Effect of modified brain histamine contents on prolactin and thyrotropin secretion in male rats

Brain histamine and histamine H3 receptors following repeated L-histidine administration in rats

In order to assess the importance of the chronic increase in precursor availability on central histaminergic mechanisms in rats, nine male Wistar rats received L-histidine orally at a dose of 1000 mg/kg, twice daily (07.00 h and 19.00 h) for 1 week; 9 rats were used as controls. Brain tissue histamine and tele-methylhistamine levels, as well as plasma histamine concentration were assayed. Binding properties and regional distribution of the autoregulatory histamine H3 receptors in brain were studied with [3H]-R-alpha-methylhistamine receptor binding and autoradiography. In L-histidine loaded rats, tissue histamine levels in cortex, hypothalamus, and rest of the brain were significantly increased by 40%-70%. Histamine concentrations in cerebellum and plasma, and tele-methylhistamine concentrations in cortex and hypothalamus did not change. The binding properties of H3 receptors in cortex were not altered. However, there were changes in the regional distribution of [3H]-R-alpha-methylhistamine binding sites, suggestive of a region-selective up-/down-regulation of histamine H3 receptors or their receptor sub-types. These results imply that following repeated L-histidine administration in the rat (1) there is enhanced synthesis of brain histamine not reflected in its functional release; (2) the excess of histamine is sequestered and stored rather than being metabolized; (3) histamine H(3) receptor binding properties are not altered, whereas receptor density is changed in selected regions. In conclusion, these results demonstrate that the neuronal mechanisms controlling histamine synthesis, storage, and release are adaptable and allow the sequestration of the excess of histamine in order to prevent excessively high neuronal activity.

Effects of intracerebroventricular injection of alpha-fluoromethylhistidine on radial maze performance in rats

The effects of alpha-fluoromethylhistidine (alpha-FMH) on spatial cognition were investigated using the eight-arm radial maze paradigm in rats. Intracerebroventricular (ICV) injection of alpha-FMH resulted in spatial memory deficits characterized by an increase in the number of total errors (TE) and a decrease in the number of initial correct responses (ICR). There was a strong correlation between increases in the number of TE and decreases in histamine contents of the cortex and hippocampus regions of the brain, which are known to participate in learning and memory. On the other hand, both histamine (50-100 ng, ICV) and thioperamide (10 microg, ICV) significantly ameliorated the memory deficit induced by alpha-FMH. However, metoprine showed no significant effect on the alpha-FMH-induced memory deficit. Pyrilamine and R-(alpha)-methylhistamine enhanced the memory deficit induced by alpha-FMH, at doses that had no appreciable effect when administered alone. In contrast, no significant influence on alpha-FMH-induced memory deficit was observed with zolantidine.

The blockade of H1 receptors attenuates the suppression of feeding and diuresis induced by inhibition of histamine catabolism

Metoprine elevates brain histamine content by blocking the conversion of histamine to methylhistamine. It suppresses food intake, increases water intake. and induces diuresis in rats. In the present experiment, to study which receptors were involved in these metoprine-induced changes, H1, H2, and H3 receptor blockers were administered to metoprine (10 mg/kg IP)-treated rats. The food and water consumption and urine excretion were measured at 10 and 24 h after the drug administration. It was found that systemic administration of the H3 receptor antagonist, thioperamide (5 mg/kg IP), supplemented the feeding suppressive effect of metoprine. In addition to this, the H1 receptor antagonist mepyramine (20 mg/kg IP) antagonized the suppression of feeding in metoprine-treated rats, whereas the H2 receptor antagonist, ranitidine (100 mg/kg IP), had no effect. Mepyramine also decreased the diuretic response to metoprine, whereas ranitidine or thioperamide were virtually without effect. The present results show that elevation of brain histamine content by inhibiting the catabolism of histamine suppresses food intake, and this effect of metoprine can be abolished by pretreatment with antihistamines. Although the blockade of H1 receptors also attenuates the diuretic response to metoprine, further studies are needed to understand the mechanisms that mediate the effects of metoprine on water balance.