Last week we discussed the paper from Wang et al (From Maiken Nedergaards lab) in our journalclub. Since our lab has healthy interest in up- and downstates in Purkinje cells, it was a good choice I think.
The authors show that by stimulating the Bergmann glia via a transgenic receptor bistability is reduced and the Purkinje cells show more up- than downstates. During the stimulation, the Bergmann glia cells show a calciumtransient and the extracellular potassium concentration decreases. Interestingly, when the Bergman glia cells were hyperpolarized, Purkinje cells spent more time in the upstate.
Now of course the question is "What causes what?" And that's where the paper goes off the rails for a bit. It claims a causal relation: calcium transients cause the extracellular potassium concentration decrease by uptake by Bergmann glia. This in turn causes a reduction in bistability. But a critical experiment, buffering calcium in Bergmann Glia, was not done. Also, there is no proof that the potassium is taken up by Bergmann glia cells. Still, the paper is interesting. It raises a lot of questions, but firmly establishes a role for glia in Purkinje cell modulation.
Another puzzling aspect is the exact mechanism of regulation. How does hyperpolarization of Bergmann glia induce calcium transients? Then, how does the calcium transient cause a reduction in extracellular potassium? Would the presumed uptake of potassium by Bergmann glia cells not depolarize the glia cells again? And how does this decrease in extracellular potassium influence the up- and downstates in Purkinje cells. Actually, the last question can be somewhat answered from the paper. It seems that by reducing the external concentration of potassium, the membrane potential for the upstate drops while the membrane potential for the downstate increases. Also, from the paper of Fernando Fernandez et al. it seems that potassium conductances play a large role in the generation and control of bistability.
A possibility that can be excluded is the hyperpolarization-induced release of GABA via the Best1 channel found in Bergmann glia. It seemed a very likely candidate: hyperpolarization forces anionic GABA out of the cell and the calcium transient opens the channel. However, bistability during Bergmann glia stimulation was not affected, thus excluding the possibility that GABA release plays a role here.
So, it seems that bistability in Purkinje cells can be controlled (somewhat) by Bergmann glia, it is influenced via extracellular potassium and the mechanism is hyperpolarization of the upstate, bringing the states closer together. Serotonin also does this by acting directly on Ih channels. But this is contested already by Fernandez et al. who didn't find any involvement for Ih channels in bistability. They show that potassium plays a big role in bistability, which is in line with the current study. Then, finally there is the possibility of GABA release from Bergmann glia. But that didn't play a role here....
The discussion on Purkinje cell bistability is here to stay. Not only the discussion whether it is really there during waking, but also how it works. Now we probably have to wait for someone to come up with the solution that will connect the pieces rather than introducing a new set of factors. I will stay on top of this, exciting times!
Fushun Wang, Qiwu Xu, Weishan Wang, Takahiro Takano, and Maiken Nedergaard (2012). Bergmann glia modulate cerebellar Purkinje cell bistability via Ca2+-dependent K+ uptake PNAS DOI: 10.1073/pnas.1120380109