0:00 Introduction: Discussion of circadian rhythms, seasonal affective disorder (SAD), and the role of light in regulating mood and sleep.
7:10 Light-Sensing Cells: TWiN explores the role of intrinsically photosensitive retinal ganglion cells (ipRGCs) in circadian entrainment, challenging the idea that sunrise/sunset wavelengths are uniquely important. They highlight the importance of blue light in activating ipRGCs.
15:00 Suprachiasmatic Nucleus (SCN): The SCN in the hypothalamus is identified as the master clock of the circadian system, controlling various physiological processes. Transplantation experiments demonstrate its central role in determining circadian period.
18:28 Serotonin and Glutamate: The median raphe nucleus (MRN), a serotonergic nucleus in the brainstem, projects to the SCN and releases both serotonin and glutamate. Surprisingly, glutamate release is not observed at all MRN terminals in the SCN.
26:09 Branch Segregation: TWiN discusses the remarkable phenomenon of branch segregation, where different branches of the same neuron can release different neurotransmitters. This study reveals that MRN neurons projecting to the SCN selectively regulate glutamate release.
37:48 Photoperiod Adaptation and Glutamate: Manipulating photoperiod (day length) leads to changes in glutamate transporter (VGLUT3) expression at MRN terminals in the SCN. Longer days increase VGLUT3, while shorter days increase it further.
41:54 VGLUT3 Knockout and Adaptation: Knocking out VGLUT3 specifically in MRN neurons projecting to the SCN impairs adaptation to changes in photoperiod. Mice lacking VGLUT3 adapt more slowly to shorter days in wheel-running activity and show a complete lack of adaptation in sleep-wake cycles.
55:21 Upstream Circuitry: Using rabies virus tracing, researchers identify brain regions that project to the MRN, including the preoptic area (POA), lateral hypothalamus, lateral habenula, and ventral tegmental area (VTA).
1:01:06 POA and Photoadaptation: Silencing neurons projecting from the POA to the MRN disrupts photoperiod adaptation and alters VGLUT3 expression in the SCN, suggesting a key role for the POA in regulating this circuit.
1:05:15 Relevance to Humans: TWiN speculates on the potential relevance of these findings to humans, noting that while the specific circuits may be conserved, the valence of light (aversive for mice, generally pleasant for humans) differs and could influence the system's function.