Collaboration between a NIDA scientist and a researcher from St. John's University in Jamaica, New York, has identified a chemical compound that prevents animal responses to cocaine that correspond to human drug liking, seeking, and relapse. While the compound is not suitable as a medication, the researchers believe its mechanism of action--restricting neurotransmitter access to the dopamine D3 receptor--may provide a basis for pharmacological treatments for addiction to cocaine and other drugs.
D3 Receptor Antagonist Blocks Cocaine Reward in Rats. Cocaine exposure lowered by 19 percent rats' reward threshold, the amount of current needed to motivate an animal to push a lever to activate electrical stimulation of its brain reward system. When rats were given the D3 antagonist before being given cocaine, however, their reward threshold remained the same as when they were given placebo.The chemical messenger dopamine plays a critical role in networks of brain cells that trigger the rewarding feelings that result when we engage in pleasurable activities, such as eating. Drugs of abuse activate hot buttons, called receptors, on these brain cells, flooding the brain's reward pathways with dopamine and producing intense pleasure. With continued drug abuse, overstimulation of these pathways changes the brain, leading to the intense craving and uncontrollable pursuit of drugs that mark addiction.
To derail this process, research has focused on the use of agonists, compounds structurally similar to dopamine that bind to and stimulate dopamine receptors and seem to inhibit drug-seeking behavior, and antagonists, which bind to and block these receptors. Until now, researchers have tried developing a treatment medication that can counter the addictive effects of abused drugs at two different dopamine receptors, called D1 and D2, with little success. Potential treatment agonists that activated these receptors produced such strong stimulation that it seemed likely they themselves would be abused. And the antagonists used to block the pleasurable effects of abused drugs at these receptors produced aversive, unpleasant effects. However, recent NIDA-funded research on use of an antagonist designed to target a less-studied dopamine receptor called D3 offers new promise. Researchers Dr. Charles Ashby, Jr., of St. John's University, and Dr. Eliot Gardner, of NIDA's Intramural Research Program, note that D3 appears to play a major role in addiction and may be the elusive target for medications that could help control addictive behavior.
Scientists have identified five subtypes of dopamine receptors, each with distinct properties and each found in varying densities in different areas of the brain. To date, most research has focused on the roles of the D1 and D2 receptors, which occur in higher densities and more places in the brain than do D3 receptors. Nevertheless, several characteristics of the D3 receptor suggested that medications that interact with it may have promise as treatments for cocaine addiction. D3 receptors are mainly concentrated in the brain's reward pathway. In addition, dopamine is attracted more strongly to the D3 receptors than to other receptors. Yet researchers found that D3agonists caused rats to resume drug-seeking behavior more quickly after a period of abstinence. This reaction suggested that a compound capable of selectively blocking this receptor--an antagonist--could be important in developing a medication to reduce or block craving.
The D3 antagonist compound used in the Ashby-Gardner studies, called SB-277011-A, is highly selective, with an 80- to 100-fold preference for D3 over D2 receptors and 66 other receptors, enzymes, and ion channels. When the compound was first developed in 1997, Drs. Ashby and Gardner saw its potential. "Until then, study of the D3 receptor was hampered because the available compounds antagonized D2 as well as D3receptors to some extent, making it difficult to sort out which was responsible for the observed effects. As a result, research on the D3 receptor and compounds that affect it had yielded inconsistent and contradictory results," explains Dr. Ashby. "We knew this compound and its unambiguous selectivity for D3 receptors would allow us to test the role of these receptors, while offering promise as a treatment for addiction."
Drs. Ashby and Gardner performed three types of animal experiments. "Each experiment used the D3 antagonist to focus on a unique aspect of addiction, and all three yielded promising results," says Dr. Ashby. "Antagonizing the D3 receptor appears to weaken cocaine's rewarding effects, reduce cocaine-induced conditioned place preference, and block reinstatement of drug-seeking behavior. And the compound we were testing was not found to be rewarding or aversive."
Cocaine's Rewarding Effects
In the first set of studies, researchers used brain stimulation reward experiments to measure the direct rewarding properties of cocaine. This type of experiment is thought to produce the closest equivalent in animals to the cocaine-induced subjective high experienced by humans. The researchers implanted brain stimulation electrodes in rats and trained the animals to press a lever to self-administer electrical stimulation that produced feelings of pleasure or euphoria. Baseline reward thresholds, the amount of current below which the animal no longer finds the stimulation rewarding enough to press the lever, were determined for each animal. Drugs of abuse, which activate the same neurons in the brain's reward system as the electrical current, increase the amount of pleasure obtained from a given amount of current and therefore decrease the reward threshold. The difference between baseline reward threshold and the reward threshold after administration of a drug gives a measure of the rewarding potency of the drug being tested.
After establishing the rats' baseline reward thresholds, researchers injected the animals with placebo, 2 mg/kg cocaine, or 3 mg/kg SB compound followed by 2 mg/kg cocaine; researchers then retested the rats. As expected, the reward threshold of animals injected with placebo remained unchanged; those injected with cocaine had an average 19-percent decrease in their reward thresholds. Rats pretreated with the D3 antagonist and then given cocaine had no change in their reward thresholds, indicating that the antagonist completely abolished the enhancing effect of cocaine on brain reward.
