Effects of orexin on catecholamine synthesis and release in adrenal medulla

食欲素-A和食欲素-B，又称下丘脑素-1和食欲素-2，最近被鉴定为下丘脑多肽，起源于大鼠大脑下丘脑外侧和后部神经元中的前增食欲素原。为了确定增食欲素-A在外周代谢过程中的作用，我们检测了增食欲素-A对培养的牛肾上腺髓质细胞儿茶酚胺合成和分泌的直接影响。1)与增食欲素A(100 Pm)孵育20分钟后，细胞由酪氨酸合成的儿茶酚胺略有增加，但对L-3，4-二羟基苯基[14&gt；C]丙氨酸的合成无明显影响。食欲素-A(100 Pm)可增强乙酰胆碱(0.3 mM)和14&gt；C-儿茶酚胺合成的刺激作用。2)增食欲素-A显著提高酪氨酸羟基酶活性，在下午1点最明显，在下午100点达到最大4蛋白激酶C激活剂β-佛波醇-12β-肉豆蔻酸盐-13α-乙酸酯不能增强增食欲素-A对酪氨酸羟基酶活性的刺激作用，而蛋白激酶C的抑制剂H-7和星形孢子素则抵消了增食欲素-A的作用。3)增食欲素A对细胞内儿茶酚胺的分泌量影响不大。4)逆转录聚合酶链式反应检测到牛肾上腺髓质细胞表达增食欲素受体1(OX_1R)，而不表达增食欲素受体2(OX_2R)，提示增食欲素A可能通过激活OX_1R-蛋白激酶C通路而激活酪氨酸羟基酶，进而刺激儿茶酚胺的合成。这一信息有助于理解增食欲素对成人和儿童肥胖者代谢稳态的影响的生化基础。

Orexin-A and orexin-B, also known as hypocretin-1 and hypocretin-2, have recently been identified as hypothalamic peptides derived from a precursor, prepro-orexin, in neurons located within and around the lateral and posterior hypothalamus in the rat brain. To determine the role of orexin-A in peripheral metabolic processes, we examined direct effects of orexin-A on catecholamine synthesis and secretion in cultured bovine adrenal medullary cells. 1) Incubation of cells with orexin-A (100 pM) for 20 min caused a small but significant increase in ^<14>C-catecholamine synthesis from [^<14>C] tyrosine, but not from L-3,4-dihydroxyphenyl [3-^<14>C] alanine. Orexin-A (100 pM) potentiated the stimulatory effects of acetylcholine (0.3 mM) ^<14>C-catecholamine synthesis. 2) Orexin-A significantly increased tyrosine hydroxylase activity, which was evident at 1 pM and maximal at 100 pM. 4β-Phorbol-12β-myristate-13α-acetate, an activator of protein kinase C, did not enhance the stimulatory effects of orexin-A on tyrosine hydroxylase activity, while H-7 and staurosporine, inhibitors of protein kinase C, nullified the effects of orexin-A. 3) Orexin-A had little effect on catecholamine secretion from the cells. 4) Orexin receptor 1 (OX_1R) but not orexin receptor 2 (OX_2R) mRNA was detected in bovine adrenal medullary cells by reverse transcriptase-polymerase chain reaction.These findings suggest that orexin-A activates tyrosine hydroxylase and then stimulates catecholamine synthesis, probably via activation of the OX_1R-protein kinase C pathway in adrenal medullary cells. This information helps to understand the biochemical basis of the effect of orexins on metabolic homeostasis in the obesity of adults as well as children.