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Presynaptic increase in striatal dopamine is the primary dopaminergic abnormality in schizophrenia, but the underlying mechanisms are not understood. Here, we hypothesized that increased expression of endogenous GDNF could induce dopaminergic abnormalities that resemble those seen in schizophrenia. To test the impact of GDNF elevation, without inducing adverse effects caused by ectopic overexpression, we developed a novel in vivo approach to conditionally increase endogenous GDNF expression. We found that a 2–3-fold increase in endogenous GDNF in the brain was sufficient to induce molecular, cellular, and functional changes in dopamine signalling in the striatum and prefrontal cortex, including increased striatal presynaptic dopamine levels and reduction of dopamine in prefrontal cortex. Mechanistically, we identified adenosine A2a receptor (A2AR), a G-protein coupled receptor that modulates dopaminergic signalling, as a possible mediator of GDNF-driven dopaminergic abnormalities. We further showed that pharmacological inhibition of A2AR with istradefylline partially normalised striatal GDNF and striatal and cortical dopamine levels in mice. Lastly, we found that GDNF levels are increased in the cerebrospinal fluid of first episode psychosis patients, and in post-mortem striatum of schizophrenia patients. Our results reveal a possible contributor for increased striatal dopamine signalling in a subgroup of schizophrenia patients and suggest that GDNF—A2AR crosstalk may regulate dopamine function in a therapeutically targetable manner.

Parkinson’s disease (PD) is a progressive neurodegenerative disease that comprises a range of motor and nonmotor symptoms. Glial cell line-derived neurotrophic factor (GDNF) promotes the survival of dopamine neuronsin vitroandin vivo, and intracranial delivery of GDNF has been tested in six clinical trials for treating PD. However, clinical trials with ectopic GDNF have yielded variable results, which could in part result from abnormal expression site and levels caused by ectopic overexpression. Therefore, an important open question is whether an increase in endogenous GDNF expression could be potent in reversing PD progression. Here, we tested the therapeutic potential of endogenous GDNF using mice in which endogenous GDNF can be conditionally upregulated specifically in cells that express GDNF naturally (conditional GDNF hypermorphic mice;GdnfcHyper). We analyzed the impact of endogenous GDNF upregulation in both neuroprotection and neurorestoration procedures, and for both motor and nonmotor symptoms in the proteasome inhibitor lactacystin (LC) model of PD. Our results showed that upregulation of endogenous GDNF in the adult striatum is not protective in LC-induced PD model in mice. Since age is the largest risk factor for PD, we also analyzed the effect of deletion of endogenous GDNF in agedGdnfconditional knock-out mice. We found that GDNF deletion does not increase susceptibility to LC-induced damage. We conclude that endogenous GDNF does not impact the outcome in the LC-induced proteasome inhibition mouse model of Parkinson’s disease.

Glial cell line-derived neurotrophic factor (GDNF) supports function and survival of dopamine neurons that degenerate in Parkinson’s disease (PD). Ectopic delivery of GDNF in clinical trials to treat PD is safe but lacks significant therapeutic effect. In pre-clinical models, ectopic GDNF is effective but causes adverse effects, including downregulation of tyrosine hydroxylase, only a transient boost in dopamine metabolism, aberrant neuronal sprouting, and hyperactivity. Hindering development of GDNF mimetic increased signaling via GDNF receptor RET by activating mutations results in cancer. Safe and effective mode of action must be defined first in animal models to develop successful GDNF-based therapies. Previously we showed that about a 2-fold increase in endogenous GDNF expression is safe and results in increased motor and dopaminergic function and protection in a PD model in young animals. Recently, similar results were reported using a novel Gdnf mRNA-targeting strategy. Next, it is important to establish the safety of a long-term increase in endogenous GDNF expression. We report behavioral, dopamine system, and cancer analysis of five cohorts of aged mice with a 2-fold increase in endogenous GDNF. We found a sustained increase in dopamine levels, improvement in motor learning, and no side effects or cancer. These results support the rationale for further development of endogenous GDNF-based treatments and GDNF mimetic., Graphical Abstract, Ectopic GDNF delivery raises concerns about efficacy in clinical trials of Parkinson’s disease and safety issues in preclinical studies. Andressoo and colleagues showed that a long-term, 2-fold increase in endogenous GDNF levels in mice is safe and improves motor and dopaminergic system function, supporting further development of endogenous GDNF-based therapies.

Aims Recently we developed a model to study alcohol-seeking behaviour after withdrawal in a social context in female mice. The model raised several questions that we were eager to address to improve methodology. Methods In our model, female mice were group-housed in automated cages with three conditioned (CS+) corners and water in both sides of one separate non-conditioned corner. Water was available with opened doors at all the time of training. We established conditioning by pairing alcohol drinking with light cues. Here, we introduced prolonged access to increasing concentrations of alcohol instead of intermittent access. To study motivation to drink alcohol, we carried out the extinction tests on withdrawal days 1 (WD1) and 10 (WD10). During tests, the light cues were present in conditioned corners, but there was no liquid in the bottles. Results We found that the number of visits and nosepokes in the CS+ corner in the alcohol group was much higher than in the water group. Also, during training, the consumption of alcohol was increasing. In the extinction tests, we found that the number of nosepokes in the CS+ corner increased in the alcohol group on both WD1 and WD10. Conclusions Our study supports that alcohol-seeking behaviour after withdrawal can be modelled and studied in group-housed animals and environments without social isolation.

Serotonergic neurons in the dorsal raphe (DR) nucleus are associated with several psychiatric disorders including depression and anxiety disorders, which often have a neurodevelopmental component. During embryonic development, GATA transcription factors GATA2 and GATA3 operate as serotonergic neuron fate selectors and regulate the differentiation of serotonergic neuron subtypes of DR. Here, we analyzed the requirement of GATA cofactor ZFPM1 in the development of serotonergic neurons usingZfpm1conditional mouse mutants. Our results demonstrated that, unlike the GATA factors, ZFPM1 is not essential for the early differentiation of serotonergic precursors in the embryonic rhombomere 1. In contrast, in perinatal and adult male and femaleZfpm1mutants, a lateral subpopulation of DR neurons (ventrolateral part of the DR) was lost, whereas the number of serotonergic neurons in a medial subpopulation (dorsal region of the medial DR) had increased. Additionally, adult male and femaleZfpm1mutants had reduced serotonin concentration in rostral brain areas and displayed increased anxiety-like behavior. Interestingly, femaleZfpm1mutant mice showed elevated contextual fear memory that was abolished with chronic fluoxetine treatment. Altogether, these results demonstrate the importance of ZFPM1 for the development of DR serotonergic neuron subtypes involved in mood regulation. It also suggests that the neuronal fate selector function of GATAs is modulated by their cofactors to refine the differentiation of neuronal subtypes.SIGNIFICANCE STATEMENTPredisposition to anxiety disorders has both a neurodevelopmental and a genetic basis. One of the brainstem nuclei involved in the regulation of anxiety is the dorsal raphe, which contains different subtypes of serotonergic neurons. We show that inactivation of a transcriptional cofactor ZFPM1 in mice results in a developmental failure of laterally located dorsal raphe serotonergic neurons and changes in serotonergic innervation of rostral brain regions. This leads to elevated anxiety-like behavior and contextual fear memory, alleviated by chronic fluoxetine treatment. Our work contributes to understanding the neurodevelopmental mechanisms that may be disturbed in the anxiety disorder.

