The Neurobiology of Activational Aspects of Motivation: Exertion of Effort, Effort-Based Decision Making, and the Role of Dopamine.

Motivational processes are complex and multifaceted, with both directional and activational aspects. Behavioral activation and exertion of effort are functions that enable organisms to overcome obstacles separating them from significant outcomes. In a complex environment, organisms make cost/benefit decisions, assessing work-related response costs and reinforcer preference. Animal studies have challenged the general idea that dopamine (DA) is best viewed as the reward transmitter and instead have illustrated the involvement of DA in activational and effort-related processes. Mesocorticolimbic DA is a key component of the effort-related motivational circuitry that includes multiple neurotransmitters and brain areas. Human studies have identified brain areas and transmitter systems involved in effort-based decision making and characterized the reduced selection of high-effort activities associated with motivational symptoms of depression and schizophrenia. Animal and human research on the neurochemistry of behavioral activation and effort-related processes makes an important conceptual contribution by illustrating the dissociable nature of distinct aspects of motivation.

Ultrastructural analysis of nigrostriatal dopaminergic terminals in a knockin mouse model of DYT1 dystonia.

DYT1 dystonia is associated with decreased striatal dopamine release. In this study, we examined the possibility that ultrastructural changes of nigrostriatal dopamine terminals could contribute to this neurochemical imbalance using a serial block face/scanning electron microscope (SBF/SEM) and three-dimensional reconstruction to analyse striatal tyrosine hydroxylase-immunoreactive (TH-IR) terminals and their synapses in a DYT1(ΔE) knockin (DYT1-KI) mouse model of DYT1 dystonia. Furthermore, to study possible changes in vesicle packaging capacity of dopamine, we used transmission electron microscopy to assess the synaptic vesicle size in striatal dopamine terminals. Quantitative comparative analysis of 80 fully reconstructed TH-IR terminals in the WT and DYT1-KI mice indicate (1) no significant difference in the volume of TH-IR terminals; (2) no major change in the proportion of axo-spinous versus axo-dendritic synapses; (3) no significant change in the post-synaptic density (PSD) area of axo-dendritic synapses, while the PSDs of axo-spinous synapses were significantly smaller in DYT1-KI mice; (4) no significant change in the contact area between TH-IR terminals and dendritic shafts or spines, while the ratio of PSD area/contact area decreased significantly for both axo-dendritic and axo-spinous synapses in DYT1-KI mice; (5) no significant difference in the mitochondria volume; and (6) no significant difference in the synaptic vesicle area between the two groups. Altogether, these findings suggest that abnormal morphometric changes of nigrostriatal dopamine terminals and their post-synaptic targets are unlikely to be a major source of reduced striatal dopamine release in DYT1 dystonia.

Comparison of dopamine release and uptake parameters across sex, species and striatal subregions.

For over four decades, fast-scan cyclic voltammetry (FSCV) has been used to selectively measure neurotransmitters such as dopamine (DA) with high spatial and temporal resolution, providing detailed information about the regulation of DA in the extracellular space. FSCV is an optimal method for determining concentrations of stimulus-evoked DA in brain tissue. When modelling diseases involving disturbances in DA transmission, preclinical rodent models are especially useful because of the availability of specialized tools and techniques that serve as a foundation for translational research. There is known heterogeneity in DA dynamics between and within DA-innervated brain structures and between males and females. However, systematic evaluations of sex- and species-differences across multiple areas are lacking. Therefore, using FSCV, we captured a broad range of DA dynamics across five sub-regions of the dorsal and ventral striatum of males and females of both rats and mice that reflect the functional heterogeneity of DA kinetics and dynamics within these structures. While numerous differences were found, in particular, we documented a strong, consistent pattern of increased DA transporter activity in females in all of the regions surveyed. The data herein are intended to be used as a resource for further investigation of DA terminal function.

Perturbation of serine enantiomers homeostasis in the striatum of MPTP-lesioned monkeys and mice reflects the extent of dopaminergic midbrain degeneration.

