1
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Power EM, Ganeshan D, Iremonger KJ. Estradiol regulates voltage-gated potassium currents in corticotropin-releasing hormone neurons. J Exp Biol 2023; 226:287072. [PMID: 36805713 PMCID: PMC10038157 DOI: 10.1242/jeb.245222] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/21/2022] [Accepted: 02/08/2023] [Indexed: 02/22/2023]
Abstract
Corticotropin-releasing hormone (CRH) neurons are the primary neural population controlling the hypothalamic-pituitary-adrenal (HPA) axis and the secretion of adrenal stress hormones. Previous work has demonstrated that stress hormone secretion can be regulated by circulating levels of estradiol. However, the effect of estradiol on CRH neuron excitability is less clear. Here, we show that chronic estradiol replacement following ovariectomy increases two types of potassium channel currents in CRH neurons: fast inactivating voltage-gated A-type K+ channel currents (IA) and non-inactivating M-type K+ channel currents (IM). Despite the increase in K+ currents following estradiol replacement, there was no overall change in CRH neuron spiking excitability assessed with either frequency-current curves or current ramps. Together, these data reveal a complex picture whereby ovariectomy and estradiol replacement differentially modulate distinct aspects of CRH neuron and HPA axis function.
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Affiliation(s)
- Emmet M Power
- Centre for Neuroendocrinology, Department of Physiology, School of Biomedical Sciences, University of Otago, Dunedin9016, New Zealand
| | - Dharshini Ganeshan
- Centre for Neuroendocrinology, Department of Physiology, School of Biomedical Sciences, University of Otago, Dunedin9016, New Zealand
| | - Karl J Iremonger
- Centre for Neuroendocrinology, Department of Physiology, School of Biomedical Sciences, University of Otago, Dunedin9016, New Zealand
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2
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Gouws JM, Sherrington A, Zheng S, Kim JS, Iremonger KJ. Regulation of corticotropin-releasing hormone neuronal network activity by noradrenergic stress signals. J Physiol 2022; 600:4347-4359. [PMID: 36040213 PMCID: PMC9825848 DOI: 10.1113/jp283328] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/06/2022] [Accepted: 08/26/2022] [Indexed: 01/11/2023] Open
Abstract
Noradrenaline is a neurotransmitter released in response to homeostatic challenge and activates the hypothalamic-pituitary-adrenal axis via stimulation of corticotropin-releasing hormone (CRH) neurons. Here we investigated the mechanism through which noradrenaline regulates activity within the CRH neuronal network. Using a combination of in vitro GCaMP6f Ca2+ imaging and electrophysiology, we show that noradrenaline induces a robust increase in excitability in a proportion of CRH neurons with many neurons displaying a bursting mode of activity. Noradrenaline-induced activation required α1 -adrenoceptors and L-type voltage-gated Ca2+ channels, but not GABA/glutamate synaptic transmission or sodium action potentials. Exposure of mice to elevated corticosterone levels was able to suppress noradrenaline-induced activation. These results provide further insight into the mechanisms by which noradrenaline regulates CRH neural network activity and hence stress responses. KEY POINTS: GCaMP6f Ca2+ imaging and on-cell patch-clamp recordings reveal that corticotropin-releasing hormone neurons are activated by noradrenaline with many neurons displaying a bursting mode of activity. Noradrenaline-induced activation requires α1 -adrenoceptors. Noradrenaline-induced Ca2+ elevations persist after blocking GABAA , AMPA, NMDA receptors and voltage-gated Na+ channels. Noradrenaline-induced Ca2+ elevations require L-type voltage-gated Ca2+ channels. Corticosterone suppresses noradrenaline-induced excitation.
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Affiliation(s)
- Julia M. Gouws
- Centre for Neuroendocrinology, Department of Physiology, School of Biomedical SciencesUniversity of OtagoDunedinOtagoNew Zealand
| | - Aidan Sherrington
- Centre for Neuroendocrinology, Department of Physiology, School of Biomedical SciencesUniversity of OtagoDunedinOtagoNew Zealand
| | - Shaojie Zheng
- Centre for Neuroendocrinology, Department of Physiology, School of Biomedical SciencesUniversity of OtagoDunedinOtagoNew Zealand
| | - Joon S. Kim
- Centre for Neuroendocrinology, Department of Physiology, School of Biomedical SciencesUniversity of OtagoDunedinOtagoNew Zealand
| | - Karl J. Iremonger
- Centre for Neuroendocrinology, Department of Physiology, School of Biomedical SciencesUniversity of OtagoDunedinOtagoNew Zealand
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3
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Ohline SM, Liu X, Ibrahim MF, Mockett BM, Empson RM, Abraham WC, Iremonger KJ, Jones PP. Altered membrane properties but unchanged intrinsic excitability and spontaneous postsynaptic currents in an aged APPswe/PS1dE9 model of Alzheimer’s disease. Front Cell Neurosci 2022; 16:958876. [PMID: 36090787 PMCID: PMC9459330 DOI: 10.3389/fncel.2022.958876] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/01/2022] [Accepted: 08/09/2022] [Indexed: 11/13/2022] Open
Abstract
Neuronal hyperexcitability in Alzheimer’s disease (AD) models is thought to either contribute to the formation of amyloid beta plaques or result from their formation. Neuronal hyperexcitability has been shown in the cerebral cortex of the widely used young APPswe/PS1dE9 mice, which have accelerated plaque formation. However, it is currently unclear if hyperexcitability also occurs in CA1 hippocampal neurons of aged animals in this model. In the present work, we have compared intrinsic excitability and spontaneous synaptic inputs from CA1 pyramidal cells of 8-month-old APPswe/PS1dE9 and wildtype control mice. We find no change in intrinsic excitability or spontaneous postsynaptic currents (PSCs) between groups. We did, however, find a reduced input resistance and an increase in hyperpolarization-activated sag current. These results are consistent with findings from other aged AD model mice, including the widely used 5xFAD and 3xTg. Together these results suggest that neuronal hyperexcitability is not a consistent feature of all AD mouse models, particularly at advanced ages.
