Ontogenesis of tyrosine hydroxylase-immunopositive structures in the rat hypothalamus. An atlas of neuronal cell bodies

Prosomeric Hypothalamic Distribution of Tyrosine Hydroxylase Positive Cells in Adolescent Rats

Most of the studies on neurochemical mapping, connectivity, and physiology in the hypothalamic region were carried out in rats and under the columnar morphologic paradigm. According to the columnar model, the entire hypothalamic region lies ventrally within the diencephalon, which includes preoptic, anterior, tuberal, and mamillary anteroposterior regions, and sometimes identifying dorsal, intermediate, and ventral hypothalamic partitions. This model is weak in providing little or no experimentally corroborated causal explanation of such subdivisions. In contrast, the modern prosomeric model uses different axial assumptions based on the parallel courses of the brain floor, alar-basal boundary, and brain roof (all causally explained). This model also postulates that the hypothalamus and telencephalon jointly form the secondary prosencephalon, separately from and rostral to the diencephalon proper. The hypothalamus is divided into two neuromeric (transverse) parts called peduncular and terminal hypothalamus (PHy and THy). The classic anteroposterior (AP) divisions of the columnar hypothalamus are rather seen as dorsoventral subdivisions of the hypothalamic alar and basal plates. In this study, we offered a prosomeric immunohistochemical mapping in the rat of hypothalamic cells expressing tyrosine hydroxylase (TH), which is the enzyme that catalyzes the conversion of L-tyrosine to levodopa (L-DOPA) and a precursor of dopamine. This mapping was also combined with markers for diverse hypothalamic nuclei [agouti-related peptide (Agrp), arginine vasopressin (Avp), cocaine and amphetamine-regulated transcript (Cart), corticotropin releasing Hormone (Crh), melanin concentrating hormone (Mch), neuropeptide Y (Npy), oxytocin/neurophysin I (Oxt), proopiomelanocortin (Pomc), somatostatin (Sst), tyrosine hidroxilase (Th), and thyrotropin releasing hormone (Trh)]. TH-positive cells are particularly abundant within the periventricular stratum of the paraventricular and subparaventricular alar domains. In the tuberal region, most labeled cells are found in the acroterminal arcuate nucleus and in the terminal periventricular stratum. The dorsal retrotuberal region (PHy) contains the A13 cell group of TH-positive cells. In addition, some TH cells appear in the perimamillary and retromamillary regions. The prosomeric model proved useful for determining the precise location of TH-positive cells relative to possible origins of morphogenetic signals, thus aiding potential causal explanation of position-related specification of this hypothalamic cell type.

Circadian peak dopaminergic activity response at the biological clock pacemaker (suprachiasmatic nucleus) area mediates the metabolic responsiveness to a high-fat diet

Among vertebrate species of the major vertebrate classes in the wild, a seasonal rhythm of whole body fuel metabolism, oscillating from a lean to obese condition, is a common biological phenomenon. This annual cycle is driven in part by annual changes in the circadian dopaminergic signalling at the suprachiasmatic nuclei (SCN), with diminution of circadian peak dopaminergic activity at the SCN facilitating development of the seasonal obese insulin-resistant condition. The present study investigated whether such an ancient circadian dopamine-SCN activity system for expression of the seasonal obese, insulin-resistant phenotype may be operative in animals made obese amd insulin resistant by high-fat feeding and, if so, whether reinstatement of the circadian dopaminergic peak at the SCN would be sufficient to reverse the adverse metabolic impact of the high-fat diet without any alteration of caloric intake. First, we identified the supramammillary nucleus as a novel site providing the majority of dopaminergic neuronal input to the SCN. We further identified dopamine D2 receptors within the peri-SCN region as being functional in mediating SCN responsiveness to local dopamine. In lean, insulin-sensitive rats, the peak in the circadian rhythm of dopamine release at the peri-SCN coincided with the daily peak in SCN electrophysiological responsiveness to local dopamine administration. However, in rats made obese and insulin resistant by high-fat diet (HFD) feeding, these coincident circadian peak activities were both markedly attenuated or abolished. Reinstatement of the circadian peak in dopamine level at the peri-SCN by its appropriate circadian-timed daily microinjection to this area (but not outside this circadian time-interval) abrogated the obese, insulin-resistant condition without altering the consumption of the HFD. These findings suggest that the circadian peak of dopaminergic activity at the peri-SCN/SCN is a key modulator of metabolism and the responsiveness to adverse metabolic consequences of HFD consumption.

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.

