H2 receptor-mediated facilitation and H3 receptor-mediated inhibition of noradrenaline release in the guinea-pig brain

The effect of histamine and related drugs on the tritium overflow evoked electrically (0.3 Hz) or by introduction of Ca²⁺ ions into Ca²⁺-free K⁺-rich (25 mmol/l) medium containing tetrodotoxin was studied in superfused guinea-pig brain cortex, cerebellum, hippocampus or hypothalamus slices and in mouse brain cortex slices preincubated with ³H-noradrenaline. The electrically evoked tritium overflow in guinea-pig cortex slices was inhibited by histamine; the H₃ receptor antagonist clobenpropit reversed the effect of histamine to a slight facilitation. The facilitatory effect of histamine (obtained in the presence of clobenpropit) was not affected by the H₁ receptor antagonist mepyramine but abolished by the H₂ receptor antagonist ranitidine. In the absence of clobenpropit, ranitidine augmented the inhibitory effect of histamine. In slices superfused in the presence of ranitidine, the evoked overflow was inhibited by histamine and, more potently, by the H₃ receptor agonist R-α-methylhistamine in a concentration-dependent manner (maximum inhibitory effect obtained for both agonists 30–35%). The concentration-response curve of histamine was shifted to the right by the H₃ receptor antagonist thioperamide. R-α-Methylhistamine inhibited the electrically evoked tritium overflow also in guinea-pig cerebellar, hippocampal and hypothalamic slices. In cortex slices superfused in the presence of clobenpropit, the H₂ receptor agonists impromidine and, less potently, R-sopromidine facilitated the evoked overflow in a concentration-dependent manner. S-Sopromidine only tended to increase the evoked overflow. The effect of impromidine was counteracted by the H₂ receptor antagonists ranitidine and cimetidine. The extent of the maximum facilitatory effect of impromidine (by 15–20%) was about the same when (i) the Ca²⁺ concentration in the medium was reduced from 1.3 to 0.98 mmol/l, (ii) the time of exposure to impromidine was reduced from 28 to 8 min or (iii) cerebellar, hippocampal or hypothalamic slices were used instead of cortical slices. The Ca²⁺-induced tritium overflow in guinea-pig cortex slices was inhibited by histamine (in the presence of ranitidine); this effect was abolished by clobenpropit. In slices superfused in the presence of clobenpropit, impromidine failed to facilitate the Ca²⁺-evoked tritium overflow. The electrically evoked tritium overflow in mouse brain cortex slices was inhibited by histamine by about 60% (both in the absence or presence of ranitidine). The inhibitory effect of histamine was abolished (but not reversed) by clobenpropit. In conclusion, noradrenaline release in the guinea-pig brain cortex is inhibited via presynaptic H₃ receptors and facilitated via H₂ receptors not located presynaptically. In the mouse brain cortex, only inhibitory H₃ receptors occur. The extent of the H₃ receptor-mediated effect is more marked in the mouse than in the guinea-pig brain cortex. Received: 25 September 1997 / Accepted: 17 November 1997