Delta-opioid receptor gene expression in the mouse forebrain: localization in cholinergic neurons of the striatum

Cholinergic mu-opioid receptor deletion alters reward preference and aversion-resistance

The endogenous opioid system has been implicated in alcohol consumption and preference in both humans and animals. The mu opioid receptor (MOR) is expressed on multiple cells in the striatum, however little is known about the contributions of specific MOR populations to alcohol drinking behaviors. The current study used mice with a genetic deletion of MOR in cholinergic cells (ChAT-Cre/Oprm1fl/fl) to examine the role of MORs expressed in cholinergic interneurons (CINs) in home cage self-administration paradigms. Male and female ChAT-Cre/Oprm1fl/fl mice were generated and heterozygous Cre+ (knockout) and Cre- (control) mice were tested for alcohol consumption in two drinking paradigms: limited access "Drinking in the Dark" and intermittent access. Quinine was added to the drinking bottles in the DID experiment to test aversion-resistant, "compulsive" drinking. Nicotine and sucrose drinking were also assessed so comparisons could be made with other rewarding substances. Cholinergic MOR deletion did not influence consumption or preference for ethanol (EtOH) in either drinking task. Differences were observed in aversion-resistance in males with Cre + mice tolerating lower concentrations of quinine than Cre-. In contrast to EtOH, preference for nicotine was reduced following cholinergic MOR deletion while sucrose consumption and preference was increased in Cre+ (vs. Cre-) females. Locomotor activity was also greater in females following the deletion. These results suggest that cholinergic MORs participate in preference for rewarding substances. Further, while they are not required for consumption of alcohol alone, cholinergic MORs may influence the tendency to drink despite negative consequences.

Cholinergic mu-opioid receptor deletion alters reward preference and aversion-resistance

Heavy alcohol use and binge drinking are important contributors to alcohol use disorder (AUD). The endogenous opioid system has been implicated in alcohol consumption and preference in both humans and animals. The mu opioid receptor (MOR) is expressed on multiple cells in the striatum, however little is known about the contributions of specific MOR populations to alcohol drinking behaviors. The current study used mice with a genetic deletion of MOR in cholinergic cells (ChAT-Cre/Oprm1fl/fl) to examine the role of MORs expressed in cholinergic interneurons (CINs) in home cage self-administration paradigms. Male and female ChAT-Cre/Oprm1fl/fl mice were generated and heterozygous Cre+ (knockout) and Cre- (control) mice were tested for alcohol and nicotine consumption. In Experiment 1, binge-like and quinine-resistant drinking was tested using 15% ethanol (EtOH) in a two-bottle, limited-access Drinking in the Dark paradigm. Experiment 2 involved a six-week intermittent access paradigm in which mice received 20% EtOH, nicotine, and then a combination of the two drugs. Experiment 3 assessed locomotor activity, sucrose preference, and quinine sensitivity. Deleting MORs in cholinergic cells did not alter consumption of EtOH in Experiment 1 or 2. In Experiment 1, the MOR deletion resulted in greater consumption of quinine-adulterated EtOH in male Cre+ mice (vs. Cre-). In Experiment 2, Cre+ mice demonstrated a significantly lower preference for nicotine but did not differ from Cre- mice in nicotine or nicotine + EtOH consumption. Overall fluid consumption was also heightened in the Cre+ mice. In Experiment 3, Cre+ females were found to have greater locomotor activity and preference for sucrose vs. Cre- mice. These data suggest that cholinergic MORs are not required for EtOH, drinking behaviors but may contribute to aversion resistant EtOH drinking in a sex-dependent manner.

The expression of delta opioid receptor mRNA in adult male zebra finches (Taenopygia guttata)

The endogenous opioid system is evolutionarily conserved across reptiles, birds and mammals and is known to modulate varied brain functions such as learning, memory, cognition and reward. To date, most of the behavioral and anatomical studies in songbirds have mainly focused on μ-opioid receptors (ORs). Expression patterns of δ-ORs in zebra finches, a well-studied species of songbird have not yet been reported, possibly due to the high sequence similarity amongst different opioid receptors. In the present study, a specific riboprobe against the δ-OR mRNA was used to perform fluorescence in situ hybridization (FISH) on sections from the male zebra finch brain. We found that δ-OR mRNA was expressed in different parts of the pallium, basal ganglia, cerebellum and the hippocampus. Amongst the song control and auditory nuclei, HVC (abbreviation used as a formal name) and NIf (nucleus interfacialis nidopallii) strongly express δ-OR mRNA and stand out from the surrounding nidopallium. Whereas the expression of δ-OR mRNA is moderate in LMAN (lateral magnocellular nucleus of the anterior nidopallium), it is low in the MSt (medial striatum), Area X, DLM (dorsolateral nucleus of the medial thalamus), RA (robust nucleus of the arcopallium) of the song control circuit and Field L, Ov (nucleus ovoidalis) and MLd (nucleus mesencephalicus lateralis, pars dorsalis) of the auditory pathway. Our results suggest that δ-ORs may be involved in modulating singing, song learning as well as spatial learning in zebra finches.

Cholinergic modulation of striatal microcircuits

The purpose of this review is to bridge the gap between earlier literature on striatal cholinergic interneurons and mechanisms of microcircuit interaction demonstrated with the use of newly available tools. It is well known that the main source of the high level of acetylcholine in the striatum, compared to other brain regions, is the cholinergic interneurons. These interneurons provide an extensive local innervation that suggests they may be a key modulator of striatal microcircuits. Supporting this idea requires the consideration of functional properties of these interneurons, their influence on medium spiny neurons, other interneurons, and interactions with other synaptic regulators. Here, we underline the effects of intrastriatal and extrastriatal afferents onto cholinergic interneurons and discuss the activation of pre‐ and postsynaptic muscarinic and nicotinic receptors that participate in the modulation of intrastriatal neuronal interactions. We further address recent findings about corelease of other transmitters in cholinergic interneurons and actions of these interneurons in striosome and matrix compartments. In addition, we summarize recent evidence on acetylcholine‐mediated striatal synaptic plasticity and propose roles for cholinergic interneurons in normal striatal physiology. A short examination of their role in neurological disorders such as Parkinson's, Huntington's, and Tourette's pathologies and dystonia is also included.

Molecular Adaptations in the Rat Dorsal Striatum and Hippocampus Following Abstinence-Induced Incubation of Drug Seeking After Escalated Oxycodone Self-Administration

Repeated exposure to the opioid agonist, oxycodone, can lead to addiction. Here, we sought to identify potential neurobiological consequences of withdrawal from escalated and non-escalated oxycodone self-administration in rats. To reach these goals, we used short-access (ShA) (3 h) and long-access (LgA) (9 h) exposure to oxycodone self-administration followed by protracted forced abstinence. After 31 days of withdrawal, we quantified mRNA and protein levels of opioid receptors in the rat dorsal striatum and hippocampus. Rats in the LgA, but not the ShA, group exhibited escalation of oxycodone SA, with distinction of two behavioral phenotypes of relatively lower (LgA-L) and higher (LgA-H) oxycodone takers. Both LgA, but not ShA, phenotypes showed time-dependent increases in oxycodone seeking during the 31 days of forced abstinence. Rats from both LgA-L and LgA-H groups also exhibited decreased levels of striatal mu opioid receptor protein levels in comparison to saline and ShA rats. In contrast, mu opioid receptor mRNA expression was increased in the dorsal striatum of LgA-H rats. Moreover, hippocampal mu and kappa receptor protein levels were both increased in the LgA-H phenotype. Nevertheless, hippocampal mu receptor mRNA levels were decreased in the two LgA groups whereas kappa receptor mRNA expression was decreased in ShA and LgA oxycodone groups. Decreases in striatal mu opioid receptor protein expression in the LgA rats may serve as substrates for relapse to drug seeking because these changes occur in rats that showed incubation of oxycodone seeking.

Delta Opioid Pharmacology in Relation to Alcohol Behaviors

Delta opioid receptors (DORs) are heavily involved in alcohol-mediated processes in the brain. In this chapter we provide an overview of studies investigating how alcohol directly impacts DOR pharmacology and of early studies indicating DOR modulation of alcohol behavior. We will offer a brief summary of the different animal species used in alcohol studies investigating DORs followed by a broader overview of the types of alcohol behaviors modulated by DORs. We will highlight a small set of studies investigating the relationship between alcohol and DORs in analgesia. We will then provide an anatomical overview linking DOR expression in specific brain regions to different alcohol behaviors. In this section, we will provide two models that try to explain how endogenous opioids acting at DORs may influence alcohol behaviors. Next, we will provide an overview of studies investigating certain new aspects of DOR pharmacology, including the formation of heteromers and biased signaling. Finally, we provide a short overview of the genetics of the DORs in relation to alcohol use disorders (AUDs) and a short statement on the potential of using DOR-based therapeutics for treatment of AUDs.

