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Herbison AE. The dendron and episodic neuropeptide release. J Neuroendocrinol 2021; 33:e13024. [PMID: 34427000 DOI: 10.1111/jne.13024] [Citation(s) in RCA: 7] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 06/23/2021] [Revised: 08/04/2021] [Accepted: 08/05/2021] [Indexed: 12/21/2022]
Abstract
The unexpected observation that the long processes of gonadotrophin-releasing hormone (GnRH) neurons not only conducted action potentials, but also operated to integrate afferent information at their distal-most extent gave rise to the concept of a blended dendritic-axonal process termed the "dendron". The proximal dendrites of the GnRH neuron function in a conventional manner, receiving synaptic inputs and initiating action potentials that are critical for the surge mode of GnRH secretion. The distal dendrons are regulated by both classical synapses and volume transmission and likely operate using subthreshold electrotonic propagation into the nearby axon terminals in the median eminence. Evidence indicates that neural processing at the distal dendron is responsible for the pulsatile patterning of GnRH secretion. Although the dendron remains unique to the GnRH neuron, data show that it exists in both mice and rats and may be a common feature of mammalian species in which GnRH neuron cell bodies do not migrate into the basal hypothalamus. This review outlines the discovery and function of the dendron as a unique neuronal structure optimised to generate episodic neuronal output.
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Affiliation(s)
- Allan E Herbison
- Department of Physiology Development and Neuroscience, University of Cambridge, Cambridge, UK
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2
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Moore AM, Prescott M, Czieselsky K, Desroziers E, Yip SH, Campbell RE, Herbison AE. Synaptic Innervation of the GnRH Neuron Distal Dendron in Female Mice. Endocrinology 2018; 159:3200-3208. [PMID: 30010812 DOI: 10.1210/en.2018-00505] [Citation(s) in RCA: 28] [Impact Index Per Article: 4.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 05/23/2018] [Accepted: 07/05/2018] [Indexed: 11/19/2022]
Abstract
GnRH neuron cell bodies are scattered throughout the basal forebrain but funnel their projections to the median eminence to release GnRH into the pituitary portal system to control fertility. Prior studies have shown that GnRH neurons located in the anterior hypothalamus send projections to the median eminence that have characteristics of both dendrites and axons. These unusual structures have been termed "dendrons." To address whether the dendron is unique to anterior hypothalamic GnRH neurons or is also a characteristic of more rostral GnRH neurons, we used viral vector‒mediated GnRH neuron‒specific tract-tracing coupled with CLARITY optical clearing. Individual rostral preoptic area GnRH neurons in female mice were identified to elaborate processes up to 4 mm in length that exhibited spines and projected all the way to the median eminence before branching into multiple short axons. The synaptic innervation patterns of distal GnRH neuron dendrons and their short axons in the vicinity of the median eminence were examined using electron microscopy. This revealed the presence of a high density of synaptic inputs to distal dendrons at the border of the median eminence. In contrast, no synapses were detected on any GnRH neuron axons. These studies demonstrate that GnRH neurons in the rostral preoptic area project dendrons to the edge of the median eminence, whereupon they branch into multiple short axons responsible for GnRH secretion. The dense synaptic innervation of these distal dendrons likely represents an efficient mechanism for controlling GnRH secretion required for fertility.
