Evidence for alterations in α2-adrenergic receptor sensitivity in rats exposed to repeated cocaine administration

Therapeutics for Substance-Using Women: The Need to Elucidate Sex-Specific Targets for Better-Tailored Treatments

In the last decade, alcohol consumption in the US has risen by 84% in women compared with 35% in men. Furthermore, research has shown that sex- and gender-related differences may disadvantage women in terms of developing a range of psychological, cognitive, and medical problems considerably earlier in their drinking history than men, and despite consuming a similar quantity of substances. While this "telescoping" process has been acknowledged in the literature, a concomitant understanding of the underlying biobehavioral mechanisms, and an increase in the development of specific treatments tailored to women, has not occurred. In the current chapter we focus on understanding why the need for personalized, sex-specific medications is imperative, and highlight some of the potential sex-specific gonadal and stress-related adaptations underpinning the accelerated progress from controlled to compulsive drug and alcohol seeking in women. We additionally discuss the efficacy of these mechanisms as novel targets for medications development, using exogenous progesterone and guanfacine as examples. Finally, we assess some of the challenges faced and progress made in terms of developing innovative medications in women. We suggest that agents such as exogenous progesterone and adrenergic medications, such as guanfacine, may provide some efficacy in terms of attenuating stress-induced craving for several substances, as well as improving the ability to emotionally regulate in the face of stress, preferentially in women. However, to fully leverage the potential of these therapeutics in substance-using women, greater focus needs to the placed on reducing barriers to treatment and research by encouraging women into clinical trials.

Reinforcing and Stimulant-Like Effects of Methamphetamine Isomers in Rhesus Macaques

Monoamine releasers such as d-methamphetamine (d-MA) can reduce cocaine use in laboratory studies and have been forwarded for the management of cocaine use disorder (CUD). However, the proven abuse liability of d-MA has limited enthusiasm for clinical use. The levorotatory isomer of MA, l-MA, appears to have lesser stimulant effects, possibly due to its preferential norepinephrine-releasing properties compared with dopamine. The present study evaluated the abuse potential of l-MA by comparing its reinforcing effects with known stimulant drugs of abuse in nonhuman primates. Adult rhesus macaques (N = 4) responded for intravenous injections of cocaine, d-MA, methcathinone (MCAT), or l-MA under a fixed-ratio (FR) schedule of reinforcement; reinforcing effectiveness was evaluated using behavioral economic demand procedures. In a separate cohort (N = 9), daily activity and food-reinforced responding were assessed during 100 days of treatment with daily dosages of l-MA (2.3 mg/kg per day, i.v.) or d-MA (0.74 mg/kg per day, i.v.) previously shown to decrease cocaine self-administration. Results show that all drugs maintained self-administration, with peak injections reaching ∼100 inj per session for cocaine, MCAT, and d-MA and ∼50 inj per session for l-MA . In demand studies, self-administration of each drug gradually decreased as FR size increased. The exponential model of demand indicated that the reinforcing effectiveness of l-MA was significantly less than the other drugs studied. Chronic l-MA treatment did not appreciably alter daily activity and only transiently suppressed food-reinforced responding. These data, coupled with previous findings that l-MA effectively reduces stimulant self-administration, suggest that l-MA, or other norepinephrine-preferring releasers, may serve as agonist medication for CUD with lesser abuse liability than common psychostimulants. SIGNIFICANCE STATEMENT: Development of pharmacotherapies for cocaine use disorder remains a formidable challenge. Agonist-based therapies show promise, but enthusiasm is tempered by the abuse liability of previously proposed medications. This study evaluated the abuse liability and chronic treatment effects of methamphetamine's levorotatory isomer (l-MA). l-MA demonstrated lower abuse liability compared with commonly abused stimulants and produced few untoward effects. In the context of recent studies demonstrating that l-MA attenuates stimulant self-administration, these findings support l-MA's potential as a pharmacotherapy for stimulant addiction.

Netrin G1: its downregulation in the nucleus accumbens of cocaine-conditioned mice and genetic association in human cocaine dependence

Netrin G1 is a presynaptic ligand involved in axonal projection. Although molecular mechanisms underlying cocaine addiction are still poorly understood, Netrin G1 might have a role as a regulator of anxiety, fear and spatial memory, behavioural traits impaired in the context of cocaine exposure. In this study, the Netrin G1 (Ntng1) expression was investigated in the nucleus accumbens of mice primarily conditioned to cocaine using a place preference paradigm. A genetic association study was then conducted on 146 multiplex families of the Collaborative study on Genetics of Alcoholism, in which seven single nucleotide polymorphisms located in the NTNG1 gene were genotyped. NTNG1 expression levels were also quantified in BA10, BA46 and the cerebellum of healthy controls (with no Axis 1 psychopathology). Decreased Ntng1 expression was initially observed in the nucleus accumbens of mice conditioned to cocaine. Significant genetic family-based associations were detected between NTNG1 polymorphisms and cocaine dependence. NTNG1 expression in BA10, BA46 and the cerebellum, however, were not significantly associated with any allele or haplotype of this gene. These results confirm that Ntng1 expression is disturbed in the nucleus accumbens of mice, after cocaine conditioning. A haplotype of NTNG1 was found to constitute a vulnerability factor for cocaine use disorder in patients, although none of its single nucleotide polymorphisms were associated with a differential expression pattern in healthy controls. The data suggest that change in the Ntng1 expression is a consequence of cocaine exposure, and that some of its genetic markers are associated with a greater risk for cocaine use disorder.

