Characterization of [3H]dopamine uptake sites and [3H]cocaine recognition sites in primary cultures of mesencephalic neurons during in vitro development

Effects of two commonly found strains of influenza A virus on developing dopaminergic neurons, in relation to the pathophysiology of schizophrenia

Influenza virus (InfV) infection during pregnancy is a known risk factor for neurodevelopment abnormalities in the offspring, including the risk of schizophrenia, and has been shown to result in an abnormal behavioral phenotype in mice. However, previous reports have concentrated on neuroadapted influenza strains, whereas increased schizophrenia risk is associated with common respiratory InfV. In addition, no specific mechanism has been proposed for the actions of maternal infection on the developing brain that could account for schizophrenia risk. We identified two common isolates from the community with antigenic configurations H3N2 and H1N1 and compared their effects on developing brain with a mouse modified-strain A/WSN/33 specifically on the developing of dopaminergic neurons. We found that H1N1 InfV have high affinity for dopaminergic neurons in vitro, leading to nuclear factor kappa B activation and apoptosis. Furthermore, prenatal infection of mothers with the same strains results in loss of dopaminergic neurons in the offspring, and in an abnormal behavioral phenotype. We propose that the well-known contribution of InfV to risk of schizophrenia during development may involve a similar specific mechanism and discuss evidence from the literature in relation to this hypothesis.

Ultrastructural characterization of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid-induced cell death in embryonic dopaminergic neurons

Developing neuronal populations undergo significant attrition by natural cell death. Dopaminergic neurons in the substantia nigra pars compacta undergo apoptosis during synaptogenesis. Following this time window, destruction of the anatomic target of dopaminergic neurons results in dopaminergic cell death but the morphology is no longer apoptotic. We describe ultrastructural changes that appear unique to dying embryonic dopaminergic neurons. In primary cultures of mesencephalon, death of dopaminergic neurons is triggered by activation of glutamate receptors sensitive to alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA), and differs ultrastructurally from both neuronal apoptosis or typical excitotoxicity. AMPA causes morphological changes selectively in dopaminergic neurons, without affecting other neurons in the same culture dishes. Two hours after the onset of treatment swelling of Golgi complexes is apparent. At 3 h, dopaminergic neurons display loss of membrane asymmetry (coinciding with commitment to die), as well as nuclear membrane invagination, irregular aggregation of chromatin, and mitochondrial swelling. Nuclear changes continue to worsen until loss of cytoplasmic structures and cell death begins to occur after 12 h. These changes are different from those described in neurons undergoing either apoptosis or excitotoxic death, but are similar to ultrastructural changes observed in spontaneous death of dopaminergic neurons in the natural mutant weaver mouse.

Sexual dimorphism in the brain metabolic response to prenatal cocaine exposure

The effects of prenatal cocaine exposure on the metabolic function of major central neuronal systems in the periweanling rat are reported in this study. Pregnant Sprague--Dawley rats were administered cocaine at either 30 or 60 mg/kg or the vehicle from gestation day (G) 8 through 22 via daily gastric intubation. Since prenatal cocaine has been shown to alter behavior in weanling rats, brain functional activity was quantified using the deoxyglucose method in male and female 21-day-old offspring (one of each gender/litter). Cocaine's effects were most significant within the limbic system where a three-way interaction between cocaine treatment, sex and brain region was seen. Within the limbic system, two regions, the rostral accumbens and the diagonal band of Broca showed reductions in metabolic activity in the exposed male offspring compared to the control offspring while no changes were seen in females. At more caudal levels of the forebrain, the accumbens (at the level of 1.2 micro rostral to Bregma) and septum showed cocaine-induced reductions in metabolism which were not dependent upon the sex of the animal. Metabolism within the hypothalamus also tended to show a significant interaction between treatment, gender and brain region (P=0.06). Two regions, including the ventromedial nucleus and lateral hypothalamic area, were metabolically depressed in the males alone while three other regions; the dorsomedial, arcuate, and medial preoptic nuclei were also metabolically depressed in the treated groups collapsed across gender. There were no significant treatment or sex-related effects or interactions within the sensory and motor systems. Chronic prenatal cocaine exposure reduced metabolism significantly in a restricted portion of the forebrain, the mesocortical-limbic system, particularly in regions associated with the medial forebrain bundle. These reductions were seen primarily in males while some regions showed changes which were independent of the sex of the animal. These cocaine-induced effects resembled, to a great extent, those seen in similarly-treated males examined as adults. The data emphasize that cocaine use during a restricted period of early pregnancy depresses function within limbic and hypothalamic regions and that many of these effects are sexually dimorphic in nature.

