# Histamine H3 receptor

Histamine H$_3$ receptors are expressed in the central nervous system and to a lesser extent the peripheral nervous system, where they act as autoreceptors in presynaptic histaminergic neurons and control histamine turnover by feedback inhibition of histamine synthesis and release. The H$_3$ receptor has also been shown to presynaptically inhibit the release of a number of other neurotransmitters (i.e. it acts as an inhibitory heteroreceptor) including, but probably not limited to dopamine, GABA, acetylcholine, noradrenaline, histamine and serotonin.
The gene sequence for H$_3$ receptors expresses only about 22% and 20% homology with both H$_1$ and H$_2$ receptors respectively.
There is much interest in the histamine H$_3$ receptor as a potential therapeutic target because of its involvement in the neuronal mechanism behind many cognitive disorders and especially its location in the central nervous system.

## Tissue distribution

* Central nervous system
* Peripheral nervous system
* Heart
* Lungs
* Gastrointestinal tract
* Endothelial cells

## Function

Like all histamine receptors, the H$_3$ receptor is a G-protein coupled receptor. The H$_3$ receptor is coupled to the G$_i$ G-protein, so it leads to inhibition of the formation of cAMP. Also, the β and γ subunits interact with N-type voltage gated calcium channels, to reduce action potential mediated influx of calcium and hence reduce neurotransmitter release. H$_3$ receptors function as presynaptic autoreceptors on histamine-containing neurons.
The diverse expression of H$_3$ receptors throughout the cortex and subcortex indicates its ability to modulate the release of a large number of neurotransmitters.
H$_3$ receptors are thought to play a part in the control of satiety.

## Isoforms

There are at least six H$_3$ receptor isoforms in the human, and more than 20 discovered so far. In rats six H$_3$receptor subtypes have been identified so far. Mice also have three reported isoforms. These subtypes all have subtle difference in their pharmacology (and presumably distribution, based on studies in rats) but the exact physiological role of these isoforms is still unclear.

## Pharmacology
### Agonists

There are currently no therapeutic products acting as selective agonists for H$_3$ receptors, although there are several compounds used as research tools which are reasonably selective agonists. Some examples are:
* (R)-α-methylhistamine
* Cipralisant (initially assessed as H$_3$ antagonist, later found to be an agonist, shows functional selectivity, activating some G-protein coupled pathways but not others)
* Imbutamine (also H4 agonist)
* Immepip
* Imetit
* Immethridine
* Methimepip
* Proxyfan (complex functional selectivity; partial agonist effects on cAMP inhibition and MAPK activity, antagonist on histamine release, and inverse agonist on arachidonic acid release)

### Antagonists

H$_3$ receptor antagonists include:
* A-304121 (No tolerance formation, silent antagonist)
* A-349,821
* ABT-239
* Betahistine (also weak H$_1$ agonist)
* Burimamide (also weak H$_2$ antagonist)
* Ciproxifan
* Clobenpropit (also H$_4$ antagonist)
* Conessine
* Failproxifan (No tolerance formation)
* Impentamine
* Iodophenpropit
* Irdabisant
* Pitolisant
* Thioperamide (also H$_4$ antagonist)
* VUF-5681 (4-[3-(1H-Imidazol-4-yl)propyl]piperidine)

## Therapeutic potential

The H3-receptor is a promising potential therapeutical target for many (cognitive) disorders that are caused by a histaminergic H3R dysfunction, because it is linked to the central nervous system and its regulation of other neurotransmitters. Examples of such disorders are: sleep disorders (including narcolepsy), Tourette syndrome, Parkinson, OCD, ADHD, ASS and drug addictions.
This receptor has been proposed as a target for treating sleep disorders. The receptor has also been proposed as a target for treating neuropathic pain.
Because of its ability to modulate other neurotransmitters, H$_3$ receptor ligands are being investigated for the treatment of numerous neurological conditions, including obesity (because of the histamine/orexinergic system interaction), movement disorders (because of H$_3$ receptor-modulation of dopamine and GABA in the basal ganglia), schizophrenia and ADHD (again because of dopamine modulation) and research is underway to determine whether H$_3$ receptor ligands could be useful in modulating wakefulness (because of effects on noradrenaline, glutamate and histamine).
There is also evidence that the H3-receptor plays an important role in Tourette syndrome. Mouse-models and other research demonstrated that reducing histamine concentration in the H3R causes tics, but adding histamine in the striatum decreases the symptoms. The interaction between histamine (H3-receptor) and dopamine as well as other neurotransmitters is an important underlying mechanism behind the disorder.

## History

* 1983: The H$_3$ receptor is pharmacologically identified.
* 1988: H$_3$ receptor found to mediate inhibition of serotonin release in rat brain cortex.
* 1997: H$_3$ receptors shown to modulate ischemic norepinephrine release in animals.
* 1999: H$_3$ receptor cloned
* 2000: H$_3$ receptors called "new frontier in myocardial ischemia"
* 2002: H$_3$$^{(-/-)}$ mice (mice that do not have this receptor)