α2A -Adrenergic receptor polymorphisms and mRNA expression levels are associated with delay discounting in cocaine users

Cocaine users characteristically display preferences for smaller immediate rewards over larger delayed rewards, and this delay discounting (DD) has been proposed as an endophenotype of cocaine addiction. Recent evidence suggests that the norepinephrine system and more specifically the α_2A -adrenergic receptor (ADRA2A) are impacted by chronic cocaine use while also being potentially involved in the neural mechanisms underlying DD. Hence, we investigated the effects of ADRA2A polymorphisms and ADRA2A mRNA expression levels on DD of cocaine users and stimulant-naïve controls. Two hundred and twenty-three participants (129 cocaine users and 94 stimulant-naïve healthy controls) completed a computerized DD paradigm and were genotyped for three single nucleotide polymorphisms (SNPs; rs1800544, rs521674 and rs602618) in the ADRA2A gene, while their peripheral ADRA2A mRNA expression was quantified in whole blood samples. The three SNPs were in near-perfect linkage disequilibrium. Accordingly, significant group*genotype interactions were found for all three ADRA2A variants revealing steeper DD in cocaine users (but not in controls) carrying the G-allele of SNP rs1800544, the T-allele of rs521674 and the C-allele of rs602618. Similarly, high ADRA2A mRNA expression levels were significantly associated with a reduced tendency to choose smaller more immediate rewards (over larger delayed rewards) in cocaine users but not in controls. As the relationship between DD and cocaine use was moderated by ADRA2A SNPs and by peripheral ADRA2A gene expression, we propose that the norepinephrine system is involved in DD deficits observed in cocaine using individuals. Consequently, pharmacological compounds targeting ADRA2As might be considered for the symptom-specific treatment of delay aversion in stimulant addiction.

Relationship between central behavioral effects and peripheral sympathetic neurotransmission functionality during acute cocaine withdrawal syndrome in adult rats

BACKGROUND

Acute cocaine withdrawal syndrome (ACWS) is characterized as a set of organic alterations triggered by abrupt discontinuation of chronic cocaine consumption, usually occurring at 24-40 hours after withdrawal. However, little is known about the relationship between central and peripheral sympathetic neurotransmission during ACWS.

OBJECTIVE AND METHODS

We investigated the mechanisms involved in central and peripheral sympathetic neurotransmission and how ACWS affects the sympathetic functionality. Cocaine was administered twice daily for 5 days in Wistar rats (at least 5 in each group): on the first and second day, 15 mg/kg/i.p.; third day, 20 mg/kg/i.p.; and finally in the last two days, 30 mg/kg/i.p. Subsequently, at 1, 24, 48 and 120 h after cocaine administration the following experiments were done: (i) at the central level, behavioral tests of open-field and elevated plus maze; and (ii) at the peripheral level, tests of catecholamine release, function of α2-adrenergic receptors (α2-ARs), imidazoline receptors (I(1,2)-Rs), L-type voltage-gated (Ca(v1.2)) Ca(2+) channels and α1-ARs.

RESULTS

During ACWS, rats showed hypolocomotion and exacerbation of anxiogenic-effects 24 h after cocaine withdrawal. Likewise, a decrease in the catecholamine release and activity of α2-ARs/I(1,2)-Rs at 24-48 h after cocaine withdrawal was observed. A decrease in Ca(v1.2) channels and α1-ARs function at 48 h after cocaine withdrawal was observed.

CONCLUSIONS

The relationship of central and peripheral sympathetic neurotransmission during ACWS possibly due to a failure in activation and/or inactivation of presynaptic α2-ARs/I(1,2)-Rs, may offer a potential target for attenuating ACWS.

Effects of abstinence from chronic cocaine self-administration on nonhuman primate dorsal and ventral noradrenergic bundle terminal field structures

Repeated exposure to cocaine is known to dysregulate the norepinephrine system, and norepinephrine has also been implicated as having a role in abstinence and withdrawal. The goal of this study was to determine the effects of exposure to cocaine self-administration and subsequent abstinence on regulatory elements of the norepinephrine system in the nonhuman primate brain. Rhesus monkeys self-administered cocaine (0.3 mg/kg/injection, 30 reinforcers/session) under a fixed-interval 3-min schedule of reinforcement for 100 sessions. Animals in the abstinence group then underwent a 30-day period during which no operant responding was conducted, followed by a final session of operant responding. Control animals underwent identical schedules of food reinforcement and abstinence. This duration of cocaine self-administration has been shown previously to increase levels of norepinephrine transporters (NET) in the ventral noradrenergic bundle terminal fields. In contrast, in the current study, abstinence from chronic cocaine self-administration resulted in elevated levels of [(3)H]nisoxetine binding to the NET primarily in dorsal noradrenergic bundle terminal field structures. As compared to food reinforcement, chronic cocaine self-administration resulted in decreased binding of [(3)H]RX821002 to α2-adrenoceptors primarily in limbic-related structures innervated by both dorsal and ventral bundles, as well as elevated binding in the striatum. However, following abstinence from responding for cocaine binding to α2-adrenoceptors was not different than in control animals. These data demonstrate the dynamic nature of the regulation of norepinephrine during cocaine use and abstinence, and provide further evidence that the norepinephrine system should not be overlooked in the search for effective pharmacotherapies for cocaine dependence.

