Downregulation of prolactin-releasing peptide gene expression in the hypothalamus and brainstem of diabetic rats

The role of nucleus of the solitary tract glucagon-like peptide-1 and prolactin-releasing peptide neurons in stress: anatomy, physiology and cellular interactions

Neuroendocrine, behavioural and autonomic responses to stressful stimuli are orchestrated by complex neural circuits. The caudal nucleus of the solitary tract (cNTS) in the dorsomedial hindbrain is uniquely positioned to integrate signals of both interoceptive and psychogenic stress. Within the cNTS, glucagon‐like peptide‐1 (GLP‐1) and prolactin‐releasing peptide (PrRP) neurons play crucial roles in organising neural responses to a broad range of stressors. In this review we discuss the anatomical and functional overlap between PrRP and GLP‐1 neurons. We outline their co‐activation in response to stressful stimuli and their importance as mediators of behavioural and physiological stress responses. Finally, we review evidence that PrRP neurons are downstream of GLP‐1 neurons and outline unexplored areas of the research field. Based on the current state‐of‐knowledge, PrRP and GLP‐1 neurons may be compelling targets in the treatment of stress‐related disorders.

Functions of two distinct "prolactin-releasing peptides" evolved from a common ancestral gene

Prolactin-releasing peptide (PrRP) is one of the RF-amide peptides and was originally identified in the bovine hypothalamus as a stimulator of prolactin (PRL) release. Independently, another RF-amide peptide was found in Japanese crucian carp and named Carassius-RFa (C-RFa), which shows high homology to PrRP and stimulates PRL secretion in teleost fish. Therefore, C-RFa has been recognized as fish PrRP. However, recent work has revealed that PrRP and C-RFa in non-mammalian vertebrates are encoded by separate genes originated through duplication of an ancestral gene. Indeed, both PrRP and C-RFa are suggested to exist in teleost, amphibian, reptile, and avian species. Therefore, we propose that non-mammalian PrRP (C-RFa) be renamed PrRP2. Despite a common evolutionary origin, PrRP2 appears to be a physiological regulator of PRL, whereas this is not a consistent role for PrRP itself. Further work revealed that the biological functions of PrRP and PrRP2 are not limited solely to PRL release, because they are also neuromodulators of several hypothalamus-pituitary axes and are involved in some brain circuits related to the regulation of food intake, stress, and cardiovascular functions. However, these actions appear to be different among vertebrates. For example, central injection of PrRP inhibits feeding behavior in rodents and teleosts, while it stimulates it in chicks. Therefore, both PrRP and PrRP2 have acquired diverse actions through evolution. In this review, we integrate the burgeoning information of structures, expression profiles, and multiple biological actions of PrRP in higher vertebrates, as well as those of PrRP2 in non-mammals.

Central administration of prolactin-releasing peptide shifts the utilities of metabolic fuels from carbohydrate to lipids in chicks

We have recently identified prolactin (PRL)-releasing peptides (PrRPs) and their stimulating effects on feeding behavior in chicks. To investigate further metabolic functions of PrRP, the present study was performed to clarify whether intracerebroventricular (ICV) injection of PrRP31, an active form of PrRP in chicks, affects heat production (HP), respiratory quotient (RQ) and plasma concentrations of metabolic fuels in chicks. The ICV injection of PrRP31 (94 and 375 pmol) did not affect HP but significantly lowered RQ. The change in RQ implies that PrRP31 shifted the utility of metabolic fuels in the body. This idea was confirmed by subsequent results in which ICV injection of PrRP31 significantly reduced glucose but increased non-esterified fatty acid concentrations in plasma. These shifts in blood metabolic fuels would not be through the increased plasma insulin, because the ICV injection of PrRP31 significantly decreased plasma insulin concentration. On the other hand, ICV injection of another orexigenic peptide, neuropeptide Y (NPY) also induced the insulin release and the metabolic effects were similar to those of PrRP31. Because ICV injection of PrRP31 increased NPY mRNA in the diencephalon, the NPY may mediate the metabolic functions of PrRP31. In summary, the present study suggests that central PrRP31 shifts the utilities of peripheral energy sources, which is not via hyperinsulinemia but via the diencephalon.

