In vitro evidence supports the presence of glucokinase-independent glucosensing mechanisms in hypothalamus and hindbrain of rainbow trout

Abstract

We previously obtained evidence in rainbow trout for the presence and response to changes in circulating levels of glucose (induced by intraperitoneal hypoglycaemic and hyperglycaemic treatments) of glucosensing mechanisms based on liver X receptor (LXR), mitochondrial production of reactive oxygen species (ROS) leading to increased expression of uncoupling protein 2 (UCP2), and sweet taste receptor in the hypothalamus, and on sodium/glucose cotransporter 1 (SGLT-1) in hindbrain. However, these effects of glucose might be indirect. Therefore, we evaluated the response of parameters related to these glucosensing mechanisms in a first experiment using pooled sections of hypothalamus and hindbrain incubated for 6 h at 15°C in modified Hanks' medium containing 2, 4 or 8 mmol l −1 D-glucose. The responses observed in some cases were consistent with glucosensing capacity. In a second experiment, pooled sections of hypothalamus and hindbrain were incubated for 6 h at 15°C in modified Hanks' medium with 8 mmol l −1 D-glucose alone (control) or containing 1 mmol l −1 phloridzin (SGLT-1 antagonist), 20 µmol l −1 genipin (UCP2 inhibitor), 1 µmol l −1 trolox (ROS scavenger), 100 µmol l −1 bezafibrate (T1R3 inhibitor) and 50 µmol l −1 geranyl-geranyl pyrophosphate (LXR inhibitor). The response observed in the presence of these specific inhibitors/ antagonists further supports the proposal that critical components of the different glucosensing mechanisms are functioning in rainbow trout hypothalamus and hindbrain.

Glucosensing capacity of rainbow trout telencephalon

“…Analyses were performed on 1 μL of cDNA using the MAXIMA SYBR Green qPCR Mastermix (Thermo Scientific, Waltham, MA, USA), in a total PCR reaction volume of 15 μL, containing 50‐500 nmol L ‐1 of each primer. The mRNA abundance of AgRP, CART, cytochrome c oxidase subunit 4 (COX4), carnitine palmitoyltransferase 1c (CPT1c), corticotrophin‐releasing factor (CRF), fructose 1,6‐bisphosphatase (FBPase), G6Pase, GK, GLUT2, α‐gustducin (Gnat3), GSase, inward rectifier channel pore type 6.x‐like (Kir6.x‐like), LXRα, NPY, PEPCK, 6‐phosphofructo 1‐kinase (PFK), POMCa1, peroxisome proliferator‐activated receptor (PPAR) type α, SGLT‐1, sterol regulatory element‐binding protein type 1c (SREBP1c), type 1 taste receptor subunit 2 (T1R2), type 1 taste receptor subunit 3 (T1R3) and UCP2a was determined as described previously in the same species . Sequences and accession numbers of the primers used for each gene expression are shown in Table .…”

Nutrient Sensing Systems in Fish: Impact on Food Intake Regulation and Energy Homeostasis

“…Hypothalamic POMC mRNA levels increased in hyperglycaemic rainbow trout (Conde-Sieira et al, 2010b; Otero-Rodiño et al, 2015). Finally, AgRP mRNA abundance did not display changes in hypothalamus of rainbow trout after hyperglycaemic treatment (Otero-Rodiño et al, 2015, 2016a). Therefore, the mRNA abundance in glucosensing central areas (hypothalamus and hindbrain) of the four neuropeptides involved in the food intake regulation is affected by changes in glycaemia, and this is compatible with the changes observed in food intake (Polakof et al, 2008a,b).…”

“…The presence of GK-independent glucosensing mechanisms and their response to changes in glucose levels has recently been assessed in different central and peripheral areas of rainbow trout (Polakof and Soengas, 2013; Otero-Rodiño et al, 2015, 2016a,b,c). These include hypothalamus (mitochondrial activity, sweet taste receptor, and LXR), hindbrain (SGLT-1), liver (sweet taste receptor), BB (sweet taste receptor, LXR, and mitochondrial activity), and intestine (sweet taste receptor, SGLT-1, and LXR).…”

“…However, few studies in fish described changes in the mRNA abundance of those neuropeptides in response to changes in glucose levels. The mRNA abundance of hypothalamic NPY decreased in hyperglycaemic-treated rainbow trout (Conde-Sieira et al, 2010b, 2012b; Aguilar et al, 2011; Otero-Rodiño et al, 2016a). A similar decline occurred in fish fed with a carbohydrate-enriched diet, such as in rainbow trout (Figueiredo-Silva et al, 2012c) and goldfish (Narnaware and Peter, 2002) whereas in the whole brain of gilthead sea bream no changes occurred (Babaei et al, 2017).…”

Hypothalamic Integration of Metabolic, Endocrine, and Circadian Signals in Fish: Involvement in the Control of Food Intake

“…The response in rainbow trout hypothalamus after IP, ICV or dietary treatments inducing changes in the levels of glucose is shown in Table 1. Moreover, recent studies (Otero-Rodiño et al, 2015, 2016) provided evidence in rainbow trout for the presence and response to changes in circulating levels of glucose of glucosensing mechanisms in hypothalamus dependent on mitochondrial activity, LXR, and sweet taste receptor. No other studies attempted to elucidate the presence of glucosensing mechanisms in fish though glucose-sensing properties have been recently described in medaka hypothalamus (Hasebe et al, 2016).…”