Histamine Receptor and its Regulation of Energy Metabolism

In a series of studies on brain functions of histamine, probes to manipulate activities of histaminergic neuronal systems were applied to assess histaminergic function in non-obese normal, and lean and obese Zucker rats. Food intake was suppressed by both activation of H₁-receptors and inhibition of H₃-receptors in the ventromedial hypothalamic nucleus (VMH) and the paraventricular nucleus, each of which is a satiety center. Feeding circadian rhythm was decreased in its amplitude through histaminergic modulation in the hypothalamus. Histamine neurons in the mesencephalic trigeminal nucleus (Me5) were involved in regulation of masticatory functions, particularly eating speed, while histamine-containing neurons in the VMH controlled intake volume of meals. Energy deficiency in the brain enhanced satiation through histaminergic activation of VMH neurons, which in turn produced glycogenolysis in the hypothalamus to maintain homeostatic control of glucose supply. A very-low-calorie conventional Japanese diet, which is a fiber rich and low energy food source, enhanced satiation by increased mastication and because of the low energy supply of the diet. Hypothalamic histamine neurons were activated by high ambient temperature and also by interleukin-1β, an endogenous pyrogen, to maintain homeostatic thermoregulation. Behavioral and metabolic abnormalities of Zucker obese rats were mediated by a deficit in hypothalamic neuronal histamine, and the Zucker rat was evaluated as an animal model of histamine deficiency. Transplantation of the lean fetal hypothalamus into the third cerebroventricle of host obese Zuckers attenuated the abnormalities.     

Anti-obesity actions of mastication driven by histamine neurons in rats

Implications of mastication in energy intake and expenditure regulated by histamine (HA) neurons were investigated in rats. Depletion of neuronal HA from the mesencephalic trigeminal sensory nucleus (Me5) reduced eating speed, but that from a satiety center of the ventromedial hypothalamus (VMH) increased both meal size and its duration leaving eating speed unaffected. Turnover of neuronal HA in the Me5 was elevated at the early phase of feeding and that in the VMH was at the later phase. This elevated turnover was abolished by gastric intubations of an isocaloric liquid diet or an equivolume of water. Mastication-induced activation of HA neurons suppressed physiological food intake through H1-receptor in the hypothalamic paraventricular nucleus (PVN) and the VMH. On the other hand, the HA neurons activation accelerated lipolysis particularly in the visceral adipose tissues and up-regulated mRNA expression of uncoupling protein family through sympathetic efferent nerve. Mastication thus plays an important role as a potent input signal to activate HA neurons. Our recent findings have evidently shown how tightly and elegantly HA neurons are concordant with leptin signaling system through a negative feedback loop.     

Hypothalamic administration of cAMP agonist/PKA activator inhibits both schedule feeding and NPY-induced feeding in rats

Following central administration, neuropeptides that decrease the level of cAMP induce feeding. Conversely, cAMP activating neuropeptides tend to elicit satiety. When the inhibitory effect of neuropeptide Y (NPY) on the hypothalamic cAMP production was blocked by pertussis toxin, the potent orexigenic effect of NPY was lost. These findings suggest that there may be a link between hypothalamic cAMP and the central regulation of food intake. In this report, we show that the injection of the membrane-permeable cAMP agonist, adenosine-3',5'-cyclic monophosphorothioate Sp-isomer (Sp-cAMP), into perifornical hypothalamus (PFH) significantly inhibited schedule-induced and NPY-induced food intake for up to 4h. This inhibitory effect was normalized within 24h. A taste aversion could not be conditioned to Sp-cAMP treatment, suggesting that the anorectic response was not due to malaise. Sp-cAMP administration significantly increased the active protein kinase A (PKA) activity in dorsomedial (DMH) and ventromedial (VMH), but not in lateral (LH) hypothalamus. Consistently, food deprivation lowered, while refeeding normalized endogenous cAMP content in DMH and VMH, but not in LH areas. No significant effect of adenosine-3',5'-cyclic monophosphorothioate Rp-isomer (Rp-cAMP, cAMP antagonist) was observed on hypothalamic PKA activity, schedule-induced, or NPY-induced food intake. These findings suggest that the increase in cAMP level and PKA activity in DMH and VMH areas may trigger a satiety signal.     

