Distributed Temporal Coding of Visual Memory Categories in Human Hippocampal Neurons Revealed by an Interpretable Decoding Model

Acid-sensing ion channels (ASICs) are emerging as fundamental players in the regulation of neural plasticity and in pathological conditions. Here we showed that lead (Pb2+), a well known neurotoxic metal ion, reversibly and concentration-dependently inhibited ASIC currents in the acutely dissociated spinal dorsal horn and hippocampal CA1 neurons of rats. In vitro expression of ASIC subunits in combination demonstrated that both ASIC1 and -3 subunits were sensitive to Pb2+. Mechanistically, Pb2+ reduced the pH sensitivity of ASICs independent of membrane voltage change. Moreover, Pb2+ inhibited the ASIC-mediated membrane depolarization and the elevation of intracellular Ca2+ concentration. In addition, we compared the effect of Pb2+ with that of Ca2+ or amiloride to explore the possible interactions of Pb2+ and Ca2+ in regulating ASICs, and we found that Pb2+ inhibited ASIC currents independent of the amiloride/Ca2+ blockade. Because ASIC1b and -3 subunits are mainly expressed in peripheral neurons, our data identified ASIC1a-containing Ca2+-permeable ASIC as a novel central target of Pb2+ action, which may contribute to Pb2+ neurotoxicity.

Oxygen and glucose deprivation (OGD) induces delayed cell death in hippocampal CA1 neurons via Ca2+/Zn2+-permeable, GluR2-lacking AMPA receptors (AMPARs). Following OGD, synaptic AMPAR currents in hippocampal neurons show marked inward rectification and increased sensitivity to channel blockers selective for GluR2-lacking AMPARs. This occurs via two mechanisms: a delayed down-regulation of GluR2 mRNA expression and a rapid internalization of GluR2-containing AMPARs during the OGD insult, which are replaced by GluR2-lacking receptors. The mechanisms that underlie this rapid change in subunit composition are unknown. Here, we demonstrate that this trafficking event shares features in common with events that mediate long term depression and long term potentiation and is initiated by the activation of N-methyl-d-aspartic acid receptors. Using biochemical and electrophysiological approaches, we show that peptides that interfere with PICK1 PDZ domain interactions block the OGD-induced switch in subunit composition, implicating PICK1 in restricting GluR2 from synapses during OGD. Furthermore, we show that GluR2-lacking AMPARs that arise at synapses during OGD as a result of PICK1 PDZ interactions are involved in OGD-induced delayed cell death. This work demonstrates that PICK1 plays a crucial role in the response to OGD that results in altered synaptic transmission and neuronal death and has implications for our understanding of the molecular mechanisms that underlie cell death during stroke.

The hyperpolarization-activated cation current, I(h), plays an important role in regulating intrinsic neuronal excitability in the brain. In hippocampal pyramidal neurons, I(h) is mediated by h channels comprised primarily of the hyperpolarization-activated cyclic nucleotide-gated (HCN) channel subunits, HCN1 and HCN2. Pyramidal neuron h channels within hippocampal area CA1 are remarkably enriched in distal apical dendrites, and this unique distribution pattern is critical for regulating dendritic excitability. We utilized biochemical and immunohistochemical approaches in organotypic slice cultures to explore factors that control h channel localization in dendrites. We found that distal dendritic enrichment of HCN1 is first detectable at postnatal day 13, reaching maximal enrichment by the 3rd postnatal week. Interestingly we found that an intact entorhinal cortex, which projects to distal dendrites of CA1 but not area CA3, is critical for the establishment and maintenance of distal dendritic enrichment of HCN1. Moreover blockade of excitatory neurotransmission using tetrodotoxin, 6-cyano-7-nitroquinoxaline-2,3-dione, or 2-aminophosphonovalerate redistributed HCN1 evenly throughout the dendrite without significant changes in protein expression levels. Inhibition of calcium/calmodulin-dependent protein kinase II activity, but not p38 MAPK, also redistributed HCN1 in CA1 pyramidal neurons. We conclude that activation of ionotropic glutamate receptors by excitatory temporoammonic pathway projections from the entorhinal cortex establishes and maintains the distribution pattern of HCN1 in CA1 pyramidal neuron dendrites by activating calcium/calmodulin-dependent protein kinase II-mediated downstream signals.

