Glucose modulates the release of histamine from the mouse hypothalamus in vitro

Changes in free amino acid and monoamine concentrations in the chick brain associated with feeding behavior

Domesticated chicks are precocial and therefore have relatively well-developed feeding behavior. The role of hypothalamic neuropeptides in food-intake regulation in chicks has been reported for decades. However, we hypothesized that nutrients and their metabolites in the brain may be involved in food intake in chicks because these animals exhibit a very frequent feeding pattern. Therefore, the purpose of this study was to examine the feeding behavior of chicks as well as the associated changes in free amino acid and monoamine concentrations in the chick brain. The feeding behavior of chicks was recorded continuously for 6 h. The next day, brain and blood samples were collected when the chicks either attempted to have food (hungry group) or turned food down (satiated group), in order to analyze the concentrations of the free amino acids and monoamines. We confirmed that the feeding behavior of neonatal chicks was characterized by short resting periods between very brief times spent on food intake. Several free amino acids in the mesencephalon were significantly lower in the satiated group than in the hungry group, while l-histidine and l-glutamine were significantly higher. Notably, there was no change in the free amino acid concentrations in other brain regions or plasma. As for monoamines, serotonin and norepinephrine were significantly lower in the mesencephalon of the hungry group compared with the satiated group, but 5 hydroxyindolacetic acid (5-HIAA) was higher. In addition, serotonin and norepinephrine levels were significantly higher in the brain stem of the hungry chicks compared with the satiated group, but levels of 5-HIAA and homovanillic acid were lower. Levels of both dopamine and its metabolite, 3,4-dihydroxyphenylacetic acid, were significantly higher in the diencephalon and telencephalon of the chicks in the hungry group. In conclusion, the changes in the free amino acids and monoamines in the brain may have some role in the feeding behavior of neonatal chicks.

L-proline is a sedative regulator of acute stress in the brain of neonatal chicks

The purpose of the present study was to clarify the central nervous system function of amino acids during acute stress. In Experiment 1, changes in free amino acid pattern were investigated in the brain of neonatal chicks exposed to either restraint with isolation-induced or fasting stress. L-proline and L-arginine were decreased in the telencephalon and diencephalon under any stress. Since the central nervous system functions of L-arginine during the stress response has recently been reported, in Experiment 2, the effect of intracerebroventricular injection of L-proline (0.5, 1.0, 2.0 micromol) during isolation-induced stress was investigated. L-proline induced sedative and hypnotic effects in a dose-dependent manner. It is suggested that L: -proline may have an important role to attenuate the stress response in the central nervous system of chicks.

Hypoxic ventilatory response during light and dark periods and the involvement of histamine H1 receptor in mice

Ventilation oscillates throughout a day in parallel with oscillations in metabolic rate. Histamine affects ventilation and the balance of the energy metabolism via H1 receptors in the brain. We tested the hypothesis that the ventilatory response to hypoxia varies between light and dark periods and that histamine H1 receptors are required for the circadian variation, using wild-type (WT) and histamine H1 receptor knockout (H1RKO) mice. Mice were exposed to hypoxic gas (7% O(2) + 3% CO(2) in N(2)) during light and dark periods. Ventilation initially increased and then declined. In WT mice, minute ventilation (.Ve) during hypoxia was higher in the dark period than in the light period, which was an upward shift along with the baseline ventilation. Hypoxia decreased the metabolic rate, whereas O2 consumption (.VO(2)) and CO(2) excretion were higher in the dark period than in the light period. However, in H1RKO mice, changes in Ve during hypoxia between light and dark periods were minimal, because .Ve was increased relative to .VO(2), particularly in the light period. In H1RKO mice, the HCO(3)(-) concentration and base excess values were increased in arterial blood, and the level of ketone bodies was increased in the serum, indicating that metabolic acidosis occurred. Respiratory compensation takes part in the .Ve increase relative to .VO(2) during hypoxia. These results suggested that changes in .Ve during hypoxia vary between light and dark periods and that H1 receptors play a role in circadian variation in .Ve through control of the acid-base status and metabolism in mice.

Somatic Ca2+ dynamics in response to choline-mediated excitation in histaminergic tuberomammillary neurons

Histaminergic tuberomammillary (TM) neurons of the posterior hypothalamus have been implicated in cognition, alertness and sleep-wakefulness cycles. Spontaneous firing of TM neurons has been associated with histamine release and wakefulness. The expression of alpha7 nicotinic acetylcholine receptors (nAChRs) in TM neurons suggests a role for endogenous choline and for nicotinic drugs in the regulation of intracellular Ca(2+) metabolism, normal TM neuronal activity and histamine release. First, we established the link between TM neuronal spontaneous firing frequency and cytosolic free Ca(2+) concentration ([Ca(2+)](i)). A strong correlation was observed: an onset of spontaneous firing (3-4Hz) was accompanied by a 20-fold increase in [Ca(2+)](i) from 56+/-18 nM to 1.0+/-0.6 microM. The same range of firing frequencies has been observed in TM neurons in vivo and is associated with wakefulness. Secondly, choline-induced activation of alpha7 nAChRs did not elevate [Ca(2+)](i) directly, i.e. in the absence of high-threshold voltage-gated Ca(2+) channel (HVGCC) activation. Cd(2+) (200 microM) completely blocked all Ca(2+) signals, but inhibited only 37+/-16% of alpha7 nAChR-mediated currents. Thirdly, the responsiveness of [Ca(2+)](i) to choline-mediated excitation was inhibited by hyperpolarization and enhanced by depolarization, sensitizing [Ca(2+)](i) at membrane voltages associated with normal TM neuronal activity. These properties of [Ca(2+)](i) define the ability of TM neurons to translate cholinergic stimuli of identical strengths into different cytosolic Ca(2+) effects, providing the physiological substrate for state-specific modulation of incoming cholinergic information and would be expected to play a very important role in determining activity profiles of TM neurons exposed to elevated concentrations of cholinergic agents, such as choline and nicotine.

