A histamine agonist is a drug which causes increased activity at one or more of the four histamine receptor subtypes.
H2 : Betazole and Impromidine are examples of agonists used in diagnostics to increase histamine.
H3 : Betahistine is a weak Histamine1 agonist and a very strong Histamine3 antagonist (paradoxically histamine increasing).
References
Histamine agonist Wikipedia(Text) CC BY-SA