Paper #2 - Action potentials contribute to neuronal signaling in C elegans

Title:  Action potentials contribute to neuronal signaling in C elegans

Year: 2008

Summary: In this paper, they undertook patch clamp measurements on C. elegans neurons, a rarely used (and presumably difficult) technique, to study the electrophysiological properties of C. elegans interneurons. They specifically looked at AVA (the 'backwards command' neuron) and RMD interneurons. AVA has an apparently very linear IV-curve in current clamp mode, consistent with previous measurements.  In contrast RMD appears bistable, with a stable depolarized state at ~ -10mV! This is often referred to as a plateau potential, in which a brief current pulse can switch the neuron's state from low to high or vice versa. Without any stimulation, RMD also appeared to natively switch between a high and a low state, whereas AVA flatlined.

I should clarify that the authors present two related but distinct phenomena here. The first, which I'll call 'comparator-like behavior', is where a linearly-varying input leads to one of two distinct outputs (states), and thus there is a point where a very small change in the input can make a huge difference in the output. They show that this is generally the case for the RMD neuron.  The other phenomenon, plateau potentials, requires this 'comparator-like behavior', but in addition, the output continues in the absence of input. For example, a single positive current pulse pushes the neuron to a depolarized state, but after the input current is stopped, the neuron stays depolarized until it receives hyperpolarizing current. As they note in the article, this is 'Schmitt trigger-like' or hysteretic behavior.

They tested whether removing Na+ from the medium (and replacing it with N-methyl-D-glucamine to retain osmolarity) eliminated comparator-like behavior, but it did not, suggesting Na+ isnt essential for comparator-like behavior. While they note it did eliminate Schmitt trigger-like behavior, after returning the worm to normal media, they did not regain Schmitt-trigger-like behavior, so it's not clear this had anything to do with the Na+. In short, Na+ doesn't seem too important for these neuronal dynamics.

In contrast, removing calcium eliminated the comparator-like behavior, suggest that this may be due to voltage-gated calcium channels. This effect is striking and very very clear.  However, knocking out single or even pairs of voltage-gated calcium channel alpha subunits did not affect this behavior. They thus suggest that perhaps other types of calcium channels, or perhaps even uncharacterized calcium channels contribute to this behavior.

 

What I didn't understand: In some of the experiments, it appears that a treatment affects plateau potentials, but since plateau potentials seem to only occur half the time (and I'm assuming somewhat randomly) and they typically only show a single trace, I don't know whether its safe to conclude the treatment affects plateau potentials or not. For now, I'm assuming nothing affects plateau potentials.

 

Questions I should look into further:

  • Why hasn't this study been done on every other neuron in C. elegans!? This type of data seems crucial, yet to my knowledge isn't available.
  • They note "in 14 out of 16 RMD neurons... the voltage response to depolarizing current ramps was linear... but the voltage response then became regenerative leading to a solitary action potential....". What about the other two RMD neurons?? Do worms differ?
  • They also note "We found bistable potentials associated with 54 out of 98 RMD action potentials." What dictates whether a potential is sustained after current is removed?? Is it some network property? The presence/absence of a neuromodulator?

 

Other notes: I chose this paper because it had some interesting commentary that followed. Shawn Lockery and Miriam Goodman wrote a commentary 8 months later, seemingly praising the paper's finding of plateau potentials, as well as a  correspondence (in the same issue) condemning the claim of action potentials as 'premature.'  The debate over whether the observed dynamics constitute action potentials seems to be predominantly a semantic one, as it's clear that C. elegans 'action potentials' are certainly distinct from many other kinds of action potentials reported so far, but yet share some striking similarities. Lockery and Goodman argue that to be called an action potential, the voltage transient must meet the following criteria:

  1. the amplitude of the voltage transient is invariant with respect to the amplitude and duration of the stimulus that generated it.
  2. It is intrinsically self-terminating
  3. it has a stereotyped waveform that is invariant to the amplitude duration and waveform of the stimulus.

And in short, Mellem et al. rebut that this definition is too narrow.