Neuronal and vascular localization of histamine N-methyltransferase in the bovine central nervous system

Histamine N-methyltransferase (HMT) (EC 2.1.1.8) plays a crucial role in the inactivation of the neurotransmitter histamine in the CNS. However, the localization of HMT remains to be determined. In the present study, we investigated immunohistochemical localization of HMT in the bovine CNS using a polyclonal antibody against bovine HMT. The HMT-like immunoreactivity was observed mainly in neurons. Strongly immunoreactive neurons were present in the oculomotor nucleus and ruber nucleus in the midbrain, the facial nucleus in the pons, the dorsal vagal nucleus and hypoglossal nucleus in the medulla oblongata and in the anterior horn as well as intermediolateral zone of the spinal cord. Intermediately immunoreactive neurons were present in the piriform cortex and the inferior olivary nucleus. The grey matter of the forebrain regions was diffusely and faintly stained. In the cerebellum and the striatum, the nerve fibres in the white matter were positive. The tuberomammillary nucleus, where histaminergic neurons are present, were weakly positive. The other immunoreactive structures in the CNS were blood vessels. Almost all of the blood vessel walls, irrespective of whether they were arterial or venous, were variably stained. The glial fibrillary acidic protein- (GFAP-) immunoreactive astrocytes were not stained. These findings indicated that histamine released from histaminergic nerve terminals or varicose fibres is methylated mainly in postsynaptic or extrasynaptic neurons rather than in astrocytes. The localization of HMT in the blood vessel wall may mean that blood-borne histamine and histamine released from mast cells associated with the blood vessels are catabolized in this structure.

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 homoestatic 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-1 beta, 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.

The effect of metoprine on glucoprivic feeding induced by 2-deoxy-D-glucose

Metoprine is a histamine N-methyltransferase inhibitor that elevates endogenous histamine (HA) levels. Because the histaminergic mechanism may be involved in the regulation of feeding behavior as well as in body glucose homeostasis, the effect of metoprine on glucoprivic feeding was studied in Wistar rats. Although metoprine treatment (10 and 20 mg/kg, IP) decreased feeding, the rats still responded to the administration of 400 mg/kg of 2-deoxy-D-glucose (2-DG) by increasing their feed intake. No difference was seen in the 6-h cumulative feed intake after administration of 2-DG between the metoprine- and solvent-treated rats. However, the response was delayed, and with 20 mg/kg metoprine the feed intake was significantly reduced during 2 h after 2-DG application. Both 2-DG and metoprine elevated plasma glucose concentration despite their opposite effects on feeding. Hypothalamic HA or its metabolite levels were not affected by 2-DG. The results suggest that the effects of metoprine and 2-DG are largely independent of each other, and that the feeding modulating function of HA is on such a level that it does not prevent the glucoprivic emergency response.

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.

Inhibition of histamine release from rat hypothalamic slices by omega-conotoxin GVIA, but not by nilvadipine, a dihydropyridine derivative

Histamine release in response to 40 mM high K+-stimulation from the rat hypothalamic slice preparations perifused in vitro was significantly inhibited by 1.0 nM-1.0 microM omega-conotoxin GVIA, a peptide modulator of N- and L-type voltage-sensitive calcium channels, but not by similar concentrations of nilvadipine, a dihydropyridine derivative of L-type calcium channel antagonist. These results indicate that the voltage-sensitive calcium channel controlling histamine release from hypothalamic slices is omega-conotoxin-sensitive but dihydropyridine-insensitive.

Changes in histamine metabolism in the brains of mice with streptozotocin-induced diabetes

Histamine (HA) metabolism in the brain of mice with streptozotocin (STZ)-induced diabetes was examined. The levels of tele-methylhistamine (t-MH), a major metabolite of brain HA, significantly increased 3 and 4 weeks after STZ injection. However, the HA turnover rates in the diabetic mice, determined from the accumulation of t-MH after the administration of pargyline, were not different from the control values when the animals were allowed free access to food. When the mice were starved for 15 h 4 weeks after STZ treatment, the brain levels of L-histidine decreased significantly, whereas HA turnover increased significantly. Such changes were not observed in starved control mice. Histidine decarboxylase or HA N-methyltransferase activity did not change after starvation in either diabetic or control mice. These results show that the histaminergic (HAergic) activity in the brains of diabetic mice remains within normal range as long as the animals are allowed free access to food. However, they also indicate that a marked enhancement of HAergic activity accompanied by a decrease in the brain L-histidine level occurs in starved diabetic mice.