Neurobiology Laboratory, National Institute of Environmental Health Sciences, NIH, Research Triangle Park, NC 27709, United States.
Carolina Institute for Developmental Disabilities and Department of Psychiatry, University of North Carolina School of Medicine, Chapel Hill, NC 27599, United States.
Neurobiol Learn Mem. 2019 Sep;163:107044. doi: 10.1016/j.nlm.2019.107044. Epub 2019 Jul 15.
Activity of hippocampal pyramidal cells is critical for certain forms of learning and memory, and work from our lab and others has shown that CA2 neuronal activity is required for social cognition and behavior. Silencing of CA2 neurons in mice impairs social memory, and mice lacking Regulator of G-Protein Signaling 14 (RGS14), a protein that is highly enriched in CA2 neurons, learn faster than wild types in the Morris water maze spatial memory test. Although the enhanced spatial learning abilities of the RGS14 KO mice suggest a role for CA2 neurons in at least one hippocampus-dependent behavior, the role of CA2 neurons in fear conditioning, which requires activity of hippocampus, amygdala, and possibly prefrontal cortex is unknown. In this study, we expressed excitatory or inhibitory DREADDs in CA2 neurons and administered CNO before the shock-tone-context pairing. On subsequent days, we measured freezing behavior in the same context but without the tone (contextual fear) or in a new context but in the presence of the tone (cued fear). We found that increasing CA2 neuronal activity with excitatory DREADDs during training resulted in increased freezing during the cued fear tests in males and females. Surprisingly, we found that only females showed increased freezing during the contextual fear memory tests. Using inhibitory DREADDs, we found that inhibiting CA2 neuronal activity during the training phase also resulted in increased freezing in females during the subsequent contextual fear memory test. Finally, we tested fear conditioning in RGS14 KO mice and found that female KO mice had increased freezing on the cued fear memory test. These three separate lines of evidence suggest that CA2 neurons are actively involved in both intra- and extra-hippocampal brain processes and function to influence fear memory. Finally, the intriguing and consistent findings of enhanced fear conditioning only among females is strongly suggestive of a sexual dimorphism in CA2-linked circuits.
海马锥体神经元的活动对于某些形式的学习和记忆至关重要，我们实验室和其他实验室的工作表明，CA2 神经元活动对于社会认知和行为是必需的。在小鼠中沉默 CA2 神经元会损害社交记忆，而缺乏 G 蛋白信号调节因子 14（RGS14）的小鼠在 Morris 水迷宫空间记忆测试中比野生型小鼠学习更快，RGS14 是一种在 CA2 神经元中高度富集的蛋白质。虽然 RGS14 KO 小鼠增强的空间学习能力表明 CA2 神经元至少在一种海马依赖行为中起作用，但 CA2 神经元在恐惧条件反射中的作用是未知的，恐惧条件反射需要海马、杏仁核和可能的前额叶皮层的活动。在这项研究中，我们在 CA2 神经元中表达兴奋性或抑制性 DREADD，并在休克音-语境配对之前给予 CNO。在随后的几天里，我们在相同的语境中测量了冻结行为（语境恐惧）或在新的语境中但存在音（提示恐惧）。我们发现，在训练过程中用兴奋性 DREADD 增加 CA2 神经元的活动导致雄性和雌性的提示恐惧测试中的冻结增加。令人惊讶的是，我们发现只有雌性在语境恐惧记忆测试中表现出冻结增加。使用抑制性 DREADD，我们发现在训练阶段抑制 CA2 神经元的活动也导致雌性在随后的语境恐惧记忆测试中冻结增加。最后，我们在 RGS14 KO 小鼠中测试了恐惧条件反射，发现雌性 KO 小鼠在提示恐惧记忆测试中的冻结增加。这三个独立的证据表明，CA2 神经元积极参与海马内外的脑过程，并影响恐惧记忆。最后，增强的恐惧条件反射仅在女性中发现的有趣且一致的发现强烈表明 CA2 相关回路中存在性别二态性。