The hippocampus is involved in episodic memory, which is composed of subjective experiences in the multisensory world; however, little is known about the subthreshold membrane potential responses of individual hippocampal neurons to sensory stimuli. Using in-vivo whole-cell patch-clamp recordings from hippocampal CA1 neurons in awake mice, we found that almost all hippocampal neurons exhibited a hyperpolarization of 1-2 mV immediately after the onset of a sound. This large-scale hyperpolarization was unaffected by the duration or pitch of the tone. The response was abolished by general anesthesia and a surgical fimbria-fornix lesion.

A substantial number of human epidemiological data, as well as experimental studies, suggest that adverse maternal stress during gestation is involved in abnormal behavior, mental, and cognition disorder in offspring. To explore the effect of prenatal stress (PS) on hippocampal neurons, in this study, we observed the dendritic field of pyramidal neurons in hippocampal CA3, examined the concentration of glutamate (Glu), and detected the expression of synaptotagmin-1 (Syt-1) and N-methyl-D-aspartate receptor 1 (NR1) in hippocampus of juvenile female offspring rats. Pregnant rats were divided into two groups: control group (CON) and PS group. Female offspring rats used were 30-day old. The total length of the apical dendrites of pyramidal neurons in hippocampal CA3 of offspring was significantly shorter in PS than that in CON (p < 0.01). The number of branch points of the apical dendrites of pyramidal neurons in hippocampal CA3 of offspring was significantly less in PS (p < 0.01). PS offspring had a higher concentration of hippocampal Glu compared with CON (p < 0.05). PS offspring displayed increased expression of Syt-1 and decreased NR1 in hippocampus compared with CON (p < 0.001 and p < 0.01, respectively). The expression of NR1 in different hippocampus subfields of offspring was significantly decreased in PS than that in CON (p < 0.05-0.01). This study shows that PS increases the Glu in hippocampus and causes apical dendritic atrophy of pyramidal neurons of hippocampal CA3 in offspring rats. The decline of NR1 in hippocampus may be an adaptive response to the increased Glu.

The hippocampus is an essential hub for episodic memory processing. However, how human hippocampal single neurons code multi-element associations remains unknown. In particular, it is debated whether each hippocampal neuron represents an invariant element within an episode or whether single neurons bind together all the elements of a discrete episodic memory. Here we provide evidence for the latter hypothesis. Using single-neuron recordings from a total of 30 participants, we show that individual neurons, which we term episode-specific neurons, code discrete episodic memories using either a rate code or a temporal firing code. These neurons were observed exclusively in the hippocampus. Importantly, these episode-specific neurons do not reflect the coding of a particular element in the episode (that is, concept or time). Instead, they code for the conjunction of the different elements that make up the episode.

Acute p38-mediated inhibition of NMDA-induced outward currents in hippocampal CA1 neurons by interleukin-1β

Activation of mitochondrial KATP channels mediates neuroprotection induced by chronic morphine preconditioning in hippocampal CA-1 neurons following cerebral ischemia

