Ontogenesis of immunoreactive tyrosine hydroxylase-containing neurons in rat hypothalamus

Hypothalamic neurons fully or partially expressing the dopaminergic phenotype: development, distribution, functioning and functional significance. A review

The hypothalamus is a key link in neuroendocrine regulations, which are provided by neuropeptides and dopamine. Until the late 1980 s, it was believed that, along with peptidergic neurons, hypothalamus contained dopaminergic neurons. Over time, it has been shown that besides dopaminergic neurons expressing the dopamine transporter and dopamine-synthesizing enzymes - tyrosine hydroxylase (TH) and aromatic L-amino acid decarboxylase (AADC) - the hypothalamus contains neurons expressing only TH, only AADC, both enzymes or only dopamine transporter. The end secretory product of TH neurons is L-3,4-dihydroxyphenylalanine, while that of AADC neurons and bienzymatic neurons lacking the dopamine transporter is dopamine. During ontogenesis, especially in the perinatal period, monoenzymatic neurons predominate in the hypothalamic neuroendocrine centers. It is assumed that L-3,4-dihydroxyphenylalanine and dopamine are released into the neuropil, cerebral ventricles, and blood vessels, participating in the regulation of target cell differentiation in the perinatal period and the functioning of target cells in adulthood.

DiI tracing of the hypothalamic projection systems during perinatal development

The hypothalamus is the higher neuroendocrine center of the brain and therefore possesses numerous intrinsic axonal connections and is connected by afferent and efferent fiber systems with other brain structures. These projection systems have been described in detail in the adult but data on their early development is sparse. Here I review studies of the time schedule and features of the development of the major hypothalamic axonal systems. In general, anterograde tracing experiments have been used to analyze short distance projections from the arcuate and anteroventral periventricular nuclei (Pe), while hypothalamic projections to the posterior and intermediate pituitary lobes (IL) and median eminence, mammillary body tracts and reciprocal septohypothalamic connections have been described with retrograde tracing. The available data demonstrate that hypothalamic connections develop with a high degree of spatial and temporal specificity, innervating each target with a unique developmental schedule which in many cases can be correlated with the functional maturity of the projection system.

Brain neurons partly expressing dopaminergic phenotype: location, development, functional significance, and regulation

In addition to catecholaminergic neurons possessing all the enzymes of catecholamine synthesis and the specific membrane transporters, neurons partly expressing the catecholaminergic phenotype have been found a quarter of a century ago. Most of them express individual enzymes of dopamine (DA) synthesis, tyrosine hydroxylase (TH), or aromatic l-amino acid decarboxylase (AADC), lacking the DA membrane transporter and the vesicular monoamine transporter, type 2. These so-called monoenzymatic neurons are widely distributed throughout the brain in ontogenesis and adulthood being in some brain regions even more numerous than dopaminergic (DA-ergic) neurons. Individual enzymes of DA synthesis are expressed in these neurons continuously or transiently in norm and pathology. It has been proven that monoenzymatic TH neurons and AADC neurons are capable of producing DA in cooperation. It means that l-3,4-dihydroxyphenylalanine (l-DOPA) synthesized from l-tyrosine in monoenzymatic TH neurons is transported to monoenzymatic AADC neurons for DA synthesis. Such cooperative synthesis of DA is considered as a compensatory reaction under a failure of DA-ergic neurons, for example, in neurodegenerative diseases like hyperprolactinemia and Parkinson's disease. Moreover, l-DOPA, produced in monoenzymatic TH neurons, is assumed to play a role of a neurotransmitter or neuromodulator affecting the target neurons via catecholamine receptors. Thus, numerous widespread neurons expressing individual complementary enzymes of DA synthesis serve to produce DA in cooperation that is a compensatory reaction at failure of DA-ergic neurons.

Molecular genetics of the developing neuroendocrine hypothalamus

Formation of the mammalian endocrine system and neuroendocrine organs involves complex regulatory networks resulting in a highly specialized cell system able to secrete a diverse array of peptide hormones. The hypothalamus is located in the mediobasal region of the brain and acts as a gateway between the endocrine and nervous systems. From an endocrinology perspective, the parvicellular neurons of the hypothalamus are of particular interest as they function as a control centre for several critical physiological processes including growth, metabolism and reproduction by regulating hormonal signaling from target cognate cell types in the anterior pituitary. Delineating the genetic program that controls hypothalamic development is essential for complete understanding of parvicellular neuronal function and the etiology of congenital disorders that result from hypothalamic-pituitary axis dysfunction. In recent years, studies have shed light on the interactions between signaling molecules and activation of transcription factors that regulate hypothalamic cell fate commitment and terminal differentiation. The aim of this review is to summarize the recent molecular and genetic findings that have advanced our understanding of the emergence of the known important hypophysiotropic signaling molecules in the hypothalamus. We have focused on reviewing the literature that provides evidence of the dependence on expression of specific genes for the normal development and function of the cells that secrete these neuroendocrine factors, as well as studies of the elaboration of the spatial or temporal patterns of changes in gene expression that drive this development.

Non-dopaminergic neurons partly expressing dopaminergic phenotype: distribution in the brain, development and functional significance

Besides the dopaminergic (DA-ergic) neurons possessing the whole set of enzymes of DA synthesis from l-tyrosine and the DA membrane transporter (DAT), the neurons partly expressing the DA-ergic phenotype have been first discovered two decades ago. Most of the neurons express individual enzymes of DA synthesis, tyrosine hydroxylase (TH) or aromatic l-amino acid decarboxylase (AADC) and lack the DAT. A list of the neurons partly expressing the DA-ergic phenotype is not restricted to so-called monoenzymatic neurons, e.g. it includes some neurons co-expressing both enzymes of DA synthesis but lacking the DAT. In contrast to true DA-ergic neurons, monoenzymatic neurons and bienzymatic non-dopaminergic neurons lack the vesicular monoamine transporter 2 (VMAT2) that raises a question about the mechanisms of storing and release of their final synthetic products. Monoenzymatic neurons are widely distributed all through the brain in adulthood being in some brain regions even more numerous than DA-ergic neurons. Individual enzymes of DA synthesis are expressed in these neurons continuously or transiently in norm or under certain physiological conditions. Monoenzymatic neurons, particularly those expressing TH, appear to be even more numerous and more widely distributed in the brain during ontogenesis than in adulthood. Most populations of monoenzymatic TH neurons decrease in number or even disappear by puberty. Functional significance of monoenzymatic neurons remained uncertain for a long time after their discovery. Nevertheless, it has been shown that most monoenzymatic TH neurons and AADC neurons are capable to produce l-3,4-dihydroxyphenylalanine (L-DOPA) from l-tyrosine and DA from L-DOPA, respectively. L-DOPA produced in monoenzymatic TH neurons is assumed to play a role of a neurotransmitter or neuromodulator acting on target neurons via catecholamine receptors. Moreover, according to our hypothesis L-DOPA released from monoenzymatic TH neurons is captured by monoenzymatic AADC neurons for DA synthesis. Such cooperative synthesis of DA is considered as a compensatory reaction under a failure of DA-ergic neurons, e.g. in neurodegenerative diseases like hyperprolactinemia and Parkinson's disease.Thus, a substantial number of the brain neurons express partly the DA-ergic phenotype, mostly individual complementary enzymes of DA synthesis, serving to produce DA in cooperation that is supposed to be a compensatory reaction under the failure of DA-ergic neurons.

[Synthesis of monoamines by non-monoaminergic neurons: illusion or reality?]

