General Knowledge & Review
MORs are associated with what type of property?
"This receptor mediates strong euphorigenic properties.." (Bailly et al., 2022)
Where does the aversion shown to 20hz stimulation happen?
In the mice that have channelrhodopsin activated
Considering this paper and what you've learned so far, what MOR signaling pathways are involved in promoting reward vs reducing aversion?
Hint: Look at Figure 5!
The VTA/NAc pathways are involved in increasing DA, which promotes reward while the Hb/IPN/DRN pathways reduce aversion
Describe the tail suspension test
A mouse is suspended by its tail (usually by tape) and its behavior is observed. Attempting to get out of the suspension is viewed as normal behavior whilst a lack of attempt or a short attempt is viewed as depressive behavior
Name one of the authors of the paper
1) Julie Bailly 6) Florence Petit
2) Florence Allain 7) Marco A Diana
3) Eric Schwartz 8) Emmanuel Darcq
4) Chloé Tirel 9) Brigitte L Kieffer
5) Charles Dupuy
What is Channelrhodopsin?
Light-gated cation channels that convert light into electrical currents for cell depolarization. They are activated by blue light!
Why is 'Total distance (m)' included in Figure 1 panel C?
To ensure that the activation of the neurons are not affecting the mice's motor functions.
In Figure 3, the authors used optogenetics to stimulate Hb-MOR neurons. What was the primary behavioral result in the Real Time Place Preference Test?
Stimulation elicited avoidance behavior, which demonstrates that activating these neurons promotes negative affect.
What does a higher marble burying score indicate?
Marble burying suggests increased levels of anxiety. Mice naturally dig/bury novel items, often in response to anxiety or compulsion.
In this paper, μ-opioid receptors (MORs) are established to mediate positive affect not only euphoria but also what?
Aversion
What was the main hypothesis of this study, written in the Introduction?
"Because MORs are inhibitory receptors, this finding led us to hypothesize that endogenous MOR signaling normally acts as a brake on Hb neurons expressing the receptor, to limit aversive states." (Bailly et al., 2022)
Where does the MHb project onto?
The DRN (Dorsal Raphe Nucleus) and the IPN (Interpeduncular Nucleus)
Activation of Hb-MOR/DRN neurons increase levels of anxiety for _____ tested parameters (hint: figure 4D)
Elevated plus maze and marble burying
Explain the open and closed arms of the elevated plus maze test
There are two arms that are enclosed by walls, and they are called the "closed arms." There are two more arms that have no walls, and they are called "open arms." More time spent in the closed arms indicates anxiety in mice.
Before diving into the results of this study, the authors explain the results of a previous study they conducted. What previous study did they conduct?
They tested naloxone-induced conditioned place aversion in wildtype mice vs mice with deleted Hb MOR neurons.
"strong aversive state". In another study, the MHb shows hyperactivity during nicotine withdrawal and naltrexone-precipitated withdrawal.
Why was c-fos expression measured in Hb-MOR neurons?
c-fos is produced when there’s a large influx of calcium in a cell, and a large influx of calcium indicates neuronal activity. c-fos expression was therefore used as a measure of cell activity; the authors used it to ensure Hb-MOR cells were being activated by optostimulation
Why do we see significant difference in the RTPT and the Tail Suspension Test in Figure 3 but not in Figure 4?
The Hb-MOR/IPN pathway is associated with avoidance and depressive like behavior whilst the Hb-MOR pathway is associated with anxiety like behavior.
What is the DRVD, DRVL, and DRV, in Figure 4B?
DRN: Dorsal raphe nucleus
DRV: Dorsal raphe ventral
DRVD: Dorsal raphe ventrodorsal
DRVL: Dorsal raphe ventrolateral
What do the red and blue dots in the representative tracking plots (Panel B in Figures 1, 3, and 4) represent?
Starting (red) and ending point (blue) of a traveling mouse. The path of one mouse is used as representative of the whole sample size.
According to the study, what is the effect of blocking endogenous mu-opioid tone specifically in the habenula using naloxone?
It induces conditioned place aversion (CPA), confirming that MORs normally inhibit aversive states in this region.
What is anterograde tracing?
Using a fluorescent tracer to identify projections from the presynaptic cell to the postsynaptic cell (in this case, the projection of Hb-MOR neurons to the IPN and DRN)
Based on Figure 5, which specific circuit (Hb-MOR/IPN or Hb-MOR/DRN) is responsible for mediating despair like behaviors?
The HB-MOR/IPN
Why was electrophysiology used in this study?
Electrophysiological recordings of the Hb-MOR neurons were taken before, during, and after blue light stimulation. This was done to ensure that the neurons were actually being activated by optostimulation.
Hedonic Homeostasis