Biomedical subjects

Peter J Morgan

Publications and source records attributed to Peter J Morgan.

14 recordsLinked to original sources

PubMed2006

What can we learn from seasonal animals about the regulation of energy balance?

Weight loss in humans requires, except during an illness, some form of imposed restriction on food intake or increase in energy expenditure. This necessitates overcoming powerful peripheral and central signals that serve to protect against negative energy balance. The identification of the systems and pathways involved has come from mouse models with genetic and targeted mutations, e.g., ob/ob and MC4 R(-/-) as well as rat models of obesity. Study of seasonal animals has shown that they undergo annual cycles of body fattening and adipose tissue loss as important adaptations to environmental change, yet these changes appear to involve mechanisms distinct from those known already. One animal model, the Siberian hamster, exhibits marked, but reversible, weight loss in response to shortening day length. The body weight is driven by a decrease in food intake with the magnitude of the loss of body weight being directly related to the length of time of exposure to short photoperiod. The most important facet of this response is that the point of energy balance is continuously re-adjusted during the transition in body weight reflecting an apparent 'sliding set point'. Studies have focused on identifying the neural basis of this mechanism. Initial studies of known genes (e.g., NPY, POMC, and AgRP) both through the measurement of gene expression in the arcuate nucleus as well as following intracerebroventricular (i.c.v.) injection indicated that the systems involved are not those involved in restoring energy balance following energy deficits. Instead, a novel mechanism of regulation is implied. Recent studies have begun to explore the neural basis of the seasonal body weight response. A discrete and novel region of the posterior arcuate nucleus, the dorsal medial posterior arcuate nucleus (dmpARC) has been identified, where a battery of gene expression changes for signalling molecules (vgf and histamine H3 receptor) and transcription factors (RXRgamma and RAR) occur in association with seasonal changes in body weight. This work provides the basis of a potentially novel mechanism of energy balance regulation.

Animals↗

PubMed2006

Photoperiodic regulation of cellular retinol binding protein, CRBP1 [corrected] and nestin in tanycytes of the third ventricle ependymal layer of the Siberian hamster.

Tanycytes in the ependymal layer of the third ventricle act both as a barrier and a communication gateway between the cerebrospinal fluid, brain and portal blood supply to the pituitary gland. However, the range, importance and mechanisms involved in the function of tanycytes remain to be explored. In this study, we have utilized a photoperiodic animal to examine the expression of three unrelated gene sequences in relation to photoperiod-induced changes in seasonal physiology and behaviour. We demonstrate that cellular retinol binding protein [corrected] (CRBP1), a retinoic acid transport protein, GPR50, an orphan G-protein-coupled receptor and nestin, an intermediate filament protein, are down-regulated in short-day photoperiods. The distribution of the three sequences is very similar, with expression located in cells with tanycyte morphology in the region of the ependymal layer where tanycytes are located. Furthermore, CRBP1 expression in the ependymal layer is shown to be independent of a circadian clock and altered testosterone levels associated with testicular regression in short photo-period. Pinealectomy of Siberian hamsters demonstrates CRBP1 expression is likely to be dependent on melatonin output from the pineal gland. This provides evidence that tanycytes are seasonally responsive cells and are likely to be an important part of the mechanism to facilitate seasonal physiology and behaviour in the Siberian hamster.

Animals↗

PubMed2005

Temporal changes in gene expression in the arcuate nucleus precede seasonal responses in adiposity and reproduction.

