Scientists have long described dopamine and opioids as the brain’s reward chemicals, the substances behind pleasure, desire, and the drive to seek out food, sex, or social connection. A new study proposes something more fundamental: that these two systems are primarily tools for managing the body’s energy budget, with feelings of motivation and pleasure emerging as byproducts rather than the main event.
The research, published in Neuroscience and Biobehavioral Reviews, was conducted by Matan Cohen and Shir Atzil at the Hebrew University of Jerusalem. The authors argue that dopamine and opioids function as opposing physiological regulators, with dopamine activating bodily systems and opioids calming them back down once a disturbance has passed.
According to the framework, dopamine does not encode pleasure directly. Instead, it raises the body’s physiological activity in response to a demand, such as hunger, stress, or physical exertion, and the rising energetic cost of that response generates the experience of motivation to act. When the demand is resolved, opioids are released to dampen the physiological response, and the resulting reduction in energetic burden is what a person subjectively experiences as relief or satisfaction.
The researchers draw on evidence from digestion, respiration, immune function, fluid regulation, sleep and temperature control, all areas where dopamine consistently activates bodily processes and opioids consistently suppress them. This pattern, they argue, is too consistent across systems to be coincidental, and points to a domain-general regulatory function rather than a narrowly defined role in reward.
The implications for understanding addiction, depression and schizophrenia are significant. The paper suggests that conditions involving disrupted dopamine or opioid signalling, including Parkinson’s disease, obesity and opioid dependence, are better understood as metabolic dysregulation than as disorders of pleasure or motivation in a purely psychological sense. Parkinson’s disease, for example, is characterised by dopamine deficiency and is associated with reduced appetite, low body weight, slowed metabolism and impaired motivation, a pattern consistent with the idea that dopamine normally drives physiological activity.
The study also reframes how the brain learns. The authors propose that behaviours and physiological responses are reinforced not because they feel good in an abstract sense, but because they reduce the body’s energetic costs. They describe this as metabolic gain, a measurable reduction in the physiological burden that follows a successful adaptation, whether that adaptation is behavioural or physiological.
The framework also extends to complex human experiences such as responses to music, money, art and social bonding. The authors suggest that the brain learns to associate these stimuli with metabolic consequences over time, which is why they come to carry subjective value.
The researchers acknowledge that other neurotransmitter systems, including serotonin, norepinephrine and GABA, are not yet incorporated into the model, and they note several findings in the existing literature that do not fit neatly into the proposed sequence. Further empirical work will be needed to test whether metabolic effort and metabolic gain can reliably predict behaviour and learning across species.