Department of Biological Science, Florida State University, Tallahassee, Florida, USA.
Program in Neuroscience, Florida State University, Tallahassee, Florida, USA.
J Comp Neurol. 2023 Oct;531(15):1510-1524. doi: 10.1002/cne.25518. Epub 2023 Jul 11.
Olfactory cues play a key role in natural behaviors such as finding food, finding mates, and avoiding predators. In principle, the ability of the olfactory system to carry out these perceptual functions would be facilitated by signaling related to an organism's physiological state. One candidate pathway includes a direct projection from the hypothalamus to the main olfactory bulb, the first stage of olfactory sensory processing. The pathway from the hypothalamus to the main olfactory bulb is thought to include neurons that express the neuropeptide orexin, although the proportion that is orexinergic remains unknown. A current model proposes that the orexin population is heterogeneous, yet it remains unknown whether the proportion that innervates the main olfactory bulb reflects a distinct subpopulation of the orexin population. Herein, we carried out combined retrograde tract tracing with immunohistochemistry for orexin-A in the mouse to define the proportion of hypothalamic input to the main olfactory bulb that is orexinergic and to determine what fraction of the orexin-A population innervates the bulb. The numbers and spatial positions of all retrogradely labeled neurons and all the orexin-A-expressing neurons were quantified in sequential sections through the hypothalamus. Retrogradely labeled neurons were found in the ipsilateral hypothalamus, of which 22% expressed orexin-A. The retrogradely labeled neurons that did and did not express orexin-A could be anatomically distinguished based on their spatial position and cell body area. Remarkably, only 7% of all the orexin-A neurons were retrogradely labeled, suggesting that only a small fraction of the orexin-A population directly innervate the main olfactory bulb. These neurons spatially overlapped with the orexin-A neurons that did not innervate the bulb, although the two cell populations were differentiated based on cell body area. Overall, these results support a model in which olfactory sensory processing is influenced by orexinergic feedback at the first synapse in the olfactory processing pathway.
嗅觉线索在寻找食物、寻找配偶和躲避捕食者等自然行为中起着关键作用。原则上，嗅觉系统执行这些感知功能的能力将通过与生物体生理状态相关的信号得到促进。一个候选途径包括从下丘脑到主嗅觉球的直接投射，这是嗅觉感觉处理的第一阶段。从下丘脑到主嗅觉球的途径被认为包括表达神经肽食欲素的神经元，尽管食欲素能神经元的比例尚不清楚。目前的模型提出，食欲素群体是异质的，但仍不清楚支配主嗅觉球的食欲素群体的比例是否反映了食欲素群体的一个独特亚群。在此，我们在小鼠中进行了逆行示踪与食欲素-A 免疫组织化学的联合研究，以确定下丘脑输入主嗅觉球的食欲素能比例，并确定支配嗅球的食欲素-A 群体的分数。在通过下丘脑的连续切片中定量了所有逆行标记神经元和所有食欲素-A 表达神经元的数量和空间位置。在同侧下丘脑中发现了逆行标记神经元，其中 22%表达食欲素-A。基于其空间位置和细胞体面积，可以在解剖学上区分表达和不表达食欲素-A 的逆行标记神经元。值得注意的是，只有 7%的所有食欲素-A 神经元被逆行标记，这表明只有一小部分食欲素-A 群体直接支配主嗅觉球。这些神经元在空间上与不支配嗅球的食欲素-A 神经元重叠，尽管这两个细胞群体基于细胞体面积而有所区分。总的来说，这些结果支持了一个模型，即嗅觉感觉处理受到嗅觉处理途径中第一突触的食欲素能反馈的影响。
