What inhibits the inhibitors? In the hippocampus, each memory trace is encoded by a specific subset of pyramidal cells. The other pyramidal cells must be actively excluded from the memory encoding process by inhibition, which is done by selective dendrite-targeting interneurons. Szőnyi et al. found that γ-aminobutyric acid–releasing (GABAergic) cells located in a small region in the brain stem called the nucleus incertus project to the hippocampus. The nucleus incertus again is innervated by several regions that respond to salient stimuli. Its GABAergic cells preferentially inhibit the dendrite-targeting interneurons in the hippocampus. The nucleus incertus is thus a central mediator between brain regions that are highly responsive to salient stimuli and the hippocampal circuitry involved in memory formation. Science, this issue p. eaaw0445 A brainstem regulatory mechanism for the selection of hippocampal neuronal assemblies during contextual learning is described. INTRODUCTION Associative learning is essential for survival, and the mammalian hippocampal neurocircuitry has been shown to play a central role in the formation of specific contextual memories. Contrary to the slow, neuromodulatory role commonly associated with brainstem systems, we discovered a highly specific, spatiotemporally precise, inhibitory ascending brainstem pathway that effectively controls hippocampal fear memory formation. Pyramidal neurons of the dorsal hippocampus CA1 region pair multisensory contextual information (see the figure, panel A, CA3) with direct sensory-related inputs (see the figure, panel A, EntCx). Each memory trace is encoded by a specific subset of pyramidal neurons. Remaining pyramidal cells must be actively excluded from the given memory-encoding process by direct dendritic inhibition, which is executed by somatostatin-positive (SOM) dendrite-targeting interneurons. SOM interneurons are activated by excitatory inputs from the medial septum (MS) upon salient environmental stimuli. Previous models suggested that the subset of memory-forming pyramidal cells escape this dendritic inhibition only by a stochastic, self-regulatory process, in which some SOM interneurons become inactive. However, we hypothesized that this process must be regulated more actively, and SOM interneurons should be inhibited precisely in time, on the basis of subcortical information; otherwise, underrecruitment of pyramidal neurons would lead to unstable memory formation. RATIONALE γ-aminobutyric acid (GABA)–releasing inhibitory neurons of the brainstem nucleus incertus (NI) seemed well suited to counterbalance the activation of SOM interneurons, as they specifically project to the stratum oriens of the hippocampus, where most SOM cells arborize. Using cell type–specific neuronal tract tracing, immunoelectron microscopy, and electrophysiological methods, we investigated the targets of NI in the mouse hippocampus, and in the MS, where excitation of SOM cells originates. We also used monosynaptic rabies tracing to identify the inputs of GABAergic NI neurons. Two-photon calcium imaging was used to analyze the response of GABAergic NI fibers to sensory stimuli in vivo. Finally, we used in vivo optogenetics combined with behavioral experiments or electrophysiological recordings to explore the role of the NI in contextual memory formation and hippocampal network activity. RESULTS We discovered that NI GABAergic neurons selectively inhibit hippocampal SOM interneurons in the stratum oriens both directly and also indirectly through inhibition of excitatory neurons in the MS (see the figure, panels A and B). We observed that NI GABAergic neurons receive direct inputs from several brain areas that process salient environmental stimuli, including the prefrontal cortex and lateral habenula, and that these salient sensory stimuli (e.g., air puffs, water rewards) rapidly activated hippocampal fibers of NI GABAergic neurons in vivo. Behavioral experiments revealed that optogenetic stimulation of NI GABAergic neurons or their fibers in hippocampus, precisely at the moment of aversive stimuli (see the figure, panel C), prevented the formation of fear memories, whereas this effect was absent if light stimulation was not aligned with the stimuli. However, optogenetic inhibition of NI GABAergic neurons during fear conditioning resulted in the formation of excessively enhanced contextual memories. Optogenetic stimulation of NI GABAergic neurons also changed memory encoding–related hippocampal theta rhythms. CONCLUSION A role of NI GABAergic neurons may be fine-tuning of the selection of memory-encoding pyramidal cells, on the basis of the relevance and/or modality of environmental inputs. They may also help filter nonrelevant everyday experiences (e.g., those to which animals have already accommodated), by regulating the sparsity of memory-encoding dorsal CA1 pyramidal neurons. NI GABAergic neuron dysfunction may also contribute to dementia-like disorders or pathological memory formation in certain types of anxiety or stress disorders. Our data represent an unexpectedly specific role of an ascending inhibitory pathway from a brainstem nucleus in memory encoding. Nucleus incertus (NI) activation prevents memory formation. NI GABAergic neurons regulate contextual memory formation by inhibiting somatostatin interneurons (SOM IN) directly in hippocampus (HIPP) (A) and indirectly through inhibition of their excitatory inputs in the medial septum (MS). Pairing optical stimulation (B) with aversive stimuli (C) eliminates fear memory formation, whereas control mice display normal fear (freezing) after exposure to the same environment a day later (D). Hippocampal pyramidal cells encode memory engrams, which guide adaptive behavior. Selection of engram-forming cells is regulated by somatostatin-positive dendrite-targeting interneurons, which inhibit pyramidal cells that are not required for memory formation. Here, we found that γ-aminobutyric acid (GABA)–releasing neurons of the mouse nucleus incertus (NI) selectively inhibit somatostatin-positive interneurons in the hippocampus, both monosynaptically and indirectly through the inhibition of their subcortical excitatory inputs. We demonstrated that NI GABAergic neurons receive monosynaptic inputs from brain areas processing important environmental information, and their hippocampal projections are strongly activated by salient environmental inputs in vivo. Optogenetic manipulations of NI GABAergic neurons can shift hippocampal network state and bidirectionally modify the strength of contextual fear memory formation. Our results indicate that brainstem NI GABAergic cells are essential for controlling contextual memories.

