Post-weaning A1/A2 β-casein milk intake modulates depressive-like behavior, brain μ-opioid receptors, and the metabolome of rats

The postnatal period is critical for brain and behavioral development and is sensitive to environmental stimuli, such as nutrition. Prevention of weaning from maternal milk was previously shown to cause depressive-like behavior in rats. Additionally, loss of dietary casein was found to act as a developmental trigger for a population of brain opioid receptors. Here, we explore the effect of exposure to milk containing A1 and A2 β-casein beyond weaning. A1 but not A2 β-casein milk significantly increased stress-induced immobility in rats, concomitant with an increased abundance of Clostridium histolyticum bacterial group in the caecum and colon of A1 β-casein fed animals, brain region-specific alterations of μ-opioid and oxytocin receptors, and modifications in urinary biochemical profiles. Moreover, urinary gut microbial metabolites strongly correlated with altered brain metabolites. These findings suggest that consumption of milk containing A1 β-casein beyond weaning age may affect mood via a possible gut-brain axis mechanism.

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Postnatal brain development is sensitive to nutritional exposures

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Consumption of A1 but not A2 β-casein milk post-weaning affects mood in rats

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Gut microbial, biochemical, and neurochemical changes accompany mood alterations

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Urinary gut microbial metabolites correlate with brain metabolites

Wheel running during chronic nicotine exposure is protective against mecamylamine-precipitated withdrawal and up-regulates hippocampal α7 nACh receptors in mice

BACKGROUND AND PURPOSE

Evidence suggests that exercise decreases nicotine withdrawal symptoms in humans; however, the mechanisms mediating this effect are unclear. We investigated, in a mouse model, the effect of exercise intensity during chronic nicotine exposure on nicotine withdrawal severity, binding of α4β2*, α7 nicotinic acetylcholine (nAChR), μ-opioid (μ receptors) and D2 dopamine receptors and on brain-derived neurotrophic factor (BDNF) and plasma corticosterone levels.

EXPERIMENTAL APPROACH

Male C57Bl/6J mice treated with nicotine (minipump, 24 mg·kg-1 ·day-1 ) or saline for 14 days underwent one of three concurrent exercise regimes: 24, 2 or 0 h·day-1 voluntary wheel running. Mecamylamine-precipitated withdrawal symptoms were assessed on day 14. Quantitative autoradiography of α4β2*, α7 nAChRs, μ receptors and D2 receptor binding was performed in brain sections of these mice. Plasma corticosterone and brain BDNF levels were also measured.

KEY RESULTS

Nicotine-treated mice undertaking 2 or 24 h·day-1 wheel running displayed a significant reduction in withdrawal symptom severity compared with the sedentary group. Wheel running induced a significant up-regulation of α7 nAChR binding in the CA2/3 area of the hippocampus of nicotine-treated mice. Neither exercise nor nicotine treatment affected μ or D2 receptor binding or BDNF levels. Nicotine withdrawal increased plasma corticosterone levels and α4β2* nAChR binding, irrespective of exercise regimen.

CONCLUSIONS AND IMPLICATIONS

We demonstrated for the first time a profound effect of exercise on α7 nAChRs in nicotine-dependent animals, irrespective of exercise intensity. These findings shed light onto the mechanism underlining the protective effect of exercise on the development of nicotine dependence.

LINKED ARTICLES

This article is part of a themed section on Nicotinic Acetylcholine Receptors. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v175.11/issuetoc.

Mu Opioid Receptors in Gamma-Aminobutyric Acidergic Forebrain Neurons Moderate Motivation for Heroin and Palatable Food

BACKGROUND

Mu opioid receptors (MORs) are central to pain control, drug reward, and addictive behaviors, but underlying circuit mechanisms have been poorly explored by genetic approaches. Here we investigate the contribution of MORs expressed in gamma-aminobutyric acidergic forebrain neurons to major biological effects of opiates, and also challenge the canonical disinhibition model of opiate reward.

METHODS

We used Dlx5/6-mediated recombination to create conditional Oprm1 mice in gamma-aminobutyric acidergic forebrain neurons. We characterized the genetic deletion by histology, electrophysiology, and microdialysis; probed neuronal activation by c-Fos immunohistochemistry and resting-state functional magnetic resonance imaging; and investigated main behavioral responses to opiates, including motivation to obtain heroin and palatable food.

RESULTS

Mutant mice showed MOR transcript deletion mainly in the striatum. In the ventral tegmental area, local MOR activity was intact, and reduced activity was only observed at the level of striatonigral afferents. Heroin-induced neuronal activation was modified at both sites, and whole-brain functional networks were altered in live animals. Morphine analgesia was not altered, and neither was physical dependence to chronic morphine. In contrast, locomotor effects of heroin were abolished, and heroin-induced catalepsy was increased. Place preference to heroin was not modified, but remarkably, motivation to obtain heroin and palatable food was enhanced in operant self-administration procedures.

CONCLUSIONS

Our study reveals dissociable MOR functions across mesocorticolimbic networks. Thus, beyond a well-established role in reward processing, operating at the level of local ventral tegmental area neurons, MORs also moderate motivation for appetitive stimuli within forebrain circuits that drive motivated behaviors.

Cocaine abstinence induces emotional impairment and brain region-specific upregulation of the oxytocin receptor binding

The key problem in treating cocaine addiction is the maintenance of a drug-free state as negative emotional symptoms during abstinence often trigger relapse. The mechanisms underpinning the emotional dysregulation during abstinence are currently not well-understood. There is evidence suggesting a role of the neuropeptide oxytocin in the modulation of drug addiction processes. However, its involvement during long-term abstinence from cocaine use remains unclear. In this study, we aimed to behaviourally characterize a mouse model of long-term cocaine withdrawal and assess the effect of chronic cocaine administration and long-term cocaine abstinence on the central oxytocinergic system and the hypothalamic-pituitary-adrenal axis. Fourteen-day escalating-dose cocaine administration (3 × 15-30 mg/kg/day) and 14-day withdrawal increased plasma corticosterone levels and oxytocin receptor (OTR) binding in piriform cortex, lateral septum and amygdala. A specific cocaine withdrawal-induced increase in OTR binding was observed in the medial septum. These biochemical alterations occurred concomitantly with the emergence of memory impairment, contextual psychomotor sensitization and an anhedonic and anxiogenic phenotype during withdrawal. Our study established a clear relationship between cocaine abstinence and emotional impairment in a novel translationally relevant model of cocaine withdrawal and demonstrated for the first time brain region-specific neuroadaptations of the oxytocin system, which may contribute to abstinence-induced negative emotional state.

A critical role of striatal A2A R-mGlu5 R interactions in modulating the psychomotor and drug-seeking effects of methamphetamine

Addiction to psychostimulants is a major public health problem with no available treatment. Adenosine A2A receptors (A2A R) co-localize with metabotropic glutamate 5 receptors (mGlu5 R) in the striatum and functionally interact to modulate behaviours induced by addictive substances, such as alcohol. Using genetic and pharmacological antagonism of A2A R in mice, we investigated whether A2A R-mGlu5 R interaction can regulate the locomotor, stereotypic and drug-seeking effect of methamphetamine and cocaine, two drugs that exhibit distinct mechanism of action. Genetic deletion of A2A R, as well as combined administration of sub-threshold doses of the selective A2A R antagonist (SCH 58261, 0.01 mg/kg, i.p.) with the mGlu5 R antagonist, 3-((2-methyl-4-thiazolyl)ethynyl)pyridine (0.01 mg/kg, i.p.), prevented methamphetamine- but not cocaine-induced hyperactivity and stereotypic rearing behaviour. This drug combination also prevented methamphetamine-rewarding effects in a conditioned-place preference paradigm. Moreover, mGlu5 R binding was reduced in the nucleus accumbens core of A2A R knockout (KO) mice supporting an interaction between these receptors in a brain region crucial in mediating addiction processes. Chronic methamphetamine, but not cocaine administration, resulted in a significant increase in striatal mGlu5 R binding in wild-type mice, which was absent in the A2A R KO mice. These data are in support of a critical role of striatal A2A R-mGlu5 R functional interaction in mediating the ambulatory, stereotypic and reinforcing effects of methamphetamine but not cocaine-induced hyperlocomotion or stereotypy. The present study highlights a distinct and selective mechanistic role for this receptor interaction in regulating methamphetamine-induced behaviours and suggests that combined antagonism of A2A R and mGlu5 R may represent a novel therapy for methamphetamine addiction.