Deoxyglucose-autoradiography of the hypothalamus after food deprivation in the mouse

[14C] deoxyglucose-autoradiography of mouse brain after food deprivation showed no specific changes in the ratio of autoradiographic optical density of the lateral nucleus of the hypothalamus (LH) versus ventromedial nucleus of the hypothalamus (VMH). The results show that even if there were differences in functional activity between these hypothalamic nuclei during conditions of hunger and satiety, they are not reflected as autoradiographically detectable in relative deoxyglucose uptake.     

Effects of myeloid sirtuin 1 deficiency on hypothalamic neurogranin in mice fed a high-fat diet

Hypothalamic inflammation has been known as a contributor to high-fat diet (HFD)-induced insulin resistance and obesity. Myeloid-specific sirtuin 1 (SIRT1) deletion aggravates insulin resistance and hypothalamic inflammation in HFD-fed mice. Neurogranin, a calmodulin-binding protein, is expressed in the hypothalamus. However, the effects of myeloid SIRT1 deletion on hypothalamic neurogranin has not been fully clarified. To investigate the effect of myeloid SIRT1 deletion on food intake and hypothalamic neurogranin expression, mice were fed a HFD for 20 weeks. Myeloid SIRT1 knockout (KO) mice exhibited higher food intake, weight gain, and lower expression of anorexigenic proopiomelanocortin in the arcuate nucleus than WT mice. In particular, KO mice had lower ventromedial hypothalamus (VMH)-specific neurogranin expression. However, SIRT1 deletion reduced HFD-induced hypothalamic neurogranin. Furthermore, hypothalamic phosphorylated AMPK and parvalbumin protein levels were also lower in HFD-fed KO mice than in HFD-fed WT mice. Thus, these findings suggest that myeloid SIRT1 deletion affects food intake through VMH-specific neurogranin-mediated AMPK signaling and hypothalamic inflammation in mice fed a HFD.     

QRFP in female rats: effects on high fat food intake and hypothalamic gene expression across the estrous cycle

Pyroglutamylated arginine-phenylalanineamide peptide (QRFP) is a neuropeptide involved in feeding behavior. Central administration of QRFP selectively increases the intake of a high fat diet in male rats. QRFP administration also stimulates the hypothalamic-pituitary-gonadal axis via gonadotrophin-releasing hormone in male and female rats. Prepro-QRFP mRNA is expressed in localized regions of the mediobasal hypothalamus which are abundant in neurotransmitters, neuropeptides and receptor systems important for food intake regulation and reproductive behaviors. The current experiments were conducted to investigate the effects of centrally administered QRFP-26 on the intake of a high fat diet (HFD, 60% kcal from fat) in female rats and to investigate alterations in hypothalamic prepro-QRFP and its receptors, GPR130a and GPR103b, mRNA levels over the estrous cycle. In Experiment 1, female rats were administered QRFP-26 (intracerebroventricular; 0.3 nmol, 0.5 nmol, 1.0 nmol) in rats consuming either a HFD or a low fat diet. All doses of QRFP-26 selectively increased the intake of the HFD in female rats. These data suggest that QRFP-26 regulates the intake of energy dense foods in female rats, which is similar to previous findings in male rats. In Experiment 2, hypothalamic levels of prepro-QRFP mRNA and its receptors were assessed during diestrus, proestrus, or estrus. The level of prepro-QRFP mRNA in the ventromedial/arcuate nucleus (VMH/ARC) of the hypothalamus was increased during proestrus, which suggests that endogenous estrogen levels regulate QRFP expression in the VMH/ARC. These data suggest that QRFP may play a role in coordinating feeding behaviors with reproductive function when energy demand is increased.     