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Affiliation(s)
- Aleisha M Moore
- Centre for Neuroendocrinology, Department of Physiology, Otago School of Medical Sciences, University of Otago, Dunedin, New Zealand
| | - Mel Prescott
- Centre for Neuroendocrinology, Department of Physiology, Otago School of Medical Sciences, University of Otago, Dunedin, New Zealand
| | - Katja Czieselsky
- Centre for Neuroendocrinology, Department of Physiology, Otago School of Medical Sciences, University of Otago, Dunedin, New Zealand
| | - Elodie Desroziers
- Centre for Neuroendocrinology, Department of Physiology, Otago School of Medical Sciences, University of Otago, Dunedin, New Zealand
| | - Siew Hoong Yip
- Centre for Neuroendocrinology, Department of Physiology, Otago School of Medical Sciences, University of Otago, Dunedin, New Zealand
| | - Rebecca E Campbell
- Centre for Neuroendocrinology, Department of Physiology, Otago School of Medical Sciences, University of Otago, Dunedin, New Zealand
| | - Allan E Herbison
- Centre for Neuroendocrinology, Department of Physiology, Otago School of Medical Sciences, University of Otago, Dunedin, New Zealand
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Naugle MM, Lozano SA, Guarraci FA, Lindsey LF, Kim JE, Morrison JH, Janssen WG, Yin W, Gore AC. Age and Long-Term Hormone Treatment Effects on the Ultrastructural Morphology of the Median Eminence of Female Rhesus Macaques. Neuroendocrinology 2016; 103:650-64. [PMID: 26536204 PMCID: PMC4860175 DOI: 10.1159/000442015] [Citation(s) in RCA: 9] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 04/27/2015] [Accepted: 10/29/2015] [Indexed: 12/26/2022]
Abstract
The median eminence (ME) of the hypothalamus comprises the hypothalamic nerve terminals, glia (especially tanycytes) and the portal capillary vasculature that transports hypothalamic neurohormones to the anterior pituitary gland. The ultrastructure of the ME is dynamically regulated by hormones and undergoes organizational changes during development and reproductive cycles in adult females, but relatively little is known about the ME during aging, especially in nonhuman primates. Therefore, we used a novel transmission scanning electron microscopy technique to examine the cytoarchitecture of the ME of young and aged female rhesus macaques in a preclinical monkey model of menopausal hormone treatments. Rhesus macaques were ovariectomized and treated for 2 years with vehicle, estradiol (E2), or estradiol + progesterone (E2 + P4). While the overall cytoarchitecture of the ME underwent relatively few changes with age and hormones, changes to some features of neural and glial components near the portal capillaries were observed. Specifically, large neuroterminal size was greater in aged compared to young adult animals, an effect that was mitigated or reversed by E2 alone but not by E2 + P4 treatment. Overall glial size and the density and tissue fraction of the largest subset of glia were greater in aged monkeys, and in some cases reversed by E2 treatment. Mitochondrial size was decreased by E2, but not E2 + P4, only in aged macaques. These results contrast substantially with work in rodents, suggesting that the ME of aging macaques is less vulnerable to age-related disorganization, and that the effects of E2 on monkeys' ME are age specific.
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Affiliation(s)
| | - Sateria A. Lozano
- Division of Pharmacology & Toxicology, University of Texas at Austin, Austin, TX
| | - Fay A. Guarraci
- Department of Psychology, Southwestern University, Georgetown, TX
| | - Larry F. Lindsey
- Center for Learning and Memory, University of Texas at Austin, Austin, TX
| | - Ji E. Kim
- Division of Pharmacology & Toxicology, University of Texas at Austin, Austin, TX
| | - John H. Morrison
- Fishberg Department of Neuroscience and the Friedman Brain Institute, Icahn School of Medicine at Mount Sinai, New York, NY
| | - William G.M. Janssen
- Fishberg Department of Neuroscience and the Friedman Brain Institute, Icahn School of Medicine at Mount Sinai, New York, NY
| | - Weiling Yin
- Division of Pharmacology & Toxicology, University of Texas at Austin, Austin, TX
| | - Andrea C. Gore
- Institute for Neuroscience, University of Texas at Austin, Austin, TX
- Division of Pharmacology & Toxicology, University of Texas at Austin, Austin, TX
- Institute for Cellular & Molecular Biology, University of Texas at Austin, Austin, TX
- Correspondence: Andrea C Gore, PhD, The University of Texas at Austin, 107 West Dean Keeton, C0875, Austin, TX, 78712, USA, ; Tel: +1-512-471-3669; Fax: +1-512-471-5002