Emotional Impairment and Persistent Upregulation of mGlu5 Receptor following Morphine Abstinence: Implications of an mGlu5-MOPr Interaction

BACKGROUND

A difficult problem in treating opioid addicts is the maintenance of a drug-free state because of the negative emotional symptoms associated with withdrawal, which may trigger relapse. Several lines of evidence suggest a role for the metabotropic glutamate receptor 5 in opioid addiction; however, its involvement during opioid withdrawal is not clear.

METHODS

Mice were treated with a 7-day escalating-dose morphine administration paradigm. Following withdrawal, the development of affective behaviors was assessed using the 3-chambered box, open-field, elevated plus-maze and forced-swim tests. Metabotropic glutamate receptor 5 autoradiographic binding was performed in mouse brains undergoing chronic morphine treatment and 7 days withdrawal. Moreover, since there is evidence showing direct effects of opioid drugs on the metabotropic glutamate receptor 5 system, the presence of an metabotropic glutamate receptor 5/μ-opioid receptor interaction was assessed by performing metabotropic glutamate receptor 5 autoradiographic binding in brains of mice lacking the μ-opioid receptor gene.

RESULTS

Withdrawal from chronic morphine administration induced anxiety-like, depressive-like, and impaired sociability behaviors concomitant with a marked upregulation of metabotropic glutamate receptor 5 binding. Administration of the metabotropic glutamate receptor 5 antagonist, 3-((2-Methyl-4-thiazolyl)ethynyl)pyridine, reversed morphine abstinence-induced depressive-like behaviors. A brain region-specific increase in metabotropic glutamate receptor 5 binding was observed in the nucleus accumbens shell, thalamus, hypothalamus, and amygdala of μ-opioid receptor knockout mice compared with controls.

CONCLUSIONS

These results suggest an association between metabotropic glutamate receptor 5 alterations and the emergence of opioid withdrawal-related affective behaviors. This study supports metabotropic glutamate receptor 5 system as a target for the development of pharmacotherapies for the treatment of opioid addiction. Moreover, our data show direct effects of μ-opioid receptor system manipulation on metabotropic glutamate receptor 5 binding in the brain.

Differential regulation of mGlu5 R and ΜOPr by priming- and cue-induced reinstatement of cocaine-seeking behaviour in mice

The key problem for the treatment of drug addiction is relapse to drug use after abstinence that can be triggered by drug-associated cues, re-exposure to the drug itself and stress. Understanding the neurobiological mechanisms underlying relapse is essential in order to develop effective pharmacotherapies for its prevention. Given the evidence implicating the metabotropic glutamate receptor 5 (mGlu5 R), μ-opioid receptor (MOPr), κ-opioid receptor (ΚOPr) and oxytocin receptor (OTR) systems in cocaine addiction and relapse, our aim was to assess the modulation of these receptors using a mouse model of cue- and priming-induced reinstatement of cocaine seeking. Male mice were trained to self-administer cocaine (1 mg/kg/infusion, i.v.) and were randomized into different groups: (1) cocaine self-administration; (2) cocaine extinction; (3) cocaine-primed (10 mg/kg i.p.); or (4) cue-induced reinstatement of cocaine seeking. Mice undergoing the same protocols but receiving saline instead of cocaine were used as controls. Quantitative autoradiography of mGlu5 R, MOPr, KOPr and OTR showed a persistent cocaine-induced upregulation of the mGlu5 R and OTR in the lateral septum and central amygdala, respectively. Moreover, a downregulation of mGlu5 R and MOPr was observed in the basolateral amygdala and striatum, respectively. Further, we showed that priming- but not cue-induced reinstatement upregulates mGlu5 R and MOPr binding in the nucleus accumbens core and basolateral amygdala, respectively, while cue- but not priming-induced reinstatement downregulates MOPr binding in caudate putamen and nucleus accumbens core. This is the first study to provide direct evidence of reinstatement-induced receptor alterations that are likely to contribute to the neurobiological mechanisms underpinning relapse to cocaine seeking.

GABAB receptor subtypes differentially modulate synaptic inhibition in the dentate gyrus to enhance granule cell output

BACKGROUND AND PURPOSE

Activation of GABAB receptors in the dentate gyrus (DG) enhances granule cell (GC) activity by reducing synaptic inhibition imposed by hilar interneurons. This disinhibitory action facilitates signal transfer from the perforant path to the hippocampus. However, as the two main molecular subtypes, GABA(B(1a,2)) and GABA(B(1b,2)) receptors, prefer axonal terminal and dendritic compartments, respectively, they may modulate the hilar pathways at different synaptic localizations. We examined their relative expression and functions in the DG.

EXPERIMENTAL APPROACH

The localization of GABAB subtypes was revealed immunohistochemically using subunit-selective antibodies in GABA(B1a)(-/-) and GABA(B1b)(-/-) mice. Effects of subtype activation by the GABAB receptor agonist, baclofen, were examined on the perforant path-stimulated GC population activities in brain slices.

KEY RESULTS

GABA(B(1a,2)) receptors were concentrated in the inner molecular layer, the neuropil of the hilus and hilar neurons at the border zone; while GABA(B(1b,2)) receptors dominated the outer molecular layer and hilar neurons in the deep layer, showing their differential localization on GC dendrite and in the hilus. Baclofen enhanced the GC population spike to a larger extent in the GABA(B1b)(-/-) mice, demonstrating exclusively disinhibitory roles of the GABA(B(1a,2)) receptors. Conversely, in the GABA(B1a)(-/-) mice baclofen not only enhanced but also inhibited the population spike during GABAA blockade, revealing both disinhibitory and inhibitory effects of GABA(B(1b,2)) receptors.

CONCLUSIONS AND IMPLICATIONS

The GABA(B(1a,2)) and GABA(B(1b,2)) receptor subtypes differentially modulate GC outputs via selective axonal terminal and dendritic locations in the hilar pathways. The GABA(B(1a,2)) receptors exclusively mediate disinhibition, thereby playing a greater role in gating signal transfer for hippocampal spatial and pattern learning.

Region-specific up-regulation of oxytocin receptor binding in the brain of mice following chronic nicotine administration

Nicotine addiction is considered to be the main preventable cause of death worldwide. While growing evidence indicates that the neurohypophysial peptide oxytocin can modulate the addictive properties of several abused drugs, the regulation of the oxytocinergic system following nicotine administration has so far received little attention. Here, we examined the effects of long-term nicotine or saline administration on the central oxytocinergic system using [(125)I]OVTA autoradiographic binding in mouse brain. Male, 7-week old C57BL6J mice were treated with either nicotine (7.8 mg/kg daily; rate of 0.5 μl per hour) or saline for a period of 14-days via osmotic minipumps. Chronic nicotine administration induced a marked region-specific upregulation of the oxytocin receptor binding in the amygdala, a brain region involved in stress and emotional regulation. These results provide direct evidence for nicotine-induced neuroadaptations in the oxytocinergic system, which may be involved in the modulation of nicotine-seeking as well as emotional consequence of chronic drug use.

