NIDA-supported researchers have weakened rats' behavioral responses to environmental cocaine cues by manipulating the neurotransmitter serotonin. Moreover, such manipulation can make the drug seem less stimulant-like to the rats. The findings suggest that medications that act on the serotonin system might help recovering cocaine abusers sustain abstinence, say Dr. Kathryn Cunningham and colleagues at the Center for Addiction Research at the University of Texas Medical Branch in Galveston and the Polish Academy of Sciences in Krakow.
While cocaine makes its primary pharmacological impact on the neurotransmitter dopamine, it also increases levels of other chemical messengers, including serotonin (5-HT). Previous research with animals has shown that 5-HT2C and 5-HT2A receptors—two proteins on brain cell surfaces that mediate serotonin's effects on cellular activity—regulate behavioral responsiveness to cocaine. For example, activating the 5-HT2C receptor reduces the animals' typical behavioral responses to cocaine—including hyperactivity, self-administration, and return to drug-seeking following abstinence. Dr. Cunningham's studies showed that the 5-HT2C receptor affected responsiveness to cocaine-associated environments, and that both receptors affected the animals' experience of the drug.
Hyperactivity and Discrimination
Dr. Cunningham and colleagues first examined the effect of manipulating 5-HT2C receptors on a behavior called conditioned hyperactivity: When researchers repeatedly move an animal from its home cage and give cocaine in a test cage, the drug-paired environment comes to evoke the same behavioral effect as the stimulant itself when saline is administered, so that the animal starts moving about restlessly as soon as it finds itself in the cage. Experience has shown that compounds that inhibit conditioned hyperactivity usually also reduce behaviors that are laboratory stand-ins for human relapse.
The Drug Discrimination Protocol
In their second experiment, Dr. Cunningham and colleagues used a drug discrimination procedure to determine whether the test compounds changed how cocaine felt to the animals. To each rat, they first gave randomly alternating infusions of cocaine and saline. After each infusion, the rat could obtain a water reward by pressing Lever A if it had received cocaine or Lever B if it had received saline. After numerous repetitions, the rat regularly pressed the correct lever—demonstrating that it had learned the challenge, wanted the reward, and could discriminate between the experiences produced by cocaine and saline. Next, the researchers observed the rat's performance when it was given a receptor blocker prior to cocaine:
- Rats given a 5-HT2A receptor blocker no longer went as reliably to Lever A, indicating that they were less able to distinguish cocaine from the placebo;
- Rats given a 5-HT2C receptor blocker pressed more on Lever A, indicating enhanced cocaine-like effects.
Dr. Cunningham and colleagues administered cocaine (15 mg/kg) to rats daily for 7 days in a test cage. Two days later, they gave some of the rats a compound that activates the 5-HT2C receptor (MK 212), some a compound that blocks it (SB 242084), and others saline, and returned the animals to the test cage. Compared with the saline-treated animals, who showed the usual conditioned hyperactivity, rats given the 5-HT2C-receptor-stimulating compound moved around less (by about 40 percent), while those that received the blocker showed an exaggerated hyperactive response (by 25 percent) to the test cage. A separate group of animals was given cocaine only in their home cage and saline in the test cage. These animals showed normal activity when tested 2 days later, which was unaffected by the 5-HT2C receptor compounds. These data strengthen the conclusion that the 5-HT2C receptor is important in the cocaine-environment link.
In another study, Dr. Cunningham's team used an experimental protocol called drug discrimination to determine whether compounds that act at the 5-HT2C and 5-HT2A receptors would alter the way cocaine made the rats feel (see textbox, below). Prior research had indicated that the two receptors oppose each others' effects on the cocaine response, and the researchers hypothesized that blocking the 5-HT2A receptors would make cocaine feel less stimulant-like to the rats, whereas inhibiting 5-HT2C receptors would enhance the drug's effects. Rats given a compound that blocks 5-HT2A receptors (SR 46349B) prior to cocaine reduced their pressing on the lever associated with cocaine's effects compared with one linked with saline. Animals pretreated with a compound that blocks 5-HT2C receptors (SDZ SER-082) increased their pressing on the cocaine lever over the saline lever. The results bore out the hypothesis.