In anticipation of seasonal climate changes, Siberian hamsters display a strategy for survival that entails profound physiological adaptations driven by photoperiod. These include weight loss, reproductive quiescence, and pelage growth with shortening photoperiod and vice versa with lengthening photoperiod. This study reports gene expression changes in the hypothalamus of Siberian hamsters switched from short days (SD) to long days (LD), and also in photorefractory hamsters. Siberian hamsters were maintained in either LD or SD for 14 wk, conditions that generate physiological states of obesity under LD and leanness under SD. After 14 wk, SD lighting was switched to LD and gene expression investigated after 0, 2, 4, and 6 wk by in situ hybridization. Genes encoding nuclear receptors (RXR/RAR), retinoid binding proteins (CRBP1 and CRABP2), and histamine H3 receptor were photoperiodically regulated with significantly lower expression in SD, whereas VGF mRNA expression was significantly higher in SD, in the dorsomedial posterior arcuate nucleus. After a SD-to-LD switch, gene expression changes of CRABP2, RAR, H3R, and VGF occurred relatively rapidly toward LD control levels, ahead of body weight recovery and testicular recrudescence, whereas CRBP1 responded less robustly and rxrgamma did not respond at the mRNA level. In this brain nucleus in photorefractory animals, the CRABP2, RAR, H3R, and VGF mRNA returned toward LD levels, whereas CRBP1 and rxrgamma remained at the reduced SD level. Thus, genes described here are related to photoperiodic programming of the neuroendocrine hypothalamus through expression responses within a subdivision of the arcuate nucleus.

Adaptation, Physiological↗

PubMed2005

Neuromedin-U is regulated by the circadian clock in the SCN of the mouse.

Neuromedin-U (NMU) has been reported to drive several physiological or behavioural responses following i.c.v. injection of the peptide into the third ventricle of rodent brains. Many of these responses are mediated through a change in corticotrophin-releasing factor (CRF) output from the paraventricular nucleus (PVN). A number of the physiological or behavioural responses are regulated in a circadian manner, e.g. feeding. We have previously reported NMU gene expression in the suprachiasmatic nucleus (SCN) and NMU-2 receptor expression in the PVN, dorsal medial hypothalamus (DMH) and other regions of the mouse brain. We therefore hypothesized that NMU would be regulated by the circadian clock and may consequently drive a circadian rhythm of CRF expression in the PVN. Here we report that NMU is regulated in a circadian manner with peak expression during the light phase of a light-dark cycle. In C3H mice held in constant darkness, the NMU rhythm free runs with a period predicted by the free running period of locomotor activity in this mouse. The NMU mRNA transcript colocalizes with cells expressing AVP in the SCN and shows a coincident rhythm of expression with AVP. On the other hand, CRF did not express a circadian rhythm of expression in a light-dark cycle, although a rhythm was evident in constant darkness with a peak of expression prior to the rise of NMU in the same conditions. This would suggest that the circadian rhythm in NMU expression in the SCN does not drive a circadian rhythm in CRF in the PVN to be translated into physiological and behavioural responses mediated by NMU.

Animals↗

PubMed2004

Photoperiodic regulation of histamine H3 receptor and VGF messenger ribonucleic acid in the arcuate nucleus of the Siberian hamster.

To survive winter the Siberian hamster has evolved profound physiological and behavioral adaptations, including a moult to winter pelage, regression of the reproductive axis, onset of daily torpor and increased capacity for thermogenesis. However, one of the most striking adaptations is the catabolism of intraabdominal and sc fat reserves contributing to the loss of up to 40% of body weight. These physiological and behavioral adaptations are photoperiodically driven, yet neither the site(s) in the brain nor the molecular mechanism(s) involved in the regulation of these profound adaptations is known. Here we report a dynamic regulation of gene expression in a dorsal region of the medial posterior area of the arcuate nucleus (dmpARC) of the Siberian and Syrian hamster brain in response to altered photoperiod. We show mRNA for the histamine H3 receptor is down-regulated and VGF is up-regulated in the dmpARC in hamsters switched from long- to short-day photoperiod. These data provide further evidence to support the view that the dmpARC is a major site to relay photoperiodic changes and as a site for the long-term regulation of seasonal physiology and behavior.

Animals↗

PubMed2004

Feeding and behavioural effects of central administration of the melanocortin 3/4-R antagonist SHU9119 in obese and lean Siberian hamsters.

