Horseman N D, Buntin J D
Department of Molecular and Cellular Physiology, University of Cincinnati College of Medicine, Ohio 45267-0576, USA.
Annu Rev Nutr. 1995;15:213-38. doi: 10.1146/annurev.nu.15.070195.001241.
Prolactin stimulates the growth and development of specialized epithelial cells lining the cropsac of pigeons and doves (family Columbidae), leading to formation of "cropmilk," which is fed to the newly hatched squab. This system of milk feeding is unique among birds. To support the feeding of cropmilk, a complex array of behavioral adaptations are also supported by high levels of prolactin secretion in columbids during parenting. These specializations include elevated food intake (hyperphagia), nest attendance, and regurgitation feeding of the squab. Although prolactin is clearly important for these behavioral adaptations, the precise physiological and mechanistic bases for these behavioral effects remain controversial. The molecular mechanisms of prolactin action in the cropsac epithelium have been studied by cloning prolactin-induced genes, by cloning and expressing the pigeon prolactin receptor, and by analyzing the transcription factors that are activated after prolactin treatment. The avian (pigeon) prolactin receptor is a member of the cytokine receptor superfamily and uniquely contains a complete duplication of the extracellular ligand-binding domain. One of the early signal-transducing actions of prolactin in cropsac epithelium is the activation of signal transducer and activator of transcription (STAT) proteins via tyrosine phosphorylation. This fundamental signaling pathway is shared with mammalian prolactin target tissues. The convergent evolution of milk feeding and the behaviors that support parenting in columbids and mammals has depended on adaptation of both conserved mechanisms and divergent physiological processes.
催乳素刺激鸽形目（鸠鸽科）鸽子和鸠类嗉囊内衬的特化上皮细胞的生长和发育，促使“嗉囊乳”形成，嗉囊乳用于喂养刚孵化出的雏鸽。这种哺乳系统在鸟类中是独一无二的。为了支持嗉囊乳的喂养，育雏期间鸠鸽类动物高水平的催乳素分泌还支持了一系列复杂的行为适应性变化。这些特化包括食物摄入量增加（食欲亢进）、在巢中照料以及对雏鸽进行反刍喂养。虽然催乳素对这些行为适应性变化显然很重要，但这些行为效应的确切生理和机制基础仍存在争议。通过克隆催乳素诱导基因、克隆和表达鸽催乳素受体以及分析催乳素处理后被激活的转录因子，对催乳素在嗉囊上皮细胞中的作用分子机制进行了研究。禽类（鸽）催乳素受体是细胞因子受体超家族的成员，独特之处在于其细胞外配体结合域完全重复。催乳素在嗉囊上皮细胞中的早期信号转导作用之一是通过酪氨酸磷酸化激活信号转导子和转录激活子（STAT）蛋白。这一基本信号通路与哺乳动物催乳素靶组织相同。鸠鸽类和哺乳动物在哺乳以及支持育雏行为方面的趋同进化依赖于保守机制和不同生理过程的共同适应。