In contrast to monoaminergic (MA-ergic) neurons possessing the whole set of the enzymes for MA synthesis from the precursor amino-acid, some, mostly peptidergic, neurons co-express only one of the enzymes of monoamine synthesis. They are widely distributed in the brain, being particularly numerous in ontogenesis and, in adulthood, under certain physiological conditions. Most monoenzymatic neurons possess one of the enzymes for dopamine (DA) synthesis, tyrosine hydroxylase (TH) or aromatic L-amino acid decarboxylase (AADC). TH and AADC are enzymatically active in a substantial number of monoenzymatic neurons, where they are capable of converting L-tyrosine to L-3,4-dihydroxy-phenylalanine (L-DOPA) and L-DOPA to dopamine (DA) (or 5-hydroxy-tryptophan, 5-HTP to serotonin), respectively. According to our data L-DOPA synthesized in monoenzymatic TH-neurons is released and taken up by monoenzymatic AADC-neurons for DA synthesis. Moreover, L-DOPA captured by dopaminergic neurons and serotoninergic neurons serves to stimulate dopamine synthesis in the former and to start DA synthesis in the latter. Cooperative synthesis of MAs is considered as a compensatory reaction under a failure of MA-ergic neurons, e.g. in neurodegenerative diseases like hyperprolactinemia and Parkinson's disease, which are developed primarily because of degeneration of DA-ergic neurons of the tuberoinfundibular system and the nigrostriatal system, respectively. Noteworthy, the neurotoxin-induced increase of prolactin secretion returns with time to a normal level due to the stimulation of DA synthesis by the tuberoinfundibular most probably monoenzymatic neurons. The same compensatory mechanism is supposed to be used under the failure of the nigrostriatal DA-ergic system that is manifested by an increased number of monoenzymatic neurons in the striatum of animals with neurotoxin-induced parkinsonism and in humans with Parkinson's disease. Expression of the enzymes of MA synthesis in non-monoaminergic neurons is controlled by intercellular signals such as classical neurotransmitters (catecholamines), etc. Thus, a substantial number of brain neurons express partly the monoaminergic phenotype, namely individual complementary enzymes of MA synthesis, serving to produce MAs in cooperation, which is considered as a compensatory reaction under the failure of MA-ergic neurons.

Dopaminergic and serotoninergic deficiencies in young adult rats prenatally exposed to the bacterial lipopolysaccharide

We have reported previously that prenatal bacterial lipopolysaccharide (LPS) exposure at the gestation window of vulnerability could consistently lead to dopamine (DA) neuron loss in the substantia nigra (SN). Thus, we suggested that prenatal LPS exposure might represent as a risk factor for the development of Parkinson's disease (PD). Here, we report that the same exposure could lead to tryptophan hydroxylase (TPH, a serotonin neuron marker) immunoreactive cell loss in the dorsal raphe nucleus (DRN). Twenty two pups born to saline or LPS-injected gravid female rats at E10.5 were used in the current study. Twelve male pups at age of 4 months (6 from each of two prenatal groups) were used for the tyrosine hydroxylase (TH) and tryptophan hydroxylase (TPH) immunochemistry studies. The other 10 (5 from each of two prenatal groups) males were used in the biochemistry studies. A 29% THir neuron loss in the substantia nigra (F(1,11)=17.573, P=0.002) and a 31% TPHir neuron loss (F(1,11)=44.005, P<0.001) in the DRN were seen. Significant DA and 5-hydroxytryptamine (5-HT) reductions (P<0.05) were found in the frontal cortex, nucleus accumbens, striatum, amygdala, hippocampus, and hypothalamus. The losses of DA and 5-HT were accompanied by the significant increases in homovanillic acid over DA and 5-hydroxyindoleacetic acid over 5-HT ratios in the most areas tested. These data further validate prenatal LPS exposure as a model of PD since DA and 5-HT changes similar to those seen in PD patients. They also suggest that prenatal LPS might be a risk factor for other diseases including mood disorders.

Long-term, homologous prolactin, administered through ectopic pituitary grafts, induces hypothalamic dopamine neuron differentiation in adult Snell dwarf mice

Pituitary prolactin (PRL) secretion is inhibited by dopamine (DA) released into the portal circulation from hypothalamic tuberoinfundibular DA (TIDA) neurons. Ames (df/df) and Snell (dw/dw) dwarf mice lack PRL, GH, and TSH, abrogating feedback and resulting in a reduced hypophysiotropic TIDA population. In Ames df/df, ovine PRL administration for 30 d during early postnatal development increases the TIDA neuron number to normal, but 30 d PRL treatment of adult df/df does not. The present study investigated the effects of homologous PRL, administered via renal capsule pituitary graft surgery for 4 or 6 months, on hypothalamic DA neurons in adult Snell dw/dw mice using catecholamine histofluorescence, tyrosine hydroxylase immunocytochemistry, and bromodeoxyuridine immunocytochemistry. PRL treatment did not affect TIDA neuron number in normal mice, but 4- and 6-month PRL-treated dw/dw had significantly increased (P < or = 0.01) TIDA (area A12) neurons compared with untreated dw/dw. Snell dwarfs treated with PRL for 6 months had more (P < or = 0.01) TIDA neurons than 4-month PRL-treated dw/dw, but lower (P < or = 0.01) numbers than normal mice. Periventricular nucleus (area A14) neuron number was lower in dwarfs than in normal mice, regardless of treatment. Zona incerta (area A13) neuron number was unchanged among phenotypes and treatments. Prolactin was unable to induce differentiation of a normal-sized A14 neuron population in dw/dw. Bromodeoxyuridine incorporation was lower (P < or = 0.01) in 6-month PRL-treated normal mice than in 6-month PRL-treated dwarfs in the subventricular zone of the lateral ventricle and in the dentate gyrus, and lower (P < or = 0.05) in 4-month untreated dwarfs than in 4-month untreated normal mice in the median eminence and the periventricular area surrounding the third ventricle. Thus, a PRL-sensitive TIDA neuron population exists in adult Snell dwarf mice when replacement uses homologous hormone and/or a longer duration. This finding indicates that there is potential for neuronal differentiation beyond early developmental periods and suggests plasticity within the mature hypothalamus.

Directed differentiation of neural cells to hypothalamic dopaminergic neurons

Hypothalamic neurons play a key role in homeostasis, yet little is known about their differentiation. Here, we demonstrate that Shh and Bmp7 from the adjacent prechordal mesoderm govern hypothalamic neural fate, their sequential action controlling hypothalamic dopaminergic neuron generation in a Six3-dependent manner. Our data suggest a temporal distinction in the requirement for the two signals. Shh acts early to specify dopaminergic neurotransmitter phenotype. Subsequently, Bmp7 acts on cells that are ventralised by Shh, establishing aspects of hypothalamic regional identity in late-differentiating/postmitotic cells. The concerted actions of Shh and Bmp7 can direct mouse embryonic stem cell-derived neural progenitor cells to a hypothalamic dopaminergic fate ex vivo.

Degeneration of dopaminergic neurons triggers an expression of individual enzymes of dopamine synthesis in non-dopaminergic neurons of the arcuate nucleus in adult rats

Non-dopaminergic neurons expressing individual complementary enzymes dopamine (DA) synthesis were shown to produce DA in cooperation [Ugrumov, M., Melnikova, V., Ershov, P., Balan, I., Calas A., 2002. Tyrosine hydroxylase- and/or aromatic L-amino acid decarboxylase-expressing neurons in the rat arcuate nucleus: ontogenesis and functional significance. Psychoneuroendocrinology 27, 533-548; Ugrumov, M.V., Melnikova, V.I., Lavrentyeva, A.V., Kudrin, V.S., Rayevsky, K.S., 2004. Dopamine synthesis by non-dopaminergic neurons expressing individual complementary enzymes of the dopamine synthetic pathway in the arcuate nucleus of fetal rats. Neuroscience 124, 629-635]. This study was aimed at testing our hypothesis that the cooperative synthesis of DA in non-dopaminergic neurons is an adaptive reaction under functional insufficiency of the dopaminergic system. Functional insufficiency of the tuberoinfundibular dopaminergic system was provoked by 6-OHDA-induced degeneration of dopaminergic neurons in the arcuate nucleus in adult rats. Bienzymatic (dopaminergic) neurons and monoenzymatic neurons expressing tyrosine hydroxylase (TH) or aromatic L-amino acid decarboxylase (AADC) were detected with a double-immunofluorescent technique on cryostat sections. The 6-OHDA-induced degeneration of dopaminergic neurons was accompanied by a significant increase of the number of monoenzymatic TH neurons and AADC neurons that appears to support our hypothesis. The reaction of bienzymatic and monoenzymatic neuron populations to the 6-OHDA administration occurred to be region-specific. The former disappeared in the dorsomedial region of the arcuate nucleus while the latter increased in the ventrolateral region. Thus, degeneration of dopaminergic neurons in the arcuate nucleus of adult rats is accompanied by the expression of individual enzymes of DA synthesis in non-dopaminergic neurons that may be an adaptive reaction.

