Catecholamine-containing neurons in the sheep brainstem and diencephalon: immunohistochemical study with tyrosine hydroxylase (TH) and dopamine-beta-hydroxylase (DBH) antibodies

Differential voluntary feed intake and whole transcriptome profiling in the hypothalamus of young sheep offered CP and phosphorus-deficient diets

A reduction in voluntary feed intake is observed in ruminants consuming nutrient-deficient diets, such as those with a low CP or P content, and has been attributed to active metabolic regulation, rather than a physical constraint. The hypothalamus is the key integrator of feed intake regulation in mammals. The objectives of this experiment were to (1) establish a model of metabolic feed intake regulation in ruminants consuming diets of variable CP and P content, and (2) determine key biochemical pathways and influential points of regulation within the hypothalamus. Merino wethers [n = 40; 23.7 ± 1.4 kg liveweight (mean ± SD)] were fed one of five dietary treatments (n = 8/treatment) for 63 days in individual pens. The treatments included targeted combinations of high (H) and low (L) CP (110 and 55 g/kg DM) and high and low P (2.5 and 0.7 g/kg DM) with 9 MJ metabolisable energy (ME) per kg DM which were fed ad libitum (UMEI; unrestricted ME intake) resulting in four experimental diets (HCP-HP-UMEI, LCP-HP-UMEI, HCP-LP-UMEI and LCP-LP-UMEI). An additional nutritional treatment (HCP-HP-RMEI) restricted intake of the HCP-HP diet to an equivalent ME intake of wethers consuming the LCP-LP-UMEI treatment. Wethers offered the LCP-HP-UMEI, HCP-LP-UMEI and LCP-LP-UMEI treatments consumed 42, 32 and 49% less total DM (P ≤ 0.05), respectively than the HCP-HP-UMEI treatment, and this was not attributable to any physical limitation of the rumen. Plasma concentrations of urea nitrogen and inorganic phosphate indicated that these nutrient deficiencies were successfully established. To assess potential mechanisms, RNA-seq was conducted on samples from the arcuate nucleus (ARC), ventromedial hypothalamus and lateral hypothalamus of the wethers, yielding a total of 301, 8 and 148 differentially expressed genes across all pairwise comparisons, respectively. The expression of NPY, AGRP and CARTPT, known for their regulatory role in mammalian feed intake regulation, had a similar transcriptional response in the ARC of wethers consuming nutrient-deficient treatments and those consuming a ME-restricted treatment, despite these wethers expressing behaviours indicative of satiated and hungry states, respectively. In addition, genes involved with glycolysis (TPI1), the citric acid cycle (CS, OGDH, GLUD1, GOT1) and oxidative phosphorylation (COX5A, ATP5MC1, ATP5F1B, ATP5MC3) were downregulated in the ARC of wethers fed a nutrient deficient (LCP-LP-UMEI) relative to the non-deficient (HCP-HP-UMEI) treatment. In summary, a model for voluntary feed intake restriction was established to determine genome-wide molecular changes in the hypothalamus of young ruminants.

Seasonality of prolactin in birds and mammals

In most animals, annual rhythms in environmental cues and internal programs regulate seasonal physiology and behavior. Prolactin, an evolutionarily ancient hormone, serves as a molecular correlate of seasonal timing in most species. Prolactin is highly pleiotropic with a wide variety of well-documented physiological effects; in a seasonal context prolactin is known to regulate annual changes in pelage and molt. While short-term homeostatic variation of prolactin secretion is under the control of the hypothalamus, long-term seasonal rhythms of prolactin are programmed by endogenous timers that reside in the pituitary gland. The molecular basis of these rhythms is generally understood to be melatonin dependent in mammals. Prolactin rhythmicity persists for several years in many species, in the absence of hypothalamic signaling. Such evidence in mammals has supported the hypothesis that seasonal rhythms in prolactin derive from an endogenous timer within the pituitary gland that is entrained by external photoperiod. In this review, we describe the conserved nature of prolactin signaling in birds and mammals and highlight its role in regulating multiple diverse physiological systems. The review will cover the current understanding of the molecular control of prolactin seasonality and propose a mechanism by which long-term rhythms may be generated in amniotes.

The Mammalian Locus Coeruleus Complex-Consistencies and Variances in Nuclear Organization

Descriptions of the nuclear parcellation of the locus coeruleus complex have been provided in approximately 80 mammal species spanning the phylogenetic breadth of this class. Within the mammalian rostral hindbrain, noradrenergic neurons (revealed with tyrosine hydroxylase and dopamine-ß-hydroxylase immunohistochemistry) have been observed within the periventricular grey matter (A4 and A6 nuclei) and parvicellular reticular nucleus (A5 and A7 nuclei), with the one exception to date being the tree pangolin, where no A4/A6 neurons are observed. The alphanumeric nomenclature system, developed in laboratory rodent brains, has been adapted to cover the variation observed across species. Cross-species homology is observed regarding the nuclear organization of noradrenergic neurons located in the parvicellular reticular nucleus (A5 and A7). In contrast, significant variations are observed in the organization of the A6 neurons of the locus coeruleus proper. In most mammals, the A6 is comprised of a moderate density of neurons, but in Murid rodents, primates, and megachiropteran bats, the A6 exhibits a very high density of neurons. In primates and megachiropterans, there is an additional moderate density of A6 neurons located rostromedial to the high-density portion. These variations are of importance in understanding the translation of findings in laboratory rodents to humans.

A Few TH-Immunoreactive Neurons Closely Appose DMX-Located Neuronal Somata Projecting to the Stomach Prepyloric Region in the Pig

Simple Summary

Although organization of the catecholaminergic system, in the porcine vagal motor nuclei of the pig, as well as distribution and chemical nature of the parasympathetic preganglionic neurons innervating the prepyloric region of the porcine stomach in the nucleus, have been well established, the question of a possible direct regulatory interaction between both neuronal systems still remains unknown. We discovered morphological foundations for direct regulatory action of the local TH-immunoreactive neurons on vagal preganglionic parasympathetic efferent neurons supplying the prepyloric region of the porcine stomach.

Abstract

The vagus nerve is responsible for efferent innervation and functional control of stomach functions. The efferent fibers originate from neurons located in the dorsal motor nucleus of the vagus (DMX) and undergo functional control of the local neuroregulatory terminals. The aim of the present study was to examine the existence of morphological foundations for direct regulatory action of the local TH-immunoreactive neurons on parasympathetic efferent neurons supplying the prepyloric region of the porcine stomach. Combined injection of neuronal retrograde tracer Fast Blue into the stomach prepyloric region with TH immunostaining was used in order to visualize spatial relationship between DMX-located stomach prepyloric region supplying neuronal stomata and local TH-IR terminals. We confirmed existence of TH-immunoreactive neural terminals closely opposing the stomach prepyloric region innervating neurons at the porcine DMX area. The observed spatial relationship points out the possibility of indirect catecholaminergic control of the stomach function exerted through preganglionic parasympathetic efferent neurons in the pig.

A Preliminary Description of the Sleep-Related Neural Systems in the Brain of the Blue Wildebeest, Connochaetes taurinus

The current study provides a detailed qualitative description of the organization of the cholinergic, catecholaminergic, serotonergic, orexinergic, and GABAergic sleep-related systems in the brain of the blue wildebeest (Connocheates taurinus), along with a quantitative analysis of the pontine cholinergic and noradrenergic neurons, and the hypothalamic orexinergic neurons. The aim of this study was to compare the nuclear organization of these systems to other mammalian species and specifically that reported for other Cetartiodactyla. In the brain of the blue wildebeest, from the basal forebrain to the pons, the nuclear organization of the cholinergic, catecholaminergic, serotonergic, and orexinergic systems, for the most part, showed a corresponding nuclear organization to that reported in other mammals and more specifically the Cetartiodactyla. Furthermore, the description and distribution of the GABAergic system, which was examined through immunostaining for the calcium binding proteins calbindin, calretinin, and parvalbumin, was also similar to that seen in other mammals. These findings indicate that sleep in the blue wildebeest is likely to show typically mammalian features in terms of the global brain activity of the generally recognized sleep states of mammals, but Cetartiodactyl-specific features of the orexinergic system may act to lower overall daily total sleep time in relation to similar sized non-Cetartiodactyl mammals. Anat Rec, 2019. © 2019 American Association for Anatomy Anat Rec, 303:1977-1997, 2020. © 2019 American Association for Anatomy.