Chronic nicotine improves short-term memory selectively in a G72 mouse model of schizophrenia

BACKGROUND AND PURPOSE

The prevalence of smoking in schizophrenia patients is exceptionally high; it is not known why but many researchers suggest that smoking constitutes a form of self-medication. Among the symptoms of schizophrenia that may be improved by nicotine are cognitive deficits. Hence, we studied the effects of long-term nicotine administration on cognition in a genetic animal model of schizophrenia susceptibility, G72-transgenic (G72Tg) mice.

EXPERIMENTAL APPROACH

The effect of long-term nicotine or saline, administered by osmotic minipumps, on different cognitive domains was assessed in G72Tg mice and controls using a battery of behavioural tests. To investigate the mechanism underlying phenotypic differences, quantitative autoradiographic mapping of nACh receptor subtypes was performed in forebrain structures to explore effects of chronic nicotine exposure on nACh receptor density in wild-type (WT) and G72Tg mice.

KEY RESULTS

Genotype significantly affected the cognitive effects of chronic nicotine administration. Whereas chronic nicotine disrupted cognitive performance in WT mice, it was effective at restoring impaired prepulse inhibition, working memory and social recognition in G72Tg mice. However, long-term spatial learning was further impaired by nicotine in transgenic animals. In contrast, associative learning was protected by G72-expression against the adverse nicotine effects seen in WT animals. G72-expression did not decisively influence nicotine-induced up-regulation of the α4β2*subtype, whereas α7nACh receptor density was differentially altered by genotype or by a genotype·treatment interaction in specific brain areas, most notably hippocampal subregions.

CONCLUSIONS AND IMPLICATIONS

Our data support the hypothesis that nicotine self-medication of schizophrenics improves cognitive symptoms, possibly by facilitating nicotine-induced α7nACh receptor activation in the hippocampus.

Knockout subtraction autoradiography: a novel ex vivo method to detect heteromers finds sparse KOP receptor/DOP receptor heterodimerization in the brain

Several methodological approaches suggest that receptor heteromers exist in cell systems, but their presence in physiological tissue is widely contentious. We describe a novel method to determine if heterodimers exist in brain tissue sections using autoradiographic binding comparisons from single and double gene knockout mice, where tissues either have a full receptor complement and can form heterodimers, or are incapable of making heterodimers. We have tested this model, which we have named Knockout Subtraction Autoradiography, to determine if heterodimerisation of the kappa (KOP) and delta opioid (DOP) receptors occurs, as evidence from binding studies in cell systems suggest they are present in the brain. Using labeling of putative KOP receptor/DOP receptor heterodimers with either [(3)H]bremazocine or with [(3)H]naltrindole, two ligands which were used to provide evidence suggesting that these opioid receptor subtypes heterodimerize, we have applied a subtraction equation model based on the principle that receptor gene double knockout of either MOP receptor/KOP receptor (DOP receptor expression only) or MOP receptor/DOP receptor (KOP receptor expression only) produces tissue incapable of making the KOP receptor/DOP receptor heterodimer. We have shown in most brain regions that the labeling fits a simple additive model of monomer labeling, but that in a few brain regions opioid receptor heterodimerization does occur. The data does not support the conclusion that KOP receptor/DOP receptor heterodimerisation is widespread in the central nervous system, but does indicate that this novel methodology can detect heterodimerisation, when ligands with distinct binding affinities for monomer and heterodimer forms exist.

Chronic methamphetamine treatment induces oxytocin receptor up-regulation in the amygdala and hypothalamus via an adenosine A2A receptor-independent mechanism

There is mounting evidence that the neuropeptide oxytocin is a possible candidate for the treatment of drug addiction. Oxytocin was shown to reduce methamphetamine self-administration, conditioned place-preference, hyperactivity and reinstatement in rodents, highlighting its potential for the management of methamphetamine addiction. Thus, we hypothesised that the central endogenous oxytocinergic system is dysregulated following chronic methamphetamine administration. We tested this hypothesis by examining the effect of chronic methamphetamine administration on oxytocin receptor density in mice brains with the use of quantitative receptor autoradiographic binding. Saline (4ml/kg/day, i.p.) or methamphetamine (1mg/kg/day, i.p.) was administered daily for 10 days to male, CD1 mice. Quantitative autoradiographic mapping of oxytocin receptors was carried out with the use of [(125)I]-vasotocin in brain sections of these animals. Chronic methamphetamine administration induced a region specific upregulation of oxytocin receptor density in the amygdala and hypothalamus, but not in the nucleus accumbens and caudate putamen. As there is evidence suggesting an involvement of central adenosine A2A receptors on central endogenous oxytocinergic function, we investigated whether these methamphetamine-induced oxytocinergic neuroadaptations are mediated via an A2A receptor-dependent mechanism. To test this hypothesis, autoradiographic oxytocin receptor binding was carried out in brain sections of male CD1 mice lacking A2A receptors which were chronically treated with methamphetamine (1mg/kg/day, i.p. for 10 days) or saline. Similar to wild-type animals, chronic methamphetamine administration induced a region-specific upregulation of oxytocin receptor binding in the amygdala and hypothalamus of A2A receptor knockout mice and no genotype effect was observed. These results indicate that chronic methamphetamine use can induce profound neuroadaptations of the oxytocinergic receptor system in brain regions associated with stress, emotionality and social bonding and that these neuroadaptations are independent on the presence of A2A receptors. These results may at least partly explain some of the behavioural consequences of chronic methamphetamine use.

Genetic deletion of the adenosine A(2A) receptor prevents nicotine-induced upregulation of α7, but not α4β2* nicotinic acetylcholine receptor binding in the brain

Considerable evidence indicates that adenosine A(2A) receptors (A(2A)Rs) modulate cholinergic neurotransmission, nicotinic acetylcholine receptor (nAChR) function, and nicotine-induced behavioural effects. To explore the interaction between A(2A) and nAChRs, we examined if the complete genetic deletion of adenosine A(2A)Rs in mice induces compensatory alterations in the binding of different nAChR subtypes, and whether the long-term effects of nicotine on nAChR regulation are altered in the absence of the A(2A)R gene. Quantitative autoradiography was used to measure cytisine-sensitive [¹²⁵I]epibatidine and [¹²⁵I]α-bungarotoxin binding to α4β2* and α7 nAChRs, respectively, in brain sections of drug-naïve (n = 6) or nicotine treated (n = 5-7), wild-type and adenosine A(2A)R knockout mice. Saline or nicotine (7.8 mg/kg/day; free-base weight) were administered to male CD1 mice via subcutaneous osmotic minipumps for a period of 14 days. Blood plasma levels of nicotine and cotinine were measured at the end of treatment. There were no compensatory developmental alterations in nAChR subtype distribution or density in drug-naïve A(2A)R knockout mice. In nicotine treated wild-type mice, both α4β2* and α7 nAChR binding sites were increased compared with saline treated controls. The genetic ablation of adenosine A(2A)Rs prevented nicotine-induced upregulation of α7 nAChRs, without affecting α4β2* receptor upregulation. This selective effect was observed at plasma levels of nicotine that were within the range reported for smokers (10-50 ng ml⁻¹). Our data highlight the involvement of adenosine A(2A)Rs in the mechanisms of nicotine-induced α7 nAChR upregulation, and identify A(2A)Rs as novel pharmacological targets for modulating the long-term effects of nicotine on α7 receptors.

The stereotypy-inducing and OCD-like effects of chronic 'binge' cocaine are modulated by distinct subtypes of nicotinic acetylcholine receptors

BACKGROUND AND PURPOSE

High rates of cigarette smoking occur in cocaine-dependent individuals, reflecting an involvement of nicotinic acetylcholine receptors (nAChRs) in cocaine-elicited behaviour. This study was designed to assess the contribution of different nAChR subtypes to the behavioural and neurochemical effects of chronic cocaine treatment.