Delta Opioid Receptors: Learning and Motivation

Delta opioid receptor (DOR) displays a unique, highly conserved, structure and an original pattern of distribution in the central nervous system, pointing to a distinct and specific functional role among opioid peptide receptors. Over the last 15 years, in vivo pharmacology and genetic models have allowed significant advances in the understanding of this role. In this review, we will focus on the involvement of DOR in modulating different types of hippocampal- and striatal-dependent learning processes as well as motor function, motivation, and reward. Remarkably, DOR seems to play a key role in balancing hippocampal and striatal functions, with major implications for the control of cognitive performance and motor function under healthy and pathological conditions.

In vivo techniques to investigate the internalization profile of opioid receptors

G-protein-coupled receptors (GPCRs) regulate a remarkable diversity of biological functions, and are thus often targeted for drug therapies. Receptor internalization is commonly observed following agonist binding and activation. Receptor trafficking events have been well characterized in cell systems, but the in vivo significance of GPCR internalization is still poorly understood. To address this issue, we have developed an innovative knock-in mouse model, where an opioid receptor is directly visible in vivo. These knockin mice express functional fluorescent delta opioid receptors (DOR-eGFP) in place of the endogenous receptor, and these receptors are expressed at physiological levels within their native environment. DOR-eGFP mice have proven to be an extraordinary tool in studying receptor neuroanatomy, real-time receptor trafficking in live neurons, and in vivo receptor internalization. We have used this animal model to determine the relationship between receptor trafficking in neurons and receptor function at a behavioral level. Here, we describe in detail the construction and characterization of this knockin mouse. We also outline how to use these mice to examine the behavioral consequences of agonist-specific trafficking at the delta opioid receptor. These techniques are potentially applicable to any GPCR, and highlight the powerful nature of this imaging tool.

Cholinergic interneurons in the dorsal and ventral striatum: anatomical and functional considerations in normal and diseased conditions

Striatal cholinergic interneurons (ChIs) are central for the processing and reinforcement of reward-related behaviors that are negatively affected in states of altered dopamine transmission, such as in Parkinson's disease or drug addiction. Nevertheless, the development of therapeutic interventions directed at ChIs has been hampered by our limited knowledge of the diverse anatomical and functional characteristics of these neurons in the dorsal and ventral striatum, combined with the lack of pharmacological tools to modulate specific cholinergic receptor subtypes. This review highlights some of the key morphological, synaptic, and functional differences between ChIs of different striatal regions and across species. It also provides an overview of our current knowledge of the cellular localization and function of cholinergic receptor subtypes. The future use of high-resolution anatomical and functional tools to study the synaptic microcircuitry of brain networks, along with the development of specific cholinergic receptor drugs, should help further elucidate the role of striatal ChIs and permit efficient targeting of cholinergic systems in various brain disorders, including Parkinson's disease and addiction.

Striatal cholinergic interneuron regulation and circuit effects

The striatum plays a central role in motor control and motor learning. Appropriate responses to environmental stimuli, including pursuit of reward or avoidance of aversive experience all require functional striatal circuits. These pathways integrate synaptic inputs from limbic and cortical regions including sensory, motor and motivational information to ultimately connect intention to action. Although many neurotransmitters participate in striatal circuitry, one critically important player is acetylcholine (ACh). Relative to other brain areas, the striatum contains exceptionally high levels of ACh, the enzymes that catalyze its synthesis and breakdown, as well as both nicotinic and muscarinic receptor types that mediate its postsynaptic effects. The principal source of striatal ACh is the cholinergic interneuron (ChI), which comprises only about 1-2% of all striatal cells yet sends dense arbors of projections throughout the striatum. This review summarizes recent advances in our understanding of the factors affecting the excitability of these neurons through acute effects and long term changes in their synaptic inputs. In addition, we discuss the physiological effects of ACh in the striatum, and how changes in ACh levels may contribute to disease states during striatal dysfunction.

The role of δ-opioid receptors in learning and memory underlying the development of addiction

Opioids are important endogenous ligands that exist in both invertebrates and vertebrates and signal by activation of opioid receptors to produce analgesia and reward or pleasure. The μ-opioid receptor is the best known of the opioid receptors and mediates the acute analgesic effects of opiates, while the δ-opioid receptor (DOR) has been less well studied and has been linked to effects that follow from chronic use of opiates such as stress, inflammation and anxiety. Recently, DORs have been shown to play an essential role in emotions and increasing evidence points to a role in learning actions and outcomes. The process of learning and memory in addiction has been proposed to involve strengthening of specific brain circuits when a drug is paired with a context or environment. The DOR is highly expressed in the hippocampus, amygdala, striatum and other basal ganglia structures known to participate in learning and memory. In this review, we will focus on the role of the DOR and its potential role in learning and memory underlying the development of addiction.

LINKED ARTICLES

This article is part of a themed section on Opioids: New Pathways to Functional Selectivity. To view the other articles in this section visit http://dx.doi.org/10.1111/bph.2015.172.issue-2.

δ-opioid and dopaminergic processes in accumbens shell modulate the cholinergic control of predictive learning and choice

Decision-making depends on the ability to extract predictive information from the environment to guide future actions. Outcome-specific Pavlovian-instrumental transfer (PIT) provides an animal model of this process in which a stimulus predicting a particular outcome biases choice toward actions earning that outcome. Recent evidence suggests that cellular adaptations of δ-opioid receptors (DORs) on cholinergic interneurons (CINs) in the nucleus accumbens shell (NAc-S) are necessary for PIT. Here we found that modulation of DORs in CINs critically influences D1-receptor (D1R)-expressing projection neurons in the NAc-S to promote PIT. First, we assessed PIT-induced changes in signaling processes in dopamine D1- and D2-receptor-expressing neurons using drd2-eGFP mice, and found that PIT-related signaling was restricted to non-D2R-eGFP-expressing neurons, suggesting major involvement of D1R-neurons. Next we confirmed the role of D1Rs pharmacologically: the D1R antagonist SCH-23390, but not the D2R antagonist raclopride, infused into the NAc-S abolished PIT in rats, an effect that depended on DOR activity. Moreover, asymmetrical infusion of SCH-23390 and the DOR antagonist naltrindole into the NAc-S also abolished PIT. DOR agonists were found to sensitize the firing responses of CINs in brain slices prepared immediately after the PIT test. We confirmed the opioid-acetylcholinergic influence over D1R-neurons by selectively blocking muscarinic M4 receptors in the NAc-S, which tightly regulate the activity of D1Rs, a treatment that rescued the deficit in PIT induced by naltrindole. We describe a model of NAc-S function in which DORs modulate CINs to influence both D1R-neurons and stimulus-guided choice between goal-directed actions.

Learning-related translocation of δ-opioid receptors on ventral striatal cholinergic interneurons mediates choice between goal-directed actions

The ability of animals to extract predictive information from the environment to inform their future actions is a critical component of decision-making. This phenomenon is studied in the laboratory using the pavlovian-instrumental transfer protocol in which a stimulus predicting a specific pavlovian outcome biases choice toward those actions earning the predicted outcome. It is well established that this transfer effect is mediated by corticolimbic afferents on the nucleus accumbens shell (NAc-S), and recent evidence suggests that δ-opioid receptors (DORs) play an essential role in this effect. In DOR-eGFP knock-in mice, we show a persistent, learning-related plasticity in the translocation of DORs to the somatic plasma membrane of cholinergic interneurons (CINs) in the NAc-S during the encoding of the specific stimulus-outcome associations essential for pavlovian-instrumental transfer. We found that increased membrane DOR expression reflected both stimulus-based predictions of reward and the degree to which these stimuli biased choice during the pavlovian-instrumental transfer test. Furthermore, this plasticity altered the firing pattern of CINs increasing the variance of action potential activity, an effect that was exaggerated by DOR stimulation. The relationship between the induction of membrane DOR expression in CINs and both pavlovian conditioning and pavlovian-instrumental transfer provides a highly specific function for DOR-related modulation in the NAc-S, and it is consistent with an emerging role for striatal CIN activity in the processing of predictive information. Therefore, our results reveal evidence of a long-term, experience-dependent plasticity in opioid receptor expression on striatal modulatory interneurons critical for the cognitive control of action.

