| Table 2Alpha-2 adrenoceptor agonists that decrease footshock-stress-induced reinstatement of drug seeking or footshock-induced reinstatement of drug CPP in non-humans and stress-induced or stress + cue-induced craving in humans |
The effect of the alpha-2 adrenoceptor agonists on footshock-induced reinstatement is centrally mediated. Ventricular injections of clonidine mimic the inhibitory effect of the drug’s systemic injections (
Shaham et al. 2000b), while systemic injections of ST-91, a charged analogue of clonidine that does not readily cross the blood–brain barrier (
Scriabine et al. 1975), had no effect on footshock-induced reinstatement. Another finding from this series of studies is that systemic injections of the alpha-2 adrenoceptor agonists had no effect on reinstatement of drug seeking induced by drug priming injections or exposure to drug-associated cues (
Erb et al. 2000;
Highfield et al. 2001).
Additional evidence for a role of alpha-2 adrenoceptors in stress-induced reinstatement is that systemic injections of the prototypical alpha-2 adrenoceptor antagonist yohimbine, which increases noradrenaline cell firing and release (
Abercrombie et al. 1988;
Aghajanian and VanderMaelen 1982), reinstate methamphetamine, cocaine, heroin, and alcohol seeking in rats (
Banna et al. 2010;
Feltenstein and See 2006;
Le et al. 2005;
See and Waters 2010;
Shepard et al. 2004), and cocaine seeking in monkeys (
Lee et al. 2004). Yohimbine also potently reinstates palatable food seeking in rats (
Ghitza et al. 2006;
Ghitza et al. 2007;
Nair et al. 2009;
Nair et al. 2011). Surprisingly, however, the evidence that yohimbine-induced reinstatement of drug seeking is mediated by central noradrenergic systems is mixed. The alpha-2 adrenoceptor agonists clonidine and guanfacine attenuate yohimbine-induced reinstatement of alcohol seeking in rats (
Le et al. 2009;
Lê et al. 2011) and cocaine seeking in monkeys (
Lee et al. 2004). In contrast, clonidine has no effect on yohimbine-induced reinstatement of cocaine seeking in rats (
Brown et al. 2009) or yohimbine-induced reinstatement of CPP in mice (
Mantsch et al. 2010). Additionally, in rats, 6-hydroxydopamine lesions of the ventral or dorsal noradrenergic bundles have no effect on yohimbine-induced reinstatement of alcohol seeking (
Le et al. 2009). Furthermore, yohimbine’s effect on reinstatement of alcohol seeking is not mimicked by RS79948, a selective alpha-2 adrenoceptor antagonist (
Le et al. 2009). In contrast, in monkeys, yohimbine’s effect on reinstatement is mimicked by RS79948 (
Lee et al. 2004), and in mice this effect of yohimbine is mimicked by another selective alpha-2 adrenoceptor antagonist, BRL44408 (
Mantsch et al. 2010).
Mantsch et al. (2010) also reported that yohimbine-induced reinstatement of cocaine CPP in mice is attenuated by the beta adrenoceptor antagonist propranolol. Finally,
Lê et al. (2011) recently reported that the alpha-1 adrenoceptor antagonist prazosin blocks yohimbine-induced reinstatement of alcohol seeking; prazosin also blocks intermittent footshock-induced reinstatement. A tentative conclusion from the above studies is that both adrenergic and non-adrenergic (possibly serotonergic) mechanisms contribute to the potent effect of yohimbine on reinstatement of drug seeking (
Le et al. 2009).
