Regulation of dopamine uptake by basic fibroblast growth factor and epidermal growth factor in cultured rat astrocytes

Kinetic Properties and Pharmacological Modulation of High- and Low-Affinity Dopamine Transport in Striatal Astrocytes of Adult Rats

Astrocytes actively participate in neurotransmitter homeostasis by bidirectional communication with neuronal cells, a concept named the tripartite synapse, yet their role in dopamine (DA) homeostasis remains understudied. In the present study, we investigated the kinetic and molecular mechanisms of DA transport in cultured striatal astrocytes of adult rats. Kinetic uptake experiments were performed using radiolabeled [3H]-DA, whereas mRNA expression of the dopamine, norepinephrine, organic cation and plasma membrane monoamine transporters (DAT, NET, OCTs and PMAT) and DA receptors D1 and D2 was determined by qPCR. Additionally, astrocyte cultures were subjected to a 24 h treatment with the DA receptor agonist apomorphine, the DA receptor antagonist haloperidol and the DA precursor L-DOPA. [3H]-DA uptake exhibited temperature, concentration and sodium dependence, with potent inhibition by desipramine, nortriptyline and decynium-22, suggesting the involvement of multiple transporters. qPCR revealed prominent mRNA expression of the NET, the PMAT and OCT1, alongside lower levels of mRNA for OCT2, OCT3 and the DAT. Notably, apomorphine significantly altered NET, PMAT and D1 mRNA expression, while haloperidol and L-DOPA had a modest impact. Our findings demonstrate that striatal astrocytes aid in DA clearance by multiple transporters, which are influenced by dopaminergic drugs. Our study enhances the understanding of regional DA uptake, paving the way for targeted therapeutic interventions in dopaminergic disorders.

Transporters involved in adult rat cortical astrocyte dopamine uptake: Kinetics, expression and pharmacological modulation

Astrocytes, glial cells in the central nervous system, perform a multitude of homeostatic functions and are in constant bidirectional communication with neuronal cells, a concept named the tripartite synapse; however, their role in the dopamine homeostasis remains unexplored. The aim of this study was to clarify the pharmacological and molecular characteristics of dopamine transport in cultured cortical astrocytes of adult rats. In addition, we were interested in the expression of mRNA of dopamine transporters as well as dopamine receptors D1 and D2 and in the effect of dopaminergic drugs on the expression of these transporters and receptors. We have found that astrocytes possess both Na+-dependent and Na+-independent transporters. Uptake of radiolabelled dopamine was time-, temperature- and concentration-dependent and was inhibited by decynium-22, a plasma membrane monoamine transporter inhibitor, tricyclic antidepressants desipramine and nortriptyline, both inhibitors of the norepinephrine transporter. Results of transporter mRNA expression indicate that the main transporters involved in cortical astrocyte dopamine uptake are the norepinephrine transporter and plasma membrane monoamine transporter. Both dopamine receptor subtypes were identified in cortical astrocyte cultures. Twenty-four-hour treatment of astrocyte cultures with apomorphine, a D1/D2 agonist, induced upregulation of D1 receptor, norepinephrine transporter and plasma membrane monoamine transporter, whereas the latter was downregulated by haloperidol and L-DOPA. Astrocytes take up dopamine by multiple transporters and express dopamine receptors, which are sensitive to dopaminergic drugs. The findings of this study could open a promising area of research for the fine-tuning of existing therapeutic strategies.

Cortical and Striatal Astrocytes of Neonatal Rats Display Distinct Molecular and Pharmacological Characteristics of Dopamine Uptake

Astrocytes are crucial in the regulation of neurotransmitter homeostasis, and while their involvement in the dopamine (DA) tripartite synapse is acknowledged, it necessitates a more comprehensive investigation. In the present study, experiments were conducted on primary astrocyte cultures from the striatum and cortex of neonatal rats. The pharmacological intricacies of DA uptake, including dependence on time, temperature, and concentration, were investigated using radiolabelled [3H]-DA. The mRNA expression of transporters DAT, NET, PMAT, and OCTs was evaluated by qPCR. Notably, astrocytes from both brain regions exhibited prominent mRNA expression of NET and PMAT, with comparatively lower expression of DAT and OCTs. The inhibition of DA uptake by the DAT inhibitor, GBR12909, and NET inhibitors, desipramine and nortriptyline, impeded DA uptake in striatal astrocytes more than in cortical astrocytes. The mRNA expression of NET and PMAT was significantly upregulated in cortical astrocytes in response to the DA receptor agonist apomorphine, while only the mRNA expression of NET exhibited changes in striatal astrocytes. Haloperidol, a DA receptor antagonist, and L-DOPA, a DA precursor, did not induce significant alterations in transporter mRNA expression. These findings underscore the intricate and region-specific mechanisms governing DA uptake in astrocytes, emphasizing the need for continued exploration to unravel the nuanced dynamics of astrocytic involvement in the DA tripartite synapse.