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Sharif A, Baroncini M, Prevot V. Role of glia in the regulation of gonadotropin-releasing hormone neuronal activity and secretion. Neuroendocrinology 2013; 98:1-15. [PMID: 23735672 DOI: 10.1159/000351867] [Citation(s) in RCA: 42] [Impact Index Per Article: 3.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 04/05/2013] [Accepted: 05/08/2013] [Indexed: 11/19/2022]
Abstract
Gonadotropin-releasing hormone (GnRH) neurons are the final common pathway for the central control of reproduction. The coordinated and timely activation of these hypothalamic neurons, which determines sexual development and adult reproductive function, lies under the tight control of a complex array of excitatory and inhibitory transsynaptic inputs. In addition, research conducted over the past 20 years has unveiled the major contribution of glial cells to the control of GnRH neurons. Glia use a variety of molecular and cellular strategies to modulate GnRH neuronal function both at the level of their cell bodies and at their nerve terminals. These mechanisms include the secretion of bioactive molecules that exert paracrine effects on GnRH neurons, juxtacrine interactions between glial cells and GnRH neurons via adhesive molecules and the morphological plasticity of the glial coverage of GnRH neurons. It now appears that glial cells are integral components, along with upstream neuronal networks, of the central control of GnRH neuronal function. This review attempts to summarize our current knowledge of the mechanisms used by glial cells to control GnRH neuronal activity and secretion.
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Affiliation(s)
- Ariane Sharif
- INSERM, Jean-Pierre Aubert Research Center, Development and Plasticity of the Postnatal Brain, Unit 837, and UDSL, School of Medicine, Lille, France.
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Abstract
The median eminence at the base of the hypothalamus serves as an interface between the neural and peripheral endocrine systems. It releases hypothalamic-releasing hormones into the portal capillary bed for transport to the anterior pituitary, which provides further signals to target endocrine systems. Of specific relevance to reproduction, a group of about 1000 neurons in mammals release the gonadotropin-releasing hormone (GnRH) peptide from neuroterminals in the median eminence. During the life cycle, there are dramatic changes in reproductive demands, and we focus this review on how GnRH terminals in the median eminence change during reproductive senescence. We discuss morphological and functional properties of the median eminence, and how relationships among GnRH terminals and their microenvironment of nerve terminals, glial cells, and the portal capillary vasculature determine the ability of GnRH peptide to be secreted and to reach its target in the anterior pituitary gland.
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Affiliation(s)
- Weiling Yin
- Division of Pharmacology and Toxicology, College of Pharmacy, The University of Texas at Austin, Austin, Texas 78712, USA
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Prevot V, Hanchate NK, Bellefontaine N, Sharif A, Parkash J, Estrella C, Allet C, de Seranno S, Campagne C, de Tassigny XD, Baroncini M. Function-related structural plasticity of the GnRH system: a role for neuronal-glial-endothelial interactions. Front Neuroendocrinol 2010; 31:241-58. [PMID: 20546773 DOI: 10.1016/j.yfrne.2010.05.003] [Citation(s) in RCA: 46] [Impact Index Per Article: 3.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 03/05/2010] [Revised: 05/17/2010] [Accepted: 05/18/2010] [Indexed: 12/18/2022]
Abstract
As the final common pathway for the central control of gonadotropin secretion, GnRH neurons are subjected to numerous regulatory homeostatic and external factors to achieve levels of fertility appropriate to the organism. The GnRH system thus provides an excellent model in which to investigate the complex relationships between neurosecretion, morphological plasticity and the expression of a physiological function. Throughout the reproductive cycle beginning from postnatal sexual development and the onset of puberty to reproductive senescence, and even within the ovarian cycle itself, all levels of the GnRH system undergo morphological plasticity. This structural plasticity within the GnRH system appears crucial to the timely control of reproductive competence within the individual, and as such must have coordinated actions of multiple signals secreted from glial cells, endothelial cells, and GnRH neurons. Thus, the GnRH system must be viewed as a complete neuro-glial-vascular unit that works in concert to maintain the reproductive axis.