Impaired Satiation and Increased Feeding Behaviour in the Triple-Transgenic Alzheimer's Disease Mouse Model

Alzheimer''s disease (AD) is associated with non-cognitive symptoms such as changes in feeding behaviour that are often characterised by an increase in appetite. Increased food intake is observed in several mouse models of AD including the triple transgenic (3×TgAD) mouse, but the mechanisms underlying this hyperphagia are unknown. We therefore examined feeding behaviour in 3×TgAD mice and tested their sensitivity to exogenous and endogenous satiety factors by assessing food intake and activation of key brain regions. In the behavioural satiety sequence (BSS), 3×TgAD mice consumed more food after a fast compared to Non-Tg controls. Feeding and drinking behaviours were increased and rest decreased in 3×TgAD mice, but the overall sequence of behaviours in the BSS was maintained. Exogenous administration of the satiety factor cholecystokinin (CCK; 8–30 µg/kg, i.p.) dose-dependently reduced food intake in Non-Tg controls and increased inactive behaviour, but had no effect on food intake or behaviour in 3×TgAD mice. CCK (15 µg/kg, i.p.) increased c-Fos protein expression in the supraoptic nucleus of the hypothalamus, and the nucleus tractus solitarius (NTS) and area postrema of the brainstem to the same extent in Non-Tg and 3×TgAD mice, but less c-Fos positive cells were detected in the paraventricular hypothalamic nucleus of CCK-treated 3×TgAD compared to Non-Tg mice. In response to a fast or a period of re-feeding, there was no difference in the number of c-Fos-positive cells detected in the arcuate nucleus of the hypothalamus, NTS and area postrema of 3×TgAD compared to Non-Tg mice. The degree of c-Fos expression in the NTS was positively correlated to food intake in Non-Tg mice, however, this relationship was absent in 3×TgAD mice. These data demonstrate that 3×TgAD mice show increased feeding behaviour and insensitivity to satiation, which is possibly due to defective gut-brain signalling in response to endogenous satiety factors released by food ingestion.     

Expression of dopaminergic receptors in the hypothalamus of lean and obese Zucker rats and food intake

As revealed by previous microdialysis studies, basal and food intake-accompanied dopamine release significantly differs in the hypothalamus of obese vs. lean Zucker rats. In the present study, we determined whether dopaminergic receptors are also compromised in obesity. Dopaminergic D(1) and D(2) receptor mRNA expression was studied in the ventromedial hypothalamus (VMH), lateral hypothalamic area (LHA), and the adenohypophysis (AH) of obese and lean Zucker rats using RT-PCR technique. In obese Zucker rats, we found an upregulation of D(1) receptor mRNA in the VMH and AH and a downregulation in the LHA, whereas D(2) receptor mRNA was downregulated in both the VMH and LHA, but not changed in the AH, compared with lean rats. Also, an increase of D(1) receptor staining was seen in the paraventricular nucleus of obese rats by immunohistochemistry. We selected the VMH to test if the observed changes in the dopamine receptor expression of obese rats induce behavioral sensitization to dopamine as expressed by hyperphagia. The overnight food-deprived rats received a single VMH injection (10 nmol) of sulpiride (D(2) receptor antagonist) or saline as control, then food was provided and 1-h food intake was measured. Food intake after sulpiride vs. saline injection was greater in obese rats but was not different in lean rats. Our data suggest that downregulation of D(2) receptor in the hypothalamus at least in the VMH induces behavior sensitization for having large meals. Low D(2) receptor expression may be causal for an exaggerated dopamine release observed in obese rats during food ingestion and for reduced satiety feedback effect of dopamine. High level of D(1) receptor expression in the VMH and low in the LHA may also contribute to the specific feeding pattern in obese rats represented by large meal size and low meal number.     