Melatonin Analogues Potently Inhibit MAO-B and Protect PC12 Cells against Oxidative Stress

Monoamine oxidase B (MAO-B) metabolizes dopamine and plays an important role in oxidative stress by altering the redox state of neuronal and glial cells. MAO-B inhibitors are a promising therapeutical approach for Parkinson’s disease (PD). Herein, 24 melatonin analogues (3a–x) were synthesized as novel MAO-B inhibitors with the potential to counteract oxidative stress in neuronal PC12 cells. Structure elucidation, characterization, and purity of the synthesized compounds were performed using ¹H-NMR, ¹³C-NMR, HRMS, and HPLC. At 10 µM, 12 compounds showed >50% MAO-B inhibition. Among them, compounds 3n, 3r, and 3u–w showed >70% inhibition of MAO-B and IC₅₀ values of 1.41, 0.91, 1.20, 0.66, and 2.41 µM, respectively. When compared with the modest selectivity index of rasagiline (II, a well-known MAO-B inhibitor, SI > 50), compounds 3n, 3r, 3u, and 3v demonstrated better selectivity indices (SI > 71, 109, 83, and 151, respectively). Furthermore, compounds 3n and 3r exhibited safe neurotoxicity profiles in PC12 cells and reversed 6-OHDA- and rotenone-induced neuronal oxidative stress. Both compounds significantly up-regulated the expression of the anti-oxidant enzyme, heme oxygenase (HO)-1. Treatment with Zn(II)-protoporphyrin IX (ZnPP), a selective HO-1 inhibitor, abolished the neuroprotective effects of the tested compounds, suggesting a critical role of HO-1 up-regulation. Both compounds increased the nuclear translocation of Nrf2, which is a key regulator of the antioxidative response. Taken together, these data show that compounds 3n and 3r could be further exploited for their multi-targeted role in oxidative stress-related PD therapy.

l-Dopa and Fluoxetine Upregulate Astroglial 5-HT2B Receptors and Ameliorate Depression in Parkinson’s Disease Mice

Here, we report the association between depressive behavior (anhedonia) and astroglial expression of 5-hydroxytryptamine receptor 2B (5-HT2B) in an animal model of Parkinson’s disease, induced by bilateral injection of 6-hydroxydopamine (6-OHDA) into the striatum. Expression of the 5-HT2B receptor at the mRNA and protein level was decreased in the brain tissue of 6-OHDA-treated animals with anhedonia. Expression of the 5-HT2B receptor was corrected by four weeks treatment with either l-3,4-dihydroxyphenylalanine (l-dopa) or fluoxetine. Simultaneously, treatment with l-dopa abolished 6-OHDA effects on both depressive behavior and motor activity. In contrast, fluoxetine corrected 6-OHDA-induced depression but did not affect 6-OHDA-induced motor deficiency. In addition, 6-OHDA downregulated gene expression of the 5-HT2B receptor in astrocytes in purified cell culture and this downregulation was corrected by both l-dopa and fluoxetine. Our findings suggest that 6-OHDA-induced depressive behavior may be related to the downregulation of gene expression of the 5-HT2B receptor but 6-OHDA-induced motor deficiency reflects, arguably, dopamine depletion. Previously, we demonstrated that fluoxetine regulates gene expression in astrocytes by 5-HT2B receptor-mediated transactivation of epidermal growth factor receptor (EGFR). However, the underlying mechanism of l-dopa action remains unclear. The present work indicates that the decrease of gene expression of the astroglial 5-HT2B receptor may contribute to development of depressive behavior in Parkinson’s disease.

