The reward systems in the brain are a group of structures that are activated whenever we experience something that rewards us, such as eating delicious food, having sex, or using addictive drugs. The medial forebrain bundle is defined as a projection pathway for axons that are both ascending from the brainstem as well as forebrain neurons that are descending to terminate in the hypothalamus and lower regions of the neuraxis.
The medial forebrain bundle is an important part of the reward system involved in the integration of reward and pleasure. Electrical stimulation of the medial forebrain bundle is thought to produce pleasure. The proof of this is the experiment with rats in Skinner boxes. Having been given the opportunity to self-stimulate, the rats repeatedly pressed the lever, activating the stimulation of the Medial forebrain bundle, subsequently forgetting to take food and water, and, ultimately, died of exhaustion.
There is an argument against the involvement of the medial forebrain bundle in reward. Haber et. al (2017) theorized that the medial forebrain bundle is not a pleasure center but a pleasure expectation center. Another research method that was used to determine the function of MFB was microdialysis. It has been shown that the enhancement of MFB electrical stimulation by this method causes the release of dopamine in the nucleus accumbens.
The mesolimbic dopamine pathway is the most important reward pathway in the brain. Dopamine is a neurotransmitter that is both excitatory and inhibitory in function. It is also a neuromodulator that has a role in addiction, motivation, and reward. Dopamine is released after joyful events and binds to dopaminergic receptors found in the nucleus accumbens in the reward pathways, specifically the mesolimbic pathway. Dopamine is also released during pleasurable brain stimulation in the aforementioned rat studies. Dopamine is activated in the VTA when positive experiences are had. The mesolimbic pathway then directs this neurotransmitter to a region known as the nucleus accumbens.
Reference
Haber, S., Knutson, B. (2017). The reward circuit: linking primate anatomy and human imaging. Neuropsychopharmacol, 35, 4–26.