Opiate dependence induces network state shifts in the limbic system.

Among current theories of addiction, hedonic homeostasis dysregulation predicts that the brain reward systems, particularly the mesolimbic dopamine system, switch from a physiological state to a new "set point." In opiate addiction, evidence show that the dopamine system principal targets, prefrontal cortex (PFC), nucleus accumbens (NAC) and basolateral amygdala complex (BLA) also adapt to repeated drug stimulation. Here we investigated the impact of chronic morphine on the dynamics of the network of these three interconnected structures. For that purpose we performed simultaneous electrophysiological recordings in freely-moving rats subcutaneously implanted with continuous-release morphine pellets. Chronic morphine produced a shift in the network state underpinned by changes in Delta and Gamma oscillations in the LFP of PFC, NAC and BLA, in correlation to behavioral changes. However despite continuous stimulation by the drug, an apparent normalization of the network activity and state occurred after 2 days indicating large scale adaptations. Blockade of µ opioid receptors was nonetheless sufficient to disrupt this acquired new stability in morphine-dependent animals. In line with the homeostatic dysregulation theory of addiction, our study provides original direct evidence that the PFC-NAC-BLA network of the dependent brain is characterized by a de novo balance for which the drug of abuse becomes the main contributor.

Role of 5-HT2C receptors in the enhancement of c-Fos expression induced by a 5-HT2B/2C inverse agonist and 5-HT 2 agonists in the rat basal ganglia.

Some non-selective serotonin2C (5-HT2C) agonists or inverse agonists enhance the product of the proto-oncogene c-Fos within the basal ganglia, a group of brain regions involved in motor behavior and in the ability of these drugs to promote abnormal movements. The role of 5-HT2C receptors in these effects is unclear. The 5-HT2C antagonist SB243,213 (1 mg/kg), which enhanced Fos per se in the striatum and the subthalamic nucleus (STN) only, was used to study the implication of 5-HT2C receptors. The agonists Ro 60-0175 (3 mg/kg) and m-CPP (1 mg/kg) and the inverse agonist SB206,553 (10 mg/kg) enhanced Fos expression in the STN and faintly in the entopeduncular nucleus (EPN, the internal globus pallidus in primate). The effects of these drugs differed mainly in the striatum regarding the magnitude (m-CPP > Ro 60-0175> SB243,213 > SB206,553) or the striatal quadrants (faint to no labeling in lateral striatum) and in the substantia nigra. None of these compounds enhanced Fos expression by themselves in the globus pallidus or in the EPN when combined with SB243,213. Their Fos effect in the STN was reduced significantly by SB243,213 only in the case of m-CPP. In the ventromedial striatum, SB243,213 reduced the effects of m-CPP while SB206,553 reduced the effects of SB243,213. The results show that opposite pharmacological agents alter similarly Fos expression in the EPN or the STN. Although some of the effects of 5-HT agents are related to targets other than 5-HT2C receptors, the study confirms the existence of multiple 5-HT2C receptor-dependent controls recruited by these drugs upon basal ganglia activity.

CRF2 receptor-deficiency eliminates opiate withdrawal distress without impairing stress coping.

The opiate withdrawal syndrome is a severe stressor that powerfully triggers addictive drug intake. However, no treatment yet exists that effectively relieves opiate withdrawal distress and spares stress-coping abilities. The corticotropin-releasing factor (CRF) system mediates the stress response, but its role in opiate withdrawal distress and bodily strategies aimed to cope with is unknown. CRF-like signaling is transmitted by two receptor pathways, termed CRF(1) and CRF(2). Here, we report that CRF(2) receptor-deficient (CRF(2)(-/-)) mice lack the dysphoria-like and the anhedonia-like states of opiate withdrawal. Moreover, in CRF(2)(-/-) mice opiate withdrawal does not increase the activity of brain dynorphin, CRF and periaqueductal gray circuitry, which are major substrates of opiate withdrawal distress. Nevertheless, CRF(2) receptor-deficiency does not impair brain, neuroendocrine and autonomic stress-coping responses to opiate withdrawal. The present findings point to the CRF(2) receptor pathway as a unique target to relieve opiate withdrawal distress without impairing stress-coping abilities.