Correlation of Plasma EGF with Striatal Dopamine Transporter Availability in Healthy Subjects

We aimed to evaluate the association between plasma epidermal growth factor (EGF) and the availability of dopamine transporter (DAT) measured from ¹²³I-FP-CIT single-photon emission computed tomography in healthy controls in this study. Volume of interest template was applied to measure specific binding ratios (SBRs) of caudate nucleus, putamen, and striatum representing DAT availability as follows; SBR = (target– cerebellum)/cerebellum. Plasma EGF was negatively correlated with the availabilities of both caudate nucleus (r = −0.261, p = 0.019), and putamen (r = −0.341, p = 0.002). After dividing subjects according to Apo E genotyping, DAT availability of caudate nucleus of Apo e4 non-carriers (n = 60) showed the positive correlation with cerebrospinal fluid (CSF) α-synuclein (r = 0.264, p = 0.042). Plasma EGF was negatively correlated with DAT availabilities of Apo e4 non-carriers. Further studies are needed to clarify underlying mechanisms of this phenomenon.

Quinine enhances the behavioral stimulant effect of cocaine in mice

The Na(+)-dependent dopamine transporter (DAT) is primarily responsible for regulating free dopamine (DA) concentrations in the brain by participating in the majority of DA uptake; however, other DA transporters may also participate, especially if cocaine or other drugs of abuse compromise DAT. Recently, such cocaine-insensitive low-affinity mono- and poly-amine OCT transporters were described in astrocytes which use DA as a substrate. These transporters are from a different transporter family and while insensitive to cocaine, they are specifically blocked by quinine and some steroids. Quinine is inexpensive and is often found in injected street drugs as an "adulterant". The present study was designed to determine the participation of OCTs in cocaine dependent behavioral and physiological changes in mice. Using FVB mice we showed, that daily single injections of quinine (10 mg/kg, i.p.) co-administered with cocaine (15 mg/kg, i.p.) for 10 days significantly enhanced cocaine-induced locomotor behavioral sensitization. Quinine had no significant effect on the time course of behavioral activation. In astrocytes from the ventral tegmental area of mice, transporter currents of quinine-sensitive monoamine transporters were also augmented after two weeks of cocaine administration. The importance of low-affinity high-capacity transporters for DA clearance is discussed, explaining the known ability of systemically administered DAT inhibitors to anomalously increase DA clearance.

Striatal astrocytes act as a reservoir for L-DOPA

L-DOPA is therapeutically efficacious in patients with Parkinson’s disease (PD), although dopamine (DA) neurons are severely degenerated. Since cortical astrocytes express neutral amino acid transporter (LAT) and DA transporter (DAT), the uptake and metabolism of L-DOPA and DA in striatal astrocytes may influence their availability in the dopaminergic system of PD. To assess possible L-DOPA- and DA-uptake and metabolic properties of striatal astrocytes, we examined the expression of L-DOPA, DA and DAT in striatal astrocytes of hemi-parkinsonian model rats after repeated L-DOPA administration, and measured the contents of L-DOPA, DA and their metabolite in primary cultured striatal astrocytes after L-DOPA/DA treatment. Repeated injections of L-DOPA induced apparent L-DOPA- and DA-immunoreactivities and marked expression of DAT in reactive astrocytes on the lesioned side of the striatum in hemi-parkinsonian rats. Exposure to DA for 4h significantly increased the levels of DA and its metabolite DOPAC in cultured striatal astrocytes. L-DOPA was also markedly increased in cultured striatal astrocytes after 4-h L-DOPA exposure, but DA was not detected 4 or 8h after L-DOPA treatment, despite the expression of aromatic amino acid decarboxylase in astrocytes. Furthermore, the intracellular level of L-DOPA in cultured striatal astrocytes decreased rapidly after removal of extracellular L-DOPA. The results suggest that DA uptaken into striatal astrocytes is rapidly metabolized and that striatal astrocytes act as a reservoir of L-DOPA that govern the uptake or release of L-DOPA depending on extracellular L-DOPA concentration, but are less capable of converting L-DOPA to DA.

