Consequence of dopamine D2 receptor blockade on the hyperphagic effect induced by cannabinoid CB1 and CB2 receptors in layers

Cannabinoid CB2 receptor orthologues; in vitro function and perspectives for preclinical to clinical translation

Cannabinoid CB2 receptor agonists are in development as therapeutic agents, including for immune modulation and pain relief. Despite promising results in rodent preclinical studies, efficacy in human clinical trials has been marginal to date. Fundamental differences in ligand engagement and signalling responses between the human CB2 receptor and preclinical model species orthologues may contribute to mismatches in functional outcomes. This is a tangible possibility for the CB2 receptor in that there is a relatively large degree of primary amino acid sequence divergence between human and rodent. Here, we summarise CB2 receptor gene and protein structure, assess comparative molecular pharmacology between CB2 receptor orthologues, and review the current status of preclinical to clinical translation for drugs targeted at the CB2 receptor, focusing on comparisons between human, mouse and rat receptors. We hope that raising wider awareness of, and proposing strategies to address, this additional challenge in drug development will assist in ongoing efforts toward successful therapeutic translation of drugs targeted at the CB2 receptor. LINKED ARTICLES: This article is part of a themed issue Therapeutic Targeting of G Protein-Coupled Receptors: hot topics from the Australasian Society of Clinical and Experimental Pharmacologists and Toxicologists 2021 Virtual Annual Scientific Meeting. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v181.14/issuetoc.

The role of central neurotransmitters in appetite regulation of broilers and layers: similarities and differences

The importance of feeding as a vital physiological function, on the one hand, and the spread of complications induced by its disorder in humans and animals, on the other hand, have led to extensive research on its regulatory factors. Unfortunately, despite many studies focused on appetite, only limited experiments have been conducted on avian, and our knowledge of this species is scant. Considering this, the purpose of this review article is to examine the role of central neurotransmitters in regulating food consumption in broilers and layers and highlight the similarities and differences between these two strains. The methodology of this review study includes a comprehensive search of relevant literature on the topic using appropriate keywords in reliable electronic databases. Based on the findings, the central effect of most neurotransmitters on the feeding of broilers and laying chickens was similar, but in some cases, such as dopamine, ghrelin, nitric oxide, and agouti-related peptide, differences were observed. Also, the lack of conducting a study on the role of some neurotransmitters in one of the bird strains made it impossible to make an exact comparison. Finally, it seems that although there are general similarities in appetite regulatory mechanisms in meat and egg-type chickens, the long-term genetic selection appropriate to breeding goals (meat or egg production) has caused differences in the effect of some neurotransmitters. Undoubtedly, conducting future studies while completing the missing links can lead to a comprehensive understanding of this process and its manipulation according to the breeding purposes of chickens.

Central dopaminergic, serotoninergic, as well as GABAergic systems mediate NMU-induced hypophagia in newborn chicken

AIM

Dopaminergic, serotoninergic, and GABAergic systems influence feeding; however, it is unknown how these chemicals interact with neuromedin U (NMU)-induced feeding in birds. In the current study, ten trials were conducted to determine the links between the above-mentioned systems and NMU.

MATERIALS AND METHODS

In the foremost experimentation, chickens were given intracerebroventricularly injections of NMU (0.1, 1, and 10 µg). NMU (10 µg), SCH23390 (5 nmol), a D1 receptor antagonist, and NMU + SCH23390 were administered in the second experiment. In subsequent experiments, instead of SCH23390, were applied AMI-193 (5 nmol D2 receptor antagonist), NGB2904 (6.4 nmol D3 receptor antagonist), L-741,742 (6 nmol D4 receptor antagonist), 6-OHDA (2.5 nmol dopamine inhibitor), SB242084 (5-HT2c receptor antagonist, 1.5 μg), 8-OH-DPAT (5-HT1A receptor agonist, 15.25 nmol), picrotoxin (GABAA receptor antagonist, 0.5 μg), and CGP54626 (GABAB receptor antagonist, 20 ng). Then, cumulative intake of food was recorded for 2 h.

