µL papain (Wellingsworth) to 5 mL HBSS. Warm in 37°C water bath to aid in dissolving. Filter sterilize before use.
8. Incubate hippocampi in 37°C water bath for 12 or 15 minutes, inverting gently every few minutes.
9. After incubation, add 5mL of NPM.
10. Centrifuge for 5 min at 1000 rpm and carefully remove the supernatant.
11. Resuspend pellet in 3 mL of NPM and dissociate cells by pipetting up and down with a 1mL pipette 3x or 5x, followed by trituration with a fire polished long stem pipette until most chunks of tissue have been dissociated. For best results with mouse neurons do not triturate with glass pipette more than 12x. If there are tissue chunks remaining, let them settle to the bottom of the conical tube and then remove them.
12. Add NPM to a final concentration of 5-10 mLs.
13. Use a hemocytometer to determine cell density (45 uL of cell suspension plus 5 uL of trypan blue). Trypan blue will stain dead cells blue. Do not include dead cells in your count. For mouse neurons, 80% of your cells should be viable at this stage.
14. Dilute cells to 25,000/ mL (75,000 cells/ mL) with NPM in a separate container (make sure to account for pipetting error). Add 2 mLs per well, over the poly-L-lysine coated coverslips. This will give a final cell density of 50,000 (150,000) cells per well.
15. Place dishes in incubator for 5-6 hrs.
16. Replace media with NBC.