Effects of orexin-A on firing activity of gastric distension-sensitive neurons in the basomedial amygdala and food intake in diet-induced obese rats

Abstract

Objective To investigate the effects of orexin-A on firing activity of gastric distension-sensitive (GD) neurons in the basomedial amygdala (BMA) and food intake in diet-indaced obese rats. Methods Healthy male Wistar rats were selected, and the diet-induced obesity (DIO) rat model and diet-induced resistant (DR) rat model were established by high-fat diet. The effects of orexin-A and an opioid receptor antagonist naloxone on BMA GD neurons were observed by recording the extracellular potentials of single neurons.The effects of orexin-A and naloxone on the food intake of different rats were observed by using BMA catheterization.The mRNA expression and protein expression of orexin-1 receptor (OX-1R) and μ opioid receptor were detected by real-time PCR and Elisa, respectively. Results After microinjection of orexin-A into the BMA, the firing frequency of GD-sensitive neurons in the normal rats was significantly increased (GD-E: (78.3±6.9)%, GD-I: (55.5±4.7)%, P<0.01), and this effect was completely blocked by OX-1R receptor antagonist SB334867, and naloxone partially blocked the discharge-promoting effect of orexin-A; Compared with the normal rats, the firing frequency of GD-sensitive neurons in the DIO(GD-E: (91.6±7.1)%, GD-I: (67.9±8.1)%) and DR(GD-E: (87.9±6.8)%, GD-I: (69.2±5.8)%)rats was significantly increased after BMA injection of orexin-A (P<0.05). After administration of orexin-A into the BMA, food intake of the normal rats, DIO rats and DR rats ((2.38±0.34) g, (3.75±0.32) g, (4.01±0.38) g, respectively) was significantly increased (P<0.01), and the food intake of DR and DIO rats were significantly higher than that of normal rats (P<0.05). After BMA was injected with naloxone, the food intake of rats was inhibited, and the food intake of the DIO rats was significantly lower than that of the DR rats (P<0.05), food intake of the DR rats was significantly lower than that of the normal rats (P<0.05). The results of real-time PCR showed that the mRNA levels of OX-1R in DIO and DR rats were( 5.85±0.45 )and (6.03±0.42) were higher than that of normal rats, and the difference was significant (P<0.05); and mRNA levels of μ-opioid receptors in DIO and DR rats((4.51±0.42) and (8.31±0.41)times) were higher than those in normal rats (P<0.05). The results of Elisa showed that the protein levels of OX-1R in DIO ((2.98±0.28) ng/μl) and DR rats ((3.05±0.31) ng/μl) were higher than those in normal rats ((1.53±0.31)ng/μl, P<0.05). The content of μ-opioid receptor protein in DR rats ((4.21±0.35)ng/μl) was higher than that of DIO rats ((2.77±0.27) ng/μl), and higher than that of normal rats((1.48±0.32) ng/μl), the difference was significant (P<0.05). Conclusion BMA orexin-A promotes the spontaneous discharge of GD-sensitive neurons and food intake in normal rats, DIO rats and DR rats, μ-opioid receptors may be involved in the regulation of this process. Key words: Orexin-A; Food intake; Gastric distension-sensitive neurons; Diet-induced obesity rat; Amygdala