Background Motor symptoms of Parkinson's disease (PD) are caused by degeneration and progressive loss of nigrostriatal dopamine neurons. Currently, no cure for this disease is available. Existing drugs alleviate PD symptoms but fail to halt neurodegeneration. Glial cell line–derived neurotrophic factor (GDNF) is able to protect and repair dopamine neurons in vitro and in animal models of PD, but the clinical use of GDNF is complicated by its pharmacokinetic properties. The present study aimed to evaluate the neuronal effects of a blood‐brain‐barrier penetrating small molecule GDNF receptor Rearranged in Transfection agonist, BT13, in the dopamine system. Methods We characterized the ability of BT13 to activate RET in immortalized cells, to support the survival of cultured dopamine neurons, to protect cultured dopamine neurons against neurotoxin‐induced cell death, to activate intracellular signaling pathways both in vitro and in vivo , and to regulate dopamine release in the mouse striatum as well as BT13's distribution in the brain. Results BT13 potently activates RET and downstream signaling cascades such as Extracellular Signal Regulated Kinase and AKT in immortalized cells. It supports the survival of cultured dopamine neurons from wild‐type but not from RET‐knockout mice. BT13 protects cultured dopamine neurons from 6‐Hydroxydopamine (6‐OHDA) and 1‐methyl‐4‐phenylpyridinium (MPP+)–induced cell death only if they express RET. In addition, BT13 is absorbed in the brain, activates intracellular signaling cascades in dopamine neurons both in vitro and in vivo, and also stimulates the release of dopamine in the mouse striatum. Conclusion The GDNF receptor RET agonist BT13 demonstrates the potential for further development of novel disease‐modifying treatments against PD. © 2019 The Authors. Movement Disorders published by Wiley Periodicals LLC. on behalf of International Parkinson and Movement Disorder Society.

Addictive drugs enhance dopamine release in the striatum, which can lead to compulsive drug-seeking after repeated exposure. Glial cell line-derived neurotrophic factor (GDNF) is an important regulator of midbrain dopamine neurons, and may play a mechanistic role in addiction-related behaviors. To elucidate the components of GDNF-signaling that contribute to addiction-related behaviors of place preference and its extinction, we utilized two genetically modified GDNF mouse models in an amphetamine induced conditioned place preference (CPP) paradigm and evaluated how the behavioral findings correlate with dopamine signaling in the dorsal and ventral striatum. We utilized two knock-in mouse strains to delineate contributions of GDNF and Ret signaling using MEN2B mice (constitutively active GDNF receptor Ret), and GDNF hypermorphic mice (enhanced endogenous GDNF expression). The duration of amphetamine-induced CPP was greatly enhanced in MEN2B mice, but not in the GDNF hypermorphic mice. The enhanced duration of CPP was correlated with increased tyrosine hydroxylase (TH) expression and dopamine content in the ventral striatum. Together, our results suggest that downstream components of GDNF signaling, in this case Ret, may mediate persistent drug-seeking behavior through increased TH expression and dopamine levels in the mesolimbic dopamine neurons. (C) 2017 Elsevier Ltd. All rights reserved.

Midbrain dopamine neuron dysfunction contributes to various psychiatric and neurological diseases, including drug addiction and Parkinson's disease. Because of its well established dopaminotrophic effects, the therapeutic potential of glial cell line-derived neurotrophic factor (GDNF) has been studied extensively in various disorders with disturbed dopamine homeostasis. However, the outcomes from preclinical and clinical studies vary, highlighting a need for a better understanding of the physiological role of GDNF on striatal dopaminergic function. Nevertheless, the current lack of appropriate animal models has limited this understanding. Therefore, we have generated novel mouse models to study conditionalGdnfdeletion in the CNS during embryonic development and reduction of striatal GDNF levels in adult mice via AAV-Cre delivery. We found that both of these mice have reduced amphetamine-induced locomotor response and striatal dopamine efflux. Embryonic GDNF deletion in the CNS did not affect striatal dopamine levels or dopamine release, but dopamine reuptake was increased due to increased levels of both total and synaptic membrane-associated dopamine transporters. Collectively, these results suggest that endogenous GDNF plays an important role in regulating the function of dopamine transporters in the striatum.SIGNIFICANCE STATEMENTDelivery of ectopic glial cell line-derived neurotrophic factor (GDNF) promotes the function, plasticity, and survival of midbrain dopaminergic neurons, the dysfunction of which contributes to various neurological and psychiatric diseases. However, how the deletion or reduction of GDNF in the CNS affects the function of dopaminergic neurons has remained unknown. Using conditionalGdnfknock-out mice, we found that endogenous GDNF affects striatal dopamine homeostasis and regulates amphetamine-induced behaviors by regulating the level and function of dopamine transporters. These data regarding the physiological role of GDNF are relevant in the context of neurological and neurodegenerative diseases that involve changes in dopamine transporter function.

Hypothalamic histaminergic neurons regulate a variety of homeostatic, metabolic and cognitive functions. Recent data have suggested a modulatory role of histamine and histamine receptors in shaping striatal activity and connected the histaminergic system to neuropsychiatric disorders. We characterized exploratory behavior and striatal neurotransmission in mice lacking the histamine producing enzyme histidine decarboxylase (Hdc). The mutant mice showed a distinct behavioral pattern during exploration of novel environment, specifically, increased frequency of rearing seated against the wall, jumping and head/body shakes. This behavioral phenotype was associated with decreased levels of striatal dopamine and serotonin and increased level of dopamine metabolite DOPAC. Gene expression levels of dynorphin and enkephalin, opioids released by medium spiny neurons of striatal direct and indirect pathways respectively, were lower in Hdc mutant mice than in control animals. A low dose of amphetamine led to similar behavioral and biochemical outcomes in both genotypes. Increased striatal dopamine turnover was observed in Hdc KO mice after treatment with dopamine precursor l-Dopa. Overall, our study suggests a role for striatal dopamine and opioid peptides in formation of distinct behavioral phenotype of Hdc KO mice.

