Jacqueline M. Ho

Coming Full Circle: Contributions of Central and Peripheral Oxytocin Actions to Energy Balance

The neuropeptide oxytocin has emerged as an important anorexigen in the regulation of energy balance. Its effects on food intake have largely been attributed to limiting meal size through interactions in key regulatory brain regions such as the hypothalamus and hindbrain. Pharmacologic and pair-feeding studies indicate that its ability to reduce body mass extends beyond that of food intake, affecting multiple factors that determine energy balance such as energy expenditure, lipolysis, and glucose regulation. Systemic administration of oxytocin recapitulates many of its effects when administered centrally, raising the questions of whether and to what extent circulating oxytocin contributes to energy regulation. Its therapeutic potential to treat metabolic conditions remains to be determined, but data from diet-induced and genetically obese rodent models as well as application of oxytocin in humans in other areas of research have revealed promising results thus far.

Hindbrain Oxytocin Receptors Contribute to the Effects of Circulating Oxytocin on Food Intake in Male Rats

Oxytocin (OT)-elicited hypophagia has been linked to neural activity in the nucleus of the solitary tract (NTS). Because plasma OT levels increase after a meal, we hypothesized that circulating OT acts at both peripheral and hindbrain OT receptors (OTRs) to limit food intake. To initially determine whether circulating OT inhibits food intake by acting at hindbrain OTRs, we pretreated rats with an OTR antagonist administered into the fourth ventricle (4V) followed by either central or systemic OT administration. Administration of the OTR antagonist into the 4V blocked anorexia induced by either 4V or i.p. injection of OT. However, blockade of peripheral OTRs also weakened the anorectic response to ip OT. Our data suggest a predominant role for hindbrain OTRs in the hypophagic response to peripheral OT administration. To elucidate central mechanisms of OT hypophagia, we tested whether OT activates NTS catecholaminergic neurons. OT (ip) increased the number of NTS cells expressing c-Fos, of which 10%-15% were catecholaminergic. Furthermore, electrophysiological studies in mice revealed that OT stimulated 47% (8 of 17) of NTS catecholamine neurons through a presynaptic mechanism. However, OT-elicited hypophagia did not appear to require activation of α1-adrenoceptors, and blockade of glucagon-like peptide-1 receptors similarly did not attenuate anorexia induced by OT. These findings demonstrate that OT elicits satiety through both central and peripheral OTRs and that although catecholamine neurons are a downstream target of OT signaling in the NTS, the hypophagic effect is mediated independently of α1-adrenoceptor signaling.

Chronic CNS oxytocin signaling preferentially induces fat loss in high-fat diet-fed rats by enhancing satiety responses and increasing lipid utilization

Based largely on a number of short-term administration studies, growing evidence suggests that central oxytocin is important in the regulation of energy balance. The goal of the current work is to determine whether long-term third ventricular (3V) infusion of oxytocin into the central nervous system (CNS) is effective for obesity prevention and/or treatment in rat models. We found that chronic 3V oxytocin infusion between 21 and 26 days by osmotic minipumps both reduced weight gain associated with the progression of high-fat diet (HFD)-induced obesity and elicited a sustained reduction of fat mass with no decrease of lean mass in rats with established diet-induced obesity. We further demonstrated that these chronic oxytocin effects result from 1) maintenance of energy expenditure at preintervention levels despite ongoing weight loss, 2) a reduction in respiratory quotient, consistent with increased fat oxidation, and 3) an enhanced satiety response to cholecystokinin-8 and associated decrease of meal size. These weight-reducing effects persisted for approximately 10 days after termination of 3V oxytocin administration and occurred independently of whether sucrose was added to the HFD. We conclude that long-term 3V administration of oxytocin to rats can both prevent and treat diet-induced obesity.

Alterations in activity and energy expenditure contribute to lean phenotype in Fischer 344 rats lacking the cholecystokinin-1 receptor gene

CCK is hypothesized to inhibit meal size by acting at CCK1 receptors (CCK1R) on vagal afferent neurons that innervate the gastrointestinal tract and project to the hindbrain. Earlier studies have shown that obese Otsuka Long-Evans Tokushima Fatty (OLETF) rats, which carry a spontaneous null mutation of the CCK1R, are hyperphagic and obese. Recent findings show that rats with CCK1R-null gene on a Fischer 344 background (Cck1r(-/-)) are lean and normophagic. In this study, the metabolic phenotype of this rat strain was further characterized. As expected, the CCK1R antagonist, devazepide, failed to stimulate food intake in the Cck1r(-/-) rats. Both Cck1r(+/+) and Cck1r(-/-) rats became diet-induced obese (DIO) when maintained on a high-fat diet relative to chow-fed controls. Cck1r(-/-) rats consumed larger meals than controls during the dark cycle and smaller meals during the light cycle. These effects were accompanied by increased food intake, total spontaneous activity, and energy expenditure during the dark cycle and an apparent reduction in respiratory quotient during the light cycle. To assess whether enhanced responsiveness to anorexigenic factors may contribute to the lean phenotype, we examined the effects of melanotan II (MTII) on food intake and body weight. We found an enhanced effect of MTII in Cck1r(-/-) rats to suppress food intake and body weight following both central and peripheral administration. These results suggest that the lean phenotype is potentially driven by increases in total spontaneous activity and energy expenditure.

