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

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.

Gestational Exposure to Sodium Valproate Disrupts Fasciculation of the Mesotelencephalic Dopaminergic Tract, With a Selective Reduction of Dopaminergic Output From the Ventral Tegmental Area

Gestational exposure to valproic acid (VPA) is known to cause behavioral deficits of sociability, matching similar alterations in human autism spectrum disorder (ASD). Available data are scarce on the neuromorphological changes in VPA-exposed animals. Here, we focused on alterations of the dopaminergic system, which is implicated in motivation and reward, with relevance to social cohesion. Whole brains from 7-day-old mice born to mothers given a single injection of VPA (400 mg/kg b.wt.) on E13.5 were immunostained against tyrosine hydroxylase (TH). They were scanned using the iDISCO method with a laser light-sheet microscope, and the reconstructed images were analyzed in 3D for quantitative morphometry. A marked reduction of mesotelencephalic (MT) axonal fascicles together with a widening of the MT tract were observed in VPA treated mice, while other major brain tracts appeared anatomically intact. We also found a reduction in the abundance of dopaminergic ventral tegmental (VTA) neurons, accompanied by diminished tissue level of DA in ventrobasal telencephalic regions (including the nucleus accumbens (NAc), olfactory tubercle, BST, substantia innominata). Such a reduction of DA was not observed in the non-limbic caudate-putamen. Conversely, the abundance of TH+ cells in the substantia nigra (SN) was increased, presumably due to a compensatory mechanism or to an altered distribution of TH+ neurons occupying the SN and the VTA. The findings suggest that defasciculation of the MT tract and neuronal loss in VTA, followed by diminished dopaminergic input to the ventrobasal telencephalon at a critical time point of embryonic development (E13-E14) may hinder the patterning of certain brain centers underlying decision making and sociability.

Postnatal genistein administration selectively abolishes sexual dimorphism in specific hypothalamic dopaminergic system in mice

As demonstrated in previous studies, early postnatal genistein (GEN) administration to mice pups of both sexes, at doses similar to that of infant soy-based formulas, may affect the development of some steroid-sensitive neuronal circuits (i.e. nitrergic and vasopressinergic systems), causing irreversible alterations in adults. Here, we investigated the hypothalamic and mesencephalic dopaminergic system (identified with tyrosine hydroxylase immunohistochemistry). GEN administration (50 mg/kg) to mice of both sexes during the first week of postnatal life specifically affected tyrosine hydroxylase immunohistochemistry in the hypothalamic subpopulation of neurons, abolishing their sexual dimorphism. On the contrary, we did not observe any effects in the mesencephalic groups. Due to the large involvement of dopamine in circuits controlling rodent sexual behavior and food intake, these results clearly indicate that the early postnatal administration of GEN may irreversibly alter the control of reproduction, of energetic metabolism, and other behaviors. These results suggest the need for a careful evaluation of the use of soy products in both human and animal newborns.

Tyrosine-hydroxylase immunoreactivity in the mouse transparent brain and adrenal glands

Working on catecholamine systems for years, the neuropharmacologist Arvid Carlsson has made a number of important and pioneering discoveries, which have highlighted the key role of these neuronal and peripheral neurotransmitters in brain functions and adrenal regulations. Since then, major advances have been made concerning the distribution of the catecholaminergic systems in particular by studying their rate-limiting enzyme, tyrosine hydroxylase (TH). Recently new methods of tissue transparency coupled with in toto immununostaining and three-dimensional (3D) imaging technologies allow to precisely map TH immunoreactive pathways in the mouse brain and adrenal glands. High magnification images and movies obtained with combined technologies (iDISCO+ and light-sheet microscopy) are presented in this review dedicated to the pioneer work of Arvid Carlsson and his collaborators.