Loss of dopaminergic midbrain neurons perturbs l-serine and d-serine homeostasis in the post-mortem caudate putamen (CPu) of Parkinson's disease (PD) patients. However, it is unclear whether the severity of dopaminergic nigrostriatal degeneration plays a role in deregulating serine enantiomers' metabolism. Here, through high-performance liquid chromatography (HPLC), we measured the levels of these amino acids in 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-treated monkeys and MPTP-plus-probenecid (MPTPp)-treated mice to determine whether and how dopaminergic midbrain degeneration affects the levels of serine enantiomers in various basal ganglia subregions. In addition, in the same brain regions, we measured the levels of key neuroactive amino acids modulating glutamatergic neurotransmission, including l-glutamate, glycine, l-aspartate, d-aspartate, and their precursors l-glutamine, l-asparagine. In monkeys, MPTP treatment produced severe denervation of nigrostriatal dopaminergic fibers (⁓75%) and increased the levels of serine enantiomers in the rostral putamen (rPut), but not in the subthalamic nucleus, and the lateral and medial portion of the globus pallidus. Moreover, this neurotoxin significantly reduced the protein expression of the astrocytic serine transporter ASCT1 and the glycolytic enzyme GAPDH in the rPut of monkeys. Conversely, concentrations of d-serine and l-serine, as well as ASCT1 and GAPDH expression were unaffected in the striatum of MPTPp-treated mice, which showed only mild dopaminergic degeneration (⁓30%). These findings unveil a link between the severity of dopaminergic nigrostriatal degeneration and striatal serine enantiomers concentration, ASCT1 and GAPDH expression. We hypothesize that the up-regulation of d-serine and l-serine levels occurs as a secondary response within a homeostatic loop to support the metabolic and neurotransmission demands imposed by the degeneration of dopaminergic neurons.

Downregulation of PACAP and the PAC1 Receptor in the Basal Ganglia, Substantia Nigra and Centrally Projecting Edinger-Westphal Nucleus in the Rotenone model of Parkinson's Disease.

Numerous in vitro and in vivo models of Parkinson's disease (PD) demonstrate that pituitary adenylate cyclase-activating polypeptide (PACAP) conveys its strong neuroprotective actions mainly via its specific PAC1 receptor (PAC1R) in models of PD. We recently described the decrease in PAC1R protein content in the basal ganglia of macaques in the 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) model of PD that was partially reversed by levodopa therapy. In this work, we tested whether these observations occur also in the rotenone model of PD in the rat. The rotarod test revealed motor skill deterioration upon rotenone administration, which was reversed by benserazide/levodopa (B/L) treatment. The sucrose preference test suggested increased depression level while the open field test showed increased anxiety in rats rendered parkinsonian, regardless of the received B/L therapy. Reduced dopaminergic cell count in the substantia nigra pars compacta (SNpc) diminished the dopaminergic fiber density in the caudate-putamen (CPu) and decreased the peptidergic cell count in the centrally projecting Edinger-Westphal nucleus (EWcp), supporting the efficacy of rotenone treatment. RNAscope in situ hybridization revealed decreased PACAP mRNA (Adcyap1) and PAC1R mRNA (Adcyap1r1) expression in the CPu, globus pallidus, dopaminergic SNpc and peptidergic EWcp of rotenone-treated rats, but no remarkable downregulation occurred in the insular cortex. In the entopeduncular nucleus, only the Adcyap1r1 mRNA was downregulated in parkinsonian animals. B/L therapy attenuated the downregulation of Adcyap1 in the CPu only. Our current results further support the evolutionarily conserved role of the PACAP/PAC1R system in neuroprotection and its recruitment in the development/progression of neurodegenerative states such as PD.

Aggression Results in the Phosphorylation of ERK1/2 in the Nucleus Accumbens and the Dephosphorylation of mTOR in the Medial Prefrontal Cortex in Female Syrian Hamsters.