The role of oxidative stress in Parkinson's disease

Oxidative stress plays an important role in the degeneration of dopaminergic neurons in Parkinson's disease (PD). Disruptions in the physiologic maintenance of the redox potential in neurons interfere with several biological processes, ultimately leading to cell death. Evidence has been developed for oxidative and nitrative damage to key cellular components in the PD substantia nigra. A number of sources and mechanisms for the generation of reactive oxygen species (ROS) are recognized including the metabolism of dopamine itself, mitochondrial dysfunction, iron, neuroinflammatory cells, calcium, and aging. PD causing gene products including DJ-1, PINK1, parkin, alpha-synuclein and LRRK2 also impact in complex ways mitochondrial function leading to exacerbation of ROS generation and susceptibility to oxidative stress. Additionally, cellular homeostatic processes including the ubiquitin-proteasome system and mitophagy are impacted by oxidative stress. It is apparent that the interplay between these various mechanisms contributes to neurodegeneration in PD as a feed forward scenario where primary insults lead to oxidative stress, which damages key cellular pathogenetic proteins that in turn cause more ROS production. Animal models of PD have yielded some insights into the molecular pathways of neuronal degeneration and highlighted previously unknown mechanisms by which oxidative stress contributes to PD. However, therapeutic attempts to target the general state of oxidative stress in clinical trials have failed to demonstrate an impact on disease progression. Recent knowledge gained about the specific mechanisms related to PD gene products that modulate ROS production and the response of neurons to stress may provide targeted new approaches towards neuroprotection.

Brain gene expression differences are associated with abnormal tail biting behavior in pigs

Knowledge about gene expression in animals involved in abnormal behaviors can contribute to the understanding of underlying biological mechanisms. This study aimed to explore the motivational background to tail biting, an abnormal injurious behavior and severe welfare problem in pig production. Affymetrix microarrays were used to investigate gene expression differences in the hypothalamus and prefrontal cortex of pigs performing tail biting, pigs receiving bites to the tail and neutral pigs who were not involved in the behavior. In the hypothalamus, 32 transcripts were differentially expressed (P < 0.05) when tail biters were compared with neutral pigs, 130 when comparing receiver pigs with neutrals, and two when tail biters were compared with receivers. In the prefrontal cortex, seven transcripts were differently expressed in tail biters when compared with neutrals, seven in receivers vs. neutrals and none in the tail biters vs. receivers. In total, 19 genes showed a different expression pattern in neutral pigs when compared with both performers and receivers. This implies that the functions of these may provide knowledge about why the neutral pigs are not involved in tail biting behavior as performers or receivers. Among these 19 transcripts were genes associated with production traits in pigs (PDK4), sociality in humans and mice (GTF2I) and novelty seeking in humans (EGF). These are in line with hypotheses linking tail biting with reduced back fat thickness and explorative behavior.

Dopamine inhibits mitochondrial motility in hippocampal neurons

Background

The trafficking of mitochondria within neurons is a highly regulated process. In an earlier study, we found that serotonin (5-HT), acting through the 5-HT1A receptor subtype, promotes axonal transport of mitochondria in cultured hippocampal neurons by increasing Akt activity, and consequently decreasing glycogen synthase kinase (GSK3β) activity. This finding suggests a critical role for neuromodulators in the regulation of mitochondrial trafficking in neurons. In the present study, we investigate the effects of a second important neuromodulator, dopamine, on mitochondrial transport in hippocampal neurons.

Methodology/Principal Findings

Here, we show that dopamine, like 5-HT, regulates mitochondrial motility in cultured hippocampal neurons through the Akt-GSK3β signaling cascade. But, in contrast to the stimulatory effect of 5-HT, administration of exogenous dopamine or bromocriptine, a dopamine 2 receptor (D2R) agonist, caused an inhibition of mitochondrial movement. Moreover, pretreatment with bromocriptine blocked the stimulatory effect of 5-HT on mitochondrial movement. Conversely, in cells pretreated with 5-HT, no further increases in movement were observed after administration of haloperidol, a D2R antagonist. In contrast to the effect of the D2R agonist, addition of SKF38393, a dopamine 1 receptor (D1R) agonist, promoted mitochondrial transport, indicating that the inhibitory effect of dopamine was actually the net summation of opposing influences of the two receptor subtypes. The most pronounced effect of dopamine signals was on mitochondria that were already moving directionally. Western blot analysis revealed that treatment with either a D2R agonist or a D1R antagonist decreased Akt activity, and conversely, treatment with either a D2R antagonist or a D1R agonist increased Akt activity.

Conclusions/Significance

Our observations strongly suggest a role for both dopamine and 5-HT in regulating mitochondrial movement, and indicate that the integrated effects of these two neuromodulators may be important in determining the distribution of energy sources in neurons.