Sex differences in guanfacine effects on drug craving and stress arousal in cocaine-dependent individuals

Currently, no FDA-approved medication exists for the treatment of cocaine use disorder. Furthermore, as women become increasingly more at risk for the consequences of cocaine addiction, the need to establish better-tailored treatment medications is paramount. We examine the effects of the alpha2 adrenergic agonist, guanfacine HCl, on responses to stress and drug cue in a group of cocaine-dependent men and women who also abuse alcohol and nicotine. Forty early abstinent treatment-seeking cocaine-dependent males and females were randomly assigned to receive either daily placebo (12 M/7 F) or guanfacine (2 or 3 mg) (15 M/6 F) for 3 weeks. In week 4, they participated in a laboratory experiment and were exposed to three 10-min guided imagery conditions (stress/stress, cue/cue, and stress/cue), one per day, consecutively in a random, counterbalanced order. Craving, negative emotion, anxiety, and cardiovascular function were assessed at baseline, immediately following imagery exposure, and at various recovery time points. Guanfacine significantly attenuated cocaine craving, alcohol craving, anxiety, and negative emotion following exposure to all three imagery conditions in females, but not males. Guanfacine did, however, reduce sympathetic tone as well as stress and cue-induced nicotine craving and systolic blood pressure (SBP) in both males and females. These findings highlight sex-specific effects of guanfacine on drug craving, anxiety, and negative mood with significant effects in women and not men. The findings suggest further evaluation of guanfacine in the treatment of cocaine use disorder with a specific focus on sex differences in treatment response.

Neurobiological mechanisms underlying relapse to cocaine use: contributions of CRF and noradrenergic systems and regulation by glucocorticoids

Considering its pervasive and uncontrollable influence in drug addicts, understanding the neurobiological processes through which stress contributes to drug use is a critical goal for addiction researchers and will likely be important for the development of effective medications aimed at relapse prevention. In this paper, we review work from our laboratory and others focused on determining the neurobiological mechanisms that underlie and contribute to stress-induced relapse of cocaine use with an emphasis on the actions of corticotropin-releasing factor in the ventral tegmental area (VTA) and a key pathway from the bed nucleus of the stria terminalis to the VTA that is regulated by norepinephrine and beta adrenergic receptors. Additionally, we discuss work suggesting that the influence of stress in cocaine addiction changes and intensifies with repeated cocaine use in an intake-dependent manner and examine the potential role of glucocorticoid hormones in the underlying drug-induced neuroadaptations. It is our hope that research in this area will inform clinical practice and medication development aimed at minimizing the contribution of stress to the addiction cycle, thereby improving treatment outcomes and reducing the societal costs of addiction.

The role of guanfacine as a therapeutic agent to address stress-related pathophysiology in cocaine-dependent individuals

The pathophysiology of cocaine addiction is linked to changes within neural systems and brain regions that are critical mediators of stress system sensitivity and behavioral processes associated with the regulation of adaptive goal-directed behavior. This is characterized by the upregulation of core adrenergic and corticotropin-releasing factor mechanisms that subserve negative affect and anxiety and impinge upon intracellular pathways in the prefrontal cortex underlying cognitive regulation of stress and negative emotional state. Not only are these mechanisms essential to the severity of cocaine withdrawal symptoms, and hence the trajectory of clinical outcome, but also they may be particularly pertinent to the demography of cocaine dependence. The ability of guanfacine to target overlapping stress, reward, and anxiety pathophysiology suggests that it may be a useful agent for attenuating the stress- and cue-induced craving state not only in women but also in men. This is supported by recent research findings from our own laboratory. Additionally, the ability of guanfacine to improve regulatory mechanisms that are key to exerting cognitive and emotional control over drug-seeking behavior also suggests that guanfacine may be an effective medication for reducing craving and relapse vulnerability in many drugs of abuse. As cocaine-dependent individuals are typically polydrug abusers and women may be at a greater disadvantage for compulsive drug use than men, it is plausible that medications that target catecholaminergic frontostriatal inhibitory circuits and simultaneously reduce stress system arousal may provide added benefits for attenuating cocaine dependence.

Differential effects of acute and chronic treatment with the α2-adrenergic agonist, lofexidine, on cocaine self-administration in rhesus monkeys

BACKGROUND

Lofexidine, an α2-adrenergic agonist, is being investigated as a treatment for reducing opioid withdrawal symptoms and blocking stress-induced relapse to cocaine taking. Opioid abusers are often polydrug abusers and cocaine is one frequent drug of choice. However, relatively little is known about lofexidine interactions with cocaine. The present study investigated the effects of acute and chronic treatment with lofexidine in a pre-clinical model of cocaine self-administration.

METHODS

Male rhesus monkeys were trained to respond for food (1g) and cocaine (0.01 mg/kg/injection) under a fixed ratio 30 (FR30) or a second order FR2 (VR16:S) schedule of reinforcement. Systematic observations of behavior were conducted during and after chronic treatment with lofexidine.

RESULTS

Acute treatment with lofexidine (0.1 or 0.32 mg/kg, IM) significantly reduced cocaine self-administration but responding for food was less effected. In contrast, chronic treatment (7-10 days) with lofexidine (0.1-0.32 mg/kg/h, IV) produced a leftward shift in the cocaine self-administration dose-effect curve, but had no effect on food-maintained responding. Lofexidine did not produce any observable side effects during or after treatment.

CONCLUSIONS

Lofexidine potentiated cocaine's reinforcing effects during chronic treatment. These data suggest that it is unlikely to be effective as a cocaine abuse medication and could enhance risk for cocaine abuse in polydrug abusers.