Like many social behaviors, aggression can be rewarding, leading to behavioral plasticity. One outcome of reward-induced aggression is the long-term increase in the speed in which future aggression-based encounters is initiated. This form of aggression impacts dendritic structure and excitatory synaptic neurotransmission in the nucleus accumbens, a brain region well known to regulate motivated behaviors. Yet, little is known about the intracellular signaling mechanisms that drive these structural/functional changes and long-term changes in aggressive behavior. This study set out to further elucidate the intracellular signaling mechanisms regulating the plasticity in neurophysiology and behavior that underlie the rewarding consequences of aggressive interactions. Female Syrian hamsters experienced zero, two or five aggressive interactions and the phosphorylation of proteins in reward-associated regions was analyzed. We report that aggressive interactions result in a transient increase in the phosphorylation of extracellular-signal related kinase 1/2 (ERK1/2) in the nucleus accumbens. We also report that aggressive interactions result in a transient decrease in the phosphorylation of mammalian target of rapamycin (mTOR) in the medial prefrontal cortex, a major input structure to the nucleus accumbens. Thus, this study identifies ERK1/2 and mTOR as potential signaling pathways for regulating the long-term rewarding consequences of aggressive interactions. Furthermore, the recruitment profile of the ERK1/2 and the mTOR pathways are distinct in different brain regions.

Prenatal Amphetamine-Induced Dopaminergic Alteration in a Gender- and Estrogen-Dependent Manner.

Prenatal exposure to amphetamine induces changes in dopamine receptors in mesolimbic areas and alters locomotor response to amphetamine during adulthood. Sex differences have been reported in amphetamine-induced brain activity and stress sensitivity. We evaluated the effects of prenatal amphetamine exposure on locomotor activity, dopamine receptors and tyrosine hydroxylase mRNA expression in nucleus accumbens and caudate-putamen in response to amphetamine challenge in adult female and male rats. The role of estrogen in the response to restraint stress was analyzed in ovariectomized, prenatally amphetamine-exposed rats. Pregnant rats were treated with D-amphetamine during days 15-21 of gestation. Nucleus accumbens and caudate-putamen were processed for mRNA determination by real-time PCR. In nucleus accumbens, higher mRNA dopamine (D3) receptor expression was found in basal and D-amphetamine-challenge conditions in female than male, and prenatal amphetamine increased the difference. No sex differences were observed in caudate-putamen. Basal saline-treated females showed higher locomotor activity than males. Amphetamine challenge in prenatally amphetamine-exposed rats increased locomotor activity in males and reduced it in females. In nucleus accumbens, estrogen diminished mRNA D1, D2 and D3 receptor expression in basal, and D1 and D3 in ovariectomized stressed rats. Estrogen prevented the increase in tyrosine hydroxylase expression induced by stress in ovariectomized prenatally exposed rats. In conclusion, estrogen modulates mRNA levels of D1, D2 and D3 receptors and tyrosine hydroxylase expression in nucleus accumbens; prenatal amphetamine-exposure effects on D3 receptors and behavioral responses were gender dependent.

Perinatal activation of ERα and ERß but not GPER-1 masculinizes female rat caudate-putamen medium spiny neuron electrophysiological properties.