L-DOPA treatment from the viewpoint of neuroprotection. Possible mechanism of specific and progressive dopaminergic neuronal death in Parkinson's disease

With regard to the mechanism of selective dopaminergic neuronal death, experimental results of studies on the neurotoxicity of MPTP and rotenone indicate that degeneration of dopamine neurons is closely related to mitochondrial dysfunction, inflammatory process and oxidative stress, particularly with regard to the generation of quinones as dopamine neuron-specific oxidative stress. Thus, it is now clear that the presence of high levels of discompartmentalized free dopamine in dopaminergic neurons may explain the specific vulnerability of dopaminergic neurons through the generation of highly toxic quinones.

Tryptamine induces cell death with ultrastructural features of autophagy in neurons and glia: Possible relevance for neurodegenerative disorders

Tryptamine derivatives are a family of biogenic amines that have been suggested to be modulators of brain function at physiological concentrations. However, pharmacological concentrations of these amines display amphetamine-like properties, and they seem to play a role in brain disorders. Amphetamines induce autophagy in nerve cells, and this type of cell death has also been involved in neurodegenerative diseases. In the present work, we clearly demonstrate for the very first time that high concentrations of tryptamine (0.1-1 mM) induce autophagy in HT22 and SK-N-SH nerve cell lines and in primary cultures of astrocytes, glial cells being less sensitive than neurons. Ultrastructural cell morphology shows all of the typical hallmarks of autophagy. There is no nuclear chromatin condensation, endoplasmic reticulum and mitochondria are swollen, and a great number of double-membraned autophagosomes and residual bodies can be shown in the cytoplasm. Autophagosomes and residual bodies contain mitochondria, membranes, and vesicles and remain unabridged until the cell membrane is disrupted and the cell dies. The same results have been found when cells were incubated with high concentrations of 5-methoxytryptamine (0.1-1 mM). Our results establish a possible link between the role of tryptamine derivatives in brain disorders and the presence of autophagic cell death in these kinds of disorders.

Effect of reactive cell density on net [2-14C]acetate uptake into rat brain: labeling of clusters containing GFAP+- and lectin+-immunoreactive cells

Astrocytic proliferation is a hallmark of brain injury, but the biological functions and metabolic activities of reactive astrocytes in vivo are poorly understood. [2-14C]Acetate, which is preferentially transported into and, therefore, metabolized by astrocytes, was used to assess injury- and trophic factor-induced changes in astrocyte metabolic activity. Local rates of net [2-14C]acetate uptake and glucose utilization (CMR(glc)), determined with [14C]deoxyglucose to assay overall metabolic activity of all brain cells, were assayed 7 days after a cannula placement; adjacent brain sections were immunostained to identify glial fibrillary acidic protein-positive (GFAP(+)) astrocytes and microglia plus macrophages (lectin-positive cells). GFAP(+) cells were abundant in tissue surrounding the cannula compared to the contralateral hemisphere, whereas lectin(+) cells were restricted to the wound boundary. CMR(glc) fell 25% in regions enriched in reactive astrocytes compared to the homologous contralateral hemisphere, whereas [14C]acetate uptake increased slightly (6%) but statistically significantly; metabolism of both tracers in 13 other brain structures was unchanged. Injection of basic fibroblast growth factor (b-FGF) into cerebral cortex or superior colliculus produced fiber-rich cell clusters containing both GFAP(+) and lectin(+) cells that had a 37% increase in [14C]acetate uptake; GFAP(+)-cell density rose in the nearby neuropil but the corresponding change in [14C]acetate uptake was small (6-8%). Sensory stimulation did not alter [14C]acetate uptake into the clusters. Thus, [14C]acetate uptake was relatively stable with respect to changes in the density of reactive astrocytes that are dispersed throughout the neuropil and to changes in cellular activity arising from sensory stimulation. In contrast, b-FGF-induced cell clusters that contain mixed cell types and numerous fibers accumulated higher levels of [14C]acetate, raising the possibility that increased uptake might be due to high numbers of activated astrocytes and, perhaps, acetate metabolism by other cell types.

