Roubos E W
Department of Cellular Animal Physiology, Nijmegen Institute for Neurosciences, University of Nijmegen, The Netherlands.
Comp Biochem Physiol A Physiol. 1997 Nov;118(3):533-50. doi: 10.1016/s0300-9629(97)00035-2.
This review is concerned with recent literature on the neural control of the pituitary pars intermedia of the amphibian Xenopus laevis. This aquatic toad adapts skin colour to the light intensity of its environment, by releasing the proopiomelanocortin (POMC)-derived peptide alpha-MSH (alpha-melanophore-stimulating hormone) from melanotrope cells. The activity of these cells is controlled by brain centers of which the hypothalamic suprachiasmatic and magnocellular nuclei, respectively, inhibit and stimulate both biosynthesis and release of alpha-MSH. The suprachiasmatic nucleus secretes dopamine, GABA, and NPY from synaptic terminals on the melanotropes. The structure of the synapses depends on the adaptation state of the animal. The inhibitory transmitters act via cAMP. Under inhibition conditions, melanotropes actively export cAMP, which might have a first messenger action. The magnocellular nucleus produces CRH and TRH. CRH, acting via cAMP, and TRH stimulate POMC-biosynthesis and POMC-peptide release. ACh is produced by the melanotrope cell and acts in an autoexcitatory feedback on melanotrope M1 muscarinic receptors to activate secretory activity. POMC-peptide secretion is driven by oscillations of the [Ca2+]i, which are initiated by receptor-mediated stimulation of Ca2+ influx via N-type calcium channels. The hypothalamic neurotransmitters and ACh control Ca2+ oscillatory activity. The structural and functional aspects of the various neural and endocrine steps in the regulation of skin colour adaptation by Xenopus reveal a high degree of plasticity, enabling the animal to respond optimally to the external demands for physiological adaptation.
本综述关注的是有关两栖动物非洲爪蟾垂体中间叶神经控制的近期文献。这种水栖蟾蜍通过从促黑素细胞释放源自阿黑皮素原（POMC）的肽α-促黑素（α-MSH，α-黑素细胞刺激素）来使皮肤颜色适应其环境的光照强度。这些细胞的活动受脑中枢控制，其中下丘脑视交叉上核和大细胞核对α-MSH的生物合成和释放分别起抑制和刺激作用。视交叉上核从促黑素细胞的突触末端分泌多巴胺、γ-氨基丁酸（GABA）和神经肽Y（NPY）。突触的结构取决于动物的适应状态。抑制性递质通过环磷酸腺苷（cAMP）起作用。在抑制条件下，促黑素细胞主动输出cAMP，其可能具有第一信使作用。大细胞核产生促肾上腺皮质激素释放激素（CRH）和促甲状腺激素释放激素（TRH）。CRH通过cAMP起作用，TRH刺激POMC的生物合成和POMC肽的释放。乙酰胆碱（ACh）由促黑素细胞产生，并对促黑素细胞的M1毒蕈碱受体起自激反馈作用以激活分泌活动。POMC肽的分泌由细胞内钙离子浓度（[Ca2+]i）的振荡驱动，这种振荡由受体介导的通过N型钙通道的钙离子内流刺激引发。下丘脑神经递质和ACh控制钙离子振荡活动。非洲爪蟾皮肤颜色适应调节中各种神经和内分泌步骤的结构和功能方面显示出高度的可塑性，使动物能够对外部生理适应需求做出最佳反应。
