Biochemical characterization of the uptake and release of [3H]dopamine by dopaminergic hypothalamic neurons: a developmental study using serum-free medium cultures

Release of endogenous dopamine in cultured mesencephalic neurons: influence of dopaminergic agonists and glucocorticoid antagonists

Several electrochemical techniques allow the measurement of dopamine release in freely moving animals and brain slices. In this report, we applied one of these techniques, coulometry, coupled to high-performance liquid chromatography (HPLC), to the study of dopamine release in primary cultures of embryonic mesencephalic dopaminergic neurons. Between day 9 and 33 of culture, concentrations of dopamine, above the detection threshold, were found in the incubation buffer (Krebs ringer buffer, KRB). Concentrations of dopamine in the incubation buffer reflected neuronal release as they were: (i) positively correlated with the number of tyrosine hydroxylase-positive dopamine neurons in the culture; (ii) tetrodotoxin (TTX) sensitive and Ca2+ dependent; (iii) increased by a depolarizing stimulus, e.g. K+ (20 mM), or by the indirect dopamine agonists amphetamine and cocaine; (iv) decreased by a hyperpolarizing stimulus, e.g. the dopamine D2-like receptor agonist quinpirole. Dopamine release in this model was also sensitive to the manipulation of glucocorticoids, potent modulators of dopamine release in vivo. Long-term treatment of the cell cultures with RU 39305, a selective antagonist of glucocorticoid receptors (GR), but not with spironolactone, a selective antagonist of mineralocorticoid receptors (MR), dose-dependently decreased K+-stimulated dopamine release. In conclusion, these results demonstrate an in vitro model that allows the studying of the release of endogenous dopamine in cell cultures and the effects of glucocorticoid hormones on the release dynamics.

Development of the mesencephalic and diencephalic catecholamine systems in human fetuses: uptake and release of catecholamines in vitro

Development of catecholamine (CA) systems of the ventral mesencephalon and diencephalon were studied in human fetuses at age 6, 8, 10 and 12 weeks, evaluating the CA specific uptake and K(+)-stimulated release with the isotopic biochemical technique. In the mesencephalon, the [3H]dopamine (DA) uptake was detected as early as 6 weeks, suggesting the existence of either CA neurons or fibers. This was followed by gradual increase of the [3H]DA uptake up to 10 weeks and a subsequent fall at 12 weeks. In the diencephalon, the uptake was first observed at 8 weeks, followed by its decrease at 10 weeks and subsequent increase at 12 weeks. The dynamic uptake is considered as a manifestation of the continuous differentiation of CA neurons and sprouting of CA fibers. In contrast to uptake, no CA release was detected in response to membrane depolarization in the diencephalon and mesencephalon at any age studied, suggesting a timing dissociation between the onset of the CA uptake and K(+)-provoked release in the course of neuron differentiation in human fetuses.

A comparison of the developing dopamine neuron phenotype in cultures of embryonic rat mesencephalon and hypothalamus

Development of the dopamine (DA) neuron phenotype was monitored in cultures of embryonic rat mesencephalon (MES) and hypothalamus (HYP) maintained for 1 to 21 days in vitro (DIV) in the absence of glial support cells. Cell counts following immunohistochemistry for tyrosine hydroxylase (TH) demonstrated that the number of DA neurons declined by 85% in MES cultures yet increased 5-fold in cultures of HYP, so that by 21 DIV equal numbers of DA neurons were present in these culture systems. After 21 DIV MES DA neurons exhibited a multipolar morphology, with numerous branching processes. HYP DA neurons were primarily fusiform in shape with fewer processes and process branch points. Double-label immunohistochemistry for TH and microtubule-associated protein 2 identified the majority of TH-positive processes in either culture system as dendrites. Individual MES but not HYP DA neurons were also found to generate axons. Western analysis showed that between 1 and 21 DIV the concentration of TH protein increased 2-fold in MES and 4-fold in HYP cultures. After 21 DIV the concentration of TH protein in MES cultures was twice that found in cultures of HYP. In the period between 1 and 21 DIV levels of tetrahydrobiopterin (BH4) increased by 6-fold in MES and 20-fold in HYP cultures. After 21 DIV BH4 content was 3-fold higher in HYP than in MES cultures. The abundance of the mRNA encoding for GTP cyclohydrolase I, the rate-limiting enzyme in BH4 biosynthesis, was similar in MES and HYP cultures despite this difference in BH4 levels. In contrast, TH mRNA was 4-fold more abundant in MES than in HYP cultures. Treatment of MES cultures with the DA neuron toxin 1-methyl-4-phenylpyridinium decreased DA cell numbers, TH protein content and BH4 levels, demonstrating that BH4 is localized primarily to DA neurons. Similar treatment of HYP cultures did not effect any of these parameters. Steady-state levels of DA and the rate of DA synthesis were both 3-fold higher in MES than in HYP cultures. A 95% decline in BH4 content produced by inhibiting BH4 biosynthesis resulted in 64% and 84% declines in the rate of MES and HYP DA synthesis, respectively. Overall, these observations indicate that, with the exception of the capacity to synthesize DA, DA neurons in MES and HYP cultures share few common properties.

