Arie H. Mulder

Drug‐induced reinstatement of heroin‐ and cocaine‐seeking behaviour following long‐term extinction is associated with expression of behavioural sensitization

The present study was designed to evaluate the relationship between reinstatement of drug‐seeking behaviour following long‐term extinction of intravenous (i.v.) drug self‐administration (an animal model for craving) and long‐term behavioural sensitization. Rats were allowed to self‐administer heroin (50 μg/kg per inj., 14 daily sessions), cocaine (500 μg/kg per inj., 10 daily sessions) or saline. Following a 3‐week extinction period, reinstatement tests were performed to evaluate priming effects of amphetamine, cocaine and heroin on non‐reinforced drug‐seeking behaviour. In addition, the occurrence of long‐term behavioural sensitization in rats with a history of heroin or cocaine self‐administration was determined. Heroin‐seeking behaviour was reinstated by heroin (0.25 mg/kg), amphetamine (1.0 mg/kg) and cocaine (10 mg/kg). In addition, animals with a history of heroin self‐administration displayed locomotor sensitization to both heroin and amphetamine. Cocaine‐seeking behaviour was reinstated by cocaine and amphetamine, but not by heroin. Interestingly, locomotor sensitization to amphetamine, but not heroin, was observed in animals with a history of cocaine self‐administration. In other words, the induction of drug‐seeking behaviour following a prolonged drug‐free period was found to be associated with the expression of long‐term behavioural sensitization. These data provide experimental evidence for a role of behavioural sensitization in the incentive motivation underlying drug‐seeking behaviour. If drug hyperresponsiveness would indeed be a crucial factor in drug‐induced craving in human addicts, pharmacological readjustment of the neuroadaptations underlying drug sensitization may prevent relapse to drug use long after detoxification.

Ethanol, like psychostimulants and morphine, causes long-lasting hyperreactivity of dopamine and acetylcholine neurons of rat nucleus accumbens: possible role in behavioural sensitization.

Abstract Repeated treatment of rats with ethanol (1 g/kg, once daily for 15 days) enhanced the locomotor effect of morphine, 3 weeks post-treatment. This ethanol-induced long-term behavioural sensitization to morphine was associated with an increase in the electrically evoked release of [3H]dopamine (DA) and [14C]acetylcholine (ACh) from nucleus accumbens slices. A similar enhanced responsiveness of accumbal dopaminergic and cholinergic neurons to depolarization was apparent 3 weeks after repeated morphine, amphetamine or cocaine administration. Prior ethanol exposure also caused a long-term enhancement of electrically evoked release of [3H]DA and [14C]ACh from slices of the caudate-putamen. Unlike the locomotor effect of morphine, that of amphetamine was not enhanced in ethanol-pretreated rats. These data indicate that ethanol administration may cause long-term behavioural sensitization associated with adaptive changes in dopaminergic and cholinergic neurons of rat nucleus accumbens and caudate-putamen. Furthermore, an enhanced reactivity of nucleus accumbens dopaminergic nerve terminals and dopamine-sensitive cholinergic neurons appears to be a common long-term neuroadaptive effect of distinct types of addictive drugs. However, since repeated ethanol exposure did not cause a long-term increase in the locomotor effect of amphetamine, these neuroadaptations may not always be sufficient to cause long-lasting behavioural (cross-)sensitization.

Morphine-induced long-term sensitization to the locomotor effects of morphine and amphetamine depends on the temporal pattern of the pretreatment regimen

Abstract The development of behavioural sensitization is thought to depend on the dose and temporal pattern of drug treatment. Previous studies have shown that two distinct morphine pretreatment regimens cause different long-term neuroadaptations in rat striatum. Therefore, in the present study the ability of these pretreatment regimens to induce long-term behavioural sensitization was investigated. One pretreatment regimen, termed “chronic”, consisted of three daily injections, for 5 days, with escalating doses (10–50 mg/kg) of morphine, and the other, termed “intermittent”, of 14 daily injections with morphine (10 mg/kg). Both intermittent and chronic morphine pretreatment caused sensitization to the locomotor effects of morphine, 3 weeks post-treatment, although the former induced a far greater level of sensitization. Moreover, 3 weeks post-treatment, intermittent, but not chronic, morphine pretreatment induced cross-sensitization to the locomotor stimulant effects of amphetamine. Behavioural sensitization following intermittent morphine pretreatment was clear-cut both 1 day and 3 weeks post-treatment, while after 9 weeks, the locomotor effects of morphine were still slightly augmented. It is concluded that intermittent morphine pretreatment is far more effective in inducing long-term behavioural sensitization than chronic morphine pretreatment.