Exposure to steroid sex hormones such as 17ß-estradiol (estradiol) during early life potentially permanently masculinize neuron electrophysiological phenotype. In rodents, one crucial component of this developmental process occurs in males, with estradiol aromatized in the brain from testes-sourced testosterone. However, it is unknown whether most neuron electrophysiological phenotypes are altered by this early masculinization process, including medium spiny neurons (MSNs) of the rat caudate-putamen. MSNs are the predominant and primary output neurons of the caudate-putamen and exhibit increased intrinsic excitability in females compared to males. Here, we hypothesize that since perinatal estradiol exposure occurs in males, then a comparable exposure in females to estradiol or its receptor agonists would be sufficient to induce masculinization. To test this hypothesis, we injected perinatal female rats with estradiol or its receptor agonists and then later assessed MSN electrophysiology. Female and male rats on postnatal day 0 and 1 were systemically injected with either vehicle, estradiol, the estrogen receptor (ER)α agonist PPT, the ERß agonist DPN, or the G-protein-coupled receptor 1 (GPER-1) agonist G1. On postnatal days 19 ± 2, MSN electrophysiological properties were assessed using whole cell patch clamp recordings. Estradiol exposure abolished increased intrinsic excitability in female compared to male MSNs. Exposure to either an ERα or ERß agonist masculinized female MSN evoked action potential firing rate properties, whereas exposure to an ERß agonist masculinized female MSN inward rectification properties. Exposure to ER agonists minimally impacted male MSN electrophysiological properties. These findings indicate that perinatal estradiol exposure masculinizes MSN electrophysiological phenotype via activation of ERα and ERß.NEW & NOTEWORTHY This study is the first to demonstrate that estradiol and estrogen receptor α and ß stimulation during early development sexually differentiates the electrophysiological properties of caudate-putamen medium spiny neurons, the primary output neuron of the striatal regions. Overall, this evidence provides new insight into the neuroendocrine mechanism by which caudate-putamen neuron electrophysiology is sexually differentiated and demonstrates the powerful action of early hormone exposure upon individual neuron electrophysiology.

Morphine exposure alters Fos expression in a sex-, age-, and brain region-specific manner during adolescence.

Data in both humans and preclinical animal models clearly indicate drug exposure during adolescence, when the "reward" circuitry of the brain develops, increases the risk of substance use and other mental health disorders later in life. Human data indicate that different neural and behavioral sequelae can be observed in early versus late adolescence. However, most studies with rodent models examine a single adolescent age compared to a mature adult age, and often only in males. Herein, we sought to determine whether the acute response to the opioid morphine would also differ across adolescence, and by sex. By quantifying Fos positive cells, a proxy for neural activity, at different stages during adolescence (pre-, early, mid-, and late adolescence) and in multiple reward regions (prefrontal cortex, nucleus accumbens, caudate/putamen), we determined that the neural response to acute morphine is highly dependent on adolescent age, sex, and brain region. These data suggest that heterogeneity in the consequences of adolescent opioid exposure may be due to age- and sex-specific developmental profiles in individual reward processing regions. In future studies, it will be important to add age within adolescence as an independent variable for a holistic view of healthy or abnormal reward-related neural development.

The estrous cycle modulates rat caudate-putamen medium spiny neuron physiology.

The neuroendocrine environment in which the brain operates is both dynamic and differs by sex. How differences in neuroendocrine state affect neuron properties has been significantly neglected in neuroscience research. Behavioral data across humans and rodents indicate that natural cyclical changes in steroid sex hormone production affect sensorimotor and cognitive behaviors in both normal and pathological contexts. These behaviors are critically mediated by the caudate-putamen. In the caudate-putamen, medium spiny neurons (MSNs) are the predominant and primary output neurons. MSNs express membrane-associated estrogen receptors and demonstrate estrogen sensitivity. However, how the cyclical hormone changes across the estrous cycle may modulate caudate-putamen MSN electrophysiological properties remains unknown. Here, we performed whole-cell patch-clamp recordings on male, diestrus female, proestrus female, and estrus female caudate-putamen MSNs. Action potential, passive membrane, and miniature excitatory post-synaptic current properties were assessed. Numerous MSN electrical properties robustly differed by cycle state, including resting membrane potential, rheobase, action potential threshold, maximum evoked action potential firing rate, and inward rectification. Strikingly, when considered independent of estrous cycle phase, all but one of these properties do not significantly differ from male MSNs. These data indicate that female caudate-putamen MSNs are sensitive to the estrous cycle, and more broadly, the importance of considering neuroendocrine state in studies of neuron physiology.

Repeated toluene exposure leads to neuroadaptation in dopamine release mechanisms within the nucleus accumbens core.