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Affiliation(s)
- Vincent Prevot
- Inserm, Jean-Pierre Aubert Research Center, U837, Development and Plasticity of the Postnatal Brain, F-59000 Lille, France.
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Yin W, Mendenhall JM, Monita M, Gore AC. Three-dimensional properties of GnRH neuroterminals in the median eminence of young and old rats. J Comp Neurol 2010; 517:284-95. [PMID: 19757493 DOI: 10.1002/cne.22156] [Citation(s) in RCA: 26] [Impact Index Per Article: 1.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/06/2023]
Abstract
The decapeptide gonadotropin-releasing hormone (GnRH), which regulates reproduction in all vertebrates, is stored in, and secreted from, large dense-core secretory vesicles in nerve terminals in the median eminence. GnRH is released from these terminals with biological rhythms that are critical for the maintenance of normal reproduction. During reproductive aging in female rats, there is a loss of GnRH pulses and a diminution of the GnRH surge. However, information about the specific role of GnRH nerve terminals is lacking, particularly in the context of aging. We sought to gain novel ultrastructural information about GnRH neuroterminals by performing three-dimensional (3D) reconstructions of GnRH neuroterminals and their surrounding microenvironment in the median eminence of young (4-5 months) and old (22-24 months) ovariectomized Sprague-Dawley female rats. Median eminence tissues were freeze-plunge embedded and serial ultrathin sections were collected on slot grids for immunogold labeling of GnRH immunoreactivity. Sequential images were used to create 3D models of GnRH terminals. These reconstructions provided novel perspectives into the morphological properties of GnRH terminals and their neural and glial environment. We also noted that the cytoarchitectural features of the median eminence became disorganized with aging. Quantitative measures showed a significant decrease in the apposition between GnRH terminal membranes and glial cells. Our data suggest reproductive aging in rats is characterized by structural organizational changes to the GnRH terminal microenvironment in the median eminence.
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Affiliation(s)
- Weiling Yin
- Division of Pharmacology and Toxicology, College of Pharmacy, University of Texas at Austin, Austin, TX 78712, USA
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Yin W, Wu D, Noel ML, Gore AC. Gonadotropin-releasing hormone neuroterminals and their microenvironment in the median eminence: effects of aging and estradiol treatment. Endocrinology 2009; 150:5498-508. [PMID: 19819960 PMCID: PMC2795719 DOI: 10.1210/en.2009-0679] [Citation(s) in RCA: 36] [Impact Index Per Article: 2.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 12/18/2022]
Abstract
The GnRH decapeptide controls reproductive function through its release from neuroendocrine terminals in the median eminence, a site where there is a convergence of numerous nerve terminals and glial cells. Previous work showed dynamic changes in the GnRH-glial-capillary network in the median eminence under different physiological conditions. Because aging in rats is associated with a diminution of GnRH release and responsiveness to estradiol feedback, we examined effects of age and estradiol treatment on these anatomical interactions. Rats were ovariectomized at young (4 months), middle-aged (11 months), or old (22-23 months) ages, allowed 4 wk to recover, and then treated with vehicle or estradiol for 72 h followed by perfusion. Immunofluorescence of GnRH was measured, and immunogold electron microscopic analyses were performed to study the ultrastructural properties of GnRH neuroterminals and their microenvironment. Although the GnRH immunofluorescent signal showed no significant changes with age and estradiol treatment, we found that the median eminence underwent both qualitative and quantitative structural changes with age, including a disorganization of cytoarchitecture with aging and a decrease in the apposition of GnRH neuroterminals to glia with age and estradiol treatment. Thus, although GnRH neurons can continue to synthesize and transport peptide, changes in the GnRH neuroterminal-glial-capillary machinery occur during reproductive senescence in a manner consistent with a disconnection of these elements and a potential dysregulation of GnRH neurosecretion.
