The Dorsal Raphe Nucleus, or DRN, is an area of the brain located along the midline of the brainstem that contains the largest number of serotonin containing (serotonergic) neurons in the brain. Serotonin participates in mood setting, sleep, social behavior, and many other things. Another cell type in this area are neurons that contain Glutamate Decarboxylase 1 (Gad1), which helps produce gamma-aminobutyeric acid (GABA), the primary inhibitory neurotransmitter in the brain. When GABAergic neurons are activated, they inhibit the neurons with which they are connected, including serotonergic neurons.
The images you see here show the DRN in a transgenic rat brain, specifically a Gad1-cre rat. This means that all of the neurons in this rat brain that have Gad1 also have a genetic “tag” called cre. The blue in the images comes from a fluorescent Nissl stain, which marks the cell bodies of every neuron in this area. The red marks the neurons that contain an enzyme called Tryptophan Hydroxylase 2 (TPH2), which marks serotonergic neurons using immunohistochemistry. The yellow comes from a cre-dependent virus injected into the area that is tagged with a yellow fluorescent protein (YFP). By cre-dependent, we mean that this virus will only transfect neurons tagged with cre, which in this rat are the Gad1 positive neurons, marking them as yellow. In orange are the neurons that were tagged by both our virus and IHC, which means they are both Gad1+ and TPH2+. So in the DRN we have Gad1 only neurons (yellow), TPH2 only neurons (red), and neurons that contain both (orange). This gives us important information about the types of neurons in this brain region.
Our cre-dependent virus has another trick up its sleeve: it can be activated by light. Because of this nifty ability, this virus is used in optogenetics, a powerful tool researchers can use to manipulate neural activity. Optogenetics employs a light sensitive virus and light to manipulate the activity of neurons by either activating or inhibiting them. The virus used here is called Halorhodopsin, and it inhibits neurons when activated. The virus reacts to specific wavelengths of light, in this case green light, and when a green laser is pointed at these neurons, the neurons are inhibited. In short, we can use this virus-laser combination to control neurons in the brain! How cool is that? And what’s even cooler, is that this very precise manipulation allows us to investigate the role of these specific neurons in learning, mood, psychiatric disorders, and so much more.
If you want to learn more about the DRN and serotonin, check out Sweet Serotonin.
Contributed by Maddie Ray, a third year PhD graduate student at Boston College, and our Featured Artist for September and October, 2018. To meet Maddie and see more of her art, click here.