You want to artificially increase function of the dopamine system in one or more regions of the rat brain. Which of the following methods would be LEAST effective at accomplishing this in rats? Select one:

a.
Optogenetics

b.
A dopamine agonist delivered via intraperitoneal injection

c.
6-hydroxydopamine injection directly into the brain

d.
Putting L-DOPA in the rats' diet

e.
A dopamine agonist delivered via intravenous injection