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Affiliation(s)
- Weiling Yin
- Division of Pharmacology and Toxicology, The University of Texas at Austin, Austin, Texas 78712, USA
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Backholer K, Smith J, Clarke IJ. Melanocortins may stimulate reproduction by activating orexin neurons in the dorsomedial hypothalamus and kisspeptin neurons in the preoptic area of the ewe. Endocrinology 2009; 150:5488-97. [PMID: 19819961 DOI: 10.1210/en.2009-0604] [Citation(s) in RCA: 86] [Impact Index Per Article: 5.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 11/19/2022]
Abstract
To further test the hypothesis that melanocortins stimulate the reproductive axis, we treated ewes with melanocortin agonist (MTII) in the luteal phase of the estrous cycle and during seasonal anestrus. Lateral ventricular infusion of MTII (10 microg/h) during the luteal phase increased LH secretion. Retrograde neuronal tracing in the brain showed few proopiomelanocortin or kisspeptin cells in the arcuate nucleus, but more than 70% of kisspeptin cells in the dorsolateral preoptic area (POA), projecting to the ventromedial POA in which GnRH cells are located. MTII infusion (20 h) was repeated in luteal phase ewes and brains were harvested to measure gene expression of preproorexin and kisspeptin. Expression of orexin in the dorsomedial hypothalamus and kisspeptin in the POA was up-regulated by MTII treatment and Kiss1 in the arcuate nucleus was down-regulated. Seasonally anestrous ewes were progesterone primed and then treated (lateral ventricular) with MTII (10 microg/h) or vehicle for 30 h, and blood samples were collected every 2 h from 4 h before infusion until 6 h afterward to monitor acute response in terms of LH levels. A rise in basal LH levels was seen, but samples collected around the time of the predicted LH surge did not indicate that an ovulatory event occurred. We conclude that melanocortins are positive regulators of the reproductive neuroendocrine system, but treatment with melanocortins does not fully overcome seasonal acyclicity. The stimulatory effect of melanocortin in the luteal phase of the estrous cycle may be via the activation of kisspeptin cells in the POA and/or orexin cells in the dorsomedial hypothalamus.
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Affiliation(s)
- Kathryn Backholer
- Department of Physiology, Monash University, Clayton, Victoria 3800, Australia
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Ramaswamy S, Guerriero KA, Gibbs RB, Plant TM. Structural interactions between kisspeptin and GnRH neurons in the mediobasal hypothalamus of the male rhesus monkey (Macaca mulatta) as revealed by double immunofluorescence and confocal microscopy. Endocrinology 2008; 149:4387-95. [PMID: 18511511 PMCID: PMC2553371 DOI: 10.1210/en.2008-0438] [Citation(s) in RCA: 197] [Impact Index Per Article: 12.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 11/19/2022]
Abstract
Kisspeptin is recognized to play a critical role in eliciting the pubertal resurgence of pulsatile GnRH release, the proximal trigger of puberty in higher primates. Expression of the kisspeptin receptor (GPR54) by GnRH neurons indicates a direct action of kisspeptin on the GnRH neuronal network. The purpose of the present study was to examine the distribution of kisspeptin cell bodies in the monkey hypothalamus and to assess the structural basis for the stimulatory action of kisspeptin on the GnRH neuronal network. Three castrated male rhesus monkeys, 39-51 months of age, were deeply anesthetized and their brains perfused transcardially with 4% paraformaldehyde in PBS. Serial 25-microm coronal sections throughout the hypothalamus were prepared, and immunopositive neurons identified using a cocktail of specific primary antibodies (sheep anti-kisspeptin at 1:120,000, and rabbit anti-GnRH at 1:100,000) detected with fluorescently tagged secondary antibodies (antisheep, Alexa Fluor 488; antirabbit, Cy3) in combination with confocal microscopy. Kisspeptin perikarya were found only in the mediobasal hypothalamus (MBH) almost exclusively in the posterior two-thirds of the arcuate nucleus. Surprisingly, kisspeptin-beaded axons made only infrequent contacts with GnRH neurons (kisspeptin and GnRH profiles abutting in a 0.5- to 1.0-mum optical section) in the MBH. In the median eminence, kisspeptin and GnRH axons were found in extensive and intimate association. GnRH contacts on kisspeptin perikarya and dendrites were observed. These findings indicate that nonsynaptic pathways of communication in the median eminence should be considered as a possible mechanism of kisspeptin regulation of GnRH release, and provide an anatomical basis for reciprocal control of kisspeptin neuronal activity by GnRH.
