Monitoring large populations of locus coeruleus neurons reveals the non-global nature of the norepinephrine neuromodulatory system (2017) et al. http://biorxiv.org/content/early/2017/02/18/109710
This is the latest installment of an ongoing series of capsule summaries of the Shea Lab journal club meetings. We are doing this to actively provoke open discussion of papers we read that are either published in a traditional journal or BioRxiv.
The brainstem nucleus locus coeruleus (LC) is nearly the sole source of the neuromodulator noradrenaline (NA) for the forebrain. The relatively small number of cells in locus coeruleus collectively broadly and diffusely innervate most of the forebrain. This projection pattern and the seemingly homogenous cellular composition of LC has inspired the conventional view that LC output carries a unitary signal, or some sort of global variable, to all brain regions.
Some recent anatomical data is leading researchers to reexamine that model. For example, this paper from the Waterhouse lab and this one from my colleagues Justus Kebschull and Tony Zador show that there is heterogeneity in LC, and that individual cells may target a more restricted brain location than was previously appreciated. This potentially provides an anatomical substrate for LC to differentially modulate target brain structures. However, this would still depend on cells targeting those structure carrying distinct functional activity patterns.
The fundamental question that this paper tries to answer is: “To what extent do different LC neurons carry correlated or distinct information?” This is a very important question that has implications for past and ongoing work in my lab involving LC and NA, so with great interest, I selected the bioRxiv preprint for discussion in a recent Shea lab journal club. The paper is very complex and contains a lot of analysis, but my intention is not to write a comprehensive review. Here I just want to comment on some what I see as some important limitations of the study as written. Also, since this paper is in the preprint stage, maybe my thoughts will be useful to the authors moving forward.
In any neural system, there are always processes occurring in parallel at many time scales. For example, very rapid firing rate fluctuations may closely track the fine temporal structure of a vibratory tactile stimulus. Or auditory system neurons may “phase lock” to the periodic structure of sound. On the other hand, some neurons vary their spiking output dramatically across the circadian cycle of sleep and wakefulness, a much slower pattern. This contrast is also found in the multiplexing of firing rate fluctuations at long and short time scales in LC neurons.
For decades, researchers have recognized that LC neuronal spiking patterns are composed of slowly varying “tonic” spiking patterns that are punctuated by “bursts” of spikes at irregular intervals. Tonic firing changes occur over minutes or more, and individual phasic bursts last several hundred milliseconds. I would argue that these observations point to a range of time scales over which LC firing is likely to change with respect to behavior. As a result, I was disappointed that the authors of this paper performed the majority of their analyses at time scales that are shorter than that.
In many ways, the analysis in this paper makes sense. When one is looking at neuronal correlations, typically they are seen on a spike to spike basis. If cell B’s spikes are reliably occurring with a short delay after each of cell A’s spikes, then that shows that A and B are correlated. The presence of gap junctions between some cells in LC makes these correlations largely expected. At this timescale, the authors found fewer correlations than one might have thought. The authors also looked for correlations over longer 200 ms windows, and found surprisingly few.
So why did this paper fall short of fulfilling my initial enthusiasm? It’s because the authors for the most part didn’t examine the relationship between firing in different cells over the timescale that I suspect is most closely related to behavior. To me, the relevant questions are: What is the relationship between phasic bursts across cells? Do they occur in a synchronous, coordinated manner? As tonic firing rates rise and fall over minutes within a single cell, does firing in other cells rise and fall in a coordinated manner? The authors quantified correlated firing over a maximum window of 200 ms, which is barely enough to encompass one of the phasic bursts. They did examine longer timescales, but only periodic activity; that analysis was not sensitive to slow, aperiodic fluctuations. So, the analysis demonstrates a surprising lack of spike to spike correlations, but they didn’t answer any of the questions above.
I imagine they may have the data that could answer these questions, and I would be very interested to know the results. To best answer to these issues however, one needs to fully explore a broad range of firing rates. That would be best achieved in an awake animal experiencing a range of brain states, but these experiments were performed in anesthetized rats. I don’t know whether they saw large swings in tonic firing rates and frequent phasic bursts under these conditions. In the spirit of constructive feedback, since this is a preprint, I recommend to the authors to describe how firing rates and bursts are coordinated over longer times. Otherwise, congratulations to them on a nice piece of work!