RESULTS

According to the results, NMU reduced feeding when compared to the control group (p < 0.05). The NMU-induced hypophagia was reduced with co-injection of NMU and SCH23390 (p < 0.05). Hypophagia was diminished with NMU and AMI-193 (p < 0.05). NMU + NGB2904 and NMU + L-741,742 co-injections had no influence (p > 0.05). 6-OHDA reduced the hypophagia (p < 0.05). NMU and SB242084 decreased the hypophagia (p < 0.05), whereas NMU and 8-OH-DPAT had no effect (p > 0.05). The effects were amplified with picrotoxin (p < 0.05). NMU with CGP54626 had no influence on the hypophagia (p > 0.05).

CONCLUSION

Thus, NMU-induced hypophagia is probably mediated by D1/D2, 5-HT2c, and GABAA receptors in neonatal chicks.

Update on Feeding Regulation by Ghrelin in Birds: Focused on Brain Network

Ghrelin is known to be a feeding stimulatory hormone in mammals, but in birds, in contrast to mammals, the feeding behavior is regulated in inhibitory manners. This is because the neuropeptides associated with the regulation in the brain are different from those in mammals, i.e., it has been shown that, in chickens, a corticotropin-releasing hormone family peptide, urocortin, which is a feeding-inhibitory peptide, is mainly involved in the inhibitory mechanism. However, feeding is also regulated by various neurotransmitters in the brain, and recently, their interaction with the mechanisms underlying feeding inhibition by ghrelin in birds has been intensively studied and clarified. This review summarizes these findings.

2-Ethylhexyl Diphenyl Phosphate Causes Obesity in Zebrafish by Stimulating Overeating via Inhibition of Dopamine Receptor D2

Obesity is a popular public health problem worldwide and is mainly caused by overeating, but little is known about the impacts of synthetic chemicals on obesity. Herein, we evaluated the obesogenic effect caused by 2-ethylhexyl diphenyl phosphate (EHDPHP) on zebrafish. Adult zebrafish were exposed to 5, 35, and 245 μg/L of EHDPHP for 21 days. Results showed that EHDPHP exposure significantly promoted the feeding behavior of zebrafish, as evidenced by shorter reaction time, increased average food intake, feeding rate, and intake frequency (p < 0.05). Transcriptomic, real-time quantitative PCR, and neurotransmitter analyses revealed that the dopamine (DA) receptor D2 (DRD2) was inhibited, which interfered with the DA neural reward regulation system, thus stimulating food addiction to zebrafish. This was further verified by the restored DRD2 after 7 days of Halo (a DRD2 agonist) treatment. A strong interaction between EHDPHP and DRD2 was identified via molecular docking. As a consequence of the abnormal feeding behavior, the exposed fish exhibited significant obesity evidenced by increased body weight, body mass index, plasma total cholesterol, triglyceride, and body fat content. Additionally, the pathways linked to Parkinson's disease, alcoholism, and cocaine addiction were also disrupted, implying that EHDPHP might cause other neurological disorders via the disrupted DA system.

Opioid receptor μ, not δ and κ modulate food intake induced by ghrelin in laying chickens

Evidence from animal studies suggests that opioidergic system and gherlin have a regulatory role in food intake, but their interaction(s) have not been studied in laying chickens. So in this study, 4 experiments (each included 4 groups) were designed. The first experiment was performed to evaluate the effect of ghrelin on the cumulative food intake. Experiments 2 to 4 were designed to investigate the possibility of μ, δ, or κ opioid receptors mediating ghrelin-induced hypophagia. All drugs were injected intracerebroventricularly (ICV) at 5 days of age. The results of this study showed that the ICV injection of 1.5 nmol ghrelin did not affect cumulative food intake. But ICV injection of ghrelin with doses of 3 and 6 nmol significantly reduced the cumulative food intake (p<0.05). However, co-injection of ghrelin with NTI, and nor-BNI did not show a significant change in decreased food intake compared to ghrelin. Also, opioid μ receptor gene expression significantly increased (p<0.05), but δ and κ opioid receptors gene expression did not significantly change. These results indicated the opioidergic system is involved in developing ghrelin-induced hypophagic effects in laying chickens. Accordingly, this effect of ghrelin to modify the nutritional behavior is possibly mediated by opioid μ receptor.

