Development of the hypothalamic 5-hydroxytryptamine system during ontogenesis in rats: uptake and release of 5-hydroxytryptamine in vitro

Brain as an endocrine source of circulating 5-hydroxytryptamine in ontogenesis in rats

This study was aimed to test the authors' hypothesis stating that the developing brain before the closure of the blood brain barrier (BBB) operates as an endocrine organ that secretes classical neurotransmitters and neuropeptides into the general circulation. 5-Hydroxytryptamine (5-HT) was selected as a marker of brain endocrine activity though it is also secreted by peripheral organs. 5-HT was detected in blood of rats in a biologically active concentration at any studied age, from the 21st embryonic day till the 30th postnatal day. The brain was proven to be a source of circulating 5-HT before the BBB closure by showing that the 5-HT concentration in blood decreased significantly after the inhibition of 5-HT synthesis in the brain of neonates. The 5-HT concentration in blood was not diminished after the BBB closure, apparently due to compensatory increase of 5-HT secretion by peripheral sources. Thus, brain-derived 5-HT is delivered to the general circulation before the BBB closure being potentially capable of providing endocrine regulation of target organs.

Influence of monoamines on differentiating gonadotropin-releasing hormone neurones in foetal mice

This study evaluated the influence of monoamines, serotonin (5-hydroxytryptamine, 5-HT) and noradrenaline, on differentiating gonadotropin-releasing hormone (GnRH)-producing neurones in foetal mice. The differentiation and migration of GnRH neurones were compared in Tg8 mice (the knocked-out gene encoding monoamine oxidase A) with increased levels of 5-HT and noradrenaline and in C3H mice with normal metabolism of monoamines in C3H mice. To achieve this, immunocytochemistry for GnRH combined with quantitative and semiquantitative image analysis were employed. GnRH neurones in foetuses at the 18th embryonic day were detected in the forebrain along the trajectory of their migration from the olfactory bulbs to the hypothalamic retrochiasmatic region. The total number of GnRH neurones in the forebrain in knockout mice was significantly lower compared to C3H mice, suggesting an inhibiting influence of monoamines on the proliferation of precursor cells. The fraction of GnRH neurones in the caudal part of the trajectory of their migration in Tg8 mice exceeded significantly those in C3H foetuses, whereas there was a reverse in the rostral part of the trajectory. These data suggest that an excess of 5-HT and noradrenaline served to accelerate the GnRH neurone migration in Tg8 mice. Moreover, an excess of 5-HT and noradrenaline provided a minor effect on the area and optical density of GnRH neurones (i.e. on GnRH neurone differentiation). Thus, an excess of 5-HT and noradrenaline appears to inhibit the proliferation of the precursor cells of GnRH neurones and stimulates the GnRH neurone migration to the place of their final location in the septo-preoptic region.

Radial migration in the cerebral cortex is enhanced by signals from thalamus

The six layered cerebral cortex derives from cells that divide in the ventricular zone and migrate to their final destination in the cortical plate (future cortex). In the mouse, cortical layer III and IV neurons undergo their final mitotic division at around E16, at which time thalamic axons are beginning to enter the cortex. We used bromodeoxyuridine-birth dating of cells in cortical slice cultures to show that the thalamus enhances the migration out of the ventricular zone of future layer III/IV cells. When cortical slices were cultured alone, less than 35% of cells born in vitro on E16 were present in the pial half of the slice after 48 h in culture. In contrast, when cortical slices were cocultured with thalamus, 69% of these cells were found in the pial half of the slice. Explants of other developing tissues did not mimic the effect of the thalamus. The thalamus had no obvious effect on cortical radial glial cells, cortical cell viability or maintenance of cortical slice structure. We found that most precursors born at a similar age but in vivo, shortly before cortical slices were isolated, migrated to the pial half of the cultured slices in the absence of a cocultured thalamic explant. Thus, E16 cortical slices cultured without thalamus permit migration of cells born in vivo and therefore already exposed to the thalamus. Our results indicate that the thalamus provides factors to E16-born cortical precursors that enhance their directed migration out of the ventricular zone to the cortical plate.

