Mamoru Kurokawa

Hypothalamic neuronal histamine: Implications of its homeostatic control of energy metabolism☆

In a series of studies on histaminergic functions in the hypothalamus, probes to manipulate activities of histaminergic neuron systems were applied to assess its physiologic and pathophysiologic implications using non-obese normal and Zucker obese rats, an animal model of genetic obesity. Food intake is suppressed by either activation of H1-receptor or inhibition of the H3-receptor in the ventromedial hypothalamus (VMH) or the paraventricular nucleus, each of which is involved in satiety regulation. Histamine neurons in the mesencephalic trigeminal sensory nucleus modulate masticatory functions, particularly eating speed through the mesencephalic trigeminal motor nucleus, and activation of the histamine neurons in the VMH suppress intake volume of feeding at meals. Energy deficiency in the brain, i.e., intraneuronal glucoprivation, activates neuronal histamine in the hypothalamus. Such low energy intake in turn accelerates glycogenolysis in the astrocytes to prevent the brain from energy deficit. Thus, both mastication and low energy intake act as afferent signals for activation of histaminergic nerve systems in the hypothalamus and result in enhancement of satiation. There is a rationale for efficacy of a very-low-calorie conventional Japanese diet as a therapeutic tool for weight reduction. Feeding circadian rhythm is modulated by manipulation of hypothalamic histamine neurons. Hypothalamic histamine neurons are activated by an increase in ambient temperature. Hypothalamic neuronal histamine controls adaptive behavior including a decrease in food intake and ambulation, and an increase in water intake to maintain body temperature to be normally constant. In addition, interleukin-1 beta, an endogenous pyrogen, enhanced turnover of neuronal histamine through prostaglandin E2 in the brain. Taken together, the histamine neuron system in the hypothalamus is essential for maintenance of thermoregulation through the direct and indirect control of adaptive behavior. Behavioral and metabolic abnormalities of obese Zucker rats including hyperphagia, disruption of feeding circadian rhythm, hyperlipidemia, hyperinsulinemia, and disturbance of thermoregulation are essentially derived from a defect in hypothalamic neuronal histamine. Abnormalities produced by depletion of neuronal histamine from the hypothalamus in normal rats mimic those of obese Zuckers. Grafting the lean Zucker fetal hypothalamus into the obese Zucker pups attenuates those abnormalities. These findings indicate that histamine nerve systems in the brain play a crucial role in maintaining homeostatic energy balance.

ENHANCED EXPRESSION OF UNCOUPLING PROTEIN 2 GENE IN RAT WHITE ADIPOSE TISSUE AND SKELETAL MUSCLE FOLLOWING CHRONIC TREATMENT WITH THYROID HORMONE

Evidence is rapidly emerging which suggests that uncoupling protein 2 (UCP2), by virtue of its ubiquitous expression, may be important for determining basal metabolic rate. To assess the functional modulation of UCP2 gene expression in relation to body weight control, we examined the effects of hyperthyroid state induced by chronic treatment with triiodothyronine (T3) on UCP2 mRNA expression in male rats. Daily subcutaneous injection of T3 (37 pmol/100 g body weight) for 7 days increased UCP2 mRNA expression in brown adipose tissue (BAT), white adipose tissue (WAT) and the soleus muscle 1.6‐, 1.6‐ and 1.7‐fold compared to the controls, respectively, and increased UCP1 mRNA expression in BAT 1.2‐fold. In contrast, the same treatment with T3 decreased both ob mRNA expression in WAT and plasma leptin level 0.5‐fold for each. The present results suggest that T3 may directly increase UCP2 expression independently of leptin action.

Zucker obese rats: defect in brain histamine control of feeding

Manipulation of hypothalamic histamine produced different effects on feeding between the Zucker obese (fa/fa) and their lean littermate rats (Fa/-). Infusion of a histamine H1-receptor antagonist into the third cerebroventricle elicited feeding in the lean and Wistar King A rats, but it did not affect feeding in the obese rats. To enhance hypothalamic neuronal histamine, thioperamide, and H3-receptor antagonist, was similarly infused. The lean and Wistar rats decreased their food intake after the infusion, but thioperamide produced no significant effect on feeding in the obese rats. Infusion of histamine into the third cerebroventricle mimicked the effects of thioperamide on feeding: reduction of food intake in the lean and Wistar rats, but no significant change in the obese rats. Hypothalamic histamine of the obese rats (0.430 nmol/g) was significantly lower than the lean (1.209 nmol/g) and Wistar rats (4.838 nmol/g). The histamine concentration of the cerebral cortex in the obese rats was also lower than the non-obese animals. The results indicate that the feeding abnormality of Zucker obese rats may be at least due to disturbance of histamine suppressive signals both at presynaptic and postsynaptic levels.