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Affiliation(s)
- Shane M. Ohline
- Department of Physiology, School of Biomedical Sciences, University of Otago, Dunedin, New Zealand
- HeartOtago, University of Otago, Dunedin, New Zealand
- Brain Health Research Centre, University of Otago, Dunedin, New Zealand
- *Correspondence: Shane M. Ohline,
| | - Xinhuai Liu
- Department of Physiology, School of Biomedical Sciences, University of Otago, Dunedin, New Zealand
| | - Mohamed F. Ibrahim
- Nuffield Department of Clinical Neurosciences, Oxford University, Oxford, United Kingdom
| | - Bruce M. Mockett
- Brain Health Research Centre, University of Otago, Dunedin, New Zealand
- Department of Psychology, University of Otago, Dunedin, New Zealand
| | - Ruth M. Empson
- Department of Physiology, School of Biomedical Sciences, University of Otago, Dunedin, New Zealand
- Brain Health Research Centre, University of Otago, Dunedin, New Zealand
| | - Wickliffe C. Abraham
- Brain Health Research Centre, University of Otago, Dunedin, New Zealand
- Department of Psychology, University of Otago, Dunedin, New Zealand
| | - Karl J. Iremonger
- Department of Physiology, School of Biomedical Sciences, University of Otago, Dunedin, New Zealand
- Brain Health Research Centre, University of Otago, Dunedin, New Zealand
| | - Peter P. Jones
- Department of Physiology, School of Biomedical Sciences, University of Otago, Dunedin, New Zealand
- HeartOtago, University of Otago, Dunedin, New Zealand
- Brain Health Research Centre, University of Otago, Dunedin, New Zealand
- Peter P. Jones,
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4
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Jamieson BB, Kim JS, Iremonger KJ. Cannabinoid and vanilloid pathways mediate opposing forms of synaptic plasticity in corticotropin-releasing hormone neurons. J Neuroendocrinol 2022; 34:e13084. [PMID: 35034400 DOI: 10.1111/jne.13084] [Citation(s) in RCA: 3] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 05/11/2021] [Revised: 12/20/2021] [Accepted: 12/23/2021] [Indexed: 01/18/2023]
Abstract
Activity-dependent release of retrograde signaling molecules form micro-feedback loops to regulate synaptic function in neural circuits. Single neurons can release multiple forms of these signaling molecules, including endocannabinoids and endovanilloids, which act via cannabinoid (CB) receptors and transient receptor potential vanilloid 1 (TRPV1) receptors. In hypothalamic corticotrophin-releasing hormone (CRH) neurons, endocannabinoids acting via CB1 receptors have been shown to play an important role in regulating excitability and hence stress hormone secretion. However, the importance of endovanilloid signaling in CRH neurons is currently unclear. Here, we show that, in response to postsynaptic depolarization, CRH neurons release endocannabinoid/endovanilloid molecules that can activate CB1 and TRPV1 receptors. Activation of CB1 receptors suppresses glutamate neurotransmission whereas activation of TRPV1 enhances spontaneous glutamate transmission. However, the excitatory effects of TRPV1 are normally masked by the inhibitory effects of CB1. When the degradation of the endocannabinoid 2-arachidonoylglycerol (2-AG) was inhibited, this revealed tonic activation of CB1 receptors, suggesting tonic endocannabinoid release. However, we found no evidence for tonic activation of TRPV1 receptors under similar conditions. These findings show that activation of CRH neurons can drive the release of signaling molecules that activate parallel endocannabinoid and endovanilloid receptor pathways to mediate opposing forms of synaptic plasticity.
