Department of Integrative Physiology and Neuroscience, Washington State University, College of Veterinary Medicine, Washington State University, Pullman, WA 99164, USA.
Endocrinology. 2013 Aug;154(8):2613-25. doi: 10.1210/en.2013-1062. Epub 2013 May 28.
Vagal afferent nerve fibers transmit gastrointestinal satiation signals to the brain via synapses in the nucleus of the solitary tract (NTS). Despite their pivotal role in energy homeostasis, little is known about the cellular mechanisms enabling fleeting synaptic events at vagal sensory endings to sustain behavioral changes lasting minutes to hours. Previous reports suggest that the reduction of food intake by the satiation peptide, cholecystokinin (CCK), requires activation of N-methyl-D-aspartate-type glutamate receptors (NMDAR) in the NTS, with subsequent phosphorylation of ERK1/2 (pERK1/2) in NTS vagal afferent terminals. The synaptic vesicle protein synapsin I is phosphorylated by pERK1/2 at serines 62 and 67. This pERK1/2-catalyzed phosphorylation increases synaptic strength by increasing the readily releasable pool of the neurotransmitter. Conversely, dephosphorylation of serines 62 and 67 by calcineurin reduces the size of the readily releasable transmitter pool. Hence, the balance of synapsin I phosphorylation and dephosphorylation can modulate synaptic strength. We postulated that CCK-evoked activation of vagal afferent NMDARs results in pERK1/2-catalyzed phosphorylation of synapsin I in vagal afferent terminals, leading to the suppression of food intake. We found that CCK injection increased the phosphorylation of synapsin I in the NTS and that this increase is abolished after surgical or chemical ablation of vagal afferent fibers. Furthermore, fourth ventricle injection of an NMDAR antagonist or the mitogen-activated ERK kinase inhibitor blocked CCK-induced synapsin I phosphorylation, indicating that synapsin phosphorylation in vagal afferent terminals depends on NMDAR activation and ERK1/2 phosphorylation. Finally, hindbrain inhibition of calcineurin enhanced and prolonged synapsin I phosphorylation and potentiated reduction of food intake by CCK. Our findings are consistent with a mechanism in which NMDAR-dependent phosphorylation of ERK1/2 modulates satiation signals via synapsin I phosphorylation in vagal afferent endings.
迷走传入神经纤维通过孤束核 (NTS) 中的突触将胃肠道饱腹感信号传递到大脑。尽管它们在能量平衡中起着关键作用，但对于使迷走感觉末梢短暂的突触事件持续几分钟到几个小时的行为变化的细胞机制知之甚少。以前的报告表明，饱食肽胆囊收缩素 (CCK) 通过 NTS 中的 N-甲基-D-天冬氨酸型谷氨酸受体 (NMDAR) 的激活来减少食物摄入，随后 NTS 迷走传入末梢中的 ERK1/2 (pERK1/2) 磷酸化。突触小泡蛋白突触素 I 在丝氨酸 62 和 67 处被 pERK1/2 磷酸化。这种 pERK1/2 催化的磷酸化通过增加神经递质的易释放池来增加突触强度。相反，钙调神经磷酸酶使丝氨酸 62 和 67 去磷酸化会减少易释放递质池的大小。因此，突触素 I 的磷酸化和去磷酸化的平衡可以调节突触强度。我们假设 CCK 引发的迷走传入 NMDAR 激活导致迷走传入末梢中 pERK1/2 催化的突触素 I 磷酸化，从而抑制食物摄入。我们发现 CCK 注射增加了 NTS 中突触素 I 的磷酸化，并且这种增加在迷走传入纤维的手术或化学消融后被消除。此外，第四脑室注射 NMDAR 拮抗剂或丝裂原活化 ERK 激酶抑制剂阻断了 CCK 诱导的突触素 I 磷酸化，表明迷走传入末梢中的突触素磷酸化取决于 NMDAR 激活和 ERK1/2 磷酸化。最后，后脑钙调神经磷酸酶的抑制增强并延长了突触素 I 的磷酸化，并增强了 CCK 对食物摄入的减少。我们的发现与一种机制一致，即 NMDAR 依赖性 ERK1/2 磷酸化通过迷走传入末梢中突触素 I 的磷酸化调节饱食信号。