Adeno-associated virus (AAV) vector-mediated delivery of human α-synuclein (α-syn) gene in rat substantia nigra (SN) results in increased expression of α-syn protein in the SN and striatum which can progressively degenerate dopaminergic neurons. Therefore, this model is thought to recapitulate the neurodegeneration in Parkinson's disease. Here, using AAV to deliver α-syn above the SN in male and female rats resulted in clear expression of human α-syn in the SN and striatum. The protein was associated with moderate behavioral deficits and some loss of tyrosine hydroxylase (TH) in the nigrostriatal areas. However, the immunohistochemistry results were highly variable and showed little to no correlation with behavior and the amount of α-syn present. Expression of green fluorescent protein (GFP) was used as a control to monitor gene delivery and expression efficacy. AAV-GFP resulted in a similar or greater TH loss compared to AAV-α-syn and therefore an additional vector that does not express a protein was tested. Vectors with double-floxed inverse open reading frame (DIO ORF) encoding fluorescent proteins that generate RNA that is not translated also resulted in TH downregulation in the SN but showed no significant behavioral deficits. These results demonstrate that although expression of wild-type human α-syn can cause neurodegeneration, the variability and lack of correlation with outcome measures are drawbacks with the model. Furthermore, design and control selection should be considered carefully because of conflicting conclusions due to AAV downregulation of TH, and we recommend caution with having highly regulated TH as the only marker for the dopamine system.

Neurotrophic factors (NTFs) hold potential as disease-modifying therapies for neurodegenerative disorders like Parkinson’s disease. Glial cell line-derived neurotrophic factor (GDNF), cerebral dopamine neurotrophic factor (CDNF), and mesencephalic astrocyte-derived neurotrophic factor (MANF) have shown neuroprotective and restorative effects on nigral dopaminergic neurons in various animal models of Parkinson’s disease. To date, however, their effects on brain neurochemistry have not been compared using in vivo microdialysis. We measured extracellular concentration of dopamine and activity of dopamine neurochemistry-regulating enzymes in the nigrostriatal system of rat brain. NTFs were unilaterally injected into the striatum of intact Wistar rats. Brain microdialysis experiments were performed 1 and 3 weeks later in freely-moving animals. One week after the treatment, we observed enhanced stimulus-evoked release of dopamine in the striatum of MANF-treated rats, but not in rats treated with GDNF or CDNF. MANF also increased dopamine turnover. Although GDNF did not affect the extracellular level of dopamine, we found significantly elevated tyrosine hydroxylase (TH) and catechol-O-methyltransferase (COMT) activity and decreased monoamine oxidase A (MAO-A) activity in striatal tissue samples 1 week after GDNF injection. The results show that GDNF, CDNF, and MANF have divergent effects on dopaminergic neurotransmission, as well as on dopamine synthetizing and metabolizing enzymes. Although the cellular mechanisms remain to be clarified, knowing the biological effects of exogenously administrated NTFs in intact brain is an important step towards developing novel neurotrophic treatments for degenerative brain diseases. Electronic supplementary material The online version of this article (10.1007/s12035-018-0872-8) contains supplementary material, which is available to authorized users.

It is about half a century ago when the so-called "Wise model" to study alcohol drinking behavior in rats was established. The model was based on voluntary intermittent access to increasing concentrations of alcohol. We aimed to establish a model of alcohol craving and used an extinction test on withdrawal days 1 and 10 to study motivation for alcohol. For this purpose, the alcohol drinking training was paired with light cues to establish conditioning. The extinction test was carried out without alcohol but in the presence of light cues and empty bottles. The outcome measures were number of visits, nosepokes, and licks in the conditioned corner where the number of nosepokes represents how much mice "want" alcohol and number of licks shows how much mice "like" alcohol. The number of nosepokes during withdrawal is a measure of craving. Late withdrawal craving was found when intermittent alcohol access was carried out in the automated cages. In this case, we observed a significant increase in the number of nosepokes on both withdrawal days 1 and 10 as compared to water control. The number of nosepokes in the withdrawal days did not correlate with alcohol dose, but number of nosepokes on withdrawal day 1 correlated with the number of nosepokes on the last training day. Although we did not observe incubation of alcohol craving after withdrawal, the craving was increased at the late time point. We conclude that we have established a new tool to study alcohol drinking behavior and craving in female mice.

Alpha-synuclein is the main component of Lewy bodies, a histopathological finding of Parkinson's disease. Prolyl oligopeptidase (PREP) is a serine protease that binds to alpha-synuclein and accelerates its aggregation in vitro. PREP enzyme inhibitors have been shown to block the alpha-synuclein aggregation process in vitro and in cellular models, and also to enhance the clearance of alpha-synuclein aggregates in transgenic mouse models. Moreover, PREP inhibitors have induced alterations in dopamine and metabolite levels, and dopamine transporter immunoreactivity in the nigrostriatal tissue. In this study, we characterized the role of PREP in the nigrostriatal dopaminergic and GABAergic systems of wild-type C57Bl/6 and PREP knockout mice, and the effects of PREP overexpression on these systems. Extracellular concentrations of dopamine and protein levels of phosphorylated dopamine transporter were increased and dopamine reuptake was decreased in the striatum of PREP knockout mice, suggesting increased internalization of dopamine transporter from the presynaptic membrane. Furthermore, PREP overexpression increased the level of dopamine transporters in the nigrostriatal tissue but decreased phosphorylated dopamine transporters in the striatum in wild-type mice. Our results suggest that PREP regulates the function of dopamine transporter, possibly by controlling the phosphorylation and transport of dopamine transporter into the striatum or synaptic membrane.