A test of context and sex-dependent dopaminergic effects on the behavior of a gregarious bird, the common waxbill Estrilda astrild

The Dopaminergic (DAergic) system has well known influences on behavioral and cognitive functions. Previous work with common waxbills (Estrilda astrild) reported context-specific DAergic effects that could have been due to social environment. Manipulating the dopamine D2-like receptor family (D2R) pathways had opposed effects on behavior depending on whether waxbills were tested alone or in a small cage with a mirror as social stimulus. Since waxbills are highly gregarious, it was hypothesized that being alone or perceiving to have a companion might explain this context-dependence. To test context-dependent DAergic effects, we compared behavioral effects of D2R manipulation in waxbills in the same familiar environment, but either alone or with a familiar, same-sex companion. We found that D2R agonism decreased movement and feeding, similarly to previous results when testing waxbills alone. However, contrary to the hypothesis of dependence on social context, we found that the behavioral effects of the D2R agonist were unchanged when waxbills were tested with a companion. The context-dependence reported earlier might thus be due to other factors, such as the stress of being in a novel environment (small cage) or with an unfamiliar social stimulus (mirror image). In tests with a companion, we also found a sex-specific social effect of D2R manipulation: D2R blocking tended to decrease aggression in males but to increase in females. Together with past work, our results suggest that DAergic effects on behavior involve different types of context- or sex-dependence.

Chronic neuroleptic treatment combined with a high fat diet elevated [3H] flunitrazepam binding in the cerebellum

Clinical and preclinical studies have shown dysfunctions in genetic expression and neurotransmission of γ-Aminobutyric acid (GABA), GABA_A receptor subunits, and GABA-synthesizing enzymes GAD₆₇ and GAD₆₅ in schizophrenia. It is well documented that there is significant weight gain after chronic neuroleptic treatment in humans. While there are limited studies on the effects of diet on GABA signaling directly, a change in diet has been used clinically as an adjunct to treatment for schizophrenic relief. In this study, rats chronically consumed either a chow diet (CD) or a 60% high-fat diet (HFD) and drank from bottles that contained one of the following solutions: water, haloperidol (1.5 mg/kg), or olanzapine (10 mg/kg) for four weeks. Rats were then euthanized and their brains were processed for GABA_A in-vitro receptor autoradiography using [³H] flunitrazepam. A chronic HFD treatment yielded significantly increased [³H] flunitrazepam binding in the rat cerebellum independent of neuroleptic treatment. The desynchronization between the prefrontal cortex and the cerebellum is associated with major cognitive and motor dysfunctions commonly found in schizophrenic symptomatology, such as slowed reaction time, motor dyscoordination, and prefrontal activations related to speech fluency and cognitive alertness. These data support the notion that there is a dietary effect on GABA signaling within the cerebellum, as well as the importance of considering nutritional intervention methods as an adjunct treatment for patients chronically treated with neuroleptics. Finally, we indicate that future studies involving the analysis of individual patient's genetic profiles will further assist towards a precision medicine approach to the treatment of schizophrenia.

Dietary stevioside supplementation increases feed intake by altering the hypothalamic transcriptome profile and gut microbiota in broiler chickens

BACKGROUND

Stevioside (STE) is a widely used sweetener. Despite the fact that chickens are insensitive to sweetness, dietary STE supplementation could increase the feed intake of broiler chickens. Stevioside might regulate the feeding behavior through functional mechanisms other than its high-potency sweetness. The present study was aimed to elucidate the potential sweetness-independent mechanism of an STE-induced orexigenic effect using the broiler chicken and considering the hypothalamic transcriptome profile and gut microbiome.