Developing brain as an endocrine organ: secretion of dopamine

This study was aimed to test our hypothesis that the developing brain operates as an endocrine organ before the establishment of the blood-brain barrier (BBB), in rats up to the first postnatal week. Dopamine (DA) was selected as a marker of the brain endocrine activity. The hypothesis was supported by the observations in rats of: (i) the physiological concentration of DA in peripheral blood of fetuses and neonates, before the BBB establishment, and its drop by prepubertal period, after the BBB development; (ii) a drop of the DA concentration in the brain for 54% and in blood for 74% on the 3rd postnatal day after the intraventricular administration of 50 μg of α-methyl-p-tyrosine, an inhibitor of DA synthesis, with no changes in the DA metabolism in peripheral DA-producing organs. Thus, the developing brain is a principal source of circulating DA which is capable of providing an endocrine regulation of peripheral organs and the brain.

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.

The influence of catecholamine on the migration of gonadotropin-releasing hormone-producing neurons in the rat foetuses

Catecholamines (CA) play an important role in the regulation of GnRH neurons in adults, and it is probable that they control GnRH-neuron development. Migration of GnRH neurons was evaluated in male and female rats at the 17th embryonic day (E17) and E21, following the daily treatment of their pregnant mothers from the 11th to the 16th and 20th day of gestation with alpha-methyl-para-tyrosine (alphaMPT), an inhibitor of catecholamine synthesis. High-performance liquid chromatography with electrochemical detection (HPLC-ED) was used to specify the alphaMPT-induced CA depletion. There was a 50-70% decrease in dopamine and noradrenaline content in the nose and in the brain of alphaMPT-treated foetuses, proving the efficacy of this pharmacological model. Immunohistochemistry was used to evaluate the percentage (%) of GnRH neurons along their migration pathway from the vomeronasal organ (VNO) in the nose to the septo-preoptic area in the forebrain which is considered as an index of neuron migration. Special attention was paid to the topographic relationships of GnRH neurons with catecholaminergic fibres. These were observed in apposition with GnRH neurons in the entrance to the forebrain. In CA-deficient foetuses, the percentage of GnRH neurons located in the rostral regions extending from the VNO to the septum was greater than in controls. However, no statistically significant difference was found in the forebrain which extended from the septum to the retrochiasmatic area. In conclusion, these data suggest that endogenous catecholamines stimulate the GnRH neuron migration in ontogenesis.

Interaction between the circadian clocks of mother and fetus

In mammals, a unidirectional communication exists between the biological clocks of the mother and fetus. As a biological clock begins oscillating in the suprachiasmatic nuclei of the fetus, redundant circadian signals entrain the fetal clock to the prevailing light-dark cycle. Recent studies have revealed an activatable dopamine system within the fetal hypothalamus which may serve as a final common pathway by which maternal signals entrain the fetus. An entrained biological clock during fetal life makes the developing mammal better prepared for life in the outside world.

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.

Neuronal expression of catechol O-methyltransferase mRNA in neonatal rat suprachiasmatic nucleus

We examined the expression profile of catechol O-methyltransferase (COMT) mRNA and its protein in the neonatal rat hypothalamus by in situ hybridization and immunohistochemistry to clarify the sites of dopamine degradation. Strong COMT mRNA expression was observed in the suprachiasmatic nucleus (SCN) throughout its rostrocaudal extent at postnatal day 1 (P1) and P2, and the mRNA levels decreased gradually until P16. COMT mRNA was predominantly localized to the ventral and medial parts of the SCN. Intense COMT immunoreactivity was demonstrated in the ventral SCN and was detected in neuronal perikarya and processes at P1. Ependymal and microglial cells also exhibited strong COMT immunoreactivity. These results indicate that COMT may directly be involved in dopaminergic signaling in the neonatal SCN.

Early and transient ontogenetic expression of the cocaine- and amphetamine-regulated transcript peptide in the rat mesencephalon: correlation with tyrosine hydroxylase expression

The ontogeny of cocaine- and amphetamine-regulated transcript (CART) expression has been analyzed by immunohistochemistry in the mesencephalon of the rat central nervous system, and compared to the pattern of tyrosine hydroxylase- (TH-) expression. CART-producing neurons were first detected on the embryonic day 11 (E11) in the ventral mesencephalic vesicle. These neurons are among the first cells of the mantle layer to differentiate. From E13, a complementary pattern of distribution was observed, dividing the mantle layer into an external TH zone and an internal CART zone. Many TH-positive neurons were found to migrate from the neuroepithelium through the area containing the CART-immunoreactive neurons to settle more laterally. These TH cells exhibited prominent leading and trailing dendrites in the immediate vicinity of CART perikarya. On E16, the number of CART neurons appeared to diminish, and they were confined near the ventricle and around the fasciculus retroflexus. On E18 and E20, only the Edinger-Westphal nucleus exhibited a strong CART staining as described in the adult brain. Thus, the very early detection of CART during prenatal ontogeny led us to speculate that this peptide might have a role in the development of specific regions of the rat brain. In particular, our observations suggest that CART-expressing neurons might help the migration of the dopaminergic neurons of the substantia nigra.