EXPERIMENTAL APPROACH

Cocaine (15 mg·kg(-1) , i.p.) was administered to male C57BL/6J mice in a chronic 'binge' paradigm, with and without the coadministration of the α7 preferring nAChR antagonist methyllycaconitine (MLA; 5 mg·kg(-1) , i.p.) or the β2* nAChR antagonist dihydro-β-erythroidine (DHβE; 2 mg·kg(-1) , i.p.). Quantitative autoradiography was used to examine the effect of cocaine exposure on α7 and α4β2* nAChRs, and on the high-affinity choline transporter.

KEY RESULTS

MLA+cocaine administration induced an intense self-grooming behaviour, indicating a likely role for α7 nAChRs in modulating this anxiogenic, compulsive-like effect of cocaine. In the major island of Calleja, a key area of action for neuroleptics, MLA+cocaine reduced choline transporter binding compared with cocaine (with or without DHβE) administration. DHβE treatment prevented the induction of stereotypy sensitisation to cocaine but prolonged locomotor sensitisation, implicating heteromeric β2* nAChRs in the neuroadaptations mediating cocaine-induced behavioural sensitisation. 'Binge' cocaine treatment region-specifically increased α4β2* nAChR binding in the midbrain dopaminergic regions: ventral tegmental area and substantia nigra pars compacta.

CONCLUSIONS AND IMPLICATIONS

We have shown a differential, subtype-selective, contribution of nAChRs to the behavioural and neurochemical sequelae of chronic cocaine administration. These data support the clinical utility of targeting specific nAChR subtypes for the alleviation of cocaine-abuse symptomatology.

The endogenous opioid system in cocaine addiction: what lessons have opioid peptide and receptor knockout mice taught us?

Cocaine addiction has become a major concern in the UK as Britain tops the European 'league table' for cocaine abuse. Despite its devastating health and socio-economic consequences, no effective pharmacotherapy for treating cocaine addiction is available. Identifying neurochemical changes induced by repeated drug exposure is critical not only for understanding the transition from recreational drug use towards compulsive drug abuse but also for the development of novel targets for the treatment of the disease and especially for relapse prevention. This article focuses on the effects of chronic cocaine exposure and withdrawal on each of the endogenous opioid peptides and receptors in rodent models. In addition, we review the studies that utilized opioid peptide or receptor knockout mice in order to identify and/or clarify the role of different components of the opioid system in cocaine-addictive behaviours and in cocaine-induced alterations of brain neurochemistry. The review of these studies indicates a region-specific activation of the µ-opioid receptor system following chronic cocaine exposure, which may contribute towards the rewarding effect of the drug and possibly towards cocaine craving during withdrawal followed by relapse. Cocaine also causes a region-specific activation of the κ-opioid receptor/dynorphin system, which may antagonize the rewarding effect of the drug, and at the same time, contribute to the stress-inducing properties of the drug and the triggering of relapse. These conclusions have important implications for the development of effective pharmacotherapy for the treatment of cocaine addiction and the prevention of relapse.

In vivo dopaminergic and behavioral responses to acute cocaine are altered in adenosine A(2A) receptor knockout mice

Adenosine, acting on adenosine A(2A) receptors (A2ARs), regulates addictive processes induced by drugs of abuse. This study investigates the role of A(2A) adenosine receptors in neurochemical and behavioral responses to an acute cocaine challenge. Changes in the extracellular levels of dopamine (DA) in the nucleus accumbens (NAc) of mice lacking A(2A) adenosine receptors and wild type (WT) littermates after an acute cocaine (20 mg/kg) administration were evaluated by in vivo microdialysis studies. Locomotor effects induced by cocaine were measured during the microdialysis procedure. Cocaine-evoked increases in extracellular DA were not sustained in mice lacking A(2A) Rs in comparison with wild-type mice (P < 0.05). Cocaine administration significantly increased ambulatory activity in both genotypes. However, overall locomotor activity was further increased, whereas rest and small local movement measures were significantly attenuated in the A(2A) R knockout mice compared with WT littermates (P < 0.05). Our findings support an important role for adenosine A(2A) R in modulating the acute effects of cocaine, as demonstrated by the decrease in cocaine-evoked dopaminergic transmission in the NAc. Furthermore, the results support an important antagonistic role of A(2A) R in vivo in regulating psychostimulant-induced hyperlocomotion.

Differential region-specific regulation of α4β2* nAChRs by self-administered and non-contingent nicotine in C57BL/6J mice

Neuronal nAChR upregulation is the hallmark of chronic nicotine exposure. Neuroplasticity to abused drugs, however, depends on whether their administration is forced by the experimenter or is under the control of the experimental animal. Neuroadaptation to chronic nicotine self-administration was examined with a yoked-control paradigm, using nose-poking as the operating procedure. Freely moving C57BL/6J mice that responded for 0.03 mg/kg/infusion of intravenous nicotine under a continuous schedule of reinforcement (FR-1), had control over the rate and amount of drug intake that a yoked littermate passively received (n = 11). The impact of response dependency on neurobiological changes in nicotinic and dopaminergic systems was subsequently assessed using quantitative autoradiography. Cytisine-sensitive [(125)I]epibatidine binding, [³H]SCH23390, [³H]raclopride and [³H]mazindol were used to label nAChRs with α4β2* subtype properties, D1 and D2 dopaminergic receptors, and dopamine transporters, respectively. During a period of 12 days, self-administration was reliably initiated and maintained in animals receiving response-contingent nicotine. Region specific changes in the density of α4β2* nAChRs were found to be dependent on the contingency of nicotine treatment. Higher levels of α4β2* receptor binding were observed in the dorsal lateral geniculate nucleus and the ventral tegmental area of self-administering mice, compared to non-contingent animals. Moreover, response-independent increases in D2 binding were observed following chronic nicotine administration. No change in D1 and DAT binding was observed among groups. These findings indicate regional specific alterations in the regulation of the nicotinic cholinergic system following contingent and non-contingent nicotine exposure, and underline the importance of response dependency on the development of nicotine addiction.

Lack of genotype effect on D1, D2 receptors and dopamine transporter binding in triple MOP-, DOP-, and KOP-opioid receptor knockout mice of three different genetic backgrounds

We investigated D1, D2 receptors and dopamine transporter (DAT) binding levels in mice lacking all three opioid receptors and wild-type (WT) mice on three different genetic backgrounds. Quantitative autoradiography was used to determine the level of radioligand binding to the D1 and D2 receptors and DAT labeled with [(3)H]SCH23390, [(3)H]raclopride, and [(3)H]mazindol, respectively in triple-opioid receptor knockout (KO) and WT maintained on C57BL/6 (B6) and 129/SvEvTac (129) as well as C57BL/6 x 129/SvPas (B6 x 129) strains. No significant genotype effect was observed in D1, D2 receptors and DAT binding in any regions analyzed in any of the strains studied, suggesting that a lack of all three opioid receptors does not influence D1, D2 receptors and DAT expression, irrespective of their genetic strain background. However, strain differences were observed in D1 binding between the three strains of mice studied. Lower levels of D1 binding were observed in the substantia nigra of B6 x 129 WT mice compared with the 129 WT mice and in the olfactory tubercle of B6 x 129 WT compared with B6 WT and 129 WT mice. Lower levels of D1 binding were observed in the caudate putamen of B6 x 129 KO mice compared with 129 KO mice. In contrast, no significant strain differences were observed in D2 and DAT binding between the three strains of mice in any regions analyzed. Overall, these results indicate a lack of modulation of the dopaminergic system by the deletion of all three opioid receptors regardless of different background strains.

