How to have the same login database for users

Hi,

Currently I have installed word press on 4 of my sub domains as I need to have different themes for each sub domain.

The problem is that, a user has to register in all the sub domains. Is it possible to have one database which is interconnected with all the database so that the user need not register in each sub domain.

I don’t want to recommend users to use Facebook as the source of login.

Please help on this as I want to create only one database for the users to register and login.