. Brainstem nucleus incertus controls contextual memory formation. Science. 2019 May 24;364(6442) PubMed.


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  1. This study adds more evidence to the fascinating role of evolutionarily ancient brainstem nuclei, such as the locus coeruleus and the nucleus incertus in higher-order cognitive functions. The nucleus incertus consists of mostly GABAergic projection neurons, the main inhibitory neurotransmitter in the brain, and modulates hippocampal theta rhythms.

    The observation of Szonyi and colleagues that optogenetic stimulation of GABAerigc neurons in the nucleus incertus during learning was associated with reduced memory formation is consistent with previous work showing that the GABAergic agonist muscimol impairs memory, but the GABAergic anatagonist picrotoxin enhances memory (McGaugh, 1989). Interestingly, locus coeruleus hyperactivity has also been related to neurodegenerative diseases, as a greater turnover of norepinephrine was associated with increases in clinical symptoms (Weinshenker 2018; Sheline et al., 1998; Jacobs et al., 2019) and increases in tau burden (Jacobs et al., 2019).

    An intriguing question is how these associations between hyperactivity in brainstem nuclei and memory performance are connected to the pathogenesis of Alzheimer’s disease. Increased excitability of neurons has indeed been associated with tau accumulation (de Calignon et al., 2012), and therefore it is plausible to assume a link between both processes, though causality remains to be established.


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    View all comments by Heidi Jacobs
  2. This is a beautiful and exciting study showing conclusively that brainstem nucleus incertus (NI) GABAergic neurons control hippocampal-dependent contextual memory formation by inhibiting hippocampal somatostatin (SOM) interneurons in the stratum orients, both directly and indirectly through inhibition of excitatory neurons in the medial septum (MS). The study represents a great showcase of applying multidisciplinary approaches, including cell-type-specific neuronal tract tracing, immunogold receptor localization with electron microscope, electrophysiology, behavioral test, and optogenetic regulation, to complementarily address a key scientific question.

    The study suggests, as the authors concluded, that a role of NI GABAergic neurons may be fine-tuning of the selection of memory-encoding pyramidal cells in the CA1 of the hippocampus, on the basis of the relevance and/or modality of environmental inputs. The NI GABAergic neurons may also help filter non-relevant everyday experiences, such as those to which animals have already accommodated, by regulating the sparsity of memory-encoding CA1 pyramidal neurons. Thus, it is conceivable that dysfunction of NI GABAergic neurons may contribute to memory deficit, as seen Alzheimer’s disease.

    In this regard, we have previously shown that human ApoE4, the major genetic risk factor for late-onset Alzheimer’s disease, induces a SOM interneuron deficit in the hippocampus of mice in an age-dependent manner (Andrews-Zwilling et al., 2010; Leung et al., 2012) and in induced pluripotent stem cell (iPSC)-derived human neuron cultures (Wang et al., 2018). Importantly, the ApoE4-induced SOM interneuron deficit is also associated with alterations of the hippocampal network activity and correlate with spatial learning and memory impairments in aged mice (Gillespie et al., 2016; Andrews-Zwilling et al., 2010). Thus, dysfunction or loss of neurons along the NI GABAergic neuron-hippocampal SOM interneuron-axis (or the NI GABAergic neuron-MS excitatory neuron-hippocampal SOM interneuron-axis) may contribute to memory impairment in Alzheimer’s disease.

    Interestingly, as shown in this study, NI GABAergic neurons project to SOM interneurons in both the CA1 and the hilus of the dentate gyrus in mouse hippocampus. It would be worthwhile to also study the potential regulatory role of NI GABAergic neuron on hilar SOM interneurons and its implication in memory process and formation.


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    View all comments by Yadong Huang
  3. Arousal depends on the activation of ascending neural pathways originating in the brainstem (pons and medulla). The nucleus incertus (NI) is a conserved area within the pontine periventricular region that consists of ascending GABAergic projection neurons and glutamatergic neurons. These GABAergic neurons disinhibit and synchronize the activity of their forebrain targets, promoting the rhythms typical of awake conscious states. Theta and gamma rhythms are among these oscillations, and these rhythms are thought to play a role in the encoding phase of learning and memory function.

    This paper effectively demonstrates that optogenetic stimulation of NI GABAergic neurons decreases hippocampal theta power in vivo. By contrast, optogenetic inhibition of NI GABAergic neurons during fear conditioning was shown by these researchers to enhance contextual memory formation. These findings demonstrate the importance of NI GABAergic neurons in hippocampus-dependent fear-memory formation implicated in many anxiety disorders such as phobias and PTSD. But this work sheds little light on the formation of episodic memories associated with less-stressful life events, such as place field formation during exploration of a novel environment or performance on a novel-object-recognition task following a delay, both of which may be less dependent upon activation of the NI.   

    The NI is also the primary site of Relaxin‐3 neurons.  It has previously been shown that corticotropin-releasing factor (CRF) depolarizes NI cells. The spontaneous firing of Relaxin-3 but not non- Relaxin‐3 neurons was shown by other investigators to be strongly modulated and phase-locked, with the initial ascending phase of hippocampal theta oscillations via postsynaptic CRFR1 in a long-lasting and non-desensitizing manner (Ma et al., 2013). NI is thus an important site for CRF modulation of hippocampal theta rhythm via effects on GABA/Relaxin‐3  transmission.

    There is a growing body of literature that suggests that AD neuropathology begins with dysregulation of function within the brainstem and that this, in turn, plays a role in the earliest stages of AD. The locus coeruleus (LC, also located in the pons) with its ascending adrenergic projections has also been implicated in AD.  Activation of the LC has been shown to suppresses feed-forward interneurons, and to enhance theta activity in rat dentate gyrus (Brown et al., 2005).  

    The LC is also one of the brain regions shown to be the most vulnerable to the occurrence of age-related tauopathy.  While it is known that dogs deprived of sleep develop tau pathology, and that stress can disturb sleep, it is nevertheless unclear to this reviewer how disruption of NI function may relate to dementia or AD neuropathology per se based on the data presented in this paper. 

    It will be important to elucidate the role of the NI in tauopathy using relevant animal models of aMCI or AD.  Increased tau phosphorylation and aggregation has been reported in the hippocampi of mice overexpressing corticotropin-releasing factor and so, dysregulation of function within the pons may eventually prove to be the missing link between neural circuit function and this specific aspect of AD neuropathology. 

    That said, there is also an emerging body of literature implicating stress and arousal networks in sleep dysregulation and a similarly impressive body of literature implicating sleep dysregulation in memory deficits and AD. More work will be needed to parse out the nature of these complex relationships.


    . Heterogeneous responses of nucleus incertus neurons to corticotrophin-releasing factor and coherent activity with hippocampal theta rhythm in the rat. J Physiol. 2013 Aug 15;591(16):3981-4001. Epub 2013 May 13 PubMed.

    . Locus ceruleus activation suppresses feedforward interneurons and reduces beta-gamma electroencephalogram frequencies while it enhances theta frequencies in rat dentate gyrus. J Neurosci. 2005 Feb 23;25(8):1985-91. PubMed.

    View all comments by Marcia Ratner

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  1. Brainstem GABAergic Inputs to Hippocampus Control Memory Formation