Knockout mice studies reveal that enkephalins primarily regulate feeding motivation and diet-induced weight gain, while β-endorphin modulates how rewarding food tastes — explaining why broad opioid-blocking drugs fail for obesity.
Enkephalins = motivation, β-endorphin = rewardMice lacking enkephalins ate less and resisted diet-induced obesity, while mice lacking β-endorphin showed altered taste pleasure but normal weight — revealing distinct peptide roles in feeding
What the researchers found
Proenkephalin knockout (PENK KO) mice showed reduced feeding motivation — fewer licking bouts for sucrose but normal bout lengths — and had lower baseline body weight on regular chow and attenuated weight gain on an energy-dense cafeteria diet.
β-endorphin-deficient (BEND KO) mice showed altered taste reward — shorter licking bouts (suggesting reduced palatability) but normal bout frequency. Critically, BEND KO mice were insensitive to shifts in sucrose concentration and hunger state, indicating β-endorphin specifically mediates orosensory reward responses under conditions of high need or high stimulus value.
PENK KO but not BEND KO mice showed resistance to diet-induced obesity, demonstrating that enkephalins — not β-endorphin — are the primary opioid peptides regulating body weight through motivational tone.
Why it matters
Obesity is a global epidemic, and the opioid system in the brain has long been a target for appetite-suppressing drugs. But drugs that broadly block mu-opioid receptors (like naltrexone) have shown limited long-term efficacy for weight control. This study explains why: enkephalins and β-endorphin have fundamentally different roles in eating behavior, and blocking both simultaneously may counterproductively disrupt the system. More targeted approaches — specifically addressing enkephalin-mediated motivation — might be more effective for obesity treatment.
How the study worked
The study used proenkephalin knockout (PENK KO) and β-endorphin-deficient (BEND KO) mice compared to wild-type controls. Palatable liquid consumption was measured using detailed lickometer analysis (bout frequency and bout length) with sucrose solutions at varying concentrations. Body weight was tracked during consumption of standard chow and an energy-dense 'cafeteria diet.' Sensitivity to sucrose concentration shifts and hunger states was assessed to distinguish motivational from hedonic components of feeding.
What this study cannot tell us
The study was conducted in genetically modified mice, and compensatory developmental changes in knockout animals may confuse interpretation. Human opioid system regulation of feeding may differ. The cafeteria diet model, while useful, doesn't fully replicate human eating patterns. The study focused on sucrose consumption as a measure of palatability, which may not generalize to all food types. Specific neural circuits mediating these peptide effects were not identified.
How to read the evidence
This is a well-designed preclinical study published in Neuropsychopharmacology using genetic knockout models with detailed behavioral analysis. The double-dissociation between PENK and BEND knockouts provides compelling mechanistic evidence, though translation to human obesity treatment requires further work.
When this study was published
Published in 2015, this study contributed important mechanistic insights to the 'wanting vs liking' framework in feeding behavior and remains relevant to ongoing opioid-system-based obesity drug development efforts.
The bigger picture
This study elegantly dissects the 'wanting' (motivation, enkephalin-mediated) versus 'liking' (hedonic reward, β-endorphin-mediated) distinction in feeding behavior — a framework influential in addiction and obesity research. By showing that these two opioid peptide systems independently control different aspects of eating, it provides a neurochemical basis for more precise pharmacological interventions. The finding also connects to the broader understanding that the opioid system modulates reward processing across many domains, not just pain.
Questions still open
- Could drugs that selectively block enkephalin signaling (without affecting β-endorphin) be more effective for obesity than current broad opioid antagonists?
- Do humans with naturally lower enkephalin levels show less diet-induced weight gain, similar to the PENK KO mice?
- How do enkephalin and β-endorphin interact with other appetite-regulating systems like GLP-1 and ghrelin?
Common questions
What's the difference between 'wanting' and 'liking' food?
Why haven't opioid-blocking drugs worked well for weight loss?
Read the original research
Involvement of Endogenous Enkephalins and β-Endorphin in Feeding and Diet-Induced Obesity.
Neuropsychopharmacology : official publication of the American College of Neuropsychopharmacology, 40(9), 2103-12
Citation
Mendez, Ian A; Ostlund, Sean B; Maidment, Nigel T; Murphy, Niall P. (2015). Involvement of Endogenous Enkephalins and β-Endorphin in Feeding and Diet-Induced Obesity.. Neuropsychopharmacology : official publication of the American College of Neuropsychopharmacology, 40(9), 2103-12. https://doi.org/10.1038/npp.2015.67