Adult islet1 Expression Outlines Ventralized Derivatives Along Zebrafish Neuraxis

Signals issued by dorsal roof and ventral floor plates, respectively, underlie the major patterning process of dorsalization and ventralization during vertebrate neural tube development. The ventrally produced morphogen Sonic hedgehog (SHH) is crucial for vertebrate hindbrain and spinal motor neuron development. One diagnostic gene for motor neurons is the LIM/homeodomain gene islet1, which has additional ventral expression domains extending into mid- and forebrain. In order to corroborate motor neuron development and, in particular, to improve on the identification of poorly documented zebrafish forebrain islet1 populations, we studied adult brains of transgenic islet1-GFP zebrafish (3 and 6 months). This molecular neuroanatomical analysis was supported by immunostaining these brains for tyrosine hydroxylase (TH) or choline acetyltransferase (ChAT), respectively, revealing zebrafish catecholaminergic and cholinergic neurons. The present analysis of ChAT and islet1-GFP label confirms ongoing adult expression of islet1 in zebrafish (basal plate) midbrain, hindbrain, and spinal motor neurons. In contrast, non-motor cholinergic systems lack islet1 expression. Additional presumed basal plate islet1 positive systems are described in detail, aided by TH staining which is particularly informative in the diencephalon. Finally, alar plate zebrafish forebrain systems with islet1 expression are described (i.e., thalamus, preoptic region, and subpallium). We conclude that adult zebrafish continue to express islet1 in the same brain systems as in the larva. Further, pending functional confirmation we hypothesize that the larval expression of sonic hedgehog (shh) might causally underlie much of adult islet1 expression because it explains findings beyond ventrally located systems, for example regarding shh expression in the zona limitans intrathalamica and correlated islet1-GFP expression in the thalamus.

Locus coeruleus complex of the family Delphinidae

The locus coeruleus (LC) is the largest catecholaminergic nucleus and extensively projects to widespread areas of the brain and spinal cord. The LC is the largest source of noradrenaline in the brain. To date, the only examined Delphinidae species for the LC has been a bottlenose dolphin (Tursiops truncatus). In our experimental series including different Delphinidae species, the LC was composed of five subdivisions: A6d, A6v, A7, A5, and A4. The examined animals had the A4 subdivision, which had not been previously described in the only Delphinidae in which this nucleus was investigated. Moreover, the neurons had a large amount of neuromelanin in the interior of their perikarya, making this nucleus highly similar to that of humans and non-human primates. This report also presents the first description of neuromelanin in the cetaceans’ LC complex, as well as in the cetaceans’ brain.

Neurochemical organization and morphology of the sleep related nuclei in the brain of the Arabian oryx, Oryx leucoryx

The Arabian oryx, Oryx leucoryx, is a member of the superorder Cetartiodactyla and is native to the Arabian Desert. The desert environment can be considered extreme in which to sleep, as the ranges of temperatures experienced are beyond what most mammals encounter. The current study describes the nuclear organization and neuronal morphology of the systems that have been implicated in sleep control in other mammals for the Arabian oryx. The nuclei delineated include those revealed immunohistochemically as belonging to the cholinergic, catecholaminergic, serotonergic and orexinergic systems within the basal forebrain, hypothalamus, midbrain and pons. In addition, we examined the GABAergic neurons and their terminal networks surrounding or within these nuclei. The majority of the neuronal systems examined followed the typical mammalian organizational plan, but some differences were observed: (1) the neuronal morphology of the cholinergic laterodorsal tegmental (LDT) and pedunculopontine tegmental (PPT) nuclei, as well as the parvocellular subdivision of the orexinergic main cluster, exhibited Cetartiodactyl-specific features; (2) the dorsal division of the catecholaminergic anterior hypothalamic group (A15d), which has not been reported in any member of the Artiodactyla studied to date, was present in the brain of the Arabian oryx; and (3) the catecholaminergic tuberal cell group (A12) was notably more expansive than previously seen in any other mammal. The A12 nucleus has been associated functionally to osmoregulation in other mammals, and thus its expansion could potentially be a species specific feature of the Arabian oryx given their native desert environment and the need for extreme water conservation.

The "Ram Effect": A "Non-Classical" Mechanism for Inducing LH Surges in Sheep

During spring sheep do not normally ovulate but exposure to a ram can induce ovulation. In some ewes an LH surge is induced immediately after exposure to a ram thus raising questions about the control of this precocious LH surge. Our first aim was to determine the plasma concentrations of oestradiol (E2) E2 in anoestrous ewes before and after the “ram effect” in ewes that had a “precocious” LH surge (starting within 6 hours), a “normal” surge (between 6 and 28h) and “late» surge (not detected by 56h). In another experiment we tested if a small increase in circulating E2 could induce an LH surge in anoestrus ewes. The concentration of E2 significantly was not different at the time of ram introduction among ewes with the three types of LH surge. “Precocious” LH surges were not preceded by a large increase in E2 unlike “normal” surges and small elevations of circulating E2 alone were unable to induce LH surges. These results show that the “precocious” LH surge was not the result of E2 positive feedback. Our second aim was to test if noradrenaline (NA) is involved in the LH response to the “ram effect”. Using double labelling for Fos and tyrosine hydroxylase (TH) we showed that exposure of anoestrous ewes to a ram induced a higher density of cells positive for both in the A1 nucleus and the Locus Coeruleus complex compared to unstimulated controls. Finally, the administration by retrodialysis into the preoptic area, of NA increased the proportion of ewes with an LH response to ram odor whereas treatment with the α1 antagonist Prazosin decreased the LH pulse frequency and amplitude induced by a sexually active ram. Collectively these results suggest that in anoestrous ewes NA is involved in ram-induced LH secretion as observed in other induced ovulators.

The "ram effect": new insights into neural modulation of the gonadotropic axis by male odors and socio-sexual interactions

Reproduction in mammals is controlled by the hypothalamo-pituitary-gonadal (HPG) axis under the influence of external and internal factors such as photoperiod, stress, nutrition, and social interactions. Sheep are seasonal breeders and stop mating when day length is increasing (anestrus). However, interactions with a sexually active ram during this period can override the steroid negative feedback responsible for the anoestrus state, stimulate luteinizing hormone (LH) secretion and eventually reinstate cyclicity. This is known as the “ram effect” and research into the mechanisms underlying it is shedding new light on HPG axis regulation. The first step in the ram effect is increased LH pulsatile secretion in anestrus ewes exposed to a sexually active male or only to its fleece, the latter finding indicating a “pheromone-like” effect. Estradiol secretion increases in all ewes and this eventually induces a LH surge and ovulation, just as during the breeding season. An exception is a minority of ewes that exhibit a precocious LH surge (within 4 h) with no prior increase in estradiol. The main olfactory system and the cortical nucleus of the amygdala are critical brain structures in mediating the ram effect since it is blocked by their inactivation. Sexual experience is also important since activation (increased c-fos expression) in these and other regions is greatly reduced in sexually naïve ewes. In adult ewes kisspeptin neurons in both arcuate and preoptic regions and some preoptic GnRH neurons are activated 2 h after exposure to a ram. Exposure to rams also activates noradrenergic neurons in the locus coeruleus and A1 nucleus and increased noradrenalin release occurs in the posterior preoptic area. Pharmacological modulation of this system modifies LH secretion in response to the male or his odor. Together these results show that the ram effect can be a fruitful model to promote both a better understanding of the neural and hormonal regulation of the HPG axis in general and also the specific mechanisms by which male cues can overcome negative steroid feedback and trigger LH release and ovulatory cycles.

Sex differences and effects of prenatal exposure to excess testosterone on ventral tegmental area dopamine neurons in adult sheep

Prenatal testosterone (T) excess in sheep results in a wide array of reproductive neuroendocrine deficits and alterations in motivated behavior. The ventral tegmental area (VTA) plays a critical role in reward and motivated behaviors and is hypothesised to be targeted by prenatal T. Here we report a sex difference in the number VTA dopamine cells in the adult sheep, with higher numbers of tyrosine hydroxylase (TH)-immunoreactive (-ir) cells in males than females. Moreover, prenatal exposure to excess T during either gestational days 30-90 or 60-90 resulted in increased numbers of VTA TH-ir cells in adult ewes compared to control females. Stereological analysis confirmed significantly greater numbers of neurons in the VTA of males and prenatal T-treated ewes, which was primarily accounted for by greater numbers of TH-ir cells. In addition, immunoreactivity for TH in the cells was denser in males and prenatal T-treated females, suggesting that sex differences and prenatal exposure to excess T affects both numbers of cells expressing TH and the protein levels within dopamine cells. Sex differences were also noted in numbers of TH-ir cells in the substantia nigra, with more cells in males than females. However, prenatal exposure to excess T did not affect numbers of TH-ir cells in the substantia nigra, suggesting that this sex difference is organised independently of prenatal actions of T. Together, these results demonstrate sex differences in the sheep VTA dopamine system which are mimicked by prenatal treatment with excess T.

