Hello, to measure neuronal activity, the fos or jun family are good options.

I recommand to use the NeuN-antiboby (millipore, MAB377). NeuN is high specific for mature neurons.

This makes the diffenrentiation of nuclei much easier then to do this on Nissl's stained slides, because no glia or endothelia cells are stained.

I have put an image of a rat brain into the attachment. there you can see how it looks like.

https://www.cell.com/cell-reports/pdf/S2211-1247(17)30321-2.pdf