In vivo modulation of the histamine release in the hypothalamus by adrenoreceptor agonists and antagonists.

The modulation of the histamine release from histaminergic neurons by noradrenergic neurons was investigated by the push-pull technique. The posterior hypothalamus of the conscious, freely moving rat was superfused with artificial CSF through a push-pull cannula and the release of endogenous histamine was determined in the superfusate. Hypothalamic superfusion with a potassium-rich CSF enhanced the release rate of histamine. Superfusion with the alpha 2-agonists noradrenaline or clonidine diminished the release rate of histamine. Moreover, clonidine abolished the potassium-induced increase in the histamine release. Superfusion with the alpha 2-antagonists yohimbine or idazoxan enhanced the release rate of histamine. It is concluded that noradrenaline released from noradrenergic neurons of the hypothalamus modulates the release of histamine from histaminergic neurons by stimulating alpha 2-adrenoreceptors located on histaminergic nerve terminals.