GABAergic inputs from direct and indirect striatal projection neurons onto cholinergic interneurons in the primate putamen

Striatal cholinergic interneurons (ChIs) are involved in reward-dependent learning and the regulation of attention. The activity of these neurons is modulated by intrinsic and extrinsic γ-aminobutyric acid (GABA)ergic and glutamatergic afferents, but the source and relative prevalence of these diverse regulatory inputs remain to be characterized. To address this issue, we performed a quantitative ultrastructural analysis of the GABAergic and glutamatergic innervation of ChIs in the postcommissural putamen of rhesus monkeys. Postembedding immunogold localization of GABA combined with peroxidase immunostaining for choline acetyltransferase showed that 60% of all synaptic inputs to ChIs originate from GABAergic terminals, whereas 21% are from putatively glutamatergic terminals that establish asymmetric synapses, and 19% from other (non-GABAergic) sources of symmetric synapses. Double pre-embedding immunoelectron microscopy using substance P and Met-/Leu-enkephalin antibodies to label GABAergic terminals from collaterals of "direct" and "indirect" striatal projection neurons, respectively, revealed that 47% of the indirect pathway terminals and 36% of the direct pathway terminals target ChIs. Together, substance P- and enkephalin-positive terminals represent 24% of all synapses onto ChIs in the monkey putamen. These findings show that ChIs receive prominent GABAergic inputs from multiple origins, including a significant contingent from axon collaterals of direct and indirect pathway projection neurons.

Response of striosomal opioid signaling to dopamine depletion in 6-hydroxydopamine-lesioned rat model of Parkinson's disease: a potential compensatory role

The opioid peptide receptors consist of three major subclasses, namely, μ, δ, and κ (MOR, DOR, and KOR, respectively). They are involved in the regulation of striatal dopamine functions, and increased opioid transmissions are thought to play a compensatory role in altered functions of the basal ganglia in Parkinson's disease (PD). In this study, we used an immunohistochemistry with tyramide signal amplification (TSA) protocols to determine the distributional patterns of opioid receptors in the striosome-matrix systems of the rat striatum. As a most striking feature of striatal opioid anatomy, MORs are highly enriched in the striosomes and subcallosal streak. We also found that DORs are localized in a mosaic pattern in the dorsal striatum (caudate-putamen), with heightened labeling for DOR in the striosomes relative to the matrix compartment. In the 6-hydroxydopamine-lesioned rat model of PD, lesions of the nigrostriatal pathways caused a significant reduction of striatal labeling for both the MOR and DOR in the striosomes, but not in the matrix compartment. Our results suggest that the activities of the striosome and matrix compartments are differentially regulated by the opioid signals involving the MORs and DORs, and that the striosomes may be more responsive to opioid peptides (e.g., enkephalin) than the matrix compartment. Based on a model in which the striosome compartment regulates the striatal activity, we propose a potent compensatory role of striosomal opioid signaling under the conditions of the striatal dopamine depletion that occurs in PD.

Impaired hippocampus-dependent and facilitated striatum-dependent behaviors in mice lacking the δ opioid receptor

Pharmacological data suggest that delta opioid receptors modulate learning and memory processes. In the present study, we investigated whether inactivation of the delta opioid receptor modifies hippocampus (HPC)- and striatum-dependent behaviors. We first assessed HPC-dependent learning in mice lacking the receptor (Oprd1(-/-) mice) or wild-type (WT) mice treated with the delta opioid antagonist naltrindole using novel object recognition, and a dual-solution cross-maze task. Second, we subjected mutant animals to memory tests addressing striatum-dependent learning using a single-solution response cross-maze task and a motor skill-learning task. Genetic and pharmacological inactivation of delta opioid receptors reduced performance in HPC-dependent object place recognition. Place learning was also altered in Oprd1(-/-) animals, whereas striatum-dependent response and procedural learning were facilitated. Third, we investigated the expression levels for a large set of genes involved in neurotransmission in both HPC and striatum of Oprd1(-/-) mice. Gene expression was modified for several key genes that may contribute to alter hippocampal and striatal functions, and bias striatal output towards striatonigral activity. To test this hypothesis, we finally examined locomotor effects of dopamine receptor agonists. We found that Oprd1(-/-) and naltrindole-treated WT mice were more sensitive to the stimulant locomotor effect of SKF-81297 (D1/D5), supporting the hypothesis of facilitated striatonigral output. These data suggest, for the first time, that delta receptor activity tonically inhibits striatal function, and demonstrate that delta opioid receptors modulate learning and memory performance by regulating the HPC/striatum balance.

Novel δ1-receptor agonist KNT-127 increases the release of dopamine and L-glutamate in the striatum, nucleus accumbens and median pre-frontal cortex

The effects of systemic δ1-agonist on neurotransmission remains obscure, since no selective δ1-agonist exists that can penetrate the blood-brain barrier. Recently, we succeeded in synthesizing a putative δ1-receptor agonist, KNT-127, which has been demonstrated the effectiveness of systemic administration against anxiety and depressive-like behavior. To clarify the functional selectivity of KNT-127 and neurotransmission regulating system of δ1-receptor, the present study investigated the interaction between KNT-127 and δ-receptor antagonists on the release of dopamine, L-glutamate and GABA in nucleus accumbens (NAc), striatum and median pre-frontal cortex (mPFC) using multi-probe microdialysis. Intraperitoneal administration of KNT-127 increased the release of dopamine and L-glutamate in three regions, but decreased and increased GABA releases in respective NAc and mPFC without affecting that in striatum. The effects of KNT-127 in the three regions were abrogated by δ1-antagonist but not by δ2-antagonist. MK801 inhibited KNT-127-induced dopamine release in striatum and NAc, but enhanced that in mPFC, inhibited KNT-127-induced mPFC GABA release without affecting KNT-127-induced GABA reduction in NAc. Muscimol enhanced KNT-127-induced dopamine release in mPFC. Sulpiride inhibited KNT-127-induced reduction of GABA release in NAc. The results indicated that KNT-127 is a selective δ1-agonist, and suggested that δ1-receptor directly activates the release of dopamine and L-glutamate in the striatum, NAc and mPFC, but not that of GABA in the three regions. δ1-receptor indirectly inhibited GABA release in NAc via activated dopaminergic transmission, while δ1-receptor indirectly enhanced GABA release in mPFC via activated glutamatergic transmission.

Shifting responsibly: the importance of striatal modularity to reinforcement learning in uncertain environments

We propose here that the modular organization of the striatum reflects a context-sensitive modular learning architecture in which clustered striosome-matrisome domains participate in modular reinforcement learning (RL). Based on anatomical and physiological evidence, it has been suggested that the modular organization of the striatum could represent a learning architecture. There is not, however, a coherent view of how such a learning architecture could relate to the organization of striatal outputs into the direct and indirect pathways of the basal ganglia, nor a clear formulation of how such a modular architecture relates to the RL functions attributed to the striatum. Here, we hypothesize that striosome-matrisome modules not only learn to bias behavior toward specific actions, as in standard RL, but also learn to assess their own relevance to the environmental context and modulate their own learning and activity on this basis. We further hypothesize that the contextual relevance or "responsibility" of modules is determined by errors in predictions of environmental features and that such responsibility is assigned by striosomes and conveyed to matrisomes via local circuit interneurons. To examine these hypotheses and to identify the general requirements for realizing this architecture in the nervous system, we developed a simple modular RL model. We then constructed a network model of basal ganglia circuitry that includes these modules and the direct and indirect pathways. Based on simple assumptions, this model suggests that while the direct pathway may promote actions based on striatal action values, the indirect pathway may act as a gating network that facilitates or suppresses behavioral modules on the basis of striatal responsibility signals. Our modeling functionally unites the modular compartmental organization of the striatum with the direct-indirect pathway divisions of the basal ganglia, a step that we suggest will have important clinical implications.

The kappa-opioid receptor agonist U-69593 prevents cocaine-induced phosphorylation of DARPP-32 at Thr(34) in the rat brain

DARPP-32 (dopamine- and cAMP-regulated phosphoprotein) is a potent endogenous inhibitor of protein phosphatase-1, which plays an important role in dopaminergic transmission. A large body of evidence supports the key role of DARPP-32-dependent signalling in mediating the actions of multiple drugs of abuse, including cocaine, which, when acutely administered, increases the Thr(34) phosphorylation of DARPP-32 in the striatal and cortical areas. In this study, we have examined the contribution of the kappa opioid system to the regulation of DARPP-32 phosphorylation at Thr(34), following acute cocaine administration, in selected rat brain areas. Results showed that a single injection of cocaine induces a significant increase in DARPP-32 phosphorylation at Thr(34) in the hippocampus, caudate putamen and prefrontal cortex. In addition, pretreatment with the kappa opioid receptor agonist U-69593 prevented cocaine effects in all the investigated areas. These data could be considered consistent with the ability of kappa opioid agonists to attenuate many behavioural and neurochemical effects of cocaine.

