Control of motivation for palatable food by kappa opioid receptor signaling and the neuropeptide Orexin-A
Authors:   , Sandoval Caballero Carolina Loreto 
Published: August 2024

Eating is a motivated behavior that provides nutrients to allow life. Animals evolved to perceive food as a positive reinforcer, resulting in increased motivation for food when the animal is hungry and decreased motivation when it is satiated. Motivation evolved to seek and eat food in an environment of food scarcity; however, it can become maladaptive in an environment with plenty food access, favoring excessive eating, weight gain, and obesity. Understanding the neuronal mechanisms regulating food motivation could improve therapies against obesity.Opioids and the hypothalamic Orexin/Dynorphin neurons are part of the brain mechanisms that regulate food intake and motivation for food. Opioids bind mainly to three G-protein coupled receptors (kappa, mu, and delta), which are expressed in brain regions that regulate food intake and motivation. Still, the contribution of each opioid receptor to food intake and motivation for food is unclear. The Orexin/Dynorphin neurons co-release the excitatory neuropeptides Orexins (e.g., Orexin-A and Orexin-B) and the inhibitory opioids Dynorphin (e.g., Dynorphin-A 1-13) into different areas, including the Oxytocin neurons from the paraventricular hypothalamic nucleus. The effects of Orexin and Dynorphin peptides in eating behavior are brain-site specific. For example, in the paraventricular hypothalamic nucleus, Dynorphin-A1-13 increases and Orexin-A decreases palatable food intake (i.e., tasty food, generally high in sugars and fats). Still, the opposite effect is observed in the ventral tegmental area. Dynorphin-A1-13 binds mainly to the kappa opioid receptor, which is expressed in the anorexigenic oxytocin neurons of the paraventricular hypothalamic nucleus; however, whether inhibition of oxytocin neurons is involved in the orexigenic effects of Dynorphin-A1-13 in the paraventricular hypothalamic nucleus is unknown. Also, in this brain region, Orexin-A blocks the orexigenic effects of Dynorphin-A1-13. Still, it is unclear whether these effects are mediated by changes in motivation for palatable food and whether the endogenous kappa opioid receptor signaling in the paraventricular hypothalamic nucleus regulates motivation for food. We hypothesize that in the paraventricular hypothalamic nucleus, Dynorphin-A1-13 signaling through the kappa opioid receptor promotes motivation for sucrose, and Orexin-A inhibits this behavior. We evaluated this hypothesis through four specific aims. [1] To determine through a meta-analysis the effects of kappa, mu, and delta opioid receptor ligands on food intake and motivation. We found that: (1) agonists increased food intake after central injections while antagonists decreased food intake after central and peripheral injections. (2) We did not find differences between the effects of opioid receptor ligands on standard rodent food intake after injections in the paraventricular hypothalamic nucleus compared to other brain sites. (3) Mu agonists had the strongest orexigenic effect compared to kappa and delta agonists, with the most significant effect on fat intake. (4) Whereas only centrally administered kappa antagonists reduce feeding, peripherally administered antagonists for all opioid receptor subtypes reduce feeding; (5) peripheral administration of antagonists decreases motivation for food regardless of food type. [2] To determine whether in the paraventricular hypothalamic nucleus signaling through kappa opioid receptor mediates the ability of Dynorphin-A1-13 to increase motivation for sucrose. We assessed motivation for sucrose using a progressive ratio schedule in which mice must perform work licks to obtain sucrose. After each sucrose reward obtention, there was a 20 sec time-out period in which licks were recorded, but sucrose could not be obtained. We found that in the paraventricular hypothalamic nucleus and relative to the vehicle: (1) bilateral administration of the kappa opioid antagonist nor-binaltorphimine increased the time to obtain sucrose rewards and decreased sucrose work and time-out licks in the early and middle stages of the sucrose progressive ratio. These effects were also found after subcutaneous administration of nor-binaltorphimine. (2) Unilateral Dynorphin-A1-13 administration increased the time to obtain sucrose rewards and decreased sucrose work licks in the middle and late stages of the sucrose progressive ratio without changing time-out licks. (3) Bilateral Dynorphin-A1-13 administration did not change the time to obtain sucrose rewards. However, it strongly decreased sucrose work licks in the early and middle stages of the sucrose progressive ratio. Also, nor-binaltorphimine did not block Dynorphin-A1-13 effects. Finally, we could not downregulate the expression of the kappa opioid receptor in the paraventricular hypothalamic nucleus and thus, these effects in motivation for sucrose were not performed. [3] To determine whether oxytocin neurons in the paraventricular hypothalamic nucleus express the kappa opioid receptor and whether blocking the kappa opioid receptor activates Oxytocin neurons in this brain region. Published data demonstrated that the kappa opioid receptor is expressed in oxytocin neurons in the paraventricular hypothalamic nucleus. Thus, we assessed whether nor-binaltorphimine activates oxytocin neurons and decreases food intake in mice. We found that nor-binaltorphimine administration in the paraventricular hypothalamic nucleus decreased cafeteria diet intake and abolished the negative correlation between the percentage of active caudal oxytocin neurons and cafeteria diet intake established after vehicle injection. [4] To determine whether activation of the kappa opioid receptor by Dynorphin-A1-13 and orexin receptors by Orexin-A in the paraventricular hypothalamic nucleus have opposing effects on motivation for sucrose. We found that in the paraventricular hypothalamic nucleus and relative to vehicle (1) Unilateral Orexin-A administration increased sucrose work and time-out licks in the middle and late stages of the sucrose progressive ratio without changing the time to obtain sucrose rewards and time-out licks. (2) Orexin-A decreases the demotivational effects of Dynorphin-A1-13.We conclude that signaling through orexin and opioid receptors modulates feeding behaviors and motivation for food. Those effects depend on variables including the dose and administration route of the drug, food tested, and circadian period in which the outcomes were measured. Dynorphin-A1-13 and nor-binaltorphimine in the paraventricular hypothalamic nucleus decreased sucrose work licks, and Orexin-A blocked the demotivational effects of Dynorphin-A1-13 over time. We reasoned that the demotivational effects of nor-binaltorphimine could involve the activation of anorexigenic oxytocin neurons, and the demotivational effects of Dynorphin-A1-13 could be explained by their signaling through other receptors besides the kappa opioid receptor, such as Mu opioid receptor. We conclude that during the active phase of the mice, Orexin-A potentiates the orexigenic mechanisms that are naturally active in mice and increases locomotion and motivation. Consequently, the orexigenic effects of Dynorphin-A1-13 could not be detected because orexigenic mechanisms reached a ceiling effect. Thus, instead of promoting motivation for sucrose, Dynorphin-A1-13 prompts allodynia that decreases feeding and motivated behaviors.