BACKGROUND: Intentionally inhaling volatile organic solvent like toluene for its intoxicating effects continues to be a public health concern. While repeated abuse of toluene has deleterious behavioral and health effects, little is known about the actions of toluene on the dopaminergic neurotransmitter system within the central nervous system. METHOD: The present study employed complementary neurochemical techniques of slice fast-scan cyclic voltammetry (FSCV) and in vivo microdialysis, to assess dopamine (DA) dynamics immediately after repeated exposure to 2000- or 4000-ppm toluene. DA D3 autoreceptor functionality, measured by FSCV with pharmacological manipulations and brain tissue content analysis with high performance liquid chromatography, were also used to account for the changes in the DA dynamics. RESULTS: Toluene-exposed mice had decreased stimulated DA release only in the nucleus accumbens core immediately after seven days of repeated exposure. DA uptake was decreased in the core only after 2000-ppm exposure. The differences in stimulated DA release were not attributed to alterations in intraneuronal DA levels as measured by tissue content analysis. Basal extracellular DA levels were not significantly different between the air- and toluene-treated mice. However, following an additional toluene exposure, mice had elevated extracellular DA levels in the nucleus accumbens during recovery. This potentiation in extracellular accumbal DA levels was further heightened following potassium stimulation. The accumbal DA D3 autoreceptor function did not appear to play a role as a potential mediator for these differences. CONCLUSION: Our FSCV and microdialysis results suggest a neuroadaptation in DA release mechanics within the nucleus accumbens, but the exact neuronal mechanism of toluene's impact remains elusive.

Estradiol rapidly modulates excitatory synapse properties in a sex- and region-specific manner in rat nucleus accumbens core and caudate-putamen.

Estradiol acutely facilitates sex differences in striatum-dependent behaviors. However, little is understood regarding the underlying mechanism. In striatal regions in adult rodents, estrogen receptors feature exclusively extranuclear expression, suggesting that estradiol rapidly modulates striatal neurons. We tested the hypothesis that estradiol rapidly modulates excitatory synapse properties onto medium spiny neurons (MSNs) of two striatal regions, the nucleus accumbens core and caudate-putamen in adult female and male rats. We predicted there would be sex-specific differences in pre- and postsynaptic locus and sensitivity. We further analyzed whether MSN intrinsic properties are predictive of estrogen sensitivity. Estradiol exhibited sex-specific acute effects in the nucleus accumbens core: miniature excitatory postsynaptic current (mEPSC) frequency robustly decreased in response to estradiol in female MSNs, and mEPSC amplitude moderately increased in response to estradiol in both male and female MSNs. This increase in mEPSC amplitude is associated with MSNs featuring increased intrinsic excitability. No MSN intrinsic electrical property associated with changes in mEPSC frequency. Estradiol did not acutely modulate mEPSC properties in the caudate-putamen of either sex. This is the first demonstration of acute estradiol action on MSN excitatory synapse function. This demonstration of sex and striatal region-specific acute estradiol neuromodulation revises our understanding of sex hormone action on striatal physiology and resulting behaviors.NEW & NOTEWORTHY This study is the first to demonstrate rapid estradiol neuromodulation of glutamatergic signaling on medium spiny neurons (MSNs), the major output neuron of the striatum. These findings emphasize that sex is a significant biological variable both in MSN sensitivity to estradiol and in pre- and postsynaptic mechanisms of glutamatergic signaling. MSNs in different regions exhibit diverse responses to estradiol. Sex- and region-specific estradiol-induced changes to excitatory signaling on MSNs explain sex differences partially underlying striatum-mediated behaviors and diseases.

Selective α-Synuclein Knockdown in Monoamine Neurons by Intranasal Oligonucleotide Delivery: Potential Therapy for Parkinson's Disease.

