T. J. De Vries

Dopaminergic mechanisms mediating the incentive to seek cocaine and heroin following long-term withdrawal of IV drug self-administration

Abstract  Rationale: The neurobiological mechanisms underlying the persistence of drug craving in detoxified addicts are still poorly understood. Objective: The present study was designed to evaluate dopaminergic mechanisms in drug-seeking behaviour following long-term (>3 weeks) extinction of IV drug self-administration in rats. Methods: To that end, we studied the effects of direct and indirect dopamine (DA) agonists on reinstatement of previously extinguished responding for heroin (50 μg/kg per injection; 14–15 daily 3-h sessions) and cocaine (500 μg/kg per injection; 10–11 daily 2-h sessions). Results: In animals with a cocaine history, priming with cocaine, the selective DA reuptake inhibitor GBR-12909 and the DA D2 receptor agonist quinpirole resulted in robust and selective reinstatement of non-reinforced nose poking behaviour in the previously drug-paired hole. In contrast, the D1 agonist SKF-82958 failed to reinstate responding and the non-selective DA agonist apomorphine even suppressed responding in these animals. In heroin-trained rats, heroin and GBR-12909 strongly reinstated responding, whereas all direct DA agonists were ineffective. Again, the two highest doses of apomorphine decreased responding in these animals. In a parallel study, the ability of DA ligands to express behavioural sensitization in animals pretreated with amphetamine or morphine was evaluated. Interestingly, all agonists that reinstated responding in the present study caused expression of locomotor sensitization and vice versa. Conclusions: The differences between direct and indirect agonists indicate a clear, but complex, involvement of DA in drug-seeking behaviour long after detoxification. Moreover, the results show an important role of D2 receptor activation in the persistence of cocaine- but not heroin-seeking behaviour. Finally, the results from both studies suggest a relationship between drug-induced reinstatement and drug hyperresponsiveness in long-term abstinent rats.

Stressor- or drug-induced sensitization of the corticosterone response is not critically involved in the long-term expression of behavioural sensitization to amphetamine

Repeated exposure to drugs of abuse induces long-lasting behavioural sensitization, which is thought to play a role in the persistence of drug-seeking behaviour. Recently, we showed that repeated exposure of rats to cocaine resulted in a long-lasting (weeks) sensitization of the hypothalamus-pituitary-adrenal axis, i.e. hypersecretion of adrenocorticotropic hormone and of the glucocorticoid corticosterone. Moreover, we found that the administration of a glucocorticoid receptor antagonist abolished the expression of psychostimulant-induced behavioural sensitization. In the present study we tested whether stressor- or drug-induced long-term hypersecretion of corticosterone is associated with the long-term expression of behavioural sensitization to psychostimulant drugs. To that end, groups of male Wistar rats were exposed once to interleukin-1beta or to footshocks, treatments that are known to induce long-term sensitization of the hypothalamus-pituitary-adrenal axis, or were treated with amphetamine or morphine, according to protocols known to induce long-lasting behavioural (locomotor) sensitization. Three weeks later, the groups and their controls were challenged with amphetamine or vehicle. Previous exposure to interleukin-1beta or footshocks enhanced adrenocorticotropic hormone and corticosterone responses, but did not affect the long-term locomotor sensitization to amphetamine. Prior amphetamine treatment enhanced the locomotor response and the adrenocorticotropic hormone and corticosterone responses to amphetamine. Prior morphine treatment resulted in long-term locomotor sensitization, whereas the adrenocorticotropic hormone and corticosterone responses to amphetamine were decreased. From these findings and the absence of within-group correlation between corticosterone and locomotor responses in interleukin-1beta and morphine-pretreated rats, we conclude that there is no correlation between sensitization of the corticosterone response and behavioural sensitization to amphetamine. Apparently, sensitization of the corticosterone response is not a prerequisite for the long-term expression of behavioural sensitization, which suggests that drug-induced long-term behavioural sensitization may involve corticosteroid receptor-dependent (central) mechanisms that occur independent of hypothalamus-pituitary-adrenal axis responsiveness.