"Taken together, the findings of these studies support the idea that the serotonin 5-HT2C receptor plays a role in linking environmental cues and the experience of cocaine, as well as the subjective effects of the drug. The 5-HT2A receptor also influences these behaviors, but in the opposite direction," says Dr. Cunningham. "From the medication development perspective, a drug with dual action at both receptors—that is, one that simultaneously stimulates the 5-HT2C receptor and blocks the 5-HT2A receptor—might be effective in reducing cue-induced craving. We know of no such compound, and our team is working to develop one." She adds that agents that stimulate 5-HT2C or inhibit 5-HT2A receptors do not fully mimic cocaine or affect other behaviors, suggesting limited side effects.
More Data Point to Serotonin
Dr. Janet Neisewander and colleagues at Arizona State University confirmed the therapeutic potential of compounds that act on the two serotonin receptors (5-HT2A and 5-HT2C) that Dr. Cunningham's team examined. Using a self-administration-extinction-trigger-exposure model of testing for relapse (see "Animal Experiments in Addiction Science (Archives)"), the researchers found that:
- Ketanserin—a compound that blocks the 5-HT2A and 5-HT2C receptors—attenuated cue-induced relapse to cocaine, but not drug-triggered relapse. Because ketanserin blocks both receptors, the investigators did not know which might have been responsible for preventing cue-induced relapse. By combining data from this experiment and the one described below, the researchers were able to zero in on the 5-HT2A receptor.
- SB 242084, a selective blocker of 5-HT2C receptors, did not affect cue-induced or cocaine-triggered drug-seeking. The findings suggested that the results seen with ketanserin most likely were due to its ability to block the 5-HT2A receptor. The researchers concluded that blocking the 5-HT2A receptor might help prevent relapse triggered by environmental cues associated with taking cocaine, the same inference as Dr. Cunningham's team.
In another experiment, Dr. Neisewander's team found that SB 242084 interfered with the ability of a drug that augments serotonin, d-fenfluramine, to prevent cue-induced cocaine seeking. Because blocking the 5-HT2C receptor made d-fenfluramine ineffective, the investigators concluded that stimulating the receptor may help prevent cue-induced relapse.
The compounds tested by Dr. Neisewander's team will not necessarily be developed as medications to prevent cocaine relapse, but the results of the study do suggest that drugs that act on serotonin may be potential pharmacotherapy candidates. The findings add to a growing number of studies that suggest the promise of a dual-action pharmacotherapeutic approach for relapse prevention—that is, a drug that simultaneously blocks 5-HT2A receptors and stimulates 5-HT2C receptors.
"Recent evidence suggests that serotonin is involved in motivation for various pleasurable experiences, including food. Researchers developing medications for obesity also are studying the effect of stimulating the 2C receptor, which may be a point of intersection for several addictions," says Dr. Cunningham.
Burmeister, J.J., et al. Differential roles of 5-HT receptor subtypes in cue and cocaine reinstatement of cocaine-seeking behavior in rats. Neuropsychopharmacology 29(4):660-668, 2004.
"It is not really surprising that serotonin is implicated in addiction given its importance to essential behaviors—including sleep, eating, mood, cognitive processes, and self-regulation—and its influence on dopamine," says Dr. Minda Lynch of NIDA's Division of Basic Neuroscience and Behavioral Research. "Serotonin influences dopamine in the brain's reward pathway and cortex, so examining the behavioral effects of serotonin-influencing compounds in animals is a reasonable approach in the investigation of potential pharmacotherapies," says Dr. Lynch. She agrees that a dual-action compound that operates on the serotonin 5-HT2C and 5-HT2A receptors might eventually help prevent relapse. "However, a great deal of further testing in animals is needed. A good next step would be to confirm the findings in animal protocols that mimic cue-induced relapse," says Dr. Lynch.
Liu, S., and Cunningham, K.A. Serotonin (2C) receptors (5-HT(2C)R) control expression of cocaineinduced conditioned hyperactivity. Drug and Alcohol Dependence 81(3):275-282, 2006. [Abstract]
Filip, M.; Bubar, M.J.; and Cunningham, K.A. Contribution of serotonin (5-HT) 5-HT2 receptorsubtypes to the discriminative stimulus effects of cocaine in rats. Psychopharmacology 183(4):482-489, 2006. [Abstract]