Von Economo Neurons and Fork Cells: A Neurochemical Signature Linked to Monoaminergic Function

The human anterior cingulate and frontoinsular cortices are distinguished by 2 unique Layer 5 neuronal morphotypes, the von Economo neurons (VENs) and fork cells, whose biological identity remains mysterious. Insights could impact research on diverse neuropsychiatric diseases to which these cells have been linked. Here, we leveraged the Allen Brain Atlas to evaluate mRNA expression of 176 neurotransmitter-related genes and identified vesicular monoamine transporter 2 (VMAT2), gamma-aminobutyric acid (GABA) receptor subunit θ (GABRQ), and adrenoreceptor α-1A (ADRA1A) expression in human VENs, fork cells, and a minority of neighboring Layer 5 neurons. We confirmed these results using immunohistochemistry or in situ hybridization. VMAT2 and GABRQ expression was absent in mouse cerebral cortex. Although VMAT2 is known to package monoamines into synaptic vesicles, in VENs and fork cells its expression occurs in the absence of monoamine-synthesizing enzymes or reuptake transporters. Thus, VENs and fork cells may possess a novel, uncharacterized mode of cortical monoaminergic function that distinguishes them from most other mammalian Layer 5 neurons.

Three-dimensional distribution of tyrosine hydroxylase, vasopressin and oxytocin neurones in the transparent postnatal mouse brain

Over the years, advances in immunohistochemistry techniques have been a critical step in detecting and mapping neuromodulatory substances in the central nervous system. The better quality and specificity of primary antibodies, new staining procedures and the spectacular development of imaging technologies have allowed such progress. Very recently, new methods permitting tissue transparency have been successfully used on brain tissues. In the present study, we combined whole-mount immunostaining for tyrosine hydroxylase (TH), oxytocin (OXT) and arginine vasopressin (AVP), with the iDISCO+ clearing method, light-sheet microscopy and semi-automated counting of three-dimensionally-labelled neurones to obtain a (3D) distribution of these neuronal populations in a 5-day postnatal (P5) mouse brain. Segmentation procedure and 3D reconstruction allowed us, with high resolution, to map TH staining of the various catecholaminergic cell groups and their ascending and descending fibre pathways. We show that TH pathways are present in the whole P5 mouse brain, similar to that observed in the adult rat brain. We also provide new information on the postnatal distribution of OXT and AVP immunoreactive cells in the mouse hypothalamus, and show that, compared to AVP neurones, OXT neurones in the supraoptic (SON) and paraventricular (PVN) nuclei are not yet mature in the early postnatal period. 3D semi-automatic quantitative analysis of the PVN reveals that OXT cell bodies are more numerous than AVP neurones, although their immunoreactive soma have a volume half smaller. More AVP nerve fibres compared to OXT were observed in the PVN and the retrochiasmatic area. In conclusion, the results of the present study demonstrate the utility and the potency of imaging large brain tissues with clearing procedures coupled to novel 3D imaging technologies to study, localise and quantify neurotransmitter substances involved in brain and neuroendocrine functions.

Estrogen-dependent effects of 5-hydroxytryptophan on cortical spreading depression in rat: Modelling the serotonin-ovarian hormone interaction in migraine aura

Background Cortical spreading depression (CSD) is the likely culprit of the migraine aura. Migraine is sexually dimorphic and thought to be a "low 5-HT" condition. We sought to decipher the interrelation between serotonin, ovarian hormones and cortical excitability in a model of migraine aura. Methods Occipital KCl-induced CSDs were recorded for one hour at parieto-occipital and frontal levels in adult male (n = 16) and female rats (n = 64) one hour after intraperitoneal (i.p.) injection of 5-hydroxytryptophan (5-HTP) or NaCl. Sixty-five oophorectomized females were treated with estradiol- (E2) or cholesterol- (Chol) filled capsules. Two weeks later we recorded CSDs after 5-HTP/NaCl injections before or 20 hours after capsule removal. Results 5-HTP had no effect in males, but decreased CSD frequency in cycling females, significantly so during estrus, at parieto-occipital (-3.5CSD/h, p < 0.001) and frontal levels (-2.5CSD/h, p = 0.014). In oophorectomized rats, CSD susceptibility increased during E2 treatment at both recording sites (+5CSD/h, p = 0.001 and +3CSD/h, p < 0.01), but decreased promptly after E2 withdrawal (-4.7CSD/h, p < 0.001 and -1.7CSD/h, p = 0.094). The CSD inhibitory effect of 5-HTP was significant only in E2-treated rats (-3.4CSD/h, p = 0.006 and -1.8CSD/h, p = 0.029). Neither the estrous cycle phase, nor E2 or 5-HTP treatments significantly modified CSD propagation velocity. Conclusion 5-HTP decreases CSD occurrence in the presence of ovarian hormones, suggesting its potential efficacy in migraine with aura prophylaxis in females. Elevated E2 levels increase CSD susceptibility, while estrogen withdrawal decreases CSD. In a translational perspective, these findings may explain why migraine auras can appear during pregnancy and why menstrual-related migraine attacks are rarely associated with an aura.