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%.

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.

The differentiation of dopaminergic neurons in situ, in vivo, and in transplants

This article summarizes results obtained from studies on the differentiation of dopaminergic neurons in animal hypothalamus and human substantia nigra in situ, in vitro, and in transplants, as well as the role of the microenvironment in regulating this process. Four stages were identified in the differentiation of dopaminergic neurons from rat hypothalamus: a) formation of neurons from neuroepithelial precursor cells, b) expression of specific synthetic products (enzymes and dopamine itself) and mechanisms for transmembrane dopamine transport (reuptake and secretion in response to membrane depolarization), c) formation of permanent and transient efferent connections, and d) formation of afferent innervation and synaptogenesis. Along with dopaminergic neurons, rat fetuses contained neurons expressing only one of the dopamine-synthesizing enzymes and probably taking part in in situ dopamine synthesis. Differentiation of dopaminergic neurons was sexually dimorphic in terms of the dynamics of neuron formation and expression of enzymes involved in dopamine synthesis. A neurotransplantation model showed that humoral factors of placental and maternal origin had no significant effect on the differentiation of the dopaminergic neurons of the hypothalamus. As regards the dopaminergic neurons of the substantia nigra, expression of their specific phenotype in human fetuses started with the synthesis of tyrosine hydroxylase and co-maturation of the specific dopamine reuptake mechanism during the sixth week of development. During the next four weeks, specific uptake increased, and this appears to be a measure of the number of neurons and the growth of their processes. These data provide the basis for regarding the period from week 6 to week 10 as optimal for transplantation of dopaminergic neurons into the striatum of patients with Parkinson's disease. Suspensions of fetal substantia nigra cells enriched with dopaminergic neurons were introduced stereotaxically into a patient's striatum through a cannula. Positron emission tomography studies showed that the transplanted neurons survived within the host brain, underwent differentiation, and started to synthesize dopamine. The results of clinical assessment performed in parallel with these studies suggested that the transplanted dopaminergic neurons were involved in regulating striatal target neurons.

Membranes from pituitary intermediate lobe cells enhance differentiation of fetal hypothalamic dopaminergic neurons in primary culture

Coculture of adult pituitary intermediate lobe (IL) cells, a target for hypothalamic dopaminergic neurons, with fetal rat hypothalamic cells accelerate differentiation of dopaminergic neurons. This involves long range diffusible as well as additional factors which may be membrane-bound. To determine whether IL membrane-bound factors contribute to the differentiating effect of IL cells, IL membranes were added to dispersed fetal hypothalamic neurons. This stimulated the outgrowth of dopaminergic neurites and elevated TH levels. Limited trypsin proteolysis of IL cell surface abolished the effect on TH levels. Addition of adenohypophyseal membranes was ineffective. Joint treatment with IL membranes, and medium conditioned (CM) over IL cells, produced the same effect on TH levels as did coculture with the same number of IL cells. The results demonstrate that IL cells express on their surface a membrane-bound factor promoting differentiation of fetal dopaminergic neurons in vitro; this factor acts in addition to diffusible activities.

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.

Hypothalamic synaptogenesis and its relationship with the maturation of hormonal secretion

1. Information obtained during the last decade has demonstrated that hypothalamic neurons release a wide variety of neuroactive substances, such as neurotransmitters, mostly monoamines and amino acids, and neuromodulators such as the peptides vasopressin (AVP) and oxytocin (OXT) and hypophysial releasing hormones. 2. Synapse formation between hypothalamic neurons was followed at different times within a given nucleus and among different nuclei during development of the mouse hypothalamus. 3. The amounts of various neurotransmitters and hormones were determined at various stages of development. 4. A correlation is presented of the biochemical and ultrastructural features and their functional implications during maturation.

