Orexinergic projections to neocortex: potential role in arousal, stress and anxiety-related disorders.

压力加剧了许多精神疾病，反复的压力有助于疾病的发病机制，如创伤后应激障碍，恐慌症和重度抑郁症。哺乳动物的大脑皮层负责我们的高级认知功能，如语言、情景记忆和复杂的感知。它与其他各种结构相互作用，如丘脑，一大群神经元要么将感觉输入传递到皮层，要么通过这些神经元介导皮质-皮质相互作用。我们的大脑并不总是关注所有的细节，也不总是分析详细的背景。其状态受神经内分泌因子调节。这些都是小分子，比如食欲素（下丘脑分泌素）。食欲素只在外侧下丘脑的神经元中产生。食欲素系统对压力反应强烈，并调节许多生理过程，这些生理过程在压力相关的精神疾病中被改变，包括睡眠/觉醒模式、食欲和认知。据报道，在严重的抑郁症和焦虑症中，食欲素水平的变化，对食欲素敏感性的遗传缺陷（食欲素1受体多态性）与焦虑谱系障碍有关，特别是在女性中，她们患压力相关精神疾病的可能性是男性的两倍。食欲能神经元在整个中枢神经系统中具有广泛的投射目标，包括其他对调节觉醒、食欲和神经内分泌功能很重要的局部（下丘脑）神经元。然而，尽管它们可能参与调节皮层觉醒和对压力的认知反应，但对皮层回路的投射作用仍然知之甚少。因此，它们可能代表有希望的药物开发目标，选择性地针对皮层，而不是皮层下机制参与产生焦虑。解决食欲能神经元和皮层回路之间的解剖和功能连接，以及该系统中的性别差异，将是开始设计焦虑的食欲能治疗的关键。在感觉皮层中，6b层是唯一的食欲素敏感层。这些神经元可以使用Ctgf-/Drd1a-Cre转基因小鼠进行选择性标记，我们在研究这种细胞群方面有相当多的专业知识。我们证明在外侧下丘脑食欲素神经元和这些细胞之间有直接的联系。我们还证明了一些食欲素敏感层6b神经元选择性地投射到参与高级认知功能的丘脑核。因此，感觉层6b神经元可能作为一个食欲门控回路，通过皮质-丘脑-皮质回路放大反馈，在调节大脑状态和认知方面发挥重要作用。在我们的初步实验中，我们通过基因沉默了整个大脑皮层中选定的Drd1a-Cre神经元群体，并观察了这些Snap25 cKO小鼠的行为。当小鼠暴露在新环境中时，这种操作不会影响昼夜节律或运动活动。然而，在三种不同的行为任务中，它导致了焦虑行为的大幅减少。这表明，一些Drd1a-Cre神经元可能是大脑皮层调节情绪行为的关键组成部分。我们将研究i)食欲素神经元与其皮质靶点之间的功能连接，ii) Ctgf-/Drd1a-Cre神经元对应激和觉醒的生理反应，以及食欲素如何调节这一反应，以及ii)这些细胞在焦虑和恐惧学习中的作用。选择性操纵这些回路为干预焦虑提供了独特的治疗途径，同时不影响自主功能。我们提出的实验将剖析这些回路的关键成分、细胞类型和性别差异，并将测试可能在治疗中利用的分子机制。

Stress exacerbates many psychiatric conditions, and repeated stress contributes to the pathogenesis of disorders such as Post Traumatic Stress Disorder, Panic and Major Depressive Disorder. The mammalian cerebral cortex is responsible for our higher cognitive functions such as language, episodic memory, and complex perception. It interacts with various other structures, such as the thalamus, a large group of neurons that either relay sensory input to the cortex or mediate cortico-cortical interactions through these neurons. Our brain is not always paying attention to all details, and it does not always analyse detailed contexts. Its state is regulated by neuroendocrine factors. These are small molecules, such as orexin (hypocretin). Orexin is exclusively produced in neurons of the lateral hypothalamus. The orexin system is highly reactive to stress and regulates many physiological processes that are altered in stress-related mental illness, including sleep/wake patterns, appetite, and cognition. Changes in orexin levels have been reported in major depression and anxiety disorders, and genetic defects in the sensitivity to orexin (orexin 1 receptor polymorphism) have been associated with anxiety spectrum disorders, particularly in women who are twice as likely as men to suffer from stress-related mental illness. Orexinergic neurons have wide projection targets across the entire central nervous system, including to other local (hypothalamic) neurons that are important for modulating arousal, appetite, and neuroendocrine functions. However, the role of projections to cortical circuits remains less well understood, although they may be involved in regulating cortical arousal and the cognitive responses to stress. Thus, they could represent promising targets for drug development that selectively target cortical, but not subcortical mechanisms involved in generating anxiety. Resolving the anatomical and functional connectivity between orexinergic neurons and cortical circuits, as well as the gender differences in this system, will be critical for starting to design orexinergic treatments for anxiety. Within sensory cortex, layer 6b is the only orexin-sensitive layer. These neurons can be selectively labelled using Ctgf-/Drd1a-Cre transgenic mice, and we have very considerable expertise in studying this cell population. We demonstrated that there is a direct link between lateral hypothalamic orexin neurons and these cells. We also demonstrated that some of the orexin-sensitive layer 6b neurons selectively project to thalamic nuclei that are involved in higher cognitive functions. Sensory layer 6b neurons might therefore function as an orexin-gated circuit that amplifies feedback via cortico-thalamo-cortical loops and play an important role in regulating brain state and cognition. In our preliminary experiments we genetically silenced a selected population of Drd1a-Cre neurons across the entire cerebral cortex and observed the behaviour of these Snap25 cKO mice. This manipulation did not influence circadian rhythms or locomotor activity when mice were exposed to a novel environment. However, it led to a strong reduction in anxiety-like behaviour, as measured in three different behavioural tasks. This suggests that some of the Drd1a-Cre neurons may act as a key component in the cortex for regulating emotional behaviours. We shall examine i) functional connectivity between orexin neurons and their cortical targets, ii) physiological responses of Ctgf-/Drd1a-Cre neurons to stress and arousal, and how this is modulated by orexin, and ii) involvement of these cells in anxiety and fear learning. The selective manipulation of these circuits presents unique therapeutic avenues for the intervention against anxiety, while not influencing autonomous functions. Our proposed experiments will dissect key components, cell type and gender differences of these circuits and shall test the molecular mechanisms that could be exploited in therapy.