Predator odor exposure of rat pups has opposite effects on play by juvenile males and females

Juvenile social play behavior is one of the earliest sexually differentiated behaviors to emerge. In rats, as with most other species that play, males engage in more rough-and-tumble play compared to females. Exposure to early life adversity is a major driver of adult health and can manifest differently in males and females. However, the effects of adverse early life exposure on play behavior in the juvenile period are poorly understood. To address this, male and female neonatal rats were exposed to predator odor (PO), for 5min/day on PN1-PN3. At the time of exposure to PO, both male and female pups suppressed ultrasonic vocalization and displayed more freezing behavior. Circulating corticosterone increased in males immediately following PO exposure but not in females. The enduring effects of PO exposure were opposite in males compared to females in that PO exposed males decreased social play, while PO exposed females increased play behavior compared to same sex controls. PO exposure did not significantly affect cell genesis in the neonatal dentate gyrus of either sex. PO exposure did not affect anxiety-like behavior assessed in the juvenile period or in adulthood, nor did it affect social interactions in adulthood. This work provides new insight into how sex may interact with adverse early life events to contribute to development of the social consequences of such exposures.

Reversible Pharmacological Induction of Motor Symptoms in MPTP-Treated Mice at the Presymptomatic Stage of Parkinsonism: Potential Use for Early Diagnosis of Parkinson's Disease

A crucial event in the pathogenesis of Parkinson's disease is the death of dopaminergic neurons of the nigrostriatal system, which are responsible for the regulation of motor function. Motor symptoms first appear in patients 20-30 years after the onset of the neurodegeneration, when there has been a loss of an essential number of neurons and depletion of compensatory reserves of the brain, which explains the low efficiency of treatment. Therefore, the development of a technology for the diagnosing of Parkinson's disease at the preclinical stage is of a high priority in neurology. In this study, we have developed at an experimental model a fundamentally novel for neurology approach for diagnosis of Parkinson's disease at the preclinical stage. This methodology, widely used for the diagnosis of chronic diseases in the internal medicine, is based on the application of a challenge test that temporarily increases the latent failure of a specific functional system, thereby inducing the short-term appearance of clinical symptoms. The provocation test was developed by a systemic administration of α-methyl-p-tyrosine (αMpT), a reversible inhibitor of tyrosine hydroxylase to MPTP-treated mice at the presymptomatic stage of parkinsonism. For this, we first selected a minimum dose of αMpT, which caused a decrease of the dopamine level in the striatum of normal mice below the threshold at which motor dysfunctions appear. Then, we found the maximum dose of αMpT at which a loss of dopamine in the striatum of normal mice did not reach the threshold level, and motor behavior was not impaired. We showed that αMpT at this dose induced a decrease of the dopamine concentration in the striatum of MPTP-treated mice at the presymptomatic stage of parkinsonism below a threshold level that results in the impairment of motor behavior. Finally, we proved that αMpT exerts a temporal and reversible influence on the nigrostriatal dopaminergic system of MPTP-treated mice with no long-term side effects on other catecholaminergic systems. Thus, the above experimental data strongly suggest that αMpT-based challenge test might be considered as the provocation test for Parkinson's disease diagnosis at the preclinical stage in the future clinical trials.

