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 antagonist of the Histamine3autoreceptor. Antagonizing H3 increases histaminergic tone.
See also
Histamine antagonist
References
↑Sakai, N.; Nishino, S. (2013). "Wake-Promoting Medications". Encyclopedia of Sleep. Elsevier. pp. 627–633. doi:10.1016/b978-0-12-378610-4.00126-1. ISBN978-0-12-378611-1. "Although centrally injected histamine or histaminergic H1 agonists promote wakefulness, the systemic administration of these compounds induces various unacceptable side effects via peripheral H1 receptor stimulation."