Addictive substances (such as opioids, cocaine, and ethanol) all produce feelings of pleasure. These pleasurable or rewarding effects are strongly reinforcing, and increase the likelihood of repeated use. The rewarding effects of both drugs and behaviors such as consuming highly palatable and calorically dense foods involve activating the brain’s dopamine and endogenous opioid signaling systems (see Figure). Multiple studies have demonstrated that neurons signaling through dopamine (often referred to as the brain’s pleasure transmitter) and endogenous opioids (dynorphins, endorphins, and enkephalins) are activated by addictive substances. The brain’s opioid system is especially important in mediating the rewarding effects of alcohol, opioids, and highly palatable, calorically dense foods.
In the simplified illustration to the right, dopamine neurons originating in the ventral tegmental area (VTA) project to the nucleus accumbens (NAc), forming part of the brain’s reward circuitry. Naturally occurring opioid peptides activate opioid receptors in both the VTA and NAc, enhancing dopamine’s actions. Opioid peptides act directly on receptors in the NAc and indirectly, through opioid receptors located on γ-aminobutyric acid (GABA) containing neurons in the VTA. Despite their structural diversity, addictive substances including opioids and alcohol, as well as highly palatable and calorically dense foods activate this reward circuitry by either mimicking (opioid analgesics such as morphine and hydrocodone; abused opioids such as heroin) or releasing (alcohol, palatable foods) endogenous opioids.