Siberian hamsters accumulate fat reserves in long photoperiods, but show a long-term decrease in food intake and body weight when exposed to a short winter photoperiod. The aim of this study was to determine the role of central melanocortin 3/4 receptors (MC3/4-R) in generating this chronic catabolic state by investigating the effects of SHU9119, a MC3/4-R antagonist, on food intake and associated behaviours. In adult male hamsters, intra-cerebroventricular infusions of SHU9119 significantly increased food intake in a dose-dependent manner. The time course of action was slow, food intake being increased between 4 and 24 h after intra-cerebroventricular administration. A similar degree of increase in food intake occurred in fat hamsters in long days and in lean hamsters chronically exposed to short days. Intra-cerebroventricular treatment with MTII (a MC3/4-R agonist) significantly decreased food intake for up to 24 h after treatment, and SHU9119 reversed these suppressive effects between 4 and 24 h after treatment, a similar time course to that observed when SHU9119 was administered alone. We conclude that endogenous melanocortin peptides acting via MC3/4-R are involved in the regulation of food intake in hamsters in both anabolic and catabolic states, but these acute studies do not provide evidence that increased activity of this hypothalamic system underlies the seasonal decrease in food intake that contributes to the long-term catabolic state in short days.

Analysis of Variance↗

PubMed2003

Photoperiodic regulation of hypothalamic retinoid signaling: association of retinoid X receptor gamma with body weight.

This study reports novel events related to photoperiodic programming of the neuroendocrine hypothalamus. To investigate photoperiod-responsive genes, Siberian hamsters were maintained in long or short photoperiods that generate physiological states of obesity or leanness. Microarray expression analysis first identified CRBP1 as a photoperiod-responsive gene, and then further studies using in situ hybridization and immunocytochemistry revealed that expression levels of several related retinoid-signaling genes were modulated in response to photoperiod changes. Genes of the retinoid-signaling pathway, encoding nuclear receptors (RXR/RAR) and retinoid binding proteins (CRBP1 and CRABP2) are photoperiodically regulated in the dorsal tuberomamillary nucleus (DTM): Their expression is significantly lower in short photoperiods and parallels body weight decreases. Studies in pinealectomized hamsters confirm that the pineal melatonin rhythm is necessary for these seasonal changes, and studies in testosterone-treated hamsters reveal that these changes in gene expression are not the secondary consequence of photoperiod-induced changes in steroid levels. Comparative studies using Syrian hamsters, which show divergent seasonal body weight responses to Siberian hamsters when exposed to short photoperiods, showed a distinct pattern of changes in retinoid gene expression in the DTM in response to a change in photoperiod. We infer that the DTM may be an important integrating center for photoperiodic control of seasonal physiology and suggest that the changes in retinoid X receptor gamma expression may be associated with seasonal changes in body weight and energy metabolism.

Animals↗

PubMed2003

Early regulation of hypothalamic arcuate nucleus CART gene expression by short photoperiod in the Siberian hamster.

Cocaine- and amphetamine-regulated transcript (CART) mRNA is expressed in a number of hypothalamic nuclei including the arcuate nucleus (ARC). An increase in CART gene expression in the ARC of juvenile female Siberian hamsters (Phodopus sungorus) 14 days after transfer to short photoperiod at weaning and prior to major divergence of body weight trajectory in this seasonal mammal implicates CART in the induction of programmed weight change. In the current series of experiments, elevated CART mRNA in short photoperiod juvenile female animals relative to long photoperiod controls was apparent throughout the caudal-rostral extent of the ARC after 14 days, but was not observed when short photoperiod exposure was limited to 4-7 days. Elevated CART gene expression was also observed in juvenile males 14 days after transfer to short photoperiod at weaning, in adult female hamsters 14 days after transfer to short photoperiod and in adult male hamsters 21 days after transfer to short photoperiod. There were no consistent trends in expression levels of other energy balance-related genes with these relatively short duration photoperiod manipulations, suggesting that CART may be involved in short photoperiod-programmed body weight regulation.