RESULTS

The analysis of RNA-Seq identified 398 differently expressed genes (160 up-regulated and 238 down-regulated) in the hypothalamus of the STE-supplemented group compared with the control group. Cluster analysis revealed several appetite-related genes were differentially expressed, including NPY, NPY5R, TSHB, NMU, TPH2, and DDC. The analysis of 16S rRNA sequencing data also indicated that dietary STE supplementation increased the relative abundance of Lactobacillales, Bacilli, Lactobacillus, and Lactobacillaceae. Meanwhile, the proportion of Ruminococcaceae, Lachnospiraceae, Clostridia, and Clostridiales was decreased after dietary supplementation with STE.

CONCLUSION

Dietary STE supplementation promoted feed intake through the regulation of the hypothalamic neuroactive ligand-receptor interaction pathway and the alteration of intestinal microbiota composition. This study provides valuable information about the sweetness-independent mechanism of the STE-induced orexigenic effect using the broiler chicken (which is insensitive to sweetness) as the animal model. © 2020 Society of Chemical Industry.

Mediatory role of the dopaminergic system through D1 receptor on glycine-induced hypophagia in neonatal broiler-type chickens

The present study aimed to examine the mediatory role of the dopaminergic system in the food intake induced by intracerebroventricular (ICV) injection of glycine in neonatal 3-h feed-deprived (FD3) meat-type chickens. In the first and second experiments, birds were ICV injected using low and high doses of glycine (50, 100 and 200 nmol) and strychnine (50, 100 and 200 nmol), respectively. In experiments 3-9, the behaviorally subeffective doses of dopamine (10 nmol), 6-OHDA (2.5 nmol), SCH 23,390 (D1 antagonist; 5 nmol), AMI-193 (D2 antagonist; 5 nmol), NGB2904 (D3 antagonist; 6.4 nmol) and L-741,742 (D4 antagonist; 6 nmol) were, respectively, co-administrated with glycine (200 nmol) in FD3 5-day-old chicks to investigate possible interplay of dopamine receptors in glycine-induced feeding behavior. Then, cumulative food intake based on body weight percentage (%BW) was determined at 30, 60 and 120 min after the injection. According to the results, dopamine significantly boosted the hypophagia induced by glycine at all-time intervals (p ≤ 0.001). These results combined with the previous findings suggest an interplay between dopamine and glycine in chicken's brain in which D1 receptor-mediated food intake induced by glycine.

β2 adrenergic receptors and leptin interplay to decrease food intake in chicken

1. The present study was designed to examine the effects of intracerebroventricular (ICV) injection of different α and [Formula: see text] adrenergic receptor antagonists on leptin-induced hypophagia in broiler chickens.2. The study consisted of six experiments. In all experiments, chickens were deprived of feed for 3 h prior to the ICV injections and thereafter were returned immediately to the individual cages and cumulative feed intake, based on the percentage of body weight, was measured at 30, 60 and 120 min post-injection.3. In experiment 1, leptin (2.5, 5 or 10 µg) were injected in birds. In experiment 2, groups received either control solution, prazosin (10 nmol), leptin (10 µg) or a co-injection of prazosin (10 nmol) and leptin (10 µg). The other experiments were conducted as experiment 2, but instead of prazosine (10 nmol), yohimbine (13 nmol) was used in experiment 3, metoprolol (24 nmol) in experiment 4, ICI 118,551 (5 nmol) in experiment 5 and SR 59230R (5 nmol) in experiment 6 were injected either in a group or in combination with leptin (10 µg).4. The results of this study revealed a dose-dependent hypophagic effect of leptin and, in experiment 5, ICV co-injection of ICI118, 551 (5 nmol) and leptin (10 µg) significantly attenuated this effect (P˂0.5). These results suggest that the hypophagic effect of leptin is probably mediated by β2 adrenergic receptors in chickens.

