Influence of histamine on the serotonergic system of rat brain.

Both histamine and 4-methylhistamine, after intraventricular injection into normal rats, reduced the levels of serotonin and increased those of 5-hydroxyindoleacetic acid in hypothalamus; after injection into tranylcypromine-treated rats, head twitches were induced which were blocked by antiserotonin agents. 2-Pyridylethylamine, an agonist of histamine H1 receptors, neither influenced serotonin level in hypothalamus nor evoked behavioural changes. It is concluded that injected histamine may release serotonin from the hypothalamus and that this produces the behavioural changes.