D1-dopamine receptor binding and tyrosine hydroxylase-immunoreactivity in the fetal and neonatal hamster suprachiasmatic nucleus

The suprachiasmatic nucleus (SCN) of the anterior hypothalamus is the site of an endogenous biological clock that regulates mammalian circadian rhythms. Circadian rhythms, although endogenously driven, are synchronized or entrained to daily environmental cues. Developmentally, the SCN begins to oscillate before birth and is entrained to the maternal circadian rhythm by a mechanism that is still unclear. Recent evidence in rats and hamsters suggests that a fetal dopaminergic system and D1-dopamine receptors may be involved in the process of entraining the fetal clock. The present study using [3H]SCH 23390 autoradiography and tyrosine hydroxylase (TH) immunocytochemistry determined the developmental time courses of the appearance of D1 receptor in, and catecholaminergic input to, the hamster SCN. [3H]SCH 23390 binding to D1-dopamine receptors was first detected in the fetal SCN on embryonic day (E) 15, the day before birth in this species, and persisted through adulthood. The TH immunoreactive fibers were first observed on day E15 coursing just ventral to the fetal SCN but TH-immunoreactive cells and fibers were not seen within the SCN until postnatal day (P) 5. The presence of D1-dopamine receptor binding in the fetal hamster SCN is consistent with the role of these receptors in entrainment of the fetal circadian pacemaker to maternal cues. However, a receptor-transmitter mismatch exists between D1-dopamine receptors and TH-immunoreactive fibers in the fetal SCN suggesting that the role of dopamine in maternal-fetal entrainment may be as a paracrine or humoral signal.

Lithium increases melanin-concentrating hormone mRNA stability and inhibits tyrosine hydroxylase gene expression in PC12 cells

Melanin-concentrating hormone (MCH) is a cyclic peptide involved in the regulation of food-intake behaviour and stress response in mammals. Expression of the MCH gene predominates in hypothalamic neurons. Mechanisms governing the regulation of expression of MCH gene in established cell lines were not explored yet. Here, we analysed the actions of nerve growth factor (NGF), dexamethasone, forskolin and lithium on MCH mRNA levels in the PC12 pheochromocytoma cell line. We compared them with those observed on tyrosine hydroxylase (TH) mRNA, constitutively expressed in PC12 cells, and neurotensin (NT) mRNA, taken as a control. In untreated cells, MCH RNA species of high molecular weight were found. Exposure of cells at a combination of NGF and lithium resulted in decreased expression of these MCH RNAs and in the transient production of mature MCH mRNA. Strikingly, after short exposure of PC12 cells to NGF, lithium per se elicited a marked increase in MCH mRNA levels whilst it exerted a potent inhibitory action on TH mRNA expression. Detailed investigations revealed that lithium enhanced MCH mRNA expression through post-transcriptional mechanisms whereas it regulated TH gene expression mainly at the level of transcription. These results demonstrate that lithium, an agent widely used for treatment of manic depressive illness, can exert an opposite effect on MCH and TH mRNA production in PC12 cells. The MCH gene system in NGF-treated PC12 cells provides a good opportunity for studying the effect of lithium on gene expression at post-transcriptional levels in a neuron-like cellular model.

Ontogeny of sex differences in the mammalian hypothalamus and preoptic area

1. There are numerous sites in the nervous system where steroid hormones dramatically influence development. Increasing interest in mechanisms in neural development is providing avenues for understanding how gonadal steroids alter the ontogeny of these regions during sexual differentiation. 2. An increasing number of researchers are examining effects of gonadal steroids on neurite outgrowth, cell differentiation, cell death, cell migration, and synaptogenesis. The interrelated timing of these events may be a key aspect influenced by gonadal steroids throughout development. 3. The preoptic area and hypothalamus are characteristically heterogeneous in terms of cell type (e.g., different neuropeptides) and cell derivation. Perhaps a major reason for the ontogeny of sexual differences in the preoptic area and hypothalamus lies in the convergence of many different cell types from diverse sources (i.e., proliferative zones surrounding the lateral and third ventricles, and the olfactory placodes) that can be influenced in an interactive manner by gonadal steroid mechanisms. 4. The characterization of multiple mechanisms (e.g., trophic, migratory, apoptotic, fate, etc.,) that contribute to permanent changes in brain structure and ultimately function is essential for unraveling the process of sexual differentiation.