Elevated Norepinephrine may be a Unifying Etiological Factor in the Abuse of a Broad Range of Substances: Alcohol, Nicotine, Marijuana, Heroin, Cocaine, and Caffeine

A wide range of commonly abused drugs have effects on the noradrenergic neurotransmitter system, including alterations during acute intoxication and chronic use of these drugs. It is not established, however, that individual differences in noradrenergic signaling, which may be present prior to use of drugs, predispose certain persons to substance abuse. This paper puts forth the novel hypothesis that elevated noradrenergic signaling, which may be raised largely due to genetics but also due to environmental factors, is an etiological factor in the abuse of a wide range of substances, including alcohol, nicotine, marijuana, heroin, cocaine, and caffeine. Data are reviewed for each of these drugs comprising their interaction with norepinephrine during acute intoxication, long-term use, subsequent withdrawal, and stress-induced relapse. In general, the data suggest that these drugs acutely boost noradrenergic signaling, whereas long-term use also affects this neurotransmitter system, possibly suppressing it. During acute withdrawal after chronic drug use, noradrenergic signaling tends to be elevated, consistent with the observation that norepinephrine lowering drugs such as clonidine reduce withdrawal symptoms. Since psychological stress can promote relapse of drug seeking in susceptible individuals and stress produces elevated norepinephrine release, this suggests that these drugs may be suppressing noradrenergic signaling during chronic use or instead elevating it only in reward circuits of the brain. If elevated noradrenergic signaling is an etiological factor in the abuse of a broad range of substances, then chronic use of pharmacological agents that reduce noradrenergic signaling, such as clonidine, guanfacine, lofexidine, propranolol, or prazosin, may help prevent or treat drug abuse in general.

Norepinephrine and impulsivity: effects of acute yohimbine

RATIONALE

Rapid-response impulsivity, characterized by inability to withhold response to a stimulus until it is adequately appraised, is associated with risky behavior and may be increased in a state-dependent manner by norepinephrine.

OBJECTIVE

We assessed effects of yohimbine, which increases norepinephrine release by blocking alpha-2 noradrenergic receptors, on plasma catecholamine metabolites, blood pressure, subjective symptoms, and laboratory-measured rapid-response impulsivity.

METHODS

Subjects were 23 healthy controls recruited from the community, with normal physical examination and ECG, and negative history for hypertension, cardiovascular illness, and axis I or II disorder. Blood pressure, pulse, and behavioral measures were obtained before and periodically after 0.4 mg/kg oral yohimbine or placebo in a randomized, counterbalanced design. Metabolites of norepinephrine [3-methoxy-4-hydroxyphenylglycol (MHPG) and vanillylmandelic acid (VMA)] and dopamine [homovanillic acid (HVA)] were measured by high-pressure liquid chromatography with electrochemical detection. Rapid-response impulsivity was measured by commission errors and reaction times on the immediate memory task (IMT), a continuous performance test designed to measure impulsivity and attention.

RESULTS

Yohimbine increased plasma MHPG and VMA but not HVA. Yohimbine increased systolic and diastolic blood pressure and pulse rate. On the IMT, yohimbine increased impulsive errors and impulsive response bias and accelerated reaction times. Yohimbine-associated increase in plasma MHPG correlated with increased impulsive response rates. Time courses varied; effects on blood pressure generally preceded those on metabolites and test performance.

CONCLUSIONS

These effects are consistent with increased rapid-response impulsivity after pharmacological noradrenergic stimulation in healthy controls. Labile noradrenergic responses, or increased sensitivity to norepinephrine, may increase risk for impulsive behavior.

Assessing learned associations between conditioned cocaine reward and environmental stimuli in the Wistar Kyoto rat

Clinical studies demonstrate that anxiety disorders increase the risk of substance use disorder. However, few studies have directly assessed anxiety as a vulnerability factor in processing of rewarding stimuli. The Wistar–Kyoto (WKY) rat has been proposed as a model of anxiety vulnerability because it exhibits extreme behavioral inhibition in novel and social environments; yet, it displays paradoxical rapid active avoidance learning that is resistant to extinction. The present study was designed to characterize the acquisition and persistence of cocaine conditioned place preference (CPP) in WKY rats. In the first of a series of three experiments, adult male WKY and Sprague Dawley (SD) rats were given six pairings of cocaine (3, 5, 10, 15 mg/kg) or saline on alternating days. SD rats developed cocaine-induced CPP to each of the four doses of cocaine tested. In contrast, WKY rats demonstrated CPP when conditioned with 3, 5, and 10 mg/kg, but displayed no preference to the 15 mg/kg dose. Next, separate groups of rats were subject to an extended CPP paradigm, which included acquisition, extinction and reinstatement phases. Rats were conditioned with cocaine and saline on alternating days using either a 6/6 (as above) or 4/4 conditioning regimen. Both SD and WKY rats acquired a lasting CPP with the 6/6 conditioning regimen. Results from the 4/4 conditioning regimen show that SD, but not WKY, rats acquired CPP. Preference scores for SD rats during the cocaine primed reinstatement test were significantly different from pretest scores indicating reinstatement of CPP in this group. Paradoxically, WKY rats demonstrated a latent sensitization to the conditioned rewarding effects of cocaine during the drug-primed reinstatement test. Taken together, WKY rats appear to be more sensitive to high doses of cocaine and need more experience with the drug to acquire a preference than SD rats.

Dexmedetomidine as a Novel Countermeasure for Cocaine-Induced Central Sympathoexcitation in Cocaine-Addicted Humans

Cocaine-induced acute hypertension is mediated largely by increased central sympathetic nerve activity. We hypothesized that dexmedetomidine, a central sympatholytic, reverses cocaine-induced increases in sympathetic nerve activity, mean arterial pressure (MAP), and heart rate (HR) in cocaine-addicted subjects. First, we conducted a dose-finding study in 15 nontreatment-seeking cocaine-addicted subjects and 12 cocaine-naive healthy controls to find doses of intravenous dexmedetomidine that lower MAP and HR in the absence of acute-cocaine challenge. We then conducted a placebo-controlled treatment trial in 26 cocaine-addicted subjects to determine whether dexmedetomidine reverses MAP and HR increases after intranasal cocaine (3 mg/kg). Skin sympathetic nerve activity (measured in the second protocol) and skin vascular resistance (measured in both protocols) served as indices of cocaine-sensitive central sympathoexcitation. In doses up to 0.6 µg/kg IV, dexmedetomidine alone caused comparable dose-dependent decreases in blood pressure in cases and controls but a 1.0 µg/kg dose was required to lower HR. In cocaine-addicted subjects, low-dose dexmedetomidine (0.4 µg/kg; n=14) abolished cocaine-induced increases in skin sympathetic nerve activity (156±26 versus −15±22%, cocaine/placebo versus cocaine/dexmedetomidine; P <0.05), skin vascular resistance (+10±2 versus −2±3 U; P <0.05), and MAP (+6±1 versus −5±2 mm Hg; P <0.01) without affecting HR (+13±2 versus +9±2 bpm; P =ns). When dexmedetomidine was increased to 1 µg/kg (high dose; n=12) to reverse cocaine-induced increases in HR, MAP did not fall further and increased paradoxically in 4 of 12 subjects. Thus, in a low nonsedating dose, dexmedetomidine constitutes a putative new treatment for cocaine-induced acute hypertension but higher sedating doses can increase blood pressure unpredictably during acute-cocaine challenge and should be avoided.