Stimulation of serotonin2C receptors elicits abnormal oral movements by acting on pathways other than the sensorimotor one in the rat basal ganglia.

Serotonin2C (5-HT(2C)) receptors act in the basal ganglia, a group of sub-cortical structures involved in motor behavior, where they are thought to modulate oral activity and participate in iatrogenic motor side-effects in Parkinson's disease and Schizophrenia. Whether abnormal movements initiated by 5-HT(2C) receptors are directly consequent to dysfunctions of the motor circuit is uncertain. In the present study, we combined behavioral, immunohistochemical and extracellular single-cell recordings approaches in rats to investigate the effect of the 5-HT(2C) agonist Ro-60-0175 respectively on orofacial dyskinesia, the expression of the marker of neuronal activity c-Fos in basal ganglia and the electrophysiological activity of substantia nigra pars reticulata (SNr) neuron connected to the orofacial motor cortex (OfMC) or the medial prefrontal cortex (mPFC). The results show that Ro-60-0175 (1 mg/kg) caused bouts of orofacial movements that were suppressed by the 5-HT(2C) antagonist SB-243213 (1 mg/kg). Ro-60-0175 (0.3, 1, 3 mg/kg) dose-dependently enhanced Fos expression in the striatum and the nucleus accumbens. At the highest dose, it enhanced Fos expression in the subthalamic nucleus, the SNr and the entopeduncular nucleus but not in the external globus pallidus. However, the effect of Ro-60-0175 was mainly associated with associative/limbic regions of basal ganglia whereas subregions of basal ganglia corresponding to sensorimotor territories were devoid of Fos labeling. Ro-60-0175 (1-3 mg/kg) did not affect the electrophysiological activity of SNr neurons connected to the OfMC nor their excitatory-inhibitory-excitatory responses to the OfMC electrical stimulation. Conversely, Ro-60-0175 (1 mg/kg) enhanced the late excitatory response of SNr neurons evoked by the mPFC electrical stimulation. These results suggest that oral dyskinesia induced by 5-HT(2C) agonists are not restricted to aberrant signalling in the orofacial motor circuit and demonstrate discrete modifications in associative territories.

Reactivity and plasticity in the amygdala nuclei during opiate withdrawal conditioning: differential expression of c-fos and arc immediate early genes.

Opiate withdrawal leads to the emergence of an aversive state that can be conditioned to a specific environment. Reactivation of these withdrawal memories has been suggested to be involved in relapse to drug-seeking of abstinent opiate addicts. Among the limbic areas that are likely to mediate these features of opiate dependence, amygdala nuclei represent critical neural substrates. Using a conditioned place aversion paradigm (CPA), we have previously shown specific opposite patterns of reactivity in the basolateral (BLA) and the central (CeA) amygdala, when comparing the experience of acute opiate withdrawal with the re-exposure to a withdrawal-paired environment. These data gave clues about the potential mechanisms by which amygdala nuclei may be involved in withdrawal memories. To extend these results, the present study aimed to assess the cellular reactivity and plasticity of amygdala nuclei during the opiate withdrawal conditioning process. For this, we have quantified c-fos and arc expression using in situ hybridization in rats, following each of the three conditioning sessions during CPA, and after re-exposure to the withdrawal-paired environment. BLA output neurons showed an increase in the expression of the plasticity-related arc gene during conditioning that was also increased by re-exposure to the withdrawal-paired environment. Interestingly, the CeA showed an opposite pattern of responding, and the intercalated cell masses (ITC), a possible inhibitory interface between the BLA and CeA, showed a persistent activation of c-fos and arc mRNA. We report here specific c-fos and arc patterns of reactivity in amygdala nuclei during withdrawal conditioning. These findings improve our understanding of the involvement of the amygdala network in the formation and retrieval of withdrawal memories. Plasticity processes within BLA output neurons during conditioning, may participate in increasing the BLA reactivity to conditioned stimuli, which could in turn (by the control of downstream nuclei) reinforce and drive the motivational properties of withdrawal over drug consumption.

Role of imidazoline receptors in the anti-aversive properties of clonidine during opiate withdrawal in rats.