Progressive neuronal death in brainstem nuclei and widespread accumulation of α-synuclein are neuropathological hallmarks of Parkinson's disease (PD). Reduction of α-synuclein levels is therefore a potential therapy for PD. However, because α-synuclein is essential for neuronal development and function, α-synuclein elimination would dramatically impact brain function. We previously developed conjugated small interfering RNA (siRNA) sequences that selectively target serotonin (5-HT) or norepinephrine (NE) neurons after intranasal administration. Here, we used this strategy to conjugate inhibitory oligonucleotides, siRNA and antisense oligonucleotide (ASO), with the triple monoamine reuptake inhibitor indatraline (IND), to selectively reduce α-synuclein expression in the brainstem monoamine nuclei of mice after intranasal delivery. Following internalization of the conjugated oligonucleotides in monoamine neurons, reduced levels of endogenous α-synuclein mRNA and protein were found in substantia nigra pars compacta (SNc), ventral tegmental area (VTA), dorsal raphe nucleus (DR), and locus coeruleus (LC). α-Synuclein knockdown by ∼20%-40% did not cause monoaminergic neurodegeneration and enhanced forebrain dopamine (DA) and 5-HT release. Conversely, a modest human α-synuclein overexpression in DA neurons markedly reduced striatal DA release. These results indicate that α-synuclein negatively regulates monoamine neurotransmission and set the stage for the testing of non-viral inhibitory oligonucleotides as disease-modifying agents in α-synuclein models of PD.

Alterations in mGlu5 receptor expression and function in the striatum in a rat depression model.

Major depressive disorder is a common form of mental illness. Many brain regions are implicated in the pathophysiology and symptomatology of depression. Among key brain areas is the striatum that controls reward and mood and is involved in the development of core depression-like behavior in animal models of depression. While molecular mechanisms in this region underlying depression-related behavior are poorly understood, the glutamatergic input to the striatum is believed to play a role. In this study, we investigated changes in metabotropic glutamate (mGlu) receptor expression and signaling in the striatum of adult rats in response to prolonged (10-12 weeks) social isolation, a pre-validated animal paradigm modeling depression in adulthood. We found that mGlu5 receptor protein levels in the striatum were increased in rats that showed typical depression- and anxiety-like behavior after chronic social isolation. This increase in mGlu5 receptor expression was seen in both subdivisions of the striatum, the nucleus accumbens and caudate putamen. At subcellular and subsynaptic levels, mGlu5 receptor expression was elevated in surface membranes at synaptic sites. In striatal neurons, the mGlu5-associated phosphoinositide signaling pathway was augmented in its efficacy after prolonged social isolation. These data indicate that the mGlu5 receptor is a sensitive substrate of depression. Adulthood social isolation leads to the up-regulation of mGlu5 receptor expression and function in striatal neurons.

Role of adenosine A2A receptors in motor control: relevance to Parkinson's disease and dyskinesia.

Adenosine is an endogenous purine nucleoside that regulates several physiological functions, at the central and peripheral levels. Besides, adenosine has emerged as a major player in the regulation of motor behavior. In fact, adenosine receptors of the A2A subtype are highly enriched in the caudate-putamen, which is richly innervated by dopamine. Moreover, several studies in experimental animals have consistently demonstrated that the pharmacological antagonism of A2A receptors has a facilitatory influence on motor behavior. Taken together, these findings have envisaged A2A receptors as a promising target for symptomatic therapies aimed at ameliorating motor deficits. Accordingly, A2A receptor antagonists have been extensively studied as new agents for the treatment of Parkinson's disease (PD), the epitome of motor disorders. In this review, we provide an overview of the effects that adenosine A2A receptor antagonists elicit in rodent and primate experimental models of PD, with regard to the counteraction of motor deficits as well as to manifestation of dyskinesia and motor fluctuations. Moreover, we briefly present the results of clinical trials of A2A receptor antagonists in PD patients experiencing motor fluctuations, with particular regard to dyskinesia. Finally, we discuss the interaction between A2A receptor antagonists and serotonin receptor agonists, since combined administration of these drugs has recently emerged as a new potential therapeutic strategy in the treatment of dyskinesia.

Neuronal loss and gliosis in the rat striatum subjected to 15 and 30 minutes of middle cerebral artery occlusion.