Treatment of cocaine withdrawal anxiety with guanfacine: relationships to cocaine intake and reinstatement of cocaine seeking in rats

RATIONALE

Successful treatment of cocaine addiction is severely impeded by the propensity of users to relapse. Withdrawal severity may serve as a key predictor of susceptibility to relapse. Therefore, the identification and treatment of cocaine withdrawal symptoms such as anxiety may improve addiction treatment outcome.

OBJECTIVES

The current study examined the role of anxiety-like behavior during cocaine withdrawal and anxiolytic treatment in reinstatement of cocaine seeking in an animal model of relapse.

METHODS

Male rats experienced daily IV cocaine self-administration. One group of animals received the norepinephrine α-2 agonist, guanfacine, or vehicle prior to anxiety testing 48 h after the last self-administration session. In the second group of rats, relationships between cocaine intake, anxiety-like behavior after withdrawal of cocaine, and reinstatement responding were investigated. The third and fourth groups of animals received guanfacine, yohimbine (norepinephrine α-2 antagonist), or vehicle once per day for 3 days 48 h after cessation of cocaine self-administration, followed by extinction and subsequent reinstatement induced by cocaine injections, cocaine-paired cues, and yohimbine administration.

RESULTS

Cocaine-withdrawn rats at 48 h demonstrated higher levels of anxiety-like behavior as measured on a defensive burying task when compared to yoked saline controls, an effect reversed by guanfacine treatment. Cocaine intake was positively correlated with measures of anxiety-like behavior during early withdrawal, and this anxiety-like behavior was significantly correlated with subsequent cocaine-primed reinstatement. Yohimbine treatment during early withdrawal increased reinstatement to conditioned cues, while guanfacine treatment reduced reinstatement to yohimbine.

CONCLUSIONS

These studies suggest an important role for noradrenergic mediation of anxiety-like behavior that emerges after withdrawal of cocaine and potential risk of relapse as modeled by reinstatement, and suggest that treatment of anxiety symptoms during early abstinence may reduce the risk of relapse.

Cannabinoid receptor involvement in stress-induced cocaine reinstatement: potential interaction with noradrenergic pathways

This study examined the role of endocannabinoid signaling in stress-induced reinstatement of cocaine seeking and explored the interaction between noradrenergic and endocannabinergic systems in the process. A well-validated preclinical model for human relapse, the rodent conditioned place preference assay, was used. Cocaine-induced place preference was established in C57BL/6 mice using injections of 15 mg/kg cocaine. Following extinction of preference for the cocaine-paired environment, reinstatement of place preference was determined following 6 min of swim stress or cocaine injection (15 mg/kg, i.p.). The role of endocannabinoid signaling was studied using the cannabinoid antagonist AM-251 (3 mg/kg, i.p.). Another cohort of mice was tested for reinstatement following administration of the cannabinoid agonist CP 55,940 (10, 20, or 40 μg/kg, i.p.). The alpha-2 adrenergic antagonist BRL-44408 (5 mg/kg, i.p.) with or without CP 55,940 (20 μg/kg) was administered to a third group of mice. We found that: (1) AM-251 blocked forced swim-induced, but not cocaine-induced, reinstatement of cocaine-seeking behavior; (2) the cannabinoid agonist CP 55,940 did not reinstate cocaine-seeking behavior when administered alone but did synergize with a non-reinstating dose of the alpha-2 adrenergic antagonist BRL-44408 to cause reinstatement. These results are consistent with the hypothesis that stress exposure triggers the endogenous activation of CB1 receptors and that activation of the endocannabinoid system is required for the stress-induced relapse of the mice to cocaine seeking. Further, the data suggest that the endocannabinoid system interacts with noradrenergic mechanisms to influence stress-induced reinstatement of cocaine-seeking behavior.

Involvement of noradrenergic neurotransmission in the stress- but not cocaine-induced reinstatement of extinguished cocaine-induced conditioned place preference in mice: role for β-2 adrenergic receptors