Serotonin uptake and release mechanisms in developing cultures of rat embryonic raphe neurons: age- and region-specific differences

The development of serotonergic neurons of the rat raphe was followed in primary neuronal cell cultures taken at embryonic days embryonic day 13 and embryonic day 14 from three different raphe sub-groups, topographically defined with respect to their position to the isthmus as rostral (R1), intermediate (R2) and caudal (R3). In neurons cultivated from embryonic day 13 raphe serotonin, immunoreactivity was detected after only two days in vitro in the rostral R1 and the intermediate R2 sub-groups. Within two weeks of cultivation the number of serotonergic neurons as well as the dendritic branching continuously increased in all three sub-groups. In cultures obtained from embryonic day 13 raphe a specific uptake of [3H]serotonin could not be detected during the first days in vitro. Specific uptake as well as regulated serotonin release, however, was clearly discernible in these cultures after nine days in vitro, indicating developmental differentiation of the initially immature serotonergic neurons in culture. In contrast, serotonergic neurons obtained from the three raphe sub-groups at embryonic day 14 took up and released [3H]serotonin, as early as after two days in culture. Basal as well as stimulated serotonin release was diminished when preincubating the cells with tetanus toxin, which cleaves synaptobrevin thereby blocking exocytosis. Our data indicate that the differential development of serotonergic neurons in the various raphe sub-groups in vivo is also sustained in culture. The differences observed when comparing neurons from embryonic days 13 and 14 suggest that a short time-period of about 24h appears to be crucial for the developmental upregulation of serotonin uptake, storage and release.

Serotonin promotes the differentiation of glutamate neurons in organotypic slice cultures of the developing cerebral cortex

The monoamines serotonin (5-HT), noradrenaline (NA), and dopamine (DA), which are present in the developing brain apparently before they assume their neurotransmitter functions, are regarded as strong candidates for a role in the maturation of the cerebral cortex. Here we sought to investigate their effects on the generation and differentiation of cortical cell types. Slice cultures, prepared from the cortices of embryonic day (E) 14, E16, and E19 rat fetuses, were kept in defined medium or in defined medium plus 5-HT for 7 d. E16 cortices were also exposed to NA or DA for the same period. At the end of this period, the proportions of the neuronal [glutamate (Glu)-, GABA-, calbindin-, calretinin-labeled], glial (GFAP), and neuroepithelial (nestin) cell types were estimated for all conditions. We found that in E16 cultures, application of 5-HT, but not of NA or DA, significantly increased the proportion of Glu-containing neurons without affecting the overall neuronal population or the proportions of any other cell types. A similar effect was observed in co-cultures of E16 cortex with slices through the midbrain raphe nuclei of E19 rats. The total amount of cortical Glu, as measured with HPLC, was also increased in these co-cultures. To investigate whether the effect of 5-HT was the result of changes in cell proliferation, we exposed slices to bromodeoxyuridine (BrdU) and found that the proportion of BrdU-labeled cells was similar in the 5-HT-treated and control slices. These results indicate that 5-HT promotes the differentiation of cortical Glu-containing neurons without affecting neuroepithelial cell proliferation.

Decreased hypothalamic serotonin levels in adult rats treated neonatally with clomipramine

Early postnatal treatment with the antidepressant drug clomipramine has repeatedly been shown to lead to behavioural and physiological changes in adult rats. To provide some neurochemical correlates to these studies we have measured a number of monoaminergic parameters in the brains of adult (one year old) rats that were treated twice daily with 15 mg/kg clomipramine from postnatal day 2-14. The most consistent finding was that the hypothalamic levels of serotonin (5-HT) were decreased and those of the dopamine metabolite dihydroxyphenylacetic acid (DOPAC) were increased in rats irrespectively whether they went through a range of behavioural and physiological tests or not. The numbers of beta-adrenoceptors in the frontal cortex and of alpha 2-adrenoceptors in the amygdala/piriform cortex were not changed. The decrease in hypothalamic 5-HT concentrations appears to be up to now the most consistent neurochemical alteration in adult rats that were neonatally treated with antidepressant drugs. It is, however, not clear what the relation is with the functional changes in these rats, that are proposed by some authors as an animal model for depression.

Early postnatal changes in sexual dimorphism of catecholamine and indoleamine content in the brain of prenatally stressed rats

Sexual dimorphism in catecholamine and indoleamine content in the brain preoptic area and mediobasal hypothalamus was studied in 10-day-old rat pups whose mothers had been exposed to the daily 1-h immobilization stress during the last week of pregnancy. Concentration of noradrenaline in the preoptic area and 5-hydroxyindoleacetic acid in the mediobasal hypothalamus of the prenatally stressed male offspring as well as dopamine content in the mediobasal hypothalamus and 5-hydroxyindoleacetic acid content in the preoptic area of the females were increased by 59%, 45%, 34%, 76%, respectively. Dopamine content in the preoptic area of the female pups was decreased. In addition, an increase of 5-hydroxytryptamine metabolism in the female preoptic area has been revealed. As a result of prenatal stress, sex-related differences in noradrenaline and 5-hydroxyindoleacetic acid concentrations in the preoptic area and those in dopamine concentration in the mediobasal hypothalamus disappeared. The suggestion is made that the early changes in sexual dimorphism of the brain catecholamines and 5-hydroxytryptamine metabolism in prenatally stressed rats may be responsible for the development of the long-term disorders of sexual differentiation of the neuroendocrine functions.