Chapter 2: Cellular properties of human hippocampal CA1 neurons. This chapter investigates the structural and functional properties of human hippocampal CA1 neurons, highlighting adaptations that support advanced cognitive processes and how these differ from mice. Human CA1 neurons are significantly larger and more complex than previously understood, with dendritic lengths reaching up to a millimeter. These larger neurons possess a greater number of oblique dendrites, which function as independent processing units, thereby substantially enhancing memory capacity. Additionally, nearly all human CA1 neurons exhibit a resonance frequency closely aligned with low theta rhythms, critical for memory encoding and spatial navigation. In contrast, mouse CA1 neurons are structurally simpler and display resonance less frequently, with a less precise alignment to theta rhythms. This suggests that mouse neurons are adapted for survival-related tasks rather than complex cognitive functions. Furthermore, the memory capacity of human CA1 neurons was found to be 11 times higher, a difference largely attributed to the role of oblique dendrites in humans. These findings provide strong evidence of species-specific adaptations in hippocampal neurons, which underpin humans' advanced cognitive abilities. Chapter 3: Synaptic communication in human pyramidal neurons compared to rodents. This chapter examines how synaptic communication in human pyramidal neurons of the cortex differs from rodents. Human neurons exhibit synaptic conductance and voltage changes three times greater than those of mice, resulting in significantly stronger and more reliable synaptic transmission. Connected human neurons respond nearly 100% of the time, compared to a 25% failure rate observed in rodents. Pharmacological studies confirmed that NMDA receptors play a critical role in this enhanced synaptic strength. Blocking NMDA receptors significantly reduced excitatory postsynaptic potentials (EPSPs) in human neurons, while the effect was less pronounced in rodents. NMDA receptor activation prolongs EPSPs and supports long-term potentiation (LTP), a fundamental mechanism for learning and memory. Interestingly, the enhanced synaptic strength and reliability are not due to a higher number of synapses per connection, as both species have an average of five synapses per pair. Instead, human neurons exhibit larger presynaptic active zones and postsynaptic densities, which enhance neurotransmitter release and receptor binding. Additionally, the human cortex contains approximately three times more total synapses than the rodent cortex, facilitating more long-range cortical connections. These features support complex and integrated neural networks that underlie human-specific cognitive functions. These adaptations illustrate how the human brain has evolved to manage more sophisticated cognitive tasks than the rodent brain. Chapter 4: Regional variability in morpho-Electric properties of human neocortical pyramidal neurons. This chapter explores the regional variability of superficial and deep pyramidal neurons in layers 2 and 3 (L2/L3) of the human neocortex. Superficial neurons, primarily involved in feedback processing, and deep neurons, responsible for feedforward projections, exhibit distinct regional patterns along the rostro-caudal axis. Superficial L2/L3 neurons show a gradient where structural complexity decreases and excitability increases from the frontal to the occipital lobe. This suggests that neurons in the frontal cortex are optimized for higher-order cognitive functions, such as decision-making and working memory. Conversely, neurons in the occipital cortex, with higher excitability, are better suited for rapid sensory information processing. Deep L2/L3 neurons exhibit less pronounced structural and excitability gradients but show a marked increase in HCN channel activity, indicated by a higher sag ratio, along the same axis. This suggests that deep neurons play a key role in modulating cortical excitability and supporting long-distance communication between cortical regions. These findings demonstrate that superficial and deep L2/L3 pyramidal neurons follow distinct specialization gradients. The regional variability of these neurons enables the human neocortex to meet diverse computational demands across different cortical areas.

Objective To explore the therapeutic effects of docosahexaenoic acid(DHA) on depression, and its influence on the expression of caspase-3 in hippocampal neuron. Methods A total of 50 SD rats were randomly divided into 5 groups (control, model, fluoxetine, DHA, combination group), 10 rats in each group. SD rats were given different stimuli for 21 days to set up unforeseeable chronic stress depression model. After 21 days, the changes of behaviors in forced swimming test and morphology change in CA1, CA2 and CA3 in hippocampal neurons were observed. The change of caspase-3 in hippocampal neuron were detected by real-time PCR and western blot. Results Behavioral results showed that the swimming immobility time in depression model group, DHA group and combination group were respectively(113.33±9.50)s, (81.67±7.68)s and(73.00±8.54)s.Compared with depression model group, the DHA group and DHA combined with fluoxetine group obviously reduced the swimming immobility time of rats(t=8.164, 9.855, P<0.01). HE stain result showed DHA could partly recovered the number of hippocampal neurons in CA1, CA2 and CA3 and reduced apoptosis. qPCR and WB results showed DHA alone or DHA combined with fluoxetine could inhibit mRNA and protein level of caspase-3 in hippocampal neurons. Conclusion DHA can improve depression to a certain extent in a rat model of unforeseeable chronic stress depression, which might work through inhibition of neuron apoptosis and caspase-3 expression in hippocampal neurons. Key words: Docosahexaenoic acid; Depression; Hippocampal neuron; Apoptosis; Caspase-3