Axonal projections from the hypothalamus to the pituitary intermediate lobe in rats during ontogenesis: DiI tracing study

This study has determined ontogenetic schedule of axonal arrival from the hypothalamus in the pituitary intermediate lobe (IL) in rats using 1,1'-dioctadecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate (DiI) as a retrograde tracer. The brains with attached pituitaries were dissected in rats from the 20th embryonic day (E20) to the 20th postnatal day (P20). Anterior lobe was mechanically detached from the IL, material fixed in paraformaldehyde, and DiI crystals were applied on the IL laying on the posterior lobe (PL). The labeling of IL + PL resulted in staining of hypothalamic magnocellular neurons, which send their axons to the PL, and hypothalamic parvocellular neurons contributing to the innervation of the IL. Therefore, the magnocellular neurons were not taken into account when identifying the neurons projecting axons to the IL. Rare fluorescent neurons projecting their axons to the IL were detected as early as on E20 in the ventral part of the periventricular nucleus (Pe) and in the rostral part of the arcuate nucleus. Few DiI-labeled neurons were seen in Pe from P1 to P3. At P5, the fluorescent neurons were accumulated giving rise to the prominent cluster in the Pe, which was enlarged on later stages and occupied all the Pe. In addition to the Pe, fluorescent neurons first appeared in the retrochiasmatic region and around the ventromedial nucleus in young rats. Thus, the axons of hypothalamic neurons of the Pe and mediobasal hypothalamus first arrive in the IL in rats at the end of intrauterine development, although the principal innervation of the IL is the postnatal event.

Dopamine synthesis by non-dopaminergic neurons expressing individual complementary enzymes of the dopamine synthetic pathway in the arcuate nucleus of fetal rats

This study was aimed to test our hypothesis about dopamine (DA) synthesis by non-DAergic neurons expressing individual complementary enzymes of the DA synthetic pathway in cooperation, i.e. L-dihydroxyphenylalanine (L-DOPA) synthesized in tyrosine hydroxylase (TH)-expressing neurons is transported to aromatic L-amino acid decarboxylase (AADC)-expressing neurons for conversion to DA. The mediobasal hypothalamus of rats at the 21st embryonic day was used as an experimental model because it contains mainly monoenzymatic TH neurons and AADC neurons (>99%) whereas the fraction of bienzymatic (DAergic) neurons does not exceed 1%. The fetal substantia nigra containing DAergic neurons served as a control. DA and L-DOPA were measured by high performance liquid chromatography in: (1) cell extracts of the cell suspension prepared ex tempora; (2) cell extracts and incubation medium after the static incubation of the cell suspension with, or without exogenous L-tyrosine; (3) effluents of the incubation medium during perifusion of the cell suspension in the presence, or the absence of L-tyrosine. Total amounts of DA and L-DOPA in the incubation medium and cell extracts after the static incubation were considered as the indexes of the rates of their syntheses. L-Tyrosine administration caused the increased L-DOPA synthesis in the mediobasal hypothalamus and substantia nigra. Moreover, L-tyrosine provoked an increase of DA synthesis in the substantia nigra and its decrease in the mediobasal hypothalamus. This contradiction is most probably explained by the L-tyrosine-induced competitive inhibition of the L-DOPA transport to the monoenzymatic AADC-neurons after its release from the monoenzymatic TH neurons. Thus, this study provides convincing evidence of cooperative DA synthesis by non-DAergic neurons expressing TH or AADC in fetal rats at the end of the intrauterine development.

Effect of intracerebroventricular injection of lipopolysaccharide on the tuberoinfundibular dopaminergic system of the rat

Numerous studies indicate that monoaminergic systems are sensitive to both peripheral and central inflammatory stimuli, and in particular dopaminergic neurons in the nigrostriatal system degenerate after local injection of lipopolysaccharide (LPS). However, data about the response of other dopaminergic groups to local inflammation are very sparse. In this study, we have examined the effect of i.c.v. injection of LPS on the tuberoinfundibular dopaminergic (TIDA) system by using biochemical and morphological parameters. Our results show that 6 h after i.c.v. injection of LPS, in parallel to a transient and intense immunoreaction to interleukin-1beta in arcuate microglial cells, there is a decrease in tyrosine hydroxylase (TH) activity in the median eminence and in the number of TH- and TH mRNA-positive cells in the arcuate nucleus, and at 12 h, an increase of prolactin levels in serum. Posterior changes were found in the TH mRNA labeling pattern, mostly in the ventrolateral region of the arcuate nucleus, but they were not accompanied by any changes in TH activity and immunoreactivity and TH-cell count. This suggests that the TIDA system is functionally susceptible to local inflammation, but the effects are transient and do not induce neurodegeneration.

Development of the neuroendocrine hypothalamus

The development of the neuroendocrine hypothalamus has been studied using a variety of neuroanatomical and molecular techniques. Here, the major findings that mold our understanding of hypothalamic development are reviewed. The rat hypothalamus is generated predominantly from the third ventricular neuroepithelium in a "lateral early to medial late" pattern dictated perhaps by the medially receding third ventricle. Neuroendocrine neurons seem to exhibit a delayed migrational strategy, showing relatively early birthdates, although they are located in the latest-generated, periventricular nuclei. Several homeobox genes seem to play a role in hypothalamic development, and gene knockout experiments implicate a number of genes of importance in the generation of the neuroendocrine cell type.

Neurons possessing enzymes of dopamine synthesis in the mediobasal hypothalamus of rats. Topographic relations and axonal projections to the median eminence in ontogenesis

We evaluated the topographic relations between tyrosine hydroxylase (TH)- and/or aromatic L-amino acid decarboxylase (AADC)-immunoreactive neurons in the arcuate nucleus (AN), as well as between TH- and/or AADC-immunoreactive axons in the median eminence (ME) in rats at the 21st embryonic day, 9th postnatal day, and in adulthood. The double-immunofluorescent technique in combination with confocal microscopy was used. Occasional bienzymatic neurons but numerous monoenzymatic TH- or AADC-immunoreactive neurons were observed in fetuses. There was almost no overlap in the distribution of monoenzymatic neurons, and therefore few appositions were observed in between. In postnatal animals, numerous bienzymatic neurons appeared in addition to monoenzymatic neurons. They were distributed throughout the AN resulting in the increased frequency of appositions. Furthermore, specialized-like contacts between monoenzymatic TH- and AADC-immunoreactive neurons appeared. The quantification of the fibers in the ME showed that there were large specific areas of the monoenzymatic TH-immunoreactive fibers and bienzymatic fibers in fetuses, followed by the gradual reduction of the former and the increase of the latter to adulthood. The specific area of the monoenzymatic AADC-immunoreactive fibers in fetuses was rather low, and thereafter increased progressively to adulthood. The fibers of all the types were in apposition in the ME at each studied age. Close topographic relations between the neurons containing individual complementary enzymes of dopamine synthesis at the level of cell bodies and axons suggest functional interaction in between.