Clonidine is used as a treatment for heroin addiction. Previous studies have reported that clonidine attenuated conditioned place aversion (CPA) to naloxone-precipitated opiate withdrawal by acting on alpha2 adrenoceptors (alpha2R). However, clonidine acts as a partial agonist both at alpha2R and at imidazoline-1 receptors (I1Rs). The current study was designed to determine the role of I1R in the induction of naloxone-induced CPA in morphine-dependent rats. Morphine dependence was induced by subcutaneous implantation of morphine pellets. Morphine-dependent rats were tested in a three-chamber place-aversion apparatus. A range of agonists were chosen on the basis of their differential selectivity for alpha2R and I1R. As expected, pretreatment with clonidine prevented naloxone-induced CPA. By contrast, pretreatment with a selective alpha2R agonist (UK14304) failed to prevent the CPA. We then tested whether the high affinity of clonidine for I1R was responsible for the difference between these two alpha2R agonists. Rilmenidine (a mixed alpha2R/I1R agonist) attenuated aversion to opiate withdrawal in a dose-dependent manner. The action of clonidine on I1R was studied by co-administering clonidine with RX821002, a specific alpha2R antagonist. Co-treatment with RX821002 and clonidine blocked naloxone-induced CPA. These results indicate that the pharmacologically protective effects of clonidine on naloxone-induced CPA are related to actions on I1RS as well as alpha2Rs.

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.

Galpha(olf) levels are regulated by receptor usage and control dopamine and adenosine action in the striatum.

In the striatum, dopamine D(1) and adenosine A(2A) receptors stimulate the production of cAMP, which is involved in neuromodulation and long-lasting changes in gene expression and synaptic function. Positive coupling of receptors to adenylyl cyclase can be mediated through the ubiquitous GTP-binding protein Galpha(S) subunit or through the olfactory isoform, Galpha(olf), which predominates in the striatum. In this study, using double in situ hybridization, we show that virtually all striatal efferent neurons, identified by the expression of preproenkephalin A, substance P, or D(1) receptor mRNA, contained high amounts of Galpha(olf) mRNA and undetectable levels of Galpha(s) mRNA. In contrast, the large cholinergic interneurons contained both Galpha(olf) and Galpha(s) transcripts. To assess the functional relationship between dopamine or adenosine receptors and G-proteins, we examined G-protein levels in the striatum of D(1) and A(2A) receptor knock-out mice. A selective increase in Galpha(olf) protein was observed in these animals, without change in mRNA levels. Conversely, Galpha(olf) levels were decreased in animals lacking a functional dopamine transporter. These results indicate that Galpha(olf) protein levels are regulated through D(1) and A(2A) receptor usage. To determine the functional consequences of changes in Galpha(olf) levels, we used heterozygous Galpha(olf) knock-out mice, which possess half of the normal Galpha(olf) levels. In these animals, the locomotor effects of amphetamine and caffeine, two psychostimulant drugs that affect dopamine and adenosine signaling, respectively, were markedly reduced. Together, these results identify Galpha(olf) as a critical and regulated component of both dopamine and adenosine signaling.

Mapping of c-fos gene expression in the brain during morphine dependence and precipitated withdrawal, and phenotypic identification of the striatal neurons involved.

The c-fos gene is expressed in the central nervous system in response to various neuronal stimuli. Using in situ hybridization, we examined the effects of chronic morphine treatment and withdrawal on c-fos mRNA in the rat brain, and particularly within identified striatal neurons. Morphine dependence was induced by subcutaneous implantation of two pellets of morphine for 6 days and withdrawal was precipitated by administration of naltrexone. Placebo animals and morphine-dependent rats showed a very weak c-fos mRNA expression in all the structures studied. Our study emphasized the spatial variations in c-fos mRNA expression, and also revealed a peak expression of c-fos mRNA at 1 h after naltrexone-precipitated withdrawal in the projection areas of dopaminergic neurons, noradrenergic neurons and in several regions expressing opiate receptors. Interestingly, morphine withdrawal induces c-fos mRNA expression in the two efferent populations of the striatum (i.e. striatonigral and striatopallidal neurons) both in the caudate putamen and nucleus accumbens. Moreover, the proportions of activated neurons during morphine withdrawal are different in the caudate putamen (mostly in striatopallidal neurons) and in the shell and core parts of the nucleus accumbens (mostly in striatonigral neurons). The activation of striatopallidal neurons suggests a predominant dopaminergic regulation on c-fos gene expression in the striatum during withdrawal. On the contrary, c-fos induction in striatonigral neurons during withdrawal seems to involve a more complex regulation like opioid-dopamine interactions via the mu opioid receptor and the D1 dopamine receptor coexpressed on this neuronal population or the implication of other neurotransmitter systems.

