Central Oxytocin and Food Intake: Focus on Macronutrient-Driven Reward

Oxytocin (Oxt) is a nonapeptide hormone best known for its role in lactation and parturition. Since 1906 when its uterine-contracting properties were described until 50 years later when its sequence was elucidated, research has focused on its peripheral roles in reproduction. Only over the past several decades have researchers focused on what functions Oxt might have in the brain, the subject of this review. Immunohistochemical studies revealed that magnocellular neurons of the hypothalamic paraventricular and supraoptic nuclei are the neurons of origin for the Oxt released from the posterior pituitary. Smaller cells in various parts of the brain, as well as release from magnocellular dendrites, provide the Oxt responsible for modulating various behaviors at its only identified receptor. Although Oxt is implicated in a variety of "non-social" behaviors, such as learning, anxiety, feeding and pain perception, it is Oxt's roles in various social behaviors that have come to the fore recently. Oxt is important for social memory and attachment, sexual and maternal behavior, and aggression. Recent work implicates Oxt in human bonding and trust as well. Human disorders characterized by aberrant social interactions, such as autism and schizophrenia, may also involve Oxt expression. Many, if not most, of Oxt's functions, from social interactions (affiliation, aggression) and sexual behavior to eventual parturition, lactation and maternal behavior, may be viewed as specifically facilitating species propagation.

Research on the neurobiological and behavioral effects of oxytocin (OT), as well as on its possible therapeutic applications, has intensified in the past decade. Accurate determination of peripheral OT levels is essential to reach meaningful conclusions and to motivate, support and inform clinical interventions. Different, but concordant, methods for measuring plasma OT have been developed over the past four decades, but since 2004 several commercially available methods have been favored in research with humans. Evaluation of these methods reveals that they lack reliability when used on unextracted samples of human fluids, and that they tag molecules in addition to OT, yielding estimates that are wildly discrepant with an extensive body of earlier findings that were obtained using methods that are well validated, but more laborious. An accurate, specific, and readily available method for measuring OT that can be adopted as the standard in the field is urgently needed for advances in our understanding of OT's roles in cognition and behavior. Copyright © 2013 Elsevier Ltd. All rights reserved.

The purpose of this study was to determine whether plasma oxytocin (OT) levels change during human sexual responses and, if so, to demonstrate the temporal pattern of change. Plasma OT levels were measured by RIA before, during, and after private self-stimulation to orgasm in normal men (n = 9) and women (n = 13). Blood samples were collected continuously through indwelling venous catheters. The subjects pressed a signal to indicate the start and finish of orgasm/ejaculation. Objective assessment of sexual arousal and orgasm was obtained by measuring blood-pulse amplitude and electromyographic activity, recorded continuously throughout testing from an anal device containing a photoplethysmograph and electromyograph electrodes connected to a polygraph located in an adjacent room. These measures allowed collection of data from men and women of changes in blood flow and muscle activity in the lower pelvic/pubic area. Plasma OT levels increased during sexual arousal in both women and men and were significantly higher during orgasm/ejaculation than during prior baseline testing. We suggest that the temporal pattern of secretion could be related to smooth muscle contractions of the reproductive system during orgasm.