Abstract
Centrally and peripherally administered oxytocin (OT) decreases food intake and activation of the endogenous OT systems, which is associated with termination of feeding. Evidence gathered thus far points to OT as a facilitator of early satiation, a peptide that reduces the need for a meal that has already begun. It is not known, however, whether OT can diminish a feeling of hunger, thereby decreasing a perceived need to seek calories. Therefore, in the current project, we first confirmed that intraperitoneal (i.p.) OT at 0.3–1 mg/kg reduces food intake in deprived and non-deprived rats. We then used those OT doses in a unique hunger discrimination protocol. First, rats were trained to discriminate between 22- and 2-h food deprivation (hungry vs. sated state) in a two-lever operant procedure. After rats acquired the discrimination, they were food-restricted for 22 h and given i.p. OT before a generalization test session. OT did not decrease 22-h deprivation-appropriate responding to match that following 2-h food deprivation, thus, it did not reduce the perceived level of hunger. In order to better understand the mechanisms behind this ineffectiveness of OT, we used c-Fos immunohistochemistry to determine whether i.p. OT activates a different subset of feeding-related brain sites under 22- vs. 2-h deprivation. We found that in sated animals, OT induces c-Fos changes in a broader network of hypothalamic and brain stem sites compared to those affected in the hungry state. Finally, by employing qPCR analysis, we asked whether food deprivation vs. sated state have an impact on OT receptor expression in the brain stem, a CNS “entry” region for peripheral OT. Fasted animals had significantly lower OT receptor mRNA levels than their ad libitum-fed counterparts. We conclude that OT does not diminish a feeling of hunger before a start of a meal. Instead OT's anorexigenic properties are manifested once consumption has already begun which is—at least to some extent—driven by changes in brain responsiveness to OT treatment in the hungry vs. fed state. OT should be viewed as a mediator of early satiation rather than as a molecule that diminishes perceived hunger.
Highlights
A nine amino acid neuropeptide, oxytocin (OT), synthesized primarily in the hypothalamic paraventricular (PVN) and supraoptic (SON) nuclei, released throughout the CNS and, via the neurohypophysis, into general circulation, has been known to regulate a number of functions, including parturition, lactation, and social behaviors
In animals subjected to 22-h food deprivation after which they gained access to the high-fat high-sugar (HFHS) chow, 1 mg/kg OT decreased consumption by approximately 25% at 2 h [F(3,32) = 3.38; p = 0.015] and 4 h [F(3,32) = 5.12; p = 0.004) of re-feeding (Figure 1B)
The fundamental drive that initiates food intake is a feeling of hunger
Summary
A nine amino acid neuropeptide, oxytocin (OT), synthesized primarily in the hypothalamic paraventricular (PVN) and supraoptic (SON) nuclei, released throughout the CNS and, via the neurohypophysis, into general circulation, has been known to regulate a number of functions, including parturition, lactation, and social behaviors. Injections of OT in the third and fourth cerebral ventricles as well as in numerous brain areas, including the hypothalamic ventromedial nucleus (VMH), dorsal vagal complex, central (CEA), and basolateral (BLA) amygdala, ventral tegmental area (VTA) and nucleus accumbens core (AcbC), and the limbic system, produce cessation of ingestive behavior [2,3,4,5,6,7,8,9,10] This early termination of feeding after OT treatment pertains to relatively bland laboratory chow, as well as those that are highly palatable [2, 3, 10, 11]. Despite its limited ability to cross the bloodbrain barrier (BBB), peripherally administered OT (including via intraperitoneal (i.p.), subcutaneous, and intravenous routes) potently decreases food intake, most likely by engaging the brainstem where the BBB protection is weak [1, 2, 9, 12,13,14]
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