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Affiliation(s)
- Bradley B Jamieson
- Centre for Neuroendocrinology, Department of Physiology, University of Otago, Dunedin, New Zealand
| | - Joon S Kim
- Centre for Neuroendocrinology, Department of Physiology, University of Otago, Dunedin, New Zealand
| | - Karl J Iremonger
- Centre for Neuroendocrinology, Department of Physiology, University of Otago, Dunedin, New Zealand
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5
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Abbasi M, Perkinson MR, Seymour AJ, Piet R, Campbell RE, Iremonger KJ, Brown CH. Local kisspeptin excitation of rat oxytocin neurones in late pregnancy. J Physiol 2022; 600:1753-1770. [PMID: 35045190 PMCID: PMC9303251 DOI: 10.1113/jp282531] [Citation(s) in RCA: 2] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/21/2021] [Accepted: 01/17/2022] [Indexed: 11/24/2022] Open
Abstract
Abstract The hormone, oxytocin, is synthesised by magnocellular neurones of the supraoptic and paraventricular nuclei and is released from the posterior pituitary gland into the circulation to trigger uterine contractions during parturition. Kisspeptin fibre density increases around the supraoptic nucleus over pregnancy and intracerebroventricular kisspeptin excites oxytocin neurones only in late pregnancy. However, the mechanism of this excitation is unknown. Here, we found that microdialysis administration of kisspeptin into the supraoptic nucleus consistently increased the action potential (spike) firing rate of oxytocin neurones in urethane‐anaesthetised late‐pregnant rats (gestation day 18–21) but not in non‐pregnant rats. Hazard analysis of action potential firing showed that kisspeptin specifically increased the probability of another action potential firing immediately after each action potential (post‐spike excitability) in late‐pregnant rats. Patch‐clamp electrophysiology in hypothalamic slices showed that bath application of kisspeptin did not affect action potential frequency or baseline membrane potential in supraoptic nucleus neurones. Moreover, kisspeptin superfusion did not affect the frequency or amplitude of excitatory postsynaptic currents or inhibitory postsynaptic currents in supraoptic nucleus neurones. Taken together, these studies suggest that kisspeptin directly activates oxytocin neurones in late pregnancy, at least in part, via increased post‐spike excitability. Key points Oxytocin secretion is triggered by action potential firing in magnocellular neurones of the hypothalamic supraoptic and paraventricular nuclei to induce uterine contractions during birth. In late pregnancy, kisspeptin expression increases in rat periventricular nucleus neurones that project to the oxytocin system. Here, we show that intra‐supraoptic nucleus administration of kisspeptin increases the action potential firing rate of oxytocin neurones in anaesthetised late‐pregnant rats, and that the increased firing rate is associated with increased oxytocin neurone excitability immediately after each action potential. By contrast, kisspeptin superfusion of hypothalamic slices did not affect the activity of supraoptic nucleus neurones or the strength of local synaptic inputs to supraoptic nucleus neurones. Hence, kisspeptin might activate oxytocin neurons in late pregnancy by transiently increasing oxytocin neuron excitability after each action potential.
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Affiliation(s)
- Mehwish Abbasi
- Brain Health Research Centre, Centre for Neuroendocrinology and Department of Physiology, University of Otago, Aotearoa, Dunedin, New Zealand
| | - Michael R Perkinson
- Brain Health Research Centre, Centre for Neuroendocrinology and Department of Physiology, University of Otago, Aotearoa, Dunedin, New Zealand
| | - Alexander J Seymour
- Brain Health Research Centre, Centre for Neuroendocrinology and Department of Physiology, University of Otago, Aotearoa, Dunedin, New Zealand
| | - Richard Piet
- Brain Health Research Institute, Department of Biological Sciences, Kent State University, Kent, OH, USA
| | - Rebecca E Campbell
- Brain Health Research Centre, Centre for Neuroendocrinology and Department of Physiology, University of Otago, Aotearoa, Dunedin, New Zealand
| | - Karl J Iremonger
- Brain Health Research Centre, Centre for Neuroendocrinology and Department of Physiology, University of Otago, Aotearoa, Dunedin, New Zealand
| | - Colin H Brown
- Brain Health Research Centre, Centre for Neuroendocrinology and Department of Physiology, University of Otago, Aotearoa, Dunedin, New Zealand
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6
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Perkinson MR, Kim JS, Iremonger KJ, Brown CH. Visualising oxytocin neurone activity in vivo: The key to unlocking central regulation of parturition and lactation. J Neuroendocrinol 2021; 33:e13012. [PMID: 34289195 DOI: 10.1111/jne.13012] [Citation(s) in RCA: 9] [Impact Index Per Article: 3.0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 05/05/2021] [Revised: 06/22/2021] [Accepted: 07/05/2021] [Indexed: 11/26/2022]
Abstract
During parturition and lactation, oxytocin neurones in the supraoptic and paraventricular nuclei fire high-frequency bursts of action potentials that are coordinated across the entire population. Each burst generates a large pulse of oxytocin release into the circulation to induce uterine contraction for parturition and mammary duct contraction for milk ejection. Bursts are stimulated by cervical stretch during parturition and by suckling during lactation. However, the mechanisms by which these stimuli are translated into episodic bursts are poorly understood, as are the mechanisms that coordinate bursts across the oxytocin neurone population. An elegant series of experiments conducted in the 1980s and 1990s used serial paired recordings to show that oxytocin neurones do not act as a syncytium during bursts; rather, they start each burst within a few hundred milliseconds of each other but with no distinct "leaders" or "followers". In addition to afferent noradrenergic inputs that relay the systemic stimuli to oxytocin neurones, bursts depend on somato-dendritic oxytocin release within the hypothalamus. Hence, bursts are considered to be an emergent property of oxytocin neurones that is bootstrapped by appropriate afferent stimulation. Although much progress was made using traditional electrophysiological recordings in head-fixed anaesthetised animals, research has effectively stalled in the last few decades. However, the emergence of new technologies to monitor neuronal activity in freely-behaving animals has reinvigorated efforts to understand the biology underpinning burst firing in oxytocin neurones. Here, we report the use of fibre photometry to monitor the dynamics of milk ejection bursts in the oxytocin neurone population of freely-behaving mice. This approach will shed light on the neural mechanisms that control the oxytocin bursts underpinning parturition and lactation.