Suckling and salsolinol attenuate responsiveness of the hypothalamic-pituitary-adrenal axis to stress: focus on catecholamines, corticotrophin-releasing hormone, adrenocorticotrophic hormone, cortisol and prolactin secretion in lactating sheep

In mammals, the responsiveness of the hypothalamic-pituitary-adrenal (HPA) axis to stress is reduced during lactation and this mainly results from suckling by the offspring. The suckling stimulus causes a release of the hypothalamic 1-metyl-6,7-dihydroxy-1,2,3,4-tetrahydroisoquinoline (salsolinol) (a derivative of dopamine), one of the prolactin-releasing factors. To investigate the involvement of salsolinol in the mechanism suppressing stress-induced HPA axis activity, we conducted a series of experiments on lactating sheep, in which they were treated with two kinds of isolation stress (isolation from the flock with lamb present or absent), combined with suckling and/or i.c.v infusion of salsolinol and 1-methyl-3,4-dihydro-isoqinoline (1-MeDIQ; an antagonistic analogue of salsolinol). Additionally, a push-pull perfusion of the infundibular nucleus/median eminence (IN/ME) and blood sample collection with 10-min intervals were performed during the experiments. Concentrations of perfusate corticotrophin-releasing hormone (CRH) and catecholamines (noradrenaline, dopamine and salsolinol), as well as concentrations of plasma adenocorticotrophic hormone (ACTH), cortisol and prolactin, were assayed. A significant increase in perfusate noradrenaline, plasma ACTH and cortisol occurred in response to both kinds of isolation stress. Suckling and salsolinol reduced the stress-induced increase in plasma ACTH and cortisol concentrations. Salsolinol also significantly reduced the stress-induced noradrenaline and dopamine release within the IN/ME. Treatment with 1-MeDIQ under the stress conditions significantly diminished the salsolinol concentration and increased CRH and cortisol concentrations. Stress and salsolinol did not increase the plasma prolactin concentration, in contrast to the suckling stimulus. In conclusion, salsolinol released in nursing sheep may have a suppressing effect on stress-induced HPA axis activity and peripheral prolactin does not appear to participate in its action.

Effects of hypothalamic dopamine on growth hormone-releasing hormone-induced growth hormone secretion and thyrotropin-releasing hormone-induced prolactin secretion in goats

The aim of the present study was to clarify the effects of hypothalamic dopamine (DA) on the secretion of growth hormone (GH) in goats. The GH-releasing response to an intravenous (i.v.) injection of GH-releasing hormone (GHRH, 0.25 μg/kg body weight (BW)) was examined after treatments to augment central DA using carbidopa (carbi, 1 mg/kg BW) and L-dopa (1 mg/kg BW) in male and female goats under a 16-h photoperiod (16 h light, 8 h dark) condition. GHRH significantly and rapidly stimulated the release of GH after its i.v. administration to goats (P < 0.05). The carbi and L-dopa treatments completely suppressed GH-releasing responses to GHRH in both male and female goats (P < 0.05). The prolactin (PRL)-releasing response to an i.v. injection of thyrotropin-releasing hormone (TRH, 1 μg/kg BW) was additionally examined in male goats in this study to confirm modifications to central DA concentrations. The treatments with carbi and L-dopa significantly reduced TRH-induced PRL release in goats (P < 0.05). These results demonstrated that hypothalamic DA was involved in the regulatory mechanisms of GH, as well as PRL secretion in goats.

Sex differences in the expression of estrogen receptor alpha within noradrenergic neurons in the sheep brain stem

In female sheep, high levels of estrogen exert a positive feedback action on gonadotropin releasing hormone (GnRH) secretion to stimulate a surge in luteinizing hormone (LH) secretion. Part of this action appears to be via brain stem noradrenergic neurons. By contrast, estrogen action in male sheep has a negative feedback action to inhibit GnRH and LH secretion. To investigate whether part of this sex difference is due to differences in estrogen action in the brain stem, we tested the hypothesis that the distribution of estrogen receptor α (ERα) within noradrenergic neurons in the brain stem differs between rams and ewes. To determine the distribution of ERα, we used double-label fluorescence immunohistochemistry for dopamine β-Hydroxylase, as a marker for noradrenergic and adrenergic cells, and ERα. In the ventrolateral medulla (A1 region), most ERα-immunoreactive (-ir) cells were located in the caudal part of the nucleus. Overall, there were more ERα-ir cells in rams than ewes, but the proportion of double-labeled cells was did not differ between sexes. Much greater numbers of ERα-ir cells were found in the nucleus of the solitary tract (A2 region), but <10% were double labeled and there were no sex differences. The majority of ERα-labeled cells in this nucleus was located in the more rostral areas. ERα-labeled cells were found in several rostral brain stem regions but none of these were double labeled and so were not quantified. Because there was no sex difference in the number of ERα-ir cells in the brain stem that were noradrenergic, the sex difference in the action of estrogen on gonadotropin secretion in sheep is unlikely to involve actions on brain stem noradrenergic cells.

Evidence that histaminergic neurons are devoid of estrogen receptor alpha in the ewe diencephalon during the breeding season

In sheep as in rat, it has been highly suggested that neuronal histamine (HA) participates to the estradiol (E2)-induced GnRH and LH surges, through H1 receptor. With the aim of determining if E2 could act directly on HA neurons, we examined here whether HA neurons express estrogen receptor alpha (ERα) in the ewe diencephalon during the breeding season. We first produced a specific polyclonal antibody directed against recombinant ovine histidine decarboxylase (oHDC), the HA synthesizing enzyme. Using both this anti-oHDC antibody and an anti-ERα monoclonal antibody in double label immunohistochemistry, we showed that HA neurons do not express ERα in diencephalon of ewes with different hormonal status. This result diverges from those obtained in rat, in which around three quarters of HA neurons express ERα in their nucleus. This discrepancy between these two mammal species may reflect difference in their neuronal network.

Functional neuroanatomy of the central noradrenergic system

The central noradrenergic neurone, like the peripheral sympathetic neurone, is characterized by a diffusely arborizing terminal axonal network. The central neurones aggregate in distinct brainstem nuclei, of which the locus coeruleus (LC) is the most prominent. LC neurones project widely to most areas of the neuraxis, where they mediate dual effects: neuronal excitation by α₁-adrenoceptors and inhibition by α₂-adrenoceptors. The LC plays an important role in physiological regulatory networks. In the sleep/arousal network the LC promotes wakefulness, via excitatory projections to the cerebral cortex and other wakefulness-promoting nuclei, and inhibitory projections to sleep-promoting nuclei. The LC, together with other pontine noradrenergic nuclei, modulates autonomic functions by excitatory projections to preganglionic sympathetic, and inhibitory projections to preganglionic parasympathetic neurones. The LC also modulates the acute effects of light on physiological functions ('photomodulation'): stimulation of arousal and sympathetic activity by light via the LC opposes the inhibitory effects of light mediated by the ventrolateral preoptic nucleus on arousal and by the paraventricular nucleus on sympathetic activity. Photostimulation of arousal by light via the LC may enable diurnal animals to function during daytime. LC neurones degenerate early and progressively in Parkinson's disease and Alzheimer's disease, leading to cognitive impairment, depression and sleep disturbance.

Mapping of tyrosine hydroxylase in the diencephalon of alpaca (Lama pacos) and co-distribution with somatostatin-28 (1-12)

Based on previous work describing the distribution of somatostatin-28 (1-12) in the male alpaca (Lama pacos) diencephalon, and owing to the well known interactions between this peptide and the catecholaminergic system, the aims of this work are (1) to describe the distribution of putative catecholaminergic cell groups in the alpaca diencephalon and (2) to study the possible morphological basis of the interactions between these substances in the diencephalon of the alpaca by using double immunohistochemistry methods. Thus, the distribution of catecholaminergic cell groups in the alpaca diencephalon agrees with that previously described in the diencephalon of other mammalian species of the same order: the A11, A12, A13, A14 and A15d cell groups have been identified; however, we have observed an additional hitherto undescribed cell group containing tyrosine hydroxylase in the medial habenula. In addition, double-labelling procedures did not reveal neurons containing tyrosine hydroxylase and somatostatin, suggesting that the hypothalamic interactions between catecholamines and somatostatin at intra-cellular level must be carried out by a somatostatin molecule other than fragment (1-12). Otherwise, the overlapping distribution patterns of these substances would suggest some interconnections between groups of chemospecific neurons. These results could be the starting point for future studies on hypothalamic functions in alpacas, for example those concerning reproductive control, since other physiological studies have suggested that this species could have different regulatory mechanisms from other mammalian species. Our results support the Manger hypothesis that the same nuclear complement of neural systems exists in the brain of species of the same order.