Tyrosine hydroxylase- and/or aromatic L-amino acid decarboxylase-expressing neurons in the rat arcuate nucleus: ontogenesis and functional significance

This study has evaluated in vivo, ex vivo and in vitro the ontogenesis and functional significance of the neurons of the arcuate nucleus (AN) expressing either individual enzymes of dopamine (DA) synthesis, tyrosine hydroxylase (TH) or aromatic L-amino acid decarboxylase (AADC) as well as both of them in rats from the 17th embryonic day (E) till adulthood. Immunocytochemistry, image analysis, confocal microscopy, high performance liquid chromatography with electrochemical detection and radioimmunoassay were used to solve this problem. Monoenzymatic TH-containing neurons were initially observed on E18 located in the ventrolateral AN whereas the neurons expressing only AADC or both AADC and TH first appeared on E20 in the dorsomedial AN. On E21, the monoenzymatic TH- or AADC-expressing neurons comprised more than 99% of the whole neuron population expressing the DA-synthesizing enzymes. In spite of an extremely small number (<1%) of the neurons expressing both enzymes (DArgic neurons), the dissected AN (ex vivo) and its primary cell culture (in vitro) contained a surprisingly high amount of DA and L-dihydroxyphenylalanine (L-DOPA) which were released in response to membrane depolarization. Furthermore, DA production in the AN of fetuses occurred to be sufficient to provide an inhibitory control of prolactin secretion, as in adults. The above data suggest that DA could be synthesized, at least in the AN of fetuses, by monoenzymatic neurons containing either TH or AADC, in co-operation. This hypothesis may be extended to adult animals as their AN contained the same populations of the neurons expressing DA-synthesizing enzymes as in fetuses though the proportion of true DArgic neurons increased up to 38%. During ontogenesis, the monoenzymatic TH- and AADC-containing neurons established axosomatic and axo-axonal junctions that might facilitate the L-DOPA transport from the former to the latter. Moreover, the monoenzymatic AADC-expressing neurons project their axons to the median eminence, thereby, providing the pathway for the DA transport toward the hypophysial portal circulation. Thus, DA appears to be synthesized in the AN not only by DArgic neurons but also by monoenzymatic TH- and AADC-expressing neurons in co-operation.

Non-dopaminergic neurons expressing dopamine synthesis enzymes: differentiation and functional significance

The development and functional significance of neurons in the arcuate nucleus expressing tyrosine hydroxylase and/or aromatic L-amino acid decarboxylase were studied in rat fetuses, neonates, and adults using immunocytochemical (single and double immunolabeling of tyrosine hydroxylase and aromatic L-amino acid decarboxylase) methods with a confocal microscope and computerized image analysis, HPLC with electrochemical detection, and radioimmunological analysis. Single-enzyme neurons containing tyrosine hydroxylase were first seen on day 18 of embryonic development in the ventrolateral part of the arcuate nucleus. Neurons expressing only aromatic L-amino acid decarboxylase or both enzymes of the dopamine synthesis pathway were first seen on day 20 of embryonic development, in the dorsomedial part of the nucleus. On days 20-21 of embryonic development, dopaminergic (containing both enzymes) neurons amounted to less than 1% of all neurons expressing tyrosine hydroxylase and/or aromatic L-amino acid decarboxylase. Nonetheless, in the ex vivo arcuate nucleus and in primary neuron cultures from this structure, there were relatively high leveLs of dopamine and L-dihydroxyphenylalanine (L-DOPA), and these substances were secreted spontaneously and in response to stimulation. In addition. dopamine levels in the arcuate nucleus in fetuses were sufficient to support the inhibitory regulation of prolactin secretion by the hypophysis, which is typical of adult animals. During development, the proportion of dopaminergic neurons increased, reaching 38% in adult rats. Specialized contacts between single-enzyme tyrosine hydroxylase-containing and aromatic L-amino acid decarboxylase-containing neurons were present by day 21 of embryonic development; these were probably involved in transporting L-DOPA from the former neurons to the latter. It was also demonstrated that the axons of single-enzyme decarboxylase-containing neurons projected into the median eminence, supporting the secretion of dopamine into the hypophyseal portal circulation. Thus, dopamine is probably synthesized in the arcuate nucleus not only by dopaminergic neurons, but also by neurons expressing only tyrosine hydroxylase or aromatic L-amino acid decarboxylase.

Differentiation of tyrosine hydroxylase-synthesizing and/or aromatic L-amino acid decarboxylase-synthesizing neurons in the rat mediobasal hypothalamus: quantitative double-immunofluorescence study

In this double-immunofluorescence study, we first quantified the neurons of the arcuate nucleus as immunoreactive (+) for tyrosine hydroxylase (TH) and/or aromatic L-amino acid decarboxylase (AADC) in rats at embryonic day 21 (E21), at postnatal day 9 (P9), and in adulthood by using conventional fluorescent or confocal microscopy. On E21, monoenzymatic (TH(+)AADC immunonegative (-) and TH(-)AADC(+)) neurons and bienzymatic (TH(+)AADC(+)) neurons accounted for 99% and 1%, respectively, of the whole neuron population expressing enzymes of dopamine synthesis. Further development was characterized by the dramatic increase in TH(+)AADC(-) dorsomedial and TH(+)AADC(+) dorsomedial populations from E21 to P9 as well as by the increase in the TH(+)AADC(+) dorsomedial population (in females) and a drop in the TH(+)AADC(-) ventrolateral and TH(+)AADC(-) dorsomedial (in males) populations from P9 to adulthood. In contrast to TH(+)AADC(-) (in males) and TH(+)AADC(+) neurons, the TH(-)AADC(+) neurons did not change in number from E21 to adulthood. Thus, in rat fetuses, the neurons synthesizing TH and/or AADC were mainly monoenzymatic, whereas during postnatal life the fraction of bienzymatic neurons increased by up to 60%.

Ontogenetic development of the diencephalic MCH neurons: a hypothalamic 'MCH area' hypothesis

The ontogeny of rat diencephalic melanin-concentrating hormone (MCH) neurons has been analysed, using the bromodeoxyuridine method to determine the period of birth of these neurons, and using in situ hybridization and immunohistochemistry to study their chemical differentiation. The spatiotemporal pattern of MCH neuron generation is complex, although it is broadly lateromedial with a peak between embryonic days (E) 12 and E13. The first expression of the MCH gene has been detected on E13 in neurons in the presumptive lateral hypothalamic area. But the adult-like pattern was observed from E18. Medial-most MCH neurons express the peptide CART (cocaine-amphetamine-regulated transcript) from E18, and the receptor neurokinin 3 (NK3) from between postnatal day (P) 0 and P5. These results are discussed and compared with data from the literature to better understand the organization of the 'MCH-containing area'.

Neurobiological mechanisms of the onset of puberty in primates

An increase in pulsatile release of LHRH is essential for the onset of puberty. However, the mechanism controlling the pubertal increase in LHRH release is still unclear. In primates the LHRH neurosecretory system is already active during the neonatal period but subsequently enters a dormant state in the juvenile/prepubertal period. Neither gonadal steroid hormones nor the absence of facilitatory neuronal inputs to LHRH neurons is responsible for the low levels of LHRH release before the onset of puberty in primates. Recent studies suggest that during the prepubertal period an inhibitory neuronal system suppresses LHRH release and that during the subsequent maturation of the hypothalamus this prepubertal inhibition is removed, allowing the adult pattern of pulsatile LHRH release. In fact, y-aminobutyric acid (GABA) appears to be an inhibitory neurotransmitter responsible for restricting LHRH release before the onset of puberty in female rhesus monkeys. In addition, it appears that the reduction in tonic GABA inhibition allows an increase in the release of glutamate as well as other neurotransmitters, which contributes to the increase in pubertal LHRH release. In this review, developmental changes in several neurotransmitter systems controlling pulsatile LHRH release are extensively reviewed.