The responsiveness of central noradrenergic systems to stressors and cocaine poses norepinephrine as a potential common mechanism through which drug re-exposure and stressful stimuli promote relapse. This study investigated the role of noradrenergic systems in the reinstatement of extinguished cocaine-induced conditioned place preference by cocaine and stress in male C57BL/6 mice. Cocaine- (15 mg/kg, i.p.) induced conditioned place preference was extinguished by repeated exposure to the apparatus in the absence of drug and reestablished by a cocaine challenge (15 mg/kg), exposure to a stressor (6-min forced swim (FS); 20-25°C water), or administration of the α-2 adrenergic receptor (AR) antagonists yohimbine (2 mg/kg, i.p.) or BRL44408 (5, 10 mg/kg, i.p.). To investigate the role of ARs, mice were administered the nonselective β-AR antagonist, propranolol (5, 10 mg/kg, i.p.), the α-1 AR antagonist, prazosin (1, 2 mg/kg, i.p.), or the α-2 AR agonist, clonidine (0.03, 0.3 mg/kg, i.p.) before reinstatement testing. Clonidine, prazosin, and propranolol failed to block cocaine-induced reinstatement. The low (0.03 mg/kg) but not high (0.3 mg/kg) clonidine dose fully blocked FS-induced reinstatement but not reinstatement by yohimbine. Propranolol, but not prazosin, blocked reinstatement by both yohimbine and FS, suggesting the involvement of β-ARs. The β-2 AR antagonist ICI-118551 (1 mg/kg, i.p.), but not the β-1 AR antagonist betaxolol (10 mg/kg, i.p.), also blocked FS-induced reinstatement. These findings suggest that stress-induced reinstatement requires noradrenergic signaling through β-2 ARs and that cocaine-induced reinstatement does not require AR activation, even though stimulation of central noradrenergic neurotransmission is sufficient to reinstate.

Evaluation of the relationship between anxiety during withdrawal and stress-induced reinstatement of cocaine seeking

The initial termination of cocaine consumption in human addicts is associated with heightened anxiety states and low levels of craving. Craving, however, tends to increase progressively over time, remains high for extended periods of time, and can be exacerbated by stressors, leading to relapse. Laboratory rats, likewise, exhibit heightened states of anxiety after withdrawal from drug, and follow a time course of cocaine seeking that parallels the time course of craving reported in humans. In addition, laboratory rats show heightened susceptibility to relapse when exposed to stressors after extended periods of withdrawal, and exhibit persistent and heightened expressions of stress-induced anxiety. The general objective of this paper is to consider the relationship between anxiety states after withdrawal from cocaine and stress-induced reinstatement of cocaine seeking in laboratory rats, and to identify the neural substrates involved. The focus of the review is on studies addressing the roles of corticotropin-releasing factor (CRF) and noradrenaline pathways of the extended amygdala circuitry, and their direct or indirect interactions with the mesocorticolimbic dopamine system, in anxiety after withdrawal from cocaine and stress-induced reinstatement of cocaine seeking. Furthermore, the effects of time after withdrawal from cocaine and amount of cocaine exposure during self-administration on the activity of CRF, noradrenaline, and dopamine pathways of the extended amygdala and mesocorticolimbic systems will be considered. The review will highlight how changing levels of activity within these systems may serve to alter the nature of the relationship between anxiety and stress-induced reinstatement of cocaine seeking at different times after withdrawal from cocaine.

Noradrenergic function in pathological gambling: blunted growth hormone response to clonidine

The noradrenergic system has been linked to impulsive behaviour in animals and humans, yet little data on noradrenergic system exist in specific impulse control disorders. To explore the role of the noradrenergic system in pathological gamblers (PG), we assessed neuroendocrine growth hormone (GH) response to the alpha2-adrenergic receptor agonist clonidine and placebo in PG and controls. The net effects of clonidine are a decrease in neurotransmission by depressing locus coeruleus activity and stimulation of GH secretion through activation of post-synaptic alpha2-adrenergic receptors in the hypothalamus. Twenty-nine PG subjects, free of other comorbid conditions, and 27 healthy controls received a double-blinded, placebo-controlled, single dose of oral clonidine (0.15 mg/kg). Data observed included GH, clonidine levels and levels of the main noradrenergic metabolite, 3-methoxy-4-hydroxy-phenylglycol (MHPG). The area under the curve for GH response to clonidine was significantly lower (separate variance t with 44.3 df = 2.626, P = 0.012, d = 0.58) in the PG group (199.6) than in the control group (426.3). PG had significantly blunted GH responses compared with controls at 120 and 150 min post-clonidine. These results are consistent with the idea that the subsensitivity of post-synaptic alpha-2 receptors is possibly attributable to higher-than-normal noradrenergic secretion in PG. This peripheral noradrenergic dysfunction could be consistent with attenuated cortico-frontal noradrenergic function as shown in positron emission tomography (PET) studies of PG.

Neural substrates of psychostimulant withdrawal-induced anhedonia

Psychostimulant drugs have powerful reinforcing and hedonic properties and are frequently abused. Cessation of psychostimulant administration results in a withdrawal syndrome characterized by anhedonia (i.e., an inability to experience pleasure). In humans, psychostimulant withdrawal-induced anhedonia can be debilitating and has been hypothesized to play an important role in relapse to drug use. Hence, understanding the neural substrates involved in psychostimulant withdrawal-induced anhedonia is essential. In this review, we first summarize the theoretical perspectives of psychostimulant withdrawal-induced anhedonia. Experimental procedures and measures used to assess anhedonia in experimental animals are also discussed. The review then focuses on neural substrates hypothesized to play an important role in anhedonia experienced after termination of psychostimulant administration, such as with cocaine, amphetamine-like drugs, and nicotine. Both neural substrates that have been extensively investigated and some that need further evaluation with respect to psychostimulant withdrawal-induced anhedonia are reviewed. In the context of reviewing the various neurosubstrates of psychostimulant withdrawal, we also discuss pharmacological medications that have been used to treat psychostimulant withdrawal in humans. This literature review indicates that great progress has been made in understanding the neural substrates of anhedonia associated with psychostimulant withdrawal. These advances in our understanding of the neurobiology of anhedonia may also shed light on the neurobiology of nondrug-induced anhedonia, such as that seen as a core symptom of depression and a negative symptom of schizophrenia.