Cloning and functional characterization of the DA2 receptor gene in Chinese mitten crab (Eriocheir sinensis)

Dopamine (DA) plays a modulatory role in numerous physiological processes such as light adaptation and food intake, and exerts these functions through DA receptors (DARs). This study presents, for the first time, isolation and characterization of the dopamine receptor 2(DA₂ receptor) cDNA from the intestinal tissue of Eriocheir sinensis, an economically important freshwater aquaculture species in China. The DA₂ receptor cDNA sequence, which was obtained by rapid amplification of cDNA ends, is 2369bp long, encode peptide of 589 amino acid, and is highly homologous to related sequences in crustaceans. Analysis of the deduced amino acid sequence and the structure of the DA₂ indicated that this receptor is a member of the family of G protein-coupled receptors (GPCRs), as it contains seven transmembrane domains and other common signatures of GPCRs. RT-PCR showed that the expression of the DA₂ receptor gene was distributed in various tissues, and high expression levels were observed in the cranial ganglia and the thoracic ganglia. Further study of the effect of photoperiod on DA₂ expression showed that constant darkness induced a significant increase in DA₂ expression in the cranial ganglia. Finally, analysis of DA₂ receptor expression under different feeding statuses showed that there was significantly greater expression in the hepatopancreas and intestines after feeding than before feeding, but there were no differences in expression between the before feeding and during feeding periods in either tissue. Our results indicate that the DA₂ receptor structurally belongs to the family of G protein-coupled receptors, and that the cranial ganglia are the main tissues in which the DA₂ receptor participates in light adaptation during dark hours. In addition, the DA₂ receptor in E. sinensis may be involved in the physiological regulation of the hepatopancreas and digestive tract after the ingestion of food. This study provides a foundation for further exploration of the light adaptation and digestive functions of the DA₂ receptor in decapods.

Dopamine D2-like receptors (DRD2 and DRD4) in chickens: Tissue distribution, functional analysis, and their involvement in dopamine inhibition of pituitary prolactin expression

Dopamine (DA) D2-like (and D1-like) receptors are suggested to mediate the dopamine actions in the anterior pituitary and/or CNS of birds. However, the information regarding the structure, functionality, and expression of avian D2-like receptors have not been fully characterized. In this study, we cloned two D2-like receptors (cDRD2, cDRD4) from chicken brain using RACE PCR. The cloned cDRD4 is a 378-amino acid receptor, which shows 57% amino acid (a.a.) identity with mouse DRD4. As in mammals, two cDRD2 isoforms, cDRD2L (long isoform, 437 a.a.) and cDRD2S (short isoform, 408 a.a.), which differ in their third intracellular loop, were identified in chickens. Using cell-based luciferase reporter assays or Western blot, we demonstrated that cDRD4, cDRD2L and cDRD2S could be activated by dopamine and quinpirole (a D2-like receptor agonist) dose-dependently, and their activation inhibits cAMP signaling pathway and stimulates MAPK/ERK signaling cascade, indicating that they are functional receptors capable of mediating dopamine actions. Quantitative real-time PCR revealed that cDRD2 and cDRD4 are widely expressed in chicken tissues with abundant expression noted in anterior pituitary, and their expressions are likely controlled by their promoters near exon 1, as demonstrated by dual-luciferase reporter assays in DF-1 cells. In accordance with cDRD2/cDRD4 expression in the pituitary, DA or quinpirole could partially inhibit vasoactive intestinal peptide-induced prolactin expression in cultured chick pituitary cells. Together, our data proves the functionality of DRD2 and DRD4 in birds and aids to uncover the conserved roles of DA/D2-like receptor system in vertebrates, such as its action on the pituitary.