Degeneration of nigrostriatal dopaminergic system is the principal lesion in Parkinson's disease. Because glial cell line-derived neurotrophic factor (GDNF) promotes survival of dopamine neurons in vitro and in vivo, intracranial delivery of GDNF has been attempted for Parkinson's disease treatment but with variable success. For improving GDNF-based therapies, knowledge on physiological role of endogenous GDNF at the sites of its expression is important. However, due to limitations of existing genetic model systems, such knowledge is scarce. Here, we report that prevention of transcription of Gdnf 3'UTR in Gdnf endogenous locus yields GDNF hypermorphic mice with increased, but spatially unchanged GDNF expression, enabling analysis of postnatal GDNF function. We found that increased level of GDNF in the central nervous system increases the number of adult dopamine neurons in the substantia nigra pars compacta and the number of dopaminergic terminals in the dorsal striatum. At the functional level, GDNF levels increased striatal tissue dopamine levels and augmented striatal dopamine release and re-uptake. In a proteasome inhibitor lactacystin-induced model of Parkinson's disease GDNF hypermorphic mice were protected from the reduction in striatal dopamine and failure of dopaminergic system function. Importantly, adverse phenotypic effects associated with spatially unregulated GDNF applications were not observed. Enhanced GDNF levels up-regulated striatal dopamine transporter activity by at least five fold resulting in enhanced susceptibility to 6-OHDA, a toxin transported into dopamine neurons by DAT. Further, we report how GDNF levels regulate kidney development and identify microRNAs miR-9, miR-96, miR-133, and miR-146a as negative regulators of GDNF expression via interaction with Gdnf 3'UTR in vitro. Our results reveal the role of GDNF in nigrostriatal dopamine system postnatal development and adult function, and highlight the importance of correct spatial expression of GDNF. Furthermore, our results suggest that 3'UTR targeting may constitute a useful tool in analyzing gene function.

Besides alcohol, alcohol-preferring AA and alcohol-avoiding ANA rats differ also with respect to other abused drugs. To study the molecular basis of these differences, we examined the expression of two transcription factors implicated in addiction, DeltaFosB and pCREB, in brain dopaminergic regions of AA and ANA rats. The effects of morphine and nicotine were studied to relate the behavioral and molecular changes induced by these drugs. Baseline FosB/DeltaFosB immunoreactivity (IR) in the nucleus accumbens core and pCREB IR in the prefrontal cortex (PFC) were elevated in AA rats. Morphine increased DeltaFosB-like IR more readily in the caudate-putamen of AA rats than in ANA rats. In the PFC morphine decreased pCREB IR in AA rats, but increased it in ANA rats. In addition to enhanced locomotor response, the development of place preference to morphine was enhanced in AA rats. The enhanced nicotine-induced locomotor sensitization found in AA compared with ANA rats seems to depend in addition to dopamine and DeltaFosB on other mechanisms. These findings suggest that enhanced sensitivity of AA rats to morphine is related to augmented morphine-induced expression of FosB/DeltaFosB and morphine-induced reduction of pCREB levels. Moreover, altered innate expression of FosB/DeltaFosB and pCREB in AA rats is likely to affect the sensitivity of these rats to abused drugs.

The Ret receptor tyrosine kinase is the common signaling receptor for the glial cell line-derived neurotrophic factor (GDNF) family ligands. The Met918Thr mutation leads to constitutive activation of Ret and is responsible for dominantly inherited cancer syndrome MEN2B. Previously, we found that the mice carrying the mutation (MEN2B mice) have profoundly increased tissue dopamine (DA) concentrations in the striatum as well as increased striatal levels of tyrosine hydroxylase (TH) and dopamine transporter. The aim of this study was to characterize the striatal dopaminergic neurotransmission in MEN2B mice and to clarify the mechanisms by which they compensate their over-production of DA. We found that tyrosine hydroxylase activity and DA synthesis are increased in MEN2B mice. Augmented effects of alpha-methyl-para-tyrosine (alphaMT, an inhibitor of TH) and tetrabenazine (VMAT2 blocker) on DA levels suggest that also storage of DA is increased in MEN2B mice. There was no difference in the basal extracellular DA concentrations or potassium-evoked DA release between the genotypes. The effects of cocaine and haloperidol were also similar between the genotypes as assessed by in vivo microdialysis. However, with in vivo voltammetry we found increase in stimulated DA release in MEN2B mice and detailed analysis of DA overflow showed that uptake of DA was also enhanced in MEN2B mice. Thus, our data show that enhanced synthesis of DA leading to increased storage and releasable pools in pre-synaptic terminals in MEN2B mice apparently also leads to increased DA release, which in turn is compensated by higher dopamine transporter activity.

Dopamine sulfate (DA-3- and DA-4-S) have been determined in the human brain, but it is unclear whether they are locally formed in the central nervous system (CNS), or transported into the CNS from peripheral sources. In the current study, permeation of the blood-brain barrier (BBB) by DA-S was studied by injecting C-13(6)-labelled regioisomers of DA-S ((13)DA-3-S and (13)DA-4-S) and dopamine (DA) subcutaneously (s.c.) in anesthetized rats, then analyzing brain microdialysis and plasma samples by UPLC-MS/MS. The results in the microdialysis samples demonstrated that brain concentrations of (13)DA-S regioisomers clearly increased after the s.c. injections. The concentration of DA did not change, indicating the permeation of DA-S through an intact BBB. The analysis of plasma samples, however, showed that DA-S only permeates the BBB to a small extent, as the concentrations in plasma were substantially higher than in the microdialysis samples. The results also showed that the concentrations of DA-3-S were around three times higher than the concentrations of DA-4-S in rat brain, as well as in the plasma samples after the s.c. injections, indicating that DA-3-S and DA-4-S permeate the BBB with similar efficiency. The fate of (13)DA-S in brain was followed by monitoring C-13(6)-labelled DA-S hydrolysis products, i.e. (13)DA and its common metabolites; however, no C-13(6)-labelled products were detected. This suggests that DA-S either permeates through the BBB back to the peripheral circulation or is dissociated or metabolized by unexpected mechanisms.

Glial cell line-derived neurotrophic factor (GDNF) has been tested in clinical trials to treat Parkinson’s disease with promising but variable results. Improvement of therapeutic effectiveness requires solid understanding of the physiological role of GDNF in the maintenance of the adult brain catecholamine system. However, existing data on this issue is contradictory. Here we show with three complementary approaches that, independent of the time of reduction, Gdnf is not required for maintenance of catecholaminergic neurons in adult mice.