Use of [3H]-GBR12935 to measure dopaminergic nerve terminal growth into the developing rat striatum

In this study we determined the temporal association between the appearance of the dopamine transporter, measured by 1-[2-(diphenyl-methoxy)ethyl]-4-(3- phenylpropyl)-piperazine ([3H]-GBR12935), a potent and selective inhibitor of dopamine uptake, and other biochemical markers of dopaminergic nerve terminal growth into the developing striatum. [3H]-GBR12935 binding was minimally detected in the rudimentary striatum of embryonic day 14 rat brains, increased to 23% of the adult level by birth, and reached the adult level during the fifth postnatal week. This finding contrasts with a slower developmental increase in [3H]-dopamine uptake, a functional measure of the transporter. Tyrosine hydroxylase activity levels followed a developmental curve similar to that of [3H]-GBR12935 binding but did not reach adult levels until the 7th postnatal week. Dopamine content increased at a slower rate, being only 10% and 92% of the adult level at birth and postnatal week 8, respectively. These results indicate that the appearance of a structural, but not optimally functional, dopamine transporter may be the earliest detectable biochemical index of dopaminergic nerve terminal growth into the striatum during development.

Differential expression and subcellular localization of secretogranin II and synaptophysin during early development of mouse hypothalamic neurons in culture

Mature neurons contain two distinct regulated secretory pathways, characterized electron microscopically by so-called large dense core vesicles and small synaptic vesicles, respectively. Each vesicle type is characterized by vesicle-specific proteins, such as the granins (chromogranins/secretogranins) for the matrix of large dense core vesicles and synaptophysin for the membrane of small synaptic vesicles. So far, no data exist on the biogenesis of these two distinct vesicle types during neuronal development. We have used secretogranin II and synaptophysin as markers for the biogenesis of these two vesicle types during the development of mouse hypothalamic neurons in culture, using immunocytochemistry and biochemical analyses. By immunofluorescence, we found that secretogranin II appears as early as synaptophysin, but in a subset of neurons only, and with different subcellular localizations. It was observed in cytoplasmic areas where little or no synaptophysin immunofluorescence was detected, such as lamellipodia, emerging neurites and growth cones. At later stages, the proportion of secretogranin II-containing varicosities remained steady whereas that of synaptophysin-containing varicosities increased dramatically. By quantitative analysis we found that the level of expression of synaptophysin increased several-fold during synaptogenesis whereas that of secretogranin II decreased. These data suggest that large dense core vesicles and small synaptic vesicles can be formed separately and expressed at different levels. They provide evidence for a differential biogenesis of these two distinct vesicle types.