Selective neuronal death or loss in certain brain regions has been well characterized in animal models of transient global cerebral ischemia. However, selective neuronal death in transient focal cerebral ischemia needs more investigation. Therefore, in this study, we studied selective neuronal death in the striatum (caudate putamen) of rats subjected to 15 or 30 min middle cerebral artery occlusion (MCAO). Neuronal death occurred in the dorsolateral field, not in the medial field in 30 min, not 15 min, MCAO-operated rats 5 days after MCAO using neuronal nuclear antigen immunohistochemistry and Fluoro-Jade B histofluorescence staining. In this group, immunoreactivity of glial fibrillary acidic protein in astrocytes was hardly shown in the dorsolateral field, although the immunoreactivity increased in the medial field. In addition, immunoreactivity of ionized calcium binding adapter molecule 1 in microglia was dramatically increased in the dorsolateral, not in the medial, field only in 30 min MCAO-operated rats. Briefly, these results show that at least 30 min of MCAO can evoke selective neuronal death, astrocytic dysfunction and microglial activation in the dorsolateral field of the rat striatum and suggest that a rat model of 30 min MCAO can be used to investigate mechanisms of neuronal death and gliosis following brief transient focal cerebral ischemic events for acute transient ischemic attack.

Dopamine D4 receptor stimulation prevents nigrostriatal dopamine pathway activation by morphine: relevance for drug addiction.

Morphine is one of the most effective drugs used for pain management, but it is also highly addictive. Morphine elicits acute and long-term adaptive changes at cellular and molecular level in the brain, which play a critical role in the development of tolerance, dependence and addiction. Previous studies indicated that the dopamine D4 receptor (D4 R) activation counteracts morphine-induced adaptive changes of the µ opioid receptor (MOR) signaling in the striosomes of the caudate putamen (CPu), as well as the induction of several Fos family transcription factors. Thus, it has been suggested that D4 R could play an important role avoiding some of the addictive effects of morphine. Here, using different drugs administration paradigms, it is determined that the D4 R agonist PD168,077 prevents morphine-induced activation of the nigrostriatal dopamine pathway and morphological changes of substantia nigra pars compacta (SNc) dopamine neurons, leading to a restoration of dopamine levels and metabolism in the CPu. Results from receptor autoradiography indicate that D4 R activation modulates MOR function in the substantia nigra pars reticulata (SNr) and the striosomes of the CPu, suggesting that these regions are critically involved in the modulation of SNc dopamine neuronal function through a functional D4 R/MOR interaction. In addition, D4 R activation counteracts the rewarding effects of morphine, as well as the development of hyperlocomotion and physical dependence without any effect on its analgesic properties. These results provide a novel role of D4 R agonist as a pharmacological strategy to prevent the adverse effects of morphine in the treatment of pain.

Dopamine transporters govern diurnal variation in extracellular dopamine tone.

The majority of neurotransmitter systems shows variations in state-dependent cell firing rates that are mechanistically linked to variations in extracellular levels, or tone, of their respective neurotransmitter. Diurnal variation in dopamine tone has also been demonstrated within the striatum, but this neurotransmitter is unique, in that variation in dopamine tone is likely not related to dopamine cell firing; this is largely because of the observation that midbrain dopamine neurons do not display diurnal fluctuations in firing rates. Therefore, we conducted a systematic investigation of possible mechanisms for the variation in extracellular dopamine tone. Using microdialysis and fast-scan cyclic voltammetry in rats, as well as wild-type and dopamine transporter (DAT) knock-out mice, we demonstrate that dopamine uptake through the DAT and the magnitude of subsecond dopamine release is inversely related to the magnitude of extracellular dopamine tone. We investigated dopamine metabolism, uptake, release, D2 autoreceptor sensitivity, and tyrosine hydroxylase expression and activity as mechanisms for this variation. Using this approach, we have pinpointed the DAT as a critical governor of diurnal variation in extracellular dopamine tone and, as a consequence, influencing the magnitude of electrically stimulated dopamine release. Understanding diurnal variation in dopamine tone is critical for understanding and treating the multitude of psychiatric disorders that originate from perturbations of the dopamine system.

Influence of caffeine on 3,4-methylenedioxymethamphetamine-induced dopaminergic neuron degeneration and neuroinflammation is age-dependent.

