The neurons isolated are likely from all the areas of the hippocampus you listed. The glutamate is used to establish the culture. If the media is being changed every day, the media should have glutamate supplemented through day 4 after recovery. Beyond day 4, it is not necessary and likely not preferred. Any glutamate can be used as long as it is of a higher purity such as FCC, USP, ACS, etc. grade.