Dopamine control of striatal gene expression during development: relevance to knockout mice for the dopamine transporter.

The aim of this study was to determine at which developmental stage and how dopamine regulates the expression of striatal dopamine receptor and neuropeptide mRNAs. For this, we studied the expression of these mRNAs, in relation to dopamine innervation, in normal mice from gestational day 13 (G13) to adult. Particularly, we investigated the adaptive changes in the expression of these markers in mice lacking the dopamine transporter during development. We detected tyrosine hydroxylase, by immunohistochemistry, in the ventral mesencephalon and the striatal anlage in both genotypes at G13, whereas the dopamine transporter appeared in the striatum of normal mice at G14. By in situ hybridization, we detected striatal dopamine D1, D2, D3 receptor, and substance P mRNAs at G13, preproenkephalin A mRNA at G14 and dynorphin mRNA at G17 in normal mice. Although the time of initial detection and the distribution were not affected in mutant mice, quantitative changes were observed. Indeed, D1 and D2 receptor as well as preproenkephalin A mRNA levels were decreased from G14 on, and dynorphin mRNA level was increased from G17 on. In contrast, substance P mRNA level was unaffected. Our data demonstrate that the influence of dopamine on striatal neurons occurs early during the development of the mesostriatal system as quantitative changes appeared in mutant mice as soon as G14. These findings bring new insights to the critical influence of dopamine on the expression of striatal dopamine receptor and neuropeptide mRNAs during development, and suggest that mesostriatal dopamine transmission functions from G14 on.

Co-stimulation of D(1)/D(5) and D(2) dopamine receptors leads to an increase in c-fos messenger RNA in cholinergic interneurons and a redistribution of c-fos messenger RNA in striatal projection neurons.

The anatomical subdivision of striatum in patch and matrix compartments plays an important role for the processing of neurotransmission through the basal ganglia in primates and rodents. Here we report that co-administration of D(1)/D(5) and D(2) receptor agonists, which induces a heterogenous and patchy pattern of c-fos messenger RNA expression in striatum, stimulates c-fos messenger RNA expression in cholinergic interneurons. Moreover, this treatment induces c-fos messenger RNA in projection neurons containing D(1)-, rather than D(2)-receptor messenger RNA. The preferential induction of c-fos messenger RNA in patches does not depend upon a higher degree of co-localization between D(1) and D(2) receptors in this area, since double in situ hybridization experiments showed a large segregation of D(1) and D(2) receptor messenger RNAs in the patch as well as the matrix compartments. By contrast, treatment with a full D(1)/D(5) receptor agonist up-regulates striatal c-fos messenger RNA homogenously and in similar proportions of D(1) and D(2) receptor messenger RNA-containing projection neurons in both medial and lateral striatum, but has only minor effects on c-fos messenger RNA expression in cholinergic interneurons. These results provide a neuroanatomical/neurochemical correlate to the well-known behavioral interaction between dopamine D(1)/D(5) agonists and dopamine D(2) agonists. They also suggest that there may be a relation between a heterogenous, patch-enriched c-fos messenger RNA expression and an increased expression of this immediate early gene in cholinergic interneurons.

Dopamine D(1) receptor-induced gene transcription is modulated by DARPP-32.

