine dose with a behavior that parallels relapse in people: They resumed pressing a lever that they had previously used to self-administer the drug (see Figure 1, top graph).
Figure 1. The figure shows three bar graphs indicating how many times rats that had learned to self-administer cocaine by pressing a cocaine-delivering lever pressed the lever after receiving a low dose of cocaine with or without a stressor, that is, a mild foot shock (top graph), with or without the stress hormone corticosterone (middle graph), or with or without a blocker of the OCT3 transporter (bottom graph).
- Text Description of Figure 1
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Figure 1. The figure shows three bar graphs indicating how many times rats that had learned to self-administer cocaine by pressing a cocaine-delivering lever pressed the lever after receiving a low dose of cocaine with or without a stressor, that is, a mild foot shock (top graph), with or without the stress hormone corticosterone (middle graph), or with or without a blocker of the OCT3 transporter (bottom graph). The number of lever presses, indicating how strongly a rat sought cocaine, after these treatments was compared with that at the very end of a period in which pressing of the lever had yielded no cocaine. The vertical (y)-axes on all graphs shows the number of lever presses a rat did during a 2-hour interval, and the horizontal (x)-axes show the treatment combinations. Blue bars show the number of times the rats pressed the lever on the last day of the drug-free period (as a control) and red bars the number of lever presses after the rats had received cocaine alone; the stressor, corticosterone, or the OCT3 blocker alone; or after receiving both cocaine and one of the three agents. As shown in the top graph, only the combination of low-dose cocaine and the stressor caused a substantial increase in lever pressing from about 10 presses before this treatment to about 50 presses after receiving the low-dose cocaine–stressor combination; the intensity of lever pressing after a rat received cocaine or the stressor alone was comparable to that immediately before the rat received one of the two agents. As shown in the middle graph, only the combination of low-dose cocaine and the stress hormone corticosterone caused a substantial increase in lever pressing from about 10 presses before this treatment to about 50 presses after receiving the low-dose cocaine–corticosterone combination; the intensity of lever pressing after a rat received cocaine or corticosterone alone was comparable to that immediately before the rat received one of the two agents. As shown in the bottom graph, only the combination of low-dose cocaine and the OCT3 blocker caused a substantial increase in lever pressing from about 10 presses before this treatment to about 40 presses after receiving the low-dose cocaine–OCT3 blocker combination; the intensity of lever pressing after a rat received cocaine or the OCT3 blocker alone was again comparable to that immediately before the rat received one of the two agents.
A Stress Hormone Underlies the Effect
Mr. Graf and colleagues turned their attention to the question of how stress sensitizes animals to cocaine’s motivational effect. One likely place to start was with the hormone corticosterone. In stressful situations, the adrenal glands release corticosterone into the blood, which carries it throughout the body and to the brain. Among corticosterone’s physiological roles is that it affects glucose metabolism and helps to restore homeostasis after stress. The Marquette researchers demonstrated that increasing cocaine’s potential to induce relapse also belongs on the list of corticosterone’s effects. Reprising their original experimental protocol with a couple of new twists, they showed that:
- Corticosterone is necessary for stress to promote relapse to cocaine seeking: The researchers removed rats’ adrenal glands, which prevented the animals from producing corticosterone. In this condition, the animals did not exhibit relapse behavior when exposed to the stressor and low-dose cocaine.
- Corticosterone in the brain reward center is sufficient by itself to increase cocaine’s potency as a relapse trigger: The researchers injected these same rats with corticosterone, bringing the hormone concentration up to stress levels in the brain reward center (nucleus accumbens, NAc). Now the animals exhibited relapse behavior when exposed to cocaine, even without the stressor (see Figure 1, middle graph).
Enhanced Dopamine Activity…
The researchers next took up the question: What does corticosterone do in the NAc to increase cocaine’s potency to induce relapse? A hypothesis that suggested itself immediately was that the hormone enhances dopamine activity. Dopamine is an important neuromodulator in the NAc, and all addictive drugs, including cocaine, produce their motivating effects by increasing dopamine concentrations in the NAc.
The Marquette team showed that, indeed, stress-level concentrations of corticosterone enhance the cocaine-induced rise in extracellular dopamine in the NAc. In this experiment, the researchers exposed two groups of rats to low-dose cocaine, then measured their NAc dopamine levels with in vivo microdialysis. One group, which was pretreated with corticosterone injections, had higher dopamine levels than the other, which was not pretreated.
The Marquette team firmed up their hypothesis with a further experiment. They reasoned that if corticosterone promotes relapse behavior by increasing dopamine activity, then preventing that enhancement should prevent the behavior. This indeed turned out to be the case. When the researchers injected animals with corticosterone but also gave them a compound (fluphenazine) that blocks dopamine activity, exposure to low-dose cocaine did not elicit relapse behavior.
…Due To Reduced Dopamine Clearance
So far the Marquette team had established that the stress hormone corticosterone promotes relapse behavior by increasing dopamine activity in the NAc. Now they moved on to the next question: How does corticosterone enhance dopamine activity?
To address this question, the researchers considered the cycle of dopamine release and reuptake. In the NAc, as elsewhere in the brain, dopamine activity depends on the concentration of the neurotransmitter in the extracellular space (space between neurons): the higher the concentration, the more activity there will be. In turn, the extracellular dopamine concentration depends on the balance between two reciprocal ongoing processes: specialized neurons releasing dopamine molecules into the space, and specialized proteins drawing molecules back inside the neurons.