Neurons expressing individual enzymes of dopamine synthesis in the mediobasal hypothalamus of adult rats: functional significance and topographic interrelations

Besides dopaminergic (DA-ergic) neurons having all enzymes of DA synthesis, tyrosine hydroxylase (TH) and aromatic l-amino acid decarboxylase (AADC), "monoenzymatic" neurons expressing only one of them were found in the brain, mostly in the mediobasal hypothalamus (MBH). The aim of this study was to test our hypothesis that DA is synthesized by monoenzymatic neurons, i.e. l-3,4-dihydroxyphenylalanine (l-DOPA), which produced in the monoenzymatic TH neurons is transported in the monoenzymatic AADC neurons for DA synthesis. Incubation of MBH in Krebs-Ringer solution with l-leucine, a competitive inhibitor of l-DOPA uptake, was used to prevent a hypothetical l-DOPA capture into AADC-containing neurons. Incubation of the substantia nigra containing DA-ergic neurons under the same conditions served as the control. According to our data, the l-leucine administration provoked a decrease of DA concentration in MBH and in the incubation medium but not in the substantia nigra and respective incubation medium, showing a decrease of cooperative synthesis of DA in MBH. This conclusion was supported by an observation of higher concentration of l-DOPA in the incubation medium under perfusion of MBH with Krebs-Ringer solution containing tolcapone, an inhibitor of catechol-O-methyltransferase, and l-leucine than under perfusion with the same solution, but without l-leucine. Functional interaction between monoenzymatic TH and AADC neurons was indirectly confirmed by finding in electron microscopy their close relations in MBH. Besides monoenzymatic AADC neurons, any AADC-possessing neurons, catecholaminergic and serotoninergic, apparently, could participate in DA synthesis together with monoenzymatic TH neurons. This idea was confirmed by the observation of close topographic relations between monoenzymatic TH neurons and those containing both enzymes, i.e. DA-ergic, noradrenergic or adrenergic. Thus, monoenzymatic neurons possessing TH or AADC and being in close topographic relations can synthesize DA in cooperation.

Dopamine-2 receptor activation suppresses PACAP expression in gonadotrophs

Pituitary adenylate cyclase-activating polypeptide (PACAP) is expressed at a high level in the fetal pituitary and decreases profoundly between embryonic day 19 and postnatal day 1 (PN1), with a further decrease from PN1 to PN4. In this series of experiments, we investigated the hypothesis that dopamine 2 receptor (Drd2) activation interrupts a cAMP-dependent feed-forward loop that maintains PACAP expression at a high level in the fetal pituitary. Using single-cell RT-PCR of pituitary cell cultures from newborn rats, Drd2 mRNA was identified in gonadotrophs that were also positive for PACAP mRNA. PACAP expression in pituitary cultures from embryonic day 19 rats was suppressed by the PACAP6-38 antagonist and by the Drd2 agonist bromocriptine. Increasing concentrations of bromocriptine inhibited cAMP production as well as cAMP signaling based on cAMP response element-luciferase activity, decreased PACAP promoter activity, and decreased PACAP mRNA levels in αT3-1 gonadotroph cells. Furthermore, blockade of dopamine receptors by injecting haloperidol into newborn rat pups partially reversed the developmental decline in pituitary PACAP mRNA that occurs between PN1 and PN4. These results provide evidence that dopamine receptor signaling regulates PACAP expression under physiological conditions and lend support to the hypothesis that a rise in hypothalamic dopamine at birth abrogates cAMP signaling in fetal gonadotrophs to interrupt a feed-forward mechanism that maintains PACAP expression at a high level in the fetal pituitary. We propose that this perinatal decline in pituitary PACAP reduces pituitary follistatin which permits GnRH receptors and FSH-β to increase to facilitate activation of the neonatal gonad.

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.

Localization and expression of VMAT2 aross mammalian species: a translational guide for its visualization and targeting in health and disease

VMAT2 is the vesicular monoamine transporter that allows DA, NE, Epi, His, and 5-HT uptake into neurons and endocrine cells. A second isoform, VMAT1, has similar structure and function, but does not recognize histamine as a substrate. VMAT1 is absent from neurons, and its major function appears to be in endocrine cells, that is, enterochromaffin cells, which scavenge 5-HT, but not histamine, from dietary sources. This chapter provides an update on the neuroanatomical distribution of VMAT2 across various mammalian species, including human, primate, pig, rat, and mouse. When necessary, VMAT1 expression is provided as a contrast. The main purpose of this chapter is to allow clinicians, in particular endocrinologists and diagnosing neuroradiologists and neuropathologists, an acquaintanceship with the possibilities for VMAT2 as a target for in vivo imaging, and drug development, based on this updated information.

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.