The role of the dopamine- and cyclic AMP-regulated phosphoprotein of M(r) 32,000 (DARPP-32) in dopaminergic regulation of gene transcription in striatum and globus pallidus was examined. Mice with targeted disruption of the gene encoding DARPP-32, its homologue, inhibitor-1, or both, were used. Pharmacological characterization showed that mutant mice had normal basal levels of dopamine D(1) and D(2) receptors and adenosine A(2A) receptors. Basal expression levels of the striatonigral-specific neuropeptides substance P and prodynorphin and the immediate early genes c-fos and NGFI-A were also unaltered in mutant mice. A full D(1) receptor agonist, SKF 82958, up-regulated the expression of these neuropeptides and immediate early genes significantly more in wild-type mice than in mice lacking DARPP-32. Moreover, the additive stimulation of SKF 82958 and quinelorane, a D(2) receptor agonist, on c-fos mRNA in globus pallidus was significantly decreased in DARPP-32 and DARPP-32/I-1 knockout mice. No changes in dopamine receptor-induced gene expression were found in I-1 knockout mice. These results demonstrate an important involvement of DARPP-32 in dopamine receptor-mediated regulation of gene expression both in striatal neurons, which are enriched in DARPP-32, and in pallidal neurons, which do not contain DARPP-32.

Differential regulation of the dopamine D1, D2 and D3 receptor gene expression and changes in the phenotype of the striatal neurons in mice lacking the dopamine transporter.

Mice with a genetic disruption of the dopamine transporter (DAT-/-) exhibit locomotor hyperactivity and profound alterations in the homeostasis of the nigrostriatal system, e.g. a dramatic increase in the extracellular dopamine level. Here, we investigated the adaptive changes in dopamine D1, D2 and D3 receptor gene expression in the caudate putamen and nucleus accumbens of DAT-/- mice. We used quantitative in situ hybridization and found that the constitutive hyperdopaminergia results in opposite regulations in the gene expression for the dopamine receptors. In DAT-/- mice, we observed increased mRNA levels encoding the D3 receptor (caudate putamen, +60-85%; nucleus accumbens, +40-107%), and decreased mRNA levels for both D1 (caudate putamen, -34%; nucleus accumbens, -45%) and D2 receptors (caudate putamen, -36%; nucleus accumbens, -33%). Furthermore, we assessed the phenotypical organization of the striatal efferent neurons by using double in situ hybridization. Our results show that in DAT+/+ mice, D1 and D2 receptor mRNAs are segregated in two different main populations corresponding to substance P and preproenkephalin A mRNA-containing neurons, respectively. The phenotype of D1 or D2 mRNA-containing neurons was unchanged in both the caudate putamen and nucleus accumbens of DAT-/- mice. Interestingly, we found an increased density of preproenkephalin A-negative neurons that express the D3 receptor mRNA in the nucleus accumbens (core, +35%; shell, +46%) of DAT-/- mice. Our data further support the critical role for the D3 receptor in the regulation of D1-D2 interactions, an action being restricted to neurons coexpressing D1 and D3 receptors in the nucleus accumbens.

Distribution, biochemistry and function of striatal adenosine A2A receptors.

It is well known that the nucleoside adenosine exerts a modulatory influence in the central nervous system by activating G protein coupled receptors. Adenosine A2A receptors, the subject of the present review, are predominantly expressed in striatum, the major area of the basal ganglia. Activation of A2A receptors interferes with effects mediated by most of the principal neurotransmitters in striatum. In particular, the inhibitory interactions between adenosine acting on A2A receptors and dopamine acting on D2 receptors have been well examined and there is much evidence that A2A receptors may be a possible target for future development of drugs for treatment of Parkinson's disease, schizophrenia and affective disorders. Our understanding of the role of striatal A2A receptors has increased dramatically over the last few years. New selective antibodies, antagonist radioligands and optimized in situ hybridization protocols have provided detailed information on the distribution of A2A receptors in rodent as well as primate striatum. Studies on the involvement of A2A receptors in the regulation of DARPP-32 and the expression of immediate early genes, such as nerve growth factor-induced clone A and c-fos, have pointed out an important role for these receptors in regulating striatopallidal neurotransmission. Moreover, by using novel selective antagonists for A2A receptors and transgenic mice lacking functional A2A receptors, crucial information on the behavioral role of striatal A2A receptors has been provided, especially concerning their involvement in the stimulatory action of caffeine and the anti-Parkinsonian properties of A2A receptor antagonists. In the present review, current knowledge on the distribution, biochemistry and function of striatal A2A receptors is summarized.

