Feed intake after inhibition of histamine catabolism

Fluid balance in rats of three different strains after inhibition of histamine catabolism

The effect of metoprine, an inhibitor of histamine (HA) catabolism, on fluid balance was studied in Wistar (W) and Long-Evans (LE) rats. AVP deficient Brattleboro (BB) rats were used to evaluate which phenomena were AVP-related. W and LE rats were quite different: LE rats were "dry" rats, they drank less, had higher plasma AVP, smaller urine volume and excreted more AVP, and responded less to salt loading and water deprivation. Furthermore, LE and W rats responded differently to metoprine. When water was provided as drinking fluid, metoprine increased water intake and urine flow in W rats, but these changes were not significant in LE rats. In contrast, when the rats drank saline, urine output and saline consumption were similarly decreased in LE and W rats. Although no metoprine-induced changes in plasma AVP were observed, urinary excretion of AVP per 24 h was reduced in metoprine treated rats. Inhibition of HA catabolism by metoprine caused only minor changes in fluid balance of AVP deficient BB rats. The results show that significant differences in fluid balance can exist between rat strains and that increased availability of HA after IP given metoprine strongly affects body fluids in normal rats, especially those of the W strain. The results provide further support to the involvement of HA in the regulation of fluid balance, but to obtain a more complete picture, other factors, such as atrial natriuretic peptide, should be studied.

Endogenous serotonin modulates histamine release in the rat hypothalamus as measured by in vivo microdialysis

In vivo microdialysis was used to study the effects of serotonergic drugs on histamine release from the suprachiasmatic nuclei region of the anterior hypothalamus in anesthetized rats. Local perfusion with serotonin (5-hydroxytryptamine, 5-HT) increased histamine release significantly and dose dependently. Methysergide (10 mg/kg i.p.), a 5-HT2C/2A receptor antagonist, given 30 min before 5-HT perfusion, blocked the 5-HT-evoked histamine release. Methysergide (10 mg/kg i.p.), given alone, also suppressed basal histamine release by 33%. Dexfenfluramine (10 microM), a 5-HT releaser and uptake blocker, administered via the microdialysis probe, significantly enhanced hypothalamic histamine release. With the same dose of dexfenfluramine, 5-HT release increased 10-fold in the same brain area. These results show for the first time that endogenous 5-HT modulates histamine release in vivo and it has a tonic stimulatory effect on the histaminergic nerve terminals of the rat anterior hypothalamus.

Modulation of TCDD-induced wasting syndrome by portocaval anastomosis and vagotomy in Long-Evans and Han/Wistar rats

Portocaval anastomosis and vagotomy operations were performed in Long-Evans (L-E) and Han/Wistar (H/W) rats to elucidate the mechanism of anorexia induced by TCDD (2,3,7,8-tetrachlorodibenzo-p-dioxin). TCDD-sensitive L-E rats were given a sublethal (5 micrograms/kg) or a lethal dose (20 micrograms/kg) by gavage 5-8 weeks after portocaval anastomosis. TCDD-resistant H/W rats were given a nonlethal dose (500 or 7200 micrograms/kg). The shunt operation did not reduce lethality from TCDD. The effect on wasting of the marginally toxic dose of 5 micrograms/kg in L-E rats was potentiated by the portocaval operation, and the lethal dose was effective in both shunted and sham-operated L-E rats. TCDD failed to decrease food intake and body weight in shunted rats of H/W strain at either dose level though it did so in sham-operated controls. The lack of effect may be due to the already reduced weight of shunted rats at the time of TCDD dosing. TCDD anorexia was not explained by changes in histamine or serotonin (5-HT) turnover in the brain. Vagotomy did not influence lethality after TCDD, although reduction in food intake was somewhat blunted in H/W rats. The results seem to indicate that the anorectic effect of TCDD is modified when portal blood bypasses the liver. The mechanisms remain to be elucidated in detail, but the results do not favor the role of liver as the only or the major initiator of TCDD anorexia. Little evidence was found to support a crucial role of vagal afferent input.

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.

FMH-induced decrease in central histamine levels produces increased feeding and body weight in rats

The present study tested the long-term effects of the histamine (H) synthesis inhibitor alpha-fluoromethylhistidine (FMH) on feeding and body weight in rats. FMH (administered via 2-week osmotic minipumps) increased feeding significantly throughout the test period. Body weights were also significantly increased toward the end of the test period. Hypothalamic H assays, performed at the end of the study, confirmed that FMH-treated rats had significantly lower H levels than controls. In general, the results suggest that H activity and feeding are inversely related.

Effect of a single dose of TCDD on the level of histamine in discrete nuclei in rat brain

Histaminergic neurones may be involved in the regulation of feed intake. Since TCDD causes anorexia in rats, histamine concentrations were measured in several hypothalamic nuclei involved in feeding regulation. Histamine concentrations were not changed in medial or lateral accumbens, suprachiasmatic, paraventricular, arcuate, ventromedial, dorsomedial or perifornical nuclei, or in lateral hypothalamic area, cortex, or pineal gland. There was, however, an increase in histamine concentration in median eminence 25 h after the administration of TCDD.

Histamine: a neurotransmitter that influences food intake?

Brain histamine levels in rats in various states of water balance were raised by blocking histamine catabolism with metoprine. Acute administration of this drug caused a significant reduction in food intake. This finding suggests a possible role of histamine in the regulation of food intake.

Changes in rat brain monoamines, monoamine metabolites and histamine after a single administration of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD)

Male Long-Evans rats were given 50 micrograms/kg 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) intraperitoneally and after 1, 4, 28 or 76 hr, noradrenaline, dopamine, dihydroxyphenylacetic acid (DOPAC), homovanillic acid (HVA), 5-hydroxytryptamine (5-HT), 5-hydroxyindoleacetic acid (5-HIAA), tryptophan and histamine were measured in the brain (dissected into ten parts) as well as in the pituitary gland. Several slight but significant changes were observed, e.g. in the hypothalamus where HVA and 5-HIAA were decreased after 4 hr, noradrenaline was decreased after 76 hr and histamine increased after 28 hr. Several late changes were also found, conspicuously tryptophan was increased in most brain areas after 76 hr and in some cases earlier; these changes may be due to starvation after hypophagia rather than TCDD directly. The results demonstrate that TCDD causes changes in brain neurotransmitter systems, but the changes are minor and it is not likely that aminergic systems are the key mediators in TCDD-induced hypophagia.