MFB - medial forebrain bundle

[Seth BH]

Regarding "the" reward system, first of all, there is no single location
that can be pointed to as a reward center; however, stimulation of the
septal nuclei, MFB and hypothalamic regions can elicit behaviors which
seem as if the subject is being rewarded (lord how radical behaviorist I
sound...) But remember that the MFB is not a nucleus of cells (and thus a
potential "center") but rather a collection of fibers from various
aminergic neurons in the brainstem which project through the lateral
hypothalamus to the telencepthalon.  You might want to look into the
nucleus accumbens as a major reinforcment "center."   

Anyway, a classic in the field is Olds & Milner (1954) Positive
reinforcement produced by electrical stimulation of the septal area and
other regions of the rat brain. J. Comp. Physiol. Psychol.  47: 419-427.

For a more recent review, check out 
Toates (1986).  Motivational systems.  Cambridge, England: Cambridge
University Press

There is a lot of literature on this, do a MedLine, Bio Abstracts or
PsychLit search and you'll probably spend most of your day just chasing
downt the refs, let alone reading them.

Good luck.

Seth Boatright-Horowitz
Dept. of Neuroscience
Brown University