Cocaine-Seeking Behavior
The second set of experiments used conditioned place preference, aiming to measure cocaine-seeking behavior evoked by environmental cues associated with cocaine. The experiments involved providing rats one of five pretreatments--placebo or varying doses of the SB compound--and then confining them to one chamber of a two-room cage. The rats were subsequently given cocaine and confined to the other chamber. Each chamber had distinct visual and tactile furnishings. Rats were then allowed to freely explore the entire cage for 15 minutes, while researchers measured their time in each chamber.
D3 Receptor Antagonist Blocks Cocaine Reward in Rats. Rats were given placebo and confined to one chamber of a two-chamber cage, then given cocaine and confined to the other chamber. They were then allowed to roam throughout the cage for 15 minutes, with researchers measuring the time spent in each chamber. This exercise was repeated with four other groups of rats, which were given either 0.3 mg/kg, 1 mg/kg, 3 mg/kg, or 10 mg/kg D3 antagonist (SB compound) and then cocaine. Rats treated with the SB compound before receiving cocaine spent significantly less time in the cocaine-associated chamber than rats pretreated with placebo.Rats given placebo and then cocaine spent roughly two thirds of their time in the chamber they associated with cocaine. However, rats pretreated with the D3 antagonist (SB compound) spent, on average, less time in the cocaine-associated chamber, with the minutes spent in that chamber decreasing as the D3 antagonist dose increased from 0.3 mg/kg to 1 mg/kg, 3 mg/kg, and up to 10 mg/kg. Rats pretreated with the highest dose of the D3 antagonist spent about 40 percent fewer minutes in the cocaine-associated chamber than did rats in the placebo-cocaine group. The results indicate that the D3 antagonist blocked the rats' motivation to seek out cocaine, eliminating their acquisition and expression of cocaine-induced conditioned preference.
Reinstatement of Drug Seeking
The final set of experiments focused on cocaine self-administration and reinstatement. The researchers implanted an intravenous catheter in the rat's external jugular vein and trained the animal to self-administer cocaine by pressing a lever. The daily 3-hour sessions continued until the rat was self-administering consistent amounts of the drug every day. The researchers then phased out the lever-pressing behavior by substituting saline for the cocaine; since pressing the lever no longer resulted in cocaine, the rats lost interest and pushed the lever much less often. At this point, the researchers gave rats that had been pretreated with placebo or the SB compound a priming dose of cocaine (1 mg/kg) normally sufficient to trigger reinstatement of the drug-seeking, lever-pressing behavior. The rats returned to the lever, and the researchers counted how many times they pressed it.
On the day before they were given the priming dose of cocaine, rats pressed the active lever an average of 7.7 times. After receiving the priming dose, the rats pretreated with placebo pressed the lever an average of 38.8 times, while rats pretreated with 3, 6, or 12 mg/kg of the D3 antagonist pressed the lever an average of 39.0, 18.6, and 14.2 times, respectively. Pretreatment with the D3 antagonist thus produced a dose-related weakening of cocaine-triggered resumption of the drug-seeking behavior.
Future of D3 Antagonist Research
The researchers are optimistic about the future of this line of research. "The SB compound has jumped through many hoops already," says Dr. Ashby. "It's been shown in our studies and other studies to block cue-induced, drug-induced, and stress-induced relapse to cocaine-seeking behavior, and acquisition and expression of heroin-induced conditioned place preference. It is neither rewarding nor aversive and has been found to work on cocaine, heroin, and nicotine. We've seen no significant adverse effects of the compound in animals. We think antagonizing the D3 receptor represents a breakthrough for addiction treatment."
D3 Receptor Antagonist Inhibits Reinstatement of Cocaine Seeking. After receiving a priming dose of cocaine, rats pretreated with placebo resumed cocaine-seeking behavior, pressing a lever to self-administer cocaine. However, rats pretreated with the D3antagonist (SB compound) pressed the lever fewer times as the SB compound dose increased."In more than 35 years in the field, this D3 antagonist research is the most promising thing I have ever seen," Dr. Gardner says. "No one else has assembled such a variety of animal evidence showing that acute administration of a compound so profoundly modifies the addictive properties of cocaine as this selective D3 antagonist."
Both Drs. Ashby and Gardner are quick to note, however, that much work lies ahead. "We don't know if these results will hold up in long-term studies," says Dr. Gardner. "We think the reason this compound is successful in animal studies is because of its D3 antagonist action. To verify this, we still need to develop other, chemically different D3antagonists and redo all the tests. If we obtain the same results with these other D3 antagonists, then we'll be more comfortable that we are on the right track and that D3 receptor antagonism is truly responsible for our findings."
More animal experiments are planned to focus on other drugs of abuse and other animal paradigms, such as progressive ratio studies that measure a drug's motivational potency. Studies with chronic administration and with other mammalian species also will be needed, as will toxicology studies. The human pharmacokinetics of the compounds also will have to be improved; for instance, the current compound has a very short half-life, lasting only about 30 minutes in primates.
"Almost without fail, people I've spoken with who are addicted to drugs express a strong desire for clinically effective anticraving, antirelapse medication," says Dr. Gardner. "We hope this research takes us in that direction."
Source
- Vorel, S.R.; Ashby, C.R., Jr.; Paul, M.; Liu, X.; Hayes, R.; Hagan, J.J.; Middlemiss, D.N.; Stemp, G.; Gardner, E.L. Dopamine D3 receptor antagonism inhibits cocaine-seeking and cocaine-enhanced brain reward in rats. Journal of Neuroscience22(21):9595-9603, 2002. [Abstract]