Autoinhibition of histamine synthesis mediated by presynaptic H 3-receptors

Abstract

The regulation of histamine synthesis was studied on rat brain slices or synaptosomes labeled with l-[ 3H]histidine. Depolarization by increased extracellular K + concentration enhanced by about twofold the [ 3H]histamine formation in slices of cerebral cortex. This stimulation was also observed, although to a lesser extent, in synaptosomes from cerebral cortex and slices from the posterior hypothalamus where most histaminergic cell-bodies are located, suggesting that it may occur in nerve endings as well as in perikarya. In the presence of exogenous histamine in increasing concentrations the K +-induced stimulation was progressively reduced by up to 60–70%. The effect of exogenous histamine appears to be receptor-mediated as shown by its saturable character, high pharmacological specificity and competitive reversai by histamine antagonists. The ec 50 value of histamine for synthesis reduction (0.34 ± 0.03 μM) was similar to its ec 50 value for release inhibition known to be mediated by H 3-receptors. In addition, whereas mepyramine and tiotidine, two potent antagonists at H 1- and H 2-receptors, respectively, were poorly effective, the H 3-receptor antagonists burimamide and impromidine reversed the histamine effect in an apparently competitive manner. These effects were observed in slices of cerebral cortex or posterior hypothalamus as well as in cortical synaptosomes. Furthermore, even in the absence of added histamine, H 3-receptor antagonists enhanced the depolarization-induced stimulation of [ 3H]histamine synthesis, indicating a participation of released endogenous histamine in the synthesis control process. The potencies of H 3-receptor antagonists were similar to those of these agents at presynaptic autoreceptors controlling [ 3H]histamine release. It is concluded that H 3-receptors control not only release but also synthesis of histamine at the Ievel of nerve endings and also, presumably, of perikarya. A relationship between the two regulatory processes, possibly via intracellular calcium, seems likely but remains to be investigated at the molecular Ievel.