Co-localization of GABA with other neuroactive substances in the basal ganglia

The dorsal striatum (caudate putamen) contains two types of GABAergic medium spiny neurons (MSNs) that are distinguished by the expression of either the opioid peptide, enkephalin, or the opioid peptide, dynorphin, as well as the tachykinin substance P. Pharmacological studies suggest that these peptides modulate local neurotransmission in the striatum in response to direct and indirect dopamine agonists. In contrast, GABA appears to have minimal impact within the striatum under these conditions. The actions of the peptide cocktail are dependent on the cellular distribution of their receptors in the striatal network. The net result of their actions is a homeostatic response that regulates striatal output and balances dopamine and glutamate receptor stimulation.

Knockin mice expressing fluorescent delta-opioid receptors uncover G protein-coupled receptor dynamics in vivo

The combination of fluorescent genetically encoded proteins with mouse engineering provides a fascinating means to study dynamic biological processes in mammals. At present, green fluorescent protein (GFP) mice were mainly developed to study gene expression patterns or cell morphology and migration. Here we used enhanced GFP (EGFP) to achieve functional imaging of a G protein-coupled receptor (GPCR) in vivo. We created mice where the delta-opioid receptor (DOR) is replaced by an active DOR-EGFP fusion. Confocal imaging revealed detailed receptor neuroanatomy throughout the nervous system of knock-in mice. Real-time imaging in primary neurons allowed dynamic visualization of drug-induced receptor trafficking. In DOR-EGFP animals, drug treatment triggered receptor endocytosis that correlated with the behavioral response. Mice with internalized receptors were insensitive to subsequent agonist administration, providing evidence that receptor sequestration limits drug efficacy in vivo. Direct receptor visualization in mice is a unique approach to receptor biology and drug design.

Selective deficits in the expression of striatal-enriched mRNAs in Huntington's disease

We have identified and cataloged 54 genes that exhibit predominant expression in the striatum. Our hypothesis is that such mRNA molecules are likely to encode proteins that are preferentially associated with particular physiological processes intrinsic to striatal neurons, and therefore might contribute to the regional specificity of neurodegeneration observed in striatal disorders such as Huntington's disease (HD). Expression of these genes was measured simultaneously in the striatum of HD R6/1 transgenic mice using Affymetrix oligonucleotide arrays. We found a decrease in expression of 81% of striatum-enriched genes in HD transgenic mice. Changes in expression of genes associated with G-protein signaling and calcium homeostasis were highlighted. The most striking decrement was observed for a newly identified subunit of the sodium channel, beta 4, with dramatic decreases in expression beginning at 8 weeks of age. A subset of striatal genes was tested by real-time PCR in caudate samples from human HD patients. Similar alterations in expression were observed in human HD and the R6/1 model for the striatal genes tested. Expression of 15 of the striatum-enriched genes was measured in 6-hydroxydopamine-lesioned rats to determine their dependence on dopamine innervation. No changes in expression were observed for any of these genes. These findings demonstrate that mutant huntingtin protein causes selective deficits in the expression of mRNAs responsible for striatum-specific physiology and these may contribute to the regional specificity of degeneration observed in HD.

Functional mu opioid receptors are expressed in cholinergic interneurons of the rat dorsal striatum: territorial specificity and diurnal variation

Striatal cholinergic interneurons play a crucial role in the control of movement as well as in motivational and learning aspects of behaviour. Neuropeptides regulate striatal cholinergic transmission and particularly activation of mu opioid receptor (MOR) inhibits acetylcholine (ACh) release in the dorsal striatum. In the present study we investigated whether this cholinergic transmission could be modulated by an enkephalin/MOR direct process. We show that mRNA and protein of MORs are expressed by cholinergic interneurons in the limbic/prefrontal territory but not by those in the sensorimotor territory of the dorsal striatum. These MORs are functional because potassium-evoked release of ACh from striatal synaptosomes was dose-dependently reduced by a selective MOR agonist, this effect being suppressed by a MOR antagonist. The MOR regulation of cholinergic interneurons presented a diurnal variation. (i) The percentage of cholinergic interneurons containing MORs that was 32% at the beginning of the light period (morning) increased to 80% in the afternoon. (ii) The MOR-mediated inhibition of synaptosomal ACh release was higher in the afternoon than in the morning. (iii) While preproenkephalin mRNA levels remained stable, enkephalin tissue content was the lowest (-32%) in the afternoon when the spontaneous (+35%) and the N-methyl-d-aspartate-evoked (+140%) releases of enkephalin (from microsuperfused slices) were the highest. Therefore, by acting on MORs present on cholinergic interneurons, endogenously released enkephalin reduces ACh release. This direct enkephalin/MOR regulation of cholinergic transmission that operates only in the limbic/prefrontal territory of the dorsal striatum might contribute to information processing in fronto-cortico-basal ganglia circuits.

Cocaine and heroin ('speedball') self-administration: the involvement of nucleus accumbens dopamine and mu-opiate, but not delta-opiate receptors

RATIONALE

The combined administration of heroin and cocaine ('speedball') is common among intravenous drug users. Dopamine receptors in the nucleus accumbens play a key role in cocaine self-administration; however, their role in speedball self-administration is unknown, as is the role of opiate receptors in this region.

OBJECTIVES

The effect of blocking dopamine D1, D2, mu-opiate or delta-opiate receptors in the nucleus accumbens on the intravenous self-administration of combined heroin and cocaine was examined in rats.

METHODS

Rats with bilateral cannulae implanted into the nucleus accumbens were trained to self-administer intravenous speedball (ratio of cocaine/heroin, 17:1) under a progressive ratio (PR) schedule. Prior to their self-administration session, rats were then microinjected with the dopamine D1 receptor antagonist SCH 23390 (1 and 6 nmol side(-1)), the D2 receptor antagonist raclopride (3 and 10 nmol side(-1)), the mu-opiate receptor antagonist CTOP (0.1, 0.3 and 1.0 nmol side(-1)), the delta-opiate receptor antagonist naltrindole (1.0, 3.0 and 10 nmol side(-1)) or a cocktail of SCH 23390 (1 nmol side(-1)) and CTOP (0.1 nmol side(-1)) into the nucleus accumbens.

RESULTS

Microinjection of SCH 23390, raclopride or CTOP into the nucleus accumbens produced dose-dependent decreases in breakpoints under the PR schedule, while naltrindole was without effect. The highest dose of SCH 23390 also significantly reduced locomotor activity measured during speedball self-administration. The combination of SCH 23390 and CTOP significantly reduced breakpoints, while not affecting locomotor activity.

CONCLUSIONS

These results indicate that dopamine and mu-opiate receptors, but not delta-opiate receptors, in the nucleus accumbens are involved in the reinforcing effects of speedball. Combined administration of D1 and mu-opiate receptor antagonists may be more selective at reducing the reinforcing effects of speedball self-administration than either drug alone.

Involvement of corticostriatal glutamatergic terminals in striatal dopamine release elicited by stimulation of delta-opioid receptors

We have previously shown that striatal dopamine release induced locally by a delta-opioid receptor agonist was totally inhibited by a glutamate N-methyl-D-aspartate receptor antagonist, indicating the involvement of glutamatergic receptors in this effect. The aim of the present study was to specify this mechanism. Firstly, we investigated the effect of [D-Pen2,D-Pen5]-enkephalin (DPDPE) on glutamate release in rats by intrastriatal microdialysis. The infusion of DPDPE (10 microm) enhanced the glutamate content in dialysate by approximately 34%, an effect which did not appear to result from inhibition of glutamate uptake. We then considered the consequences of a unilateral thermocoagulation of the frontal cortex on either glutamate or dopamine release induced by stimulation of delta-opioid receptors 2 days later. This lesion, which decreased the glutamate content in ipsilateral striatum by approximately 30%, totally prevented the increase in dialysate levels of glutamate induced by DPDPE. Moreover, whereas DPDPE (10 microm) was found to increase the striatal dopamine release in intact animals by approximately 59%, this effect was also completely suppressed by the cortical lesion. Finally, we studied the effect of the lesion on the [3H]-DPDPE binding to striatal membranes prepared from the whole striatum. In the ipsilateral striatum a significant decrease in this [3H]-DPDPE binding (by approximately 18%) was found 2 days after the lesion. Our results indicate that the increase in striatal dopamine release induced by DPDPE probably depends on glutamate release from corticostriatal glutamatergic afferents in response to the stimulation of delta-opioid receptors located on terminals of these neurons.