Intravenous angiotensin and salt appetite in rats.

Circulating angiotensin II is crucial for the activation of salt appetite after sodium depletion. We tested if angiotensin (ANG) II infused intravenously at 50 ng/kg/min overnight (chronic) can mimic the rapid salt appetite similar to furosemide and overnight sodium depletion. In experiment 1, rats received chronic ANG II or vehicle infusions all night with access to water and chow but no saline solution. In the morning, the infusions continued, but half of the vehicle-infused group was switched to ANG II (acute). Thirty minutes after the switch, all rats received 10 mg/kg furosemide SC. One hour later they were provided water and 0.3 M NaCl to drink. Rats infused with vehicle or acute ANG drank little, but the chronic ANG group drank 11+/-1 ml of saline in 90 min. In experiment 2, the furosemide was omitted, and a group receiving a chronic infusion of phenylephrine at 6.25 microg/kg/min was included. The chronic ANG group drank 10+/-1 ml saline in 90 min, but the phenylephrine group, which also incurred a significant negative sodium balance overnight, drank little. Thus, an overnight infusion of ANG II is sufficient to mimic the robust expression of salt appetite as observed after furosemide and overnight sodium depletion.

Effects of sleep disruption and high fat intake on glucose metabolism in mice.

Poor sleep quality or quantity impairs glycemic control and increases risk of disease under chronic conditions. Recovery sleep may offset adverse metabolic outcomes of accumulated sleep debt, but the extent to which this occurs is unclear. We examined whether recovery sleep improves glucose metabolism in mice subjected to prolonged sleep disruption, and whether high fat intake during sleep disruption exacerbates glycemic control. Adult male C57BL/6J mice were subjected to 18-h sleep fragmentation daily for 9 days, followed by 1 day of recovery. During sleep disruption, one group of mice was fed a high-fat diet (HFD) while another group was fed standard laboratory chow. Insulin sensitivity and glucose tolerance were assessed by insulin and glucose tolerance testing at baseline, after 3 and 7 days of sleep disruption, and at the end of the protocol after 24h of undisturbed sleep opportunity (recovery). To characterize changes in sleep architecture that are associated with sleep debt and recovery, we quantified electroencephalogram (EEG) recordings during sleep fragmentation and recovery periods from an additional group of mice. We now report that 9 days of 18-h daily sleep fragmentation significantly reduces rapid eye movement sleep (REMS) and non-rapid eye movement sleep (NREMS). Mice respond with increases in REMS, but not NREMS, during the daily 6-h undisturbed sleep opportunity. However, both REMS and NREMS increase significantly during the 24-h recovery period. Although sleep disruption alone has no effect in this protocol, high fat feeding in combination with sleep disruption impairs glucose tolerance, effects that are reversed by recovery sleep. Insulin sensitivity modestly improves after 3 days of sleep fragmentation and after 24h of recovery, with significantly greater improvements in mice exposed to HFD during sleep disruption. Improvements in both glucose tolerance and insulin sensitivity are associated with NREMS rebound, raising the possibility that this sleep phase contributes to restorative effects of recovery sleep on glycemic control.

Acute sleep disruption- and high-fat diet-induced hypothalamic inflammation are not related to glucose tolerance in mice

Chronic insufficient sleep is a major societal problem and is associated with increased risk of metabolic disease. Hypothalamic inflammation contributes to hyperphagia and weight gain in diet-induced obesity, but insufficient sleep-induced neuroinflammation has yet to be examined in relation to metabolic function. We therefore fragmented sleep of adult male C57BL/6 J mice for 18 h daily for 9 days to determine whether sleep disruption elicits inflammatory responses in brain regions that regulate energy balance and whether this relates to glycemic control. To additionally test the hypothesis that exposure to multiple inflammatory factors exacerbates metabolic outcomes, responses were compared in mice exposed to sleep fragmentation (SF), high-fat diet (HFD), both SF and HFD, or control conditions. Three or 9 days of high-fat feeding reduced glucose tolerance but SF alone did not. Transient loss of body mass in SF mice may have affected outcomes. Comparisons of pro-inflammatory cytokine concentrations among central and peripheral metabolic tissues indicate that patterns of liver interleukin-1β concentrations best reflects observed changes in glucose tolerance. However, we demonstrate that SF rapidly and potently increases Iba1 immunoreactivity (-ir), a marker of microglia. After 9 days of manipulations, Iba1-ir remains elevated only in mice exposed to both SF and HFD, indicating a novel interaction between sleep and diet on microglial activation that warrants further investigation.