Breen Tracy L, Conwell Irene M, Wardlaw Sharon L
Department of Medicine, Columbia University College of Physicians and Surgeons, 630 West 168th Street New York, NY 10032, USA.
Brain Res. 2005 Jan 25;1032(1-2):141-8. doi: 10.1016/j.brainres.2004.11.008.
Agouti-related protein (AGRP) and proopiomelanocortin (POMC) have opposing effects on melanocortin receptor (MC-R) signaling and energy balance, and are important targets for leptin and insulin in the hypothalamus. While food intake and leptin have documented effects on POMC and AGRP gene expression, and insulin has effects on POMC gene expression, little is known about their effects on POMC or AGRP peptide release. Here we have examined the effects of fasting, leptin, and insulin on the release of AGRP and the POMC-derived peptide gamma(3)-MSH from the perifused rat hypothalamus in vitro. In the first experiment, fasting (48 h) resulted in a significant overall decrease in gamma(3)-MSH release measured every 20 min during a 3-h baseline perifusion period and after depolarization with 56 mM KCl (p = 0.02); there was a trend towards an overall increase in the release of AGRP but this was not significant. When the ratio of gamma(3)-MSH/AGRP release was calculated at each time point, there was an overall decrease in gamma(3)-MSH/AGRP with fasting (p < 0.01). Further examination of the ratio of gamma(3)-MSH/AGRP revealed a 34% reduction (p < 0.05) in the basal area under the curve (AUC) and a 33% reduction (p < 0.01) in the post-KCl stimulated AUC in fasted vs. fed animals. In the second experiment, perifusion of hypothalamic slices with 10(-8) or 10(-7) M leptin for 2 h resulted in a significant decrease in the release of AGRP noted primarily after depolarization with KCl (p < 0.01); no effect was seen on gamma(3)-MSH release. Similarly, in a third experiment, perifusion with 10(-7) M insulin caused a significant decrease in AGRP release (p < 0.001) without affecting gamma(3)-MSH release. Thus, there is a significant decrease in gamma(3)-MSH and the ratio of gamma(3)-MSH to AGRP released during fasting, consistent with a net inhibition of hypothalamic MC-R signaling. In contrast, short-term treatment with leptin and insulin may inhibit MC-R signaling primarily by decreasing the release of AGRP.
刺鼠相关蛋白（AGRP）和阿黑皮素原（POMC）对黑皮质素受体（MC-R）信号传导和能量平衡具有相反的作用，并且是下丘脑瘦素和胰岛素的重要作用靶点。虽然食物摄入量和瘦素对POMC和AGRP基因表达有明确的影响，胰岛素对POMC基因表达有影响，但关于它们对POMC或AGRP肽释放的影响却知之甚少。在这里，我们研究了禁食、瘦素和胰岛素对体外灌流的大鼠下丘脑释放AGRP和POMC衍生肽γ(3)-促黑素（γ(3)-MSH）的影响。在第一个实验中，禁食（48小时）导致在3小时的基线灌流期以及用56 mM KCl去极化后，每20分钟测量一次的γ(3)-MSH释放量总体显著下降（p = 0.02）；AGRP释放量有总体增加的趋势，但不显著。当计算每个时间点的γ(3)-MSH/AGRP释放率时，禁食会导致γ(3)-MSH/AGRP总体下降（p < 0.01）。对γ(3)-MSH/AGRP比率的进一步检查发现，禁食动物与进食动物相比，曲线下基础面积（AUC）降低了34%（p < 0.05），KCl刺激后的AUC降低了33%（p < 0.01）。在第二个实验中，用10(-8)或10(-7) M瘦素灌流下丘脑切片2小时，导致AGRP释放量显著下降，主要在KCl去极化后观察到（p < 0.01）；对γ(3)-MSH释放没有影响。同样，在第三个实验中，用10(-7) M胰岛素灌流导致AGRP释放量显著下降（p < 0.001），而不影响γ(3)-MSH释放。因此，禁食期间γ(3)-MSH以及γ(3)-MSH与AGRP的释放比率显著下降，这与下丘脑MC-R信号传导的净抑制一致。相比之下，短期用瘦素和胰岛素治疗可能主要通过减少AGRP的释放来抑制MC-R信号传导。