Brain sex differences and the organisation of juvenile social play behaviour

Juvenile social play behaviour is one of the earliest forms of non-mother directed social behaviour in rodents. Juvenile social play behaviour is sexually dimorphic, with males exhibiting higher levels compared to females, making it a useful model to study both social development and sexual differentiation of the brain. As with most sexually dimorphic behaviour, juvenile play behaviour is organised by neonatal steroid hormone exposure. The developmental organisation of juvenile play behaviour also appears to be influenced by the early maternal environment. This review will focus briefly on why and how rats play, some brain regions controlling play behaviour, and how neurotransmitters and the social environment converge within the developing brain to influence sexual differentiation of juvenile play behaviour.

[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.

Regulation of progestin receptor expression in the developing rat brain by a dopamine d1 receptor antagonist

Steroid receptors within the developing brain influence a variety of cellular processes that endure into adulthood, altering both behaviour and physiology. Therefore, it is important to understand how steroid receptor expression is regulated during early brain development. Most studies indicate that oestradiol, by acting upon oestrogen receptors, increases the expression of progestin receptors in the developing brain. We have recently observed an additional mechanism by which dopamine can increase the expression of progestin receptors in developing female rat brain. That is, we found that a dopamine D1 receptor agonist can further increase progestin receptor expression by activating oestrogen receptors in a ligand-independent manner within restricted areas of female brain; however, it is unclear whether dopamine D1 receptors are involved in the normally occurring expression of progestin receptors in developing male and female brain. To investigate this, we examined whether a dopamine D1 receptor antagonist can disrupt the normal developmental expression of progestin receptors in both male and female rat brain. We report that treatment with a dopamine D1 receptor antagonist reduces progestin receptor expression within some, but not all, regions of the developing rat brain in a sex-specific manner. Some of the current findings also suggest that dopamine might be acting to prevent sex differences in progestin receptor expression in some areas while contributing to a sex difference in other areas.

Three types of tyrosine hydroxylase-positive CNS neurons distinguished by dopa decarboxylase and VMAT2 co-expression

1. We investigate here for the first time in primate brain the combinatorial expression of the three major functionally relevant proteins for catecholaminergic neurotransmission tyrosine hydroxylase (TH), aromatic acid acid decarboxylase (AADC), and the brain-specific isoform of the vesicular monoamine transporter, VMAT2, using highly specific antibodies and immunofluorescence with confocal microscopy to visualize combinatorial expression of these proteins. 2. In addition to classical TH, AADC, and VMAT2-copositive catecholaminergic neurons, two unique kinds of TH-positive neurons were identified based on co-expression of AADC and VMAT2. 3. TH and AADC co-positive, but VMAT2-negative neurons, are termed "nonexocytotic catecholaminergic TH neurons." These were found in striatum, olfactory bulb, cerebral cortex, area postrema, nucleus tractus solitarius, and in the dorsal motor nucleus of the vagus. 4. TH-positive neurons expressing neither AADC nor VMAT2 are termed "dopaergic TH neurons." We identified these neurons in supraoptic, paraventricular and periventricular hypothalamic nuclei, thalamic paraventicular nucleus, habenula, parabrachial nucleus, cerebral cortex and spinal cord. We were unable to identify any dopaergic (TH-positive, AADC-negative) neurons that expressed VMAT2, suggesting that regulatory mechanisms exist for shutting off VMAT2 expression in neurons that fail to biosynthesize its substrates. 5. In several cases, the corresponding TH phenotypes were identified in the adult rat, suggesting that this rodent is an appropriate experimental model for further investigation of these TH-positive neuronal cell groups in the adult central nervous system. Thus, no examples of TH and VMAT2 co-positive neurons lacking AADC expression were found in rodent adult nervous system. 6. In conclusion, the adult mammalian nervous system contains in addition to classical catecholaminergic neurons, cells that can synthesize dopamine, but cannot transport and store it in synaptic vesicles, and neurons that can synthesize only L-dopa and lack VMAT2 expression. The presence of these additional populations of TH-positive neurons in the adult primate CNS has implications for functional catecholamine neurotransmission, its derangement in disease and drug abuse, and its rescue by gene therapeutic maneuvers in neurodegenerative diseases such as Parkinson's disease.