Opioid receptor gene expression in dopamine transporter knock-out mice in adult and during development

Dopamine transporter knock-out mice display locomotor hyperactivity due to increased extracellular striatal levels of dopamine. Hyperdopaminergic activity within this mesolimbic pathway is involved in the rewarding properties of morphine which are also increased in these mice. Due to the hyperdopaminergia, profound alterations in gene expression for dopamine receptors and neuropeptides are observed in the caudate putamen and nucleus accumbens. Here we investigated (1) the levels of mu-, delta- and kappa-opioid receptors mRNAs in normal mice from gestational day 13 (G13) to adult, and (2) the adaptive changes in the expression of these receptors in mice lacking the dopamine transporter. Our results show that, in wild-type mice, mu-opioid receptor mRNA expression appears early during development (G13) with a homogeneous distribution that evolves towards a patchy distribution in adult. Delta-opioid receptor mRNA appears only at G17 and kappa-opioid receptor mRNA is not observed before adulthood. The levels of delta-opioid receptor mRNA are not modified during development in knock-out mice compared to wild-type, but are increased in adult caudate putamen (+39%, P<0.05) and nucleus accumbens (+66%, P<0.05) at a time when these receptors are believed to be functional. The mu- and kappa-opioid receptors mRNA levels are not modified in the adult knock-out mice. In addition, we observed no differences in any opioid receptor mRNA level in dopamine transporter knock-out mice during prenatal ontogeny compared to wild-type. Our results constitute a detailed neuroanatomical description of opioid receptor mRNA expression from the time of their appearance during prenatal development until adulthood. Furthermore, we show here that chronic constitutive hyperdopaminergia only affects delta-opioid receptor mRNA levels in adult. Even if the propensity of knock-out mice to show increased rewarding properties to morphine seems to be mainly due to the substantial and further increase in hyperdopaminergic activity following drug treatment, the involvement of increased delta-opioid receptor mRNA levels in this behavior remains to be elucidated.

Effects of perinatal buprenorphine and methadone exposures on striatal cholinergic ontogeny

The effects of exposure to various doses of buprenorphine, methadone or water during the perinatal period were studied on striatal cholinergic development in the rat. Rats were exposed to buprenorphine (0.3 or 3.0 mg/kg/day), methadone (9 mg/kg/day) and/or water prenatally, postnatally or both pre- and postnatally via maternally implanted osmotic minipumps. The effects of buprenorphine varied with the dose used. There were some similarities between the effects of perinatal buprenorphine and perinatal methadone, such as a reduction in striatal acetylcholine (ACh) content in 4-day-old pups exposed prenatally to methadone or buprenorphine (0.3 mg/kg/day). However, differences were also observed between the effects of the two drugs. Unlike methadone, the 0.3-mg/kg/day dose of buprenorphine produced a sex-related increase in striatal ACh in male postnatal day (PND) 21 pups. The 3-mg/kg/day dose of buprenorphine produced a completely different range of results, such as decreased striatal ACh content in 4-day-old pups exposed to the drug postnatally and in 21-day-old pups exposed both pre- and postnatally. Differences in the effects of the two drugs may be related to the different affinities and efficacies of the drugs at different opioid receptor subtypes.

Antiparkinsonian action of a delta opioid agonist in rodent and primate models of Parkinson's disease

The opioid peptides localized in striatal projection neurons are of great relevance to Parkinson's disease, not only as a consequence of their distribution, but also due to the pronounced changes in expression seen in Parkinson's disease. It has long been suspected that increased expression of enkephalin may represent one of the many mechanisms that compensate for dopamine (DA) depletion in Parkinson's disease. Here we demonstrate that a systemically delivered, selective delta opioid agonist (SNC80) has potent antiparkinsonian actions in both rat and primate models of Parkinson's disease. In rats treated with either the D2-preferring DA antagonist haloperidol (1 mg/kg) or the selective D1 antagonist SCH23390 (1 mg/kg), but not a combination of D1 and D2 antagonists, SNC80 (10 mg/kg) completely reversed the catalepsy induced by DA antagonists. In rats rendered immobile by treatment with reserpine, SNC80 dose-dependently reversed akinesia (EC(50) 7.49 mg/kg). These effects were dose-dependently inhibited (IC(50) 1.05 mg/kg) by a selective delta opioid antagonist (naltrindole) and by SCH23390 (1 mg/kg), but not by haloperidol (1 mg/kg). SNC80 also reversed parkinsonian symptoms in the MPTP-treated marmoset. At 10 mg/kg (ip), scores measuring bradykinesia and posture were significantly reduced and motor activity increased to levels comparable with pre-MPTP-treatment scores. Any treatment that serves to increase delta opioid receptor activation may be a useful therapeutic strategy for the treatment of Parkinson's disease, either in the early stages or as an adjunct to dopamine replacement therapy. Furthermore, enhanced enkephalin expression observed in Parkinson's disease may serve to potentiate dopamine acting preferentially at D1 receptors.

Alterations in cortical and basal ganglia levels of opioid receptor binding in a rat model of l-DOPA-induced dyskinesia

Opioid receptor-binding autoradiography was used as a way to map sites of altered opioid transmission in a rat model of l-DOPA-induced dyskinesia. Rats with unilateral 6-hydroxydopamine lesions of the nigrostriatal pathways sustained a 3-week treatment with l-DOPA (6 mg/kg/day, combined with 12 mg/kg/day benserazide), causing about half of them to develop dyskinetic-like movements on the side of the body contralateral to the lesion. Autoradiographic analysis of mu-, delta-, and kappa-opioid binding sites was carried out in the caudate-putamen (CPu), the globus pallidus (GP), the substantia nigra (SN), the primary motor area, and the premotor-cingulate cortex. The dopamine-denervating lesion alone caused an ipsilateral reduction in opioid radioligand binding in the CPu, GP, and SN, but not in the cerebral cortex. Chronic l-DOPA treatment affected opioid receptor binding in both the basal ganglia and the cerebral cortex, producing changes that were both structure- and receptor-type specific, and closely related to the motor response elicited by the treatment. In the basal ganglia, the most clear-cut differences between dyskinetic and nondyskinetic rats pertained to kappa opioid sites. On the lesioned side, both striatal and nigral levels of kappa binding densities were significantly lower in the dyskinetic group, showing a negative correlation with the rats' dyskinesia scores on one hand and with the striatal expression of opioid precursor mRNAs on the other hand. In the cerebral cortex, levels of mu and delta binding site densities were bilaterally elevated in the dyskinetic group, whereas kappa radioligand binding was specifically increased in the nondyskinetic cases and showed a negative correlation with the rats' dyskinesia scores. These data demonstrate that bilateral changes in cortical opioid transmission are closely associated with l-DOPA-induced dyskinesia in the rat. Moreover, the fact that dyskinetic and nondyskinetic animals often show opposite changes in opioid radioligand binding suggests that the motor response to l-DOPA is determined, at least in part, by compensatory adjustments of brain opioid receptors.

Exofocal alterations in opioidergic receptor densities following focal cerebral ischemia in the mouse

In previous studies of our group, we have reported differential alterations in opioidergic receptor subtypes densities in infarcted and periinfarcted brain tissue following middle cerebral artery occlusion (MCAO) in mice. Other studies have also described subcortical alterations consecutive to focal cortical ischemia. For a better understanding of ischemic processes in exofocal areas, we have investigated the evolution of opioidergic receptors following focal cortical ischemia through the quantification of relative binding densities, B(max) and K(d) values for the mu, delta, and kappa subtypes. Our results demonstrate that opioid receptor subtypes exhibit adaptations at distance from the ischemic core, mainly in the striatum, the thalamus, and the substantia nigra. Indeed, mu and delta B(max) values were increased in ventral thalamic nuclei, while kappa relative binding densities were transiently increased in nucleus medialis dorsalis and nucleus lateralis, pars posterior. Moreover, the B(max) of mu and delta receptors were transiently decreased at 6 h post-MCAO in ipsi- and contralateral patches and matrices of the striatum. Conversely, the mu B(max) values were increased in ipsi- and contralateral substantia nigra, pars compacta, and pars reticulata, 24 h following MCAO. In contralateral substantia nigra, pars compacta, kappa B(max) was found to be decreased at 24 h post-MCAO. These alterations could reflect neuronal dysfunction in exofocal brain structures, consecutively to the degeneration of defined neuroanatomical pathways. Our study indicates that opioidergic receptors could be used as markers of the neuronal reorganization that take place in subcortical areas following an ischemic insult of the brain cortex.

