Sexual behavior causes activation and functional alterations of mesolimbic systems: Neurobiology of motivation and reward

There is increasing evidence that the various drugs of abuse converge upon common reward pathways in the brain. While it is clear that these circuits are involved in drug-induced reward and reinforcement, less is known about how these systems function when activated by normal motivated behaviors such as sexual behavior. Like drugs of abuse, sexual behavior is a rewarding and reinforcing behavior. Sexual behavior, however, is generally not considered an "addictive" behavior. Thus, understanding how neural circuits are activated by normal motivated behaviors may lead to a better understanding of the pathology of drug addiction. The first set of studies investigated the mechanisms by which sexual behavior activates the mesolimbic dopamine (DA) system---a critical component of the neural circuitry regulating motivation and reward. These studies found that both sexual experience and sex-associated environmental cues cause endogenous opioid release and activation of DA producing neurons in the ventral tegmental area (VTA). In addition, a large population of non-dopaminergic neurons was activated by sexual behavior. Therefore, the second set of studies explored the anatomical relationship between these sex-activation neurons and other components of the limbic system. These studies suggest that the medial prefrontal cortex (mPFC) may contribute to the activation of this cell population. The final set of studies investigated whether repeated endogenous activation of the mesolimbic system causes similar functional changes as repeated administration of drugs of abuse. Indeed, sexually experienced animals displayed a robust sensitization to the locomotor effects of amphetamine. Taken together, these studies suggest that sexual behavior activates the mesolimbic DA pathway and that repeated endogenous activation of this system results in long-term changes in its function.