Mouse strain differences in locomotor, sensitisation and rewarding effect of heroin; association with alterations in MOP-r activation and dopamine transporter binding

There is growing agreement that genetic factors play an important role in the risk to develop heroin addiction, and comparisons of heroin addiction vulnerability in inbred strains of mice could provide useful information on the question of individual vulnerability to heroin addiction. This study examined the rewarding and locomotor-stimulating effects of heroin in male C57BL/6J and DBA/2J mice. Heroin induced locomotion and sensitisation in C57BL/6J but not in DBA/2J mice. C57BL/6J mice developed conditioned place preference (CPP) to the highest doses of heroin, while DBA/2J showed CPP to only the lowest heroin doses, indicating a higher sensitivity of DBA/2J mice to the rewarding properties of heroin vs C57BL/6J mice. In order to investigate the neurobiological substrate underlying some of these differences, the effect of chronic 'intermittent' escalating dose heroin administration on the opioid, dopaminergic and stress systems was explored. Twofold higher mu-opioid receptor (MOP-r)-stimulated [35S]GTPgammaS binding was observed in the nucleus accumbens and caudate of saline-treated C57BL/6J mice compared with DBA/2J. Heroin decreased MOP-r density in brain regions of C57BL/6J mice, but not in DBA/2J. A higher density of dopamine transporters (DAT) was observed in nucleus accumbens shell and caudate of heroin-treated DBA/2J mice compared with heroin-treated C57BL/6J. There were no effects on D1 and D2 binding. Chronic heroin administration decreased corticosterone levels in both strains with no effect of strain. These results suggest that genetic differences in MOP-r activation and DAT expression may be responsible for individual differences in vulnerability to heroin addiction.

Chronic cocaine-induced cardiac oxidative stress and mitogen-activated protein kinase activation: the role of Nox2 oxidase

Chronic cocaine exposure is associated with severe cardiac complications, but the mechanisms of cocaine cardiotoxicity remain unclear, and current therapies are unsatisfactory. We investigated the hypothesis of oxidative stress-mediated cardiotoxicity and the role of NADPH oxidase in this process in a mouse model of chronic escalating "binge" cocaine administration (milligrams per kilogram): days 1 to 4 at 3 x 15 mg, days 5 to 8 at 3 x 20 mg, days 9 to 12 at 3 x 25 mg, and days 13 to 14 at 3 x 30 mg. Compared with vehicle controls, chronic binge cocaine administration significantly increased the cardiac NADPH-dependent O(2)(.) production (1.96- +/- 0.4-fold) as detected by tiron (an O(2)(.) scavenger)-inhibitable lucigenin chemiluminescence and dihydroethidium fluorescence. Cocaine-induced reactive oxygen species (ROS) production was associated with significant increases ( approximately 2-fold) in the protein expressions of Nox2 (an isoform of NADPH oxidase) and its regulatory subunits: p22(phox), p67(phox), p47(phox), p40(phox), and Rac1, and in p47(phox) phosphorylation as detected by immunoblotting (all p < 0.03). Increased Nox2 activity was accompanied by the activation of extracellular signal-regulated kinase 1/2, p38 mitogen-activated protein kinase (MAPK), and c-Jun NH(2)-terminal kinase, notably in the cardiomyocytes. Cell culture experiments revealed that cocaine-induced ROS production was primarily a direct action of cocaine on cardiac myocytes, which caused severe oxidative damage to myocytes and cell death as detected by terminal deoxynucleotidyl transferase dUTP nick-end labeling assay. These could be inhibited by inhibitors to protein kinase C (bisindolymaleimide) or by depletion of Nox2 using small interfering RNA. In conclusion, chronic cocaine administration directly causes severe myocardial oxidative stress through the activation of Nox2 oxidase. Increased ROS production contributes to MAPK activation and the subsequent myocyte damage. Inhibitors to NADPH oxidase or antioxidants may have therapeutic potential in the treatment of cocaine cardiotoxicity.

Decrease of D2 receptor binding but increase in D2-stimulated G-protein activation, dopamine transporter binding and behavioural sensitization in brains of mice treated with a chronic escalating dose 'binge' cocaine administration paradigm

Understanding the neurobiology of the transition from initial drug use to excessive drug use has been a challenge in drug addiction. We examined the effect of chronic 'binge' escalating dose cocaine administration, which mimics human compulsive drug use, on behavioural responses and the dopaminergic system of mice and compared it with a chronic steady dose (3 x 15 mg/kg/day) 'binge' cocaine administration paradigm. Male C57BL/6J mice were injected with saline or cocaine in an escalating dose paradigm for 14 days. Locomotor and stereotypy activity were measured and quantitative autoradiographic mapping of D(1) and D(2) receptors, dopamine transporters and D(2)-stimulated [(35)S]GTPgammaS binding was performed in the brains of mice treated with this escalating and steady dose paradigm. An initial sensitization to the locomotor effects of cocaine followed by a dose-dependent increase in the duration of the locomotor effect of cocaine was observed in the escalating but not the steady dose paradigm. Sensitization to the stereotypy effect of cocaine and an increase in cocaine-induced stereotypy score was observed from 3 x 20 to 3 x 25 mg/kg/day cocaine. There was a significant decrease in D(2) receptor density, but an increase in D(2)-stimulated G-protein activity and dopamine transporter density in the striatum of cocaine-treated mice, which was not observed in our steady dose paradigm. Our results document that chronic 'binge' escalating dose cocaine treatment triggers profound behavioural and neurochemical changes in the dopaminergic system, which might underlie the transition from drug use to compulsive drug use associated with addiction, which is a process of escalation.

Behavioural and biochemical responses to morphine associated with its motivational properties are altered in adenosine A(2A) receptor knockout mice

BACKGROUND AND PURPOSE

The purinergic system through the A(2A) adenosine receptor regulates addiction induced by different drugs of abuse. The aim of the present study was to investigate the specific role of A(2A) adenosine receptors (A(2A)Rs) in the behavioural and neurochemical responses to morphine associated with its motivational properties.

EXPERIMENTAL APPROACH

Mice lacking A(2A)Rs (A(2A) knockout (KO) mice) and wild-type littermates were used to evaluate behavioural responses induced by morphine. Antinociception was assessed using the tail-immersion and the hot-plate tests. Place-conditioning paradigms were used to evaluate the rewarding effects of morphine and the dysphoric responses of morphine withdrawal. Microdialysis studies were carried out to evaluate changes in the extracellular levels of dopamine in the nucleus accumbens of A(2A) KO mice after morphine administration.

KEY RESULTS

The acute administration of morphine induced a similar enhancement of locomotor activity and antinociceptive responses in both genotypes. However, the rewarding effects induced by morphine were completely blocked in A(2A) KO mice. Also, naloxone did not induce place aversion in animals lacking the A(2A)Rs.

CONCLUSIONS AND IMPLICATIONS

Our findings demonstrate that the rewarding and aversive effects associated with morphine abstinence were abolished in A(2A) KO mice, supporting a differential role of the A(2A) adenosine receptor in the somatic and motivational effects of morphine addiction. This study provides evidence for the role of A(2A)Rs as general modulators of the motivational properties of drugs of abuse. Pharmacological manipulation of these receptors may represent a new target in the management of drug addiction.

Adenosine A2A receptor signaling regulation of cardiac NADPH oxidase activity

Cardiac tissues express constitutively an NADPH oxidase, which generates reactive oxygen species (ROS) and is involved in redox signaling. Myocardial metabolism generates abundant adenosine, which binds to its receptors and plays important roles in cardiac function. The adenosine A2A receptor (A2AR) has been found to be expressed in cardiac myocytes and coronary endothelial cells. However, the role of the A2AR in the regulation of cardiac ROS production remains unknown. We found that knockout of A2AR significantly decreased (39+/-8%) NADPH-dependent O2- production in mouse hearts compared to age (10 weeks)-matched wild-type controls. This was accompanied by a significant decrease in Nox2 (a catalytic subunit of NADPH oxidase) protein expression, and down-regulation of ERK1/2, p38MAPK, and JNK phosphorylation (all P<0.05). In wild-type mice, intraperitoneal injection of the selective A2AR antagonist SCH58261 (3-10 mg/kg body weight for 90 min) inhibited phosphorylation of p47phox (a regulatory subunit of Nox2), which was accompanied by a down-regulated cardiac ROS production (48+/-8%), and decreased JNK and ERK1/2 activation by 54+/-28% (all P<0.05). In conclusion, A2AR through MAPK signaling regulates p47phox phosphorylation and cardiac ROS production by NADPH oxidase. Modulation of A2AR activity may have potential therapeutic applications in controlling ROS production by NADPH oxidase in the heart.