A single administration of interleukin-1 or amphetamine induces long-lasting increases in evoked noradrenaline release in the hypothalamus and sensitization of ACTH and corticosterone responses in rats

Single administration of the cytokine interleukin‐1β (IL‐1) or the psychostimulant amphetamine causes long‐term sensitization of the hypothalamus pituitary adrenal (HPA) axis, i.e. enhanced adrenocorticotropine hormone (ACTH) and corticosterone responses weeks later. HPA responses to these stimuli involve activation of hypothalamic corticotropin‐releasing hormone (CRH) neurons by noradrenergic projections to the paraventricular nucleus (PVN). In search of the underlying mechanisms, we studied the temporal pattern of HPA sensitization in relation to (1) the reactivity of noradrenergic projections to the PVN and (2) altered secretagogue production in hypothalamic CRH neurons. Single exposure to IL‐1 or amphetamine induced cross‐sensitization of ACTH and corticosterone responses 11 and 22 days later, but not after 42 days. Amphetamine‐induced HPA sensitization was not accompanied by increased costorage of arginine vasopressin (AVP) in CRH terminals, as found previously after IL‐1 pretreatment. The reactivity of noradrenergic terminals was assessed by measuring the electrically evoked release of [3H]‐noradrenaline from superfused PVN slices. Single administration of amphetamine and IL‐1 induced a long‐lasting (up to 22 days) increase (up to 165%) of evoked noradrenaline release. This indicates that single exposure to psychostimulants or to cytokines can induce a long‐lasting increase in stimulus–secretion coupling in brainstem noradrenergic neurons that innervate the PVN. This common, long‐lasting functional change may underlie, at least in part, IL‐1‐ and amphetamine‐induced HPA cross‐sensitization. In addition, increased AVP signalling by hypothalamic CRH neurons appears to play a role in IL‐1‐induced, but not in amphetamine‐induced, HPA sensitization.

Unrestricted free-choice ethanol self-administration in rats causes long-term neuroadaptations in the nucleus accumbens and caudate putamen.

Abstract In the present study, the reactivity of striatal dopamine and dopamine-sensitive neurons in superfused striatal slices of ethanol-experienced rats was compared to that of ethanol-naive rats, 3 weeks after oral ethanol self-administration. During the acquisition phase (17 days), rats were offered increasing concentrations of ethanol (from 2 to 10%, 24 h per day) on an alternate-day schedule in a free choice with water. Following 2 weeks of unrestricted 10% ethanol consumption, the highest and lowest drinkers (representing about 25% of the upper and lower extremes of the total population) were selected. Preliminary experiments revealed that both groups of rats displayed a profound increase in ethanol consumption and preference 3 weeks after cessation of ethanol self-administration (deprivation effect). This deprivation effect was associated with an increase in electrically evoked release of [3H]dopamine from superfused nucleus accumbens slices, whereas the evoked [3H]dopamine release from caudate putamen slices remained unchanged. In slices of the caudate putamen, but not in nucleus accumbens slices, postsynaptic dopamine D1 receptor-stimulated cyclic AMP production was also enhanced. In addition, prior ethanol consumption enhanced the electrically evoked release of [14C]acetylcholine release in both striatal regions. Interestingly, the magnitude of these long-term neuroadaptations correlated with the amount of daily ethanol consumption, i.e. neuronal hyperresponsiveness in the striatum was more profound in the high than in the low ethanol drinkers. These data show for the first time that unrestricted free-choice ethanol consumption in rats is associated with a long-term increase in dopaminergic and cholinergic neurotransmission in the nucleus accumbens and caudate putamen. These (and other) neuroadaptations may underlie the enhanced motivation to self-administer ethanol and the maintenance of ethanol consumption long after deprivation.

Bremazocine reduces unrestricted free-choice ethanol self-administration in rats without affecting sucrose preference

Abstract It has been postulated that opioid systems in the brain may play a role in ethanol reinforcement. In this respect, μ- and δ-opioid receptors may mediate the rewarding effects whereas κ receptors are thought to mediate the aversive effects of opioids. Accordingly, long-acting benzomorphans such as bremazocine, that simultaneously act as μ and δ receptor antagonists and κ receptor agonists may be particularly effective in reducing ethanol self-administration. Therefore, we studied the effect of bremazocine on oral ethanol self-administration in rats using a paradigm [unrestricted free-choice drinking of 10% (v/v) ethanol], previously shown to cause long-term neuroadaptations in the nucleus accumbens and caudate putamen. Bremazocine (0.1 mg/kg, once daily for five consecutive days) reduced ethanol drinking by about 50% during the active period of the animals, whereas the intake of sucrose (3–10% w/v) was affected neither in naive nor in ethanol-experienced rats. This effect of bremazocine appeared not to be secondary to its acute sedative effect or the slight increase in total fluid consumption. Unlike bremazocine, the selective κ-opioid receptor agonist U50,488H (10 mg/kg, once daily) inhibited ethanol drinking only during the first of 5 treatment days and the opioid receptor antagonist naltrexone (0.3–10 mg/kg, once daily) only caused a modest (about 20%) suppression of ethanol drinking during the first hours after drug injection. Thus, bremazocine appears to be far more potent than the clinically applied drug naltrexone in this respect. Our data further support the role of opioid receptors in ethanol reinforcement and indicate that long-acting mixed-action opioids such as bremazocine may be useful as adjuvants for the clinical management of ethanol addiction.

