dapi stains all the cells (chromatin). Calbindin (CALB1) is a classical cytoplasmic marker of horizontal cells. Labeling can be done with anti-CALB1 either before or after mounting wholemounts on the slide

Have a look at related work as https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5352302/

https://www.jneurosci.org/content/31/17/6504

Article Live imaging of developing mouse retinal slices

I used to do horizontal cell injections blind. Learn how to recognize the surface of the retina with the sharp electrode (noise in the voltage) and go down by 5-um steps for about 100 um. Use the buzz to penetrate the cells and a brief flash of light to confirm impalement. You may want to move back slowly by 1-2 um to hold the cell better and longer. Stability of the prep is everything!

@ Igor Nasokin @ Michael Eisner

Do you mean that I can prelabel HC with Calb1 and use them for dye injections just like that with DAPI? I guess immunostaining and prelabelling must be quite different in this case.

30 min of DAPI incubation is sufficient to label the somata of most of the retinal cells, which is very helpful for dye injections in the whole-mount retina.

Do you think Calb1 might do a similar job?

If yes, could you please let me know the concentration and the incubation time for whole-mount retina?

@ Christophe P. Ribelayga

Thank you for your suggestions.

I might as well give this a try.