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.