Dopamine Synthesis by Non-Dopaminergic Neurons in the Arcuate Nucleus of Rat Fetuses

The aim of the present work was to verify the hypothesis that non-dopaminergic neurons expressing individual complementary dopamine synthesis enzymes can perform the co-located synthesis of dopamine. According to this hypothesis, neurons expressing tyrosine hydroxylase use L-tyrosine for the synthesis of L-dihydroxyphenylalanine (L-DOPA), which then enters neurons expressing aromatic amino acid decarboxylase, which converts L-DOPA to dopamine. Experiments were performed using the mediobasal hypothalamus of rat fetuses, which mostly contains single-enzyme neurons (>99%) and occasional double-enzyme neurons (<1%). Controls were obtained from the fetal substantia nigra, which is enriched with dopaminergic neurons. High-performance liquid chromatography was used to measure levels of dopamine and L-DOPA in cell extracts and the incubation medium after incubation in the presence and absence of exogenous L-tyrosine. Addition of L-tyrosine to the medium led to increases in the level of synthesis and release of L-DOPA in the mediobasal hypothalamus and substantia nigra. In addition, L-tyrosine increased dopamine synthesis in the substantia nigra and decreased dopamine synthesis in the mediobasal hypothalamus. This regional difference in levels of dopamine synthesis is probably due to inhibition of the uptake of L-DOPA from the intercellular medium by neurons in the mediobasal hypothalamus containing aromatic amino acid decarboxylase, due to the competitive binding of the L-DOPA transporter by L-tyrosine. Thus, these results provide the first evidence for the co-located synthesis of dopamine by non-dopaminergic neurons expressing single complementary enzymes involved in the synthesis of this neurotransmitter.

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.

Progesterone receptor and dopamine synthesizing enzymes in hypothalamic neurons of the guinea pig: an immunohistochemical triple-label analysis

Interactions among gonadal steroid hormones and the dopamine synthesizing enzymes, tyrosine hydroxylase (TH) or aromatic L-amino acid decarboxylase (AADC), participate in hypothalamic functions. Several findings suggest that the expression patterns of the progesterone receptor (PR), TH and AADC overlap in the guinea pig brain. However, it remained to be determined whether or not these two enzymes coexist in the same neurons which contain the PR. To test this hypothesis and quantify these colocalization relationships in the hypothalamus, we used a triple-labeling immunofluorescence procedure. Only PR/AADC-immunoreactive cells were seen in the preoptic area but no PR/TH cells and, therefore, no triple immunoreactive cells were found. An occasional colocalization between PR and the two enzymes was observed throughout the rostrocaudal extent of the arcuate nucleus with the greatest concentration of triple-labeled cells in the medial subdivision. In this region, quantitative estimation of cellular immunoreactivity showed that the triple immunoreactive cells represented about 29% of PR/TH cells, 9% of PR/AADC cells and 22% of TH/AADC cells in spite of a very low percentage in relation to total populations of neurons expressing only PR, TH or AADC. Thus, the PR are only present in monoenzymatic AADC expressing neurons in the preoptic area while they can be observed in neurons expressing both enzymes in the arcuate nucleus.

The vesicular amine transporter family (SLC18): amine/proton antiporters required for vesicular accumulation and regulated exocytotic secretion of monoamines and acetylcholine

The vesicular amine transporters (VATs) are expressed as integral proteins of the lipid bilayer membrane of secretory vesicles in neuronal and endocrine cells. Their function is to allow the transport of acetylcholine (by the vesicular acetylcholine transporter VAChT; SLC18A3) and biogenic amines (by the vesicular monoamine transporters VMAT1 and VMAT2; SLC18A1 and SLC18A2) into secretory vesicles, which then discharge them into the extracellular space by exocytosis. Transport of positively charged amines by members of the SLC18 family in all cases utilizes an electrochemical gradient across the vesicular membrane established by proton pumping into the vesicle via a vacuolar ATPase; the amine is accumulated in the vesicle at the expense of the proton gradient, at a ratio of one translocated amine per two translocated protons. The members of the SLC18 family have become important histochemical markers for chemical coding in neuroendocrine tissues and cells. The structural basis of their remarkable ability to transport positively charged amines against a very large concentration gradient, as well as potential disease association with impaired transporter function and expression, are under intense investigation.

