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Sorry for the simple question, not a neuroscientist just trying to understand a paper for school. In a study with mice, there was 2-photon calcium imaging done, and part of it read:

We used single- and multi-plane imaging approaches to record the activity of populations of excitatory neurons and two inhibitory classes, Somatostatin (Sst) and Vasoactive Intestinal Peptide (Vip) expressing interneurons, across multiple cortical depths and two visual areas (VSIp and VISl)

First I thought this meant that SST & VIP were inhibitory neurons. But when I Google it says they're hormones.

So is this saying they recorded activity from inhibitory interneurons that were ... releasing? producing? ... SST & VIP ? What does "expressing" mean in this sense.

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Gene Expression

"Expressing" refers to gene expression - in biology we say a cell is "expressing" a gene when the products of that gene are present. "Sst-expressing cell" is a way of saying "cell that has somatostatin protein present", or at least "has mRNA present for producing somatostatin protein" - biologists are not always explicit about whether they mean they measure mRNA or protein when they talk of expression, and in modern days often a "Sst-expressing cell" is identified by producing some reporter protein like green fluorescent protein (GFP) under a somatostatin promoter (or, most likely, dependent on a strain of mouse that uses Cre-recombinase to only produce GFP in cells where Cre is produced under a Sst promoter).

Classifying Interneurons

There are lots of different interneurons; they are difficult to classify. Here are a couple papers to read:

Markram, H., Toledo-Rodriguez, M., Wang, Y., Gupta, A., Silberberg, G., & Wu, C. (2004). Interneurons of the neocortical inhibitory system. Nature reviews neuroscience, 5(10), 793-807.

DeFelipe, J., López-Cruz, P. L., Benavides-Piccione, R., Bielza, C., Larrañaga, P., Anderson, S., ... & Ascoli, G. A. (2013). New insights into the classification and nomenclature of cortical GABAergic interneurons. Nature Reviews Neuroscience, 14(3), 202-216.

One way to classify some of these interneurons is by their expression of particular genes. Using that scheme, it then becomes common for, say, "Parvalbumin-expressing cells" (=cells that have parvalbumin protein/mRNA) to get shortened into just "Parvalbumin cells" or "Parvalbumin interneurons" or "PV+ cells", etc.

It's not necessary that these proteins that are used to name cell types are particularly crucial to their function; for PV cells, it seems like it is, because one feature of PV cells is that they have very fast electrophysiological kinetics, and some of those fast kinetics are assisted by the calcium binding of parvalbumin. For somatostatin interneurons, it's less clear; somatostatin is certainly an important gut hormone, but it's not particularly clear why somatostatin-expressing neurons form a particular important subclass of cells in the brain, as most known functions of somatostatin are restricted to development or the gut.

Back to the Question

So is this saying they recorded activity from inhibitory interneurons that were ... releasing? producing? ... SST & VIP ? What does "expressing" mean in this sense.

They're saying they recorded activity from inhibitory interneurons that have been grouped in the past according to their production of SST and VIP mRNA or protein. They aren't necessarily interested in either SST protein or VIP protein, but these have served as markers/names for particular cell populations. The primary neurotransmitter that brain SST and VIP interneurons produce is neither somatostatin nor vasoactive intestinal peptide; the primary neurotransmitter for both is GABA. You'd be able to find out a lot more about those cell populations by reading other papers containing terms like "Sst interneuron" or "VIP interneuron".

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