Kisspeptin and seasonality of reproduction

Wild and domesticated species display seasonality in reproductive function, controlled predominantly by photoperiod. Seasonal alterations in breeding status are caused by changes in the secretion of gonadotropin-releasing hormone (GnRH) that are mediated by upstream neuronal afferents that regulate the GnRH cells. In particular, kisspeptin appears to play a major role in seasonality of reproduction, transducing the feedback effect of gonadal steroids as well as having an independent (nonsteroid dependent) circannual rhythm. A substantial body of data on this issue has been obtained from studies in sheep and hamsters and this is reviewed here in detail. Kisspeptin function is upregulated during the breeding season in sheep, stimulating reproductive function, but contradictory data are found in Siberian and Syrian hamsters. The relative quiescence of kisspeptin cells in the nonbreeding season can be counteracted by administration of the peptide, leading to activation of reproductive function. Although there is a major role for melatonin in the transduction of photoperiod to the reproductive system, kisspeptin cells do not appear to express the melatonin receptor, so the means by which seasonality changes the level of kisspeptin activity remains unknown.

Melatonin-dependent timing of seasonal reproduction by the pars tuberalis: pivotal roles for long daylengths and thyroid hormones

Most mammals living at temperate latitudes exhibit marked seasonal variations in reproduction. In long-lived species, it is assumed that timely physiological alternations between a breeding season and a period of sexual rest depend upon the ability of day length (photoperiod) to synchronise an endogenous timing mechanism called the circannual clock. The sheep has been extensively used to characterise the time-measurement mechanisms of seasonal reproduction. Melatonin, secreted only during the night, acts as the endocrine transducer of the photoperiodic message. The present review is concerned with the endocrine mechanisms of seasonal reproduction in sheep and the evidence that long day length and thyroid hormones are mandatory to their proper timing. Recent evidence for a circadian-based molecular mechanism within the pars tuberalis of the pituitary, which ties the short duration melatonin signal reflecting long day length to the hypothalamic increase of triiodothyronine (T3) through a thyroid-stimulating hormone/deiodinase2 paracrine mechanism is presented and evaluated in this context. A parallel is also drawn with the golden hamster, a long-day breeder, aiming to demonstrate that features of seasonality appear to be phylogenetically conserved. Finally, potential mechanisms of T3 action within the hypothalamus/median eminence in relationship to seasonal timing are examined.

Differential expression of dopaminergic cell markers in the adult zebrafish forebrain

Although the simultaneous presence of tyrosine hydroxylase (TH), aromatic amino acid decarboxylase (AADC), dopamine transporter (DAT), and vesicular monoamine transporter 2 (VMAT2) is considered as a phenotypic signature of dopamine (DA) neurons, it has been suggested that they are not uniformly expressed in all dopaminergic brain nuclei. Moreover, in nonmammalian vertebrates, two tyrosine hydroxylase genes (TH1 and TH2) are found, and they exhibit different expression patterns in zebrafish brains. Here we present a detailed description of the distribution of TH1, TH2, AADC, DAT, and VMAT2 transcripts, in relation to TH and DA immunoreactivity to better characterize dopaminergic nuclei in the adult zebrafish forebrain. TH2-positive cells in the hypothalamus are strongly DA immunoreactive (DAir), providing direct evidence that they are dopaminergic. DAir cells are also found in most TH1-positive or TH-immunoreactive (THir) nuclei. However, the DAir signal was weaker than THir in the olfactory bulb, telencephalon, ventral thalamus, pretectum, and some posterior tubercular and preoptic nuclei. These cell populations also exhibited low levels of VMAT2 transcripts, suggesting that low DA is due to a lower vesicular DA accumulation. In contrast, cell populations with low levels of AADC did not always have low levels of DA. DAT transcripts were abundantly expressed in most of the TH1- or TH2-positive territories. In addition, DAT and/or VMAT2 transcripts were found in some periventricular cell populations such as in the telencephalon without TH1 or TH2 expression. Thus, expression patterns of dopaminergic cell markers are not homogeneous, suggesting that the gene regulatory logic determining the dopaminergic phenotype is unexpectedly complex.

Mapping of tyrosine hydroxylase in the alpaca (Lama pacos) brainstem and colocalization with CGRP

The distribution of tyrosine hydroxylase (TH) in the brainstem of alpaca (Lama pacos) has been analysed using immunohistochemical methods. The following catecholaminergic cell nuclei have been detected: A1, C1, A2, C2 and area postrema in the medulla oblongata; A5, A6d, A7sc and A7d in the pons; as have several mesencephalic groups: A8, A9l, A9m, A9v, A9pc, A10, A10c, A10d and A10dc. This nuclear parcellation differs from that found in rodents, but agrees with the results reported in other members of the Artiodactyla order, such as giraffe or pig, and with the catecholaminergic distribution detected in species of other mammalian orders. Thus, these findings support the hypothesis that the animals included in the same order show the same nuclear complement in the neuromodulatory systems. In addition, it seems that other species share the same catecholaminergic groups as the alpaca, suggesting that a specific nuclear disposition was important and worth maintaining throughout evolution. Moreover, the distribution of TH has been compared with that of CGRP by double immunohistochemistry. Double-labelled neurons were very isolated and observed only in a few catecholaminergic groups: A1 and C2 in the medulla oblongata, A6d, A7sc and A7d in the pons, and A9l in the mesencephalon. However, interaction between TH and CGRP may be possible in more brainstem regions, particularly the area postrema. This interaction may prove important in the regulation of the specific cardiovascular control of alpacas given their morphological characteristics.

Two tyrosine hydroxylase genes in vertebrates New dopaminergic territories revealed in the zebrafish brain

Tyrosine hydroxylase (TH) is the rate limiting enzyme for dopamine synthesis, catalyzing transformation of l-tyrosine to l-DOPA. Two TH genes (TH1 and TH2) have been reported to exist in the genome of some teleost fishes, TH1 being orthologous to the mammalian TH gene (Candy and Collet, 2005). Here we show that two TH genes are commonly found in genomes of jawed vertebrates. Our analyses of molecular phylogeny and gene synteny strongly suggest that the two TH genes emerged as a consequence of a whole genome duplication before the divergence of jawed vertebrates, and that TH2 was secondarily lost in eutherians (placental mammals). The distribution of TH1 and TH2 transcripts revealed that TH1 and TH2 are differentially expressed in the zebrafish adult brain, as often observed for duplicated genes. In particular we found that TH2 transcripts were much more abundant than TH1 in the hypothalamus, and that the TH2 cells along the periventricular zone are devoid of TH immunoreactivity, due to the lack of affinity of the available anti-TH antibodies. Although these neurons have been considered to be dopamine-uptaking cells in previous studies, the expression of other monoaminergic markers such as aromatic amino acid decarboxylase (AADC), dopamine transporter (DAT), and vesicular monoamine transporter 2 (VMAT2) suggests that these TH2 cells are dopamine-synthesizing neurons.

Organization of cholinergic, putative catecholaminergic and serotonergic nuclei in the diencephalon, midbrain and pons of sub-adult male giraffes

The current study describes the nuclear organization and neuronal morphology of the cholinergic, putative catecholaminergic and serotonergic systems within the diencephalon, midbrain and pons of the giraffe using immunohistochemistry for choline acetyltransferase, tyrosine hydroxylase and serotonin. The giraffe has a unique phenotype (the long neck), a large brain (over 500 g) and is a non-domesticated animal, while previous studies examining the brains of other Artiodactyls have all been undertaken on domesticated animals. The aim of the present study was to investigate possible differences in the nuclear organization and neuronal morphology of the above-mentioned systems compared to that seen in other Artiodactyls and mammals. The nuclear organization of all three systems within the giraffe brain was similar to that of other Artiodactyls. Some features of interest were noted for the giraffe and in comparison to other mammals studied. The cholinergic neuronal somata of the laterodorsal tegmental nucleus were slightly larger than those of the pedunculopontine tegmental nucleus, a feature not described in other mammals. The putative catecholaminergic system of the giraffe appeared to lack an A15 dorsal nucleus, which is commonly seen in other mammals but absent in the Artiodactyls, had a large and expanded substantia nigra pars reticulata (A9 ventral), a small diffuse portion of the locus coerueleus (A6d), an expansive subcoeruleus (A7sc and A7d), and lacked the A4 nucleus of the locus coeruleus complex. The nuclear organization of the serotonergic system of the giraffe was identical to that seen in all other eutherian mammals studied to date. These observations in the giraffe demonstrate that despite significant changes in life history, phenotype, brain size and time of divergence, species within the same order show the same nuclear organization of the systems investigated.

