In mammals, NK3R is the specific receptor for NKB, which played an important role in reproduction. Recently, two NK3R isoforms, namely NK3Ra and NK3Rb, have been identified in fish. However, little is known about the pituitary actions of the two NK3R isoforms in fish. In this study, both NK3Ra and NK3Rb were isolated from grass carp pituitary. Although their sequence similarity was only 61.6%, the two NK3R isoforms displayed similar ligand selectivity and binding affinity to TAC3 gene products (NKBa, NKBRPa and NKBRPb). In addition, both NK3Ra and NK3Rb displayed similar signaling pathways, including PKA, PKC, MAPK and Ca²⁺ cascades. Tissue distribution indicated that both NK3Ra and NK3Rb were highly detected in grass carp pituitary. Further study found that NK3Ra was mainly located in pituitary LHβ cells, while NK3Rb was only detected in pituitary SLα cells. Furthermore, NK3Ra and NK3Rb activation could induce LHβ and SLα promoter activity, respectively. These results suggested that the two NK3R isoforms displayed different pituitary actions in fish. Using grass carp pituitary cells as model, we found that PACAP could significantly reduce NK3Ra, but induce NK3Rb mRNA expression coupled with cAMP/PKA and PLC/PKC pathways. Interestingly, PACAP could also significantly inhibit LHβ, but stimulate SLα mRNA expression in grass carp pituitary cells. Furthermore, NK3R antagonist could not only inhibit LHβ mRNA expression, but also block PACAP-induced SLα mRNA expression in grass carp pituitary cells. These results suggested that NK3Ra and NK3Rb could mediate PACAP-reduced LHβ and -induced SLα mRNA expression in grass carp pituitary, respectively.

在哺乳动物中，NK3R是NKB的特异性受体，在繁殖过程中发挥了重要作用。最近，在鱼类中发现了两种 NK3R 亚型，即 NK3Ra 和 NK3Rb。然而，关于鱼中两种 NK3R 同种型的垂体作用知之甚少。在本研究中，NK3Ra 和 NK3Rb 均从草鱼垂体中分离得到。虽然它们的序列相似性仅为 61.6%，但两种 NK3R 亚型显示出相似的配体选择性和对 TAC3 基因产物（NKBa、NKBRPa 和 NKBRPb）的结合亲和力。此外，NK3Ra 和 NK3Rb 都表现出相似的信号通路，包括 PKA、PKC、MAPK 和 Ca ²⁺级联。组织分布表明NK3Ra和NK3Rb均在草鱼垂体中高度检测到。进一步研究发现，NK3Ra主要位于垂体LHβ细胞中，而NK3Rb仅在垂体SLα细胞中检测到。此外，NK3Ra 和 NK3Rb 激活可分别诱导 LHβ 和 SLα 启动子活性。这些结果表明，两种 NK3R 亚型在鱼类中表现出不同的垂体作用。以草鱼垂体细胞为模型，我们发现PACAP可以显着降低NK3Ra，但诱导NK3Rb mRNA表达与cAMP/PKA和PLC/PKC途径相结合。有趣的是，PACAP 还可以显着抑制 LHβ，但会刺激草鱼垂体细胞中 SLα mRNA 的表达。此外，NK3R 拮抗剂不仅可以抑制 LHβ mRNA 的表达，还可以阻断 PACAP 诱导的草鱼垂体细胞中 SLα mRNA 的表达。这些结果表明 NK3Ra 和 NK3Rb 可以分别介导 PACAP 降低的草鱼垂体中 LHβ 和 SLα mRNA 的表达。