Adrenocorticotropic hormone activation of adenylate cyclase in raphe neurons: multiple regulatory pathways control serotonergic neuronal differentiation

The RN46A cell line was derived from embryonic day 13 rat medullary raphe cells by infection with a retrovirus encoding the temperature-sensitive mutant of SV40 large T antigen (tsT-ag). The RN46A cell line is neuronally restricted and constitutively differentiates following a shift to nonpermissive temperature. Differentiated RN46A cells express low levels of tryptophan hydroxylase (TPH) but no detectable levels of serotonin (5-HT). Treatment of cultures with the adrenocorticotropic hormone peptide ACTH4-10 up-regulates the expression of TPH immunoreactivity in differentiated RN46A cells, but 5-HT synthesis requires initial treatment with ACTH4-10, followed by partial membrane depolarizing conditions. Up-regulation of TPH by ACTH4-10 is apparently due to activation of adenylate cyclase, whereas the increased 5-HT synthesis with membrane depolarization can be blocked with the voltage-sensitive Ca(2+)-channel blockers nifedipine and omega-conotoxin. ACTH4-10 treatment also markedly up-regulates the expression of the 5-HT reuptake transporter, as do dibutyryl cyclic AMP and forskolin; chronic membrane depolarization has no effect on 5-HT reuptake. The expression of the high-affinity 5-HT1A receptor is increased threefold by ACTH4-10 treatment during differentiation and fivefold by differentiation under partial membrane depolarizing conditions. Combining ACTH4-10 treatment and membrane depolarization does not increase expression of the 5-HT1A receptor further. 5-HT release is constitutive in ACTH-treated RN46A cells and linked to spontaneous synaptic vesicle fusion in RN46A cells. Considered with previous results, these data indicate that multiple effectors, ACTH, brain-derived neurotrophic factor, and membrane depolarization, have both distinct and overlapping effects that regulate specific elements of the serotonergic neuronal phenotype during differentiation and maturation.

In vitro release of [3H]5-hydroxytryptamine from fetal and maternal brain by drugs of abuse

Cortical synaptosomes were prepared from pregnant dams (GD-17) and rat fetuses (ED-17), loaded with [3H]5-HT and assayed to evaluate release mediated by cocaine (COC), fenfluramine (FEN) and 3,4-methylenedioxymethamphetamine (MDMA). COC and FEN elicited a high-affinity (10(-9) M) release response in fetal tissue which was not apparent in the dam. MDMA-induced release was similar in magnitude in both tissue types. Consequently, the release of 5-HT from developing neurons may be one mechanism by which COC and FEN elicit their teratogenetic effects in utero.

Development of the suprachiasmatic nucleus in rats during ontogenesis: serotonin-immunopositive fibers

The innervation of the suprachiasmatic nucleus by serotoninergic fibers has been studied in rats from the 22nd embryonic until the 21st postnatal day. The serotoninergic fibers were detected with the pre-embedding immunocytochemical technique at the electron-microscopic level using antibodies to serotonin. Serotonin-immunopositive fibers were always identified as axons as they contained numerous synaptic vesicles both in fetuses and postnatal rats. Moreover, immunopositive dense core vesicles appeared in the axons after birth. From the end of fetal life onwards, the serotonin-immunopositive axons made specialized contacts with the immunonegative neurons, mainly with their dendrites and to a lesser extent with cell bodies and axons. In fetuses, only immature synapses (presynapses) were observed. After birth, presynapses were replaced by typical synapses with the thickened pre- and postsynaptic membranes, accumulations of dense materials at the pre- and postsynaptic membranes as well as with an accumulation of synaptic vesicles at the presynaptic membrane. The functional significance of the serotoninergic input to the suprachiasmatic nucleus in ontogenesis is discussed.

Ontogenesis of the hypothalamic catecholaminergic system in rats: synthesis, uptake and release of catecholamines

The development of the hypothalamic catecholaminergic system during ontogenesis in rats has been studied with glyoxylic acid histofluorescent method in vivo and with isotopic biochemical technique in vitro. It has been demonstrated that at the 15th fetal day the catecholaminergic system was functionally inactive at least in its ability for the uptake and K(+)-stimulated release of catecholamines. Since the 16th fetal day, hypothalamic neuronal elements gained an ability for synthesis of catecholamines, their specific uptake and K(+)-evoked release. Over the subsequent two days, the intensity of the fluorescent intraneuronal product rose considerably showing the increase of either synthesis or accumulation of catecholamines. Simultaneously, the values of the uptake and K(+)-stimulated release of the exogenous radioactively-labelled dopamine increased significantly. The intensity of the fluorescence of the hypothalamic neuronal elements dropped from 20th fetal until the ninth postnatal day, whereas the specific uptake doubled over the same period reaching its adult level. By the 21st postnatal day the reaccumulation of the fluorescent product occurred.