Animals↗

PubMed2003

Digging deep--structure-function relationships in the melatonin receptor family.

The melatonin receptor family is a small group of receptors within the G protein-coupled receptor (GPCR) superfamily. The group comprises of three subtypes which bind melatonin and one member, the melatonin related receptor (MRR), that shares >40% sequence identity with the other melatonin receptors but does not bind melatonin. Identification of two subtypes expressed in the mouse suprachiasmatic nucleus, one of which (MT1) inhibits neuronal firing and the other (MT2) mediating the phase advancing properties of melatonin has given renewed interest to the development of subtype specific compounds for each of the mammalian melatonin receptors. Towards this goal site-directed and chimaeric receptor mutagenesis studies have been performed which have provided some insight into the structure-function relationships of the melatonin receptors. Furthermore, these studies may lead to the identification of the ligand for the orphan MRR.

Amino Acid Sequence↗

PubMed2002

Differential coupling of the extreme C-terminus of G protein alpha subunits to the G protein-coupled melatonin receptors.

Melatonin receptors interact with pertussis toxin-sensitive G proteins to inhibit adenylate cyclase. However, the G protein coupling profiles of melatonin receptor subtypes have not been fully characterised and alternative G protein coupling is evident. The five C-terminal residues of Galpha subunits confer coupling specificity to G protein-coupled receptors. This report outlines the use of Galphas chimaeras to alter the signal output of human melatonin receptors and investigate their interaction with the C-termini of Galpha subunits. The Galphas portion of the chimaeras confers the ability to activate adenylate cyclase leading to cyclic AMP production. Co-transfection of HEK293 cells expressing MT(1) or MT(2) melatonin receptors with Galphas chimaeras and a cyclic AMP activated luciferase construct provided a convenient and sensitive assay system for identification of receptor recognition of Galpha C-termini. Luciferase assay sensitivity was compared with measurement of cyclic AMP elevations by radioimmunoassay. Differential interactions of the melatonin receptor subtypes with Galpha chimaeras were observed. Temporal and kinetic parameters of cyclic AMP responses measured by cyclic AMP radioimmunoassay varied depending on the Galphas chimaeras coupled. Recognition of the C-terminal five amino acids of the Galpha subunit is a requisite for coupling to a receptor, but it is not the sole determinant.

Cell Line↗

PubMed2002

Decrease of food intake by MC4-R agonist MTII in Siberian hamsters in long and short photoperiods.

We investigated the role of the hypothalamic melanocortin system in the regulation of food intake in the Siberian hamster, which shows a profound seasonal decrease in food intake and body weight in short photoperiod (SP). In male hamsters maintained in long photoperiod (LP), intracerebroventricular injection of melanotan II (MTII) just before lights off significantly decreased food intake relative to vehicle treatment over the 6-h observation period. Similar effects were observed in age-matched hamsters after exposure to a short daylength for 9 wk, when body weight had significantly decreased. There was no clear difference in either the magnitude of response or the dose required for half-maximal inhibition of food intake in hamsters in SP compared with those in LP. MTII significantly increased grooming in both LP and SP. Our results indicate that the melanocortin system is a potent short-term regulator of food intake. However, the lack of differential response or sensitivity to MTII treatment in the obese (LP) vs. lean (SP) states does not support the hypothesis that changes in this melanocortin pathway underlie the long-term decrease in food intake that occurs in this seasonal model.

Animals↗

PubMed2002

The pars tuberalis as a target of the central clock.

