Pleasure center
Pleasure center is the general term for the set of brain structures, predominantly the nucleus accumbens, theorized to produce great pleasure when stimulated electrically.  Some references state that the septum pellucidium is generally considered to be the pleasure center  while others mention the hypothalamus when referring to pleasure center for intracranial stimulation.

# History
The pleasure center was discovered in the 1950s by two brain researchers named James Olds and Peter Milner who were investigating whether rats might be made uncomfortable by electrical stimulation of certain areas of their brain, particularly the limbic system.  In the experiment, an electrical current was given to the rat if they entered a certain corner of a cage, with the theory that they would stay away from that corner if the effect was uncomfortable.  Instead, they came back quickly after the first stimulation and even more quickly after the second.  In later experiments, they allowed the rats to press the stimulation lever themselves, to the effect that they would press it as much as seven-hundred times per hour.  This region soon came to be known as the pleasure center.

# Recent
The nucleus accumbens, part of the limbic system, plays a role in sexual arousal and the "high" derived from certain recreational drugs. These responses are heavily modulated by dopaminergic projections from the limbic system. Rats in Skinner boxes with metal electrodes implanted into their nucleus accumbens will repeatedly press a lever which activates this region, and will do so in preference over food and water, eventually dying from exhaustion.
The limbic system is also tightly connected to the prefrontal cortex. Some scientists contend that this connection is related to the pleasure obtained from solving problems. In a now-obsolete practice, to cure severe emotional disorders, this connection was sometimes surgically severed, a procedure of psychosurgery, called a prefrontal lobotomy (this is actually a misnomer). Patients who underwent this procedure often became passive and lacked all motivation.
In rodent physiology, scientists reason that the medial forebrain bundle is the pleasure center of rats.  If a rat is given the choice between stimulating the forebrain or eating, it will choose stimulation to the point of exhaustion.

# Sources
-  The Science of Love  Understanding Love and its Effects on Mind and Body. Prometheus Books. 1991. ISBN 0-87975-648-9. Unknown parameter |Author= ignored (|author= suggested) (help).mw-parser-output cite.citation{font-style:inherit}.mw-parser-output q{quotes:"\"""\"""'""'"}.mw-parser-output code.cs1-code{color:inherit;background:inherit;border:inherit;padding:inherit}.mw-parser-output .cs1-lock-free a{background:url("")no-repeat;background-position:right .1em center}.mw-parser-output .cs1-lock-limited a,.mw-parser-output .cs1-lock-registration a{background:url("")no-repeat;background-position:right .1em center}.mw-parser-output .cs1-lock-subscription a{background:url("")no-repeat;background-position:right .1em center}.mw-parser-output .cs1-subscription,.mw-parser-output .cs1-registration{color:#555}.mw-parser-output .cs1-subscription span,.mw-parser-output .cs1-registration span{border-bottom:1px dotted;cursor:help}.mw-parser-output .cs1-hidden-error{display:none;font-size:100%}.mw-parser-output .cs1-visible-error{display:none;font-size:100%}.mw-parser-output .cs1-subscription,.mw-parser-output .cs1-registration,.mw-parser-output .cs1-format{font-size:95%}.mw-parser-output .cs1-kern-left,.mw-parser-output .cs1-kern-wl-left{padding-left:0.2em}.mw-parser-output .cs1-kern-right,.mw-parser-output .cs1-kern-wl-right{padding-right:0.2em}
-  Kandel ER, Schwartz JH, Jessell TM. Principles of Neural Science, 4th ed. McGraw-Hill, New York (2000). ISBN 0-8385-7701-6
-  Liebowitz, Michael, R. (1983). The Chemistry of Love. Boston: Little, Brown, & Co.
-  Whitters, W.L. & Jones-Whitter, P. (1980). Human Sexuality - A Biological Perspective. New York: Van Nostrand.