Peripheral ghrelin participates in the glucostatic signaling mediated by the ventromedial and lateral hypothalamus neurons

Employing immunohistochemistry techniques, we examined the c-fos expression in different hypothalamic areas, when plasma glucose levels were modified by the administration of insulin and 2-deoxyglucose (2-DG) respectively. Subsequently, the hypoglycemia produced by an injection of insulin significantly increased feeding concomitant to higher c-fos expression in the arcuate nucleus (ARC), paraventricular nucleus (PVN), dorsomedial hypothalamus (DMH) and lateral hypothalamus (LH), while no statistical changes in the ventromedial hypothalamus (VMH) were found. Also, the glucopenia induced by 2-DG administration produced similar stimulatory effects on appetite and the neuronal activity affecting all the hypothalamic areas studied, including the VMH. The peripheral blockade of the orexigenic hormone ghrelin with a specific antibody (AGA) significantly decreased food intake as induced from acute hypoglycemia and glucopenia. Curiously, the conjoint AGA and insulin or 2-DG administration produced a differential effect on the hypothalamic neurons analyzed, by increasing the number of c-fos positive neurons in the ARC, PVN and DMH, but not in the VMH and LH. This outcome suggests an interactive effect of the glucostatic pathways involving these two areas with the ghrelin signaling.     

Ceruletide acts in the abdomen, not in the brain, to produce satiety

Ceruletide (caerulein), a decapeptide extracted from the skin of the frog, Hyla caerulea, is very similar in structure to the C-terminal octapeptide of cholecystokinin (CCK-8). Although ceruletide and CCK-8 act through similar or identical receptors to produce the same visceral effects, previous studies in the rat suggested that peripherally administered ceruletide acted directly on the ventromedial hypothalamic (VMH) area to decrease food intake, but peripherally administered CCK-8 acted at a vagally innervated abdominal site to decrease food intake. Since it is unprecedented for these two peptides to produce the same effect by acting at different sites, we investigated the site of action of ceruletide's satiety effect in the rat and compared it to the site of action of CCK-8. The major results were: (1) intraperitoneal administration of ceruletide and CCK-8 inhibited food intake, but intraventricular administration did not; (2) the satiety effect of ceruletide and CCK-8 was not changed by bilateral lesions of the VMH; and (3) the satiety effect of ceruletide and CCK-8 was abolished or markedly reduced by bilateral abdominal vagotomy. We conclude that ceruletide acts at the same vagally innervated abdominal site to produce satiety as CCK-8 does and that neither peptide acts directly on the VMH area.     

Orexins (hypocretins): novel hypothalamic peptides with divergent functions.

The hypothalamus is the most important region in the control of food intake and body weight. The ventromedial "satiety center" and lateral hypothalamic "feeding center" have been implicated in the regulation of feeding and energy homeostasis by various studies of brain lesions. The discovery of orexin peptides, whose neurons are localized in the lateral hypothalamus and adjacent areas, has given us new insight into the regulation of feeding. Dense fiber projections are found throughout the brain, especially in the raphe nucleus, locus coeruleus, paraventricular thalamic nucleus, arcuate nucleus, and central gray. Orexins mainly stimulate food intake, but by the virtue of wide immunoreactive projections throughout the brain and spinal cord, orexins interact with various neuronal pathways to potentiate divergent functions. In this review, we summarize recent progress in the physiological, neuroanatomical, and molecular studies of the novel neuropeptide orexins (hypocretins).     

Effects of L-lysine deficient diet on the hypothalamic interstitial norepinephrine and diet-induced thermogenesis in rats in vivo

Rats readily recognize an amino acid deficient diet, presumably via central mechanisms that involve hypothalamic circuits. Presently, effects of the essential amino acid L-lysine deficiency on the ventromedial (VMH) and lateral (LH) hypothalamus norepinephrine (NE) release were evaluated in free moving rats. Microdialysis measurement was undertaken once in 48 h (12:00 noon-14:00) in rats that had free access to food and drink. Significant decline in the food intake and VMH NE release were found in rats fed L-lysine diet. No changes were identified in LH NE release. Additionally, no significant differences in diet-induced spatial thermogenesis between normal and L-lysine deficient non-stressed rats were found in vivo. The results suggested that the VMH NE release was specifically involved in the integration of signals about amino acid deficiency. However, the decrease in VMH NE was not translated into changes of thermogenic responses to diet.     

