How to write $I$ and $I_0$ as a symbol?

How could we write both $I$ and $I_0$ as a symbol in Wolfram Mathematica? I consider $I$ as a variable and $I_0$ as a parameter in my system of equations ($Y=C_0 + I, I=I_0+bY$, Y and I are endogenous variables). I have tried to define as escIesc. It does not work to solve the system of equations.