Expression and functional characterization of the extraneuronal monoamine transporter in normal human astrocytes

In this study we examined the functional expression of the extraneuronal monoamine transporter (EMT) in normal human astrocytes (NHA). RT-PCR with EMT-specific primers demonstrated the presence of EMT mRNA in NHA. The RT-PCR products were subjected to restriction-site analysis using three different enzymes (HinfI, SacI and BclI). The restriction patterns with the three enzymes were identical and were exactly as expected from the known restriction map of human EMT cDNA. DNA sequencing was performed for the RT-PCR products from NHA. Sequence analysis demonstrated that the sequences of RT-PCR products were identical to that of EMT. The extract of NHA was immunoblotted with anti-EMT polyclonal antibody raised against EMT polypeptides. Western blotting indicated that anti-EMT polyclonal antibody recognized a band of 63 kDa. Immunocytochemical staining using anti-EMT polyclonal antibody in NHA revealed that the plasma membrane, as well as intracellular, perinuclear compartments, presumably endoplasmic reticulum or Golgi membranes, showed a considerable level of immunoreactivity. We examined the time course of temperature-dependent [3H]MPP+ uptake in NHA for 60 min. Temperature-dependent [3H]MPP+ uptake increased in a time-dependent manner for the initial 45 min and almost reached a plateau level (8.70 +/- 0.59 pmol/mg protein) at 60 min. In the presence of 3 micro m decynium22 (D22) (the most potent EMT inhibitor), temperature-dependent [3H]MPP+ uptake was strongly reduced by 61% (3.39 +/- 0.76 pmol/mg protein at 60 min). D22-sensitive [3H]MPP+ uptake was saturable over a MPP+ concentration of 6.25-200 micro m. Km for this process was 78.01 +/- 7.64 micro m and Vmax was 295.4 +/- 12.8 pmol/mg protein/min. D22-sensitive [3H]MPP+ uptake was reduced when the astrocyte membrane potential was depolarized by increasing the concentration of K+ in the uptake buffer or by adding Ba2+ to the uptake buffer. These results provide evidence that the MPP+ transport activity in NHA is potential-sensitive. Moreover, D22-sensitive [3H]MPP+ uptake was independent of extracellular Na+. D22-sensitive [3H]MPP+ uptake was inhibited by D22, various organic cations, steroids and monoamine neurotransmitters. Our results showed that the EMT is functionally expressed in NHA and may also play a key role in the disposition of cationic drugs, neurosteroids, the neurotoxin MPP+ and monoamine neurotransmitters in the brain.

Functional expression of the norepinephrine transporter in cultured rat astrocytes

We assessed the functional expression of the norepinephrine (NE) transporter (NET) in cultured rat cortical astrocytes. Specific [3H]NE uptake increased in a time-dependent manner, and this uptake involves temperature- and Na+-sensitive mechanisms. The Na+-dependent [3H]NE uptake was saturable, and the Km for the process was 539.3 +/- 55.4 nm and the Vmax was 1.41 +/- 0.03 pmol/mg protein/min. Ouabain, a Na+-K+ ATPase inhibitor, significantly inhibited Na+-dependent [3H]NE uptake. The selective NE uptake inhibitor nisoxetine, the tricyclic antidepressants desipramine and imipramine, and the serotonin and NE reuptake inhibitor (SNRI) milnacipran very potently inhibited Na+-dependent [3H]NE uptake. On the other hand, GBR-12935 (a selective dopamine uptake inhibitor), fluvoxamine (a selective serotonin reuptake inhibitor), venlafaxine (a SNRI) and cocaine had weaker inhibitory activities. RT-PCR demonstrated that astrocytes expressed mRNA for the cloned NET protein, which was characterized as neuronal NET. Western blots indicated that anti-NET polyclonal antibody recognized a major band of 80 kDa in astrocytes. These data indicate that the neuronal NET is functionally expressed in cultured rat astrocytes. Glial cells may exert significant control of noradrenergic activity by inactivating NE that escapes neuronal re-uptake in sites distant from terminals, and are thus cellular targets for antidepressant drugs that inhibit NE uptake.

Cocaine modulates mu-opioid receptor mRNA but not c-fos mRNA levels in primary cortical astrocytes

Cocaine is known to modulate the opioid system in several brain regions, including the cortex. Glial cells that are derived from the neonatal cortex have been shown to express opioid peptides and opioid receptors. In this study we investigated the effects of cocaine on c-fos and mu-opioid receptor mRNA levels in primary cortical astrocyte cultures, using RT-PCR and quantitative solution hybridization assays. Astrocyte cultures from 1-day-old Fischer rats were untreated or treated with cocaine for 30min, 2h, or 5h. While c-fos mRNA levels did not change at any time, mu-opioid receptor mRNA levels decreased by 75% after 2 and 5h of cocaine treatments. Our data suggest that cocaine differentially modulates c-fos and opioid signaling in astrocyte cell culture.