Previous studies have demonstrated that caffeine administration to adult mice potentiates glial activation induced by 3,4-methylenedioxymethamphetamine (MDMA). As neuroinflammatory response seems to correlate with neurodegeneration, and the young brain is particularly vulnerable to neurotoxicity, we evaluated dopamine neuron degeneration and glial activation in the caudate-putamen (CPu) and substantia nigra pars compacta (SNc) of adolescent and adult mice. Mice were treated with MDMA (4 × 20 mg/kg), alone or with caffeine (10 mg/kg). Interleukin (IL)-1ß, tumor necrosis factor (TNF)-α, neuronal nitric oxide synthase (nNOS) were evaluated in CPu, whereas tyrosine hydroxylase (TH), glial fibrillary acidic protein, and CD11b were evaluated in CPu and SNc by immunohistochemistry. MDMA decreased TH in SNc of both adolescent and adult mice, whereas TH-positive fibers in CPu were only decreased in adults. In CPu of adolescent mice, caffeine potentiated MDMA-induced glial fibrillary acidic protein without altering CD11b, whereas in SNc caffeine did not influence MDMA-induced glial activation. nNOS, IL-1ß, and TNF-α were increased by MDMA in CPu of adults, whereas in adolescents, levels were only elevated after combined MDMA plus caffeine. Caffeine alone modified only nNOS. Results suggest that the use of MDMA in association with caffeine during adolescence may exacerbate the neurotoxicity and neuroinflammation elicited by MDMA. Previous studies have demonstrated that caffeine potentiated glial activation induced by 3,4-methylenedioxymethamphetamine (MDMA) in adult mice. In this study, caffeine was shown to potentiate MDMA-induced dopamine neuron degeneration in substantia nigra pars compacta, astrogliosis, and TNF-α levels in caudate-putamen of adolescent mice. Results suggest that combined use of MDMA plus caffeine during adolescence may worsen the neurotoxicity and neuroinflammation elicited by MDMA.

Central HIV-1 Tat exposure elevates anxiety and fear conditioned responses of male mice concurrent with altered mu-opioid receptor-mediated G-protein activation and ß-arrestin 2 activity in the forebrain.

Co-exposure to opiates and HIV/HIV proteins results in enhanced CNS morphological and behavioral deficits in HIV(+) individuals and in animal models. Opiates with abuse liability, such as heroin and morphine, bind preferentially to and have pharmacological actions through µ-opioid-receptors (MORs). The mechanisms underlying opiate-HIV interactions are not understood. Exposure to the HIV-1 transactivator of transcription (Tat) protein causes neurodegenerative outcomes that parallel many aspects of the human disease. We have also observed that in vivo exposure to Tat results in apparent changes in morphine efficacy, and thus have hypothesized that HIV proteins might alter MOR activation. To test our hypothesis, MOR-mediated G-protein activation was determined in neuroAIDS-relevant forebrain regions of transgenic mice with inducible CNS expression of HIV-1 Tat. G-protein activation was assessed by MOR agonist-stimulated [(35)S]guanosine-5'-O-(3-thio)triphosphate ([(35)S]GTPγS) autoradiography in brain sections, and in concentration-effect curves of MOR agonist-stimulated [(35)S]GTPγS binding in membranes isolated from specific brain regions. Comparative studies were done using the MOR-selective agonist DAMGO ([D-Ala(2), N-MePhe(4), Gly-ol]-enkephalin) and a more clinically relevant agonist, morphine. Tat exposure reduced MOR-mediated G-protein activation in an agonist, time, and regionally dependent manner. Levels of the GPCR regulatory protein ß-arrestin-2, which is involved in MOR desensitization, were found to be elevated in only one affected brain region, the amygdala; amygdalar ß-arrestin-2 also showed a significantly increased association with MOR by co-immunoprecipitation, suggesting decreased availability of MOR. Interestingly, this correlated with changes in anxiety and fear-conditioned extinction, behaviors that have substantial amygdalar input. We propose that HIV-1 Tat alters the intrinsic capacity of MOR to signal in response to agonist binding, possibly via a mechanism involving altered expression and/or function of ß-arrestin-2.