Evidence for neurotensin autoreceptors and relationship of neurotensin and its receptors with tyrosine hydroxylase-positive neurons in rat primary hypothalamic cultures

Neurotensin is present in high quantity in the hypothalamus, where it regulates pituitary hormone secretion. A relationship between dopaminergic and neurotensinergic systems has been suggested in the hypothalamus in studies showing an effect of neurotensin on tuberoinfundibular dopaminergic neurons. In order to determine the anatomical basis of such interactions, primary cultures of rat hypothalamic neurons were used. Tyrosine hydroxylase and neurotensin containing cells were identified by immunocytochemistry and neurotensin binding sites by [125I]Tyr3-neurotensin autoradiography. Colocalization studies showed that neurotensin immunoreactivity was present in 16% of tyrosine hydroxylase-positive cells, and that these neurotensin/tyrosine hydroxylase neurons represented more than half (58%) of the neurotensinergic population. Five percent of the tyrosine hydroxylase-positive cells had neurotensin binding sites, suggesting that only a restricted number of hypothalamic dopaminergic neurons is responsive to neurotensin. Neurotensin binding sites were also found on some neurotensin-positive cells, demonstrating for the first time the presence of autoreceptors for this peptide on neurons. These results in primary cultures provide a cellular basis for direct effects of neurotensin on a subpopulation of hypothalamic dopaminergic cells, and support the possibility of an autocrine action of neurotensin in the hypothalamus.

Postnatal maturation of rat hypothalamoneurohypophysial neurons: evidence for a developmental decrease in calcium entry during action potentials

Action potentials and voltage-gated currents were studied in acutely dissociated neurosecretory cells from the rat supraoptic nucleus during the first three postnatal weeks (PW1-PW3), a period corresponding to the final establishment of neuroendocrine relationships. Action potential duration (at half maximum) decreased from 2.7 to 1.8 ms; this was attributable to a decrease in decay time. Application of cadmium (250 microM) reduced the decay time by 43% at PW1 and 21% at PW3, indicating that the contribution of calcium currents to action potentials decreased during postnatal development. The density of high-voltage-activated calcium currents increased from 4.4 to 10.1 pA/pF at postnatal days 1-5 and 11-14, respectively. The conductance density of sustained potassium current, measured at +20 mV, increased from 0.35 (PW1) to 0.53 (PW3) nS/pF. The time to half-maximal amplitude did not change. Conductance density and time- and voltage-dependent inactivation of the transient potassium current were stable from birth. At PW1, the density and time constant of decay (measured at 0 mV) were 0.29 nS/pF (n = 12) and 17.9 ms (n = 10), respectively. Voltage-dependent properties and density (1.1 nS/pF) of the sodium current did not change postnatally. During PW1, fitting the mean activation data with a Boltzmann function gave a half-activation potential of -25 mV. A double Boltzman equation was necessary to adequately fit the inactivation data, suggesting the presence of two populations of sodium channels. One population accounted for approximately 14% of the channels, with a half-inactivation potential of -86 mV; the remaining population showed a half-inactivation potential of -51 mV. A mathematical model, based on Hodgkin-Huxley equations, was used to assess the respective contributions of individual currents to the action potential. When the densities of calcium and sustained potassium currents were changed from immature to mature values, the decay time of the action potentials generated with the model decreased from 2.85 to 1.95 ms. A similar reduction was obtained when only the density of the potassium current was increased. Integration of the calcium currents generated during mature and immature action potentials demonstrated a significant decrease in calcium entry during development. We conclude that the developmental reduction of the action potential duration 1) is a consequence of the developmentally regulated increase in a sustained potassium current and 2) leads to a reduction of the participation of calcium currents in the action potential, resulting in a decreased amount of calcium entering the cell during each action potential.

Co-culture of hypothalamic neurons and melanotrope cells: a model to study synaptogenesis between central neurons and endocrine cells

As a first step towards elucidating mechanisms involved in neuroendocrine synaptogenesis, we developed a model of co-culture based on hypothalamic-intermediate pituitary interactions. Dissociated hypothalamic neurons from fetal rats at embryonic day 15 were cultured in a defined medium together with melanotrope cells of the pituitary intermediate lobe from neonatal rats. In these co-cultures, establishment of synaptic contacts between GABAergic or dopaminergic neurons and an endocrine target cell the melanotrope cell, was studied by morphofunctional approaches. Using double immunostaining with antibodies directed against glutamate decarboxylase or tyrosine hydroxylase and alpha-melanocyte-stimulating hormone, we demonstrated morphological contacts between GABAergic or dopaminergic neurons and melanotrope cells as early as three days in vitro. Furthermore, using an antibody directed against synapsin I, we showed a modification of synapsin I immunoreactivity from diffuse to punctate distribution correlated with the establishment of contacts and the observation of characteristic neuroendocrine synapses by electron microscopy. These results were further confirmed by electrophysiological studies. Patch-clamp recordings demonstrated that, at six days in vitro, some melanotrope cells displayed GABAergic synaptic currents, which occurred either spontaneously and/or could be evoked chemically by 50 mM KCl or 100 microM kainate. The proportion of the melanotrope cells receiving functional synaptic inputs increased until 10 days in culture, a stage at which virtually all melanotrope cells in contact with neurons possessed functional synapses. The results presented here describe the establishment of neuroendocrine synapses in vitro, studied by combining morphofunctional and electrophysiological approaches.