Lack of involvement ofδ-opioid receptors in mediating the rewarding effects of cocaine

The non-selective opioid antagonist naltrexone and the partial agonist buprenorphine have been reported to reduce cocaine self-administration (SA) and relapse in both humans and rhesus monkeys. Data suggesting an involvement ofδ-opioid receptors in modulating the conditioned rewarding effects of cocaine were also recently presented. In view of such findings, the present SA and place conditioning studies were conducted to examine the influence of the selectiveδ-opioid receptor antagonist naltrindole upon the rewarding effects of cocaine. Sprague-Dawley rats were trained to self-administer cocaine (1.0 mg/kg per infusion) on an FR2 schedule of reinforcement. Dose-response and antagonist testing commenced once stable rates of cocaine SA were achieved. For antagonist testing, rats received naltrindole (0.03–10.0 mg/kg, IP) 30 min prior to the start of 2-h SA sessions. SA behavior in response to cocaine delivery (0.25 and 1.0 mg/kg per infusion) was then determined. Naltrindole in doses of 0.03–3.0 mg/kg did not alter the number of cocaine infusions taken by the rats. A higher dose of naltrindole (10.0 mg/kg), which markedly depressed locomotor activity, resulted in a 16% reduction of cocaine (0.25 mg/kg per infusion) SA behavior. When SA sessions were terminated and naltrindole (1.0 mg/kg) was administered repeatedly for 3 days, no alterations in the re-acquisition of cocaine SA were seen. Place conditioning studies also failed to find an effect of naltrindole (0.1–3.0 mg/kg) on cocaine (10 mg/kg) — induced conditioned place preferences. Naltrindole, by itself, did not induce significant place conditioning. These data fail to indicate a role ofδ-opioid receptors in modulating either the positive reinforcing or conditioned rewarding effects of cocaine. Furthermore, they suggest that the therapeutic actions of naloxone, naltrexone and buprenorphine on cocaine SA behavior may not result from the specific blockade ofδ-opioid receptors.

Individual differences in the effects of serotonergic anxiolytic drugs on the motivation to self-administer cocaine

Numerous clinical studies have indicated that lifetime anxiety is highly prevalent in drug addicts. In the treatment of drug abuse, dually diagnosed drug addicts may benefit from pharmacological intervention strategies that alleviate the psychiatric symptomatology. We have previously shown that rats with different coping strategies in a stressful environment show strong differences in the motivation to self-administer cocaine. That is, cocaine self-administration under a progressive ratio (PR) schedule of reinforcement was enhanced in high grooming (HG) rats as compared with low grooming (LG) rats. To identify the pharmacological basis of these differences, we tested the acute effects of several anxiolytic drugs on cocaine self-administration in HG and LG rats under a PR schedule of reinforcement. Chlordiazepoxide increased PR responding in both the HG and LG rats, while the selective corticotrophin releasing hormone 1 receptor antagonist R121919 had no effect on cocaine self-administration under the PR schedule. Interestingly, buspirone and fluoxetine decreased PR responding in HG rats only and thereby abolished the individual differences in PR responding between HG and LG rats. In support of the differential effects of the serotonergic drugs on PR responding in HG and LG rats, we found that the in vitro electrically evoked release of [3H]serotonin from mesocorticolimbic brain slices was reduced in the medial prefrontal cortex, substantia nigra and nucleus accumbens core, and increased in the nucleus accumbens shell of HG rats relative to LG rats. These findings show that serotonergic anxiolytics abolish the pre-existing individual differences in cocaine self-administration between HG and LG rats, which show differences in the reactivity of serotonergic neurons. This suggests that the effectiveness of pharmacological interference may depend on the neurochemical and motivational state of the individual.

d-Cycloserine administered directly to infralimbic medial prefrontal cortex enhances extinction memory in sucrose-seeking animals

