Paper trail expansion: odor afterimages in the olfactory bulb

OR: I couldn't call this blog Trail of Papers, and not publish, now could I?

My work of the last two years has just been published in PNAS. I know you, dear reader, are busy, so here are four key panels from the paper.

(With this publication, I am on the post-doc job market. While I am applying to a few specific labs, I'm interested in seeing how this blog could influence my career. If any American labs are looking to hire someone to do chronic, in-vivo recordings using optogenetics, send me an e-mail. If you want to know what I think like, this blog is probably an accurate representation. Here is my CV.)

Mitral cells have an odor afterimage

So I was recording the responses of mitral cells in the olfactory bulb while mice smelled odors, and I noticed something odd. Some cells continued to respond to odors after the end of the odor, and this post-odor response could last for a few seconds:

PSTH from a mitral cell responding to ethyl butyrate. The cell was excited during the odor, but inhibited afterwards.

In the example above, the cell switched its firing from excitation to inhibition between the odor to the post-odor. Other cells could switch the other way, from inhibition to excitation; or could have more subtle shifts in phase between the odor and post-odor (Fig. 3 of the paper). Around 30% of cells had some sort of post-odor response, and these post-odor responses could last for over ten breaths after the odor was gone.

To see whether these post-odor responses were odor specific, we performed some population analyses, and found that the post-odor responses were unique for each odor. However, as you can see above, these post-odor responses were not simply recapitulation of the odor responses. Given that these post-odor responses are different from the odor response, but contain odor information, we dubbed these responses odor afterimages.

Most glomeruli do not have afterimages

Some labs have previously reported that olfactory sensory neurons in drosophila can have long-lasting "super sustained" or "ultra prolonged" responses, and recordings from the olfactory nerve of rats also sometimes have off responses. To see whether our mitral cells' odor afterimages were simply the result of sustained input from the receptors, Sam (the 2nd author) performed calcium imaging of glomeruli, which contain the terminals of olfactory sensory neurons. While 5-10% of cells could have sustained activity, most did not:

Response of three glomeruli to acetophenone. Some glomeruli did have a post-odor response, but most did not.

Given that some glomeruli did have post-odor responses, we tried to compare the post-odor activity in glomeruli and mitral cells using a template matching prediction algorithm. The glomeruli prediction was only above chance for one breath after the odor, showing that the post-odor responses did not contain much post-odor information. In contrast, the mitral cell responses lasted for over ten breaths. So while some of the olfactory bulb inputs may be long-lasting, there seems to be more information in the mitral cell responses than can be explained by simple peripheral processes.

Olfactory bulb activity in the absence of sniffing

Since we found that glomerular activity was not as strong as mitral cell activity during the post-odor, we investigated what types of feedback there could be during the post-odor. To look at this, we did my favourite crazy experiment: we recorded from an olfactory bulb while blocking the nostril. In this condition, all the activity in the bulb with the blocked nostril (ipsilateral), would have to come from the contralateral, unblocked nostril, via cortical connections. When we recorded from the bulb with the nostril blocked, almost all spiking activity was gone, as has been seen previously. However, we did find LFP activity:

LFP activity in the mitral cell layer of the OB in response to an odor with the nostril open (top) or blocked (bottom). With the nostril open, the odor elicits a large LFP deflection early, and gamma frequency oscillations after 50-100 milliseconds. With the nostril blocked, there is an LFP deflection, but it is delayed, and contains less gamme frequency. Breathing activity shown above LFP (inspiration is up). Total time ~1 sec.

There are a couple interesting things about the blocked nostril LFP.  First, the sign of the activity is flipped, which may signal a switch from excitation to inhibition, or some other process. Second, the activity with the nostril blocked is delayed. This could reflect the time it takes for information to get processed in the contralateral olfactory bulb, and then relayed to the ipsilateral olfactory bulb. The gamma frequency activity in  the open nostril condition also seems to coincide with the onset of activity in the blocked condition. This is not a particularly conclusive or informative result, but I love the idea of olfactory responses absent odor input, and the strange nature of these responses.

Ringing in the olfactory bulb

Since we did not see strong input from the glomeruli into the olfactory bulb, we hypothesized that the odor afterimage was maintained centrally rather than from the periphery. This gave us the idea that perhaps we could generate an "artificial afterimage." So we took some Thy1-ChR2 mice, which express ChR2 in mitral cells, stimulated the heck out of the olfactory bulb, and found that mitral cells could continue firing for up to ten breaths after the end of photostimulation:

PSTH of firing in a mitral cell in a Thy1-ChR2 mouse during and after photostimulation at 20Hz. It was difficult to record spikes during photostimulation, perhaps due to inhibition. Afterwords, the cell had rebound activity for a few seconds.

Some cells were inhibited after light stimulation, others were excited, and others were completely uneffected. I'm not current on ringing activity in other brain areas, but I think it's interesting that if you put the olfactory system into an active state, it takes a few seconds for the brain to calm down. We did most of these Thy1-ChR2 experiments in anesthetized mice, because in the one awake mouse I stimulated, the mouse started freaking out and drooling. I don't want to know what it smells like when your entire olfactory bulb is activated.

So those are my favourite four figures of the paper. There are a lot more interesting tidbits scattered throughout, and I hope the paper is readable to a general audience. Hopefully my blogging over the last few years has improved my writing style. I suggest you check it out.