Synaptic mechanisms of amygdala-dependent behaviors

A)杏仁核内抑制性突触传递 用来保护机体免受危险伤害的恐惧行为，在精神疾病期间可能会变得不适应。由于杏仁核负责控制恐惧行为，我们正试图了解它的突触属性与其恐惧调节功能之间的关系。为了分析这种关系，我们进行了并行的行为和生理研究。在行为实验中，我们使用听觉恐惧条件反射作为恐惧学习的简单模型。在生理学研究中，我们分析了杏仁核感觉输入的突触特性，这被认为是听觉恐惧条件反射的原因。在前些年，我们发现，前脑受限的小GTPase Rap1基因敲除的小鼠在恐惧学习中受到损害，并减少了焦虑。利用全细胞记录，我们还发现，这种突变选择性地增强了皮质中的突触传递，但不增加了丘脑到杏仁外侧核的输入。这些发现导致了一个问题，即杏仁核输入中突触传递的增强如何抑制杏仁核功能。 在上一财年，我们试图通过探索杏仁核抑制的机制来解决这个问题。我们比较了刺激杏仁核传入对谷氨酸能神经元、兴奋性神经元和抑制性神经元的突触传递的影响，发现杏仁核中间神经元的输入比主神经元的输入更容易增强。此外，在这种刺激过程中，通过突触前GABAB受体对主神经元的输入受到强烈抑制，而对中间神经元的输入则不受抑制。主神经元和中间神经元附近突触外GABA的差异性释放是GABAB受体在主神经元输入时优先募集的机制。 通过这些发现，我们揭示了杏仁核内抑制优势的一种新机制。我们未来的计划是调查这种机制在类似于病态恐惧的情况下是否会受到损害。 B)针对青春期发展阶段的情绪行为进行建模 在前些年，我们发现在小鼠杏仁核输入的突触传递经历了发育后期的变化(丘脑输入杏仁核的突触可塑性丧失和突触传递效率增加)，这些变化发生在出生后第35天到出生后56天之间，这一时间对应于人类的青春期。青春期被认为是情绪发展的关键时期。它的特点是情绪反应和冒险行为增强。然而，青春期情绪性的动物模型还没有建立，这种现象背后的神经元底物也还没有找到。 早些时候，我们发现在青春期，丘脑发生了戏剧性的变化，但杏仁核的皮质输入没有发生变化。在上一财年，我们试图确定一种啮齿动物的行为范式，它将模拟青少年高度的情绪性，并可能与杏仁核的功能有关。为此，我们比较了成年和晚期幼年小鼠在巴甫洛夫恐惧条件反射变体中的行为。我们发现，幼年小鼠比成年小鼠具有更高的听觉恐惧条件反射泛化水平。这种概括不是由对厌恶刺激的非联想敏感化引起的，也与对条件刺激的新颖性成分的知觉无关。此外，这种形式的对听觉刺激的恐惧概括被取消了，因为这种刺激与厌恶的无条件刺激的呈现明显不成对，这表明尽管青少年有更高的恐惧概括，但他们拥有避免非适应性恐惧的机制。鉴于丘脑-杏仁核输入特性的生理变化与恐惧泛化的行为变化之间的发育一致性，未来的工作重点是一个问题，即丘脑输入杏仁核的可塑性增强是否是导致幼年小鼠恐惧泛化增加的突触机制。

a) Inhibitory synaptic transmission in amygdala Fear behaviors, which serve to protect the organism from dangers, can become maladaptive during mental illness. As amygdala is responsible for control of fear behaviors, we are trying to understand how its synaptic properties relate to its fear-regulating functions. To analyze this relationship we perform parallel behavioral and physiological studies. In the behavioral experiments we employ auditory fear conditioning as a simple model of fear learning. In the physiological studies, we analyze synaptic properties of the sensory inputs to amygdala, which are believed to be responsible for the auditory fear conditioning. In previous years we discovered that mice with the forebrain-restricted knockout of a small GTPase Rap1 are impaired in fear learning and have reduced anxiety. Using whole-cell recording, we also found that this mutation selectively enhanced synaptic transmission in the cortical, but not in the thalamic input to lateral amygdala. These findings resulted in a question as how enhancement of synaptic transmission in amygdala inputs could suppress amygdala function. During the last fiscal year we were trying to address this question by exploring mechanisms of amygdala inhibition. We compared how stimulation of amygdala afferents affects synaptic transmission in glutamatergic inputs to excitatory and to inhibitory neurons and found that the inputs to amygdala interneurons were potentiated much easier than the input to principle neurons. Furthermore, during such stimulation, the inputs to principle neurons were strongly inhibited via presynaptic GABAb-receptor, whereas inputs to interneurons were not. The mechanism responsible for this preferential recruitment of GABAb receptors on inputs to principle neurons was differential release of extrasynaptic GABA in the vicinity of principle neurons and interneurons. With these findings we revealed a novel mechanism that underlies inhibitory dominance inside amygdala. Our future plan is to investigate whether this mechanism becomes impaired during conditions that resemble pathological states of fear. b) Modeling emotional behaviors specific to the period of adolescent development In previous years we discovered that in mice synaptic transmission in the amygdala inputs undergoes late developmental changes (loss of synaptic plasticity and increase in the efficacy of synaptic transmission in the thalamic input to amygdala), which occur between postnatal day 35 and postnatal day 56, the time corresponding to adolescence in humans. Adolescence is considered a critical period for emotional development. It is characterized by enhanced emotional responses and risk-taking behaviors. Yet, animal models of adolescent emotionality have not been established and neuronal substrates underlying this phenomenon have not been found. Following our earlier finding of dramatic changes taking place in the thalamic, but not in the cortical input to amygdala during adolescence, during the last fiscal year, we were trying to identify a rodent behavioral paradigm which would model heightened adolescent emotionality and which could be linked to amygdala functions. To this end we compared behaviors of adult and late juvenile mice in variants of a Pavlovian fear conditioning. We discovered that juvenile mice had higher levels of generalization of auditory fear conditioning than their adult counterparts. This generalization was not caused by non-associative sensitization to aversive stimulation, neither was it related to the perception of the novelty component of conditional stimulus. Furthermore, this form of fear generalization to an auditory stimulus was abolished by explicitly unpaired presentation of this stimulus with the aversive unconditional stimulus, indicating that despite higher fear generalization adolescents possess mechanisms to avoid non-adaptive fear. Given the developmental coincidence between the physiological changes in the properties of the thalamo-amygdala input and behavioral changes in fear generalization, out future work is focused on a question as whether enhanced plasticity in the thalamic input to amygdala is the synaptic mechanism that can account for the increased fear generalization in juvenile mice.