In vivo release by histamine agonists and antagonists of endogenous catecholamines in the cat hypothalamus

The posterior hypothalamus of anaesthetized cats was superfused through a push-pull cannula with histamine agonists and antagonists and the release of endogenous catecholamines was determined in the superfusate. Hypothalamic superfusion with histamine, 2-methylhistamine (H1-agonist), dimaprit (H2-agonist) or metiamide (H2-antagonist) enhanced the release of the catecholamines dopamine, noradrenaline and adrenaline. The releasing effects of these substances depended on the presence of calcium ions. Superfusion with 2-pyridylethylamine (H1-agonist) was virtually ineffective, while superfusion with 2-thiazolethylamine (H1-agonist) enhanced the rates of release of noradrenaline and adrenaline without influencing the release of dopamine. Superfusion with mepyramine (H1-antagonist) inhibited the release of noradrenaline and adrenaline without affecting the release of dopamine. Hypothalamic superfusion with a concentration of procaine which was equi-anaesthetic to that of mepyramine was ineffective. Ranitidine (H2-antagonist) did not alter the rates of release of the catecholamines. The releasing effect of histamine was inhibited on hypothalamic superfusion with mepyramine and ranitidine. Ranitidine also inhibited the releasing effects of dimaprit and 2-methylhistamine thus indicating that the releasing action of the latter compound was mainly due to stimulation of H2-receptors. These data suggest that blockade of H1-receptors of the posterior hypothalamus reduces the release of noradrenaline and adrenaline, while stimulation of H1-receptors seems to increase the rates of release of these two catecholamines. Stimulation of H2-receptors enhances the release of all three catecholamines. Thus, dopaminergic neurones of the hypothalamus seem to possess H2-receptors, while noradrenergic and adrenergic neurones possess H1- and H2-receptors.