Of note in this context is that dopamine interacts with other neurotransmitter systems that have also been associated with obesity, such as the serotonin (reviewed in [23,55,58,59]) and opiate system [37,38,83], as well as with the neuroendocrine hormone insulin [43,84,85,86,87,88]. Here, INS is linked to obesity due to melanocortin 4 receptor deficiency.