Antiparkinson potential of delta-opioid receptor agonists

delta-Opioid receptors, present in very high concentrations in striatum and overlying cortex, are thought to be involved in a number of processes, including analgesia, mood, reward, modulation of neuronal excitability, and alterations in neurotransmitter release. Given the localization of the receptors in motor circuits in brain, we thought it of interest to study the antiparkinson potential of delta-opioid receptor agonists. Rats were given unilateral 6-hydroxydopamine lesions of the nigrostriatal tract, and following recovery, were tested for rotational activity. Tonazocine mesylate is a nonpeptide, partial delta-opioid receptor agonist with mu-receptor antagonist properties. Tonazocine (0.1-10 mg/kg) evoked a dose-related, ipsilateral rotation, consistent with augmentation of dopaminergic function on the unlesioned side. The rotation evoked by tonazocine was blocked by the selective delta-opioid receptor antagonist naltrindole, suggesting that the effect was mediated by delta-opioid receptors. The full delta-opioid receptor agonist (+)-4-¿9-alpha-R)-alpha-(2S,5RO-4-allyl-2, 5-dimethyl-1-piperaziny l)-3-methoxybenzyl-N,N-diethylbenzamide (SNC-80) produced both contralateral and ipsilateral rotation. Tonazocine additionally augmented the effects of L-3,4 dihydroxyphenylalanine (L-DOPA) on reserpine-induced suppression of motor activity. Binding affinities and efficacies of tonazocine and SNC-80 against mu-, kappa-, and delta-opioid receptors were also confirmed and compared to standards. These data suggest therapeutic potential of agents interacting with delta-opioid receptors, and indicate some differences in the activities of tonazocine and SNC-80.

Delta opioid receptors regulate calcium-dependent, amphetamine-evoked glutamate levels in the rat striatum: an in vivo microdialysis study

Blockade of opioid receptors decreases amphetamine-induced behaviors and dopamine release in the striatum. Use of selective opioid receptor ligands has indicated that these effects are mediated by delta opioid receptors (DORs). However, the site of action of delta receptors and the influence of delta receptor antagonists on other neurotransmitters released by amphetamine are unknown. Therefore, the effect of reverse microdialysis of the selective delta opioid antagonist, naltrindole, on extracellular striatal glutamate levels evoked by amphetamine (2.5 mg/kg, i.p.) was investigated. Naltrindole (10-100 microM) decreased amphetamine-evoked glutamate levels in a concentration-dependent manner. The selective delta agonist, [D-Pen(2,5)]-enkephalin (100, 500 microM), reversed the effect of naltrindole, confirming that delta receptors mediated this effect. The amphetamine-evoked increase in extracellular glutamate levels was determined to be 39% calcium-sensitive by lowering the calcium concentration in the perfusate. Under these conditions, naltrindole had no effect on the calcium-independent component of amphetamine-evoked glutamate levels. These data indicate that intrastriatal DORs modulate a calcium-dependent, amphetamine-evoked component of extracellular glutamate levels that may depend on activation of a transsynaptic basal ganglia-thalamo-cortical loop.

Localization of the delta-opioid receptor and dopamine transporter in the nucleus accumbens shell: implications for opiate and psychostimulant cross-sensitization

Opiate- and psychostimulant-induced modulation of dopamine transmission in the nucleus accumbens shell (AcbSh) is thought to play a key role in their potent reinforcing and locomotor effects. To investigate the cellular basis for potential functional interactions involving opiates active at the delta-opioid receptor (DOR) and psychostimulants that bind selectively to the dopamine transporter (DAT), we examined the electron microscopic localization of their respective antisera in rat AcbSh. DOR immunoperoxidase labeling was seen primarily, and DAT immunogold particles exclusively, in axon terminals. In these terminals, DOR immunoreactivity was prominently associated with discrete segments of the plasma membrane and the membranes of nearby small synaptic and large dense core vesicles. DAT immunogold particles were almost exclusively distributed along nonsynaptic axonal plasma membranes. Thirty-nine percent DOR-labeled profiles (221/566) either apposed DAT-immunoreactive terminals or also contained DAT. Of these 221 DOR-labeled profiles, 13% were axon terminals containing DAT and 15% were dendritic spines apposed to DAT-immunoreactive terminals. In contrast, 70% were morphologically heterogeneous axon terminals and small axons apposed to DAT-immunoreactive terminals. Our results indicate that DOR agonists in the AcbSh can directly modulate the release of dopamine, as well as postsynaptic responses in spiny neurons that receive dopaminergic input, but act principally to control the presynaptic secretion of other neurotransmitters whose release may influence or be influenced by extracellular dopamine. Thus, while opiates and psychostimulants mainly have differential sites of action, cross-sensitization of their addictive properties may occur through common neuronal targets.

Presynaptic kappa-opioid and muscarinic receptors inhibit the calcium-dependent component of evoked glutamate release from striatal synaptosomes

In addition to cytosolic efflux, reversal of excitatory amino acid (EAA) transporters evokes glutamate exocytosis from the striatum in vivo. Both kappa-opioid and muscarinic receptor agonists suppress this calcium-dependent response. These data led to the hypothesis that the calcium-independent efflux of striatal glutamate evoked by transporter reversal may activate a transsynaptic feedback loop that promotes glutamate exocytosis from thalamo- and/or corticostriatal terminals in vivo and that this activation is inhibited by presynaptic kappa and muscarinic receptors. Corollaries to this hypothesis are the predictions that agonists for these putative presynaptic receptors will selectively inhibit the calcium-dependent component of glutamate released from striatal synaptosomes, whereas the calcium-independent efflux evoked by an EAA transporter blocker, L-trans-pyrrolidine-2,4-dicarboxylic acid (L-trans-PDC), will be insensitive to such receptor ligands. Here we report that a muscarinic agonist, oxotremorine (0.01-10 microM), and a kappa-opioid agonist, U-69593 (0.1-100 microM), suppressed the calcium-dependent release of glutamate that was evoked by exposing striatal synaptosomes to the potassium channel blocker 4-aminopyridine. The presynaptic inhibition produced by these ligands was concentration dependent, blocked by appropriate receptor antagonists, and not mimicked by the delta-opioid agonist [D-Pen2,5]-enkephalin. The finding that glutamate efflux evoked by L-trans-PDC from isolated striatal nerve endings was entirely calcium independent supports the notion that intact basal ganglia circuitry mediates the calcium-dependent effects of this agent on glutamate efflux in vivo. Furthermore, because muscarinic or kappa-opioid receptor activation inhibits calcium-dependent striatal glutamate release in vitro as it does in vivo, it is likely that both muscarinic and kappa receptors are inhibitory presynaptic heteroceptors expressed by striatal glutamatergic terminals.

Enkephalin regulates acute D2 dopamine receptor antagonist-induced immediate-early gene expression in striatal neurons

Projection neurons of the striatum release opioid peptides in addition to GABA. Our previous studies showed that the opioid peptide dynorphin regulates that subtype of projection neurons which sends axons to the substantia nigra/entopeduncular nucleus, as indicated by an inhibitory action of dynorphin/agonists on D1 dopamine receptor-mediated immediate-early gene induction in these neurons. The other subtype of striatal projection neurons projects to the globus pallidus and contains the opioid peptide enkephalin. Here, we investigated whether enkephalin regulates the function of striatopallidal neurons, by analysing opioid effects on immediate-early gene induction by D2 dopamine receptor blockade that occurs in these neurons. Thus, the effects of systemic and intrastriatal administration of various opioid receptor agonists and antagonists on immediate-early gene expression (c-fos, zif 268) induced by the D2 receptor antagonist eticlopride were examined with in situ hybridization histochemistry. Intrastriatal infusion of enkephalin (delta and mu), but not dynorphin (kappa), receptor agonists suppressed immediate-early gene induction by eticlopride in a dose-dependent manner. This suppression was blocked by the opioid receptor antagonist naloxone, confirming the involvement of opioid receptors. Repeated treatment with D2 receptor antagonists produces increased enkephalin expression and diminished immediate-early gene inducibility in striatopallidal neurons, as well as behavioral effects that are attenuated compared to those of acute treatment (e.g., reduced akinesia). Naloxone reversed such behavioral recovery (i.e. reinstated akinesia), but did not significantly affect suppressed immediate-early gene induction. Our results indicate that enkephalin acts, via mu and delta receptors in the striatum, to inhibit acute effects of D2 receptor blockade in striatopallidal neurons. Moreover, the present findings suggest that increased enkephalin expression after repeated D2 receptor antagonist treatment is an adaptive response that counteracts functional consequences of D2 receptor blockade, but is not involved in suppressed immediate-early gene induction. Together with our earlier findings of the role of dynorphin, these results indicate that opioid peptides in the striatum serve as negative feedback systems to regulate the striatal output pathways in which they are expressed.