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.

Influence of serotonin on the development and migration of gonadotropin-releasing hormone neurones in rat foetuses

This study used a pharmacological approach to evaluate the consequences of the metabolic perturbations of neurotransmitters on brain development. Pregnant rats received p-chlorophenylalanine (pCPA), an inhibitor of serotonin (5-hydroxytryptamine, 5-HT) synthesis, or saline (control) from the 11th day of gestation once or daily up to the 15th, 17th and 20th day, followed by processing of the forebrain and/or nasal cranium of foetal males and females for high-performance liquid chromatography of monoamines, radioimmunoassay of gonadotropin-releasing hormone (GnRH) and quantitative and semiquantitative immunocytochemistry for GnRH. The pCPA treatment resulted in a 50-70% depletion of 5-HT in the nasal crania and forebrains at any studied age. Radioimmunoassay showed no change in GnRH content in 5-HT deficient foetuses at E16 compared to controls, being higher in both cases in the rostral forebrain than in the hypothalamus. In controls at E21, the GnRH content in the hypothalamus exceeded that in the rostral forebrain, whereas in the 5-HT deficient group the opposite was found. These data suggest that 5-HT provided a stimulating effect on GnRH neurone migration, and this was confirmed by quantification of GnRH-immunoreactive neurones in the forebrain along the trajectory of their migration. At E18 and E21, the fractions of GnRH neurones in the rostral part of the trajectory in pCPA-treated foetuses were greater than those in control foetuses but the opposite was true for the caudal part of the trajectory. Moreover, 5-HT appeared to control the proliferation of the precursor cells of GnRH neurones and their differentiation, as derived from the observations of the increased number of GnRH neurones in the forebrain of foetuses of both sexes, as well as the region-specific decreased neuronal size and content of GnRH in 5-HT-deficient females. Thus, 5-HT appears to contribute to the regulation of the origin, differentiation and migration of GnRH neurones.

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.

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%.

Neuronal promoter of human aromatic L-amino acid decarboxylase gene directs transgene expression to the adult floor plate and aminergic nuclei induced by the isthmus

In order to analyze the regulatory sequences involved in the neuronal expression of aromatic L-amino acid decarboxylase (AADC), we have generated transgenic mice carrying the LacZ gene under the control of a 3.6-kb human aadc genomic fragment flanking the neuronal alternative first exon. A series of double labeling experiments were performed to compare the pattern of transgene expression to that of specific markers for catecholaminergic and serotonergic neurons. In the adult brain parenchyma, transgene expression was observed in the substantia nigra (SN), the ventral tegmental area (VTA) and the dorsal, medial and pontine raphe nuclei. A large degree of co-expression was observed with tyrosine-hydroxylase (TH) in the SN and VTA, and with serotonin (5-HT) in the dorsal raphe nucleus. Moreover, expression was observed in cells that were both TH- and 5-HT-negative, in particular in the ventral tegmental decussation and the dorsal tip of the VTA. Transgene expression was also observed in the walls of central cavities. Cells positive for both beta-gal and PSA-NCAM were localized in the ventral ependyma of the third and fourth ventricle, and of the central canal of the spinal cord, in what appears to be the adult floor plate. Transgene expressing, PSA-NCAM negative, cells located along the ventral midline of the spinal cord seemed to have migrated out of the ependyma. Our data thus reveal the complexity of aadc gene regulation. The present transgene provides a unique marker for monoaminergic nuclei induced by the isthmus and for the adult floor plate.