Dopamine as a prolactin (PRL) inhibitor

Dopamine is a small and relatively simple molecule that fulfills diverse functions. Within the brain, it acts as a classical neurotransmitter whose attenuation or overactivity can result in disorders such as Parkinson's disease and schizophrenia. Major advances in the cloning and characterization of biosynthetic enzymes, transporters, and receptors have increased our knowledge regarding the metabolism, release, reuptake, and mechanism of action of dopamine. Dopamine reaches the pituitary via hypophysial portal blood from several hypothalamic nerve tracts that are regulated by PRL itself, estrogens, and several neuropeptides and neurotransmitters. Dopamine binds to type-2 dopamine receptors that are functionally linked to membrane channels and G proteins and suppresses the high intrinsic secretory activity of the pituitary lactotrophs. In addition to inhibiting PRL release by controlling calcium fluxes, dopamine activates several interacting intracellular signaling pathways and suppresses PRL gene expression and lactotroph proliferation. Thus, PRL homeostasis should be viewed in the context of a fine balance between the action of dopamine as an inhibitor and the many hypothalamic, systemic, and local factors acting as stimulators, none of which has yet emerged as a primary PRL releasing factor. The generation of transgenic animals with overexpressed or mutated genes expanded our understanding of dopamine-PRL interactions and the physiological consequences of their perturbations. PRL release in humans, which differs in many respects from that in laboratory animals, is affected by several drugs used in clinical practice. Hyperprolactinemia is a major neuroendocrine-related cause of reproductive disturbances in both men and women. The treatment of hyperprolactinemia has greatly benefited from the generation of progressively more effective and selective dopaminergic drugs.

Pharmacological characterization and visualization of the glial serotonin transporter

Astrocytes contain transport systems that are capable of removing various neurotransmitters from the synaptic cleft by transporters present in the plasma membrane. Glial serotonin transporter (SERT) plays an important role in the re-uptake of 5-hydroxytryptamine (5-HT). We examined the pharmacological characterization of 5-HT uptake into rat cortical synaptosomes and cultured rat astrocytes, and the immunodetection of glial SERT proteins using specific site-directed monoclonal antibodies (MoAb). Furthermore, using a reverse transcriptase-polymerase chain reaction (RT-PCR) method, we addressed the expression of SERT mRNA in cultured rat astrocytes. We investigated the inhibitory effects of various monoamine uptake inhibitors on the uptake of [3H]5-HT into cultured astrocytes and cortical synaptosomes. Tricyclic antidepressants (clomipramine and imipramine) as well as selective serotonin re-uptake inhibitors (fluvoxamine, fluoxetine and zimelidine) were very potent inhibitors of [3H]5-HT uptake in both preparations. In contrast, the inhibitory effects of NE uptake inhibitors (nisoxetine and desipramine) and cocaine were weaker than those of 5-HT uptake inhibitors. In addition, dopamine (DA) uptake inhibitors (nomifensine and GBR-12935) exhibited a Ki value in the low micromolar range. The inhibitory potencies were in the order 5-HT uptake inhibitors (clomipramine, fluvoxamine, fluoxetine, imipramine and zimelidine) > NE uptake inhibitors (nisoxetine and desipramine) = cocaine > DA uptake inhibitors (nomifensine and GBR-12935). There was no difference in the order of the inhibitory effects of various monoamine uptake inhibitors between the two preparations. A correlation analysis of the potencies of various monoamine uptake inhibitors in the inhibition of [3H]5-HT into cultured astrocytes and cortical synaptosomes produced a highly significant correlation coefficient of 0.9893 (P < 0.0001). Immunocytochemical staining using anti-SERT MoAb in cultured astrocytes revealed that the plasma membrane, as well as intracellular, perinuclear compartments, presumably endoplasmic reticulum or golgi membranes, showed a considerable level of immunoreactivity. Extracts of astrocytes and synaptosomes from the cortex were immunoblotted with anti-SERT MoAb. SDS-PAGE/Western blots indicate that anti-SERT MoAb recognized two bands of 120 and 73 kDa in both preparations. RT-PCR demonstrated that astrocytes in cultured expressed mRNA for the cloned SERT protein, which has been characterized as the neuronal SERT. These pharmacological experiments indicate that this uptake process takes place through glial SERT that is very similar to neuronal SERT. Furthermore, the present data also indicate that the presence of the mRNA and protein for the neuronal SERT were established in cultured rat astrocytes, and the polypeptide portion of SERT in astrocytes and frontal cortex could be the same gene product.