Mu- and delta-opioid receptor agonists inhibit DARPP-32 phosphorylation in distinct populations of striatal projection neurons

In the striatum, DARPP-32 (dopamine- and cAMP-regulated phosphoprotein of 32 kDa) is highly expressed by virtually all projection medium-sized spiny neurons. cAMP-dependent phosphorylation of DARPP-32 is stimulated via activation of dopamine D1 receptors in striatonigral neurons, and via activation of adenosine A2A receptors in striatopallidal neurons. In this study, we have examined the contribution of mu-, delta- and kappa-opioid receptors to the regulation of DARPP-32 phosphorylation, in rat striatal slices. The results show that, at low concentrations (100 pm-1 nm), the mu-opioid agonist, Tyr-D-Ala-Gly-N-Me-Phe-glycinol (DAMGO), inhibits the increase in DARPP-32 phosphorylation induced by activation of D1, but not by activation of A2A receptors. Conversely, the delta-receptor agonist, Tyr-D-Pen-Gly-Phe-D-Pen (DPDPE), inhibits DARPP-32 phosphorylation induced by activation of A2A, but not by activation of D1 receptors. The kappa-receptor agonist, U50488, does not affect DARPP-32 phosphorylation induced by either D1 or A2A agonists. Thus, mu-opioid receptors interact with dopamine D1 receptors on striatonigral neurons, whereas delta-opioid receptors interact with adenosine A2A receptors on striatopallidal neurons. These results suggest that regulation of DARPP-32 phosphorylation is involved in mediating some of the effects exerted by enkephalin on striatal medium-sized spiny neurons.

Expression of mu, kappa, and delta opioid receptor messenger RNA in the human CNS: a 33P in situ hybridization study

The existence of at least three opioid receptor types, referred to as mu, kappa, and delta, is well established. Complementary DNAs corresponding to the pharmacologically defined mu, kappa, and delta opioid receptors have been isolated in various species including man. The expression patterns of opioid receptor transcripts in human brain has not been established with a cellular resolution, in part because of the low apparent abundance of opioid receptor messenger RNAs in human brain. To visualize opioid receptor messenger RNAs we developed a sensitive in situ hybridization histochemistry method using 33P-labelled RNA probes. In the present study we report the regional and cellular expression of mu, kappa, and delta opioid receptor messenger RNAs in selected areas of the human brain. Hybridization of the different opioid receptor probes resulted in distinct labelling patterns. For the mu and kappa opioid receptor probes, the most intense regional signals were observed in striatum, thalamus, hypothalamus, cerebral cortex, cerebellum and certain brainstem areas as well as the spinal cord. The most intense signals for the delta opioid receptor probe were found in cerebral cortex. Expression of opioid receptor transcripts was restricted to subpopulations of neurons within most regions studied demonstrating differences in the cellular expression patterns of mu, kappa, and delta opioid receptor messenger RNAs in numerous brain regions. The messenger RNA distribution patterns for each opioid receptor corresponded in general to the distribution of opioid receptor binding sites as visualized by receptor autoradiography. However, some mismatches, for instance between mu opioid receptor receptor binding and mu opioid receptor messenger RNA expression in the anterior striatum, were observed. A comparison of the distribution patterns of opioid receptor messenger RNAs in the human brain and that reported for the rat suggests a homologous expression pattern in many regions. However, in the human brain, kappa opioid receptor messenger RNA expression was more widely distributed than in rodents. The differential and region specific expression of opioid receptors may help to identify targets for receptor specific compounds in neuronal circuits involved in a variety of physiological functions including pain perception, neuroendocrine regulation, motor control and reward.

Chronic morphine exposure and spontaneous withdrawal are associated with modifications of dopamine receptor and neuropeptide gene expression in the rat striatum

The influence of chronic morphine and spontaneous withdrawal on the expression of dopamine receptors and neuropeptide genes in the rat striatum was investigated. Morphine dependence was induced by subcutaneous implantation of two morphine pellets for 6 days. Rats were made abstinent by removal of the pellets 1, 2 or 3 days before they were killed. The mRNA levels coding for D1- and D2-dopamine receptors, dynorphin, preproenkephalin A and substance P were determined by quantitative in situ hybridization. The caudate putamen and the nucleus accumbens showed equivalent modifications in dopamine receptor and neuropeptide gene expression. After 6 days of morphine, a decrease in D2-dopamine receptor and neuropeptide mRNA levels was observed (-30%), but there was no change in D1-dopamine receptor mRNA. In abstinent rats, both D1- and D2-dopamine receptor mRNA levels were decreased 1 day after withdrawal (-30% compared with chronic morphine). In contrast, neuropeptide mRNA levels were unaffected when compared with those observed after 6 days of morphine. During the second and third day of withdrawal, there was a gradual return to the levels seen in the placebo-treated group, for both dopamine receptor and neuropeptide mRNAs. Phenotypical characterization of striatal neurons expressing mu and kappa opioid receptor mRNAs showed that, in striatonigral neurons, both mRNAs were colocalized with D1-receptor and Dyn mRNAs. Our results suggest that during morphine dependence, dopamine and morphine exert opposite effects on striatonigral neurons, and that effects occurring on striatopallidal neurons are under dopaminergic control. We also show that withdrawal is associated with a down regulation of the postsynaptic D1 and D2 receptors.

Opioid receptor gene expression in the rat brain during ontogeny, with special reference to the mesostriatal system: an in situ hybridization study

The three main types of opioid receptors micro, delta and kappa are found in the central nervous system and periphery. In situ hybridization study was undertaken to determine the expression of mu, delta, kappa-opioid receptors mRNAs in the brain during pre- and postnatal development, especially in the mesostriatal system. By G13, mu and kappa-opioid receptor mRNA were detectable in the telencephalon; mu-opioid receptor mRNA was found in the striatal neuroepithelium and cortical plate and kappa-opioid receptor mRNA in the corroidal fissure. By G15, kappa-opioid receptor mRNA was detectable in the nucleus accumbens and dorsal striatum, and in the substantia nigra and ventral tegmental area, suggesting an early expression of the corresponding receptor on dopaminergic terminal fibers. For the mu-opioid receptor mRNA in the striatum, patches appeared at G20. Delta-opioid receptor mRNA was first detected at G21, in many areas including the accumbens nucleus and the dorsal striatum. At P8, delta-opioid receptor mRNA was detected in large-sized cells of the striatum, possibly cholinergic, suggesting a possible modulation by opioids of the striatal cholinergic neurons. Our results demonstrate the early appearance of mu and kappa-opioid receptor mRNA (G13) and the relatively late development of delta-opioid receptor mRNA (G21) in the brain. We also show a distinct pattern of expression for mu, delta and kappa-opioid receptor mRNAs in the mesostriatal system during the development.

Maximal densities of mu, delta, and kappa receptors are differentially altered by focal cerebral ischaemia in the mouse

Though opioids are known to have neuroprotective properties, little information is available on the functional state of opioidergic receptors following focal cerebral ischaemia. The present study investigated the evolution of the Bmax and Kd for [3H]DAMGO, [3H]DADLE, and [3H]U69,593, respectively, for the mu, delta, and kappa opioidergic receptors after permanent focal cerebral ischaemia in mice. While the various Kd were unchanged, mu and delta Bmax values were precociously decreased in frontoparietal cortices, earlier than kappa receptors, reflecting infarct extension with time. The Bmax values for mu and delta receptors were also altered in non-infarcted tissues, such as tissues at risk (e.g., temporal auditory cortex) and exofocal (e.g., contralateral and non-infarcted) cortices. These results suggest that, in non-infarcted areas, the observed changes reflect functional modifications to focal ischaemia.

Cellular sites for activation of delta-opioid receptors in the rat nucleus accumbens shell: relationship with Met5-enkephalin

The shell compartment of the nucleus accumbens (AcbSh) is prominently involved in the rewarding aspects of delta-opioid receptor (DOR) agonists, including one of its putative endogenous ligands, Met5-enkephalin (Enk). We examined the ultrastructural immunocytochemical localization of an antipeptide DOR antiserum and an antibody against Enk to determine the major cellular sites for DOR activation and the spatial relationship between DOR and Enk in this region. Sixty percent of DOR-immunoreactive profiles were axon terminals and small unmyelinated axons, whereas the remainder were mainly dendrites and dendritic spines. In axons and terminals, DOR labeling was distributed along plasma and vesicular membranes. DOR-containing terminals were mainly large and primarily formed symmetric synapses or occasionally asymmetric synapses. DOR immunoreactivity also was associated with terminals that were small and formed punctate symmetric or nonrecognizable synapses. Dual immunoperoxidase and immunogold labeling showed that 35% of DOR-labeled axons apposed other terminals that contained Enk. In addition, 25% of the DOR-labeled terminals contained Enk. Thirty-five percent of DOR labeling was observed within dendrites and dendritic spines. DOR-labeled spines showed intense immunoreactivity within asymmetric postsynaptic junctions, which were formed by terminals that lacked Enk immunoreactivity. DOR-labeled spines, however, were apposed to Enk-containing terminals in 13% of all associations between dually labeled profiles. These results provide ultrastructural evidence that activation of DOR in the AcbSh is primarily involved in modulating the presynaptic release of mainly inhibitory, but also excitatory, neurotransmitters. These data also suggest that DOR plays a role in determining the postsynaptic response to excitatory afferents.