Degeneration of nigrostriatal dopaminergic system is the principal lesion in Parkinson’s disease. Because glial cell line-derived neurotrophic factor (GDNF) promotes survival of dopamine neurons in vitro and in vivo, intracranial delivery of GDNF has been attempted for Parkinson’s disease treatment but with variable success. For improving GDNF-based therapies, knowledge on physiological role of endogenous GDNF at the sites of its expression is important. However, due to limitations of existing genetic model systems, such knowledge is scarce. Here, we report that prevention of transcription of Gdnf 3’UTR in Gdnf endogenous locus yields GDNF hypermorphic mice with increased, but spatially unchanged GDNF expression, enabling analysis of postnatal GDNF function. We found that increased level of GDNF in the central nervous system increases the number of adult dopamine neurons in the substantia nigra pars compacta and the number of dopaminergic terminals in the dorsal striatum. At the functional level, GDNF levels increased striatal tissue dopamine levels and augmented striatal dopamine release and re-uptake. In a proteasome inhibitor lactacystin-induced model of Parkinson’s disease GDNF hypermorphic mice were protected from the reduction in striatal dopamine and failure of dopaminergic system function. Importantly, adverse phenotypic effects associated with spatially unregulated GDNF applications were not observed. Enhanced GDNF levels up-regulated striatal dopamine transporter activity by at least five fold resulting in enhanced susceptibility to 6-OHDA, a toxin transported into dopamine neurons by DAT. Further, we report how GDNF levels regulate kidney development and identify microRNAs miR-9, miR-96, miR-133, and miR-146a as negative regulators of GDNF expression via interaction with Gdnf 3’UTR in vitro. Our results reveal the role of GDNF in nigrostriatal dopamine system postnatal development and adult function, and highlight the importance of correct spatial expression of GDNF. Furthermore, our results suggest that 3’UTR targeting may constitute a useful tool in analyzing gene function., Author Summary Intracranial delivery of GDNF has been attempted for Parkinson’s disease (PD) treatment but with variable success. For improving GDNF-based therapies, knowledge on physiological role of endogenous GDNF at the sites of its expression is important. However, due to limitations of existing genetic model systems, such knowledge is scarce. Here, we utilize an innovative genetic approach by targeting the 3’UTR regulation of Gdnf in mice. Such animals express elevated levels of Gdnf exclusively in natively Gdnf-expressing cells, enabling dissection of endogenous GDNF functions in vivo. We show that endogenous GDNF regulates dopamine system development and function and protects mice in a rodent PD model without side effects associated with ectopic GDNF applications. Further, we report how GDNF levels regulate kidney development and identify microRNAs which control GDNF expression. Our study highlights the importance of correct spatial expression of GDNF and opens a novel approach to study gene function in mice.

Effects of ethanol on the accumbal extracellular concentrations of dopamine, as well as of the amino acid transmitters gamma-amino butyric acid (GABA), glutamate and taurine, were studied in the alcohol-insensitive (alcohol-tolerant, AT) and alcohol-sensitive (alcohol-nontolerant, ANT) rats selected for low and high sensitivity to ethanol-induced motor impairment. Ethanol (2 or 3 g/kg ip) enhanced the output of dopamine and its metabolites in freely moving rats of both lines as measured by in vivo microdialysis. The effect of ethanol on the metabolites of dopamine tended to be stronger in the ANT rats. The smaller dose of ethanol decreased the output of GABA only in the AT rats, whereas the larger dose of ethanol decreased the output of GABA in rats of both lines to a similar degree. Ethanol at the dose of 2 g/kg slightly, but statistically, significantly decreased the output of glutamate in rats of both lines, but the larger dose of ethanol decreased the output of glutamate only in the AT rats. Ethanol at the dose of 2 g/kg induced a small transient increase in the output of taurine within 2 h after its administration in rats of both lines, but the larger dose of ethanol was without significant effect. These results confirm the previous findings that ethanol suppresses the release of GABA more in the AT than ANT rats. Thus, among the neurotransmitter systems we studied, the effects of ethanol might be the most relevant on GABAergic transmission regarding the sensitivity towards ethanol. However, our findings suggest that glutamate is also involved in this respect.

Prostatic acid phosphatase (PAP), the first diagnostic marker and present therapeutic target for prostate cancer, modulates nociception at the dorsal root ganglia (DRG), but its function in the central nervous system has remained unknown. We studied expression and function of TMPAP (the transmembrane isoform of PAP) in the brain by utilizing mice deficient in TMPAP (PAP-/- mice). Here we report that TMPAP is expressed in a subpopulation of cerebral GABAergic neurons, and mice deficient in TMPAP show multiple behavioral and neurochemical features linked to hyperdopaminergic dysregulation and altered GABAergic transmission. In addition to increased anxiety, disturbed prepulse inhibition, increased synthesis of striatal dopamine, and augmented response to amphetamine, PAP-deficient mice have enlarged lateral ventricles, reduced diazepam-induced loss of righting reflex, and increased GABAergic tone in the hippocampus. TMPAP in the mouse brain is localized presynaptically, and colocalized with SNARE-associated protein snapin, a protein involved in synaptic vesicle docking and fusion, and PAP-deficient mice display altered subcellular distribution of snapin. We have previously shown TMPAP to reside in prostatic exosomes and we propose that TMPAP is involved in the control of GABAergic tone in the brain also through exocytosis, and that PAP deficiency produces a distinct neurological phenotype.

Nicotine was administered chronically to NMRI mice in their drinking water in gradually increasing concentrations to measure gross motor activity and brain nicotine concentrations over 24 h on the 50th day of nicotine administration. Also, the striatal postmortem tissue concentrations and accumbal extracellular concentrations of dopamine (DA) and its metabolites 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) were measured to study the role of dopaminergic systems in nicotine-induced hyperactivity in mice. The cerebral nicotine concentration was at its highest at the end of the dark period. The activity of nicotine-treated mice and their striatal DA metabolism were parallelly increased at 2 to 3 h after midnight and in the forenoon. Microdialysis experiments carried out in the forenoon showed that the extracellular levels of DA and DOPAC were elevated in the nucleus accumbens of these mice. Nicotine did not alter the circadian rhythmicity of activity in the mice. Rather, our findings suggest that the mice consume more nicotine when active and this might lead to enhanced release and metabolism of DA and further, to enhanced motor behavior. These findings support the suggestions that nicotine's effects on limbic and striatal DA are critical for its stimulating effects.

The alcohol-preferring AA (Alko Alcohol) rats are more rapidly sensitized to the locomotor activity-stimulating effects of small doses of morphine than the alcohol-avoiding ANA (Alko Non-Alcohol) rats. To study the involvement of dopaminergic and serotonergic transmission in this behaviour, the effects of acute morphine (1 mg/kg) challenge on the concentrations of dopamine (DA), 5-hydroxytryptamine (5-HT, serotonin) and their metabolites were estimated in three dopaminergic areas in AA and ANA rats on the fourth day after a 3-day morphine or saline treatment. Acute administration of morphine enhanced DA metabolism in the caudate-putamen in the AA, but not in the ANA, rats; in the nucleus accumbens and in the olfactory tubercle the acute effect of morphine was similar in rats of both lines. Morphine pretreatment did not significantly enhance acute morphine's effects on DA metabolites in any of the brain areas studied in rats of either line. Acute administration of morphine enhanced brain 5-HT metabolism in the AA rats but not in the ANA rats, but after repeated treatment it induced no enhancement of 5-HT metabolism. With the methods used, no significant differences were found between the AA and ANA rats in the effects of repeated morphine on cerebral dopaminergic or serotonergic mechanisms which could account for the different behavioural sensitization found previously in rats of these lines. However, both monoamines studied might be involved in the acute locomotor stimulatory effects of morphine.

