Neuropeptide B (NPB) is a peptide hormone that was initially described in 2002. In humans, the biological effects of NPB depend on the activation of two G protein-coupled receptors, NPBWR1 (GPR7) and NPBWR2 (GPR8), and, in rodents, NPBWR1. NPB and its receptors are expressed in the central nervous system (CNS) and in peripheral tissues. NPB is also present in the circulation. In the CNS, NPB modulates appetite, reproduction, pain, anxiety, and emotions. In the peripheral tissues, NPB controls secretion of adrenal hormones, pancreatic beta cells, and various functions of adipose tissue. Experimental downregulation of either NPB or NPBWR1 leads to adiposity. Here, we review the literature with regard to NPB-dependent control of metabolism and energy homeostasis.
Peptides that regulate appetite play a prominent role in controlling energy homeostasis and whole-body metabolism. Such peptides are found in brain regions that are involved in the modulation of appetite. In addition, such peptides are present in the circulation and in numerous peripheral tissues. There is growing evidence indicating that peptides that control appetite (e.g., kisspeptin, orexins, spexin, adropin, apelin, phoenixin, ghrelin, amylin, and pancreatic peptides) also modulate the endocrine activity of endocrine glands as well as lipid and glucose metabolism. Moreover, some peptides are involved in regulating the endocannabinoid system and, through it, food intake, e.g., hemopressin, a small peptide derived from the α-chain of hemoglobin, reduces appetite through increased levels of endocannabinoids. On the other hand, endogenous cannabinoids can also increase the secretion of feeding-regulated hypothalamic neuropeptides. Thus, peptide hormones and their receptors may be of interest in therapy for obesity and obesity-related diseases such as type 2 diabetes. Almost 20 years after the discovery of neuropeptide B (NPB), there is growing evidence that this peptide modulates food intake, body weight, and lipid and glucose metabolism. In our narrative review, we discuss current findings regarding the role of NPB and its receptors in controlling food intake and energy homeostasis.
By analyzing human genomic sequences in the Celera database, in 2002, Fuji et al. identified a new neuropeptide composed of 23 or 29 amino acids that was uniquely modified with bromine. This peptide was termed neuropeptide B (NPB). The same study showed that NPB interacts with NPBWR1 (GPR7) and less potently with NPBWR2 (GPR8). At the same time, NPB as a ligand of NPBWR1 and NPBWR2 was reported by two independent laboratories. Both NPBWR1 and NPBWR2 belong to the G protein-coupled receptor superfamily. It is important to note that humans express both types of receptors, while rodents express only NPBWR1. It should be pointed out that both types of NPB receptors interact with another ligand, termed neuropeptide W. The intracellular signaling of NPBWR1 and NPBWR1 encompasses the modulation of cAMP, calcium, phospholipase C, or MAP kinase signaling. The expression of NPB and its receptors in the CNS and various peripheral tissues was reported.
The initial study showed that NPB mRNA is expressed in brain regions that are crucially relevant in the regulation of food intake. NPB mRNA was reported in the dorsomedial, paraventricular, and arcuate nuclei. In their pioneer work, Tanaka et al. investigated the effects of NPB administration on food intake in mice. Initially, the authors investigated the effects of NPB administration on appetite during the light phase. However, they did not observe any influence of NPB on appetite control in animals. In contrast, i.c.v. administration of NPB during the dark phase led to stimulated food intake during the first 2 h. In contrast, after 2 more hours, NPB caused appetite suppression. The same study evaluated the effects on appetite of co-administration of NPB and corticotropin-releasing factor (CRF), a well-known suppressor of food intake. Tanaka et al. reported that CRF significantly enhanced the suppression of appetite induced by NPB, suggesting an interaction between CRF and urocortin systems. In summary, this study showed, for the first time, that the effects of NPB on food intake are biphasic.
