Friday, 2 June 2017

Endocannabinoid control of potassium...there is no CB3

We have now identified that anandamide can save oligodendrocytes. It is thought that it effects potassium channels and has been suggested to be activing via a TASK channel.

However anadandamide does other things. 

Anandamide from the sanscript for "inner bliss" was the first natural moleucle found to bind to and stimulate the cannabinoid one receptor. It is quite weak at this and so doesn't get you high. It is also destroyed in the body pretty quickly by a molecule called fatty acid amide hydrolase.

No sooner was CB1 discovered then CB2 was discovered. Then they made knockout mice and did cannabinoid receptor assays in them and they found that anandamide did things and this was called CB3.  This function could affect the blood vessels, but  the only problem was there was no CB3. The genome had been sequenced, all genes where known and there was nothing that looked like a another cannabinoid receptor.


Anandamide was also reported to bind to the transient receptor potential cation channel subfamily V member 1 (TrpV1) also known as the vanilloid receptor. This is the heat sensitive receptor that signals the response via chilli peppers. However this is not expressed by oligodendrocytes so it ain't this. Likewise they don't express CB1 or CB2, so what is it?


The next suggestion was that it is GPR55, this is a G-protein coupled receptor that we found and it is about 30% similar to CB1 again not expressed by oligodendrocytes and then a more recent suggestion that it is a potassium channel, a big conductance, calcium activated potassium channel. This opens to allow potassium to leak out of the cell, reducing potassium levels with the cell. It is expressed by the oligodendrocyte. This is how anandamide works!


Bondarenko AI, Panasiuk O, Okhai I, Montecucco F, Brandt KJ, Mach F. Direct activation of Ca2+ and voltage-gated potassium channels of large conductance by anandamide in endothelial cells does not support the presence of endothelial atypical cannabinoid receptor. Eur J Pharmacol. 2017. pii: S0014-2999(17)30207-8.
Endocannabinoid anandamide induces endothelium-dependent relaxation commonly attributed to stimulation of the G-protein coupled endothelial anandamide receptor. The study addressed the receptor-independent effect of anandamide on large conductance Ca2+-dependent K+ channels expressed in endothelial cell line EA.hy926. Under resting conditions, 10µM anandamide did not significantly influence the resting membrane potential. In a Ca2+-free solution the cells were depolarized b
y ~10mV. Further administration of 10µM anandamide hyperpolarized the cells by ~8mV. In voltage-clamp mode, anandamide elicited the outwardly rectifying whole-cell current sensitive to paxilline but insensitive to GDPβS, a G-protein inhibitor. Administration of 70µM Mn2+, an agent used to promote integrin clustering, reversibly stimulated whole-cell current, but failed to further facilitate the anandamide-stimulated current. In an inside-out configuration, anandamide (0.1-30µM) facilitated single BKCa channel activity in a concentration-dependent manner within a physiological Ca2+ range and a wide range of voltages, mainly by reducing mean closed time. The effect is essentially eliminated following removal of Ca2+ from the inside of the cell (cytosolic) face and pre-exposure to cholesterol-reducing agent methyl-β-cyclodextrin. O-1918 (3µM), a cannabidiol analog used as a selective antagonist of endothelial anandamide receptor, reduced BKCa channel activity in inside-out patches (electrotrode to measure electrical currents moving from the insideto the outside of the cell). These results do not support the existence of endothelial cannabinoid receptor and indicate that anandamide acts as a direct BKCa opener. The action does not require cell integrity or integrins and is caused by direct modification of BKCa channel activity.

There is no CB3 it is BKCa.


The answer has just been published, but we have known this for years, more on this soon.

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