In keeping in line with the neurons closest to my heart’s delight, my favorite poster of Day 2 was on substantia nigra compacta dopamine neurons, and on a role that K-ATP channels may play in these neurons.
Poster E8 – ATP-sensitive potassium (K-ATP) channels are coupled to voltage-gated calcium channels in dopamine substantia nigra neurons – Sylvie Kutterer / Jochen Roeper
This group had previously published a few papers on the role that K-ATP channels play in dopamine neurons. They had first shown that K-ATP channels were differentially expressed in dopamine neurons and could underlie the degeneration of dopamine neurons in Parkinson’s disease. These ion channels were initially found to be present in pancreatic beta cells, they open in response to mitochondrial complex I inhibition, an increase in oxidative stress (ROS) and depletion of ATP. Lastly, in a 2012 paper, this same group has shown that these channels mediate dopamine neuron bursting and exploratory behavior in mice.
This poster seemed to be a continuation of the K-ATP story, but more specifically to the last point in attempting to understand what ion channels mediate burst firing in dopamine neurons. The author started by verifying the presence of K-ATP currents using perforated patch clamp electrophysiology. These single channel openings could not be observed in the K-ATP full knockout mice or in the presence of glibenclamide – an established antidibetic drug that belongs to the class of sulfonyureas, which blocks K-ATP channels by binding to SUR1 which forms part of the K-ATP channel complex. Additionally, they used NN414 and Tolbutamide which are specific K-ATP channel blockers and agonists respectively to block and enhance the K-ATP channels and observe the effect on the cell’s firing. In the absence of K-ATP, there is an increase in cell firing frequency and precision, showing their involvement in firing regulation. They next examined if the K-ATP channels were electrically coupled with other voltage-gated ion channels, as it is well established that other ion channels are involved in the precision and frequency of the cell’s firing. In the presence of apamin and TTX, there was little to no enhancement in the open probability of the K-ATP channels. However, in the presence of the L-type calcium channel blocker – isipiridine, they observed a decrease in the K-ATP open channel probability. The same was observed with the T-type channel blocker – NNC 55-0396. Finally, an enhancement of L-type channel activity with Bay-K 8644 resulted in an increase in the K-ATP channel open probability.
Thus, the major conclusion from this body of work is the coupling of the voltage-gated calcium ion channels to the K-ATP channels. In the pathologic state, one can imagine that in response to oxidative stress, the K-ATP channels would have an increase in their open channel probability… which would subsequently affect firing, resulting in the cell bursting more frequently which is linked to the T-type calcium channels which mediate burst firing and L-type calcium channels.
While I look forward to the completion of this story, as it does seem to go in hand with other literature that implicate the susceptibility of these neurons to death in Parkinson’s due to calcium influx, I did have a couple of questions. The synergistic role of the K-ATP channels with the calcium channels seem to contradict the mechanism of action of glibenclamide on these channels – their block results in an influx of calcium and subsequent release of insulin. However, while one can argue that there could simply be synergistic coupling with the L- and T-type calcium channels and negative coupling with the N-type calcium channels, I would have liked to see the response of the K-ATP channels in the presence of omega-conotoxin. Lastly, I was not sure about the specificity of the T-type channel blocker, as this was my first time coming across this blocker. I would have liked to see the experiment repeated in more popularly known T-type channel blockers. It was also interesting that these channels were indirectly shown to mediate firing in these neurons, as L-type calcium channels are believed to be the main mediators of firing. Another source of evidence especially via either single cell RT-PCR or direct single channel recordings will add more robustness to this last claim.