Tyrosine hydroxylase-expressing and/or aromatic L-amino acid decarboxylase-expressing neurons in the mediobasal hypothalamus of perinatal rats: differentiation and sexual dimorphism

In this quantitative and semiquantitative immunocytochemical study, the authors evaluated the differentiation of neurons expressing tyrosine hydroxylase (TH) and/or aromatic L-amino acid decarboxylase (AADC) in the mediobasal hypothalamus (MBH) of male and female rats on embryonic day 18 (E18), E20, and postnatal day 9 (P9). Four neuronal populations were distinguished according to either enzyme expression or neuron location. The earliest and most prominent first population was represented by TH-immunoreactive (IR)/AADC-immunonegative (IN) neurons that were detected initially at E18 and always were located in the ventrolateral region of the MBH. The second population of TH-IN/AADC-IR neurons was observed first at E20 and, after that time, was distributed dorsomedially. The third minor population of TH-IR/AADC-IR neurons initially was detected at E20 and was located dorsomedially. The fourth population was represented by TH-IR/AADC-IN neurons that were distributed in the dorsomedial region at any studied age. The numbers of TH-IR and AADC-IR neurons increased from their initial detection at E18 and E20 until P9. The area of TH-IR and AADC-IR neurons also increased from E18 to E20 and from E20 to P9, respectively. Both TH-IR and AADC-IR neurons showed sex differences in the neuron number, size, and optic density (OD). The numbers of TH-IR neurons in males exceeded those of females at E20 and at P9, although, at P9, sexual dimorphism was a characteristic only of the ventrolateral population. The area and OD of TH-IR neurons from females exceeded those from males in the entire mediobasal hypothalamus (MBH) at E18 and E20 but only in its dorsomedial region at P9. Sexual dimorphism also was an attribute of AADC-IR neurons at E20 and P9. Their number, size, and OD were significantly higher in females than in males. Thus, the MBH of perinatal rats contained two major populations of TH-IR/AADC-IN or TH-IN-AADC-IR neurons and a minor population of TH-IR/AADC-IR neurons. The differentiating neurons expressing either enzyme showed sexual dimorphism.

Effects of prolactin on expression of Fos-related antigens in tyrosine hydroxylase-immunoreactive neurons in subdivisions of the arcuate nucleus

Dual immunohistochemistry was employed to determine the effects of prolactin on expression of Fos and its related antigens (FRA) in tuberoinfundibular dopamine (TIDA) neurons located in the dorsomedial (DM) and ventrolateral (VL) subdivisions of the arcuate nucleus (ARC) in the male rat. Systemic administration of the DA receptor antagonist haloperidol caused a sustained (up to 12 h) increase in plasma prolactin concentrations that was accompanied by a transient increase (at 3 h) in the percentage of tyrosine hydroxylase (TH)-immunoreactive (IR) neurons containing FRA-IR nuclei in the DM-ARC. In contrast, haloperidol caused a prolonged (1. 5 to 12 h) decrease in the percentage of TH-IR neurons with FRA-IR nuclei in the VL-ARC. Haloperidol had no effect, however, on the overall number of TH-IR neurons in either of these regions. Co-administration of prolactin antisera (PRL-AB) blocked haloperidol-induced increases in both plasma prolactin concentrations and the percentage of TH-IR neurons expressing FRA in the DM-ARC, but had no effect on haloperidol-induced inhibition of FRA expression in TH-IR neurons in the VL-ARC. Intracerebroventricular (i.c.v.) administration of prolactin also increased the percentage of TH-IR neurons containing FRA-IR nuclei in the DM-ARC, but this effect was of longer duration (up to 6 h) than that of haloperidol in all but the most caudal portion of the DM-ARC. In the VL-ARC, prolactin caused a transient increase (at 1.5 h) in the percentage of TH-IR containing FRA-IR nuclei. These results demonstrate that prolactin regulates immediate early gene expression in TIDA neurons in male rats, and reveal that there are temporal differences in the responsiveness of discrete subpopulations of these neurons to prolactin. Prolactin causes a short-lived increase in FRA expression in TIDA neurons in the VL-ARC which is followed by a more prolonged activation of FRA expression in TIDA neurons in the DM-ARC.

Dopamine turnover in the mediobasal hypothalamus in rat fetuses

In this study, the dopamine turnover in the mediobasal hypothalamus, the key compartment of the neuroendocrine regulation of reproduction, was evaluated in fetal male and female rats. High-performance liquid chromatography with electrochemical detection was used to measure 3,4-dihydroxyphenylalanine, dopamine and 3,4-dihydroxyphenylacetic acid in the mediobasal hypothalamus of fetuses on the 21st day of intrauterine development and in primary cell culture (cell extracts and culture medium) of the same brain region, explanted at the 17th fetal day and maintained for seven days. The same technique was applied to determine dopamine release from fetal neurons of the mediobasal hypothalamus in response to an excess of K+ in the perifusion system or in culture. L-3,4-Dihydroxyphenylalanine, dopamine and 3,4-dihydroxyphenylacetic acid were detected both ex vivo and in culture. The ratios of the concentrations of L-3,4-dihydroxyphenylalanine/dopamine and 3,4-dihydroxyphenylacetic acid/dopamine were significantly higher in vitro than ex vivo, showing a lower rate of dopamine production and a higher rate of its degradation in the experiments in vitro. Moreover, it has been demonstrated that an excess of K+, i.e. a membrane depolarization, resulted in a highly increased release of dopamine in the perifusion system and in culture. The dopaminergic activity in the developing mediobasal hypothalamus showed sexual dimorphism that was manifested in a greater concentration of 3,4-dihydroxyphenylalanine and dopamine, at least in cell extracts of cultures, as well as in a higher rate of dopamine release, both in the perifusion system and in culture in males compared to females. Thus, dopamine is synthesized and released in response to a membrane depolarization in the mediobasal hypothalamus of rats as early as the end of intrauterine development, suggesting its contribution to the inhibitory control of pituitary prolactin secretion.

Differential innervation of individual melanotropes suggests a role for nonsynaptic inhibitory regulation of the developing and adult rat pituitary intermediate lobe

Dopamine and GABA were detected in intermediate lobe axons around birth, and early axons were closely apposed to glial cells and processes, possibly using them for guidance. In the adult, axons containing colocalized dopamine and GABA were distributed in a distinct pattern within the lobe, with plexuses located dorsally and ventrally. Axons preferentially followed glial processes in interlobular septa, yet were also interspersed between melanotropes. Individual melanotropes were contacted by varying numbers of axon terminals, with some devoid of contacts. Boutons contained both small clear vesicles and large dense-cored vesicles; membrane specializations were not well-developed. From these findings we concluded that in addition to direct synaptic inhibition, dopamine and GABA could stimulate their receptors by mechanisms similar to "parasynaptic" [Schmitt (1984) Neuroscience, 13:991-1001] or "volume" [Agnati et al. (1995) Neuroscience, 69:711-726] transmission as proposed for the CNS. Humoral agents passing into the intermediate lobe from portal vessels, thus acting as classical hormones, further regulate the melanotropes. Moreover, approximately 50% of the axonal elements were closely apposed to glia, suggesting that glia could have regulatory roles. Previous studies from our laboratory [Chronwall et al. (1987) Endocrinology, 120:1201-1211; Chronwall et al. (1988) Endocrinology, 123:1992:1202] demonstrated heterogeneity in proopiomelanocortin (POMC) biosynthesis among individual melanotropes, prompting the hypothesis that the degree of innervation could govern the expression of certain molecules. We combined immunohistochemistry and in situ hybridization histochemistry to evaluate whether melanotrope molecular heterogenity is spatially correlated with axons and terminals. Tentatively, melanotropes expressing low levels of POMC and alpha1A subunit P/Q type Ca2+ channel mRNAs often were apposed to axons, whereas those with low levels of D2L receptor mRNA rarely were contacted by axons, suggesting that innervation could be one of the factors inducing and maintaining heterogeneity.

Spatiotemporal patterns of secretomotor neuron generation in the parvicellular neuroendocrine system

Spatiotemporal patterns of parvicellular neurosecretory neuron generation (birthdates) were determined in the young adult male rat using a triple fluorescence labeling method. The six classic phenotypes were identified in histological sections with rabbit antisera to neurotransmitters (or related enzymes), nuclear bromodeoxyuridine was detected with a mouse monoclonal antibody, and an axonal projection to the median eminence was determined with the fluorescent retrograde tracer fast blue. The vast majority of triply labeled neurons are generated between embryonic days 12-14, during the time when magnocellular neurosecretory neurons are also generated. This pattern of neurogenesis is distinct from the well-known 'outside-in' pattern of hypothalamic neurogenesis, where the peak of lateral zone birthdates occurs on embryonic days 12 and 13, the peak of medial zone birthdates occurs on embryonic days 14 and 15, and the peak of periventricular zone birthdates occurs on embryonic days 16 and 17. Thus, neuroendocrine motoneurons may constitute 'pioneer neurons' for the various anatomically distinct regions of the periventricular zone. In addition, many intermixed neurons that express the same neurotransmitters as parvicellular neurosecretory neurons but do not send an axon to the median eminence, also appear to be generated between embryonic days 12 and 14. What these results imply about mechanisms underlying neuroendocrine motor zone differentiation is discussed.