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Affiliation(s)
- Suresh Ramaswamy
- Department of Cell Biology and Physiology, University of Pittsburgh, Pittsburgh, Pennsylvania 15213, USA
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Baroncini M, Allet C, Leroy D, Beauvillain JC, Francke JP, Prevot V. Morphological evidence for direct interaction between gonadotrophin-releasing hormone neurones and astroglial cells in the human hypothalamus. J Neuroendocrinol 2007; 19:691-702. [PMID: 17680884 DOI: 10.1111/j.1365-2826.2007.01576.x] [Citation(s) in RCA: 58] [Impact Index Per Article: 3.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 11/27/2022]
Abstract
In rodents, there is compelling evidence indicating that dynamic cell-to-cell communications involving cross talk between astroglial cells (such as astrocytes and specialised ependymoglial cells known as tanycytes) and neurones are important in regulating the secretion of gonadotrophin-releasing hormone (GnRH), the neurohormone that controls both sexual maturation and adult reproductive function. However, whether such astroglial cell-GnRH neurone interactions occur in the human brain is not known. In the present study, we used immunofluorescence to examine the anatomical relationship between GnRH neurones and glial cells within the hypothalamus of five women. Double-staining experiments demonstrated the ensheathment of GnRH neurone perikarya by glial fibrillary acidic protein (GFAP)-immunoreactive astrocyte processes in the periventricular zone of the tuberal region of the hypothalamus. GFAP immunoreactivity did not overlap that of GnRH at the GnRH neurone's projection site (i.e. the median eminence of the hypothalamus). Rather, human GnRH neuroendocrine fibres were found to be closely associated with vimentin or nestin-immunopositive radial glial processes likely belonging to tanycytes. In line with these light microscopy data, ultrastructural examination of GnRH-immunoreactive neurones showed numerous glial cells in direct apposition to pre-embedding-labelled GnRH cell bodies and/or dendrites in the infundibular nucleus, whereas postembedding immunogold-labelled GnRH nerve terminals were often seen to be enwrapped by glial cell processes in the median eminence. GnRH nerve button were sometimes visualised in close proximity to fenestrated pituitary portal blood capillaries and/or evaginations of the basal lamina that delineate the pericapillary space. In summary, these data demonstrate that GnRH neurones morphologically interact with astrocytes and tanycytes in the human brain and provide evidence that glial cells may contribute physiologically to the process by which the neuroendocrine brain controls the function of GnRH neurones in humans.
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Affiliation(s)
- M Baroncini
- INSERM U837-Development and Plasticity of the Postnatal Brain, Jean-Pierre Aubert Research Center, Place de Verdun, 59045 Lille, France
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Campbell RE. Defining the gonadotrophin-releasing hormone neuronal network: transgenic approaches to understanding neurocircuitry. J Neuroendocrinol 2007; 19:561-73. [PMID: 17532792 DOI: 10.1111/j.1365-2826.2007.01561.x] [Citation(s) in RCA: 27] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 11/30/2022]
Abstract
The gonadotrophin-releasing hormone (GnRH) neurones are the final downstream effector neurones driving the central regulation of reproductive function and fertility in all mammalian species. Although it is abundantly clear that successful fertility relies upon the communication of a variety signals regarding internal and external cues to the GnRH neuronal population, how this is achieved remains poorly understood. A range of technical limitations has posed significant hurdles to defining, with any certainty, the complexities of the synaptic neuronal network regulating GnRH neurones. However, recent advances in transgenic technology have opened up new avenues to reconsider questions aimed at understanding this critical network. This article addresses some of the latest advances that use transgenic mouse models as tools to understand the neuronal circuitry underpinning the regulation of the GnRH neurones. By incorporating standard morphological and viral tract tracing techniques with innovative transgenic tools, recent studies have uncovered previously unappreciated qualities of the GnRH neurone, including extensive dendritic lengths, numerous somal and dendritic spines and plasticity over pubertal development, along with beginning to define the primary and higher-order afferents that make up the GnRH neuronal network.
