Youhei Kurose

A transient ghrelin surge occurs just before feeding in a scheduled meal-fed sheep

Ghrelin, a gastric-derived peptide, has recently been identified as an endogenous natural ligand for the growth hormone (GH) secretagogue receptor. However, secretory characteristics of ghrelin are still obscure in ruminants. To investigate the diurnal rhythm in ghrelin secretion and its relationship to GH secretion, plasma ghrelin and GH concentrations were determined in Suffolk rams fed with a roughage diet once daily (Experiment 1). Abrupt increases (P<0.05) in plasma ghrelin occurred just before a meal-feeding compared with that at 1h before feeding, then rapidly fell with a minimum during the feeding. A pulsatile surge (P<0.01) in plasma GH concentrations, which seemed to follow a single surge in plasma ghrelin, was observed during the feeding. In Experiment 2, plasma ghrelin and GH were determined in sheep subjected to a pseudo-feeding of 2h to determine whether feed ingestion itself influences ghrelin and GH secretions. Compared with those at 1h before feeding, a tendency of increases (P<0.1) in plasma ghrelin and significant increases (P<0.05) in GH occurred just before and during a pseudo-feeding, respectively. Plasma ghrelin temporally declined within 1h after the start of the pseudo-feeding, and increased again and maintained higher levels during the last period of the pseudo-feeding. These results suggest that the transient surge of ghrelin secretion just before a scheduled meal feeding would not be due to the ingestion of feed, and that a pulsatile increase in plasma GH during the actual- or pseudo-feeding could be induced by the transient ghrelin surge.

A transient surge of ghrelin secretion before feeding is modified by different feeding regimens in sheep

Ghrelin is a recently identified orexigenic hormone secreted by the stomach and has been implicated in meal-time hunger. Several experiments demonstrate a transient surge in ghrelin secretion shortly before a scheduled meal, suggesting from the involvement of cephalic mechanisms. If ghrelin secretion is stimulated by hunger in sheep, plasma levels of ghrelin should be modified by different feeding regimens that affect hunger drive. To test this hypothesis, we investigated changes in plasma ghrelin concentrations in fed Suffolk rams ad libitum and in rams either twice or four times daily. Plasma ghrelin levels increased (P<0.05) abruptly just before every feeding period in sheep fed twice and four times daily and then fell shortly after feeding. Peak levels of the pre-prandial ghrelin surge were higher (P<0.01) in animals fed twice daily than in animals fed four times daily, leading to greater (P<0.05) areas under response curves over 12h. In contrast, the plasma ghrelin levels remained relatively low and constant in sheep fed ad libitum, with no evidence of surges in plasma ghrelin levels. These results confirm that the transient surge in plasma ghrelin levels occurs just before feeding and demonstrate that this can be modified by the feeding regimen in sheep.

HISTAMINE REGULATES FOOD INTAKE THROUGH MODULATING NORADRENALINE RELEASE IN THE PARA-VENTRICULAR NUCLEUS

Histamine is one of the neurotransmitters suppressing appetite, but the interactions of histaminergic neurons with other neurons in satiety centers have not been clarified. Noradrenaline in the para-ventricular nucleus (PVN) has been shown to stimulate feeding. This study was designed to determine whether histamine modulates noradrenaline release via histamine H1-receptors in the PVN. Freely-fed rats were i.c.v. injected with an histamine H1-receptor antagonist, triprolidine (82 microg), and/or an alpha 2-adrenoceptor antagonist, rauwolscine (0, 20 and 40 microg), and food intake (n=8 each) over 2 h was measured. Food intake was significantly (p<0.01) increased in rats injected with triprolidine alone. The triprolidine-elicited increase in food intake was suppressed by rauwolscine at a dose of 40 microg. The noradrenaline content in perfusates collected by a microdialysis probe aimed at the PVN was significantly (p<0.05) increased by the presence of triprolidine in the perfusates. The noradrenaline concentrations in perfusates collected from the PVN were elevated after tyramine (a noradrenaline uptake inhibitor) administration, but not when both tyramine and histamine were given. In conclusion, these results suggest that histamine inhibits noradrenaline release from hypothalamic nerve terminals in the PVN, and thus suppresses feeding behavior.