Role of gonadal steroids in determining sexual differences in expression of Fos-related antigens in tyrosine hydroxylase-immunoreactive neurons in subdivisions of the hypothalamic arcuate nucleus

Dual immunohistochemistry was employed to examine the role of gonadal steroids in determining sexual differences in the expression of Fos and its related antigens (FRA) in tuberoinfundibular dopaminergic (TIDA) neurons located in the dorsomedial (DM-) and ventrolateral (VL-) subdivisions of the arcuate nucleus (ARC). In the DM-ARC, there was no sexual difference in the number of tyrosine hydroxylase (TH)-immunoreactive (-IR) perikarya, but the number of these containing FRA-IR was greater in females than in males in all but the most caudal region. In the VL-ARC, there were more TH-IR perikarya in males than in females, but there was no sexual difference in the numbers of those containing FRA-IR throughout the entire rostrocaudal extent of this nucleus. Ovariectomy decreased the number of TH-IR perikarya containing FRA-IR in the DM-ARC, but not in the VL-ARC, whereas orchidectomy increased the number of TH-IR perikayra containing FRA-IR in both the DM-ARC and VL-ARC. These gonadectomy-induced effects were reversed by estrogen and testosterone, respectively. These results reveal gonadal steroid-dependent sexual differences in the regulation of immediate early gene expression in anatomically discrete subpopulations of TIDA neurons. In females, estrogen stimulates FRA expression in TIDA neurons in the DM-ARC, whereas in males, testosterone inhibits FRA expression in TIDA neurons in both the DM-ARC and the VL-ARC.

Neuronal dopamine subpopulations maintained in hypothalamic slice explant cultures exhibit distinct tyrosine hydroxylase mRNA turnover rates

Changes in mRNA stability have been shown to regulate critical intracellular processes. In this investigation, we studied tyrosine hydroxylase (TH) mRNA turnover in functionally and anatomically distinct dopaminergic (DA) populations of the rat hypothalamus. To this end, long-term slice explant cultures from postnatal, preoptic area/hypothalami, containing three anatomically discrete DA populations, were generated and maintained under defined conditions. The organotypic cultures were treated with the transcription inhibitors 5,6-dichloro-1-D-ribofuranosylbenzimidazole or actinomycin D and processed for in situ hybridization histochemistry. Relative TH mRNA content per cell was quantitated. Single-cell analysis showed marked differences in basal TH mRNA turnover rates between DA neuronal populations. Anterior and midhypothalamic DA neurons exhibited half-time turnovers of 9-12 and 11-23 hr, respectively. In contrast, in the caudal hypothalamus, DA neurons of the arcuate nucleus had a significantly lower baseline level and more rapid turnover (6-7 hr) of TH mRNA. This investigation shows that basal turnover of a phenotypic mRNA, TH mRNA in DA neurons, is not an intrinsic property of the phenotypic marker. Furthermore, we found that destabilization of TH mRNA in the caudal hypothalamus corresponds to the known rhythmic output displayed by arcuate DA cells and, as such, may be critical for normal function of this population. We propose that intrinsic differences in the post-transcriptional regulation of TH permits neuronal subpopulations, which subserve different physiological functions, an additional mechanism to control DA biosynthesis in response to their unique needs.

G-protein expression in melanotropes changes coincident with innervation of the developing rat pituitary intermediate lobe

The two isoforms of the dopamine D2 receptor, the D2short and the D2long differ in a 29 amino acid insert in the third cytoplasmic loop with which G proteins interact. We have previously reported that in rat melanotropes, expression of D2short increases markedly at the end of the first postnatal week which is concurrent with innervation of the intermediate lobe. Using immunohistochemistry, this study examined expression of G alpha i1/2, G alpha i3, G alpha o and G alpha s proteins before and after dopaminergic innervation. G alpha i3 increased through gestational day 20, and then remained level to postnatal day 6. At this time, coinciding with the induction of D2short expression, G alpha i3 immunoreactive intensity increased markedly, possibly indicating co-regulation of these proteins. On postnatal day 6, G alpha s immunoreactive intensity increased in some, but not all, melanotropes. The resulting heterogeneity in Gs expression persisted in the adult. G alpha i1/2 immunoreactivity did not change and G alpha o was detected only subsequent to the event of innervation. Thus, dopamine released from axons and acting through D2 receptor stimulation could increase G alpha i3 immunoreactivity and decrease G alpha s immunoreactive intensity in some melanotropes.

Melanotrope dopamine D2 receptor isoform expression in the developing rat pituitary

This study measured melanotrope mRNA and protein expression for the dopamine D2 receptor, and its long isoform, in relation to the appearance of dopamine in axons of the postnatal rat pituitary intermediate lobe. At postnatal day 2, prior to the onset of dopaminergic innervation, D2 receptor (D2T) mRNA was expressed heterogeneously in a subpopulation of melanotropes which also expressed the long isoform (DL). The D2L mRNA appeared to be predominant during early postnatal development, since the D2T probe, which did not discriminate between the isoforms, and the D2L probe hybridized generally to the same cells, as demonstrated in serial sections. Immunohistochemical methods, using two different antisera for the D2T receptor, however, indicated a low level of protein in most melanotropes. Localization of D2L protein corresponded well to D2T receptor mRNA distribution. At day 10, representing a time when dopamine is present in axons throughout the lobe, both D2T receptor mRNA and protein were detected in a significantly larger population of melanotropes than those expressing D2L mRNA and protein. This suggests the appearance of detectable short isoform (D2S) mRNA in virtually all melanotropes and implicates dopamine as a possible signal for increasing D2S isoform mRNA expression.

Glial-like cells of the rat pituitary intermediate lobe change morphology and shift from vimentin to GFAP expression during development

This study demonstrated morphological changes in glial-like cells of the rat pituitary intermediate lobe during early postnatal development, and a subsequent shift in protein expression from vimentin to GFAP. Vimentin immunoreactivity was detected in the lobe at embryo day 14 and was localized in radially-oriented, bipolar cells whose processes spanned the thickness of the intermediate lobe. At electron microscopical resolution, processes contained intermediate filaments, cell nuclei were indented while secretory vesicles characteristic of the endocrine cells were not found. Vimentin immunoreactive intensity began to decrease at postnatal day 5. By postnatal day 7, vimentin-positive, stellate cells were observed, with few radial processes found by day 10. The intensity of vimentin immunoreactivity decreased through day 25. Within the lobe parenchyma, vimentin was localized in glial-like cells since double-label immunohistochemistry revealed no colocalization of beta-endorphin and vimentin, or fibronectin and vimentin. Dopamine-containing axons were in close apposition to vimentin-positive processes. GFAP immunoreactivity first appeared on postnatal day 20 and, by day 25, stellate cell bodies with three to six extended processes were evident. Cells were primarily distributed in the caudal third of the lobe. The characteristic adult pattern of cell clusters in latero-dorsal and ventral portions of the lobe was fully established by postnatal day 55. The transition from vimentin to GFAP expression and concurrent morphological changes resemble those described for radial glial during cerebral cortical development.