d-Cycloserine (DCS), a co-agonist at the N-methyl-D-aspartate (NMDA) receptor, has proven to be an effective adjunct to cognitive behavioral therapies that utilize extinction. This pharmacological-based enhancement of extinction memory has been primarily demonstrated in neuropsychiatric disorders characterized by pathological fear (e.g. posttraumatic stress disorder and various phobias). More recently, there has been an interest in applying such a strategy in the disorders of appetitive learning (e.g. substance abuse and other addictions), but these studies have generated mixed results. Here we first examined whether extinction memory encoding in a sucrose self-administration model is dependent on NMDA receptors. The NMDA antagonist (±)-3-(2-carboxypiperazin-4-yl)propyl-1-phosphonic acid (5mg/kg, i.p.) administered 2h prior to the first extinction training session effectively inhibited extinction memory recall 24h later, without affecting the expression of the conditioned sucrose-seeking response while the drug was on board. This profile of effects suggests a specific effect on extinction memory consolidation. Next, we sought to enhance extinction memory using the co-agonist DCS (10 μg/side) by infusion directly into infralimbic medial prefrontal cortex, a brain site implicated in extinction memory recall in conditioned fear models. Indeed, infusion of DCS immediately after the first extinction training session effectively enhanced extinction memory recall 24h later. Collectively, these data suggest that the neurobiological mechanisms and the neurocircuitry mediating extinction memory are similar regardless of the valence (aversive or appetitive) of the conditioned behavior, and that similar pharmacological strategies for treatment may be applied to neuropsychiatric disorders characterized by a failure to inhibit pathological emotional memories.

Neuroadaptive changes in mesocorticolimbic dopamine and acetylcholine neurons following cocaine or saline self-administration are dependent on pre-existing individual differences

Previously, we demonstrated that stress-induced self-grooming behaviour in rats predicted an enhanced motivation to self-administer cocaine as determined under a progressive ratio schedule of reinforcement. The enhanced motivation of high grooming (HG) rats was associated with a reduced reactivity of dopaminergic neurons in the medial prefrontal cortex and amygdala, but not nucleus accumbens. In the present study, we studied the effect of cocaine and saline self-administration on these pre-existing differences in neurochemical profile by determining the electrically evoked release of [3H]dopamine and [14C]acetylcholine from superfused slices of the nucleus accumbens shell and core, medial prefrontal cortex and amygdala of HG and low grooming (LG) rats. Although HG and LG rats did not differ in acquisition of cocaine and saline self-administration, both conditions induced substantially different neuroadaptations in these rats. Differences in depolarisation-induced dopamine and acetylcholine release were maintained in the medial prefrontal cortex, emerged in the nucleus accumbens and dissipated in the amygdala. These results indicate that altered reactivity of mesocorticolimbic dopaminergic and cholinergic neurons due to exposure to cocaine and environmental stimuli (saline) is dependent on pre-existing neurochemical differences and displays region-specificity. These pre-existing differences and the cocaine- and environmental-induced neuroadaptations seem to act in concert to produce an enhanced motivational state to self-administer cocaine.

Changes in rat striatal ppENK expression and adenylate cyclase-coupled opioid receptors upon haloperidol treatment during different developmental stages

Abstract This study was designed to evaluate the effects of a chronic treatment with the classical neuroleptic drug haloperidol on the preproenkephalin (ppEnk) mRNA synthesis and its consequences for opioid and dopamine (DA) receptor-regulated adenylate cyclase in the developing and adult rat striatum. In situ hybridization experiments revealed that the tonic inhibitory effect of DA on striatal ppEnk mRNA synthesis gradually develops postnatally and seems to be dependent on the presence of adenylate cyclase-coupled D 2 receptors. In addition, postnatal treatment with haloperidol resulted in a clear reduction of D 1 DA receptor-stimulated cAMP production and a profound desensitization of δ-opioid receptors inhibitory coupled to adenylate cyclase. These adaptive changes may underlie the well-known increase in locomotor and reinforcing effects of μ opioids upon chronic neuroleptic treatment. The basal ganglia contain high levels of dopamine (DA) and enkephalins and numerous functional relationships between both neurotransmitter systems have been reported. DA seems to exert a tonic inhibition on the availability of endogenous enkephalins in the nucleus accumbens and striatum, since chronic treatment with DA antagonists or destruction of dopaminergic neurons by 6-OHDA has been shown to cause an increased expression, content and release of these neuropeptides (1,2,3). A major role of the D 2 receptor in the regulation of the enkephalin mRNA expression and peptide levels has been consistently reported. Enkephalin-containing neurons in the striatum can be detected as early as embryonic day 16 (E16) in rat brain (4). Interestingly, we found recently that DA receptors in the rat striatum develop rather asynchroniously. Whereas the stimulatory coupling of D 1 receptors to adenylate cyclase occurs in the prenatal period (E17), D 2 receptor-mediated inhibition of cAMP production can not be detected until the second postnatal week (5). Since the cAMP pathway seems to play an important role in the regulation of the expression of the preproenkephalin gene (6,7,8), we decided to study the ontogeny of the dopaminergic control of enkephalin synthesis. In addition, we determined the consequences of an enhanced enkephalin availability in the striatum, induced by chronic haloperidol treatment, for the effect of Met-enkephalin on DA-regulated adenylate cyclase activity during different developmental stages.