Catecholaminergic innervation of the suprachiasmatic nucleus in the adult rat: ultrastructural relationships with neurons containing vasoactive intestinal peptide or vasopressin

Catecholaminergic fibers in the suprachiasmatic nucleus of adult rats were investigated by use of light- and electron-microscopic immunocytochemistry. The suprachiasmatic nucleus receives a modest density of tyrosine hydroxylase-containing axons, homogeneously distributed in the nucleus and forming varicosities throughout its entire rostro-caudal extension. Immunolabeling with antibodies against dopamine showed that this catecholamine input comprises a dopaminergic component. Many tyrosine hydroxylase-positive cells were localized at the immediate periphery of the suprachiasmatic nucleus. With electron-microscopic examination, dendrites of these neurons were found within the limits of the nucleus as well as at a border zone between the suprachiasmatic nucleus proper and the optic tract where they received unlabeled synapses, providing a morphological support for a possible role of dopaminergic neurons in the integration and/or transfer of light-related signals. More than 91% of catecholaminergic axonal varicosities were found to establish morphologically defined synapses with dendrites. To investigate whether these synapses might be shared with neurons of one or both of the two main peptidergic populations of the nucleus, namely vasoactive intestinal peptide- and vasopressin-containing neurons, we carried out double-labelling experiments combining immunoperoxidase and immunogold-silver labeling. Results showed only a few cases of direct association of the catecholaminergic terminals with these peptidergic categories. In both types of dually stained sections, catecholaminergic synapses were preferentially made with unlabeled dendrites. The homogeneous distribution of tyrosine hydroxylase-immunoreactive fibers in the suprachiasmatic nucleus could therefore reflect a lack of significant catecholaminergic innervation of both vasoactive intestinal peptide- and vasopressin-synthesizing neurons.

Development of catecholamine systems in the brain of the lizard Gallotia galloti

For a better insight into general and derived traits of developmental aspects of catecholaminergic (CA) systems in amniotes, we have studied the development of these systems in the brain of a lizard, Gallotia galloti, with tyrosine hydroxylase (TH)- and dopamine (DA) immunohistochemical techniques. Two main groups of TH-immunoreactive (THi) perikarya appear very early in development: one group in the midbrain which gives rise to the future ventral tegmental area, substantia nigra and retrorubral cell groups, and another group in the tuberomammillary hypothalamus. Somewhat later in development, TH/DA-immunoreactive cells are observed in the thalamus, rostrodorsal hypothalamus and spinal cord, and, with another delay, in the suprachiasmatic nucleus, the periventricular organ, and the pretectal posterodorsal nucleus. CA cell groups that appear rather late in development include the cells in the olfactory bulb, the locus coeruleus and the caudal brainstem. As expected, the development of immunoreactive fibers stays behind that of the cell bodies, but reaches the adult-like pattern just prior to hatching. The present study revealed considerable variation in the relation between the state of cytodifferentiation and first expression of TH/DA immunoreactivity between CA cell groups. Catecholamine cells in the midbrain and tuberomammillary hypothalamus are still migrating, immature (absence of dendrites) and express only TH immunoreactivity at the time of first detection. Cells which appear at later developmental stages lie already further away from the ventricle, possess two or more dendritic processes, and generally express both TH- and DA immunoreactivity.

Distribution of histamine-, 5-hydroxytryptamine-, and tyrosine hydroxylase-immunoreactive neurons and nerve fibers in developing rat brain