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Affiliation(s)
- Michael R Perkinson
- Department of Physiology, Brain Health Research Centre, Centre for Neuroendocrinology, University of Otago, Dunedin, New Zealand
| | - Joon S Kim
- Department of Physiology, Brain Health Research Centre, Centre for Neuroendocrinology, University of Otago, Dunedin, New Zealand
| | - Karl J Iremonger
- Department of Physiology, Brain Health Research Centre, Centre for Neuroendocrinology, University of Otago, Dunedin, New Zealand
| | - Colin H Brown
- Department of Physiology, Brain Health Research Centre, Centre for Neuroendocrinology, University of Otago, Dunedin, New Zealand
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7
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Nair BB, Khant Aung Z, Porteous R, Prescott M, Glendining KA, Jenkins DE, Augustine RA, Silva MSB, Yip SH, Bouwer GT, Brown CH, Jasoni CL, Campbell RE, Bunn SJ, Anderson GM, Grattan DR, Herbison AE, Iremonger KJ. Impact of chronic variable stress on neuroendocrine hypothalamus and pituitary in male and female C57BL/6J mice. J Neuroendocrinol 2021; 33:e12972. [PMID: 33896057 DOI: 10.1111/jne.12972] [Citation(s) in RCA: 15] [Impact Index Per Article: 5.0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 11/22/2020] [Revised: 03/02/2021] [Accepted: 03/24/2021] [Indexed: 12/18/2022]
Abstract
Chronic stress exerts multiple negative effects on the physiology and health of an individual. In the present study, we examined hypothalamic, pituitary and endocrine responses to 14 days of chronic variable stress (CVS) in male and female C57BL/6J mice. In both sexes, CVS induced a significant decrease in body weight and enhanced the acute corticosterone stress response, which was accompanied by a reduction in thymus weight only in females. However, single-point blood measurements of basal prolactin, thyroid-stimulating hormone, luteinising hormone, growth hormone and corticosterone levels taken at the end of the CVS were not different from those of controls. Similarly, pituitary mRNA expression of Fshb, Lhb, Prl and Gh was unchanged by CVS, although Pomc and Tsh were significantly elevated. Within the adrenal medulla, mRNA for Th, Vip and Gal were elevated following CVS. Avp transcript levels within the paraventricular nucleus of the hypothalamus were increased by CVS; however, levels of Gnrh1, Crh, Oxt, Sst, Trh, Ghrh, Th and Kiss1 remained unchanged. Oestrous cycles were lengthened slightly by CVS and ovarian histology revealed a reduction in the number of preovulatory follicles and corpora lutea. Taken together, these observations indicate that 14 days of CVS induces an up-regulation of the neuroendocrine stress axis and creates a mild disruption of female reproductive function. However, the lack of changes in other neuroendocrine axes controlling anterior and posterior pituitary secretion suggest that most neuroendocrine axes are relatively resilient to CVS.
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Affiliation(s)
- Betina B Nair
- Centre for Neuroendocrinology, Departments of Anatomy and Physiology, University of Otago, Dunedin, New Zealand
| | - Zin Khant Aung
- Centre for Neuroendocrinology, Departments of Anatomy and Physiology, University of Otago, Dunedin, New Zealand
| | - Robert Porteous
- Centre for Neuroendocrinology, Departments of Anatomy and Physiology, University of Otago, Dunedin, New Zealand
| | - Melanie Prescott
- Centre for Neuroendocrinology, Departments of Anatomy and Physiology, University of Otago, Dunedin, New Zealand
| | - Kelly A Glendining
- Centre for Neuroendocrinology, Departments of Anatomy and Physiology, University of Otago, Dunedin, New Zealand
| | - Danielle E Jenkins
- Centre for Neuroendocrinology, Departments of Anatomy and Physiology, University of Otago, Dunedin, New Zealand
| | - Rachael A Augustine
- Centre for Neuroendocrinology, Departments of Anatomy and Physiology, University of Otago, Dunedin, New Zealand
| | - Mauro S B Silva
- Centre for Neuroendocrinology, Departments of Anatomy and Physiology, University of Otago, Dunedin, New Zealand
| | - Siew H Yip
- Centre for Neuroendocrinology, Departments of Anatomy and Physiology, University of Otago, Dunedin, New Zealand
| | - Gregory T Bouwer
- Centre for Neuroendocrinology, Departments of Anatomy and Physiology, University of Otago, Dunedin, New Zealand
| | - Colin H Brown
- Centre for Neuroendocrinology, Departments of Anatomy and Physiology, University of Otago, Dunedin, New Zealand
| | - Christine L Jasoni
- Centre for Neuroendocrinology, Departments of Anatomy and Physiology, University of Otago, Dunedin, New Zealand
| | - Rebecca E Campbell