Hypoxia during early developmental period induces long-term changes in the dopamine content and release in a mesencephalic cell culture

The present study was conducted to elucidate the long-term effects of exposure to hypoxia of dopaminergic neurons during the early developmental period. Primary mesencephalic cell cultures prepared from fetal rats and containing 0.5-2% of dopaminergic neurons were exposed to hypoxia between in vitro days 1 and 6, the putative critical developmental period. Changes in the content, release and uptake of dopamine were found to depend on the degree of hypoxia and on the duration of exposure. Following moderate hypoxia (7 h, 5% O2) on two consecutive days between in vitro days 1 and 3, the cultures showed a small increase in the dopamine levels, by 16%. After severe hypoxia (0% O2/95% N2 for 24 h), during the same time window, the cellular dopamine content was elevated by 100%. Moreover, severe hypoxia produced long-lasting modulations of the dopaminergic system. On in vitro day 14, cells exhibited increased levels of 3,4-dihydroxyphenylacetic acid and homovanillic acid (by 34% and 55%, respectively), and elevations of both the spontaneous and potassium-stimulated dopamine release by 70%. The dopamine transport and metabolism of cells exposed to hypoxia between in vitro days 4 and 6 remained unchanged with regard to long-term effects. The present study provides strong evidence for the induction of long-term changes in dopaminergic cells due to hypoxia during the critical developmental period in mesencephalic culture. The developmental period capable of inducing long-lasting changes in dopamine metabolism is restricted to in vitro days 1-3.

Expression and regulation of the human dopamine transporter in a neuronal cell line

Human cocaine users exhibit increased striatal [3H]WIN35428 binding to the dopamine transporter (DAT). However, the nature of the changes induced in the DAT are complex and may not result from a simple increase in number of DAT molecules. To better understand the regulation of DAT inhibitor binding sites and their relationship to the overall process of dopamine uptake, a neuronal model system expressing the human DAT has been developed. Initial experiments were attempted with native dopaminergic neurons so as to allow examination of DAT interactions with vesicular release and storage mechanisms. Dissociated fetal rat mesencephalic neurons, of various ages and mixtures with target cells, were grown to confluence. However, [3H]WIN35428 binding was of low affinity at all levels of maturity. Following this, a simpler model was assessed, using DAT cDNA transfected into neuroblastoma-derived Neuro2A cells. Initially, no specific and little non-specific [3H]WIN35428 or [3H]paroxetine binding was found in non-transfected cells. After transfection with the human DAT inserted in the pcDNA vector, both DAT binding and dopamine uptake were significantly and stably present. Treatment with (-)cocaine, 10-6 M for 24 h, increased DAT binding and uptake, which did not occur in parallel COS-7 experiments. Other experiments with Neuro2A cells also found that dopamine uptake was down-regulated by treatment with a PKC activator. These results suggest that the transfected Neuro2A neurons should be useful for ongoing experiments examining the regulation of the DAT by assorted treatments.

Differential potencies of cocaine and its metabolites, cocaethylene and benzoylecgonine, in suppressing the functional expression of somatostatin and neuropeptide Y producing neurons in cultures of fetal cortical cells

Using aggregate cultures derived from 17-day-old fetal rat cortex, we addressed the question: Does cocaine alter the functional expression of neuropeptide Y (NPY) and somatostatin (SRIF) neurons and, if so, are cocaethylene (CE) and benzoylecgonine (BZE) as active as cocaine? NPY/SRIF production in response to brain-derived neurotrophic factor (BDNF) or phorbol-12-myristate-13-acetate (PMA) was used as a functional criterion. A 5-day exposure to cocaine did not affect basal or stimulated (BDNF or PMA) production of NPY but it markedly suppressed BDNF- or PMA-stimulated production of SRIF. Exposure to CE led to a drastic suppression of basal as well as stimulated (BDNF or PMA) production of both NPY and SRIF. These effects of cocaine and CE were concentration dependent (1-100 microM). BZE did not alter any of these functional parameters. Next, we evaluated the fate of cocaine, CE, and BZE in the culture medium. Cocaine was converted to BZE, whereas BZE was not converted to cocaine. CE was converted to cocaine and BZE, with substantial amounts of cocaine and CE remaining in the medium after 72 hr (approximately 20% each). In summary, cocaine, CE, and BZE exhibited differential potencies in suppressing the expression of cultured NPY and SRIF neurons: CE was more potent than cocaine and BZE was inactive. SRIF neurons were more susceptible than NPY neurons to the effects of cocaine. The higher potency of CE may be due to a property of the compound and/or to CE serving as a source for a slow, continuous formation of cocaine by the brain cells themselves.