Effect of dioxins on regulation of tyrosine hydroxylase gene expression by aryl hydrocarbon receptor: a neurotoxicology study

BACKGROUND

Dioxins and related compounds are suspected of causing neurological disruption. Epidemiological studies indicated that exposure to these compounds caused neurodevelopmental disturbances such as learning disability and attention deficit hyperactivity disorder, which are thought to be closely related to dopaminergic dysfunction. Although the molecular mechanism of their actions has not been fully investigated, a major participant in the process is aryl hydrocarbon receptor (AhR). This study focused on the effect of 2, 3, 7, 8-tetrachlorodibenzo-p-dioxin (TCDD) exposure on the regulation of TH, a rate-limiting enzyme of dopamine synthesis, gene expression by AhR.

METHODS

N2a-Rbeta cells were established by transfecting murine neuroblastoma Neuro2a with the rat AhR cDNA. TH expression induced by TCDD was assessed by RT-PCR and Western blotting. Participation of AhR in TCDD-induced TH gene expression was confirmed by suppressing AhR expression using the siRNA method. Catecholamines including dopamine were measured by high-performance liquid chromatography. A reporter gene assay was used to identify regulatory motifs in the promoter region of TH gene. Binding of AhR with the regulatory motif was confirmed by an electrophoretic mobility shift assay (EMSA).

RESULTS

Induction of TH by TCDD through AhR activation was detected at mRNA and protein levels. Induced TH protein was functional and its expression increased dopamine synthesis. The reporter gene assay and EMSA indicated that AhR directly regulated TH gene expression. Regulatory sequence called aryl hydrocarbon receptor responsive element III (AHRE-III) was identified upstream of the TH gene from -285 bp to -167 bp. Under TCDD exposure, an AhR complex was bound to AHRE-III as well as the xenobiotic response element (XRE), though AHRE-III was not identical to XRE, the conventional AhR-binding motif.

CONCLUSION

Our results suggest TCDD directly regulate the dopamine system by TH gene transactivation via an AhR-AHRE-III-mediated pathway. The AhR- mediated pathway could have a particular AhR-mediated genomic control pathway transmitting the effects of TCDD action to target cells in the development of dopaminergic disabilities.

Neural connectivity in the mediobasal hypothalamus of the sheep brain

The ventromedial nucleus of the hypothalamus (VMN) and the arcuate nucleus (ARC) are two centres regulating energy balance and food intake, but inter-connectivity of these nuclei is not well defined in non-rodent species. In this study, we performed retrograde tracing and immunohistochemistry in the ovine brain with ewes receiving FluoroGold (FG) injections into either ARC or VMN for the mapping of retrogradely labelled cells. Strong reciprocal connections were found between the two regions. The distribution of the FG labelled neurons in other regions of the hypothalamus and brain stem was also mapped. Some of the cells projecting from ARC to VMN were immunopositive for neuropeptide Y, galanin, adrenocorticotropin (marker of pro-opiomelanocortin cells) or tyrosine hydroxylase (marker of dopaminergic cells). Melanin-concentrating hormone and orexin neurons in the lateral hypothalamic area were also found to provide input to the VMN and ARC. This observed interconnectivity between regions important for metabolic regulation and other neuroendocrine functions presumably allows coordinated functions. Input to both the ARC and VMN from other brain regions, such as brain stem cell groups, provides a further level of regulation. These data provide a substrate upon which further understanding of appetite regulation and neuroendocrine function can be derived in this species.

Distribution and morphology of putative catecholaminergic and serotonergic neurons in the medulla oblongata of a sub-adult giraffe, Giraffa camelopardalis

The current study details the nuclear parcellation and appearance of putative catecholaminergic and serotonergic neurons within the medulla oblongata of a sub-adult giraffe, using immunohistochemistry for tyrosine hydroxylase and serotonin. We hypothesized that the unusual phenotype of the giraffe, this being the long neck and potential axonal lengthening of these neurons, may pose specific problems in terms of the efficient functioning of these systems, as several groups of catecholaminergic and serotonergic neurons, especially of the medulla, are known to project to the entire spinal cord. This specific challenge may lead to observable differences in the nuclear parcellation and morphology of these systems in the giraffe. Our personal observations in the giraffe reveal that, as with other Artiodactyls, the spinal cord extends to the caudal end of the sacral vertebrae. Within the giraffe medulla we found evidence for five putative catecholaminergic (neurons containing tyrosine hydroxylase) and five serotonergic nuclei. In terms of both morphological appearance of the neurons and nuclear parcellation we did not find any evidence for features that may be considered affected by the phenotype of the giraffe. The nuclear parcellation and appearance of both the putative catecholaminergic and serotonergic systems in the medulla of the giraffe studied are strikingly similar to that seen in previous studies of other Artiodactyls. We interpret these findings in terms of a growing literature detailing order specific phylogenetic constraints in the evolution of these neuromodulatory systems.

Distribution and morphology of cholinergic, putative catecholaminergic and serotonergic neurons in the brain of the Egyptian rousette flying fox, Rousettus aegyptiacus

Over the past decade much controversy has surrounded the hypothesis that the megachiroptera, or megabats, share unique neural characteristics with the primates. These observations, which include similarities in visual pathways, have suggested that the megabats are more closely related to the primates than to the other group of the Chiropteran order, the microbats, and suggests a diphyletic origin of the Chiroptera. To contribute data relevant to this debate, we used immunohistochemical techniques to reveal the architecture of the neuromodulatory systems of the Egyptian rousette (Rousettus aegypticus), an echolocating megabat. Our findings revealed many similarities in the nuclear parcellation of the cholinergic, putative catecholaminergic and serotonergic systems with that seen in other mammals including the microbat. However, there were 11 discrete nuclei forming part of these systems in the brain of the megabat studied that were not evident in an earlier study of a microbat. The occurrence of these nuclei align the megabat studied more closely with primates than any other mammalian group and clearly distinguishes them from the microbat, which aligns with the insectivores. The neural systems investigated are not related to such Chiropteran specializations as echolocation, flight, vision or olfaction. If neural characteristics are considered strong indicators of phylogenetic relationships, then the data of the current study strongly supports the diphyletic origin of Chiroptera and aligns the megabat most closely with primates in agreement with studies of other neural characters.

Signaling mechanisms for alpha2-adrenergic inhibition of PACAP-induced growth hormone secretion and gene expression grass carp pituitary cells

Pituitary adenylate cyclase-activating polypeptide (PACAP) is a potent growth hormone (GH)-releasing factor in lower vertebrates. However, its functional interactions with other GH regulators have not been fully characterized. In fish models, norepinephrine (NE) inhibits GH release at the pituitary cell level, but its effects on GH synthesis have yet to be determined. We examined adrenergic inhibition of PACAP-induced GH secretion and GH gene expression using grass carp pituitary cells as a cell model. Through activation of pituitary alpha2-adrenoreceptors, NE or the alpha2-agonist clonidine reduced both basal and PACAP-induced GH release and GH mRNA expression. In carp pituitary cells, clonidine also suppressed cAMP production and intracellular Ca2+ levels and blocked PACAP induction of these two second messenger signals. In GH3 cells transfected with a reporter carrying the grass carp GH promoter, PACAP stimulation increased GH promoter activity, and this stimulatory effect could be abolished by NE treatment. In parallel experiments, clonidine reduced GH primary transcript and GH promoter activity without affecting GH mRNA stability, and these inhibitory actions were mimicked by inhibiting adenylate cyclase (AC), blocking protein kinase A (PKA), removing extracellular Ca2+ in the culture medium, or inactivating L-type voltage-sensitive Ca2+ channels (VSCC). Since our recent studies have shown that PACAP can induce GH secretion in carp pituitary cells through cAMP/PKA- and Ca2+/calmodulin-dependent mechanisms, these results, taken together, suggest that alpha2-adrenergic stimulation in the carp pituitary may inhibit PACAP-induced GH release and GH gene transcription by blocking the AC/cAMP/PKA pathway and Ca2+ entry through L-type VSCC.