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Affiliation(s)
- R E Campbell
- Centre for Neuroendocrinology and Department of Physiology, University of Otago School of Medical Sciences, Dunedin, New Zealand.
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Abstract
The process of reproductive senescence in many female mammals, including humans, is characterized by a gradual transition from regular reproductive cycles to irregular cycles to eventual acyclicity, and ultimately a loss of fertility. In the present review, the role of the hypothalamic gonadotropin-releasing hormone (GnRH) neurons is considered in this context. GnRH neurons provide the primary driving force upon the other levels of the reproductive axis. With respect to aging, GnRH cells undergo changes in biosynthesis, processing and release of the GnRH decapeptide. GnRH neurons also exhibit morphologic and ultrastructural alterations that appear to underlie these biosynthetic properties. Thus, functional and morphologic changes in the GnRH neurosecretory system may play causal roles in the transition to acyclicity. In addition, GnRH neurons are regulated by numerous inputs from neurotransmitters, neuromodulators and glia. The relationship among GnRH cells and their inputs at the cell body (thereby affecting GnRH biosynthesis) and the neuroterminal (thereby affecting GnRH neurosecretion) is crucial to the function of the GnRH system, with age-related changes in these relationships contributing to the reproductive senescent process. Therefore, the aging hypothalamus is characterized by changes intrinsic to the GnRH cell, as well as its regulatory inputs, which summate to contribute to a loss of reproductive competence in aging females.
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Affiliation(s)
- Weiling Yin
- Division of Pharmacology and Toxicology, College of Pharmacy and Institute for Neuroscience and Institute for Cellular and Molecular Biology, University of Texas at Austin, Austin, TX 78712, USA
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Abstract
Puberty is the attainment of fertility, a process encompassing morphological, physiological and behavioural development. The increased hypothalamic secretion of the gonadotrophin-releasing hormone decapeptide (GnRH) is essential for the activation of the pituitary-gonadal axis at puberty. The GnRH secretory network initially develops and is temporarily active during species-specific periods of fetal/neonatal development, so puberty is the secondary reactivation of an existing system. From a neurobiological perspective, the timing of puberty is therefore a function of changes in the neural systems controlling GnRH release. The large variability between individuals in the onset and progression of puberty indicates that the timing of puberty is not simply a function of chronological age. Rather, the neurotransmitter and neuromodulatory systems that impact upon the GnRH secretory network convey information about metabolic fuels, energy stores and somatic development and, for many species, information about season and social environment. The clear links demonstrated between metabolic fuel availability and reproductive function in many animal models provides evidence that the earlier onset of pubertal development observed in girls in certain US study populations is likely to relate to the increasing prevalence of overweight and obesity in adolescents.
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Affiliation(s)
- Francis J P Ebling
- School of Biomedical Sciences and Institute of Neuroscience, University of Nottingham Medical School, Queen's Medical Centre, Nottingham, NG7 2UH, UK.
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Affiliation(s)
- Tony M Plant
- Department of Cell Biology and Physiology, University of Pittsburgh School of Medicine, Pittsburgh, Pennsylvania 15261, USA.