Transient expression of S-100 by melanotropes of the rat pituitary intermediate lobe during development

S-100, an acidic calcium-binding protein, is co-localized with vimentin in glial-like cells in the adult rat pituitary intermediate lobe. S-100 and melanotrope markers were not co-localized in the adult. During development, S-100 and vimentin were not co-localized but appeared in cells with different morphological characteristics. S-100 was co-localized with POMC mRNA and beta-endorphin during prenatal time and the first three postnatal weeks. This was demonstrated by double-label immunohistochemistry, using combinations of antisera against S-100, vimentin and beta-endorphin, and in situ hybridization histochemistry for POMC mRNA combined with immunohistochemistry for S-100. In the second and third weeks of postnatal development, S-100 was observed in fewer melanotropes and more frequently in stellate cells, which also expressed vimentin. Thus, S-100 appeared to be transiently expressed in melanotropes during prenatal and early postnatal development. S-100 serves as a neurotrophic and glial maturation factor in the CNS. Since S-100 expression in melanotropes coincides with the onset of dopaminergic innervation and morphological changes in glial-like cells of the lobe, it could have similar functions in the rat pituitary intermediate lobe.

Intracranial transplantation and survival of tuberomammillary histaminergic neurons

Investigations were undertaken to determine whether fetal histaminergic neurons in the tuberomammillary nucleus of the posterior hypothalamus survive intracranial transplantation to adult hosts. Two methods of transplantation were utilized. Grafts were placed either into the delayed cavity of a fimbria-fornix lesion or directly into the hippocampus using stereotaxic techniques. The tissue was taken from rat fetuses at embryonic days 16-17 and grafted into adult rats of either the Sprague-Dawley or the Fischer 344 strain. Routine histology and immunohistochemistry were used to evaluate the grafts. All transplants to Sprague-Dawley rats showed signs of rejection, while no signs of rejection were seen in any of the Fischer 344 rats. Transplants placed directly into the delayed fimbria-fornix cavity did not grow as well or contain as many surviving neurons as the intraparenchymal grafts. The largest number of surviving histamine-positive neurons was obtained with grafts of posterolateral blocks of hypothalamus from fetal day 17 placed directly into the CA1 region of the rostral hippocampal formation of Fischer 344 hosts. Histamine-immunoreactive cell bodies with neuritic outgrowth were found in all Fischer 344 rats that received hypothalamic grafts. Cell bodies exhibited histamine immunoreactivity evenly throughout the cytoplasm and had morphological characteristics resembling histaminergic neurons in situ. Axonal outgrowth extended throughout the grafted hypothalamic tissue, and was sometimes seen in the host hippocampal tissue as well. It is concluded that fetal histaminergic neurons survive transplantation to the adult hippocampal formation, and that this allograft procedure can supplement current strategies to investigate the function of histaminergic tuberomamillary neurons in the central nervous system.

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.

Development of the tuberoinfundibular system in rats: birthdates of the tyrosine hydroxylase-immunopositive neurons

The present study has attempted to determine the birthdates of the tyrosine hydroxylase (TH)-immunopositive neurons in the arcuate nucleus (AN) of rats, thus evaluating the time of the last mitotic divisions of their cell precursors. For this aim, 'long-survival' [3H]thymidine autoradiography was used in combination with immunocytochemistry of TH, the first enzyme of catecholamine synthesis. According to our data, some TH-immunopositive neurons in the AN were born as early as the 13th fetal day. At day 14, the number of double-labeled neurons almost tripled while reaching a maximum at day 15 of the intrauterine development. The fraction of the double-labeled neurons dropped on the following day, and practically disappeared by fetal day 17, showing cessation of the production of the TH-immunopositive neurons. The birth of the TH-immunopositive neurons is considered as the onset of the development of the tuberoinfundibular dopaminergic system.

Development of tyrosine hydroxylase-, dopamine- and dopamine β-hydroxylase-immunoreactive neurons in a teleost, the three-spined stickleback

The development of catecholaminergic neuronal systems in the brain of a teleost, the three-spined stickleback, was studied through embryonic to early larval stages by immunocytochemistry using specific antibodies against dopamine, tyrosine hydroxylase and dopamine beta-hydroxylase. By analysing the spatiotemporal patterns of development for the catecholaminergic nuclei, possible homologies with nuclei in amniote brains have been identified. The noradrenergic neurons in the isthmus region of the rostral rhombencephalon originate in the same manner as the A4-A7 + subcoeruleus group in mammals. Their developmental characteristics show the largest similarities with the subcoeruleus group of birds and mammals, although some features are shared with developing A6 (locus coeruleus) neurons. Catecholaminergic neurons never appear during development in the ventral mesencephalon of the three-spined stickleback. A group of large dopaminergic neurons that accompany the cerebrospinal fluid (CSF)-contacting neurons follows the border between the hypothalamus and the ventral thalamus into the caudal hypothalamus, where they are continuous with the dopaminergic neurons in the posterior tuberculum. They are thus topologically comparable with the dopaminergic neurons of the zona incerta in mammals. The dopaminergic CSF-contacting neurons that line the median, lateral and posterior recesses of the third ventricle do not contain tyrosine hydroxylase-immunoreactivity at any developmental stage. This indicates that they take up and accumulate exogenous dopamine or L-dihydroxyphenylalanine, and do not synthesize dopamine from tyrosine at any developmental stage. Tyrosine hydroxylase-immunoreactive neurons appear in the pineal organ on the day of hatching (120 h post-fertilization). They were still observed in 240-h-old larvae, but are absent in the pineal organ of adult sticklebacks. The initial appearance and subsequent differentiation of catecholaminergic neurons in the stickleback embryo follow essentially the same spatial and temporal pattern as in amphibian, avian and mammalian embryos. This observation supports the hypothesis that morphologically, topologically and chemically similar monoaminergic neurons in different vertebrate classes are homologous.

Interaction of opioid peptide-containing terminals with dopaminergic perikarya in the rat hypothalamus

Both direct pituitary and indirect CNS mechanisms have been postulated for the influence of opiate agonists on prolactin secretion. By examining the interactions between terminals of neurons containing opioid peptides and hypothalamic TH-positive cell bodies, this paper addressed the anatomical basis for the latter mechanism. Initial electron microscopic studies directly demonstrated contact between opioid peptide terminals and dopaminergic cell bodies and provided some visual criteria for assessing opioid-dopamine interactions at the light microscopic level. Using these guidelines, we examined the rates of contact on both A12 and A14 neurons of each of the three opioid peptide families: pro-enkephalin, pro-dynorphin, and pro-opiomelanocortin (POMC). For A14 neurons, many of which project to the posterior pituitary, contact rates were estimated at 15, 20, and 5% for dynorphin, Met-enkephalin, and ACTH (a POMC derivative), respectively. In contrast, the A12 dopamine neurons, which regulate prolactin secretion by inhibition, showed a roughly 70% contact rate with dynorphin axons (P less than 0.001) with Met-enkephalin and ACTH contact rates remaining low at 20 and 5% respectively. Contact frequency varied significantly during the estrus cycle only with dynorphin contacts on A12 neurons. Proestrus and diestrus (less so) showed a small but significant (P less than 0.05) elevation in contact rates versus estrus, male, lactating and pregnant groups. No other significant difference emerged among these groups. On the basis of these observations, we conclude that dynorphin represents a significant and specific factor in the innervation of A12 dopamine neurons. This relationship may account for some if not most of the influence of opiate agonists and antagonists on prolactin secretion.