Although the general patterns of the developing histaminergic system in the rat brain are known, no comparative studies between the development of the brain histaminergic system and the development of other neuroactive substances have yet been published. Interestingly, separate immunohistochemical studies on the development of the 5-HT system and on the catecholaminergic system in the rat imply common features in the different aminergic systems. Therefore, the spatial distribution of histamine-immunoreactive (HA-ir) neurons and nerve fibers was compared to the distribution of 5-hydroxytryptamine (5-HT)-, and tyrosine hydroxylase-immunoreactive (TH-ir) ones in the developing rat brain between embryonic days 12 (E12) and 20 (E20) by using a double-immunostaining method. The high-pressure liquid chromatography (HPLC) fluorometric method was used for determination of histamine concentration in different brain regions during the same period of development and synthetic oligonucleotide probes complementary to the rat histidine decarboxylase (HDC) to determine the origin of HA in the brain during the development with in situ hybridization. The immunohistochemical results revealed co-localization of HA and 5-HT within a subgroup of cells in the developing raphe nuclei between E14 and E18. From E18 onwards HA immunoreactivity started to gradually disappear from the rhombencephalon, and was totally abolished by E20, while 5-HT-ir cells continued to establish their adult positions. No significant colocalization of HA and TH immunoreactivities was detected. The biochemical results were in agreement with the immunohistochemical ones and confirmed that histamine detected in the early developing brain is authentic. A positive in situ hybridization signal for HDC was detected in a small area in the ventrolateral pons in the same areas as HA- and HDC-ir cell bodies at E16, suggesting that at least some HA may be synthesized locally. These results confirm that HA is one of the first neurotransmitters to appear in the developing brain. In addition, the transient co-localization of HA and 5-HT immunoreactivities and the transient HDC expression at E16 within the developing pontine raphe nuclei may imply an interesting and a more general role for HA in modification of brain development.

Development of the suprachiasmatic nucleus in rats during ontogenesis: tyrosine hydroxylase immunopositive cell bodies and fibers

This study has evaluated differentiation of tyrosine hydroxylase-immunopositive neurons in the suprachiasmatic nucleus as well as the innervation of this nucleus by tyrosine hydroxylase-immunopositive axons in rats during ontogenesis. Tyrosine hydroxylase-containing structures were detected with electron-microscopic pre-embedding immunocytochemistry at the 22nd fetal day as well as at the second, ninth and 21st postnatal days. Rare uni- and bipolar small tyrosine-hydroxylase-immunopositive neurons were observed in the suprachiasmatic nucleus both in fetuses and postnatal rats. These neurons underwent differentiation over the perinatal period that was mainly manifested in the increase of their size as well as in the development of the Golgi complex, granular endoplasmic reticulum and the onset of the dense core vesicle production. Concomitantly, tyrosine hydroxylase-immunopositive neurons, cell bodies and dendrites, became innervated by immunonegative axons first making presynapses, and, then, symmetric (Gray-type II) and asymmetric (Gray-type I) synapses. In addition to cell bodies and dendrites, tyrosine hydroxylase-immunopositive axons were regularly observed in ventral, ventrolateral and ventromedial regions of the suprachiasmatic nucleus in fetuses and postnatal rats. Tyrosine hydroxylase-immunopositive axons were observed either in simple appositions with the immunonegative neurons or making presynapses in fetuses and symmetric and asymmetric synapses in postnatal animals. The nature of the tyrosine hydroxylase-immunopositive axons and the functional significance in the suprachiasmatic nucleus in ontogenesis are discussed.

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.

D1-dopamine receptors activate c-fos expression in the fetal suprachiasmatic nuclei

The existence of an activatable dopamine system within the hypothalamic suprachiasmatic nuclei (SCN), the site of a biological clock, was investigated in rats during fetal life. In situ hybridization studies revealed that D1-dopamine receptor mRNA was highly expressed in the fetal SCN and not expressed in other hypothalamic regions. Cocaine injected into pregnant rats or directly into rat fetuses on day 20 of gestation selectively activated c-fos gene expression in the fetal SCN; cocaine did not induce c-fos expression elsewhere in the fetal brain or in the maternal SCN. This cocaine-induced activation of c-fos expression in fetal SCN was mediated in part through D1-dopamine receptors, as the cocaine-induced activation was partially blocked by the D1-dopamine receptor antagonist SCH 23390. In addition, the selective D1-dopamine receptor agonist SKF 38393 induced high levels of c-fos expression in the fetal SCN. The presence of an activatable dopamine system within the fetal SCN provides a mechanism through which maternal signals could entrain the fetal biological clock and through which maternally administered psychotropic drugs could alter normal development of the circadian timing system.

Histamine and tyrosine hydroxylase in developing rat brain

The distribution of histamine and tyrosine hydroxylase in fetal rat brain was investigated immunocytochemically in order to determine possible colocalization of these two substances. Embryonic rat brains were fixed with carbodiimide and processed for immunofluorescence studies with antisera against histamine and tyrosine hydroxylase either in the same sections or in consecutive sections. Histamine and tyrosine hydroxylase showed no colocalisation in the developing rat brain. However, fibre networks immunoreactive for histamine and tyrosine hydroxylase were often found in the same areas. The results of the study suggest that the catecholaminergic and histaminergic neurones develop separately in the rat brain. Based on the location of developing histamine-immunoreactive neurones, a more intimate relationship between histamine- and serotonin-containing neurones in the developing rat brain is plausible.