Adenosine A2A receptors in ventral striatum, hypothalamus and nociceptive circuitry implications for drug addiction, sleep and pain

Adenosine A2A receptors localized in the dorsal striatum are considered as a new target for the development of antiparkinsonian drugs. Co-administration of A2A receptor antagonists has shown a significant improvement of the effects of l-DOPA. The present review emphasizes the possible application of A2A receptor antagonists in pathological conditions other than parkinsonism, including drug addiction, sleep disorders and pain. In addition to the dorsal striatum, the ventral striatum (nucleus accumbens) contains a high density of A2A receptors, which presynaptically and postsynaptically regulate glutamatergic transmission in the cortical glutamatergic projections to the nucleus accumbens. It is currently believed that molecular adaptations of the cortico-accumbens glutamatergic synapses are involved in compulsive drug seeking and relapse. Here we review recent experimental evidence suggesting that A2A antagonists could become new therapeutic agents for drug addiction. Morphological and functional studies have identified lower levels of A2A receptors in brain areas other than the striatum, such as the ventrolateral preoptic area of the hypothalamus, where adenosine plays an important role in sleep regulation. Although initially believed to be mostly dependent on A1 receptors, here we review recent studies that demonstrate that the somnogenic effects of adenosine are largely mediated by hypothalamic A2A receptors. A2A)receptor antagonists could therefore be considered as a possible treatment for narcolepsy and other sleep-related disorders. Finally, nociception is another adenosine-regulated neural function previously thought to mostly involve A1 receptors. Although there is some conflicting literature on the effects of agonists and antagonists, which may partly be due to the lack of selectivity of available drugs, the studies in A2A receptor knockout mice suggest that A2A receptor antagonists might have some therapeutic potential in pain states, in particular where high intensity stimuli are prevalent.

Digital autoradiography using room temperature CCD and CMOS imaging technology

CCD (charged coupled device) and CMOS imaging technologies can be applied to thin tissue autoradiography as potential imaging alternatives to using conventional film. In this work, we compare two particular devices: a CCD operating in slow scan mode and a CMOS-based active pixel sensor, operating at near video rates. Both imaging sensors have been operated at room temperature using direct irradiation with images produced from calibrated microscales and radiolabelled tissue samples. We also compare these digital image sensor technologies with the use of conventional film. We show comparative results obtained with (14)C calibrated microscales and (35)S radiolabelled tissue sections. We also present the first results of (3)H images produced under direct irradiation of a CCD sensor operating at room temperature. Compared to film, silicon-based imaging technologies exhibit enhanced sensitivity, dynamic range and linearity.

Reduced response to the formalin test and lowered spinal NMDA glutamate receptor binding in adenosine A2A receptor knockout mice

Adenosine is a neuromodulator with complex effects on pain pathways. Mice lacking the adenosine A2A receptor are hypoalgesic, and have altered analgesic responses to receptor-selective opioid agonists. These and other findings suggest a role for the adenosine A2A receptor in sensitizing afferent fibres projecting to the spinal cord. To test this hypothesis formalin (20 microl, 5%) was injected into the paw and nociceptive responses were measured in wildtype and adenosine A2A receptor knockout mice. There was a significant reduction in nociception associated with sensory nerve activation in the knockout mice as measured by time spent biting/licking the formalin-injected paw and number of flinches seen during the first phase, but only the number of flinches was reduced during the second inflammatory phase. In addition, the selective adenosine A2A antagonist SCH58261 (3 and 10 mg/kg) also antagonised both phases of the formalin test. We also labelled NMDA glutamate and NK1 receptors in spinal cord sections as an indirect measure of nociceptive transmission from peripheral sites to the spinal cord. [3H]-Substance P binding to NK1 receptors was unaltered but there was a substantial reduction in binding of [3H]-MK801 to NMDA glutamate receptors in all regions of the spinal cord from knockout mice. The decrease in NMDA glutamate receptor binding may reflect reduced peripheral sensory input to the spinal cord during development and could relate to the hypoalgesia in this genotype. These results support a key role for the adenosine A2A receptor in peripheral nociceptive pathways.

Preprodynorphin mediates locomotion and D2 dopamine and mu-opioid receptor changes induced by chronic 'binge' cocaine administration

Evidence suggests that the kappa-opioid receptor (KOP-r) system plays an important role in cocaine addiction. Indeed, cocaine induces endogenous KOP activity, which is a mechanism that opposes alterations in behaviour and brain function resulting from repeated cocaine use. In this study, we have examined the influence of deletion of preprodynorphin (ppDYN) on cocaine-induced behavioural effects and on hypothalamic-pituitary-adrenal axis activity. Furthermore, we have measured mu-opioid receptor (MOP-r) agonist-stimulated [(35)S]GTPgammaS, dopamine D(1), D(2) receptor and dopamine transporter (DAT) binding. Male wild-type (WT) and ppDYN knockout (KO) mice were injected with saline or cocaine (45 mg/kg/day) in a 'binge' administration paradigm for 14 days. Chronic cocaine produced an enhancement of locomotor sensitisation in KO. No genotype effect was found on stereotypy behaviour. Cocaine-enhanced MOP-r activation in WT but not in KO. There was an overall decrease in D(2) receptor binding in cocaine-treated KO but not in WT mice. No changes were observed in D(1) and DAT binding. Cocaine increased plasma corticosterone levels in WT but not in KO. The data confirms that the endogenous KOP system inhibits dopamine neurotransmission and that ppDYN may mediate the enhancement of MOP-r activity and the activation of the hypothalamic-pituitary-adrenal axis after chronic cocaine treatment.

Upregulation of opioid receptor binding following spontaneous epileptic seizures

Animal and limited human data suggest an important anticonvulsant role for opioid peptides and their receptors. We aimed to provide direct human in vivo evidence for changes in opioid receptor availability following spontaneous seizures. We scanned nine patients within hours of spontaneous temporal lobe seizures and compared their postictal binding of the non-subtype selective opioid receptor PET radioligand [11C]diprenorphine (DPN), quantified as a volume-of-distribution (VD), with interictal binding and with binding changes in 14 healthy controls, controlling for a range of behavioural variables associated with opioid action. A regionally specific increase of opioid receptor availability was evident in the temporal pole and fusiform gyrus ipsilateral to the seizure focus following seizures (Z 5.01, P < 0.001, 16 432 mm3). Within this region, there was a negative correlation between VD and log10 time since last seizure (r = -0.53, P < 0.03), compatible with an early increase and gradual return to baseline. [11C]DPN VD did not undergo systematic changes between time points in controls. This study provides direct human in vivo evidence for changes in opioid receptor availability over a time course of hours following spontaneous seizures, emphasizing an important role of the opioid system in seizure control.

Paracetamol inhibits nitric oxide synthesis in murine spinal cord slices

Paracetamol is an effective analgesic but its mechanism of action is unclear. We investigated the effect of paracetamol and the analgesic adjuvant caffeine on the activity of NO synthase in mouse spinal cord and cerebellar slices in vitro, by measuring the conversion of [(3)H]arginine to [(3)H]citrulline. Paracetamol (100 microM) had no effect on NO synthase activity in cerebellum, but in the spinal cord both paracetamol (100 microM) and caffeine (30 microM) attenuated glutamate (5 mM)-induced [(3)H]citrulline production and in combination they abolished it. In conclusion paracetamol inhibits spinal cord NO synthesis and this may be related to its analgesic effects.

