Synaptic mechanisms of amygdala-dependent behaviors

a）杏仁核中的抑制性突触传递 恐惧行为可以保护机体免受危险，但在精神疾病期间可能会变得适应不良。由于杏仁核负责控制恐惧行为，我们正试图了解它的突触特性如何与其恐惧调节功能相关。为了分析这种关系，我们进行了平行的行为和生理研究。在行为实验中，我们采用听觉恐惧条件反射作为恐惧学习的一个简单模型。在生理学研究中，我们分析了杏仁核的感觉输入的突触特性，这被认为是负责听觉恐惧条件反射。在过去的几年里，我们发现，前脑限制性敲除小GTTRap 1的小鼠在恐惧学习中受损，焦虑减少。使用全细胞记录，我们还发现，这种突变选择性地增强了皮层的突触传递，但不是在丘脑输入到外侧杏仁核。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。 在上一个财政年度，我们试图通过探索杏仁核抑制机制来解决这个问题。我们比较了杏仁核传入刺激如何影响兴奋性神经元和抑制性神经元的突触传递，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，在这样的刺激，主神经元的输入强烈抑制通过突触前GABAb受体，而输入中间神经元没有。负责这种优先招聘的GABAb受体的输入到主神经元的机制是差异释放突触外GABA的附近的主神经元和中间神经元。 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。我们未来的计划是研究这种机制是否会在类似于病理性恐惧状态的条件下受损。 B）塑造青少年发展时期特有的情感行为 在过去的几年中，我们发现，在小鼠杏仁核中的突触传递输入经历了晚期发育变化（突触可塑性的损失和增加的突触传递的功效，在丘脑输入杏仁核），这发生在出生后35天和出生后56天，时间对应于青春期的人类。青春期被认为是情感发展的关键时期。它的特点是增强的情绪反应和冒险行为。然而，青少年情绪化的动物模型尚未建立，这种现象背后的神经基质还没有找到。 在上一个财政年度，我们发现青少年时期丘脑发生了巨大的变化，但杏仁核的皮质输入没有发生变化。在此之后，我们试图确定一种啮齿动物的行为模式，这种模式可以模拟青少年情绪的增强，并且可能与杏仁核功能有关。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的变体中的行为。我们发现，幼年小鼠比成年小鼠具有更高水平的听觉恐惧条件反射泛化能力。这种泛化不是由对厌恶刺激的非联想敏感化引起的，也与条件刺激的新奇成分感知无关。此外，这种形式的恐惧概括的听觉刺激被取消明确的不成对的介绍，这种刺激与厌恶的无条件刺激，表明尽管较高的恐惧概括青少年拥有的机制，以避免非适应性的恐惧。鉴于丘脑-杏仁核输入的生理变化与恐惧泛化的行为变化之间的发育巧合，我们未来的工作集中在一个问题上，即丘脑-杏仁核输入的可塑性增强是否是可以解释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.