N-methyl-D-aspartate-evoked release of [3H]acetylcholine in striatal compartments of the rat: regulatory roles of dopamine and GABA

The N-methyl-D-aspartate-evoked release of [3H]acetylcholine previously formed from [3H]choline was estimated in striosome- (identified by [3H]naloxone binding) or matrix-enriched areas of the rat striatum using an in vitro microsuperfusion procedure. Experiments were performed in either the absence or the presence of dopaminergic and/or GABAergic receptor antagonists. Although the cell bodies of the cholinergic interneurons were mainly found in the matrix, in the absence of magnesium, N-methyl-D-aspartate (50 microM) stimulated the release of [3H]acetylcholine in both striatal compartments. These responses were blocked by either magnesium, dizocilpine maleate, 7-chlorokynurenate or tetrodotoxin. N-Methyl-D-aspartate responses were concentration-dependent, but the 1 mM N-methyl-D-aspartate response was higher in striosomes than in the matrix. The co-application of D-serine (10 microM) enhanced the 10 microM N-methyl-D-aspartate response in both compartments, but reduced those induced by 1 mM N-methyl-D-aspartate, this reduction being higher in striosomes. The blockade of dopaminergic transmission with the D2 and D1 dopaminergic receptor antagonists, (-)-sulpiride (1 microM) and SCH23390 (1 microM), was without effect on the 50 microM N-methyl-D-aspartate-evoked release of [3H]acetylcholine, but markedly enhanced the 1 mM N-methyl-D-aspartate+D-serine-evoked response in striosomes and to a lesser extent in the matrix. Disinhibitory responses of similar amplitude were observed not only in striosomes but also in the matrix when (-)-sulpiride was used alone, while SCH23390 alone enhanced the 1 mM N-methyl-D-aspartate+D-serine response only in striosomes and to a lower extent than (-)-sulpiride. These results indicate that D2 receptors are mainly involved in the inhibitory effect of dopamine on the 1 mM N-methyl-D-aspartate+D-serine-evoked release of [3H]acetylcholine. They also show that the stimulation of D1 receptors can either reduce (striosomes) or enhance (matrix) this response, since in the latter case the effect induced by the combined application of the D1 and D2 receptor antagonists was smaller than that observed with the D2 receptor antagonist alone. Indicating that released GABA facilitates N-methyl-D-aspartate responses, the blockade of GABAA receptors with bicuculline (5 microM) reduced the 50 microM N-methyl-D-aspartate-evoked release of [3H]acetylcholine in both striatal compartments and the 1 mM N-methyl-D-aspartate+D-serine response in the matrix. These effects result from an inhibition by GABA of the evoked release of dopamine, since the reducing effects of bicuculline on N-methyl-D-aspartate responses were not observed under the complete blockade of dopaminergic transmission by the D1 and D2 receptor antagonists. Further demonstrating a facilitatory role of GABA in the control of N-methyl-D-aspartate-evoked release of [3H]acetylcholine, in the presence of bicuculline, (-)-sulpiride and SCH23390 alone or in combination enhanced, in both compartments, the responses induced not only by 1 mM N-methyl-D-aspartate+D-serine, but also by 50 microM N-methyl-D-aspartate.

GABA- and glutamate-evoked responses in the rat ventral pallidum are modulated by dopamine

Microiontophoresis was used to investigate the influence of dopamine on GABA- and glutamate-induced responses from ventral pallidal neurons recorded extracellularly in chloral hydrate-anaesthetized rats. Modulation was determined by comparing dopamine-induced alterations in amino acid-induced activity ('signal') with dopamine-induced effects on spontaneous firing ('noise'). A dopamine ejection current-response curve was generated to determine the current levels that did not alter spontaneous firing ('subthreshold') and those that produced approximately 50% of the maximal dopamine-induced response (ECur50). Co-iontophoresis of dopamine with GABA generally diminished the inhibitory influence of GABA on pallidal neuron firing; 70% of neurons tested with ECur50 dopamine demonstrated a decrease in the signal-to-noise ratio whereas 10% displayed an increase. At subthreshold dopamine ejection currents, 59% of neurons responded with a decrease and 18% responded with an increase in the GABA signal-to-noise ratio. When ECur50 dopamine was co-iontophoresed with glutamate, 84% of the neurons displayed a decrease in the signal-to-noise ratio for glutamate-evoked excitations whereas 11% demonstrated an increase. Subthreshold dopamine ejection currents decreased the signal-to-noise ratio in 62% of the ventral pallidal neurons excited by glutamate and increased the ratio in 23%. These data illustrate that dopamine substantially alters GABA- and glutamate-evoked responses even at ejection currents that are below those necessary to change spontaneous firing. Thus, it appears that neuromodulation is an important means by which dopamine influences ventral pallidal neuronal activity.

The projection from the striatum to the nucleus basalis in the rat: an electron microscopic study

Previous studies have shown that the striatum provides synaptic inputs to the globus pallidus and entopeduncular nucleus in which GABA is co-localized with the peptides enkephalin and substance P. The aim of this study in the rat was to determine whether the striatal projections also make synaptic contact with the cholinergic neurons of the nucleus basalis, which lie near to the pallidal areas in the rat brain. The anterograde tracer biocytin was injected into different parts of the striatum, and brain sections were stained for biocytin and choline acetyltransferase immunoreactivity by using a dual colour method. Terminals labelled with biocytin by anterograde transport and which made synaptic contact with choline acetyltransferase-positive soma and dendrites were identified by light-electron microscopic correlation methods. In the cases where the biocytin injections had been made in the dorsal or lateral striatum, biocytin-labelled terminals made synaptic contact with cholinergic cells in the region between the main termination zones in the globus pallidus and the entopeduncular nucleus. In the cases where the injections had been made in the ventromedial and posterior striatum, there was greater overlap between choline acetyltransferase-positive structures and biocytin-labelled terminals in the main termination zones in the globus pallidus or entopeduncular nucleus, but relatively few of these terminals made synaptic contacts on to the cholinergic neurons. The results therefore indicate that the cholinergic nucleus basalis cells receive a relatively sparse synaptic input from all parts of the striatum. It has recently been shown that the cholinergic cells of the nucleus basalis selectively express high levels of substance P and opioid receptor messenger RNAs, while the non-cholinergic pallidal cells have much higher levels of GABA(A) receptor subunit messenger RNAs. It is concluded that the cholinergic neurons of the nucleus basalis in the rat may be selectively responsive to the peptidergic components of the striatal outputs, and that they are most likely to be influenced by both the limbic and sensorimotor parts of the striatum.

Morphine modulation of GABA- and glutamate-induced changes of ventral pallidal neuronal activity

Microiontophoresis was used to investigate the influence of morphine on the GABA- and glutamate-evoked responses of ventral pallidal neurons recorded extracellularly from chloral hydrate-anesthetized rats. Of the GABA-sensitive neurons (50 of 69 tested) in the ventral pallidum, all displayed a decreased firing rate when GABA was applied, whereas all of the glutamate-sensitive neurons (29 of 40 tested) increased neuronal activity in the presence of glutamate. The majority of ventral pallidal cells tested (65 of 83) were sensitive to iontophoretically applied morphine, and both increases and decreases in neuronal activity were observed. The ability of morphine to alter the ratio between amino acid-evoked activity ("signal") and spontaneous firing ("noise") was used as an indicator of morphine modulation. A morphine subthreshold ejection current, i.e. one that did not change spontaneous firing rate, and a morphine ejection current that produced approximately 50% of the maximum opioid-induced neuronal response were chosen for this evaluation. When morphine was co-iontophoresed with GABA or glutamate, attenuation of the amino acid signal-to-noise ratio was generally seen, though some potentiations were observed. These changes were independent of the direction of morphine-induced changes in spontaneous firing rate. Both sub- and suprathreshold ejection currents were capable of affecting GABA- and glutamate-evoked responses. These data suggest that morphine is a robust ventral pallidal neuromodulator. As ventral pallidal amino acid activity is important in the integration of sensorimotor information, opioid modulation of amino acid transmission in the ventral pallidum may have a profound effect on this integration.

Neostriatal cell subtypes and their functional roles

The neostriatum is considered to be involved in the induction of purposeful movements or in the suppression of other movements through the internal segment of the globus pallidus (GP; the entopeduncular nucleus in the rodents) and the substantia nigra (SN) to which the striatal spiny neurons project. To understand how the striatum fulfills these functions, it is necessary to know the physiological and morphological characteristics of its constituent neurons. Aspiny interneurons in the striatum are considered to receive various excitatory inputs and to contribute importantly to determining whether spiny projection neurons fire or not. Both spiny and aspiny striatal cells have been shown to be heterogeneous in their physiological, chemical and connection characteristics. In this article, how these cell subtypes are organized in the local circuitry of the striatum and their physiological roles in the basal ganglia are discussed.