Hypothalamic neuronal histamine regulates feeding circadian rhythm in rats

To clarify involvement of hypothalamic neuronal histamine in feeding circadian rhythm, we analyzed rat behavioral patterns using chemical probes which affect endogenous histaminergic activity. Sustained infusion of alpha-fluoromethylhistidine (FMH), a specific suicide inhibitor of a histamine-synthesizing enzyme, into the rat third cerebral ventricle disrupted light-dark cycles of feeding, drinking, and ambulatory behavior. Food and water intake and ambulatory activity during the 12-h light period increased, and those during the 12-h dark period decreased after the infusion. The ratio of the light period to the 24-h total period (L/T ratio) increased in all behavioral parameters. Assessed by 3-h cumulative analysis, amplitudes of circadian rhythmicity decreased in all behavioral parameters, whereas only the acrophase of ambulatory activity shifted forward after FMH infusion. Chlorpheniramine, an H1-antagonist, selectively increased food intake during the light and decreased it during the dark period. Consequently, the antagonist increased the L/T ratio in food intake, but did not affect the ratio in water intake or ambulatory activity. Famotidine, an H2-antagonist, did not affect the ratio in any parameter. Thioperamide, an antagonist of auto-inhibitory effects on histamine synthesis and release at presynaptic H3-receptor sites, decreased food intake during the dark, but did not affect the L/T ratio in any parameter. These findings indicate that neuronal histamine may regulate feeding circadian rhythm through the hypothalamic histamine H1-receptor in rats.

SATIATION AND MASTICATORY FUNCTION MODULATED BY BRAIN HISTAMINE IN RATS

Abstract Both the ventromedial hypothalamus (VMH) and the mesencephalic trigeminal sensory nucleus (Me5) are densely innervated by histaminergic neurons. The depletion of neuronal histamine (HA) from the Me5 by the bilateral microinfusion of 448 nmol/rat α-fluoromethylhistidine (FMH), a specific suicide inhibitor of histidine decarboxylase, reduced the eating speed and prolonged meal duration, while leaving the meal size unaffected. HA depletion from the VMH increased the size of the meal and prolonged its duration, but not the eating speed. When the HA turnover rate was measured at 15 min after the scheduled feeding following fasting for less than 24 hr, the rate increased in the region including the Me5, but not in the hypothalamus. The turnover rate reached higher levels at 60 min in both regions. Gastric intubation of an isocaloric liquid diet or an equivolume of water with the liquid diet abolished the increase in HA turnover both in the Me5 region and the hypothalamus. The present findings indicate that brain HA thus modulates satiation through both the VMH and masticatory function as well as due to the action of the Me. The HA function activated by mastication began earlier in the Me5 and later in the hypothalamus due to a signal originating from the oral proprioceptors and initiated by chewing.

Tumor necrosis factor-α regulates in vivo expression of the rat UCP family differentially

Abstract A family of uncoupling proteins (UCPs), free fatty acid anion transporters, plays a crucial role in energy homeostatic thermoregulation. Tumor necrosis factor-α (TNF-α), a member of the cytokine family, is well known as an endogenous pyrogen. To evaluate the interaction of TNF-α with UCPs in thermogenesis, effects of TNF-α on rat UCP gene expression were examined in intrascapular brown adipose tissue (BAT), epididymal white adipose tissue (WAT) and soleus muscle (Muscle). Administration of TNF-α elevated rectal temperature by 0.7°C as well as serum leptin which peaked at 6 h, compared with saline controls. BAT UCP1 mRNA expression was increased by 1.2-fold at 6 h after the TNF-α treatment and decreased by 0.8-fold at 16 h after the treatment. In contrast to UCP1 expression in BAT, UCP2 mRNA expressions in BAT, WAT, and Muscle was increased to reach maximum levels of 1.3-, 1.6- and 1.8-fold, respectively, at 16 h after the treatment. UCP3 mRNA in Muscle, but not in BAT or WAT, was exclusively up-regulated by 1.7-fold at 16 h after the treatment. These results indicate that TNF-α up-regulates UCP gene expression differentially and tissue dependently, and add novel insights into thermogenesis under conditions of malignancy and inflammation.