Dopaminergic mechanisms mediating the long-term expression of locomotor sensitization following pre-exposure to morphine or amphetamine

Abstract The role of dopaminergic mechanisms in opiate- and psychostimulant-induced long-term locomotor sensitization was investigated. To that aim, rats were behaviourally sensitized with morphine or amphetamine and 3 weeks after cessation of treatment challenged with various direct and indirect dopamine agonists. Both morphine- and amphetamine-pretreated rats displayed sensitization of the locomotor effects of amphetamine, cocaine, and the selective dopamine reuptake inhibitor GBR-12909. Sensitization of the locomotor stimulant effects of the dopamine D2/D3 receptor agonist quinpirole was observed in amphetamine- but not morphine-pretreated rats. In contrast, morphine-, but not amphetamine-pretreated rats appeared hyposensitive to the locomotor inhibitory effects of a low, presumably D2-autoreceptor selective, dose of quinpirole. Neither pretreatment induced sensitization to the dopamine D1/D2 agonist apomorphine or the dopamine D1 agonist SKF-82958. In fact, the locomotor stimulant effects of SKF-82958 appeared to be decreased in animals pre-exposed to amphetamine. These results suggest that functional changes in presynaptic dopamine release mechanisms represent common neuroadaptations involved in the long-term expression of morphine- and amphetamine-induced locomotor sensitization. Presynaptic dopamine D2 and postsynaptic D2 and/or D3 receptors are differentially involved in the expression of morphine- and amphetamine-induced locomotor sensitization. In a parallel study, we report that all of the drugs that elicited sensitized locomotor responses in morphine- or amphetamine-pretreated rats caused reinstatement of previously extinguished heroin- or cocaine-seeking behaviour, respectively. Taken together, these data suggest a marked relationship between drug-seeking behaviour and drug sensitization.

Repeated and chronic morphine administration causes differential long-lasting changes in dopaminergic neurotransmission in rat striatum without changing its δ- and κ-opioid receptor regulation

Repeated, once daily morphine treatment (14 days) as well as chronic morphine administration (6 days) caused a rebound reduction in the electrically evoked release of [3H]dopamine from superfused rat striatal slices 1 day after the last subcutaneous injection. Interestingly, whereas [3H]dopamine release remained significantly reduced for at least 3 weeks following morphine withdrawal in chronically treated (tolerant/dependent) rats, neurotransmitter release from dopaminergic nerve terminals gradually increased above control values following cessation of repeated morphine administration. Postsynaptically, dopamine D1 receptor-stimulated adenylate cyclase appeared to be sensitized 1-3 days but was unchanged 3 weeks after chronic morphine treatment. In contrast, such an enhanced postsynaptic dopamine D1 receptor efficacy did not occur 1-3 days following repeated morphine administration, but appeared to develop slowly resulting in a profound increase of dopamine-sensitive adenylate cyclase 3 weeks after the last injection. The inhibitory effect of dynorphin A-(1-13) on [3H]dopamine release, as well as that of [Met5]enkephalin on dopamine D1 receptor-stimulated adenylate cyclase appeared to be unchanged subsequent to repeated or chronic morphine treatment. These data indicate that, long after cessation of drug treatment, chronic morphine treatment causes a reduction whereas repeated morphine administration gradually induces an enhancement of opioid receptor-regulated dopaminergic neurotransmission due to local adaptive changes within the rat striatum. Such distinct long-lasting alterations of dopaminergic neurotransmission induced by different temporal patterns of morphine administration in projection areas of mesencephalic dopaminergic neurons may be related to the enduring effects of drug abuse such as behavioural sensitization and drug craving.

κ1- and κ2-opioid receptors mediating presynaptic inhibition of dopamine and acetylcholine release in rat neostriatum

