Structural characteristics of endogenous sugar acids and relations to feeding modulation

Physiological Significance of 2-Buten-4-Olide (2-B40), an Endogenous Satiety Substance Increased in the Fasted State

A sugar acid, 2-B4O, has been found to increase from 3.5 to 13 microM in rat serum at 36 h after food deprivation. Injections of 2-B4O (2.5 microM) into the rat III cerebral ventricle (III ICV) suppress food intake and single neuronal activity in the lateral hypothalamic area (LHA). 2-B4O is effective even in 72 h food-deprived rats. 2-B4O hyperpolarizes glucose-sensitive neurons in the LHA via Na+-K+ pump activation, but depolarizes glucoreceptor neurons in the ventromedial nucleus (VMH) via closure of ATP-sensitive K channels. The plasma levels of glucose, corticosterone, and catecholamines, and the firing rate in both parvocellular neurons in the paraventricular nucleus (PVN) and sympathetic efferent nerves, all increase 2-B4O intravenous (iv) injection, indicating activation of the hypothalamo-pituitary-adrenal axis. A 2-B4O iv injection facilitates emotional and spatial learning and memory, and pretreatment with anti-acidic fibroblast growth factor (aFGF) antibody ICV eliminates these effects. aFGF is released from ependymal cells in the III cerebral ventricle in response to the glucose increase in CSF induced by 2-B4O iv injection. 2-B4O also suppresses the clinical symptoms of experimental allergic encephalomyelitis (EAE) in Lewis rats [induced by immunization with a myelin basic protein (MBP)], a model for human multiple sclerosis. After immunization with MBP, the delayed-type hypersensitivity response to MBP is also reduced in 2-B4O-treated rats. 2-B4O thus suppresses autoimmune responses. These results indicate that 2-B4O is not only a powerful satiety substance, but also effective as an activator of the hypothalamo-pituitary-adrenal axis and sympathetic efferent outflow, and as a memory facilitation and a modulator of immune functions.

Effects of age and blood sugar levels on the proboscis extension of the blow fly Phormia regina

In some insects the proboscis is extended to imbibe a sugar solution if the concentration of sugar applied to the chemosensilla exceeds the behavioural threshold value. Recently, I found a reversal of the threshold values of this "proboscis extension reflex" (PER) in the blow fly (Phormia regina M.) for glucose and fructose. It depended on maturation and physiological conditions, both of which are explicable in terms of changing concentration of haemolymph trehalose. The direct injection of trehalose into the fly haemocoele brought about a dramatic shift of the threshold values of PER measured on tarsi or labellar sensilla, suggesting a strong dependence of PER on the blood sugar level. Using the tip-recording method, the dose-response (impulse frequency) curves for glucose and fructose were obtained on individual largest labellar chemosensilla. The curves for glucose and fructose crossed at one point because the former had a steeper gradient and higher maximum response than the latter. Injection experiments with trehalose were also carried out to test for changes in gustatory response. The shifting of the behavioural dose-response curves for glucose and fructose two hours after injection of 1 M trehalose (2 µl) into the haemocoele of the fly was associated with significant reduction in responsiveness of labellar chemosensilla to glucose, but less so to fructose. No change in responsiveness was found following injection of mannose. A hypothesis to explain the reversal relation of the PER thresholds, based on a shift in the firing rate in gustatory sensilla and possibly also interneurons, is discussed.

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.

Histaminergic control of energy balance in rats

Manipulating neuronal histamine in the hypothalamus, its effects on brain functions were assessed in nonobese normal rats and Zucker rats. Alpha-fluoromethylhistidine (FMH), an inhibitor of histamine synthesis, induced feeding dose-dependently after 2.24 mumol infusion at 1100 h, when hypothalamic histamine was normally high. This dose of FMH selectively decreased hypothalamic histamine, but not other neurotransmitters. Thioperamide, an antagonist of autoinhibitory H3-receptors, decreased food intake after infusion at 1940 h, when hypothalamic histamine was normally low. Bilateral microinfusion of 224 nmol FMH or 26 nmol chlorpheniramine, an H1-antagonist, into the ventromedial hypothalamus (VMH) and the paraventricular nucleus (PVN), elicited feeding. However, Zucker obese rats showed no significant responses to chlorpheniramine, thioperamide or histamine. Concentration of their hypothalamic histamine was excessively lower than that of the nonobese. Contents of hypothalamic histamine were lowered at 4 degrees C and raised at 31 degrees C. FMH attenuated increase in histamine, and then disrupted adaptive behavior. These findings indicate that neuronal histamine may convey the suppressive signal of food intake through H1-receptors in the VMH and/or the PVN, and play critical roles in homeostatic control of adaptive behavior.

Structural and stereoisomeric specificity of serum-borne sugar acids related to feeding control by rats

Specificity of chemical structures and stereoisomers among serum-borne short-chain organic acids in rats were assessed for their effects on feeding behavior and humoral factors by infusion into the rat third cerebroventricle. Infusion of glyceric acid (1.0 mumol), 3,4-dihydroxybutanoic acid gamma-lactone (3,4-DB) or 3,4,5-trihydroxypentanoic acid gamma-lactone (2.50 mumol) immediately before the dark phase decreased food intake for, at most, 24 h. These acids did not significantly affect drinking or ambulation. Initial feeding, not necessarily accompanied by periprandial drinking, was induced after infusion of 2,4-dihydroxybutanoic acid gamma-lactone, 2,4,5-trihydroxypentanoic acid gamma-lactone (2,4,5-TP) or exogenous 2,4,5,6-tetrahydroxyhexanoic acid gamma-lactone (2.50 mumol) in the light phase. Of these acids, 3,4-DB most potently suppressed and 2,4,5-TP most potently enhanced feeding. Of these, the 2S,4S-isomer and the 3S-isomer were the most potent of 2,4,5-TP and 3,4-DB, respectively. Only the 2S,4S-isomer of 2,4,5-TP induced hypoglycemia with hyperinsulinemia, whereas opposite effects were produced by the 3S-isomer of 3,4-DB. The results indicate that the positions of the hydroxyl groups on 4-butanolide and the S- and S,S-stereoisomers are important in modulating food intake through the hypothalamus.