Circulating leptin response to feeding and exogenous infusion of insulin in sheep exposed to thermoneutral and cold environments

Leptin has been shown to regulate feed intake and energy expenditure. Insulin stimulates leptin secretion in rodents, but its action on leptin secretion is still obscure in ruminants. If insulin stimulates leptin secretion in ruminants, circulating leptin concentrations may change during exposure to cold, because of fluctuating insulin secretion and action in the cold environment. The present experiment was designed to determine whether feeding or exogenous administration of insulin affects circulating leptin levels in sheep exposed to thermoneutral and cold environments. Suffolk rams that were shorn and fed a diet once daily were subjected to a thermoneutral (20 degrees C) or cold (0 degrees C) environment for at least 1 week. Overall mean concentrations of plasma leptin in the feeding experiment were lower (P<0.05) in the cold environment than in the thermoneutral environment. Plasma leptin levels remained relatively unchanged after feeding in both environments, though plasma insulin response to feeding in both environments increased (P<0.01). The euglycemic clamps (insulin infusion rate: 4 mUkgBW(-1)min(-1) for 2 h) increased (P<0.01) circulating leptin concentrations in the thermoneutral, but not in the cold environment. These results suggest that lower circulating leptin levels in ruminants exposed to the cold environment could be partly due to the depressed insulin action on leptin secretion.

Ghrelin differentially modulates the GH secretory response to GHRH between the fed and fasted states in sheep

The effect of energy balance on the growth hormone (GH) secretory responsiveness to growth hormone-releasing hormone (GHRH) has not been determined in ruminant animals. Therefore, we examined the effects of intravenous injections of 0, 3.3, and 6.6 microg ghrelin/kg body weight (BW), with and without GHRH at 0.25 microg/kg BW, on GH secretory responsiveness in both the fed and fasted sheep. The injections were carried out at 48 h (Fasting state) and 3h (Satiety state) after feeding. Blood samples were taken every 10 minutes, from 30 minutes before to 120 minutes after the injection. Low (3.3 microg/kg BW) and high (6.6 microg/kg BW) doses of ghrelin stimulated GH secretion significantly (P<.05) greater in the Satiety state than in the Fasting state. Growth hormone-releasing hormone plus both doses of ghrelin stimulated GH secretion significantly (P<.05) greater in the Satiety state than in the Fasting state. Ghrelin and GHRH exerted a synergistic effect in the Satiety state, but not in the Fasting state. Plasma ghrelin levels were maintained significantly (P<.05) greater in the Fasting state than in the Satiety state except the temporal increases after ghrelin administration. Plasma free fatty acid (FFA) concentrations were significantly (P<.01) greater in the Fasting state than in the Satiety state. In conclusion, the present study has demonstrated for the first time that ghrelin differentially modulates GH secretory response to GHRH according to feeding states in ruminant animals.

The differences in feeding-inhibitory responses to peripheral and central leptin between non-lactating and lactating rats

This study was conducted to examine the contributions of central and peripheral leptin to hyperphagia in lactation. Lactating rats were mated at 7-8 weeks of age and housed singly with their litters. In experiment 1, food intakes were significantly (P<0.01) greater (350% on average) in lactation than in non-lactation throughout a day. Cerebrospinal fluid (CSF) leptin levels remained constant despite plasma leptin levels being significantly (P<0.05) greater in non-lactation than in lactation. In experiment 2, CSF leptin levels were not altered by i.v. injections of leptin (0.2 and 0.4 mg/kg body weight) despite that plasma leptin levels were dose dependently (P<0.01) increased. Moreover, i.v. administration of leptin significantly (P<0.05) decreased food intake in non-lactating rats but not in lactating rats. In experiment 3, nocturnal food intakes were temporarily (P<0.05) reduced in non-lactating and lactating rats. I.c.v. administration of a leptin antagonist (15 μg) blocked the reductions of food intakes. I.c.v. administration of leptin (10 μg) significantly (P<0.05) decreased cumulative food intakes during 24 h in both the physiological states. In conclusion, this study has presented new evidence that the hyperphagia of lactating rats could be partly due to depressed sensitivity of neurons contacting blood leptin. In contrast, the responsiveness of leptin receptors contacting CSF leptin may not differ between non-lactating and lactating rats. Furthermore, the levels of CSF leptin remained constant independent of those of blood leptin. Therefore, the expression of hypothalamic leptin receptors contacting CSF could be involved in the difference in food intake between non-lactating and lactating rats.

