Well, there have been experiments on humans. http://en.wikipedia.org/wiki/Pleasure_center#Human_experiments

At its most frequent, the patient self-stimulated throughout the day, neglecting personal hygiene and family commitments. >A chronic ulceration developed at the tip of the finger used to adjust the amplitude dial and she frequently tampered with >the device in an effort to increase the stimulation amplitude. At times, she implored her to limit her access to the >stimulator, each time demanding its return after a short hiatus. During the past two years, compulsive use has become >associated with frequent attacks of anxiety, depersonalization, periods of psychogenic polydipsia and virtually complete >inactivity.