Chronic Intermittent Sucrose Consumption Facilitates the Ability to Discriminate Opioid Receptor Blockade with Naltrexone in Rats

Effect of intranasal oxytocin on palatable food consumption and c-Fos immunoreactivity in relevant brain areas in rats

Peripheral and central injections of oxytocin (OT) in laboratory animals decrease eating for energy and palatability, but the hypophagic response is dependent on the administration route. Human studies rely on intranasal (IN) administration of the peptide, the route underutilized in OT animal feeding studies thus far. Therefore, we examined the effect of IN OT on various aspects of food consumption in rats: (a) overnight deprivation-induced standard chow intake, (b) episodic (2-h) consumption of calorie-dense and palatable high-fat high-sugar (HFHS) chow, (c) 2-h episodic intake of palatable and calorie-dilute sucrose and Intralipid solutions, and (d) 2-h sucrose solution intake in rats habituated to ingesting this solution daily for several weeks. Finally, we assessed c-Fos changes in response to the acute IN OT administration in rats habituated to daily sugar consumption. We found that IN 20μg OT decreased deprivation-induced intake of standard chow and HFHS chow in nondeprived rats without affecting water consumption. IN OT also reduced 2-hour episodic fluid consumption of sucrose, but not Intralipid. In the habitual sugar consumption paradigm, acute IN OT diminished sucrose solution intake in animals accustomed to the 2-hour/day sucrose meal regimen. In rats habitually consuming sucrose, IN OT altered c-Fos immunoreactivity in brain areas related to energy homeostasis and reward, including the central nucleus of the amygdala, the hypothalamic paraventricular and the arcuate nuclei. We conclude that IN OT is an effective appetite suppressant for carbohydrate/sugar diets in rats and its effects involve feeding-related brain circuits.

Endogenous opiates and behavior: 2022

This paper is the forty-fifth consecutive installment of the annual anthological review of research concerning the endogenous opioid system, summarizing articles published during 2022 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides and receptors as well as effects of opioid/opiate agonists and antagonists. The review is subdivided into the following specific topics: molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors (1), the roles of these opioid peptides and receptors in pain and analgesia in animals (2) and humans (3), opioid-sensitive and opioid-insensitive effects of nonopioid analgesics (4), opioid peptide and receptor involvement in tolerance and dependence (5), stress and social status (6), learning and memory (7), eating and drinking (8), drug abuse and alcohol (9), sexual activity and hormones, pregnancy, development and endocrinology (10), mental illness and mood (11), seizures and neurologic disorders (12), electrical-related activity and neurophysiology (13), general activity and locomotion (14), gastrointestinal, renal and hepatic functions (15), cardiovascular responses (16), respiration and thermoregulation (17), and immunological responses (18).

Behavioral plasticity: Role of neuropeptides in shaping feeding responses

Behavioral plasticity refers to changes occurring due to external influences on an organism, including adaptation, learning, memory and enduring influences from early life experience. There are 2 types of behavioral plasticity: "developmental", which refers to gene/environment interactions affecting a phenotype, and "activational" which refers to innate physiology and can involve structural physiological changes of the body. In this review, we focus on feeding behavior, and studies involving neuropeptides that influence behavioral plasticity - primarily opioids, orexin, neuropeptide Y, and oxytocin. In each section of the review, we include examples of behavioral plasticity as it relates to actions of these neuropeptides. It can be concluded from this review that eating behavior is influenced by a number of external factors, including time of day, type of food available, energy balance state, and stressors. The reviewed work underscores that environmental factors play a critical role in feeding behavior and energy balance, but changes in eating behavior also result from a multitude of non-environmental factors, such that there can be no single mechanism or variable that can explain ingestive behavior.