Ontogeny of tyrosine hydroxylase mRNA signal levels in central dopaminergic neurons: development of a gender difference in the arcuate nuclei

Using in situ hybridization and immunocytochemistry, this study examined the tyrosine hydroxylase (TH) mRNA signal levels and immunostaining in the arcuate nuclei of the hypothalamus, zona incerta and substantia nigra of male and female rats during neonatal, peripubertal and adult life. The catalytic activity of TH in the stalk-median eminence was also investigated using the in vitro accumulation of 3,4 dihydroxyphenylalanine (DOPA) after inhibiting aromatic amino acid decarboxylase activity. In the arcuate nuclei, TH mRNA levels increased 3.5-fold between 5 and 15 days of age and remained at a steady level between 15 and 35 days of age in both male and female rats. Similar to TH mRNA levels in the arcuate nuclei, TH activity in the stalk-median eminence increased 2-3 fold between days 10 and 15 of age and remained at a steady level between 15 and 35 days of age in both sexes. A later increase in TH mRNA levels in the arcuate nuclei and catalytic activity in the stalk-median eminence was observed between 35 and 40 days in females, but not males. During adulthood, TH mRNA levels and enzyme activity were 2.7-fold higher in the arcuate nuclei and stalk-median eminence, respectively, of diestrous females vs males. These data suggest that the changes in TH mRNA in the arcuate nuclei may contribute to the developmental alterations, as well as the adult gender differences, in enzyme activity in the stalk-median eminence. Circulating progesterone levels were low (1-10 ng/ml) between days 5 and 25 of age and increased 6-fold between 25 and 35 days of age in both males and females. Progesterone levels increased 2-fold in females, but not males, between days 35 and 40 and were 4-fold higher in diestrous females as compared to adult males. Circulating prolactin levels were low (2-3 ng/ml) between days 5 and 15, increased 15-fold between days 15 and 25 and increased an additional 2- to 3-fold between days 35 and 70 in both males and females. TH mRNA signal levels increased between days 5 and 15 of age in dopaminergic perikarya in the zona incerta and the substantia nigra of both female and male rats. The TH mRNA levels remained constant between days 15 and 70 in the zona incerta, whereas TH mRNA levels declined with age in the substantia nigra.(ABSTRACT TRUNCATED AT 400 WORDS)

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.

Development of the dorsal pancreatic primordium transplanted into the third ventricle of rats

The dorsal pancreatic primordia of 12.5-day-old rat embryos transplanted into the third ventricle of adult female rats were immunohistochemically examined 10, 20 and 40 days after transplantation. On day 10, the grafts grew into an epithelial sacculus (S) with a thick subepithelial tissue (ST). Tubular and vesicular structures with a single cuboidal epithelium were found within the wall of the S, but they underwent thereafter a regression without allowing the primordia to differentiate into the exocrine acinar tissues. In contrast with this, pancreatic hormone-containing cells existed in the ST, and were arranged like the islands of a mature animal. The tissue also has smooth muscle fibers and neurons. When the primordium was grafted along with its root connected to the duodenum, gut-like tubular structures differentiated, showing mucosa with villi and crypts, submucous mesenchymal tissue and muscle layers. The mucosa possesses epithelial cells immunoreactive for the pancreatic hormones, and the muscle layers have the myenteric plexuses. These findings seem to provide further evidence that in the rat pancreas, pancreatic-hormone-containing cells differ from the acinar cells in origin.

Prenatal development of mesencephalic and diencephalic dopaminergic systems in the male and female rat

Previous results had suggested mesencephalic dopaminergic neurons in cultures of gestational day (GD) 14 rat embryonic brains to be characterized by an early maturation and acquisition of sex-related differences in transmitter uptake. Therefore development of dopaminergic systems was reexamined in the rat in vivo with special emphasis on the prenatal period, mesencephalo-hypothalamic relationships, and possible sex differences. Perfusion-fixed brains of GD 14, 17, 20, 21 and newborn rats were sectioned or processed as whole-mounts and immunostained for tyrosine hydroxylase (TH). Total numbers of mesencephalic TH-immunoreactive cell bodies as assessed by a stereological method rose between GD 14 and 17 and fell again between GD 17 and 21. As early as GD 14, a prominent mesencephalo-hypothalamic projection was observed coming off the medial forebrain bundle and terminating in the retrochiasmatic region. Two additional TH-immunoreactive fiber bundles leaving the medial forebrain bundle, one rostral and one caudal to the former, and terminating in the paraventricular and premammillary region, respectively, were noticed on GD 17. Careful examination of developing TH-immunoreactive neurons in the lower brainstem ascertained that there was no interference from ascending catecholaminergic fibers other than dopaminergic of mesencephalic origin during this early prenatal period. All 3 mesencephalo-hypothalamic projections had largely disappeared by GD 20 and were no longer detectable as distinct fiber bundles thereafter. There were subtle sex differences in numbers and distribution of both mesencephalic and diencephalic TH-immunoreactive neurons present at GD 17, which thus occurred prior to manifestation of other well-known sexual dimorphisms of the rat brain.(ABSTRACT TRUNCATED AT 250 WORDS)

Appearance of neurons with glucocorticoid receptors and neurovascular links in the embryonal rat hypothalamus grafted in the third ventricle

We have investigated the appearance of the transmitter phenotypes of hypothalamic neurons in grafts transplanted into the third ventricle of adult female rats. The grafts were the mediobasal hypothalamus and the preoptic area of 12.5-day-old rat embryos, and were examined 40-100 days later. Wheat germ agglutinin (WGA) was injected into the jugular vein of several animals for the examination of the existence of neurovascular associations. Three days after the injection, WGA appeared to have been incorporated into the neurons in the paraventricular, periventricular, and arcuate nuclei of the host animals. In the grafts, WGA was also seen incorporated in certain neurons which were found immunoreactive for tyrosine hydroxylase (TH), rat corticotropin-releasing factor (rCRF), substance P (SP), or somatostatin (SRIH). Neurons immunoreactive for neuropeptide Y (NPY) and ACTH did not seem to incorporate WGA. These findings suggest that the neurons containing TH, rCRF, SP, or SRIH link with fenestrated capillaries developed in the grafts. The immunoreactivity for glucocorticoid receptor (GR) was detected mainly in the nucleus of certain neurons and glial cells in the grafts as well as in the host hypothalamic neurons. In the grafts, strong GR immunoreactivity was detected in the cells immunoreactive for TH, NPY, and rCRF as in the host animals. It is concluded that the undifferentiated hypothalamic neurons differentiate to synthesize GR as well as definitive peptides and TH in the grafts.

Topographic relations between tyrosine hydroxylase- and luteinizing hormone-releasing hormone-immunoreactive fibers in the median eminence of adult rats

Employing electron microscopic double immunolabeling, we determined a close apposition of tyrosine hydroxylase (TH) and luteinizing hormone-releasing hormone (LHRH) nerve fibers in the rat median eminence (ME). These axo-axonic contacts occurred frequently in the internal and palisade zones, i.e. at the level of the fiber preterminals. In the superficial area of the ME, major TH fibers abutted on the basal lamina and some were projected into the pericapillary space of the portal vessels. Conversely, LHRH fibers were arrested by the endfeet of tanycytes in reaching the basal lamina.

Age- and dose-dependent effects of neonatal monosodium glutamate (MSG) administration to female rats

The age- and dose-dependent effects of neonatal MSG were evaluated in pre- and postpubertal female rats. The neurotoxic action of MSG was assessed by examining monoamine content in microdissected regions of the mediobasal hypothalamus. MSG was administered at a dose of 4 mg/g on postnatal days 2 and 4 (MSG-Lo) or on postnatal days 2, 4, 6 and 8 (MSG-Hi). MSG-Hi treatment significantly reduced dopamine (DA) content in the arcuate nucleus (ANH) and lateral median eminence (LME) on postnatal day 21 when compared to NaCl-injected controls. DA content relative to controls was not altered in the ANH or LME postnatal or postnatal day 60 in MSG-Hi, however, norepinephrine (NE) was significantly (p less than 0.05) decreased on both postnatal day 21 and 60 in the LME. MSG-Lo treatment significantly (p less than 0.05) reduced ANH NE content on postnatal day 60 compared to controls. Both MSG-Hi and MSG-Lo treatment increased 5-hydroxyindoleacetic acid content in the preoptic area (POA) on postnatal day 60 relative to the controls. Developmental changes independent of MSG treatment were noted in the hypothalamus. DA and 3,4-dihydroxyphenylacetic acid (DOPAC) content in the POA were 2-3-fold higher on postnatal day 21 compared to postnatal day 60. In contrast, DA content increased with age in the ANH, LME and medial ME. NE content in the ANH increased as a function of age in controls, but not in MSG-treated rats. The effects of MSG treatment on the postnatal development and maturation of neurons in the mediobasal hypothalamus were discussed in relation to the direct neurotoxicity of MSG.

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.