Feeding suppression by fibroblast growth factor-1 is accompanied by selective induction of heat shock protein 27 in hypothalamic astrocytes

It has been suggested that fibroblast growth factor (FGF)-1 serves as a physiological satiety factor in the hypothalamus, although the molecular mechanism underlying such a function is poorly understood. To gain additional insight into this issue, we used a Sendai virus (SeV) gene expression system in rats to explore genes differentially expressed subsequent to expression of FGF-1. Using cDNA arrays, we determined that infusion of FGF-1/SeV into one lateral ventricle induced selective expression of heat shock protein (HSP) 27 in the hypothalamus. Whereas FGF-1 expression was restricted to the ependymal cell layer of the cerebral ventricles, HSP27 was more widely expressed in astrocytes residing in the surrounding periventricular region. Similarly, infusion of FGF-1 polypeptide into a lateral ventricle induced dose-dependent HSP27 expression in periventricular astrocytes surrounding the third ventricle, with maximum mRNA levels being attained 6 h after infusion. This induction of HSP27 was accompanied by a significant suppression of feeding behaviour. Interestingly, suppression of feeding caused by intracerebro ventricular infusion of ciliary neurotrophic factor was also accompanied by induction of HSP27 in periventricular astrocytes, but suppression of feeding caused by infusion of leptin was not. It therefore appears that suppression of feeding by FGF-1 is accompanied by selective induction of HSP27 expression in hypothalamic astrocytes surrounding the third ventricle, and that this response may be a key component of the mechanism by which appetite is regulated by FGF-1.

Regulation of serotonin transporter gene expression in human glial cells by growth factors

The aims of this study were to identify monoamine transporters expressed in human glial cells, and to examine the regulation of their expression by stress-related growth factors. The expression of serotonin transporter mRNA was detected by reverse transcriptase-polymerase chain reaction in normal human astrocytes, whereas the dopamine transporter (DAT) and the norepinephrine transporter (NET) were not detected. The cDNA sequence of the "glial" serotonin transporter in astrocytes was consistent with that reported for the "neuronal" serotonin transporter (SERT). Moreover, we also demonstrated SERT expression in glial fibrillary acidic protein-positive cells by immunocytochemical staining in normal human astrocytes. Serotonin transporter gene expression was also detected in glioma-derived cell lines (A172, KG-1-C and KGK). Addition of basic fibroblast growth factor (bFGF) or epidermal growth factor (EGF) for 2 days increased serotonin transporter gene expression in astrocytes and JAR (human choriocarcinoma cell line). Basic fibroblast growth factor, but not epidermal growth factor, increased specific [3H]serotonin uptake in astrocytes in a time (1-4 days)- and concentration (20-100 ng/ml)-dependent manner. The expression of genes for basic fibroblast growth factor and epidermal growth factor receptors was detected in astrocytes. These findings suggest that the expression of the serotonin transporter in human glial cells is positively regulated by basic fibroblast growth factor.

Intracerebroventricular infusion of fibroblast growth factor-1 increases Fos immunoreactivity in periventricular astrocytes in rat hypothalamus

When released into the third ventricle of the brain, fibroblast growth factor (FGF)-1 acts as a feeding suppressor. To identify the cells that respond to FGF-1 in this process, we investigated the effects of intracerebroventricular administration of FGF-1 on Fos protein expression in the rat hypothalamus. Two hours after infusion of FGF-1, significantly more Fos signals were observed in the periventricular zone of the third ventricle and its surroundings. Double immunohistochemical studies using antibodies against Fos, glial fibrillary acidic protein, and neuronal nuclei revealed that the FGF-1-induced Fos signals in the periventricular zone were in astrocytes, not in neurons. FGF-1-induced Fos expression was not found in neurons in such hypothalamic nuclei as the lateral hypothalamic area, paraventricular nucleus, ventromedial hypothalamic nucleus, dorsomedial hypothalamic nucleus, or arcuate nucleus. These results suggest a possible involvement of periventricular astrocytes in the early stages of FGF-1-induced feeding suppression.