The anorexigenic activity of the NPB/NPBW1 system was additionally confirmed by Ishii et al., who found that GPR7−/− male mice ate more food than wild-type GPR7 mice. It is worth noting that GPR7−/− mice had reduced NPY mRNA and increased POMC mRNA expression in the hypothalamus. Of note, NPY promotes food intake, while POMC has the opposite effect. Another animal study showed that i.c.v. administration of NPB (during the light phase) in male rats promoted feeding behavior. Stimulation of food intake was detected 30 min after NPB administration and lasted at least 4 h. In contrast, NPB did not affect water intake. It is important to note that, in contrast to NPBW1−/− mice, NPB−/− mice had a normal feeding behavior. Studies addressing the role of NPB in appetite regulation are not restricted to rodents. For instance, it was found that i.p. injection of NPB stimulated mRNA expression of NPY and CCK1 in the hypothalamus of Nile tilapia Oreochromis niloticus. Since NPY stimulates food intake and CCK1 suppresses appetite, it is difficult to define the role of NPB in controlling feeding behavior in tilapia, and more studies need to be conducted.
Discussing the contribution of NPB to appetite modulation, it is worth noting that a human study was conducted on circulating NPB in blood in patients with anorexia nervosa (AN). Grzelak et al. reported that patients who suffer from AN are characterized by lower levels of NPB in the circulation compared to healthy controls, suggesting the use of NPB in diagnosing AN. Nevertheless, as pointed out in this work, NPB levels were evaluated in only 30 healthy controls and 46 patients with anorexia; therefore, these results should be interpreted cautiously. The downregulation of circulating NPB levels in patients with anorexia was independently confirmed by a study of 30 healthy controls and 30 patients with AN. Importantly, this study additionally showed that increased NPB levels are not affected by body weight normalization after hospitalization. More studies are needed to elucidate the potential role of NPB in the diagnosis of AN.
In summary, animal studies have shown that i.c.v. administration of NPB during the dark phase biphasically modulates food intake. NPB promotes food intake during the first 2 h, followed by appetite suppression. In contrast, rat studies showed that NPB displays orexigenic effects during the light phase. The role of NPB in controlling feeding behavior is complex; therefore, more studies are needed.
Beside its role in feeding behavior, in the CNS, NPB modulates locomotion and analgesia. An i.c.v. injection of NPB in rats significantly increased locomotion in an open-field test in both the bright and dark phases. On the other hand, Hirashima et al. demonstrated that i.c.v. injection of NPB in mice reduced locomotor activity during the dark period, but not during the light phase. The activity of mice was measured using an infrared activity monitor. In experiments using Npb−/− mice, no significant differences in activity levels were found compared to littermate controls.
In the CNS, NPB impacts analgesia. Tanaka et al. reported that i.c.v. injection of NPB in rats reduced licking duration in the formalin test, which indicates an analgesic role of the peptide against chemically induced pain. These effects could be conferred via NPB and NPBWR1, which are found in the periaqueductal gray matter and amygdala, areas that are also known to express opioid receptors. It is worth mentioning that NPBWR1 binds non-selective opioid ligands such as β-endorphin. The analgesic effect of NPB was also observed after intrathecal injection in the formalin test, and mechanical allodynia was inducible by carrageenan injection. However, NPB had no effect on the level of thermal hyperalgesia induced by paw carrageenan injection in rats and NPB−/− mice.
The pain response is tightly connected to anxiety. The role of NPB in regulating anxiety has been investigated using the cued and contextual fear test and elevated plus maze test. NPBWR1−/− mice had similar behavior in the contextual fear test compared to wild-type mice. However, unlike wild-type mice, NPBWR1−/− mice showed behavioral changes in social interactions. The role of NPB in the context of social behavior was evaluated by Watanabe et al. They showed that genetic changes in NPWR1 (single-nucleotide polymorphism at nucleotide 404 resulted in an amino acid change, Y135F) modulated emotional responses to facial expression. The 404AT subjects were less submissive to angry faces than 404AA subjects.