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Richter TA, Keen KL, Terasawa E. Synchronization of Ca(2+) oscillations among primate LHRH neurons and nonneuronal cells in vitro. J Neurophysiol 2002; 88:1559-67. [PMID: 12205175 DOI: 10.1152/jn.2002.88.3.1559] [Citation(s) in RCA: 38] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022] Open
Abstract
Periodic release of luteinizing hormone-releasing hormone (LHRH) from the hypothalamus is essential for normal reproductive function. Pulsatile LHRH release appears to result from the synchronous activity of LHRH neurons. However, how the activity of these neurons is synchronized to release LHRH peptide in a pulsatile manner is unclear. Because there is little evidence of physical coupling among LHRH neurons in the hypothalamus, we hypothesized that the activity of LHRH neurons might be coordinated by indirect intercellular communication via intermediary (nonneural) cells rather than direct interneural coupling. In this study, we used an in vitro preparation of LHRH neurons derived from the olfactory placode of monkey embryos to assess whether nonneuronal cells, play a role in coordinating LHRH neuronal activity. We found that cultured LHRH neurons and nonneuronal cells both exhibit spontaneous oscillations in the concentration of intracellular Ca(2+) ([Ca(2+)](i)) at similar frequencies. Moreover, [Ca(2+)](i) oscillations in both types of cell were periodically synchronized. Synchronized [Ca(2+)](i) oscillations spread as intercellular Ca(2+) waves across fields of cells that included LHRH neurons and nonneuronal cells, although waves spread at a higher velocity among LHRH neurons. These results suggest that LHRH neurons and nonneuronal cells are functionally integrated and that nonneuronal cells could be involved in synchronizing the activity of the LHRH neurosecretory network.
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Affiliation(s)
- T A Richter
- Wisconsin National Primate Research Center, University of Wisconsin, Madison, Wisconsin 53715-1261, USA
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Plant TM. Neurobiological bases underlying the control of the onset of puberty in the rhesus monkey: a representative higher primate. Front Neuroendocrinol 2001; 22:107-39. [PMID: 11259134 DOI: 10.1006/frne.2001.0211] [Citation(s) in RCA: 68] [Impact Index Per Article: 3.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 11/22/2022]
Abstract
The purpose of this article is to discuss our understanding of the neurobiological mechanisms that govern the timing of the onset of puberty in the rhesus monkey, a representative higher primate, and, whenever possible, to place findings obtained from studies of this macaque in perspective with those for the human situation. Specifically, the dynamics in the postnatal ontogeny of hypothalamic GnRH gene expression and release are described, and the roles of neuropeptide Y and gamma-aminobutyric acid in imposing the restraint on pulsatile GnRH release during juvenile development are examined. Finally, the hypothesis that circulating leptin provides the signal that times the reaugmentation of pulsatile GnRH release at the termination of juvenile development, and therefore triggers the onset of primate puberty, is discussed.
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Affiliation(s)
- T M Plant
- Department of Cell Biology and Physiology, University of Pittsburgh School of Medicine, Pennsylvania 15261, USA. plant1+@pitt.edu
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El Majdoubi M, Sahu A, Ramaswamy S, Plant TM. Neuropeptide Y: A hypothalamic brake restraining the onset of puberty in primates. Proc Natl Acad Sci U S A 2000; 97:6179-84. [PMID: 10811877 PMCID: PMC18578 DOI: 10.1073/pnas.090099697] [Citation(s) in RCA: 112] [Impact Index Per Article: 4.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/18/2022] Open
Abstract
The adult reproductive axis is driven by an intermittent discharge of gonadotropin-releasing hormone (GnRH) generated by a network of hypothalamic neurons known as the GnRH pulse generator. Although this signal generator is operational in infant primates, puberty in these species is delayed by activation shortly after birth of a central neural mechanism that holds GnRH release in check during juvenile development. Here, we show that, in the male rhesus monkey, the postnatal pattern in GnRH pulse generator activity is inversely related to that in neuropeptide Y (NPY) gene and protein expression in the mediobasal hypothalamus and that central administration of an NPY Y(1) receptor antagonist to juvenile animals elicits precocious GnRH release. Cell imaging indicated that the developmentally regulated NPY neurons may be located in regions dorsal to the arcuate nucleus. These findings lead us to propose that NPY is a fundamental component of the neurobiological brake restraining the onset of puberty in primates.
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Affiliation(s)
- M El Majdoubi
- Department of Cell Biology and Physiology, University of Pittsburgh School of Medicine, Pittsburgh, PA 15261, USA
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