Metabolic state, neurohormones, and vagal stimulation, not increased serotonin, orchestrate postprandial drowsiness

Several theories have arisen to explain postprandial sleepiness. Of these, the most prevalent theory assumes that blood flow is redistributed after a meal. This premise fails, however, because cardiac regulation strictly controls cerebral blood flow (CBF) and brain oxygenation. Another theory proposes that an elevated ratio of free l-tryptophan to large neutral amino acids (LNAA) in the blood increases cerebral serotonin (5HT) levels, which in turn induces drowsiness. But this theory does not explain why fatty meals, which reduce extracellular 5HT, induce more intense sleepiness than meals of carbohydrates, or why protein-rich meals, which actually lower cerebral 5HT, do not differ significantly from carbohydrate meals in promoting sleep. Some studies fail to confirm the presumptive role of 5HT in mood or drowsiness. Reviewing the history of theory and experimentation in this field, we conclude that 5HT is not the principal determinant in postprandial sleepiness. We propose instead that the arcuate nucleus (ARC) modulates satiety in response to metabolic indicators of energy state, postprandial neuropeptide secretion from the gut, and vagus nerve stimulation. The ARC integrates these satiety signals and forwards them to the ventromedial hypothalamus (VMH), which indirectly stimulates the sleep centers (i.e., the ventrolateral preoptic nucleus (VLPO) and median preoptic nucleus (MnPO)) by inhibiting the lateral hypothalamic area (LHA), which coordinates arousal centers. Neuropeptides or satiety signals may activate sleep centers directly to provoke postprandial somnolence. This model may resolve contradictions and inconsistencies in data that previous theories could not explain.     

Peptide signals regulating food intake and energy homeostasis

The adiposity hormone leptin has been shown to decrease food intake and body weight by acting on neuropeptide circuits in the hypothalamus. However, it is not clear how this primary hypothalamic action of leptin is translated into a change in food intake. We hypothesize that the behavioral effect of leptin ultimately involves the integration of neuronal responses in the forebrain with those in the nucleus tractus solitarius in the caudal brainstem, where ingestive behavior signals are received from the gastrointestinal system and the blood. One example is the peptide cholecystokinin, which is released from the gut following ingestion of a meal and acts via vagal afferent nerve fibers to activate medial nucleus tractus solitarius neurons and thereby decrease meal size. While it is established that leptin acts in the arcuate nucleus in the hypothalamus to stimulate anorexigenic neurons that inhibit food intake while simulataneously inhibiting orexigenic neurons that increase food intake, the mechanisms linking these effects with regions of the caudal brainstem that integrate cues related to meal termination are unclear. Based on an increasing body of supportive data, we hypothesize that this integration involves a pathway comprising descending projections from neurons from the paraventricular nucleus to neurons within the nucleus tractus solitarius that are activated by meal-related satiety factors. Leptin's anorexic effect comprises primarily decreased meal size, and at subthreshold doses for eliciting an effect on food intake, leptin intensifies the satiety response to circulating cholecystokinin. The location of neurons subserving the effects of intracerebroventricular administration of leptin and intraperitoneal injection of cholecystokinin on food intake has been identified by analysis of Fos expression. These studies reveal a distribution that includes the paraventricular nucleus and regions within the caudal brainstem, with the medial nucleus tractus solitarius having the most pronounced Fos expression in response to leptin and cholecystokinin, and support the hypothesis that the long-term adiposity signal leptin and the short-term satiety signal cholecystokinin act in concert to maintain body weight homeostasis.     