Role of antioxidants in the nitric oxide-elicited inhibition of dopamine uptake in cultured mesencephalic neurons. Insights into potential mechanisms of nitric oxide-mediated neurotoxicity

Under aerobic conditions the addition of (C2N5)2N(N[O]NO)-.Na+(DEA/NO), S-nitroso-N-acetyl penicillamine and nitric oxide (NO)-saturated buffer, but not S-nitroso-L-glutathione, to dopamine solutions resulted in dopamine o-semiquinone formation that was dependent on the formation of a NO/oxygen intermediate. High pressure liquid chromatography (HPLC) electrochemical analysis of dopamine demonstrated that the DEA/NO-induced oxidation of dopamine was abrogated in the presence of the antioxidants, ascorbate and glutathione. NO spontaneously released from DEA/NO decreased [3H]dopamine accumulation in primary cultures of mesencephalic neurons in a dose-dependent fashion. In contrast, [3H] gamma-aminobutyric acid uptake by mesencephalic neurons tested under the same conditions was unchanged. When DEA/NO was added to incubation buffer that contained [3H]dopamine and the antioxidant, ascorbate or glutathione, [3H]dopamine uptake was also inhibited. These data excluded that oxidation of extracellular [3H]dopamine by the intermediates of the NO/O2 reaction could have caused this decrease. Instead, NO may have acted directly on a not yet identified target operative in the regulation of dopamine storage and release. Analysis of the rate constants for the NO reaction with ascorbate, glutathione and dopamine revealed that dopamine quinone formation was delayed by the presence of antioxidants. Since the formation of NO as well as neurotransmitter release are activated during ischemia reperfusion injury, it is possible that prolonged NO exposure could deplete antioxidants and facilitate the oxidation of dopamine and thereby cause neurotoxicity.

Rat mesencephalic neuronal cells cultured for different periods as a model of dopamine transporter ontogenesis

Ventral mesencephalic neurons contained only low-affinity and sodium-independent binding sites of [3H]WIN 35,428 (marker of dopamine transporter) during the first 10 d in primary cultures. These sites were present in cytosol, and they are not very probably related to dopamine transporter. After 12 d in culture, membrane-bound, high-affinity, and sodium-dependent [3H]WIN 35,428 binding sites were detected. In membranes prepared from cells 14 d in culture, cocaine displaced [3H]WIN 35,428 binding with similar potency to that in striatal membranes of adult rat brain. The high-affinity [3H]WIN 35,428 binding sites in mesencephalic neuronal cell cultures are very probably related to dopamine transporter. The development of high-affinity [3H]WIN 35,428 binding sites in neurons cultured for different time periods could be a useful model of dopamine transporter ontogenesis.

Similar properties of fetal and adult amine transporters in the rat brain

A variety of drug classes, including psychomotor stimulants and antidepressants, interact with monoamine transporters in order to exert their effects. Although these transporters have been extensively characterized in the adult brain, little is known about uptake mechanisms in the fetal system. High affinity dopamine (DA) and serotonin (5-HT) uptake in the striatum and frontal cortex, respectively, were examined in rat fetuses (embryonic day 20; E-20). These results were then compared to uptake in adult rat synaptosomal preparations of the same regions. The data indicate that the fetal (E-20) uptake mechanism is sodium-dependent. Furthermore, the potency of various agents to inhibit transporter function was assessed. These drugs produced a concentration-dependent inhibition of uptake, and the resulting IC50 values were not significantly different from those obtained in the adult preparations. Our results provide evidence that the affinity of monoamine uptake inhibitors for fetal (E-20) DA and 5-HT transporters is similar to that observed with adult transporters. This observation has broad implications when considering neuronal development and in utero exposure to drugs that exert their effects through these transporters.

