(First day's entry)
Daily blogging is rough.
This is the first medium size meeting I've been to (800 attendees, 2-3 sessions), and it is uniquely exhausting. While SFN is 40 times larger, and tiring in its sheer scope, there is something exhilarating about being surrounded by so many people, like walking through Shibuya or Times Square. If you get tired of taste or olfaction, you can take in a barrel cortex or retinal development talk. Six hours a day of chemosensory talks can be repetitive.
Since I went to so many talks today, I will briefly comment on two. The day started off with Cori Bargmann. I got in a little late, but the gist of the first part is that one of the C elegans neurons, AWC, is activated when an odor turns off (which sounds familiar to me), with a time constant of 10s of seconds. They looked at the mechanisms of this, but to be honest I got lost in the worm alphabet soup.
In the next section, they looked at behaviour. C elegans have simple motor behaviour: they can move forward, reverse, turn, or pirouette. Using a microfluidic device, they measured how the worms turned in response to odors turning on and off. They found that worms tend to move forward during odor presentation, and when the odor turned off, the worms started turning, with a time constant similar to AWC. They then tried imaging the AWC in the behaving worm, to see if AWC activity was correlated with turning or moving forward. However, that AWC was similarly active when the worm moved forward or turned, which means AWC neurons are simply sensory and not motor.
Next, she moved to the timescale of a few seconds. Worms move by wiggling back and forth in sinusoidal patterns. These sine waves in effect give the worm a metered sampling of the world, a sort of sniff. One worm behaviour that may rely on this sinusoidal sniffing is turning along odor edges; worms within an odor stripe will move parallel to the stripe, or turn inward, but not outward. Since AWC neurons fire in response to changes in odor, they tested how AWC neurons fired to stimuli at 1Hz, and found the AWC neuron could follow the stimulus. Then they performed reverse correlation on the calcium signal, using an L-N model, and found that AWC neurons could linearly follow integrate over a 1s window. (GCaMP's 0.5s non-linearity was also detected)
Isaacson is looking at how experience an anesthesia can modify odor representations in the olfactory bulb. They now have 2p awake imaging working in the bulb, by injecting AAV-flex-GCaMP in Pcdh21-cre mice. In slices, they validated that GCaMP could encode APs linearly over 0-40Hz.
Using this, they imaged responses in the bulb. They found that in awake mice, individual cells responded to a relatively narrow range of odors (imaging over 4s, ignoring sniff phase). When they recorded in anesthetized mice, the responses increase in both magnitude, and broadness. They also lost inhibitory odor responses. They hypothesized that anesthesia might be selectively effecting GABAergic granule cells, and so applied gabazine in awake mice, and saw a similar effect to anesthesia. To look at this another way, they then imaged activity in the granule cells using GAD-cre mice with AAV-flex-GCaMP. The granule cells had sparse tuning. However, when they anesthetized these mice, the found the granule cells simply stopped responding to odors, which would explain the loss of inhibition.
In the 2nd half of the talk, he asked the question of how stable odor representations are. They imaged mice daily over 7 days, and found the response magnitudes decreased each day. Then they split their odor space in two; for group A, they imaged those odors every day; for group B, they imaged only on days 1 and 7. The group A responses all decreased, while the group B responses maintained their strength. Thus repeatedly presenting an odor selectively attenuates its response. This also held for single cell analyses. To look at glomerular inputs, they imaged OMP-SpH mice, and found that these responses were maintained over 7 days. They looked at how long it took for these responses to recover, and found that they began to recover after one week, but took a full 2 months to completely recover. Finally, they looked at how awareness effected this by imaging mice in both awake and anesthetized states. While the awake responses decreased, in the same mice, the anesthetized responses remained stable. Thus, the adaptation they observe in awake mice requires some sort of awareness.