It is very difficult to separate the activity of glial and neurons in some instances, making this an even more difficult question.

You can block microglia activity with diazoxide that acts on its Katp channel. It is quite specific.

Thanks Nicole,

I'd not heard of diazoxide before. Wikipedia tells me that it is a potassium channel activator, which causes local relaxation in smooth muscle by increasing membrane permeability to potassium ions. Perhaps most critically, it also states that the drug acts by switching off voltage-gated calcium ion channels which inhibits the generation of action potentials. The implication being that it significantly alters neuronal function and would seem therefore not be perfect for the job of only inhibiting microglia.

RW.

I am also wondering your question. Maybe you know my PLoS ONE paper using minocycline as a possible microglial inhibitor in human subjects. Your research focus is very much interesting for me. I know that you have recently published a very innovative paper regarding re-modeling of microglia under chronic stress (Cereb Cortex in press). If your paper will appear more earlier in pubmed, I would have cited it. In your paper, the effect of minocycline is very interesting because minocycline effects not only stressed microglia but also non-stressed microglia. Your finding will support my result in healthy human subject. Hope to have further communicate with you. Takahiro

I do not know of any available agent that selectively suppresses microglia function. There are multiple agents that suppress microglia activity *in addition* to other known effects. Some of those are naltrexone/naloxone, dextromethorphan, 3-hydroxymorphinan, ibudilast, flurocitrate, and pentoxifyline. It is possible that dextro-naltrexone and dextro-naloxone may be more specific to microglia function -- check recent work by Linda Watkins, Mark Hutchinson, and their colleagues. I imagine there is more than one pharma group that is currently working on selective microglia inhibitors, so we may see new agents in the next few years.

I agree with Jarred. Also, I find it very interesting that you so strongly state that Minocycline is the best and most well characterised for altering Microglial activation. It is surely the most commonly used, but in primary microglial cultures stimulated with LPS (mM-pM) at various doses and treatments we have not been able to decrease many markers of activation, aside from small decreases in TNF and IL1B. This suggests to us that the actions on other glia and neurons (or signalling between) is critical for the 'anti-' inflammatory effects of Mino. As many of the original in vitro papers used mixed glial cultures (or poorly characterised cultures) this may explain the strong feeling amongst researchers on its activity. What are your thoughts after more reading on the various (potential) inhibitors ?

Dear Bobbi,

In broad terms I agree with you that minocycline as an anti-inflammatory agent appears to the be relatively modest in terms of its potency. Like you, we have also found that minocycline is not a broad spectrum anti-inflammatory, rather it seems quite specific in which inflammatory actions it does and does not inhibit.

I would also mention that when I referred to microglial activation I was not only referring to activation in inflammatory context. Certainly, this was a mistake on my part as this is the most widely accepted notion. Foremost in my mind at the time of writing, however, were several studies from the field of neuroscience that have revealed microglia can significantly transform morphologically, in an experience dependent manner, and in a way that does not appear related to their inflammatory status. Consistent with this concept we and others have found that minocycline can provoke profound structural remodelling of microglia in what appear to be otherwise healthy animals.

I hope that that gets me of the hook!

RW.

Dear Takahiro

I absolutely think the work your research group is in involved in is fascinating. Minocycline, certainly in the rodent models that we have used appears to exert extremely considerable effects on microglial structure in healthy animals.

RW

Dear Rohan, If you were on a hook, that is certainly an extricating answer.

A quick thought...- morphology was the first observable indication of activation state in microglia, and then it became assumed that ramified and ameboid were completely synonymous with inactive and cytotoxic. More recently we have the idea of phenotypic markers evolving, and after a short hiatus improvements in analysis methods (automated software) morphology is reemerging as an 'important' characteristic. Also, microglia have very altered morphology during early development and within areas such as the white matter. (Deep breath)….So can you speculate for me as to what you think minocycline is doing to the microglia in healthy brains? Perhaps you simply need to point me to your most appropriate paper, but also do you have any more left-of-centre ideas ?

Are you using mice for your research? If yes, you might consider using a transgenic mice for that purpose. Our lab generated a transgenic mouse that express Cd11b-driven TK, so that proliferating microglia can be blocked by injecting a drug (ganciclovir). Original publication here: http://www.ncbi.nlm.nih.gov/pubmed/16276506

Dear Jean-Nicolas. Many thanks for your suggestion. Regards,

Rohan.

You're welcome.

You should look into palmitoylethanolamide, a lipid signaling PPAR alpha agonist, which clearly inhibits overactive glia cells and mast cells. WE use it frequently in neuropathic pains and it has a very impressive effect in some patients.

Here is my review on the influenace of PEA on non-neuronal cells

Jan keppel Hesselink - As the initial question was asked as to whether we could separate out the effects of Mino on glia and non-glia, could you speculate/inform me a little regarding the effects of PEA on non-glial cells in vivo ?? Thanks !! :-)

Oh Bobbi, I could do much more, there are so many papers on PEA's effects on non-neural cells in vivo, if you want I share with you my dropbox on full PEA papers, just yell your email adress and I will add you. PEA's main biological activity goes via PPAR alpha activation, so be my guest to guess the effects....;)

Thanks Jan !! My email is [email protected] :-)

Hi Rohan, we have also seen clear effects of minocycline on peripheral inflammatory markers, so if systemic inflammation is a contributor in your system it would also be important to bear that in mind.

A word of caution before using minocycline in vivo. It is highly acidic and needs to be titrated to neutral pH before use. If this is not done and injected intraperitoneally it can cause severe peritonitis which could have an influence on what your studying.

Also have you considered indomthacin or aspirin as a downregulator?

Totally agree Gareth. As soon as we tried an i.p injection we knew we didn't want to go that way. We deliver our Mino in drinking water ~1mg/mL. At that conc it does not have a major impact on the pH of the solution and we find the animals typically drink at a rate not that different from water alone.

I think that the other major consideration is the "injury" model in that minocycline can have multiple effects including blocking apoptosis. The conflicting data in the literature with minocycline to minimize neuronal damage and microglia activation demonstrates this limitation of identifying cause and effect. The existing publications that conflict may also represent the level of contribution of peripheral monocytes to the brain macrophage population. It is likely that systemic minocycline will down regulate the peripheral cell and thus have effects yet, not demonstrate a down regulatory effect if the injury relies on resident microglia cells in the absence of recruitment of peripheral monocytes. Just a thought for consideration. Using a model of brain injury that induces neuronal apoptosis and activation of resident microglia with no indication of recruitment of peripheral monocytes we find no down regulation with systemic delivery of minocycline.