A map of the major nuclei of the fetal sheep brainstem

The fetal sheep has been used to investigate a wide range of developmental and pathological processes such as the effect of severe hypoxia, asphyxia, or intrauterine infection on the brain but, until now, there has been no complete description of the normal anatomical organisation of neuronal groups to facilitate interpretation of these studies. In this paper, we describe the major nuclei of the fetal sheep brainstem based on a study of 5 fetal sheep at 140 days of gestation (G140: term is G147). Nuclei were identified with the aid of brain atlases available for other species, and from the previously published, partial descriptions available for the sheep. Fifty-five distinct nuclei were identified after Nissl (thionin) staining, and their caudal and rostral margins were defined. This paper provides an easy reference to the position of the major nuclei within the fetal sheep brainstem, and can be used as a guide for future studies examining the organisation of neuronal populations under normal and pathological conditions in this animal model.

Mechanisms for gonadotropin-releasing hormone potentiation of growth hormone rebound following norepinephrine inhibition in goldfish pituitary cells

In the goldfish, norepinephrine (NE) inhibits growth hormone (GH) secretion through activation of pituitary alpha(2)-adrenergic receptors. Interestingly, a GH rebound is observed after NE withdrawal, which can be markedly enhanced by prior exposure to gonadotropin-releasing hormone (GnRH). Here we examined the mechanisms responsible for GnRH potentiation of this "postinhibition" GH rebound. In goldfish pituitary cells, alpha(2)-adrenergic stimulation suppressed both basal and GnRH-induced GH mRNA expression, suggesting that a rise in GH synthesis induced by GnRH did not contribute to its potentiating effect. Using a column perifusion approach, GnRH given during NE treatment consistently enhanced the GH rebound following NE withdrawal. This potentiating effect was mimicked by activation of PKC and adenylate cyclase (AC) but not by induction of Ca(2+) entry through voltage-sensitive Ca(2+) channels (VSCC). Furthermore, GnRH-potentiated GH rebound could be alleviated by inactivation of PKC, removal of extracellular Ca(2+), blockade of VSCC, and inhibition of Ca(2+)/calmodulin (CaM)-dependent protein kinase II (CaMKII). Inactivation of AC and PKA, however, was not effective in this regard. These results, as a whole, suggest that GnRH potentiation of GH rebound following NE inhibition is mediated by PKC coupled to Ca(2+) entry through VSCC and subsequent activation of CaMKII. Apparently, the Ca(2+)-dependent cascades are involved in GH secretion during the rebound phase but are not essential for the initiation of GnRH potentiation. Since GnRH has been previously shown to have no effects on cAMP synthesis in goldfish pituitary cells, the involvement of cAMP-dependent mechanisms in GnRH potentiation is rather unlikely.

Over-expression of AhR (aryl hydrocarbon receptor) induces neural differentiation of Neuro2a cells: neurotoxicology study

BACKGROUND

Dioxins and related compounds are suspected of causing neurological disruption in human and experimental animal offspring following perinatal exposure during development and growth. The molecular mechanism(s) of the actions in the brain, however, have not been fully investigated. A major participant in the process of the dioxin-toxicity is the dioxin receptor, namely the aryl hydrocarbon receptor (AhR). AhR regulates the transcription of diverse genes through binding to the xenobiotic-responsive element (XRE). Since the AhR has also been detected in various regions of the brain, the AhR may play a key role in the developmental neurotoxicity of dioxins. This study focused on the effect of AhR activation in the developing neuron.

METHODS

The influence of the AhR on the developing neuron was assessed using the Neuro2a-AhR transfectant. The undifferentiated murine neuroblastoma Neuro2a cell line (ATCC) was stably transfected with AhR cDNA and the established cell line was named N2a-Ralpha. The activation of exogenous AhR in N2a-Ralpha cells was confirmed using RNAi, with si-AhR suppressing the expression of exogenous AhR. The neurological properties of N2a-Ralpha based on AhR activation were evaluated by immunohistochemical analysis of cytoskeletal molecules and by RT-PCR analysis of mRNA expression of neurotransmitter-production related molecules, such as tyrosine hydroxylase (TH).

RESULTS

N2a-Ralpha cells exhibited constant activation of the exogenous AhR. CYP1A1, a typical XRE-regulated gene, mRNA was induced without the application of ligand to the culture medium. N2a-Ralpha cells exhibited two significant functional features. Morphologically, N2a-Ralpha cells bore spontaneous neurites exhibiting axon-like properties with the localization of NF-H. In addition, cdc42 expression was increased in comparison to the control cell line. The other is the catecholaminergic neuron-like property. N2a-Ralpha cells expressed tyrosine hydroxylase (TH) mRNA as a functional marker of catecholaminergic neurotransmitter production. Thus, exogenous AhR induced catecholaminergic differentiation in N2a-Ralpha cells.

CONCLUSION

The excessive activation of AhR resulted in neural differentiation of Neuro2a cells. This result revealed that dioxins may affect the nervous system through the AhR-signaling pathway. Activated AhR may disrupt the strictly regulated brain formation with irregular differentiation occurring rather than cell death.

Tyrosine hydroxylase expression in the olfactory/respiratory epithelium in early sheep fetuses (Ovis aries)

Transient expression of tyrosine hydroxylase (TH, the first enzyme in catecholamine synthesis) has been shown in different brain and peripheral structures of various species. TH-immunoreactive neurons have been reported in the nasal region of human and rat fetuses migrating to the forebrain with GnRH neurons during embryogenesis. In the present study, immunohistochemical analysis and in situ hybridization were performed in fetal sheep and in vitro sheep embryo olfactory placode cultures to confirm this population in this species. On embryonic days 33 to 35, TH-immunoreactive cells as well as TH cDNA-hybridized cells were found in the olfactory and respiratory epithelium and were spatially separated from GnRH-immunoreactive neurons. In days 40 to 44 of gestation, TH-immunoreactive neurons were no longer observed in the olfactory epithelium, and TH-immunoreactive fibers were found on the trajectories of the olfactory nerves. At this stage, some TH-immunoreactive fibers were also labeled for GnRH. TH-immunoreactive cells were also found in primary cultures of olfactory placodes of fetal sheep at 10 to 18 days in vitro. Some of them coexpressed GnRH. These results imply that olfactory epithelium is also able to give rise to TH expressing cells in fetal sheep, but this expression is suppressed earlier in ontogenesis than in humans due to some unidentified factors not present in the primary cultures of olfactory placode. The role of TH expression remains unclear as in other previously described examples.

Expression of pituitary adenylate cyclase activating polypeptide type 1 receptor (PAC1R) in the ewe hypothalamus: distribution and colocalization with tyrosine hydroxylase-immunoreactive neurones

We have examined the distribution of the pituitary adenylate cyclase activating polypeptide type I receptor (PAC1R) in the ewe hypothalamus by reverse transcription-polymerase chain reaction, in situ hybridization and immunohistochemistry. PAC1R mRNA was highly expressed in the mediobasal hypothalamus of the ewe, particularly in the arcuate nucleus and ventromedial hypothalamus, compared to other hypothalamic regions. Similar results were obtained from immunohistochemistry using a specific PAC1R antibody. Intense immunolabelling was observed in the arcuate nucleus, external zone of the median eminence and ventromedial hypothalamus. Only relatively weak immunolabelling was observed in other hypothalamic regions, including the paraventricular nucleus and supraoptic nucleus. In the ewe, PACAP acts via the arcuate nucleus to suppress prolactin secretion. Therefore we examined whether PAC1R was present on the tuberoinfundibular dopamine (TIDA) neurones in this nucleus. Dual immunofluorescence labelling for PAC1R and tyrosine hydroxylase revealed that 21.2 +/- 1.7% of dopaminergic neurones in the arcuate nucleus (A12 cell group) also stained for PAC1R. By contrast, other hypothalamic dopaminergic cell groups (A11, A13, A14 and A15) exhibited little (< 3%) or no colocalization. Overall, our results indicate that, in the ewe hypothalamus, PAC1R is most concentrated in the arcuate nucleus, where it is localized on a substantial proportion of dopaminergic neurones. These observations, together with previous in vivo studies, suggest that PACAP could act directly on TIDA neurones via PAC1R to increase dopamine release and consequently inhibit prolactin secretion in the sheep.