Changes in blood pancreatic polypeptide and ghrelin concentrations in response to feeding in sheep

The roles of pancreatic polypeptide (PP) have not been determined in ruminant animals. The aim of the present study was to examine the role of PP in the regulation of ghrelin secretion in sheep. Two experiments were conducted using four 2-yr-old Suffolk wethers fed a maintenance diet of alfalfa hay cubes. In Exp. 1, the effects of feeding on blood ghrelin and PP concentrations were examined in scheduled-fed sheep. Blood samples were collected every 10 min from 30 min before to 360 min after feeding. Plasma PP concentrations were transiently increased from the preprandial average value to the values from 30 to 60 min after feeding and gradually decreased (P < 0.05) to stable values from 150 to 180 min. The values from 30 to 60 min were greater (P < 0.05) than those from 150 to 360 min. In contrast, plasma ghrelin concentrations were gradually decreased (P < 0.01) by feeding. The values from 60 to 360 min were less (P < 0.01) than the preprandial average value. In Exp. 2, the effects of continuous PP infusion on ghrelin secretion were examined in feed-deprived sheep. The animals were deprived of feed for 48 h before PP infusion. The PP-treated group intravenously received synthetic bovine PP at a rate of 10 pmol.kg(-1 )of BW.min(-1) for 180 min. Blood samples were collected every 10 min from 30 min before to 180 min after the commencement of PP infusion. Plasma PP concentrations reached a plateau within 30 min after the commencement of PP infusion. Plasma ghrelin concentrations were decreased (P = 0.002, 0.016, 0.007) by PP infusion at 160, 170, and 180 min, respectively. In conclusion, plasma ghrelin and PP concentrations were decreased and increased, respectively, in response to feeding in ruminant animals. Furthermore, PP could depress ghrelin secretion.

CENTRAL SEROTONIN MODULATES INSULIN SECRETORY RESPONSE TO GLUCOSE

セロトニンは,脳において神経伝達物質として存在する.中枢セロトニンは,内分泌系に影響することが示唆されている.本研究の目的は,中枢セロトニン神経の活性とグルコースに対する末梢インズリン分泌反応との関係を明らかにすることである.実験動物としてWistar系雄ラット(体重351～400g)を使用した.セロトニン合成阻害薬P-クロロフェニルアラニン(pCPA,1mg)を脳内のセロトニン合成を阻害する目的で側脳室へ投与した、グルコースに対する末梢インスリン分泌反応を,グルコースクランプ法によって,pCPA投与群および生理食塩水投与群において比較検討した、グルコース注入率(GIR)および血糖値は両者間で差がないにもかかわらず,血清インスリン濃度平均増加量(MSII)は,pCPA投与群の方が有意に低かった.グルコース注入に対するインスリン分泌の指標値(MPII/GIR)は,pCPA投与群の方が有意に低かった.本研究は,脳内のセロトニンの合成阻害による欠乏,すなわちセロトーン神経の不活化が,グルコースに対する末梢インスリン分泌反応を抑制することを明確に例証した.

Involvement of Dopamine in the Mechanism of Benzodiazepine-Induced Eating

The present study was conducted to examine whether dopaminergic activity in the striatum is involved in the mechanism of benzodiazepine-induced eating. The changes in dopaminergic activity were measured in Wistar rats weighing 350 to 400g using microdialysis method. Relative concentrations of dopamine in the dialysate collected from the striatum decreased after the injection of diazepam (160μg), a benzodiazepine agonist, into the lateral ventricle of the rats. The concentrations reached a minimum level between 40 and 60min, and returned to a normal level within 120min after the administration. The levels of dopamine in control rats maintained almost constant levels. Food intake was significantly (p<0.01) increased following an intracerebroventricular injection of diazepam (160μg). Bilateral injections of dopamine (20μg)into the striatum did not change food intake. However, diazepam-induced eating was significantly (p<0.01) suppressed by the administration of dopamine. These results suggested that benzodiazepines may decrease striatal dopaminergic activity, and stimulate feeding behavior.