The resistance of CA3 neurons to ischaemia and the ischaemic tolerance conferred by ischaemic preconditioning (IPC) are two well-established endogenous neuroprotective mechanisms. Elucidating the molecules involved may help us find new therapeutic targets. Thus, we determined whether dynamin-related protein 1 (Drp-1) is involved in these processes. In vivo, we subjected rats to either 10min severe global ischaemia using a four-vessel occlusion (4-VO) model or 2min IPC before the onset of 4-VO. In vitro, we performed oxygen glucose deprivation (OGD) studies in rat hippocampal neurons. Drp-1 was silenced or inhibited by siRNA or pharmacological inhibitor Mdivi1. To assess whether mitochondrial Drp-1 alters neuronal vulnerability to ischaemic injury, various approaches were used including western blot, immunohistochemistry, immunofluorescence staining and electron microscopy. Hippocampal function was assessed using an open-field test. Mitochondrial dynamin-related protein 1 (mtDrp-1) was selectively induced by ischaemia in hippocampal CA3 neurons. In hippocampal CA1 neurons, mtDrp-1 was not affected by ischaemia but significantly up-regulated by IPC. Suppression of Drp-1 increased the vulnerability of cells to OGD and global ischaemia. Inhibition of Drp-1 in vivo resulted in loss of acquisition and encoding of spatial information, and also prevented ischaemia-induced mitophagy in CA3. Thus mitochondrial-mediated injury was amplified and resistance to ischaemic injury lost. Our findings that Drp-1 increases the resistance of neurons of hippocampal CA3 affected by global ischaemia and contributes to the tolerance conferred by IPC highlight Drp-1 as a potential therapeutic target for brain ischaemic stroke.

The protein-tyrosine kinase Src is known to potentiate the function of NMDA receptors, which is necessary for the induction of long-term potentiation in the hippocampus. With recombinant receptors composed of NR1-1a/NR2A or NR1-1a/2B subunits, Src reduces voltage-independent inhibition by the divalent cation Zn2+. Thereby the function of recombinant NMDA receptors is potentiated by Src only when the Zn2+ level is sufficient to cause tonic inhibition. Here we investigated whether the Src-induced potentiation of NMDA receptor function in neurons is caused by reducing voltage-independent Zn2+ inhibition. Whereas chelating extracellular Zn2+ blocked the Src-induced potentiation of NR1-1a/2A receptors, we found that Zn2+ chelation did not affect the potentiation of NMDA receptor (NMDAR) currents by Src applied into hippocampal CA1 or CA3 neurons. Moreover, Src did not alter the Zn2+ concentration-inhibition relationship for NMDAR currents in CA1 or CA3 neurons. Also, chelating extracellular Zn2+ did not prevent the upregulation of NMDA single-channel activity by endogenous Src in membrane patches from spinal dorsal horn neurons. Taking these results together we conclude that Src-induced potentiation of NMDAR currents is not mediated by reducing Zn2+ inhibition in hippocampal and dorsal horn neurons.

Activation of ATP-sensitive K channels protects hippocampal CA1 neurons from hypoxia by suppressing p53 expression

Mutations in the presenilin 1 (PS1) gene are responsible for the early onset of familial Alzheimer disease (FAD). Accumulating evidence shows that PS1 is involved in gamma-secretase activity and that FAD-associated mutations of PS1 commonly accelerate Abeta(1-42) production, which causes Alzheimer disease (AD). Recent studies suggest, however, that PS1 is involved not only in Abeta production but also in other processes that lead to neurodegeneration. To better understand the causes of neurodegeneration linked to the PS1 mutation, we analyzed the development of tau pathology, another key feature of AD, in PS1 knock-in mice. Hippocampal samples taken from FAD mutant (I213T) PS1 knock-in mice contained hyperphosphorylated tau that reacted with various phosphodependent tau antibodies and with Alz50, which recognizes the conformational change of PHF tau. Some neurons exhibited Congo red birefringence and Thioflavin T reactivity, both of which are histological criteria for neurofibrillary tangles (NFTs). Biochemical analysis of the samples revealed SDS-insoluble tau, which under electron microscopy examination, resembled tau fibrils. These results indicate that our mutant PS1 knock-in mice exhibited NFT-like tau pathology in the absence of Abeta deposition, suggesting that PS1 mutations contribute to the onset of AD not only by enhancing Abeta(1-42) production but by also accelerating the formation and accumulation of filamentous tau.

The hippocampus is crucial for forming associations between environmental stimuli. However, it is unclear how neural activities of hippocampal neurons dynamically change during the learning process. To address this question, we developed an associative memory task for rats with auditory stimuli. In this task, the rats were required to associate tone pitches (high and low) and ports (right and left) to obtain a reward. We recorded the firing activity of neurons in rats hippocampal CA1 during the learning process of the task. As a result, many hippocampal CA1 neurons increased their firing rates when the rats received a reward after choosing either the left or right port. We referred to these cells as “reward-direction cells.” Furthermore, the proportion of the reward-direction cells increased in the middle-stage of learning but decreased after the completion of learning. This result suggests that the activity of reward-direction cells might serve as “positive feedback” signal that facilitates the formation of associations between tone pitches and port choice.