Cerebral dopaminergic mechanisms were studied in the nucleus accumbens and caudate-putamen of alcohol-preferring AA (Alko Alcohol) and alcohol-avoiding ANA (Alko Non-Alcohol) rats after 4-day repeated morphine treatment. This treatment has been shown to enhance the locomotor activity stimulating effect of morphine in the AA but not in the ANA rats. Morphine (1 or 3 mg/kg) or saline was administered subcutaneously once daily and the extracellular concentrations of dopamine (DA), 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) were measured, in freely moving rats by in vivo microdialysis on days 1 and 4. Morphine increased accumbal DA, DOPAC and HVA similarly in rats of both lines, and no sensitization of DA release or metabolism was seen in rats of either line given morphine repeatedly. In the caudate-putamen, morphine increased DA, DOPAC and HVA significantly only in the AA rats. During repeated treatment, the morphine-induced elevation of DA metabolites, but not that of DA, was enhanced similarly in rats of both lines. These results suggest that the effects of acute morphine administration on nigrostriatal dopaminergic mechanisms are stronger in the AA than in the ANA rats, whereas the effects of morphine on mesolimbic dopaminergic systems do not differ. Furthermore, in rats of both lines, repeated morphine treatment enhanced the responses of the nigrostriatal dopaminergic systems similarly, but no enhancement occurred in the mesolimbic systems of rats of either line. These findings do not support the critical role of accumbal dopaminergic systems in morphine-induced behavioural sensitization.

The aim of the study was to investigate the importance of the interaction between central dopaminergic and cholinergic mechanisms for ethanol reinforcement. This was done by comparing the effects of nicotine on locomotor activity and release of dopamine in the nucleus accumbens of the alcohol-preferring Alko Alcohol (AA) and alcohol-avoiding alko non-alcohol (ANA) rats. Nicotine was administered acutely (0.25, 0.50 or 0.75 mg/kg, s.c.) or repeatedly once daily (0.5 mg/kg, s.c.) for 8 days. An acute dose of nicotine increased locomotor activity and the extracellular levels of dopamine, 3,4-dihydroxyphenylacetic acid (DOPAC), and homovanillic acid (HVA) measured with in vivo microdialysis suggesting stimulation of dopamine release by nicotine. No difference in the stimulation of locomotor activity or in the increase in the extracellular concentrations of dopamine or its metabolites by nicotine was found between the rat lines. The concentrations of nicotine in the plasma were also identical. The rats treated repeatedly with nicotine showed a progressive increase in locomotion. On the challenge day, 1 week after termination of nicotine or saline injections, rats previously treated with nicotine were activated more by nicotine than saline-treated rats. This behavioral sensitization was not accompanied by an increase in the amplitude of the neurochemical response to nicotine, but the duration of the increase in the levels of DOPAC was longer in the nicotine than saline-treated animals. The increases in locomotor activity and metabolite levels were, however, similar in both rat lines. These data suggest that differences in the interaction of central dopaminergic and cholinergic mechanisms probably do not contribute to the difference in ethanol self-administration between the AA and ANA rat lines.

The effects of selective opioid receptor antagonists, beta-funaltrexamine (selective for mu receptor), naloxonazine (microliter) and naltrindole (delta) on morphine-induced changes in striatal and limbic dopamine (DA) metabolism were studied in rats. beta-Funaltrexamine (20 micrograms intracerebroventricularly) and naloxonazine (15 mg/kg intraperitoneally) were given 24 hr before morphine (15 mg/kg subcutaneously), and the rats were decapitated 60 min. after morphine. Naltrindole (1 mg/kg intraperitoneally) was given twice, 15 min. before and after morphine. Morphine significantly increased the concentrations of DA metabolites 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA). This effect was significantly antagonized by pretreatment with beta-funaltrexamine but not by naloxonazine or naltrindole. However, naloxonazine attenuated the antinociceptive effect of morphine in the hot-plate test. The concentration of DA was not significantly altered by any of the drugs studied. These results show that selective blockade of mu-opioid receptors totally blocks the increase of striatal and limbic DA metabolism induced by morphine. It seems that mu 2-subtype of mu-opioid receptor predominantly mediates this effect. Blockade of delta-opioid receptor did not alter these effects of morphine.

The structure and projection patterns of adult mesodiencephalic dopaminergic (DA) neurons are one of the best characterized systems in the vertebrate brain. However, the early organization and development of these nuclei remain poorly understood. The induction of midbrain DA neurons requires sonic hedgehog (Shh) from the floor plate and fibroblast growth factor 8 (FGF8) from the isthmic organizer, but the way in which FGF8 regulates DA neuron development is unclear. We show that, during early embryogenesis, mesodiencephalic neurons consist of two distinct populations: a diencephalic domain, which is probably independent of isthmic FGFs; and a midbrain domain, which is dependent on FGFs. Within these domains, DA progenitors and precursors use partly different genetic programs. Furthermore, the diencephalic DA domain forms a distinct cell population, which also contains non-DA Pou4f1+ cells. FGF signaling operates in proliferative midbrain DA progenitors, but is absent in postmitotic DA precursors. The loss of FGFR1/2-mediated signaling results in a maturation failure of the midbrain DA neurons and altered patterning of the midbrain floor. In FGFR mutants, the DA domain adopts characteristics that are typical for embryonic diencephalon, including the presence of Pou4f1+ cells among TH+ cells, and downregulation of genes typical of midbrain DA precursors. Finally, analyses of chimeric embryos indicate that FGF signaling regulates the development of the ventral midbrain cell autonomously.

Prolyl oligopeptidase (PREP, EC 3.4.21.26) inhibitors have potential as cognition enhancers, but the mechanism of action behind the cognitive effects remains unclear. Since acetylcholine (ACh) and dopamine (DA) are known to be associated with the regulation of cognitive processes, we investigated the effects of two PREP inhibitors on the extracellular levels of ACh and DA in the rat striatum using in vivo microdialysis. KYP-2047 and JTP-4819 were administered either as a single systemic dose (50 μmol/kg∼17 mg/kg i.p.) or directly into the striatum by retrodialysis via the microdialysis probe (12.5, 37.5 or 125 μM at 1.5 μl/min for 60 min). PREP inhibitors had no significant effect on striatal DA levels after systemic administration. JTP-4819 significantly decreased ACh levels both after systemic (by ∼25%) and intrastriatal (by ∼30–50%) administration. KYP-2047 decreased ACh levels only after intrastriatal administration by retrodialysis (by ∼40–50%) when higher drug levels were reached, indicating that higher brain drug levels are needed to modulate ACh levels than to inhibit PREP. This result does not support the earlier hypothesis that the positive cognitive effects of PREP inhibitors in rodents would be mediated through the cholinergic system. In vitro specificity studies did not reveal any obvious off-targets that could explain the observed effect of KYP-2047 and JTP-4819 on ACh levels, instead confirming the concept that these compounds have a high selectivity towards PREP.