The pars tuberalis (PT) of the pituitary has emerged from being a gland of obscure and unknown function to a tissue of central importance to our understanding of how photoperiod regulates seasonal responses. The discovery of melatonin receptors on this gland first pointed to its involvement in seasonal physiology. However, the more recent demonstration of the expression of clock genes in the PT, such as Per1, has heightened interest in the gland. Recent work shows how photoperiod, through the hormone melatonin, affects the timing and amplitude of expression of the Per1 gene, as well as other genes such as Icer. The effect of photoperiod and melatonin on the expression of Per1 in the PT is distinct to its effects on the SCN, and this probably reflects distinct functions of the clock genes in the two tissues - acting as part of the biological clock in the SCN, but as an interval timing system within the PT. The changes in amplitude of Per1 gene expression in response to altered length of photoperiod have provided the first clues as to how the durational melatonin signal is decoded within the neuroendocrine system.

Animals↗

PubMed2002

Hypothalamic bHLH transcription factors are novel candidates in the regulation of energy balance.

The basic helix-loop-helix transcription factors, neurological basic-helix-loop-helix-2 (Nhlh-2), neurogenic differentiation-1 (NeuroD-1) and single minded-1 (Sim-1) could have roles in energy balance regulation, although supporting evidence is inconclusive. This study in mice provides further evidence that Nhlh-2 and NeuroD-1 are involved in energy balance regulation. In situ hybridization was used to study the expression of the genes in relation to physiological status and genetic background within hypothalamic nuclei that are involved in energy balance regulation. These studies show reduced expression of Nhlh-2 mRNA in the arcuate (ARC) nucleus and NeuroD-1 mRNA in the paraventricular (PVN) nucleus in obese ob/ob and 24 h food-deprived mice relative to respective controls, suggesting regulation by leptin. Interestingly, Nhlh-2 mRNA expression is reduced in obese db/db mice, whereas NeuroD-1 remains unchanged, suggesting different mechanisms of regulation by leptin of these two genes. To study the role of leptin in the regulation of these genes, leptin was injected intraperitoneally in obese ob/ob mice and mRNA expression evaluated after 1 h or 4 h, or after twice-daily injection for 7 days. None of these regimes restored Nhlh-2 or NeuroD-1 to wild-type mRNA levels. These latter data suggest either that the regulation of the Nhlh-2 and NeuroD-1 genes by leptin is indirect or that the apparent leptin insensitivity of the gene expression reflects a developmental deficit that is a consequence of the phenotype of the obese ob/ob mice. The relationship between Nhlh-2 and candidate energy balance-related genes was studied by dual in situ hybridization. Nhlh-2 mRNA was coexpressed in a subpopulation (30%) of ARC neurons expressing pro-opiomelanocortin (POMC) mRNA, suggesting a potential functional relationship.

Animals↗

PubMed2002

Pituitary adenylate cyclase-activating polypeptide acts as a paracrine regulator of melatonin-responsive cells of the ovine pars tuberalis.

The pars tuberalis (PT) region of the anterior pituitary plays a physiological role in seasonal animals. The primary signal transduction mechanism of the melatonin receptor in this tissue is an inhibition of cAMP signaling. However, nothing is known about the endocrine signals that activate cAMP synthesis in the cells of the PT, as previous studies relied on the pharmacological tool, forskolin, to stimulate cAMP synthesis. Here we show that pituitary adenylate cyclase-activating polypeptide (PACAP) activates cAMP synthesis in the cells of the PT. The pharmacology of cAMP activation by PACAP peptides suggests that cAMP activation is mediated by the type I PACAP receptor. PACAP treatment of PT cells results in cellular responses that are consistent with cAMP activation in these cells, including activation of MAPK and elevation of melatonin receptor mt1 mRNA expression. These responses can be inhibited by melatonin, demonstrating that activation of cAMP occurs within the melatonin-responsive cells. However, although PACAP activates cAMP in the cells of the PT, the effect of PACAP may not be direct, as colocalization in situ hybridization studies demonstrates that the type I PACAP receptor and the melatonin mt1 receptor do not colocalize on the cells of the PT.

Adenylyl Cyclases↗