The above findings on the potent inhibitory effect of alpha-2 adrenoceptor agonists on footshock stress-induced reinstatement of drug seeking have led to several laboratory studies on the effect of these agonists on stress-induced craving and relapse in humans. The first human study on the effect of an alpha-2-adrenergic agonist (lofexidine) on stress and drug craving was conducted in opioid-dependent individuals in naltrexone treatment (
Sinha et al. 2007). Naltrexone is an opiate receptor antagonist that is approved for the treatment of opioid addiction, but it is not used widely because of poor compliance and high relapse rates (
Julius 1976). In rats, naltrexone has no effect on stress-induced reinstatement of heroin or alcohol seeking (
Le et al. 1999;
Liu and Weiss 2002;
Shaham and Stewart 1996). This finding raises the possibility that high relapse rates during naltrexone treatment occur because naltrexone has no effect on stress-induced drug craving and relapse. Indeed, naltrexone-treated opioid-dependent individuals show high levels of guided imagery stress-induced drug craving, physiological arousal, and emotional distress, supporting the notion that naltrexone treatment may not be effective in decreasing stress-related drug craving (
Hyman et al. 2007). In a small laboratory and clinical outcomes study, we found that daily administration of 2.4 mg of lofexidine for 4 weeks decreased the guided imagery stress-induced opiate craving, anger ratings, and basal heart rates, as well as improved opiate relapse outcomes in naltrexone-treated opioid-dependent individuals (
Sinha et al. 2007).
In a follow-up study, we examined whether chronic 4-week administration of the alpha-2 adrenoceptor agonist guanfacine (up to 3 mg/daily dosing) would decrease guided imagery, stress, cue, and stress + cue-induced drug craving, anxiety, and physiological arousal in cocaine-dependent individuals who also use alcohol and nicotine (Fox et al. under review). Guanfacine (extended release) has been recently approved for attention deficit hyperactivity disorder in children (
Sallee and Eaton 2010). Guanfacine decreased basal heart rate and blood pressure. While the placebo group reported significant increases in cocaine and nicotine craving and anxiety following drug cue-related compared with stress-related imagery, such increases were not observed in the guanfacine group. Subjects treated with guanfacine also reported lower nicotine craving, fear, and arousal following drug cue and combined stress + drug cue imagery. These preliminary findings have led to a large scale dose–response study with guanfacine in cocaine-dependent individuals who are also nicotine dependent, which is currently under way.
In another recent study,
Jobes et al. (2011) assessed the effect of clonidine on stress- and cue-induced craving in cocaine users that were randomly assigned to three groups receiving clonidine 0, 0.1, or 0.2 mg orally under double-blind conditions. The stress and cue manipulations were standard auditory imagery scripts of stress-related and drug cue-related situations. Each subject received clonidine or placebo followed 3 h later by exposure to two pairs of scripts (neutral/stress and neutral/drug).
Jobes et al. (2011) reported that both clonidine doses decreased stress-induced cocaine craving while only the high clonidine dose decreased cue-induced craving.
In conclusion, studies using the reinstatement model in rats and mice indicate that stress-induced activation of central noradrenaline systems mediates stress-induced reinstatement of drug seeking. These preclinical studies have led to three human laboratory studies that demonstrated that alpha-2 adrenoceptor agonists (clonidine, lofexidine, and guanfacine), which decrease brain nor-adrenaline cell firing and release, decreased stress-induced drug craving in drug addicts. A question for future research is whether chronic treatment with alpha-2 adrenoceptor agonists would also prevent stress-induced relapse in the addict’s environment. Ongoing studies at both Yale and National Institute on Drug Abuse (NIDA) intramural research program will provide an answer to this question in the near future. Finally, a surprising dissociation has emerged between the rat reinstatement studies and the human laboratory studies. In the rat studies, the alpha-2 adrenoceptor agonists selectively decreased footshock stress-induced reinstatement of drug seeking but not cue or drug priming-induced reinstatement (
Erb et al. 2000;
Highfield et al. 2001). In contrast, in the human studies, the alpha-2 adrenoceptor agonists consistently decreased cue-induced drug craving (Fox et al. under review;
Jobes et al. 2011;
Sinha et al. 2007). A possible reason for these different findings is that in the rat, cue exposure likely primarily causes an appetitive motivational state with little or no stress component (
Feltenstein and See 2006;
See 2005). In contrast, in the human, laboratory cue exposure primarily induces an alpha-2 adrenoceptor agonist sensitive stress-like physiological (e.g., increased cortisol and heart rate) and psychological (e.g., increased subjective ratings of anxiety, anger, and irritability) states that are very similar to those induced by exposure to stressors like guided imagery stress or the Trier Social Stress Test (
Back et al. 2010;
Sinha et al. 1999;
Sinha et al. 2000).