Inhibition of noradrenaline release in the rat brain cortex via presynaptic H₃ receptors

Summary

The effects of histamine and related drugs on the evoked tritium overflow from superfused rat brain cortex slices preincubated with³H-noradrenaline were determined. Tritium overflow was stimulated electrically (3 Hz; slices superfused with normal physiological salt solution) or by introduction of CaCl₂ 1.3 mmol/l (slices superfused with Ca²⁺-free medium containing K⁺ 20 mmol/l).

Histamine slightly decreased the electrically evoked^H overflow in slices superfused in the presence of desipramine. The degree of inhibition obtained with histamine was doubled when both desipramine and phentolamine were present in the superfusion medium (pIC₁₅ 6.46). Under the latter condition, the evoked overflow was inhibited by the H₃ receptor agonist R-(−)-α-methylhistamine and its S-(+) enantiomer (pIC₁₅ 7.36 and 5.09, respectively), but was not affected by the H₂ receptor agonist dimaprit and the H₁ receptoragonist 2-thiazolylethylamine (both at up to 32 µmol/l). The concentration-response curve of histamine was shifted to the right by the H3 receptor antagonists thioperamide, impromidine and burimamide (apparent pA₂ 8.37, 6.86 and 7.05, respectively), by the H₂ receptor antagonist ranitidine (apparent pA₂ 4.27) and was not affected by the H₁ receptor antagonist dimetindene (32 µmol/l). The inhibitory effect of R-(−)-α-methylhistamine on the evoked overflow was also counteracted by thioperamide. Given alone, none of the five histamine receptor antagonists affected the evoked overflow. In the absence of desipramine plus phentolamine, impromidine and burimamide facilitated the electrically evoked³H overflow whereas thioperamide had no effect. The facilitatory effects of impromidine and burimamide were abolished by phentolamine, but not affected by desipramine. The concentration-response curve of noradrenaline for its inhibitory effect on the evoked overflow was shifted to the right by impromidine and burimamide, but not influenced by thioperamide (apparent pA₂ 5.24, 5.04 and <6.5, respectively; experiments carried out in the presence of desipramine). In slices superfused with Ca²⁺-free K⁺-rich medium containing tetrodotoxin, desipramine plus phentolamine, the tritium overflow evoked by introduction of Ca²⁺ was inhibited by histamine; the concentration-response curve of histamine was shifted to the right by thioperamide.

The present study shows that the inhibitory effect of histamine on noradrenaline release in the rat brain cortex involves presynaptic H₃ receptors and that the degree of inhibition is increased in the presence of phentolamine. The H₃ receptor antagonists impromidine and burimamide are weak α₂-adrenoceptor antagonists.