Has the sun set on kappa3-opioid receptors?

Mu-opioid receptor agonists are a mainstay of clinical analgesia, despite the significant unwanted effects and dependence liability associated with drugs like morphine. The quest for opioids that produce analgesia with fewer undesirable effects has lead to the putative identification of multiple opioid receptor subtypes, despite the identification of only four opioid-related receptor genes. One such putative receptor subtype is the kappa3 receptor, activation of which supposedly produces analgesia in animals. In the present issue of this Journal, Olianas and co-workers have demonstrated that the prototypic kappa3 agonist naloxone benzoylhydrazone is actually a partial agonist at the cloned mu, delta, and kappa opioid receptors and an antagonist at opioid-like NOP receptors. Together with a recent study that showed that high-affinity naloxone benzoylhydrazone binding is abolished in triple mu/delta/kappa receptor knockout mice, the present study provides strong evidence that in vivo effects attributed to kappa3 receptor activation probably just reflect the combined actions of a particularly nonselective opioid drug. Indeed, molecular identification of any of the proposed subtypes of mu, delta, and kappa opioid receptors has proven elusive, suggesting that it is perhaps time to retire the notion of opioid receptor subtypes until definitive evidence for their existence is provided.

Modulation of paracetamol antinociception by caffeine and by selective adenosine A2 receptor antagonists in mice

This study investigated the involvement of adenosine receptors in the interaction between paracetamol and caffeine in mice, using the adenosine A2A receptor antagonist 5-amino-7-(2-phenylethyl)-2-(2-furyl)pyrazolo[4,3-e]-1,2,4-triazolo[1,5-c]pyrimidine (SCH58261) and the adenosine A2B receptor antagonist 1-propyl-8-p-sulfophenylxanthine (PSB1115), in the tail immersion and hot-plate tests. Paracetamol (10-200 mg/kg) was antinociceptive in both tests, but, in contrast to previous studies, caffeine (10 mg/kg) was pronociceptive in the tail immersion test, and reduced the effects of paracetamol in both tests. SCH58261 (3 mg/kg) was antinociceptive in both tests and in its presence paracetamol (50 mg/kg) had no further effect. PSB1115 (10 mg/kg) had little effect alone but potentiated the effect of paracetamol (50 mg/kg) in the hot-plate test and abolished it in the tail immersion test. These results suggest that adenosine A2B receptors may be involved in the action of paracetamol in a pathway-dependent manner, and also support the existence of pronociceptive adenosine A2A receptors.

Interaction with vesicle luminal protachykinin regulates surface expression of delta-opioid receptors and opioid analgesia

Opioid and tachykinin systems are involved in modulation of pain transmission in the spinal cord. Regulation of surface opioid receptors on nociceptive afferents is critical for opioid analgesia. Plasma-membrane insertion of delta-opioid receptors (DORs) is induced by stimulus-triggered exocytosis of DOR-containing large dense-core vesicles (LDCVs), but how DORs become sorted into the regulated secretory pathway is unknown. Here we report that direct interaction between protachykinin and DOR is responsible for sorting of DORs into LDCVs, allowing stimulus-induced surface insertion of DORs and DOR-mediated spinal analgesia. This interaction is mediated by the substance P domain of protachykinin and the third luminal domain of DOR. Furthermore, deletion of the preprotachykinin A gene reduced stimulus-induced surface insertion of DORs and abolished DOR-mediated spinal analgesia and morphine tolerance. Thus, protachykinin is essential for modulation of the sensitivity of nociceptive afferents to opioids, and the opioid and tachykinin systems are directly linked by protachykinin/DOR interaction.

An investigation of binding sites for paracetamol in the mouse brain and spinal cord

Quantitative autoradiography has been used to assess whether [3H]paracetamol (3 microM) binds specifically to any area of the murine brain and spinal cord and to investigate whether paracetamol (1-100 microM) competes for binding to the nociceptin opioid peptide (NOP) receptor or to the nitrobenzylthioinosine (NBTI)-sensitive adenosine transporter in the brains of mice. [3H]paracetamol binding was homogenous and, although there was some indication of specific binding overall, this binding in most individual regions failed to reach statistical significance. However, thoracic segments of the spinal cord were found to have significantly higher specific binding than cervical and lumbar regions. Paracetamol did not significantly compete for binding to the NOP receptor or to the NBTI-sensitive adenosine transporter, showing that it does not mediate its effect via these sites. Although paracetamol did bind specifically to the murine brain and spinal cord, the binding was not region-specific, suggesting binding is not related to any particular neurotransmitter system.

Autoradiography in opioid triple knockout mice reveals opioid and opioid receptor like binding of naloxone benzoylhydrazone

Naloxone benzoylhydrazone (NalBzoH) is a ligand used to study opioid receptors. It has been suggested to act at a novel kappa3 receptor but also appears to bind to classical opioid receptors, and possibly the ORL1 receptor. We have used opioid receptor triple knockout mice, deficient in genes coding for the mu, delta and kappa-receptor, to characterise the relative contributions of opioid and ORL1 activity to the binding of this ligand, by carrying out receptor autoradiography with [3H]NalBzoH. As competing ligands we have used diprenorphine and nociceptin at 1 microM, alone or in combination, to determine the contribution of opioid and ORL1 receptor binding. At 4 nM [3H]NalBzoH showed labelling in wild-type brains indicative of broad spectrum classical opioid receptor binding. In the triple knockout brains all labelling was completely absent, suggesting that at this concentration there is no binding to ORL1 sites. However at 50 nM [3H]NalBzoH showed labelling in triple knockout brains with a distribution pattern indicative of ORL1 labelling. Quantitative analysis showed that nociceptin displaced typically 30% of the residual labelling in knockout brains whilst diprenorphine had relatively little effect. The data show that at 50 nM NalBzoH no binding was detected other than to classical opioid receptors or to ORL1 in an approximate ratio of 2:1.

Characterization of [3H]ZM 241385 binding in wild-type and adenosine A2A receptor knockout mice

The binding of the adenosine A(2A) receptor antagonist [3H] 4-(2-[7-amino-2-(2-furyl)[1,2,4]-triazolo[2,3-a][1,3,5]triazin-5-ylamino]ethyl)phenol ([3H]ZM 241385) to mouse brain and spinal cord was investigated. In brain homogenates, single-site binding was observed with a Bmax of 299+/-28 fmol mg(-1) protein and a Kd of 0.75+/-0.08 nM. In autoradiographic studies, there was a high density of specific binding of [3H]ZM 241385 in the striatum, with a very low density in the cortex and no binding elsewhere in the brain or in the spinal cord. All specific binding of [3H]ZM 241385 was lost in genetically modified mice lacking the adenosine A(2A) receptor, confirming the selectivity of this radioligand.