- Centre for Neuroendocrinology, Departments of Anatomy and Physiology, University of Otago, Dunedin, New Zealand
| | - Stephen J Bunn
- Centre for Neuroendocrinology, Departments of Anatomy and Physiology, University of Otago, Dunedin, New Zealand
| | - Greg M Anderson
- Centre for Neuroendocrinology, Departments of Anatomy and Physiology, University of Otago, Dunedin, New Zealand
| | - David R Grattan
- Centre for Neuroendocrinology, Departments of Anatomy and Physiology, University of Otago, Dunedin, New Zealand
| | - Allan E Herbison
- Centre for Neuroendocrinology, Departments of Anatomy and Physiology, University of Otago, Dunedin, New Zealand
| | - Karl J Iremonger
- Centre for Neuroendocrinology, Departments of Anatomy and Physiology, University of Otago, Dunedin, New Zealand
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8
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Perkinson MR, Kim JS, Brown CH, Iremonger KJ. Characterisation of an Oxytocin‐Ires‐Cre Mouse Model for Recording Oxytocin Neuron Bursts in Behaving Mice. FASEB J 2020. [DOI: 10.1096/fasebj.2020.34.s1.03560] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/11/2022]
Affiliation(s)
- Michael R. Perkinson
- Brain Health Research Centre
- Centre for Neuroendocrinology
- School of Biomedical Sciences
- University of Otago
| | - Joon S. Kim
- Brain Health Research Centre
- Centre for Neuroendocrinology
- School of Biomedical Sciences
- University of Otago
| | - Colin H. Brown
- Brain Health Research Centre
- Centre for Neuroendocrinology
- School of Biomedical Sciences
- University of Otago
| | - Karl J. Iremonger
- Brain Health Research Centre
- Centre for Neuroendocrinology
- School of Biomedical Sciences
- University of Otago
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9
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Focke CMB, Iremonger KJ. Rhythmicity matters: Circadian and ultradian patterns of HPA axis activity. Mol Cell Endocrinol 2020; 501:110652. [PMID: 31738971 DOI: 10.1016/j.mce.2019.110652] [Citation(s) in RCA: 22] [Impact Index Per Article: 5.5] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 08/01/2019] [Revised: 10/29/2019] [Accepted: 11/11/2019] [Indexed: 12/15/2022]
Abstract
Oscillations are a fundamental feature of neural and endocrine systems. The hypothalamic-pituitary-adrenal (HPA) axis dynamically controls corticosteroid secretion in basal conditions and in response to stress. Across the 24-h day, HPA axis activity oscillates with both an ultradian and circadian rhythm. These rhythms have been shown to be important for regulating metabolism, inflammation, mood, cognition and stress responsiveness. Here we will discuss the neural and endocrine mechanisms driving these rhythms, the physiological importance of these rhythms and health consequences when they are disrupted.
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Affiliation(s)
- Caroline M B Focke
- Centre for Neuroendocrinology, Department of Physiology, University of Otago, Dunedin, New Zealand
| | - Karl J Iremonger
- Centre for Neuroendocrinology, Department of Physiology, University of Otago, Dunedin, New Zealand.
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10
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Kim JS, Iremonger KJ. Temporally Tuned Corticosteroid Feedback Regulation of the Stress Axis. Trends Endocrinol Metab 2019; 30:783-792. [PMID: 31699237 DOI: 10.1016/j.tem.2019.07.005] [Citation(s) in RCA: 21] [Impact Index Per Article: 4.2] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 02/11/2019] [Revised: 05/23/2019] [Accepted: 07/08/2019] [Indexed: 01/28/2023]
Abstract
Activity of the hypothalamic-pituitary-adrenal (HPA) axis is tuned by corticosteroid feedback. Corticosteroids regulate cellular function via genomic and nongenomic mechanisms, which operate over diverse time scales. This review summarizes recent advances in our understanding of how corticosteroid feedback regulates hypothalamic stress neuron function and output through synaptic plasticity, changes in intrinsic excitability, and modulation of neuropeptide production. The temporal kinetics of corticosteroid actions in the brain versus the pituitary have important implications for how organisms respond to stress. Furthermore, we will discuss, some of the technical limitations and missing links in the field, and the potential implications these may have on our interpretations of corticosteroid negative feedback experiments.
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Affiliation(s)
- Joon S Kim
- Centre for Neuroendocrinology, Department of Physiology, University of Otago, Dunedin, New Zealand
| | - Karl J Iremonger
- Centre for Neuroendocrinology, Department of Physiology, University of Otago, Dunedin, New Zealand.