Potent cocaine analogs inhibit [3H]dopamine uptake in rat mesencephalic cells in primary cultures: pharmacological selectivity of embryonic cocaine sites

The cellular localization of the cocaine binding sites in primary cultures of embryonic rat mesencephalic cells was previously reported to differ from that observed in adult rat brain. In order to know whether this different localization was associated with a different pharmacological selectivity, we tested the effect of new cocaine analogs on tritiated dopamine ([3H]DA) uptake in primary cultures of rat embryonic mesencephalic cells. In these cultures, [3H]DA was taken up by a nomifensine-sensitive, but desipramine and fluoxetine-insensitive process, reflecting selective uptake by the dopaminergic transporter. 3 beta-(4-Chlorophenyl)tropan-2 beta-carboxylic acid methyl ester (RTI-COC-31) was by far the most potent inhibitor of the [3H]DA uptake, presenting an IC50 of 3.8 nM, while the corresponding analog with an unsubstituted phenyl ring (WIN 35,065-2) was 38 times less potent. The enantiomer of WIN 35,065-2, namely WIN 35,065-3, was 30 times less potent than the former. A similar pattern was found for the relative ability of these compounds to inhibit binding of the radiolabeled cocaine derivative [125I]RTI-55 to membranes prepared from mesencephalic cultures. The order of potencies found for the three cocaine analogs on mesencephalic cultures was similar to that previously obtained in [3H] WIN 35,428 binding experiments and [3H]DA uptake inhibition in adult rat striatum, suggesting that the pharmacological selectivity of cocaine sites functionally related to the DA transporter in cultured embryonic neurons does not differ from that obtained in adult rat brain.

K(+)-evoked dopamine release depends on a cytosolic Ca2+ pool regulated by N-type Ca2+ channels

Membrane depolarization evoked by 25-40 mM K+ elicited an immediate increase of somatic and neuritic [Ca2+]i in cultured dopaminergic neurons as measured by digital fluorescence microscope imaging. The rise of neuritic [Ca2+]i was inhibited by N-type but not L-type Ca2+ channel blockers, while the rise of somatic [Ca2+]i was prevented by both L- and N-type Ca2+ channel blockers. Similarly, depolarization-induced [3H]dopamine release was selectively attenuated by N-type Ca2+ channel blockers. The present results suggest that [3H]dopamine release from mesencephalic neuronal cell cultures relates to a Ca(2+)-dependent mechanism regulated by N-type channels located in the vicinity of the exocytotic sites within neuritic processes.

A high percentage yield of tyrosine hydroxylase-positive cells from rat E14 mesencephalic cell culture

In the ventral mesencephalon of the E14 rat fetus, 90% of the dopaminergic, tyrosine hydroxylase positive (TH+) cells are localized in 1.0 mm3 of tissue. This same ventral mesencephalic region also contains 90% of the dopamine content of the E14 ventral brainstem (2.2 +/- 0.3 nmol/mg protein). When cells were prepared for culturing from this localized area, and plated at a density of 2.5 x 10(5) cells/cm2, 17-21% of the cells were TH+, at 4 and 12 h, and at 1, 5, 7 and 10 days after plating. The percentage of TH+ cells was also 17-21% when examined at 4 h, 12 h or 5 days after plating at densities ranging from 7.8 x 10(3) to 2.5 x 10(5) cells/cm2. However, cell survival at a density of less than 6.2 x 10(4) cells/cm2 was poor after 5 days in culture. Based on the degree of neurite elongation and complexity, cell maturation appeared to be complete at 5 days in culture (DIV5), and appeared to be maintained at this level up to DIV10. By DIV14, neurite retraction was evident, and the cells were more rounded. These signs may indicate the inception of senescence in the cultures. A benztropine-sensitive, concentration-dependent dopamine uptake mechanism was demonstrated in the cultures at DIV7, and DA could be released from preloaded cells using 50 mM K+. Five morphological subtypes of TH+ cells were identified in the cultures. This primary culture of the ventral mesencephalic, dopaminergic area, with a high percentage of TH+ cells, is suitable for use in acute biochemical and cellular studies, between DIV 5 and DIV10.