Dopamine as a prolactin (PRL) inhibitor

Dopamine is a small and relatively simple molecule that fulfills diverse functions. Within the brain, it acts as a classical neurotransmitter whose attenuation or overactivity can result in disorders such as Parkinson's disease and schizophrenia. Major advances in the cloning and characterization of biosynthetic enzymes, transporters, and receptors have increased our knowledge regarding the metabolism, release, reuptake, and mechanism of action of dopamine. Dopamine reaches the pituitary via hypophysial portal blood from several hypothalamic nerve tracts that are regulated by PRL itself, estrogens, and several neuropeptides and neurotransmitters. Dopamine binds to type-2 dopamine receptors that are functionally linked to membrane channels and G proteins and suppresses the high intrinsic secretory activity of the pituitary lactotrophs. In addition to inhibiting PRL release by controlling calcium fluxes, dopamine activates several interacting intracellular signaling pathways and suppresses PRL gene expression and lactotroph proliferation. Thus, PRL homeostasis should be viewed in the context of a fine balance between the action of dopamine as an inhibitor and the many hypothalamic, systemic, and local factors acting as stimulators, none of which has yet emerged as a primary PRL releasing factor. The generation of transgenic animals with overexpressed or mutated genes expanded our understanding of dopamine-PRL interactions and the physiological consequences of their perturbations. PRL release in humans, which differs in many respects from that in laboratory animals, is affected by several drugs used in clinical practice. Hyperprolactinemia is a major neuroendocrine-related cause of reproductive disturbances in both men and women. The treatment of hyperprolactinemia has greatly benefited from the generation of progressively more effective and selective dopaminergic drugs.

Differential expression of catecholamine synthetic enzymes in the caudal ventral pons

The analysis of colocalization of multiple catecholamine biosynthetic enzymes within the ventrolateral part of the medulla oblongata of the rat revealed distinct subpopulations of neurons within the C1 region (Phillips et al., J Comp Neurol 2001, 432:20-34). In extending this study to include the caudal pons, it was shown for the first time that the A5 cell group could be distinguished by the presence of immunoreactivity to tyrosine hydroxylase (TH), aromatic l-amino acid decarboxylase (AADC), and dopamine beta hydroxylase (DBH). A novel cell group was also identified. The cells within this new group were immunoreactive to DBH but not TH, AADC, or phenylethanolamine N-methyltransferase (PNMT) and will be referred to as the TH-, DBH+ cell group. The TH-, DBH+ neurons were not immunoreactive for either the dopamine or noradrenaline transporters, suggesting that these neurons do not take up these transmitters. A5 neurons were immunoreactive for the noradrenaline transporter but not the dopamine transporter (as previously shown). Retrograde tracing with cholera toxin B revealed that the TH-, DBH+ neurons do not project to the thoracic spinal cord or to the rostral ventrolateral medulla, but A5 neurons do. A calbindin immunoreactive cell group is located in a region overlapping TH-, DBH+ cell group. However, only a few neurons were immunoreactive for both markers. The physiological role of the TH-, DBH+ cell group remains to be determined.

Effect of neonatal treatment with monosodium glutamate on dopaminergic and L-DOPA-ergic neurons of the medial basal hypothalamus and on prolactin and MSH secretion of rats

The effect of neonatal treatment with monosodium L-glutamate (MSG) on the dopaminergic systems of the medial basal hypothalamus has been investigated using tyrosine hydroxylase (TH) and aromatic L-amino acid decarboxylase (AADC) immunocytochemistry. Changes in plasma levels of prolactin (PRL) and alpha-melanocyte-stimulating hormone (MSH) have also been determined in intact and in MSG-treated rats after inhibition of TH by alpha-methyl-p-tyrosine (alpha-MpT) or without inhibition of enzyme activity. Monosodium glutamate resulted in a 40% reduction in the number of TH immunopositive tuberoinfundibular neurons, but no change in the number of AADC-positive tuberoinfundibular nerve cells, indicating that this reduction has occurred mainly in TH-positive but AADC-negative elements, i.e., in L-DOPA-ergic neurons. In contrast, MSG did not cause changes in the number of TH and AADC immunoreactive neurons of the periventriculohypophysial and tuberohypophysial dopaminergic systems, and it did not influence basal plasma PRL levels. alpha-methyl-p-tyrosine has increased plasma PRL concentrations in both control and MSG-treated rats of both sexes, but significantly higher responses were detected in females. None of the treatments had any effect on plasma MSH level. These findings suggest that MSG affects primarily L-DOPA-ergic neurons located in the ventrolateral part of the arcuate nucleus, but not dopaminergic neurons situated in the dorsomedial part of the arcuate nucleus; neither PRL nor MSH secretion is altered by MSG; a significant sex difference exists in the pituitary PRL response to inhibition of TH, and this response is not affected by MSG.

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.