Higher brain structures like the orbital frontal cortex, prelimbic area and selected prefrontal area receive and interpret incoming sensory information. When a rewarding sensory input or thought is detected pyramidal cells in the prelimbic area sends signals to the core of nucleus accumbens where the feelings of motivation, urge or craving are generated so that the person has sufficient motivation (energy) to engage in a behavior to secure the targeted rewarding object. The molecule that mediates this signal is glutamate. The prefrontal brain simultaneously sends a signal to the ventral tegmental area (VTA) to activate dopamine neurons in the VTA. When these dopamine neurons are activated dopamine is released in the core of nucleus accumbens and augments the glutamate function mentioned above. Together they generate powerful motivation for us to engage in foraging and other reward seeking behavior. Once the rewards have been fulfilled animals and humans feel satisfaction, pleasure and/or excitement. When these circuits are overheated mammals feel uncontrollable urges to engage in a behavior even if the behavior might bring about unbearable pain down the road (loss of money in gambling, for example). We call these addictions: behavioral addictions (gambling, shopping, shoplifting) or drug addictions.

Figure 1 - Potential mechanisms underlying the effects of accumbens injections of BDNF or adeno-associated viral vector, which encodes CRE-recombinase to knock down local BDNF protein production, on cocaine reward and relapse.From the following articleA role for BDNF in cocaine reward and relapse

Geoffrey Schoenbaum, Thomas A Stalnaker & Yavin ShahamNature Neuroscience 10, 935 - 936 (2007)doi:10.1038/nn0807-935BACK TO ARTICLE

Exogenous BDNF can act locally in the nucleus accumbens (NAc) or act after retrograde transport to accumbens projection areas such as the hippocampus, basolateral amygdala (BLA), prefrontal cortex (mPFC) and VTA. Similarly, the viral vector can either act at the accumbens injection site to decrease local BDNF production or it can act distally after retrograde transport to the hippocampus, BLA, mPFC and VTA. These projection regions are also involved in cocaine's behavioral effects, and BDNF mRNA expression in these areas is substantially higher than it is in the accumbens. Blue arrows, direction of transport of exogenous BDNF; red arrows, direction of transport of viral encapsulated BDNF.