Food deprivation increases the mRNA expression of micro-opioid receptors in the ventral medial hypothalamus and arcuate nucleus

Activation of micro-opioid receptors makes animals hyperphagic and increases their preference for a high-fat diet. Previous studies have suggested that this receptor population plays a role in mediating the hyperphagia that is associated with food deprivation. In this paper, we tested the hypothesis that food deprivation will increase the expression of micro-opioid receptors in the ventral medial hypothalamus and arcuate nucleus (VMH/ARC). Food deprivation resulted in a significant increase in the mRNA expression of micro-opioid receptors in the VMH/ARC and the lateral hypothalamus (LH) after 48 h of fasting but not after 24 or 12 h of fasting in either the light or dark. We did not observe a change in the mRNA expression of kappa- or delta-opioid receptors after food deprivation. When food-deprived animals were given a choice between a low-fat diet and a high-fat diet, they were hyperphagic and consumed significantly more of the high-fat diet. When the micro-opioid receptors were blocked with beta-funaltrexamine (selective mu-opioid receptor antagonist), prior to giving food-deprived animals access to both a low-fat and high-fat diet, it significantly decreased the percentage of high-fat diet consumed. These data demonstrate that hypothalamic micro-opioid receptors may contribute to the hyperphagia and increased preference for a high-fat diet that is associated with food deprivation.     

DISTRIBUTION OF GAMMA-AMINOBUTYRIC ACID (GABA) IN THE RAT HYPOTHALAMUS: FUNCTIONAL CORRELATES OF GABA WITH ACTIVITIES OF APPETITE CONTROLLING MECHANISMS

—In the hypothalamus, the highest GABA content (approx. 26 nmol/mm³) was constantly observed in the lateral hypothalamic area (LHA). In other parts of the hypothalamus uneven distribution of GABA was also observed, but areas showing high concentration of GABA did not coincide with the locations of various hypothalamic nuclei. In the LHA, which is known to contain a feeding centre, the anterior part (6.4 and 6.0 mm anterior (A 6.4 and A 6.0) respectively to the vertical zero plane of de Groot) showed a remarkably high content of GABA. The GABA content in the LHA at A 6.4 was decreased during the initial phase of insulin hypoglycemia and, in contrast, showed a significant increase following hyperglycemia induced by alloxan administration. In the ventromedial nucleus (VMH) of the hypothalamus, which is known to contain a satiety centre, the GABA content was increased during the initial phase of insulin hypoglycemia. The results suggest that both certain parts of the LHA and VMH contain or receive GABA-inhibitory neurons and that these neurons may play important physiological roles in controlling functional states of the feeding and satiety centres in the hypothalamus.     

Cholecystokinin-8S levels in discrete hypothalamic nuclei of weanling rats exposed to maternal protein malnutrition

Perinatal malnutrition and growth retardation at birth are suggested to be important risk factors for the development of overweight and syndrome X in later life. Underlying mechanisms are unknown. Body weight and food intake are regulated, e.g. by hypothalamic neuropeptidergic systems which are thought to be highly vulnerable to persisting malorganization due to perinatal malnutrition. To investigate possible consequences for hypothalamic cholecystokinin-8S (CCK-8S) in the offspring, pregnant Wistar rats were fed an 8% protein diet during pregnancy and lactation (low-protein group; LP) while control mothers (CO) received a 17% protein isocaloric standard diet. LP offspring displayed underweight at birth (P < 0.05) and during suckling (P < 0.001), while leptin levels were not altered. At weaning, under basal conditions CCK-8S was decreased in LP offspring in the paraventricular hypothalamic nucleus and arcuate hypothalamic nucleus (P < 0.05), as well as in the dorsomedial hypothalamic nucleus, lateral hypothalamic area and ventromedial hypothalamic nucleus (P < 0.01). In summary, these data indicate (1) an inhibition of the satiety peptide CCK-8S in main regulators of body weight and food intake in low-protein malnourished newborn rats; (2) no direct relationship of hypothalamic CCK-8S to circulating leptin at this age; and (3) no neurochemical signs of hypothalamic CCKergic dysregulation in this animal model at the age of weaning.