Cells in the ventral tegmental area (VTA) facilitate motivated behaviors, and the activity of VTA neurons is regulated by dense projections from the lateral hypothalamic area (LHA). Orexin (Orx) neurons in the lateral and perifornical hypothalamus play important roles in arousal, feeding, and energy metabolism. Orx cells contribute substantially to the LHA projection to the rat midbrain. However, the morphological features of Orx fibers in the VTA and whether they synapse onto dopamine (DA) or gamma-aminobutyric acid (GABA) neurons have not yet been investigated. We utilized immunoperoxidase and immunogold-silver staining to examine the morphological features and synaptic incidence of Orx-labeled axons in the VTA. We then combined immunoperoxidase labeling for Orx with immunogold-silver labeling for GABA or for tyrosine hydroxylase (TH) in DA neurons. Electron microscopic analysis revealed that most Orx-labeled axons in the VTA were passing fibers. The less common Orx varicosities were occasionally apposed to TH- or GABA-labeled dendrites without synapsing. Only a small proportion of Orx-positive axons synapsed onto dendrites or soma. The synapses included both asymmetric and symmetric types and targeted TH- and GABA-labeled profiles with equal frequency. These findings suggest that most Orx fibers in the VTA are axons passing to caudal brainstem structures. However, Orx does mediate some direct synaptic influence on VTA DA and GABA neurons. Additional nonsynaptic effects are suggested by the presence of numerous dense-cored vesicles. These studies have important implications for understanding the mechanisms whereby Orx can alter behavior through regulating VTA DA and GABA cell activity.

译文

:腹侧被盖区(VTA)中的细胞有助于激发行为,而VTA神经元的活动受下丘脑外侧区(LHA)的密集投射所调节。下丘脑外侧和下丘脑中的食欲素(Orx)神经元在唤醒,进食和能量代谢中起重要作用。 Orx细胞在很大程度上促进了LHA投射到大鼠中脑。但是,尚未研究VTA中Orx纤维的形态特征以及它们是否突触到多巴胺(DA)或γ-氨基丁酸(GABA)神经元上。我们利用免疫过氧化物酶和免疫金银染色来检查VTA中Orx标记的轴突的形态特征和突触发生率。然后,我们在DA神经元中结合了Orx的免疫过氧化物酶标记和GABA或酪氨酸羟化酶(TH)的免疫金银标记。电子显微镜分析显示,VTA中大多数Orx标记的轴突都通过了纤维。不太常见的Orx静脉曲张偶尔与TH或GABA标记的树突并置而不发生突触。只有一小部分的Orx阳性轴突突触到树突或躯体上。突触包括不对称和对称类型以及具有相同频率的靶向TH和GABA标记的图谱。这些发现表明,VTA中的大多数Orx纤维都是传递至尾脑干结构的轴突。但是,Orx确实介导了对VTA DA和GABA神经元的一些直接突触影响。由于存在许多致密囊泡,提示了其他非突触作用。这些研究对于理解Orx可以通过调节VTA DA和GABA细胞活性来改变行为的机制具有重要意义。

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