Opposite tonic modulation of dopamine and adenosine on c-fos gene expression in striatopallidal neurons.

The impulse flow-dependent dopamine release in the striatum was acutely blocked by unilateral lesion of the medial forebrain bundle with 6-hydroxydopamine. Within 45 min this disruption reduced the striatal extracellular dopamine levels by 80% as determined by in vivo voltammetry. A strong induction of c-fos messenger RNA was detected in the ipsilateral dorsolateral striatum 75 min after 6-hydroxydopamine injection by in situ hybridization. Double labelling demonstrates that this induction was confined to neurons expressing the dopamine D2 receptor messenger RNA. At this time-point, there were no changes in the striatal levels of either tyrosine hydroxylase immunoreactivity or dopamine D2 receptor messenger RNA. The c-fos messenger RNA expression induced by acute 6-hydroxydopamine injection was abolished by intraperitoneal pretreatment with the dopamine D2 receptor agonist, quinelorane (2 mg/kg) and strongly reduced by administration of the selective adenosine A2A receptor antagonist SCH-58261 (5 mg/kg). The results reported here show, by using a novel methodological approach, that an acute decrease of dopamine release causes an induction of c-fos messenger RNA in dopamine D2 receptor-containing striatopallidal neurons. This, together with previous findings, demonstrates that the c-fos gene expression is tonically inhibited by the impulse flow-dependent dopamine release via D2 receptors. In addition, this study provides evidence that endogenous adenosine, acting via adenosine A2A receptors, induces striatal c-fos messenger RNA when extracellular dopamine levels are strongly reduced. Thus endogenous dopamine and adenosine exert opposite effects on the activity of the D2-containing striatopallidal neurons.

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.

Subpopulations of cortical GABAergic interneurons differ by their expression of D1 and D2 dopamine receptor subtypes.

D1 and D2 receptors have been described in different populations of efferent pyramidal neurons of the rat frontal cortex. Combined in situ hybridization and immunocytochemistry show here that these two subtypes are expressed in cortical GABAergic interneurons, with D1 and D2 mainly found in a subpopulation containing parvalbumin, whereas only 10% of the calbindin neurons express D1 receptors. These data indicate that various DA agonists may influence inhibitory circuits by distinct dopamine receptor subtypes.

Cellular distribution of adenosine A2A receptor mRNA in the primate striatum.

The cellular expression of adenosine A2A receptor mRNA in the adult monkey and human striatum was examined by using single and double in situ hybridization with ribonucleotide probes. Analysis on adjacent sections demonstrated a homogeneous overlapping expression of adenosine A2A receptor and preproenkephalin A mRNAs throughout nucleus caudatus, putamen, and nucleus accumbens. By contrast, high expression of preproenkephalin A mRNA but no expression of adenosine A2A receptor mRNA was found in the nucleus basalis of Meynert. Double in situ hybridization demonstrated an extensive colocalization of adenosine A2A receptor and preproenkephalin A mRNAs in approximately 50% of the medium-sized spiny neurons of the monkey nucleus caudatus, putamen, and nucleus accumbens. A small number of neurons (4-12%) that contained adenosine A2A receptor mRNA but not preproenkephalin A mRNA was found along the ventral borders of the striatum. Virtually all adenosine A2A receptor mRNA-containing neurons co-expressed dopamine D2 receptor mRNA, whereas only very few adenosine A2A receptor mRNA containing neurons co-expressed dopamine D1 receptor or substance P mRNAs. In addition, a sub-population of adenosine A2A receptor mRNA-expressing neurons that also contained preproenkephalin A mRNA was found in the septum in monkeys. These results demonstrate that there is a high expression of adenosine A2A receptor mRNA in the primate striatum that is extensively co-localized with dopamine D2 receptor and preproenkephalin A mRNAs. It is concluded that adenosine A2A receptors are likely to be important for the parallel organization of primate striatal neurotransmission and that these receptors could be a target for drug therapy in Parkinson's disease.