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11
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Bittar TP, Nair BB, Kim JS, Chandrasekera D, Sherrington A, Iremonger KJ. Corticosterone mediated functional and structural plasticity in corticotropin-releasing hormone neurons. Neuropharmacology 2019; 154:79-86. [PMID: 30771372 DOI: 10.1016/j.neuropharm.2019.02.017] [Citation(s) in RCA: 8] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/25/2018] [Revised: 01/22/2019] [Accepted: 02/12/2019] [Indexed: 01/01/2023]
Abstract
Corticosteroid stress hormones drive a multitude of adaptations in the brain. Hypothalamic corticotropin-releasing hormone (CRH) neurons control the circulating levels of corticosteroid stress hormones in the body and are themselves highly sensitive to corticosteroids. CRH neurons have been shown to undergo various adaptions in response to acute stress hormone elevations. However, their structural and physiological changes under chronically elevated corticosterone are less clear. To address this, we determined the structural and functional changes in CRH neurons in the paraventricular nucleus of the hypothalamus following 14 days of corticosterone treatment. We find that prolonged corticosterone elevation reduces CRH neuron intrinsic excitability as measured by summation of subthreshold postsynaptic depolarisations and spiking output. We find that under normal conditions, CRH neurons have a relatively compact and simple dendritic arbor, with a low density of somatic and dendritic spines. Interestingly, the axon originated from a proximal dendrite close to the soma in approximately half of the CRH neurons reconstructed. While prolonged elevation in corticosterone levels did not result in any changes to gross dendritic morphology, it induced a significant reduction in both somatic and dendritic spine density. Together these data reveal the morphological features of hypothalamic CRH neurons and highlight their capacity to undergo functional and morphological plasticity in response to chronic corticosterone elevations. This article is part of the Special Issue entitled 'Hypothalamic Control of Homeostasis'.
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Affiliation(s)
- Thibault P Bittar
- Centre for Neuroendocrinology, Department of Physiology, University of Otago, Dunedin, New Zealand
| | - Betina B Nair
- Centre for Neuroendocrinology, Department of Physiology, University of Otago, Dunedin, New Zealand
| | - Joon S Kim
- Centre for Neuroendocrinology, Department of Physiology, University of Otago, Dunedin, New Zealand
| | - Dhananjie Chandrasekera
- Centre for Neuroendocrinology, Department of Physiology, University of Otago, Dunedin, New Zealand
| | - Aidan Sherrington
- Centre for Neuroendocrinology, Department of Physiology, University of Otago, Dunedin, New Zealand
| | - Karl J Iremonger
- Centre for Neuroendocrinology, Department of Physiology, University of Otago, Dunedin, New Zealand.
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12
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Jamieson BB, Nair BB, Iremonger KJ. Regulation of hypothalamic corticotropin-releasing hormone neurone excitability by oxytocin. J Neuroendocrinol 2017; 29. [PMID: 28872712 DOI: 10.1111/jne.12532] [Citation(s) in RCA: 33] [Impact Index Per Article: 4.7] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 05/28/2017] [Revised: 08/30/2017] [Accepted: 08/31/2017] [Indexed: 02/01/2023]
Abstract
Oxytocin (OT) is a neuropeptide that exerts multiple actions throughout the brain and periphery. Within the brain, OT regulates diverse neural populations, including neural networks controlling responses to stress. Local release of OT within the paraventricular nucleus (PVN) of the hypothalamus has been suggested to regulate stress responses by modulating the excitability of neighbouring corticotropin-releasing hormone (CRH) neurones. However, the mechanisms by which OT regulates CRH neurone excitability are unclear. In the present study, we investigated the morphological relationship between OT and CRH neurones and determined the effects of OT on CRH neurone excitability. Morphological analysis revealed that the processes of OT and CRH neurones were highly intermingled within the PVN, possibly allowing for local cell-to-cell cross-talk. Whole-cell patch-clamp recordings from CRH neurones were used to study the impact of OT on postsynaptic excitability and synaptic innervation. Bath-applied OT did not alter CRH neurone holding current, spiking output or any action potential parameters. Recordings of evoked excitatory and inhibitory postsynaptic currents (EPSCs/IPSCs) revealed no net effect of OT on current amplitude; however, subgroups of CRH neurones appeared to respond differentially to OT. Analysis of spontaneous EPSC events uncovered a significant reduction in spontaneous EPSC frequency but no change in spontaneous EPSC amplitude in response to OT. Together, these data demonstrate that OT exerts a subtle modulation of synaptic transmission onto CRH neurones providing one potential mechanism by which OT could suppress CRH neurone excitability and stress axis activity.
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Affiliation(s)
- B B Jamieson
- Department of Physiology, Centre for Neuroendocrinology, University of Otago, Dunedin, New Zealand
| | - B B Nair
- Department of Physiology, Centre for Neuroendocrinology, University of Otago, Dunedin, New Zealand
| | - K J Iremonger
- Department of Physiology, Centre for Neuroendocrinology, University of Otago, Dunedin, New Zealand
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Iremonger KJ, Bains JS. Asynchronous presynaptic glutamate release enhances neuronal excitability during the post-spike refractory period. J Physiol 2016; 594:1005-15. [PMID: 26662615 DOI: 10.1113/jp271485] [Citation(s) in RCA: 12] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/25/2015] [Accepted: 12/04/2015] [Indexed: 01/11/2023] Open
Abstract
KEY POINTS Many excitatory synapses in the brain release glutamate with both synchronous and asynchronous components. Immediately following an action potential, neurons display a reduced excitability due to the post-spike afterhyperpolarization (AHP). This gives rise to a relative refractory period. When an action potential is evoked by glutamate synaptic input possessing asynchronous release, the delayed glutamate release events act to depolarize the neuron during the AHP and overcome the relative refractory period. These results demonstrate a new role for asynchronous release in regulating post-spike excitability and the relative refractory period in central neurons. ABSTRACT Post-spike afterhyperpolarizations (AHPs) functionally inhibit neuronal excitability for tens to hundreds of milliseconds following each action potential. This imposes a relative refractory period during which synaptic excitation is less effective at evoking spikes. Here we asked whether some synapses have mechanisms in place that allow them to overcome the AHP and drive spiking in target cells during this period of reduced excitability. We examined glutamate synapses onto oxytocin and vasopressin neurons in the paraventricular nucleus of the hypothalamus. These synapses can display pronounced asynchronous glutamate release following a single presynaptic spike, with the time course of release being similar to that of the post-spike AHP. To test whether asynchronous release is more effective at overcoming the relative refractory period, we evoked a single action potential with either a brief synchronous depolarization or an asynchronous potential and then assessed excitability at multiple time points following the spike. Neurons receiving asynchronous depolarizing synaptic inputs had a shorter relative refractory period than those receiving synchronous depolarizations. Our data demonstrate that synapses releasing glutamate in an asynchronous and delayed manner are ideally adapted to counter the AHP. By effectively overcoming the relative refractory period, the kinetics of excitatory synaptic input can play an important role in controlling post-spike excitability.