Differential c-Fos expression in the newborn lamb nucleus tractus solitarius and area postrema following ingestion of colostrum or saline

Visceral stimuli and the gut-brain axis play a crucial role in the control of ingestion even in the neonate. The aim of this study was to assess the neuronal activation in the nucleus tractus solitarius (NTS) and the area postrema (AP) following nutritional and non-nutritional stimulations. Lambs received a single gastric infusion of colostrum or saline at 5% birth weight or were sham infused. Infusion of either liquid led to c-Fos-like immunoreactivity (c-FLI) in the NTS and AP. Differences were observed along the sections of the NTS rostro-caudal axis according to the nature of the stimulation, suggesting a specificity of certain afferents and/or NTS areas for nutritional or non-nutritional signals. In the AP, the neuronal activation induced by colostrum was much higher than that induced by saline. A higher number of TH-immunoreactive cells were activated following colostrum infusion, suggesting a specific involvement of the catecholaminergic pathway in the treatment of meal-related stimuli. In spite of functional convergence, the two medullary structures observed responded differently according to the stimulation, indicating a complementary role in the integration of visceral signals.

Tyrosine hydroxylase and acetylcholinesterase in the domestic pig mesencephalon: An immunocytochemical and histochemical study

The mesencephalon of the young domestic pig was studied by tyrosine hydroxylase (TH) immunocytochemistry and acetylcholinesterase (AChE) histochemistry with focus on the substantia nigra (SN), the ventral tegmental area (VTA), and related areas. The purpose was to obtain information on the organization of the mesencephalic, TH immunoreactive (TH-i), and dopaminergic areas of the pig, in order to provide the necessary background for the possible use of the pig as an alternative large animal experimental model for research on Parkinson's disease, including the use of encapsulated pig dopaminergic neurons for intracerebral xenotransplantation. Significant findings in the pig, compared to observations in other species, included the presence of prominent bundles of TH-i dendrites passing in a dorsoventral direction from pars compacta into pars reticulata at middle and caudal levels of the SN, and the presence of a distinct TH-i substantia nigra pars lateralis (SNL). Caudally in the pig mesencephalon, the retrorubral field (RRF) was found to be very extensive. The view of the RRF, SN, and VTA as parts of the same integrated system was indicated by the crisscrossing of TH-i dendrites at the transitions between these areas. Estimation of the number of TH-i neurons in the SN and the VTA showed that these nuclei were of equal size in the pig. Further, it was found that TH-i nerve cells were present in the midline between the VTA in the interfascicular and rostral linear groups. TH-i nerve cells were also present in the otherwise serotoninergic dorsal raphe nuclei, just as other TH-i cells formed a perirubral cell group. AChE-positive neurons were present in both SN and VTA, and appeared to have the same size and morphology as the TH-i neurons in these areas. Within both nuclei, there were local differences in the AChE staining density, but perhaps more significantly were some marked differences in the structure of the AChE-positive neuropil of the two areas. We anticipate that the present description of the cellular organization of the TH-i dopaminergic areas in the domestic pig ventral mesencephalon will be useful for the development of a nonprimate, large animal, experimental model of Parkinson's disease.

Has dopamine a physiological role in the control of sexual behavior? A critical review of the evidence

The role of dopaminergic systems in the control of sexual behavior has been a subject of study for at least 40 years. Not surprisingly, reviews of the area have been published at variable intervals. However, the earlier reviews have been summaries of published research rather than a critical analysis of it. They have focused upon the conclusions presented in the original research papers rather than on evaluating the reliability and functional significance of the data reported to support these conclusions. During the last few years, important new knowledge concerning dopaminergic systems and their behavioral functions as well as the possible role of these systems in sexual behavior has been obtained. For the first time, it is now possible to integrate the data obtained in studies of sexual behavior into the wider context of general dopaminergic functions. To make this possible, we first present an analysis of the nature and organization of sexual behavior followed by a summary of current knowledge about the brain structures of crucial importance for this behavior. We then proceed with a description of the dopaminergic systems within or projecting to these structures. Whenever possible, we also try to include data on the electrophysiological actions of dopamine. Thereafter, we proceed with analyses of pharmacological data and release studies, both in males and in females. Consistently throughout this discussion, we make an effort to distinguish pharmacological effects on sexual behavior from a possible physiological role of dopamine. By pharmacological effects, we mean here drug-induced alterations in behavior that are not the result of the normal actions of synaptically released dopamine in the untreated animal. The conclusion of this endeavor is that pharmacological effects of dopaminergic drugs are variable in both males and females, independently of whether the drugs are administered systemically or intracerebrally. We conclude that the pharmacological data basically reinforce the notion that dopamine is important for motor functions and general arousal. These actions could, in fact, explain most of the effects seen on sexual behavior. Studies of dopamine release, in both males and females, have focused on the nucleus accumbens, a structure with at most a marginal importance for sexual behavior. Since accumbens dopamine release is associated with all kinds of events, aversive as well as appetitive, it can have no specific effect on sexual behavior but promotes arousal and activation of non-specific motor patterns. Preoptic and paraventricular nucleus release of dopamine may have some relationship to mechanisms of ejaculation or to the neuroendocrine consequences of sexual activity or they can be related to other autonomic processes associated with copulation. There is no compelling indication in existing experimental data that dopamine is of any particular importance for sexual motivation. There is experimental evidence showing that it is of no importance for sexual reward.

The premammillary hypothalamic area of the ewe: anatomical characterization of a melatonin target area mediating seasonal reproduction

Recent evidence suggests that the ovine premammillary hypothalamic area (PMH) is an important target for the pineal hormone, melatonin, and its role in seasonal reproduction. In rodents, the PMH is a complex region consisting of several cell groups with differing neurochemical content and anatomical connections. Therefore, to obtain a better understanding of the potential neural targets for melatonin in this area of the sheep brain, we have undertaken a detailed anatomical characterization of the PMH, including its nuclear divisions and the location of neuropeptide/neurotransmitter cells within them. By combining immunocytochemistry for NeuN, a neuronal marker, with Nissl staining in anestrous, ovariectomized, estradiol-treated ewes, we identified three nuclei within the PMH: a caudal continuation of the hypothalamic arcuate nucleus (cARC), the ventral division of the premammillary nucleus (PMv), and the ventral tuberomammillary nucleus (TMv). The cARC contained neurons that were immunoreactive for tyrosine hydroxylase, dynorphin, estrogen receptor alpha, cocaine- and amphetamine-regulated transcript peptide (CART), and nitric oxide synthase (NOS). The PMv was also characterized by the presence of cells that contained NOS and CART, although the size of these cells was larger than that of their corresponding phenotype in the cARC. By contrast, in the TMv, of the markers examined in the present study, only fibers immunoreactive for orexin were seen. Thus, the ovine PMH is a heterogeneous region comprised of three subdivisions, each with distinct morphological and neurochemical characteristics. This anatomical map of the PMH provides a basis for future studies to determine the functional contribution of each component to the influence of melatonin on seasonal reproduction.

Evidence that estrogen receptor alpha, but not beta, mediates seasonal changes in the response of the ovine retrochiasmatic area to estradiol

In ewes, anestrus results from a reduction in LH pulsatility due to an increased sensitivity of the hypothalamic estradiol negative feedback system. Considerable evidence has implicated the A15 group of dopaminergic neurons in the retrochiasmatic area in this seasonally dependent estradiol effect. Moreover, estradiol administered to the retrochiasmatic area in ovariectomized anestrous ewes inhibits LH secretion. However, A15 neurons do not appear to contain the classical estrogen receptors (ERalpha). Therefore, we tested the hypothesis that beta-estrogen receptors mediate the action of estradiol in the retrochiasmatic area by comparing the effects of estradiol and genistein, a selective ERbeta agonist. We also examined whether there are seasonal changes in response of the retrochiasmatic area to these agonists and if these effects are mediated by dopamine. To test these hypotheses, ovariectomized ewes were implanted with bilateral guide cannulae targeting the retrochiasmatic area. Crystalline agonists were administered via microimplants inserted down the cannulae. Blood samples taken before and 4 days after microimplant insertion were analyzed for LH concentrations, pulse frequency, and amplitude. Genistein treatment produced no significant change in LH levels in either season. Estradiol treatment decreased both mean LH concentrations and pulse frequency in anestrous but not breeding-season ewes. Administration of the dopamine antagonist sulpiride to ovariectomized ewes with estradiol microimplants in the retrochiasmatic area returned LH pulse frequency to levels indistinguishable from controls. From these data, we hypothesize that estradiol acts on local ERalpha-containing neurons in this area to stimulate a dopaminergic pathway that inhibits LH secretion during anestrus.

