F. Hogenboom

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.

A single exposure to morphine induces long-lasting behavioural and neurochemical sensitization in rats

Repeated exposure to drugs of abuse causes persistent behavioural sensitization and associated adaptations in striatal neurotransmission, which is thought to play an important role in certain aspects of drug addiction. Remarkably, even a single exposure to psychostimulant drugs such as amphetamine or cocaine can be sufficient to elicit long‐lasting sensitization. The present study was designed to evaluate whether long‐lasting behavioural and neurochemical sensitization can also be evoked by a single exposure to morphine, an opiate drug of abuse. Rats were pretreated once with morphine (2, 10 or 30 mg/kg). Three weeks later, the locomotor effects of morphine and amphetamine, as well as the electrically evoked release of [3H]dopamine and [14C]acetylcholine from slices of nucleus accumbens and caudate–putamen, was assessed. In morphine‐pretreated rats, the psychomotor effects of morphine and amphetamine were sensitized. In addition, the electrically evoked release of [3H]dopamine and [14C]acetylcholine was augmented in slices of nucleus accumbens and caudate–putamen from morphine‐pretreated animals. Although the sensitization of the locomotor effect of morphine was less profound than previously observed after repeated intermittent morphine treatment, the enduring behavioural and neurochemical consequences of a single and repeated intermittent morphine treatment appear to be highly comparable. We therefore conclude that a single exposure to morphine induces long‐lasting behavioural sensitization and associated neuroadaptations.

Interactions between CB1 cannabinoid and μ opioid receptors mediating inhibition of neurotransmitter release in rat nucleus accumbens core

We examined the occurrence of functional interactions between CB1 cannabinoid and mu opioid receptors in the core of rat nucleus accumbens (NAc core). To that end, receptor-mediated inhibition of depolarization (4-aminopyridine)-induced [3H]glutamate release and glutamate (NMDA) receptor-stimulated [14C]acetylcholine (ACh) and [3H]GABA release was studied in superfused NAc core slices. The inhibitory effects of the mu receptor agonist morphine and the CB1 receptor agonist HU210 on the release of these neurotransmitters were selectively antagonized by the mu receptor antagonist naloxone and the CB1 receptor antagonist SR141716A, respectively. Surprisingly, naloxone prevented the antagonistic action of SR141716A at CB1 receptors and SR141716A abolished that of naloxone at mu receptors mediating inhibition of [3H]glutamate and [3H]GABA release. Therefore, these antagonists seem to allosterically interact, indicating the involvement of physically associated mu opioid and CB1 cannabinoid receptors. Such an interaction between antagonists was not observed at the receptors mediating inhibition of [14C]ACh release. Moreover, dose-response curves of the agonists showed that mu and CB1 receptors mediating inhibition of [3H]glutamate release display a non-additive interaction, whereas these receptors synergistically interact regarding their inhibitory control of [3H]GABA release. Finally, the apparent allosteric interaction between antagonists was also observed regarding the effects of other receptor-selective agonists and antagonists at mu opioid and CB1 cannabinoid receptors (mediating inhibition of NMDA-induced [3H]GABA release) and must therefore be a unique property of the receptors involved. These data suggest the existence of physically associated mu opioid and CB1 cannabinoid receptors, whereby activation of these receptors results in either a non-additive (glutamate release) or a synergistic (GABA release) effect. It is proposed that these allosterically interacting mu and CB1 receptors in the NAc core may represent G-protein coupled heterodimeric receptor complexes.

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.

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.

Insulin Modulates Cocaine-Sensitive Monoamine Transporter Function and Impulsive Behavior

Because insulin acutely enhances the function of dopamine transporters, the tyrosine kinase receptors activated by this hormone may modulate transporter-dependent neurochemical and behavioral effects of psychoactive drugs. In this respect, we examined the effects of insulin on exocytotic monoamine release and the efficacy of the monoamine transporter blocker cocaine in rat nucleus accumbens. Whereas insulin reduced electrically evoked exocytotic [3H]dopamine release in nucleus accumbens slices, the hormone potentiated the release-enhancing effect of cocaine thereon. The phosphatidylinositol 3-kinase inhibitor LY294002 abolished these effects, indicating the involvement of insulin receptors. Similar insulin effects were observed on the release of [3H]norepinephrine in nucleus accumbens slices, but not on that of [3H]serotonin, and were also apparent in medial prefrontal cortex slices. As might then be expected, insulin also potentiated the dopamine and norepinephrine release-enhancing effects of the selective monoamine uptake inhibitors GBR12909 and desmethylimipramine, respectively. In subsequent behavioral experiments, we investigated the role of insulin in motor impulsivity that depends on monoamine neurotransmission in the nucleus accumbens. Intracranial administration of insulin in the nucleus accumbens alone reduced premature responses in the five-choice serial reaction time task and enhanced the stimulatory effect of peripheral cocaine administration on impulsivity, resembling the observed neurochemical effects of the hormone. In contrast, cocaine-induced locomotor activity remained unchanged by intra-accumbal insulin application. These data reveal that insulin presynaptically regulates cocaine-sensitive monoamine transporter function in the nucleus accumbens and, as a consequence, impulsivity. Therefore, insulin signaling proteins may represent targets for the treatment of inhibitory control deficits such as addictive behaviors.

Lack of α2-adrenoceptor autoregulation of noradrenaline release in rat nucleus accumbens slices

Abstract The role of release-inhibitory α2-adrenoceptors on or near noradrenergic nerve terminals was investigated in slices of rat nucleus accumbens and medial prefrontal cortex, representing major projection areas of mesocorticolimbic dopamine neurons. As expected, the electrically evoked release of [3H]noradrenaline from superfused medial prefrontal cortex slices was strongly inhibited (to about 25% of control values) by exogeneous noradrenaline and by the α2-adrenoceptor agonist oxymetazoline, while blockade of α2-adrenoceptors with phentolamine or rauwolscine caused a large increase (to almost 250% of control values) in neurotransmitter release. In striking contrast and much to our surprise, these drugs did not at all affect the electrically evoked release of [3H]noradrenaline from nucleus accumbens slices. These results indicate that α2-adrenoceptor autoregulation of noradrenaline release may not be a major feature of noradrenergic varicosities in rat nucleus accumbens. Thus, the functioning or presence of these receptors may vary with the brain region innervated by locus coeruleus neurons and may not represent a ubiquitous feature of noradrenergic neurons.