Ret is the canonical signaling receptor for glial cell line-derived neurotrophic factor (GDNF), which has been shown to have neuroprotective effects when administered prior to neurotoxic challenge. A missense Meth918Thr mutation causes the constitutive activation of Ret, resulting in multiple endocrine neoplasia type 2 B (MEN2B). To clarify the role of Ret signaling in neuroprotection, we studied the effects of the neurotoxins 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) and 6-hydroxydopamine (6-OHDA) on the dopaminergic system of mice carrying the MEN2B mutation. We found that MEN2B mice were significantly more resistant to nigral tyrosine hydroxylase (TH)-positive cell loss induced by unilateral striatal 6-OHDA than Wt mice. However, 6-OHDA caused profound dopamine (DA) depletion in the striatum of both MEN2B and Wt mice. Systemic MPTP caused similar DA depletion and a decrease in TH-immunostaining in the striatum of MEN2B and Wt mice. Neither neurotoxin induced a compensatory increase in striatal metabolite/DA ratios in the MEN2B mice, possibly contributing to an increased amphetamine-induced turning behavior observed in behavioral assessments of these mice. Thus, our data suggest that activated Ret protects DA cell bodies in the substantia nigra pars compacta, but does not protect DA axons in the striatum.

In vertebrates the development and function of the nervous system is regulated by neurotrophic factors (NTFs). Despite extensive searches no neurotrophic factors have been found in invertebrates. However, cell ablation studies in Drosophila suggest trophic interaction between neurons and glia. Here we report the invertebrate neurotrophic factor in Drosophila , DmMANF, homologous to mammalian MANF and CDNF. DmMANF is expressed in glia and essential for maintenance of dopamine positive neurites and dopamine levels. The abolishment of both maternal and zygotic DmMANF leads to the degeneration of axonal bundles in the embryonic central nervous system and subsequent nonapoptotic cell death. The rescue experiments confirm DmMANF as a functional ortholog of the human MANF gene thus opening the window for comparative studies of this protein family with potential for the treatment of Parkinson's disease.

Ret is the common signaling receptor for glial cell line-derived neurotrophic factor (GDNF) and other ligands of the GDNF family that have potent effects on brain dopaminergic neurons. The Met918Thr mutation leads to constitutive activity of Ret receptor tyrosine kinase, causing the cancer syndrome called multiple endocrine neoplasia type B (MEN2B). We used knock-in MEN2B mice with the Ret-MEN2B mutation to study the effects of constitutive Ret activity on the brain dopaminergic system and found robustly increased concentrations of dopamine (DA) and its metabolites in the striatum, cortex, and hypothalamus. The concentrations of brain serotonin were not affected and those of noradrenaline were slightly increased only in the lower brainstem. Tyrosine hydroxylase (TH) protein levels were increased in the striatum and substantia nigra/ventral tegmental area (SN/VTA), and TH mRNA levels were increased in SN/VTA of MEN2B mice, suggesting that constitutive Ret activity increases DA levels by increasing its synthesis. Also, the striatal DA transporter protein levels in the MEN2B mice were increased, which agrees with increased sensitivity of these mice to the stimulatory effects of cocaine. In the SN pars compacta of homozygous MEN2B mice, we found a 26% increase in the number of TH-positive cells, but no differences were found in the VTA. Thus, we show here that the constitutive Ret activity in mice is sufficient to increase the number of dopaminergic neurons and leads to profound elevation of brain DA concentration. These data clearly suggest that Ret activity per se can have a direct biological function that actively changes and shapes the brain dopaminergic system.

Transcription factor DeltaFosB has been implicated in the psychomotor responses and rewarding effects of drugs of abuse. In the present study, we compared the effects of cocaine on the expression of DeltaFosB-like proteins by immunohistochemistry in striatal brain areas of alcohol-preferring (AA) and alcohol-avoiding (ANA) rats. Cocaine was administered using a previously verified treatment paradigm that sensitized the locomotor response to cocaine in AA but not in ANA rats. We also studied the rewarding effects of cocaine with a conditioned place preference (CPP) paradigm in both lines of rats. Cocaine treatment increased the FosB/DeltaFosB immunoreactivity (IR) in the nucleus accumbens of AA rats but not in ANA rats. In addition, after repeated saline injections the accumbal FosB/DeltaFosB IR was significantly greater in saline-injected AA rats than in ANA rats. In the caudate-putamen cocaine significantly increased FosB/DeltaFosB IR, but no differences were found between the rats of two lines. In the CPP experiment, AA rats treated with cocaine 2.5 mg/kg preferred the cocaine-associated compartment, in contrast to ANA rats, which did not show such a preference. In conclusion, our findings show that AA rats are more sensitive to cocaine than ANA rats, and suggest that one possible mediator for this increased sensitivity could be the increased expression of fosB-derived proteins in the nucleus accumbens of AA rats.

Evidence for an interaction between nicotine and morphine has been found in several studies. In the present study mice withdrawn from a 7-week oral nicotine treatment were administered morphine, following which their locomotor activities were recorded and the concentrations of dopamine, 5-hydroxytryptamine (5-HT), noradrenaline and their metabolites were measured in the caudate putamen (CPu) and nucleus accumbens (NAc). For comparison, the effect of cocaine on locomotor activity was studied in mice withdrawn from nicotine. Morphine (15 mg/kg s.c.) enhanced locomotor activity significantly more in the nicotine-withdrawn mice than in the controls, whereas cocaine (20 mg/kg i.p.) stimulated the locomotor activity similarly in the nicotine-withdrawn and in the control mice. Morphine (10 mg/kg s.c.) elevated dopamine and 5-HT metabolites to the same degree in the NAc of the nicotine-withdrawn and the control mice. However, in the CPu morphine enhanced the metabolism of dopamine and also that of 5-HT in the nicotine-withdrawn mice but not in the controls. In addition, the basal concentrations of dopamine metabolites were reduced in the CPu of the nicotine-withdrawn mice. Thus, the enhancement of morphine-induced locomotor activation in the nicotine-withdrawn mice could be related to nicotine-induced changes in the regulation of the nigrostriatal dopaminergic and serotonergic systems.