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.

Somatostatin co-localizes with tyrosine hydroxylase in the nerve cells of discrete hypothalamic regions in rats

The co-expression of somatostatin (SOM)- and tyrosine hydroxylase (TH)-like immunoreactivities in nerve cells of the rat hypothalamus was investigated by the simultaneous application to the same sections of immuno-beta-galactosidase staining and the peroxidase-antiperoxidase (PAP) method. SOM-like immunoreactive cells stained blue with immuno-beta-galactosidase staining and TH-like immunoreactive cells stained brown with the PAP method. Double-labeled cells with overlapping blue and brown immunoreaction products were frequently identified in the preoptic periventricular nucleus (pope). These double-labeled cells were seen in clusters within the ventral half of the rostral pope. The periventricular hypothalamic nucleus at the level of the anterior hypothalamic nucleus contained only scattered nerve cells with both SOM- and TH-like immunoreactivities, despite the presence of many nerve cells immunoreactive for either SOM or TH in this nucleus. Double-labeled cells were also observed in some regions of the medial-basal hypothalamus, including the boundary between the ventromedial hypothalamic nucleus and the arcuate hypothalamic nucleus, and areas dorsal and lateral to the ventromedial hypothalamic nucleus. These findings may provide insight into the mechanisms underlying previously described catecholamine-mediated modulation of SOM release from the hypothalamus.

On the distribution and morpho-functional characteristics of 5-HT-immunoreactive cells in the hypothalamus of fetuses and neonatal rats

This study attempted to visualize serotonin (5-HT)-immunoreactive (IR) neurons in the hypothalamus of intact fetuses (E18) and neonatal rats (P9) as well as after their pretreatment with some drugs interfering with the 5-HT metabolism and uptake in the serotoninergic neurons (L-tryptophan, pargyline, 5-hydroxytryptophan, fluoxetine). The 5-HT-IR cells were not observed in the hypothalamus of normal, untreated fetuses and neonatal rats. However, two large accumulations of 5-HT-IR neurons appeared in the anterolateral hypothalamus and in the dorsomedial nucleus after the subsequent injections of the monoamine oxidase inhibitor, pargyline, and the amino acid precursor of the 5-HT synthesis, L-tryptophan. A significantly less intensive reaction was observed after injections either of the second precursor of the 5-HT synthesis, 5-hydroxytryptophan instead of L-tryptophan, or pargyline only. Immunostaining, provoked by the pargyline and L-tryptophan pretreatment, was completely blocked by the injection of the specific 5-HT uptake inhibitor, fluoxetine. It means that the 5-HT immunostaining of the hypothalamic neurons may be accounted for by their capacity to take up specifically 5-HT from the environment rather than by its intraneuronal synthesis from L-tryptophan. Nevertheless, the 5-HT synthesis from 5-hydroxytryptophan in these cells cannot be excluded. The uptake of extracellular 5-HT into catecholaminergic neurons can be excluded as nomifensine, the specific inhibitor of the uptake to these neurons, did not modify the immunostaining.

Ontogenesis of tyrosine hydroxylase-immunopositive structures in the rat hypothalamus. Fiber pathways and terminal fields

The innervation of the hypothalamus and septal region by catecholaminergic fibers was studied in rats from the 12th fetal day until the 9th postnatal day. Catecholaminergic fibers were visualized with preembedding immunocytochemistry using antibodies to tyrosine hydroxylase. An intensification of diaminobenzidine product with silver and gold was additionally applied to increase the sensitivity and resolution power of the routine immunocytochemical technique. It has been demonstrated that, from the 13th fetal day, the hypothalamus and the septal region receive catecholaminergic fibers either belonging to the hypothalamic neurons or coming with the medial forebrain bundle from the outside of the hypothalamus. As the development of the hypothalamus proceeds, these fibers form the extensive networks within some neurosecretory centers either containing (the zona incerta, periventricular nucleus, etc.) or almost lacking (suprachiasmatic and paraventricular nuclei) the catecholaminergic neurons. In the former case, they terminate on the processes or perikarya of catecholaminergic neurons, while in the latter case their varicosities surround the immunonegative presumptive neurons in a basket-like manner. Moreover, from the 18th fetal day catecholaminergic fibers penetrate between the ependymal cells towards the 3rd ventricle and the primary capillary plexus of the hypophysial portal circulation, apparently providing the release of catecholamines to the cerebrospinal fluid and portal blood, respectively. The data obtained in this study are considered as the morphological basis for the involvement of the hypothalamic catecholamines in neuroendocrine regulations during ontogenesis.