Forming distinct representations and memories of multiple contexts and episodes is thought to be a crucial function of the hippocampal-entorhinal cortical network. The hippocampal dentate gyrus (DG) and CA3 are known to contribute to these functions, but the role of the entorhinal cortex (EC) is poorly understood. Here, we show that Ocean cells, excitatory stellate neurons in the medial EC layer II projecting into DG and CA3, rapidly form a distinct representation of a novel context and drive context-specific activation of downstream CA3 cells as well as context-specific fear memory. In contrast, Island cells, excitatory pyramidal neurons in the medial EC layer II projecting into CA1, are indifferent to context-specific encoding or memory. On the other hand, Ocean cells are dispensable for temporal association learning, for which Island cells are crucial. Together, the two excitatory medial EC layer II inputs to the hippocampus have complementary roles in episodic memory.
The brain codes continuous spatial, temporal, and sensory changes in daily experience. Recent studies suggest the brain also tracks experience as segmented subdivisions (events), but the neural basis for encoding events remains unclear. We designed a maze for mice composed of 4 materially indistinguishable lap events, and report hippocampal CA1 neurons whose activity is modulated not only by spatial location, but also lap number. These “event-specific rate remapping” (ESR) cells remain lap-specific even when the maze length is unpredictably altered within trials, suggesting ESR cells treat lap events as fundamental units. The activity pattern of ESR cells is reused to represent lap events when the maze geometry is altered from square to circle, suggesting it helps transfer knowledge between experiences. ESR activity is separately manipulable from spatial activity, and may therefore constitute an independent hippocampal code: an “event code” dedicated to organizing experience by events as discrete and transferable units.
Entorhinal-hippocampal circuits in the mammalian brain are crucial for an animal's spatial and episodic experience, but the neural basis for different spatial computations remain unknown. Medial entorhinal cortex layer II contains pyramidal island and stellate ocean cells. Here, we performed cell type-specific Ca 2+ imaging in freely exploring mice using cellular markers and a miniature head-mounted fluorescence microscope. We found that both oceans and islands contain grid cells in similar proportions, but island cell activity, including activity in a proportion of grid cells, is significantly more speed modulated than ocean cell activity. We speculate that this differential property reflects island cells' and ocean cells' contribution to different downstream functions: island cells may contribute more to spatial path integration, whereas ocean cells may facilitate contextual representation in downstream circuits.speed | grid cell | calcium imaging | entorhinal | hippocampus
Neuronal ensembles that hold specific memory (memory engrams) have been identified in the hippocampus, amygdala, or cortex. However, it has been hypothesized that engrams of a specific memory are distributed among multiple brain regions that are functionally connected, referred to as a unified engram complex. Here, we report a partial map of the engram complex for contextual fear conditioning memory by characterizing encoding activated neuronal ensembles in 247 regions using tissue phenotyping in mice. The mapping was aided by an engram index, which identified 117 cFos+ brain regions holding engrams with high probability, and brain-wide reactivation of these neuronal ensembles by recall. Optogenetic manipulation experiments revealed engram ensembles, many of which were functionally connected to hippocampal or amygdala engrams. Simultaneous chemogenetic reactivation of multiple engram ensembles conferred a greater level of memory recall than reactivation of a single engram ensemble, reflecting the natural memory recall process. Overall, our study supports the unified engram complex hypothesis for memory storage.
Reproducible high resolution spectra of nonconducting minerals of interest in biological calcification have been obtained by x-ray photoelectron spectroscopy, utilizing a technique we call biased referencing, a combination of gold decoration and electron charge neutralization of the samples. Minerals examined were hydroxyapatite [Ca10(PO4)6(OH)2], fluorapatite [Ca10(PO4)6F2], calcium carbonate (CaCO3), octacalcium phosphate [Ca8H2(PO4)6 ⋅ 5H2O], monetite (CaHPO4), brushite (CaHPO4 ⋅ 2H2O), calcium pyrophosphate (Ca2P2O7), and an amorphous calcium phosphate synthesized at neutral pH (Ca/P=1.47). Samples were pressed into thin wafers onto which a small gold dot was affixed by vacuum deposition. Biased referencing provided calibration of sample core level ionizations (O 1s, Ca 2p, P 2p, F 1s, and C 1s) to the Fermi level of Au 4f7/2 set at a conductive value of 84.0 eV. Measured binding energies represent initial data for these gold-decorated insulators and may be used to compare data similarly generated from mineralized biological tissues.
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