H3 Receptors, a new hope!
Histamine is an important compound which is released in the inflammatory processes occuring in the body. But is it possible that we can alter the binding mechanism and find cure in certain disorders?
Well, at first it seems difficult, but it is not. We’ll see it how this happens and is now under research for treating disorders.
There are 4 types of Histamine Receptors with each one having a different function and locations. The four receptors are separated on the basis of the affinity of the histamine molecule towards the receptors and thus deciding the functionality of the histamine. The two broad categories are — High affinity and Low affinity receptors.
Further, each category has 2 receptors each; i.e. Low affinity receptors are H1 and H2 histamine receptors, whereas High affinity receptors are H3 and H4(as shown in the diagram below).
Now we all know the anti-inflammatory drugs we take for example Cetirizine, Ranitidine, Cimetidine, etc; thus help us relieve from the allergic conditions like urticarias.
But something interesting that has came up in recent researches about H3 receptors. H3 receptors are G protein coupled receptors(just like other Histamine receptors) which are situated in the CNS(Central Nervous System) predominantly. These H3 receptors are Presynaptic receptors, i.e. they are located at the neuron present before the synapse, which help in autoregulating the neurotransmitters such as Acetylcholine, Dopamine, Serotonin, GABA(Gamma-Amino-Butyric-Acid) & Norepinephrine. This H3 receptor is found in high density in the Basal Ganglia, Hippocampus, Cortical Areas of Brain which are associated in cognition.
H3 Receptors regulate the release of the neurotransmitters by influencing the amount of intracellular calcium. It does this by blocking the influx of calcium.
From the above diagram, it can be seen, that “Epilepsy” occurs because of stimulation of these receptors and histamine presynaptic neurons. Thus Anti-H3 Receptor drugs or H3R blockers can be used in treatment of epilepsy(still under research). The drugs which come under H3R are — Clobenpropit, Ciproxifan, Betahistine, Pitolisant.
Similarly, a new research is in progress which states the use of “H3R blockers in Alzheimer’s Disease”. The Histaminergic system is involved in Alzheimer’s Disease(AD) but the histamine levels are a doubtful cause for a proper conclusion. In one report, they said the increased Histamine levels in the Hippocampus, basal ganglia and fronto-temporal cortex are associated with the occurrence of AD{as seen in the scientific paper published by Cacabelos et al., 1989}, and in other it is said that the reduced levels of Histamine in the Hippocampus, Hypothalamus and temporal lobes/regions is associated with the occurrence of AD{as seen in the scientific paper published by Mazurkiewicz-Kwilecki and Nsonwah, 1989; Panula et al., 1998}.
So, any final verdict about its surety is still questionable. Some incidences have shown improvements in epilepsy when H3R blockers/H3R antagonists are used along with H1R antagonists, but for Alzheimer’s, all we can do is wait and see which is right and which works in the favour of the patients.
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