Roles of oxytocin neurones in the control of stress, energy metabolism, and social behaviour

Oxytocin neurones are activated by stressful stimuli, food intake and social attachment. Activation of oxytocin neurones in response to stressful stimuli or food intake is mediated, at least in part, by noradrenaline/prolactin-releasing peptide (PrRP) neurones in the nucleus tractus solitarius, whereas oxytocin neurones are activated after social stimuli via medial amygdala neurones. Activation of oxytocin neurones induces the release of oxytocin not only from their axon terminals, but also from their dendrites. Oxytocin acts locally where released or diffuses and acts on remote oxytocin receptors widely distributed within the brain, resulting in anxiolytic, anorexic and pro-social actions. The action sites of oxytocin appear to be multiple. Oxytocin shows anxiolytic actions, at least in part, via serotoninergic neurones in the median raphe nucleus, has anorexic actions via pro-opiomelanocortin neurones in the nucleus tractus solitarius and facilitates social recognition via the medial amygdala. Stress, obesity and social isolation are major risk factors for mortality in humans. Thus, the oxytocin-oxytocin receptor system is a therapeutic target for the promotion of human health.

Carnitine and carnitine orotate affect the expression of the prolactin-releasing peptide gene

Carnitine is involved in fatty acid metabolism in mammals and is widely used as a nutritional supplement; carnitine orotate is a more absorbable form of carnitine. We investigated the effects of carnitine and carnitine orotate on mouse prolactin-releasing peptide (PrRP) mRNA expression. Twenty-four female mice were randomly divided into four groups of six; control mice were orally drenched with physiological saline solution (250 mg/kg body weight) and treatment mice were orally drenched with carnitine (250 mg/kg) or carnitine orotate (250 or 750 mg/kg), once a day, for 20 days from parturition. The carnitine or carnitine orotate was dissolved in saline solution before administration. The hypothalamus, pituitary and ovary were sampled on day 21 after parturition, and PrRP mRNA levels in these tissues were measured by semi-quantitative PCR, with glyceraldehyde 3-phosphate dehydrogenase as a control. Expression of PrRP in mice treated with carnitine and carnitine orotate was significantly increased in the ovary and significantly reduced in the pituitary gland. Compared with the control, hypothalamus PrRP mRNA increased significantly in the carnitine and low-dose carnitine orotate groups and decreased significantly in the high-dose carnitine orotate group. We conclude that carnitine and carnitine orotate regulate expression of PrRP in the pituitary gland and ovaries.

Metabolic and stress-related roles of prolactin-releasing peptide

In the modern world, improvements in human health can be offset by unhealthy lifestyle factors, including the deleterious consequences of stress and obesity. For energy homeostasis, humoral factors and neural afferents from the gastrointestinal tract, in combination with long-term nutritional signals, communicate information to the brain to regulate energy intake and expenditure. Energy homeostasis and stress interact with each other, and stress affects both food intake and energy expenditure. Prolactin-releasing peptide, synthesized in discrete neuronal populations in the hypothalamus and brainstem, plays an important role in integrating these responses. This review describes how prolactin-releasing peptide neurons receive information concerning both internal metabolic states and environmental conditions, and play a key role in energy homeostasis and stress responses.

Endogenous prolactin-releasing peptide regulates food intake in rodents

Food intake is regulated by a network of signals that emanate from the gut and the brainstem. The peripheral satiety signal cholecystokinin is released from the gut following food intake and acts on fibers of the vagus nerve, which project to the brainstem and activate neurons that modulate both gastrointestinal function and appetite. In this study, we found that neurons in the nucleus tractus solitarii of the brainstem that express prolactin-releasing peptide (PrRP) are activated rapidly by food ingestion. To further examine the role of this peptide in the control of food intake and energy metabolism, we generated PrRP-deficient mice and found that they displayed late-onset obesity and adiposity, phenotypes that reflected an increase in meal size, hyperphagia, and attenuated responses to the anorexigenic signals cholecystokinin and leptin. Hypothalamic expression of 6 other appetite-regulating peptides remained unchanged in the PrRP-deficient mice. Blockade of endogenous PrRP signaling in WT rats by central injection of PrRP-specific mAb resulted in an increase in food intake, as reflected by an increase in meal size. These data suggest that PrRP relays satiety signals within the brain and that selective disturbance of this system can result in obesity and associated metabolic disorders.