Biphasic role of dopamine on female sexual behaviour via D2 receptors in the mediobasal hypothalamus

Dopamine has been implicated in the control of sexual behaviour, but its role seems quite complex and controversial. The aim of the present experiments was to investigate the effects of dopamine (DA) acting on D2 receptors in the mediobasal hypothalamus (MBH) on sexual behaviour in female sheep. To achieve this, the D2 agonist, quinpirole, was administered bilaterally via microdialysis probes into the MBH of ovariectomized ewes either before or after oestradiol (E2) administration. Quinpirole (100 ng/ml) infused for 6 h just before E2 hastened the onset of oestrus behaviour and the luteinizing hormone surge, whereas the same treatment given 6-12 h or 18-21 h after E2 decreased the intensity of sexual receptivity without affecting LH or prolactin secretion. We then tested the hypothesis that E2 stimulates the onset of oestrus partly by decreasing DA activation of D2 receptors. In this case the D2 antagonists pimozide or spiperone (100 ng/ml) were infused into the MBH via microdialysis probes for 11 h in the absence of E2 administration. A significant number of ewes showed induction of receptivity with both antagonists, although its intensity was significantly lower than that induced by E2. These treatments generally did not significantly alter extracellular concentrations of monoamines or aminoacids although quinpirole modulated the ability of sexual interactions to increase noradrenaline release. These experiments show that DA acts via D2 receptors in the MBH to control female sexual behaviour in a biphasic manner: the onset of sexual motivation and receptivity requiring an initial increase in activation followed by a decrease. This dual action could explain some of the controversies concerning DA action on sexual behaviour.

Long-term alterations in body weight do not affect the expression of melanocortin receptor-3 and -4 mRNA in the ovine hypothalamus

The pro-opiomelanocortin-derived peptides and the melanocortin receptors are implicated in various functions within the CNS including the regulation of food intake. In the present study, we used in situ hybridization, with specific 35S-labelled ovine riboprobes to map the expression of melanocortin receptor-3 (MC3-R) and -4 (MC4-R) mRNA in the diencephalon and brainstem of normal female sheep. Furthermore, we examined the effect of long-term alterations in energy balance on the distribution and expression of MC3-R and MC4-R mRNA in food-restricted and ad libitum-fed ovariectomized female sheep. The distribution of melanocortin receptors generally resembled that of the rat. A high number of MC3-R-labelled cells were seen in the ventral division of the lateral septum and the medial preoptic area. In the hypothalamus, a moderate number of MC3-R-labelled cells was observed in the lateral hypothalamic area while other nuclear groups had low to intermediate numbers of MC3-R-labelled cells. The distribution of MC4-R mRNA was generally similar to that of MC3-R mRNA in the septal/preoptic and hypothalamic regions, with a high number of labelled cells present in the intermediate division of the lateral septum. Within the hypothalamus, no MC4-R mRNA expression was observed in the arcuate nucleus. There was more widespread distribution of moderate to low numbers of MC4-R mRNA-expressing cells in the brainstem compared to that of MC3-R mRNA. Unlike findings in the rat, only a low number of cells expressed melanocortin receptor mRNA in the ovine hypothalamic nuclei associated with feeding behavior. The number of melanocortin receptor-labelled cells and the level of expression (silver grains/cell) in the hypothalamic feeding centers was similar in food-restricted and ad libitum-fed animals. These findings suggest that long-term alterations in metabolic status do not change the melanocortin receptor mRNA distribution and/or expression in the sheep hypothalamus.

Noradrenaline and dopamine regulation of prolactin secretion in sheep: role in prolactin homeostasis but not photoperiodism

The role of noradrenaline (NA) and dopamine (DA) in the hypothalamic control of prolactin (PRL) secretion was investigated in hypothalamic intact (control) and hypothalamo-pituitary disconnected (HPD) Soay rams. The animals were exposed to alternating 16-weekly periods of short (8 L : 16D) and long days (16 L : 8D) to induce marked cyclical changes in PRL secretion in both groups (as demonstrated previously). Selective NA and DA receptor antagonists (dose: 1.2 micromol/kg) were administered under short days (low endogenous PRL secretion), and agonists (dose: 0.0012-0.12 micromol/kg) were administered under long days (high endogenous PRL secretion). The acute changes in blood PRL concentrations were measured over 4 h as the index of responsiveness. Under short days, treatment with WB4101 (alpha-1 adenoceptor antagonist), and rauwolscine (alpha-2 antagonist), consistently increased PRL secretion in control, but not in HPD rams. The treatments produced similar acute, drug-specific behavioural effects in both groups. Propranolol (beta antagonist) had no effect on PRL secretion, while sulpiride (DA D-2 antagonist) induced a marked increase in blood PRL concentrations in control rams (> 4 h), and a transient effect in HPD rams (15 min). Under long days, when endogenous PRL secretion was increased, phenylephrine (alpha-1 agonist) produced no effects, while bromocriptine (DA D-2 agonist) robustly decreased PRL concentrations in both control and HPD rams, even at the lowest treatment dose. Overall, the positive responses to the antagonists in the control rams, support the view that DA (acting via D-2 receptors), and to a lesser extent NA (acting via alpha-1/alpha-2 receptors), negatively regulate PRL secretion. In contrast, the lack of responses to the antagonists in the HPD rams, support the view that neither DA, nor NA, mediate the photoperiodic control of PRL secretion.

Tyrosine hydroxylase protein content in the medulla oblongata of the foetal sheep brain increases in response to acute but not chronic hypoxia

We have investigated the effect of lowering foetal arterial PO(2) either acutely or chronically on tyrosine hydroxylase (TH) protein content in the dorsal and ventral medullary regions of the brainstem of the sheep foetus during late gestation. TH protein content increased in both the dorsal and ventral medullary regions of the foetal brainstem after exposure to acute hypoxia when compared to normoxia. In contrast there was no increase in the TH protein content of either the dorsal or ventral medullary regions in the brainstem of foetal sheep which were chronically hypoxaemic throughout late gestation as a consequence of experimental restriction of placental growth. The differences between the TH responses to acute and chronic hypoxaemia in the foetal sheep brainstem may be important in the mediation of physiological adaptations to these intrauterine stimuli and for the generation of an appropriate physiological response to hypoxia in the newborn period.

Neuroregulation of growth hormone secretion in domestic animals

Growth hormone (GH) is essential for postnatal somatic growth, maintenance of lean tissue at maturity in domestic animals and milk production in cows. This review focuses on neuroregulation of GH secretion in domestic animals. Two hormones principally regulate the secretion of GH: growth hormone-releasing hormone (GHRH) stimulates, while somatostatin (SS) inhibits the secretion of GH. A long-standing hypothesis proposes that alternate secretion of GHRH and SS regulate episodic secretion of GH. However, measurement of GHRH and SS in hypophysial-portal blood of unanesthetized sheep and swine shows that episodic secretion of GHRH and SS do not account for all episodes of GH secreted. Furthermore, the activity of GHRH and SS neurons decreases after steers have eaten a meal offered for a 2-h period each day (meal-feeding) and this corresponds with reduced secretion of GH. Together, these data suggest that other factors also regulate the secretion of GH. Several neurotransmitters have been implicated in this regard. Thyrotropin-releasing hormone, serotonin and gamma-aminobutyric acid stimulate the secretion of GH at somatotropes. Growth hormone releasing peptide-6 overcomes feeding-induced refractoriness of somatotropes to GHRH and stimulates the secretion of GHRH. Norepinephrine reduces the activity of SS neurons and stimulates the secretion of GHRH via alpha(2)-adrenergic receptors. N-methyl-D,L-aspartate and leptin stimulate the secretion of GHRH, while neuropeptide Y stimulates the secretion of GHRH and SS. Activation of muscarinic receptors decreases the secretion of SS. Dopamine stimulates the secretion of SS via D1 receptors and inhibits the secretion of GH from somatotropes via D2 receptors. Thus, many neuroendocrine factors regulate the secretion of GH in livestock via altering secretion of GHRH and/or SS, communicating between GHRH and SS neurons, or acting independently at somatotropes to coordinate the secretion of GH.