Dopamine synthesis precedes dopamine uptake in embryonic rat mesencephalic neurons

We have measured [3H]dopamine ([3H]DA) uptake and tyrosine hydroxylase-immunopositive immunostaining in cells acutely dissociated from the embryonic ventral mesencephalon (MSC). DA and its metabolites as well as catechol-O-methyltransferase (COMT) and monoamine oxidase (MAO) activities were determined in homogenates taken from the MSC and striatum (STR). In the embryonic ventral MSC measurable DA and tyrosine hydroxylase (TH) immunostaining were present as early as embryonic day (E) 12.5. At E14 the number of TH+ neurons was about 50% of the values at E18. In the MSC, DA concentration increased sharply at E16 and reached a plateau before birth that was 10-fold lower than adult values. In the STR, DA was first detected at E16, suggesting that DA fibers reach the STR at this embryonic stage. High-affinity DA uptake appeared in the MSC only at E16, concomitantly with the arrival of DA fibers in the STR, increased sharply between E16 and E18, and reached a plateau before birth. This uptake mechanism was not selective for catecholamine uptake inhibitors. Thus, DA synthesis in the MSC preceded the onset of high-affinity uptake mechanism, which could be correlated to the beginning of striatal DA innervation. Measurable MAO and COMT activities were detected as early as E13 (MSC) and E15 (STR), but not DA metabolites, which appeared later. We conclude that the high-affinity DA uptake mechanism in MSC DA neurons develops coincident with the arrival of DA fibers to the STR. The sharp increase of DA uptake between E16 and E18 is due only in part to an increase in the number of TH+ cells. These results support the hypothesis that in vivo the target STR neurons regulate the maturation of MSC DA cells.

Functional maturation of somatostatin neurons and somatostatin receptors during development of mouse hypothalamus in vivo and in vitro

Ontogenesis of somatostatin (SRIF) neurons and receptors was studied in fetal hypothalamic cell cultures kept in serum-free medium, and compared to the in vivo developmental pattern. Initial rise in neuronal content of SRIF occurred later in vitro than in vivo. In vitro, K(+)-induced SRIF release was only present after synaptogenesis. SRIF binding sites were measurable as early as 1 day after birth and at an equivalent time in culture, after 6 days in vitro (DIV); their affinity was in the nanomolar range. In cultured cells, binding reached a maximum at two weeks in vitro and decreased sharply thereafter as a consequence of binding site occupancy by the endogenous ligand. Indeed, pretreatment with cysteamine decreased SRIF concentration in the neuronal cultures and twice as many binding sites as in control cultures of 21 DIV were measured. Competition kinetics using unlabelled SMS 201-995 to displace [125I]SRIF revealed two distinct binding sites in the neuronal preparations (IC50 = 11 +/- 3 pM and 4.5 +/- 0.8 nM). In contrast, only the lower affinity site was present on glial cell preparations (1.7 +/- 0.4 nM). SRIF inhibited adenylate cyclase activity in glia and neurons, and the onset of SRIF coupling to the second messenger occurred earlier in vitro than in vivo. Pertussis toxin pretreatment was equally effective in neuronal and glial cell preparations to decrease SRIF binding and to inhibit adenylate cyclase activity.

Different developmental potentials of diencephalic and mesencephalic dopaminergic neurons in vitro

Morphological and functional differentiation of dopamine (DA) neurons was compared in dissociated cultures from gestational day 14 rat mesencephalon and diencephalon. Numbers of tyrosine hydroxylase-immunoreactive (TH-IR) neurons relative to all neurons were 4 and 1.7 times higher in mesencephalic than in diencephalic cultures at 6 and 13 days in vitro (DIV), respectively. Morphological maturation of diencephalic DA neurons was retarded in comparison to mesencephalic DA neurons. There were gross differences in soma size and length of processes between the two types of DA neurons, the appearance of which was strongly reminiscent of DA cell types described in vivo. Functional maturation of DA neurons was quantified by measuring uptake and Ca2+-dependent K+-stimulated release of [3H]DA per TH-IR neuron. As early as 6 DIV, DA uptake by mesencephalic DA neurons was saturable, was sensitive to benztropine and reserpine, and could be displaced by unlabeled DA. Twenty to 30% of the radioactivity accumulated could be released upon depolarization within a period of 5 min. At 6 DIV, influx of [3H]DA into diencephalic DA neurons was almost insensitive to benztropine, reserpine and unlabeled DA. Even after 13 DIV, diencephalic DA uptake was characterized by a markedly lower initial velocity, a longer time needed to reach saturation, a lower uptake capacity, and a lower sensitivity to benztropine than mesencephalic DA uptake. The releasable pool was very small and did not increase between DIV 6 and 13. The results demonstrate that mesencephalic DA neurons in vitro differentiate considerably faster than diencephalic DA neurons and gain functional competence very early in brain development. Comparison with data on adult nigrostriatal and hypothalamic DA systems suggests that the in vitro differences reflect a fundamental regional diversity of DA neurons.

