Nixon Joshua P, Smale Laura
Department of Zoology, Michigan State University, 203 Natural Science Building, East Lansing, MI 48824-1115, USA.
Behav Brain Funct. 2007 Jun 13;3:28. doi: 10.1186/1744-9081-3-28.
The orexins (hypocretins) are a family of peptides found primarily in neurons in the lateral hypothalamus. Although the orexinergic system is generally thought to be the same across species, the orexins are involved in behaviors which show considerable interspecific variability. There are few direct cross-species comparisons of the distributions of cells and fibers containing these peptides. Here, we addressed the possibility that there might be important species differences by systematically examining and directly comparing the distribution of orexinergic neurons and fibers within the forebrains of species with very different patterns of sleep-wake behavior.
We compared the distribution of orexin-immunoreactive cell bodies and fibers in two nocturnal species (the lab rat, Rattus norvegicus and the golden hamster, Mesocricetus auratus) and two diurnal species (the Nile grass rat, Arvicanthis niloticus and the degu, Octodon degus). For each species, tissue from the olfactory bulbs through the brainstem was processed for immunoreactivity for orexin A and orexin B (hypocretin-1 and -2). The distribution of orexin-positive cells was noted for each species. Orexin fiber distribution and density was recorded and analyzed using a principal components factor analysis to aid in evaluating potential species differences.
Orexin-positive cells were observed in the lateral hypothalamic area of each species, though there were differences with respect to distribution within this region. In addition, cells positive for orexin A but not orexin B were observed in the paraventricular nucleus of the lab rat and grass rat, and in the supraoptic nucleus of the lab rat, grass rat and hamster. Although the overall distributions of orexin A and B fibers were similar in the four species, some striking differences were noted, especially in the lateral mammillary nucleus, ventromedial hypothalamic nucleus and flocculus.
The orexin cell and fiber distributions observed in this study were largely consistent with those described in previous studies. However, the present study shows significant species differences in the distribution of orexin cell bodies and in the density of orexin-IR fibers in some regions. Finally, we note previously undescribed populations of orexin-positive neurons outside the lateral hypothalamus in three of the four species examined.
食欲素（下丘脑泌素）是一类主要在下丘脑外侧神经元中发现的肽。尽管一般认为食欲素能系统在不同物种间是相同的，但食欲素参与的行为表现出相当大的种间变异性。关于含这些肽的细胞和纤维分布的直接跨物种比较很少。在此，我们通过系统检查和直接比较具有非常不同睡眠 - 觉醒行为模式的物种前脑内食欲素能神经元和纤维的分布，探讨可能存在重要物种差异的可能性。
我们比较了两种夜行性物种（实验室大鼠，褐家鼠和金黄仓鼠，金仓鼠）和两种昼行性物种（尼罗草鼠，尼罗多齿鼠和八齿鼠，八齿鼠）中食欲素免疫反应性细胞体和纤维的分布。对于每个物种，从嗅球到脑干的组织进行处理，以检测食欲素A和食欲素B（下丘脑泌素 -1和 -2）的免疫反应性。记录每个物种中食欲素阳性细胞的分布。使用主成分因子分析记录和分析食欲素纤维的分布和密度，以帮助评估潜在的物种差异。
在每个物种的下丘脑外侧区均观察到食欲素阳性细胞，尽管在该区域内的分布存在差异。此外，在实验室大鼠和草鼠的室旁核以及实验室大鼠、草鼠和仓鼠的视上核中观察到食欲素A阳性但食欲素B阴性的细胞。尽管四种物种中食欲素A和B纤维的总体分布相似，但仍注意到一些显著差异，特别是在外侧乳头体核、腹内侧下丘脑核和绒球。
本研究中观察到的食欲素细胞和纤维分布在很大程度上与先前研究中描述的一致。然而，本研究表明在食欲素细胞体分布以及某些区域中食欲素免疫反应纤维密度方面存在显著的物种差异。最后，我们注意到在所检查的四个物种中的三个物种的下丘脑外侧区之外存在先前未描述的食欲素阳性神经元群体。