There is evidence that NPB is involved in sleep/wakefulness. An i.c.v. injection of NPB in mice during the dark period decreased time in the waking state and increased time in slow-wave sleep, whereas no change in paradoxical sleep time was observed. Moreover, NPBWR1+/− and NPBWR1−/− mice did not present any abnormalities compared with wild-type mice, indicating a modulatory role of NPB and NPBWR1 in the sleep/wakefulness pattern.
In summary, NPB plays a role in the regulation of locomotion and decreases locomotor activity during the dark phase. Moreover, during the dark period, NPB decreases the waking state time. It also plays an analgesic role in chemically induced pain and decreases social anxiety.
Few studies have addressed the role of NPB and its receptors in the regulation of reproductive functions. In the rat CNS, the expression of NPB mRNA was detected in the hypothalamus, which is known as the crucial center of the reproductive hypothalamus–pituitary–gonadal regulatory axis. More detailed analysis showed that, in the hypothalamus, NPB-immunoreactive cells are present in the paraventricular nucleus (PVN), ventromedial hypothalamic nucleus (VMH), dorsomedial hypothalamic nucleus (DMH), and arcuate nucleus (ARC). Many regions within the hypothalamus also have high levels of NPBWR1 mRNA expression. Of note, VMH activity is involved in sexual behavior, while ARC is importantly involved in GnRH and prolactin release from the anterior pituitary, LH surge, lactation, and growth hormone release.
Although NPB-expressing neurons were identified in rats, there is no information regarding sex differences with respect to the occurrence of NPB neurons in other mammals. However, importantly, Ishi et al. demonstrated that NPBWR1-deficient male mice exhibited a sex-specific phenotype of adult-onset obesity. Additional studies showing the importance of NPB in reproduction and the expression of NBP in the CNS of medaka teleost fish (Oryzias latipes) were published by Kikuchi et al. and Hiraki-Kajiyma et al. Both studies confirmed that NPB is preferentially expressed in the female medaka brain, in populations of Vs/Vp and PMm/PMg neurons, whose expression is known to be estrogen-dependent and associated with female sexual receptivity. NPB neurons in PMm/PMg regions are critically dependent on estrogen. Behavioral studies indicated that NPB is a direct mediator of estrogen action in female mating behavior, acting in a female-specific and reversible manner. Moreover, Hiraki-Kajiyma et al., using NPB-deficient medaka, showed that NPB/NPBWR2 signaling is involved in female sexual receptivity. The female-specific neurons located in PMm/PMg neurons are found in the region that is considered homologous to SON/PVN in the mammalian brain. It is possible that the role of NPB in female sexual receptivity may be conserved across vertebrates; however, this hypothesis needs to be investigated.
Interesting data were obtained in non-mammalian vertebrates, using the chicken as a model, to examine the functionality of the NPB and NPW system and its interaction with the pituitary gland. It was found that NPB mRNA is widely expressed in chicken tissues, including the hypothalamus, while chicken NPB receptor isoforms cNPBWR1 and cNPBWR2 were predominantly expressed in the brain and pituitary. One study confirmed NPB immunoreactivity detected in a population of cells in the rat anterior pituitary. In another study, the mRNA expression of NPB and its two receptors was detected in the rat anterior pituitary gland. Intermediate levels of NPBWR1 and NPBWR2 expression in the rat pituitary gland were also detected. In vivo, i.c.v. injection of NPB into the lateral cerebral ventricle of male rats increased prolactin but decreased GH in the circulation.
mRNA expression of NPB was detected in peripheral reproductive glands including the ovary, uterus, placenta, testes, and mammary gland in rats. Immunohistochemical detection of NPB in reproductive peripheral glands including ovarian thecal cells, granulosa and lutein cells, and oocytes and in Leydig cells of the testes was also reported. For the first time, the functional role of NPB in the regulation of gonadal hormone secretion was analyzed in pigs. Yang et al. confirmed NPB mRNA expre