Immunocytochemical localization of thyroid hormone nuclear receptors in cultured hypothalamic dopaminergic neurons

By means of a monoclonal antibody against the rat liver L-triiodothyronine nuclear receptor and a polyclonal anti-tyrosine hydroxylase serum, it has been possible to demonstrate thyroid hormone nuclear receptors in immunoreactive tyrosine hydroxylase cell nuclei in fetal rat hypothalamic cultures. After 8 days in vitro, the ratio of tyrosine hydroxylase cells that were immunoreactive for the thyroid hormone receptor to those not stained for this receptor (64% to 36% respectively) remains unchanged despite an increase in the number of tyrosine hydroxylase-positive cells with time (from day 8 to day 21) in culture. The presence of thyroid hormone nuclear receptor in dopaminergic neurons is correlated with a morphological effect of L-triiodothyronine in this neuronal population. Our results demonstrate, for the first time, the presence of triiodothyronine nuclear receptors in fetal rat dopaminergic neurons and the existence of a cellular heterogeneity in the distribution of the thyroid hormone receptor. The presence of these receptors in fetal hypothalamic dopaminergic neurons suggests that some effects of L-triiodothyronine on the maturation of DA neurons may result from a direct effect of this hormone through an interaction with its specific nuclear receptors.

Electrophysiology and ultrastructure of mouse hypothalamic neurons in culture: a correlative analysis during development

The development of the electrical activity of hypothalamic neurons in dissociated cell cultures obtained from 14 day old mice foetuses was studied using patch extracellular and intracellular recording techniques. Electrophysiological data were compared with morphological observations obtained by electron microscopy. During patch recording, excitability of the cells was tested by the application of a 40 mM KCl solution. Tetrodotoxin (TTX, 10(-6) M in the delivery pipette) and Co2+ (10(-2) M in the delivery pipette) were applied to the recorded cell by pressure in order to study the involvement of sodium and calcium currents in the electrical activity during the in vitro development. From the first day of incubation, TTX and Co2+ were able to block reversibly the spontaneous electrical activity. However, only TTX application inhibited action potentials which suggests that calcium currents could be poorly involved in the action potential generation at the beginning of neuronal differentiation. Three different phases were found in the electrophysiological development of hypothalamic neurons in culture. The first phase (between the 1st and the 5th day of incubation) was characterized by an increase in the ratio of the spontaneously active cells (15% at day 1 and 90% at day 5). This increase paralleled the increase of the ratio of excitable cells. During this period no post-synaptic activity was detected. Morphologically, at 36 h, no synaptic contact was observed and growth cones were found to be very primitive. The second phase, between the 6th and the 9th day of culture, was characterized by a decrease in the ratio of spontaneously active cells and by the appearance, in a few cases, of a postsynaptic potential activity. During this phase the majority of the silent cells were excitable. At this stage neurons formed well differentiated neurites and growth cones. Synaptogenesis had already started and several stages of synapse formation could be seen. The third phase of the development, from 10 days of incubation, was characterized by an increase in post synaptic potential activity. During this period, numerous mature synapses could be observed although most of the synaptic contacts were located on neurites. In addition, some synapses were apposed onto degenerated structures. In conclusion, hypothalamic neurons in culture appear to differentiate in 3 steps: a primitive stage during which spontaneous electrical activity and excitability develop without any synaptic contact; a 2nd stage during which synaptic contacts develop, followed by a third stage of synapse maturation where mature synapses are formed whereas transient synapses degenerate.