We studied the effects of nicotine and epibatidine given in combination with dopamine uptake inhibitor, nomifensine, on striatal extracellular dopamine and its metabolites by using brain microdialysis in freely moving rats. Nomifensine (3 mg/kg) elevated extracellular dopamine in the caudate-putamen, and clearly more in the nucleus accumbens. In the caudate-putamen, nicotine (0.5 mg/kg) and epibatidine (0.6 microg/kg but not 3.0 microg/kg) enhanced nomifensine's effect on dopamine. The effect of nomifensine on accumbal dopamine was enhanced by nicotine, but inhibited by epibatidine at 0.6 microg/kg. The larger dose of epibatidine had no effect. Thus, the effects of the smaller epibatidine dose (0.6 microg/kg) on the dopamine output in the caudate-putamen but not in the accumbens resemble those of nicotine 0.5 mg/kg. Discrepancies in the effects of epibatidine and nicotine are most probably due to differences in their affinities to nicotinic receptor subtypes regulating dopamine release. Further, different responses to low concentrations of epibatidine between the brain areas suggest that there are differences in the nicotinic regulation of nigrostriatal and mesolimbic dopaminergic pathways.

The role of nicotinic acetylcholine receptor (nAChR) activation in accumbal dopamine (DA) release during chronic continuous nicotine treatment was studied by in vivo microdialysis in freely-moving mice. Nicotine was administered chronically to NMRI mice in their drinking water. On the 50th day of nicotine administration microdialysis samples were collected at 20 min intervals and their DA, 3,4-dihydroxyphenylacetic acid, homovanillic acid and 5-hydroxyindoleacetic acid contents were measured using a high-performance liquid chromatographic-electrochemical procedure. After collection of four baseline samples the nicotinic antagonist mecamylamine (2 mg/kg, s.c.) was given. The steady-state DA output was larger in the nicotine-treated mice than in the control mice. Mecamylamine reduced the DA output in the nicotine-treated but not in the control mice. Thus, after continuous 50-day administration nicotine still continues to activate nAChRs regulating accumbal DA release.

We developed a rapid step-gradient HPLC method for determination of glutamate, glycine and taurine, and a separate method for determination of gamma-aminobutyric acid (GABA) in striatal microdialysates. The amino acids were pre-column derivatized with o-phthalaldehyde-2-mercaptoethanol by using an automated refrigerated autoinjector. Separation of the amino acids was established with a non-porous ODS-II HPLC column, late-eluting substances were washed out with a one-step low-pressure gradient. Concentrations of the amino acids were determined with a fixed-wavelength fluorescence detector. The detection limit for GABA was 80 fmol in a 15 microl sample, detection limits for glutamate, glycine and taurine were not determined because their concentrations in striatal perfusates were far above their detection limits. Total analysis time was less than 12 min, including the wash-out step. The methods described are relatively simple, sensitive, inexpensive, and fast enough to keep up with the microdialysis sampling.

The effects of withdrawal from repeated nifedipine treatment on morphine-induced analgesia, hyperthermia and catalepsy as well as on cerebral [3H]nitrendipine binding and on morphine-induced changes in striatal and limbic dopamine and 5-hydroxytryptamine metabolism were studied in rats. Repeated administration of nifedipine (5 mg/kg i.p., twice daily for 14 days) decreased [3H]nitrendipine binding in several brain areas of the rats at 24 h after the last dose but did not change the nociceptive response or rectal temperature of the animals. Further, the antinociceptive potency of acute morphine (2.5 mg/kg s.c.) was significantly reduced in rats withdrawn for 24 h from repeated nifedipine treatment. However, withdrawal from repeated nifedipine treatment failed to affect either the hyperthermia induced by this dose of morphine or the catalepsy and the elevation of dopamine or 5-hydroxytryptamine metabolites induced by 15 mg/kg of morphine. Taken together, these data show that withdrawal from repeated treatment with dihydropyridine calcium channel antagonists selectively reduces the effects of opioids on the nociceptive response.

KATAJAMAKI, J., A. HONKANEN, T. P. PIEPPONEN, I.-B. LINDEN, A. ZHARKOVSKY AND L. AHTEE. Conditioned place preference induced by a combination of l -dopa and a COMT inhibitor, entacapone, in rats. PHARMACOL BIOCHEM BEHAV 60 (1) 23–26, 1998.—The interaction of dopamine (DA) precursor l-dopa and catechol-O-methyltransferase (COMT) inhibitor, entacapone, was examined in rats using conditioned place preference (CPP) paradigm to assess reinforcement, and by measuring DA metabolism in the striatum and the limbic forebrain. Neither l-dopa (100 mg/kg IP) nor entacapone (30 mg/kg IP) alone induced CPP, but in combination they induced significant CPP. Entacapone alone had no effect on limbic or striatal DA concentrations, while it reduced the concentrations of the COMT products 3-methoxytyramine (3-MT), a metabolite reflecting DA release, and homovanillic acid (HVA) in both brain areas. l-dopa elevated limbic but not striatal 3-MT. l-dopa also slightly elevated limbic DA but had no effect on striatal DA concentration. l-Dopa–induced increase of 3-MT was attenuated by entacapone. Our results show for the first time that l-dopa is able to produce CPP in intact animals. This effect may be related to the findings that l-dopa increases synaptic DA concentrations in the limbic areas, and entacapone may enhance this elevation as it prevents the synaptic metabolism of DA.

Mammalian MANF and CDNF proteins are evolutionarily conserved neurotrophic factors that can protect and repair mammalian dopaminergic neurons in vivo. In Drosophila, the sole MANF protein (DmManf) is needed for the maintenance of dopaminergic neurites and dopamine levels. Although both secreted and intracellular roles for MANF and CDNF have been demonstrated, very little is known about the molecular mechanism of their action. Here, by using a transgenic rescue approach in the DmManf mutant background we show that only full-length MANF containing both the amino-terminal saposin-like and carboxy-terminal SAP-domains can rescue the larval lethality of the DmManf mutant. Independent N- or C-terminal domains of MANF, even when co-expressed together, fail to rescue. Deleting the signal peptide or mutating the CXXC motif in the C-terminal domain destroys the activity of full-length DmManf. Positively charged surface amino acids and the C-terminal endoplasmic reticulum retention signal are necessary for rescue of DmManf mutant lethality when DmManf is expressed in a restricted pattern. Furthermore, rescue experiments with non-ubiquitous expression reveals functional differences between the C-terminal domain of human MANF and CDNF. Finally, DmManf and its C-terminal domain rescue mammalian sympathetic neurons from toxin-induced apoptosis in vitro demonstrating functional similarity of the mammalian and fly proteins. Our study offers further insights into the functional conservation between invertebrate and mammalian MANF/CDNF proteins and reveals the importance of the C-terminal domain for MANF activity in vivo.