中文翻译：

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脑干未定核控制情境记忆的形成


是什么抑制了抑制剂？在海马体中，每个记忆痕迹都是由特定的锥体细胞子集编码的。其他锥体细胞必须通过抑制主动排除在记忆编码过程之外，这是由选择性树突靶向中间神经元完成的。 Szőnyi 等人。发现释放γ-氨基丁酸（GABAergic）的细胞位于脑干中一个称为未定核的小区域，投射到海马体。不定核再次受到对显着刺激做出反应的几个区域的神经支配。其 GABA 能细胞优先抑制海马中树突靶向的中间神经元。因此，不定核是对显着刺激高度敏感的大脑区域和参与记忆形成的海马回路之间的中心调节器。科学，本期第 14 页。 eaaw0445 描述了在情境学习期间选择海马神经元组件的脑干调节机制。引言 联想学习对于生存至关重要，哺乳动物海马神经回路已被证明在特定情境记忆的形成中发挥着核心作用。与脑干系统通常相关的缓慢的神经调节作用相反，我们发现了一种高度特异性、时空精确的抑制性上升脑干通路，可以有效控制海马恐惧记忆的形成。背侧海马 CA1 区域的锥体神经元将多感觉上下文信息（见图，A 图，CA3）与直接感觉相关输入（见图，A 图，EntCx）配对。每个记忆痕迹都由锥体神经元的特定子集编码。 剩余的锥体细胞必须通过直接树突抑制主动从给定的记忆编码过程中排除，这是由生长抑素阳性（SOM）树突靶向中间神经元执行的。 SOM 中间神经元在显着的环境刺激下被内侧隔膜 (MS) 的兴奋性输入激活。先前的模型表明，形成记忆的锥体细胞亚群只能通过随机的自我调节过程来逃避这种树突抑制，在这个过程中，一些 SOM 中间神经元变得不活跃。然而，我们假设这个过程必须受到更积极的调节，并且基于皮层下信息，SOM中间神经元应该被及时精确地抑制；否则，锥体神经元募集不足将导致记忆形成不稳定。 γ-氨基丁酸 (GABA) 释放脑干不定核 (NI) 的抑制性神经元似乎非常适合平衡 SOM 中间神经元的激活，因为它们专门投射到海马体的方向层，大多数 SOM 细胞在那里形成树状结构。使用细胞类型特异性神经元束示踪、免疫电子显微镜和电生理学方法，我们研究了小鼠海马体和 MS 中 NI 的靶标，其中 SOM 细胞的兴奋源自此处。我们还使用单突触狂犬病追踪来识别 GABA NI 神经元的输入。双光子钙成像用于分析 GABAergic NI 纤维对体内感觉刺激的反应。最后，我们利用体内光遗传学结合行为实验或电生理记录来探索 NI 在情境记忆形成和海马网络活动中的作用。 结果我们发现，NI GABA 能神经元可直接或间接通过抑制 MS 中的兴奋性神经元来选择性抑制海马 SOM 中间神经元（见图 A 和 B）。我们观察到 NI GABA 能神经元接收来自处理显着环境刺激的多个大脑区域的直接输入，包括前额皮质和外侧缰核，并且这些显着感觉刺激（例如，吹气、水奖励）快速激活 NI GABA 能神经元的海马纤维体内。行为实验表明，恰好在厌恶刺激的时刻，对海马中的 NI GABA 能神经元或其纤维进行光遗传学刺激（见图 C），可以阻止恐惧记忆的形成，而如果光刺激未对齐，则不会产生这种效果随着刺激。然而，在恐惧调节过程中，NI GABA能神经元的光遗传学抑制导致了过度增强的情境记忆的形成。 NI GABA能神经元的光遗传学刺激也改变了与记忆编码相关的海马θ节律。结论 NI GABA 能神经元的作用可能是根据环境输入的相关性和/或形态微调记忆编码锥体细胞的选择。它们还可以通过调节记忆编码背侧 CA1 锥体神经元的稀疏性来帮助过滤不相关的日常经历（例如，动物已经适应的那些经历）。 NI GABA能神经元功能障碍也可能导致痴呆样疾病或某些类型的焦虑或应激障碍中的病理性记忆形成。 我们的数据代表了脑干核的上行抑制通路在记忆编码中的出人意料的特定作用。不定核 (NI) 激活会阻止记忆形成。 NI GABA 能神经元通过直接抑制海马 (HIPP) 中的生长抑素中间神经元 (SOM IN) (A) 和间接通过抑制内侧隔膜 (MS) 中的兴奋性输入来调节情境记忆形成。将光刺激（B）与厌恶刺激（C）配对可以消除恐惧记忆的形成，而对照小鼠在一天后暴露于相同环境后表现出正常的恐惧（冻结）（D）。海马锥体细胞编码记忆印迹，指导适应性行为。印迹形成细胞的选择受到生长抑素阳性树突靶向中间神经元的调节，该中间神经元抑制记忆形成所需的锥体细胞。在这里，我们发现，小鼠不定核（NI）释放γ-氨基丁酸（GABA）的神经元选择性地抑制海马中的生长抑素阳性中间神经元，既单突触又通过抑制皮层下兴奋性输入间接抑制。我们证明NI GABA能神经元从处理重要环境信息的大脑区域接收单突触输入，并且它们的海马投射被体内显着环境输入强烈激活。 NI GABA能神经元的光遗传学操作可以改变海马网络状态并双向改变情境恐惧记忆形成的强度。我们的结果表明，脑干 NI GABA 能细胞对于控制情境记忆至关重要。