I think H3 receptor is an Auto Receptor which can regulate its own secretion & action so activation of H3 might result into decreased release of Histamine as well as reduced Histaminergic response. Totally agree with Ganesh Prasad Mishra.

I would like to add, that besides already mentioned effects (narcolepsy, cognitive disorders), H3 receptors in CNS are also included in hypothalamic neuronal networks controling appetite. E.g. It is demonstrated that H3-knock out mice tend to become obese.

Beside that, H3 receptors in CNS can also presinaptically negatively modulate the release of many other neurotrnasmitters.

Also, inverse agonists of those receptors (eg. Tiprolisant / Pitolisant) reduce the sleeping cycles in mice model of narcolepsy, as well as in patients (Lin J-S et al. An Inverse Agonist of the Histamine H3 receptor Improves Wakefulness in Narcolepsy: Studies in Orexin -/- Mice and Patients. Neurobiol Dis 2008; 30:74).

Also, release of peptides from nerves in response to inflammation is also probably modulated by histamine acting on presynaptic H3 receptors. H1 and H3 receptors play important roles in appetite and satiety; antipsychotic drugs that block these receptors cause significant weight gain. They play role in analgesia as well.

However, it is not yet known whether the H3 receptor has a similar role in humans; research is underway to determine whether H3 agonists are useful in the treatment of obesity. (Bertram G. Katzung, Chapter 16 of Basic and Clinical Pharmacology 13th edition)