Noradrenaline modulates neuronal responses to GABA in vestibular nuclei

The effects of noradrenaline (NA) on the inhibitory responses to GABA were studied in vivo in neurons of the vestibular nuclei of the rat using extracellular recordings of single unit electrical activity and a microiontophoretic technique of drug application in loco. NA application influenced GABA-evoked inhibitions in 82% of tested neurons, depressing them in 42% and enhancing them in 40% of cases. The more frequent action of NA on GABA responses was depressive in lateral and superior vestibular nuclei (50% of neurons) and enhancing in the remaining nuclei (56% of neurons). The most intense effect of NA application was the enhancement of GABA responses induced in a population of lateral vestibular nucleus neurons, characterized by a background firing rate significantly higher than that of other units. The alpha(2) noradrenergic receptor agonist clonidine mimicked the enhancing action of NA on GABA responses; this action was blocked by application of the specific alpha(2) antagonist yohimbine. The beta adrenergic agonist isoproterenol induced either depressive or enhancing effects on GABA responses; the former more than the latter were totally or partially blocked by application of the beta antagonist timolol. It is concluded that NA enhances GABA responses by acting on noradrenergic alpha(2) and to a lesser extent beta receptors, whereas depressive action involves beta receptors only. These results confirm the hypothesis that the noradrenergic system participates in the regulation of the vestibulospinal and the vestibulo-ocular reflexes and suggest that conspicuous changes of NA content in brain due to aging or stress could lead to a deterioration in the mechanisms of normal vestibular function.

Withdrawal from chronic administration of cocaine decreases delta opioid receptor signaling and increases anxiety- and depression-like behaviors in the rat

Chronic administration of cocaine has been shown to attenuate the functional capacity of delta opioid receptors to inhibit adenylyl cyclase activity. Abuse and withdrawal from cocaine in humans is associated with increases in anxiety and depression. Since recent research supports the role of delta opioid receptors in anxiety- and depression-like behaviors in rodents, we hypothesized that functional desensitization of delta opioid receptors contributes to anxiety- and depression-like behavioral phenotypes following short-term withdrawal from chronic administration of cocaine. To test this hypothesis, delta opioid receptor signaling and behaviors were evaluated 24h after 14days of binge-pattern cocaine administration (15mg/kg three times daily at 1h intervals) in male Sprague-Dawley rats. Results showed that the inhibition of adenylyl cyclase by delta opioid receptor agonists was attenuated in the frontal cortex, nucleus accumbens and caudate putamen 24h after cessation of cocaine administration. One day withdrawal from chronic administration of cocaine resulted in increased anxiety- and depression-like behaviors as measured by the elevated plus maze and the forced swim test respectively, and no change in locomotor activity. The anxiety- and depression-like behaviors were dose-dependently reduced by acute administration of the selective delta opioid receptor agonist, SNC80. These results demonstrate that early withdrawal from cocaine resulted in increased anxiety and depression, which accompanies the desensitization of delta opioid receptor function. Furthermore, cocaine-induced anxiety- and depression-like behaviors were reversible by the delta opioid receptor agonist SNC80.

Betaxolol, a selective beta(1)-adrenergic receptor antagonist, diminishes anxiety-like behavior during early withdrawal from chronic cocaine administration in rats

BACKGROUND

Anxiety has been indicated as one of the main symptoms of the cocaine withdrawal syndrome in human addicts and severe anxiety during withdrawal may potentially contribute to relapse. As alterations in noradrenergic transmission in limbic areas underlie withdrawal symptomatology for many drugs of abuse, the present study sought to determine the effect of cocaine withdrawal on beta-adrenergic receptor (beta(1) and beta(2)) expression in the amygdala.

METHODS

Male Sprague Dawley rats were administered intraperitoneal (i.p.) injections of cocaine (20 mg/kg) once daily for 14 days. Two days following the last cocaine injection, amygdala brain regions were micro-dissected and processed for Western blot analysis. Results showed that beta(1)-adrenergic receptor, but not beta(2)-adrenergic receptor expression was significantly increased in amygdala extracts of cocaine-withdrawn animals as compared to controls. This finding motivated further studies aimed at determining whether treatment with betaxolol, a highly selective beta(1)-adrenergic receptor antagonist, could ameliorate cocaine withdrawal-induced anxiety. In these studies, betaxolol (5 mg/kg via i.p. injection) was administered at 24 and then 44 h following the final chronic cocaine administration. Anxiety-like behavior was evaluated using the elevated plus maze test approximately 2 h following the last betaxolol injection. Following behavioral testing, betaxolol effects on beta(1)-adrenergic receptor protein expression were examined by Western blotting in amygdala extracts from rats undergoing cocaine withdrawal.

RESULTS

Animals treated with betaxolol during cocaine withdrawal exhibited a significant attenuation of anxiety-like behavior characterized by increased time spent in the open arms and increased entries into the open arms compared to animals treated with only saline during cocaine withdrawal. In contrast, betaxolol did not produce anxiolytic-like effects in control animals treated chronically with saline. Furthermore, treatment with betaxolol during early cocaine withdrawal significantly decreased beta(1)-adrenergic receptor protein expression in the amygdala to levels comparable to those of control animals.

CONCLUSIONS

The present findings suggest that the anxiolytic-like effect of betaxolol on cocaine-induced anxiety may be related to its effect on amygdalar beta(1)-adrenergic receptors that are up-regulated during early phases of drug withdrawal. These data support the efficacy of betaxolol as a potential effective pharmacotherapy in treating cocaine withdrawal-induced anxiety during early phases of abstinence.