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Affiliation(s)
- Karl J Iremonger
- Centre for Neuroendocrinology, Department of Physiology, University of Otago School of Medical Sciences, Dunedin, New Zealand
| | - Jaideep S Bains
- Hotchkiss Brain Institute & Department of Physiology and Pharmacology, University of Calgary, 3330 Hospital Drive NW, Calgary, Alberta, T2N 4N1, Canada
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Abstract
Gonadotropin-releasing hormone (GnRH) neurons integrate synaptic information in their dendrites in order to precisely control GnRH secretion and hence fertility. Recent discoveries concerning the structure and function of GnRH neuron dendrites have shed new light on the control of GnRH neuron excitability and GnRH secretion. This work suggests that GnRH neurons have a unique projection to the median eminence that possesses both dendritic and axonal properties. We propose that this 'dendron' projection allows GnRH neurons to multitask and integrate information in ways that would not be possible in a classically envisioned axon projection.
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Affiliation(s)
- Karl J Iremonger
- Centre for Neuroendocrinology, Department of Physiology, University of Otago School of Medical Sciences, Dunedin, New Zealand
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Iremonger KJ, Wamsteeker Cusulin JI, Bains JS. Changing the tune: plasticity and adaptation of retrograde signals. Trends Neurosci 2013; 36:471-9. [PMID: 23706151 DOI: 10.1016/j.tins.2013.04.007] [Citation(s) in RCA: 28] [Impact Index Per Article: 2.5] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/28/2013] [Revised: 04/18/2013] [Accepted: 04/18/2013] [Indexed: 11/25/2022]
Abstract
Retrograde signaling is a fundamental means by which neurons communicate. The acceptance of this statement has required a revision of how we view transmission and storage of information at the synapse. Although there is a substantial body of literature on the diverse molecules that serve as retrograde signals, less is known about how retrograde signal capacity can be modified. Is retrograde signaling plastic? How does this plasticity manifest? Are there behavioral correlates that may bias a neuron towards 'changing its tune', retrogradely speaking, of course? Here, we review recent findings that retrograde signaling is a highly labile process that adds additional layers of complexity that must be untangled to understand information processing in the nervous system.
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Affiliation(s)
- Karl J Iremonger
- Centre for Neuroendocrinology and Department of Physiology, University of Otago, PO Box 913, Dunedin 9054, New Zealand
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Iremonger KJ. It takes more than one opioid receptor to inhibit a POMC neuron. J Physiol 2013; 591:1595-6. [PMID: 23547192 DOI: 10.1113/jphysiol.2013.251637] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/08/2022] Open
Affiliation(s)
- Karl J Iremonger
- Centre for Neuroendocrinology, Department of Physiology, University of Otago School of Medical Sciences, Dunedin, New Zealand.
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Iremonger KJ, Constantin S, Liu X, Herbison AE. Glutamate regulation of GnRH neuron excitability. Brain Res 2010; 1364:35-43. [PMID: 20807514 DOI: 10.1016/j.brainres.2010.08.071] [Citation(s) in RCA: 74] [Impact Index Per Article: 5.3] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/23/2010] [Revised: 08/18/2010] [Accepted: 08/21/2010] [Indexed: 11/28/2022]
Abstract
The gonadotropin-releasing hormone (GnRH) neuronal network is the master controller of the reproductive axis. It is widely accepted that the amino acid transmitters GABA and glutamate play important roles in controlling GnRH neuron excitability. However, remarkably few studies have examined the functional role of direct glutamate regulation of GnRH neurons. Dual-labeling investigations have shown that GnRH neurons express receptor subunits required for AMPA, NMDA and kainate signaling in a heterogeneous manner. Electrophysiological and calcium imaging studies have confirmed this heterogeneity and shown that while the majority of adult GnRH neurons express AMPA/kainate receptors, only small sub-populations have functional NMDA or metabotropic glutamate receptors. Accumulating evidence suggests that one important role of direct glutamate signaling at GnRH neurons is for their activation at the time of puberty. Whereas in vivo studies have indicated the importance of NMDA signaling within the whole of the GnRH neuronal network, including afferent neurons and glia, investigations at the level of the GnRH neuron suggest that peripubertal changes in AMPA receptor expression may be dominant in the mouse. The sources of glutamatergic inputs to the GnRH neurons are only just beginning to be examined and include the anteroventral periventricular nucleus as well as the possibility that GnRH neurons may use glutamate as a neurotransmitter in recurrent collateral innervation. It is expected that a full understanding of the glutamatergic regulation of GnRH neurons will provide significant insight into the mechanisms underlying their control of reproductive function.