1 The effects of selective opioid receptor agonists and antagonists on N‐methyl‐D‐aspartate (NMDA, 10 μM)‐induced release of [3H]‐dopamine and [14C]‐acetylcholine (ACh) from superfused neostriatal slices were studied to investigate the possible occurrence of functional κ‐opioid receptor subtypes in rat brain. 2 The κ receptor agonists (−)‐ethylketocyclazocine ((−)‐EKC), U69593 and the endogenous opioid peptide dynorphin A1–13 caused a naloxone‐reversible inhibition of NMDA‐induced [3H]‐dopamine release, with pD2 values of about 9, 8.5 and 8.2, respectively, whereas both the μ agonist Tyr‐D‐Ala‐Gly‐(NMe)Phe‐Gly‐ol (DAMGO) and theδ agonist D‐Pen2‐D‐Pen5‐enkephalin (DPDPE) were ineffective in this respect. The inhibitory effect of submaximally effective concentrations of dynorphin A1–13, U69593 and (−)‐EKC on NMDA‐induced [3H]‐dopamine release were not changed by the δ1/δ2‐opioid receptor antagonist naltrindole (up to a concentration of 1 μM), but reversed by the κ receptor antagonist nor‐binaltorphimine (nor‐BNI), with an IC50 as low as 0.02 nM, indicating the involvement of U69593‐sensitive κ1‐opioid receptors. 3 NMDA‐induced [14C]‐ACh release was reduced in a naloxone‐reversible manner by DPDPE (pD2 about 7.2), dynorphin A1–13 (pD2 6.7) and EKC (pD2 6.2), but not by U69593 and DAMGO. The inhibitory effect of a submaximally effective concentration of DPDPE, unlike those of dynorphin A1–13 and (−)‐EKC, on NMDA‐induced [14C]‐ACh release was antagonized by naltrindole with an IC50 of 1 nM, indicating the involvement of δ‐opioid receptors in the inhibitory effect of DPDPE. On the other hand, the inhibitory effects of dynorphin A1–13 and (−)‐EKC on [14C]‐ACh release were readily antagonized by nor‐BNI with an IC50 of about 3 nM. A 100 fold higher concentration of nor‐BNI also antagonized the inhibitory effect of DPDPE, indicating the involvement of U69593‐insensitive κ2‐opioid receptors in the inhibitory effects of dynorphin A1–13 and (−)‐EKC. 4 Although naloxone benzoylhydrazone (NalBzoH), displaying high affinity towards the putative κ3‐opioid receptor, antagonized the inhibitory effects of dynorphin A1–13 and (−)‐EKC on [3H]‐dopamine and [14C]‐ACh release as well as that of U69593 on [3H]‐dopamine release, it displayed a low apparent affinity (IC50 about 100 nM) in each case. 5 In conclusion, whereas activation of κ1‐opioid receptors causes presynaptic inhibition of NMDA‐induced dopamine release, κ2 receptor activation results in inhibition of ACh release in rat neostriatum. As such, this study is the first to provide unequivocal in vitro evidence for the existence of functionally distinct κ‐opioid receptor subtypes in the brain.

Pharmacological profile of various κ‐agonists at κ‐, μ‐ and δ‐opioid receptors mediating presynaptic inhibition of neurotransmitter release in the rat brain

1 The potency, relative efficacy and selectivity of a series of κ‐opioid receptor agonists at the μ‐, δ‐ and κ‐opioid receptors mediating inhibition of electrically‐induced (radiolabelled) neurotransmitter release from superfused rat brain slices was determined. 2 With regard to their potencies at κ‐receptors mediating inhibition of striatal [3H]‐dopamine release, the highest pD2 value (8.7) was found for bremazocine and the lowest (7.1) for U50488; the pD2 values for ethylketocyclazocine (EKC), tifluadom, U69593 and PD117302 were between 8.0 and 8.3. There were no marked differences between the relative efficacies of the κ‐agonists (maximum inhibition being 60–70%). In contrast to the other κ‐agonists, at a concentration of 1 μm, PD117302 caused a significant (25–40%) increase of the spontaneous efflux of tritium. 3 None of the κ‐agonists significantly affected striatal [14C]‐acetylcholine (ACh) release, with the exception of a slight inhibitory effect of EKC. The δ‐receptor‐mediated inhibitory effect of [d‐Ala2, d‐Leu5] enkephalin (DADLE) on [14C]‐ACh release was antagonized in a concentration‐dependent manner by bremazocine (0.1 and 1.0 μm) and also partially by EKC (1 μm), but not by the other κ‐agonists. The pA2 value for bremazocine as an antagonist at the δ‐receptors involved was 8.0, compared to 7.6 for naloxone. 4 None of the κ‐agonists significantly affected cortical [3H]‐noradrenaline (NA) release, with the notable exception of tifluadom, which strongly inhibited release by activating μ‐receptors. The μ‐receptor‐mediated inhibitory effect of Tyr‐d‐Ala‐Gly‐(NMe)Phe‐Gly‐ol (DAMGO) on [3H]‐NA release was antagonized in a concentration‐dependent manner by bremazocine and EKC, but not by the other κ‐agonists. The pA2 value for bremazocine as an antagonist at the μ‐receptors involved was 8.2, compared to 8.6 for naloxone. 5 Thus, whereas U69593 and PD117302 display high potency and selectivity towards κ‐opioid receptors, the potent benzomorphan κ‐agonists bremazocine and EKC also appear to be strong μ‐opioid receptor antagonists.

Mifepristone prevents the expression of long-term behavioural sensitization to amphetamine

Three weeks following intermittent amphetamine exposure (2.5 mg/kg/day for 5 days), rats showed an enhanced locomotor response to an amphetamine challenge. Mifepristone (20 mg/kg) given 45 min prior to the challenge completely prevented the expression of amphetamine hyperresponsiveness. The glucocorticoid antagonist did not affect the locomotor response to amphetamine in drug-naive rats. These data demonstrate for the first time that glucocorticoid receptor antagonist treatment may prevent long-term hyperreactivity to drugs of abuse in individuals with a drug history.

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.