Hypothalamic histamine modulates adaptive behavior of rats at high environmental temperature

Histamine content in the rat hypothalamus was lower at 4°C and higher at 31°C compared to that at 21°C. Pretreatment with α-fluoromethylhistidine, a ‘suicide’ inhibitor of histidine decarboxylase, attenuated both the increased level of hypothalamic histamine and rat adaptive behavior at 31°C. Increase of histamine content in the hypothalamus appears to be an important factor contributing to rat adaptive behavior to high environmental temperature.

MOLECULAR CLONING OF RAT UNCOUPLING PROTEIN 2 CDNA AND ITS EXPRESSION IN GENETICALLY OBESE ZUCKER FATTY (FA/FA) RATS

We isolated rat UCP2 cDNA, which has been proposed to play an important role in mammalian thermogenesis and body weight regulation. The nucleotide sequence of the cDNA revealed that the rat UCP2 protein is composed of 309 amino acid residues, and is 99% and 95% identical to the mouse and human proteins, respectively. The molecular weight of rat UCP2, calculated from the predicted amino acid sequence, was 33,369, and the UCP2 protein of this size was detected when the cDNA was expressed in vitro. Northern blot analysis revealed that the corresponding mRNA is approximately 1.7 kb in size, and is expressed in a variety of rat organs, with predominant expression in the heart, lung and spleen. UCP2 mRNA levels in the heart, liver, muscle and epididymal adipose tissue of Zucker fatty (fa/fa) rats were comparable to those in the lean littermates, while ob mRNA level markedly increased in the epididymal adipose tissue of Zucker (fa/fa) rats.

A physiological role of brain histamine during energy deficiency

Histaminergic activation in the rat hypothalamus was investigated under a deficit in energy supply. Fasting of rats for 24 h increased hypothalamic histamine (HA) content. Intraperitoneal (IP) injection of insulin (2 U/kg) increased pargyline-induced accumulation of tele-methylhistamine (t-MH) leaving steady-state HA and t-MH levels unaffected, which implies enhancement of HA turnover rate. The insulin infusion induced hypoglycemia both in rats with and without pargyline pretreatment. Infusion of 2-deoxy-D-glucose (2-DG) into the third cerebroventricle also produced an increase in pargyline-induced accumulation of t-MH and no change in steady-state HA and t-MH levels. The 2-DG infusion induced hyperglycemia. Hypothalamic glycogen content decreased after 24 h starvation, but this decrease was prevented by depletion of HA by alpha-fluoromethylhistidine. Absolute glycogen contents in the cortex were lower than those in the hypothalamus, and were not affected by fasting or depletion of HA. The results indicate that activation of hypothalamic HA in response to glucoprivation may modulate homeostatic control of energy supply in the brain.

VLCD-Induced Weight Loss Improves Heart Rate Variability in Moderately Obese Japanese:

To evaluate the effects of weight reduction on the autonomic nervous system in obese patients, we investigated heart rate variability (HRV) based on 24-hr ambulatory electrocardiogram (ECG) recordings before and after weight reduction. To aim for weight reduction, 16 obese patients were treated with the very-low-calorie conventional Japanese diet (VLCD-CJ) therapy combined with behavior therapy. Percent weight reduction was 17.8% ± 1.5% (means ± SEM), but mean blood pressure did not change significantly after VLCD-CJ therapy. The mean normal R-R interval (mNN) of the 24-hr ECG and all other five time-domain indices increased after weight reduction. Spectral analysis revealed that weight reduction increased the high frequency (HF) component, but decreased the ratio of low to high (LF/HF) components. Rate of change in mNN or HF correlated positively with reduction rate of body mass index, but not that in LF/HF. Analysis of daily fluctuations in each HRV parameter showed that significant improvement after weight loss occurred mainly during the nocturnal period, but an HF component was improved throughout the day and night periods. These findings indicate that functional impairment of the autonomic nervous system in obese subjects, particularly in the nocturnal period, is improved by effective weight reduction after VLCD-CJ therapy.