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Affiliation(s)
- Karl J Iremonger
- Centre for Neuroendocrinology, Department of Physiology, University of Otago School of Medical Sciences, Dunedin 9054, New Zealand
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Iremonger KJ, Benediktsson AM, Bains JS. Glutamatergic synaptic transmission in neuroendocrine cells: Basic principles and mechanisms of plasticity. Front Neuroendocrinol 2010; 31:296-306. [PMID: 20347860 DOI: 10.1016/j.yfrne.2010.03.002] [Citation(s) in RCA: 16] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 12/14/2009] [Revised: 03/21/2010] [Accepted: 03/23/2010] [Indexed: 11/27/2022]
Abstract
Glutamate synapses drive the output of neuroendocrine cells in the hypothalamus, but until recently, relatively little was known about the fundamental properties of transmission at these synapses. Here we review recent advances in the understanding of glutamate signals in magnocellular neurosecretory cells (MNCs) in the paraventricular (PVN) and supraoptic nuclei (SON) of the hypothalamus that serve as the last step in synaptic integration before neurohormone release. While these synapses exhibit many similarities with other glutamate synapses described throughout the brain, they also exhibit a number of unique properties that are particularly well suited to the physiology of this system and will be discussed here. In addition, a number of recent studies begin to provide insights into new forms of synaptic plasticity that may be common in other brain regions, but in these cells, may serve important adaptive roles.
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Affiliation(s)
- Karl J Iremonger
- Hotchkiss Brain Institute and Department of Physiology and Pharmacology, University of Calgary, Calgary, AB, Canada T2N 4N1
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Abstract
Classically, the release of glutamate in response to a presynaptic action potential causes a brief increase in postsynaptic excitability. Previous reports indicate that at some central synapses, a single action potential can elicit multiple, asynchronous release events. This raises the possibility that the temporal dynamics of neurotransmitter release may determine the duration of altered postsynaptic excitability. In response to physiological challenges, the magnocellular neurosecretory cells (MNCs) in the paraventricular nucleus of the hypothalamus (PVN) exhibit robust and prolonged increases in neuronal activity. Although the postsynaptic conductances that may facilitate this form of activity have been investigated thoroughly, the role of presynaptic release has been largely overlooked. Because the specific patterns of activity generated by MNCs require the activation of excitatory synaptic inputs, we sought to characterize the release dynamics at these synapses and determine whether they contribute to prolonged excitability in these cells. We obtained whole-cell recordings from MNCs in brain slices of postnatal day 21-44 rats. Stimulation of glutamatergic inputs elicited large and prolonged postsynaptic events that resulted from the summation of multiple, asynchronously released quanta. Asynchronous release was selectively inhibited by the slow calcium buffer EGTA-AM and potentiated by brief high-frequency stimulus trains. These trains caused a prolonged increase in postsynaptic spike activity that could also be eliminated by EGTA-AM. Our results demonstrate that glutamatergic terminals in PVN exhibit asynchronous release, which is important in generating large postsynaptic depolarizations and prolonged spiking in response to brief, high-frequency bursts of presynaptic activity.
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Affiliation(s)
- Karl J. Iremonger
- Hotchkiss Brain Institute and Department of Physiology and Biophysics, University of Calgary, Calgary, Alberta, Canada T2N 4N1
| | - Jaideep S. Bains
- Hotchkiss Brain Institute and Department of Physiology and Biophysics, University of Calgary, Calgary, Alberta, Canada T2N 4N1
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Iremonger KJ, Anderson TR, Hu B, Kiss ZHT. Cellular mechanisms preventing sustained activation of cortex during subcortical high-frequency stimulation. J Neurophysiol 2006; 96:613-21. [PMID: 16554516 DOI: 10.1152/jn.00105.2006] [Citation(s) in RCA: 61] [Impact Index Per Article: 3.4] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022] Open
Abstract
Axonal excitation has been proposed as a key mechanism in therapeutic brain stimulation. In this study we examined how high-frequency stimulation (HFS) of subcortical white matter tracts projecting to motor cortex affects downstream postsynaptic responses in cortical neurons. Whole cell recordings were performed in the primary motor cortex (M1) and ventral thalamus of rat brain slices. In M1, neurons showed only an initial depolarization in response to HFS, after which the membrane potential returned to prestimulation levels. The prolonged suppression of excitation during stimulation was neither associated with GABAergic inhibition nor complete action potential failure in stimulated axons. Instead we found that HFS caused a depression of excitatory synaptic currents in postsynaptic neurons that was specific to the stimulated subcortical input. These data are consistent with the hypothesis that axonal HFS produces a functional deafferentation of postsynaptic targets likely from depletion of neurotransmitter.
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Affiliation(s)
- Karl J Iremonger
- Department of Clinical Neurosciences, Hotchkiss Brain Institute, University of Calgary, Calgary, Canada
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