Periodic Reporting for period 1 - 5HT-DREADD (In vivo pharmacogenetic investigation of 5-HT mechanisms in emotional learning)
Reporting period: 2016-07-01 to 2018-06-30
In this project, I introduced these receptors to serotonin cells in the dorsal raphe nucleus (DRN), the key source of serotonin innervation in the brain. However, these are not the only cells in that area as they are controlled by local inhibitory gamma aminobutyric acid (GABA) neurons. I therefore also explored how expressing these receptors and selectively activating these local GABA neurons affected the functionality of the serotonin system and the subsequent effect on behaviour. This selectivity is achieved by delivering the new genetic code to the DRN with viruses containing the information for these receptors (hM3Dq) as well as a fluorescent “tag”, or just this “tag” (mCherry) to act as a control. Different lines of mice then allow for this to either be expressed only in serotonin neurons (SERT-Cre mice) or the GABA neurons (vGAT-Cre mice).
Once serotonin-selective and functional expression of these receptors was confirmed, I tested the effect of manipulating neuronal activity within the DRN in various anxiety models. On the elevated plus maze, I found that increasing the excitability of serotonin neurons through expression and activation of these receptors in the SERT-Cre mice caused an increase in time spent in the open arms. This suggests that it makes them less anxious. vGAT-Cre mice did not show this effect. In two other tests of anxiety (the light and dark box and novelty supressed feeding), however, I saw no changes in the SERT-Cre mice. Surprisingly, vGAT-Cre mice spent a significantly reduced time in the light compartment of the light and dark box while at the same time exhibiting increased exploration of the central area of an open field, suggesting increased and decreased anxiety, respectively. These findings suggest that chemogenetic activation of serotonin neurons reduces anxiety, while chemogenetic activation of GABA neurons in the DRN increases anxiety, at least under certain conditions, and might be doing this independently of serotonin.
I then set out to characterise whether chemogenetically activating serotonin neurons in the raphe would have an impact on the acquisition and retrieval of fear memories. Surprisingly, I found no effects using this system in several variations of fear conditioning, a well-used test to investigate negative emotional memory formation and retrieval. These results suggested that chemogenetically activating serotonin neurons in the DRN is not sufficient to enhance nor disrupt the acquisition and retrieval of emotional fear memories.
Together, the data obtained in this project indicate the successful chemogenetic targeting of DRN serotonin neurons to explore the role of serotonin circuitry involved in anxiety and fear behaviour but that the role of DRN GABA neurons may be more important to the control of emotional behaviour than the serotonin neurons themselves. These results will help better understand the mechanisms underlying the control of emotional behaviour and may contribute to identify better candidates for treatments aimed at ameliorating psychiatric conditions in which emotional behaviour is altered.