Mr. Graf and colleagues discovered that corticosterone interferes with the removal of dopamine molecules from the extracellular space back into cells. It shares this effect with cocaine, but achieves it by a different mechanism.
In this experiment, the researchers measured real-time changes in dopamine concentration in the NAc in response to electrical stimulation of dopamine release in the area. This technique allowed the team to measure both A) the rate of increase in dopamine concentration, indicating the amount of dopamine released; and B) the rate of decrease in dopamine concentration, indicating the rate of dopamine clearance. The scientists measured stimulation-induced increases and decreases in extracellular dopamine concentrations under three conditions: at baseline, after giving the animals a compound that blocks the dopamine transporter (DAT), which is the mechanism whereby cocaine inhibits dopamine removal; and, last, after injecting the animals with corticosterone. They found that:
- As happens with cocaine, the clearance of extracellular dopamine decreased after DAT blockade.
- Clearance of extracellular dopamine decreased further after corticosterone.
A Candidate Mechanism
One question remained outstanding to complete the picture of how stress potentiates the response to cocaine: What is the mechanism whereby corticosterone reduces dopamine clearance?
Mr. Graf and colleagues noted that previous research provides a likely answer: Corticosterone has been shown to inhibit the functioning of the organic cation transporter 3 (OCT3), which is another of the specialized proteins that, like DAT, remove dopamine from the extracellular space. To confirm this hypothesis, the researchers resorted again to their initial experimental protocol. This time, they injected rats with a compound (normetanephrine) that blocks OCT3, followed by low-dose cocaine. The animals responded by resuming their previously abandoned lever pressing behavior, proving that OCT3 blockade is sufficient to potentiate the response to cocaine (see Figure 1, bottom graph).
The Marquette researchers say that further studies will be required to definitively establish that OCT3 plays the role their evidence points to. Taken together, however, their experiments trace a complete pathway connecting stress to an animal’s enhanced responses to cocaine (see Figure 2):
- Stress raises corticosterone levels.
- Corticosterone blocks OCT3, inhibiting dopamine clearance and thereby raising dopamine activity in the NAc.
- When a stressed animal is exposed to cocaine, the resulting dopamine surge builds on the foundation of this already higher-than-normal level of dopamine activity.
- The added elevation of the dopamine surge increases the animal’s motivation to seek the drug.
Figure 2. Stress Amplifies Cocaine’s Effect on Dopamine Release in the Nucleus Accumbens (NAc) The schematic illustrates how stress may enhance cocaine’s motivational effect and increase the risk for relapse. A) Cocaine binds to the dopamine transporter (DAT) on dopamine-releasing neurons in the NAc, reducing dopamine (DA) clearance and, in turn, increasing extracellular dopamine. B) Stress causes release of corticosterone, which inhibits the OCT3 transporter, further reducing dopamine clearance and increasing extracellular dopamine. The resulting heightened dopamine stimulation of medium spiny neurons (MSNs) enhances drug seeking.
- Text Description of Figure 2
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Figure 2. The figure shows a schematic illustrating how the combination of cocaine and stress produces drug seeking by reducing clearance of dopamine from the outside of nerve cells (neurons). The left panel (A) shows the effect of cocaine alone on the dopamine transporter (DAT) on dopamine-releasing neurons; the DAT blockade causes increased accumulation of dopamine outside the neuron. The right panel (B) shows the effect of the cocaine–stress combination on dopamine accumulation: cocaine again blocks DAT, increasing dopamine levels outside of the neuron, and stress triggers production of corticosterone, which blocks the OCT3 transporter, which, in turn, further reduces dopamine clearance. This combined DAT–OCT3 blockade of dopamine clearance from neurons causes dopamine accumulation to reach levels high enough to stimulate activity in medium spiny neurons (MSNs), leading to drug seeking.
Stress–Relapse Connection Unraveled
“Our findings show that stress doesn’t just cause relapse behavior by itself, but interacts with other ongoing behaviors to influence relapse,” Dr. Gasser says. “This insight provides a better picture of how stress can affect addiction. It helps us understand why treating cocaine addiction is so difficult and will help in designing therapies whether they be based on pharmacotherapy or counseling.” The researchers believe—and are testing as a hypothesis—that stress increases the power of environmental drug-associated cues to trigger relapse, just as it does the power of low-dose cocaine.
Although researchers have long known that stress plays an important role in relapse, pinning down its role experimentally has been a challenge, says Dr. Susan Volman, program officer and health science administrator at NIDA’s Behavioral and Cognitive Science Research Branch. “This study provides a perspective of stress as a stage-setter or modulator for relapse, and it gets all the way down to the molecular mechanism. Based on this team’s findings, OCT3 offers a potential new target for developing pharmacological therapies to help with treating addiction,” Dr. Volman says.
This work was supported by NIH grants DA017328, DA15758, and DA025679.
Source:
Graf, E.N.; Wheeler, R.A.; Baker, D.A. et al. Corticosterone acts in the nucleus accumbens to enhance dopamine signaling and potentiate reinstatement of cocaine seeking. Journal of Neuroscience 33(29):11800-11810, 2013. Full text