Animal models and treatments for addiction and depression co-morbidity

The high rates of co-morbidity of drug addiction with depression may be attributable to shared neurobiology. Here, we discuss shared neurobiological substrates in drug withdrawal and depression, with an emphasis on changes in brain reward circuitry that may underlie anhedonia, a core symptom of depression and drug withdrawal. We explored experimentally whether clinical antidepressant medications or other treatments would reverse the anhedonia observed in rats undergoing spontaneous nicotine or amphetamine withdrawal, defined operationally as elevated brain reward thresholds. The co-administration of selective serotonin reuptake inhibitors with a serotonin-1A receptor antagonist, or the tricyclic antidepressant desipramine, or the atypical antidepressant bupropion ameliorated nicotine or amphetamine withdrawal in rats. Thus, increases in monoaminergic neurotransmission, or neuroadaptations induced by increased monoaminergic neurotransmission, ameliorated depression-like aspects of drug withdrawal. Further, chronic pretreatment with the atypical antipsychotic clozapine, that has some efficacy in the treatment of the depression-like symptoms of schizophrenia, attenuated nicotine and amphetamine withdrawal. Finally, a metabotropic glutamate 2/3 receptor antagonist reversed threshold elevations associated with nicotine withdrawal. The effects of these pharmacological manipulations are consistent with the altered neurobiology observed in drug withdrawal and depression. Thus, these data support the hypothesis of common substrates mediating the depressive symptoms of drug withdrawal and those seen in psychiatric patients. Accordingly, the anhedonic state associated with drug withdrawal can be used to study the neurobiology of anhedonia, and thus contribute to the identification of novel targets for the treatment of depression-like symptoms seen in various psychiatric and neurological disorders.

Restraint-induced corticosterone secretion and hypothalamic CRH mRNA expression are augmented during acute withdrawal from chronic cocaine administration

Stress responses during cocaine withdrawal likely contribute to drug relapse and may be intensified as a consequence of prior cocaine use. The present study examined changes in stressor-induced activation of the hypothalamic-pituitary-adrenal (HPA) axis during acute withdrawal from chronic cocaine administration. Adult male Sprague-Dawley rats received daily administration of cocaine (30 mg/kg, i.p.) or saline for 14 days. Twenty-four hours after the last injection, rats in each group were sacrificed under stress-free conditions or following 30 min of immobilization. Plasma corticosterone (CORT) was measured in trunk-blood using radioimmunoassay, corticotropin-releasing hormone (CRH) mRNA levels in the paraventricular nucleus (PVN) of the hypothalamus were measured using in situ hybridization and glucocorticoid receptor (GR) protein expression in the pituitary gland and dissected brain regions was measured using Western blot analysis. Basal CRH mRNA in the PVN was unaltered as a result of prior cocaine administration. However, a significant increase in CRH mRNA was observed 90 min following the termination of restraint in cocaine withdrawn, but not saline-treated, rats. Basal CORT was also unaffected by prior cocaine administration, but the CORT response measured immediately after restraint was significantly augmented in cocaine-withdrawn rats. Differences in GR protein expression in number of regions implicated in negative feedback regulation of HPA function, including the hypothalamus, were not observed. These findings indicate that the HPA response to stressors is intensified during early withdrawal from cocaine administration and may be independent of changes in GR-mediated negative feedback.

Alpha-noradrenergic receptors modulate the development and expression of cocaine sensitization

The increased activity and stereotyped behaviors that result from repeated administration of cocaine is called cocaine sensitization. This sensitized response has been postulated as one of the basic pathophysiological mechanisms in drug addiction. Recent evidence indicates that noradrenergic neurotransmission might be implicated in some of the behavioral effects of cocaine. The present article examined the role of alpha-adrenergic receptor agonists and antagonists in the development and expression of cocaine sensitization. Rats were injected once per day, for 7 consecutive days, with the alpha-1 receptor antagonist prazosin (0.5 mg/kg, i.p.) 15 min before cocaine administration (15 mg/kg, i.p.). After 8 days, animals received a cocaine challenge (15 mg/kg, i.p.) and were tested for locomotion. Following a 7-day withdrawal period rats received a second cocaine challenge. One day after the last challenge, rats were reinstated to the initial protocol for 1 day. In another set of experiments, rats were injected twice per day with the alpha-2 receptor antagonists yohimbine (5 mg/kg, i.p.), idazoxan (0.25 mg/kg, i.p.), or with the alpha-2 agonist clonidine (0.025 mg/kg, i.p.), followed by cocaine injections (15 mg/kg, i.p.), for 7 consecutive days. Thereafter, the protocol was similar to that following prazosin administration. The results demonstrated that the alpha-1 receptor antagonist prazosin blocked the development and expression of cocaine sensitization. On the other hand, both alpha-2 antagonists failed to inhibit the development or the expression of cocaine sensitization. Instead, they produced an increase in locomotor activity during the first day of experimentation. The alpha-2 agonist clonidine attenuated the acute response to cocaine on day 1 and retarded the increased locomotor activity on the following 2 days. There was a dramatic increase in the level of sensitization after the first cocaine challenge. However, it inhibited the expression of cocaine sensitization during the reinstatement protocol. These results suggest that alpha adrenoreceptors play an important role in modulating different stages of cocaine sensitization and probably cocaine addiction.

Psychostimulant withdrawal as an inducing condition in animal models of depression

A large body of evidence indicates that the withdrawal from high doses of psychostimulant drugs in humans induces a transient syndrome, with symptoms that appear isomorphic to those of major depressive disorder. Pharmacological treatment strategies for psychostimulant withdrawal in humans have focused mainly on compounds with antidepressant properties. Animal models of psychostimulant withdrawal have been shown to demonstrate a wide range of deficits, including changes in homeostatic, affective and cognitive behaviors, as well as numerous physiological changes. Many of these behavioral and physiological sequelae parallel specific symptoms of major depressive disorder, and have been reversed by treatment with antidepressant drugs. These combined findings provide strong support for the use of psychostimulant withdrawal as an inducing condition in animal models of depression. In the current review we propound that the psychostimulant withdrawal model displays high levels of predictive and construct validity. Recent progress and limitations in the development of this model, as well as future directions for research, are evaluated and discussed.