Quantitative autoradiography of adenosine receptors and NBTI-sensitive adenosine transporters in the brains of mice deficient in the preproenkephalin gene

There is a large body of evidence indicating important interactions between the adenosine and the opioid systems in regulating pain, opioid dependence and withdrawal. Mice lacking the proenkephalin gene and therefore lacking the endogenous enkephalin peptides have been successfully developed and exhibit decreased locomotor activity, are hyperalgesic and show enhanced anxiety and aggression. In addition, an upregulation of mu and delta receptors was also observed in the brains of knockout mice. To investigate if there are any compensatory alterations in adenosine systems in the brains of mutant mice, we have carried out quantitative autoradiographic mapping of A(1) and A(2A) adenosine receptors and nitrobenzylthioinosine (NBTI)-sensitive adenosine transporters in the brains of wild-type and homozygous enkephalin knockout mice. Adjacent coronal brain sections were cut from brains of +/+ and -/- mice for the determination of binding of [(3)H]1,3-dipropyl-8-cyclopentylxanthine ([(3)H]DPCPX), [(3)H]2-[p-(2-carbonylethyl)phenylethylamino]-5'-N-ethylcarboxamidoadenosine ([(3)H]CGS21680) or [(3)H]NBTI to A(1) and A(2A) adenosine receptors and NBTI-sensitive adenosine transporters, respectively. A small but significant increase in [(3)H]DPCPX and [(3)H]NBTI binding but no significant change in [(3)H]CGS21680 binding was detected in enkephalin knockout brains. The results provide further evidence of functional interactions in the brain between opioid receptors and A(1) adenosine receptors as well as NBTI-sensitive adenosine transporters but not A(2A) receptors.

International Narcotics Research Conference-35th Meeting. Current developments in medicinal chemistry in the opioid field 17-24 July 2004, Kyoto, Japan

Research in the opioid field underwent a lull in the 1990's but much new research has recently been stimulated, firstly by the cloning of all of the opioid receptors and then by the development of gene knockout mice and their phenotype characterization. These developments have led to a a reappraisal of the potential utility of opioid agonists and antagonists not only for the treatment of pain but also for mood-related conditions and peripheral indications. Medicinal chemistry groups are still moving the field forward, and the pharmaceutical industry continues to keep a watching brief. Opioids are no longer exclusively the province of pain scientists, and potential for treatment of limbic disorders, itch and addiction are only now beginning to be considered seriously.

Enhanced morphine withdrawal and micro -opioid receptor G-protein coupling in A2A adenosine receptor knockout mice

Much evidence supports the hypothesis that A2A adenosine receptors play an important role in the expression of morphine withdrawal and that the dopaminergic system might also be involved. We have evaluated morphine withdrawal signs in wild-type and A2A receptor knockout mice and shown a significant enhancement in some withdrawal signs in the knockout mice. In addition, micro -opioid and dopamine D2 receptor autoradiography, as well as micro -opioid receptor-stimulated guanylyl 5'-[gamma-[35S]thio]-triphosphate ([35S]GTPgammaS) autoradiography was carried out in brain sections of withdrawn wild-type and knockout mice. No significant changes in D2 and micro -opioid receptor binding were observed in any of the brain regions analysed. However, a significant increase in the level of micro receptor-stimulated [35S]GTPgammaS binding was observed in the nucleus accumbens of withdrawn knockout mice. These data indicate that the A2A receptor plays a role in opioid withdrawal related to functional receptor activation.

Region selective up-regulation of micro-, delta- and kappa-opioid receptors but not opioid receptor-like 1 receptors in the brains of enkephalin and dynorphin knockout mice

The role of endogenous opioid peptides and receptors has recently been investigated using knockout mice. Although the affinities of opioid peptides for opioid receptors has been known for many years there is still some uncertainty over which receptor is the endogenous target for each peptide. To address this issue we have studied using quantitative autoradiography the levels of all four opioid receptor subtypes (micro, delta, kappa and opioid receptor-like 1 [ORL1]) in brains sectioned from enkephalin and dynorphin knockouts, as well as from double knockouts. Because receptor up-regulation has been observed when its cognate ligand-peptide is genetically ablated, regional changes in receptor binding in knockout mice may reflect areas where the peptide ligand is tonically active at its receptor or played a role in receptor regulation. In addition, the study aimed to correlate previously observed behaviour in these animals with receptor modulation. Marked region-specific up-regulation of the micro, delta, and kappa opioid receptors but not ORL1 receptors was observed in proenkephalin and prodynorphin knockouts. In proenkephalin knockouts this was most pronounced for the micro- and delta-receptor and in prodynorphin knockouts for the kappa-receptor. Combinatorial double knockouts did not show any changes in addition to those observed in single knockouts. The largest changes were observed in limbic regions and our results suggest that proenkephalin peptides are tonically active at micro and delta-receptors predominantly in these areas. Prodynorphin peptides appear to regulate mostly the kappa-receptor but they are also modulators of micro- and delta-receptors.

Quantitative autoradiography of dopamine receptors in the brains of μ-opioid receptor knockout mice

Given the existence of functional interactions between opioidergic and dopaminergic systems, we have analyzed by quantitative autoradiography the possible long-term adaptive changes in the expression of D(1)- and D(2)-like dopamine receptors in the brains of mice lacking the micro-opioid receptor gene. An overall significant increase in D(1) and D(2) receptors (7.4 and 12.6%, respectively) across all cerebral regions examined was obtained in mutant mice relative to wild-type mice. However, region by region comparisons failed to reach significance in any individual brain area. These results indicate that only moderate changes in D(1)- and D(2)-like dopamine receptors densities occur in the brains of micro-opioid receptor knockout mice.

Effects of single and dual administration of cocaine and ethanol on opioid and ORL1 receptor expression in rat CNS: an autoradiographic study

The co-abuse of cocaine and ethanol is common among human addicts and has been reported to produce a stronger increase of euphoria as compared to either drug given alone. Both cocaine and ethanol increase the extracellular dopamine concentration in the nucleus accumbens, a terminal region in the mesolimbic dopamine pathway. In addition, both cocaine and ethanol affect the endogenous opioid system, which in turn alters the activity of the mesolimbic dopamine pathway. We have carried out quantitative autoradiography mapping of the opioid receptors as well as the opioid receptor-like 1 receptor in the brains of rats treated with both single and dual cocaine and ethanol. Rats received acute cocaine, ethanol or both drugs in combination. Ethanol alone or in combination with cocaine modulated the receptor densities in rat central nervous system. The kappa receptor densities were generally decreased, while both the mu and the opioid receptor-like 1 receptors were up-regulated. The mu opioid receptor levels were mainly increased in non-cortical regions, whereas the opioid receptor-like 1 receptors were increased in cortical structures. No changes in delta opioid receptors were observed. Cocaine alone did not influence the receptor levels in any of the treatment groups.

Quantitative autoradiography of adenosine receptors in brains of chronic naltrexone-treated mice

1. Manipulation of micro opioid receptor expression either by chronic morphine treatment or by deletion of the gene encoding micro opioid receptors leads to changes in adenosine receptor expression. Chronic administration of the opioid receptor antagonist naltrexone leads to upregulation of micro receptor binding in the brain. 2. To investigate if there are any compensatory alterations in adenosine systems in the brains of chronic naltrexone-treated mice, we carried out quantitative autoradiographic mapping of A(1) and A(2A) adenosine receptors in the brains of mice treated for 1 week with naltrexone (8 mg(-1) kg(-1) day(-1)), administered subcutaneously via osmotic minipump. 3. Adjacent coronal brain sections were cut from chronic saline- and naltrexone-treated mice for the determination of binding of [(3)H] D-Ala(2)-MePhe(4)-Gly-ol(5) enkephalin ([(3)H] DAMGO), [(3)H]1,3-dipropyl-8-cyclopentylxanthine ([(3)H] DPCPX) or [(3)H] 2-[p-(2-carbonylethyl)phenylethylamino]-5'-N-ethylcarboxamidoadenosine ([(3)H] CGS21680) to micro, A(1) and A(2A) receptors, respectively. 4. A significant increase in micro and A(1) receptor binding was detected in chronic naltrexone-treated brains. The changes in micro receptors were significant in several regions, but changes in A(1) were relatively smaller but showed significant upregulation collectively. No significant change in A(2A) receptor binding was detected in chronic naltrexone-treated brains. 5. The results show that blockade of opioid receptors causes upregulation of A(1) receptors, but not A(2A) receptors, by as yet undefined mechanisms.