Sweet Serotonin

The Dorsal Raphe Nucleus, or DRN, is an area of the brain located along the midline of the brainstem, which is found towards the back of your brain. The DRN contains the largest number of serotonin-containing neurons, called serotonergic neurons, in the brain. You may recognize serotonin from the drug advertisements on TV, as it is a popular target for treating depression. Unsurprisingly, serotonin is heavily implicated in regulating mood. However,  serotonergic neurons aren’t just involved in mood; they also participate in learning, anxiety, patience, and so much more. My lab is specifically interested in how these neurons are involved in aversive learning, and to do this we target an enzyme called Tryptophan Hydroxylase 2 (TPH2), which is involved in the synthesis of serotonin.  

These images show the DRN of a transgenic rat brain. This specific rat was a TPH2-cre rat, which means that all of the neurons in its brain that have TPH2 (and thus are serotonergic) also have a special “tag” called cre. This tag allows us to target these specific TPH2 neurons, without targeting the untagged (non TPH2) neurons. The blue in the images comes from a fluorescent Nissl stain, which nonspecifically marks the cell bodies in this area so we can see all of the neurons, independent of whether they have TPH2 or not. The yellow comes from a virus injected into the area that contains a yellow fluorescent protein (YFP). This virus is especially interesting because it is a cre-dependent virus, meaning it is only expressed in the neurons tagged with cre. Since this rat was a TPH2-cre rat, that means that this virus will only be expressed in the TPH2-positive neurons, so only TPH2 neurons are yellow.

But what if this virus doesn’t work properly, and targets all of the neurons? To ensure that this virus is transfecting only the TPH2 neurons, and not every single neuron in the area, we can use immunohistochemistry (IHC). IHC is a popular tool in neuroscience that uses antibodies to mark specific proteins. By doing this, you can selectively stain only the neurons that contain this specific protein, for instance TPH2, while the neurons without the protein will not be stained. For this IHC, I targeted the TPH2 neurons with a red fluorescent tag. Therefore, all of the red you see in these images are TPH2 neurons. So if this virus works, we should see overlap, or colocalization, between the YFP (yellow), and the IHC (red), resulting in orange. This orange tells us that the neurons targeted by the virus are in fact TPH2 neurons and our virus works! Ultimately, using this combination of techniques (TPH2-cre rat, a cre-dependent virus, and TPH2 IHC), we are able to target these specific neurons. Once we can target them, we can start manipulating them by either deleting them, or activating or inhibiting them without actually deleting them using a technique called optogenetics. We’ll discuss optogenetics in more detail in my next post.

Contributed by Maddie Ray, a third year PhD graduate student at Boston College, and our Featured Artist for the fall of 2018. To meet Maddie and see more of her art, click here.