The temporoammonic input to the hippocampal CA1 region displays distinctly different synaptic plasticity compared to the Schaffer collateral input in vivo: significance for synaptic information processing

Interplay of hippocampal long-term potentiation and long-term depression in enabling memory representations

Hippocampal long-term potentiation (LTP) and long-term depression (LTD) are Hebbian forms of synaptic plasticity that are widely believed to comprise the physiological correlates of associative learning. They comprise a persistent, input-specific increase or decrease, respectively, in synaptic efficacy that, in rodents, can be followed for days and weeks in vivo. Persistent (>24 h) LTP and LTD exhibit distinct frequency-dependencies and molecular profiles in the hippocampal subfields. Moreover, causal and genetic studies in behaving rodents indicate that both LTP and LTD fulfil specific and complementary roles in the acquisition and retention of spatial memory. LTP is likely to be responsible for the generation of a record of spatial experience, which may serve as an associative schema that can be re-used to expedite or facilitate subsequent learning. In contrast, LTD may enable modification and dynamic updating of this representation, such that detailed spatial content information is included and the schema is rendered unique and distinguishable from other similar representations. Together, LTP and LTD engage in a dynamic interplay that supports the generation of complex associative memories that are resistant to generalization. This article is part of a discussion meeting issue 'Long-term potentiation: 50 years on'.

Morphological and Functional Alterations in the CA1 Pyramidal Neurons of the Rat Hippocampus in the Chronic Phase of the Lithium-Pilocarpine Model of Epilepsy

Epilepsy is known to cause alterations in neural networks. However, many details of these changes remain poorly understood. The objective of this study was to investigate changes in the properties of hippocampal CA1 pyramidal neurons and their synaptic inputs in a rat lithium-pilocarpine model of epilepsy. In the chronic phase of the model, we found a marked loss of pyramidal neurons in the CA1 area. However, the membrane properties of the neurons remained essentially unaltered. The results of the electrophysiological and morphological studies indicate that the direct pathway from the entorhinal cortex to CA1 neurons is reinforced in epileptic animals, whereas the inputs to them from CA3 are either unaltered or even diminished. In particular, the dendritic spine density in the str. lacunosum moleculare, where the direct pathway from the entorhinal cortex terminates, was found to be 2.5 times higher in epileptic rats than in control rats. Furthermore, the summation of responses upon stimulation of the temporoammonic pathway was enhanced by approximately twofold in epileptic rats. This enhancement is believed to be a significant contributing factor to the heightened epileptic activity observed in the entorhinal cortex of epileptic rats using an ex vivo 4-aminopyridine model.

Shedding light on cholecystokinin's role in hippocampal neuroplasticity and memory formation

The hippocampus is a crucial brain region involved in the process of forming and consolidating memories. Memories are consolidated in the brain through synaptic plasticity, and a key mechanism underlying this process is called long-term potentiation (LTP). Recent research has shown that cholecystokinin (CCK) plays a role in facilitating the formation of LTP, as well as learning and memory consolidation. However, the specific mechanisms by which CCK is involved in hippocampal neuroplasticity and memory formation are complicated or poorly understood. This literature review aims to explore the role of LTP in memory formation, particularly in relation to hippocampal memory, and to discuss the implications of CCK and its receptors in the formation of hippocampal memories. Additionally, we will examine the circuitry of CCK in the hippocampus and propose potential CCK-dependent mechanisms of synaptic plasticity that contribute to memory formation.

Spatially Resolved Transcriptomic Signatures of Hippocampal Subregions and Arc-Expressing Ensembles in Active Place Avoidance Memory

The rodent hippocampus is a spatially organized neuronal network that supports the formation of spatial and episodic memories. We conducted bulk RNA sequencing and spatial transcriptomics experiments to measure gene expression changes in the dorsal hippocampus following the recall of active place avoidance (APA) memory. Through bulk RNA sequencing, we examined the gene expression changes following memory recall across the functionally distinct subregions of the dorsal hippocampus. We found that recall induced differentially expressed genes (DEGs) in the CA1 and CA3 hippocampal subregions were enriched with genes involved in synaptic transmission and synaptic plasticity, while DEGs in the dentate gyrus (DG) were enriched with genes involved in energy balance and ribosomal function. Through spatial transcriptomics, we examined gene expression changes following memory recall across an array of spots encompassing putative memory-associated neuronal ensembles marked by the expression of the IEGs Arc, Egr1, and c-Jun. Within samples from both trained and untrained mice, the subpopulations of spatial transcriptomic spots marked by these IEGs were transcriptomically and spatially distinct from one another. DEGs detected between Arc+ and Arc- spots exclusively in the trained mouse were enriched in several memory-related gene ontology terms, including "regulation of synaptic plasticity" and "memory." Our results suggest that APA memory recall is supported by regionalized transcriptomic profiles separating the CA1 and CA3 from the DG, transcriptionally and spatially distinct IEG expressing spatial transcriptomic spots, and biological processes related to synaptic plasticity as a defining the difference between Arc+ and Arc- spatial transcriptomic spots.

Long-term depression induction and maintenance across regions of the apical branch of CA1 dendrites

Well known as the center for learning and memory, hippocampus is the crucial brain region to study synaptic plasticity in the context of cellular fundamental mechanisms such as long-term depression (LTD) and long-term potentiation (LTP). However, despite years of extensive research, the key to our LTD queries and their induction mechanisms has not been fully understood. Previously, we reported the induction of late-LTD (L-LTD) in the distally located synapses of apical branch of hippocampal CA1 dendrites using strong low-frequency stimulation (SLFS). In contrast synapses at the proximal site could not express L-LTD. Thus, in the present study, we wanted to investigate whether or not synapses of apical dendritic branch at the proximal location could induce and maintain LTD and its related properties in in vitro rat hippocampal slices. Results indicated that the SLFS in the distal and proximal region triggered the plasticity related proteins (PRP) synthesis in both regions, as evident by the induction and maintenance of L-LTD in the distal region by virtue of synaptic and cross-tagging. In addition, the application of emetine at the time of proximal input stimulation prevented the transition of early-LTD (E-LTD) into L-LTD at the distal region, proving PRP synthesis at the proximal site. Further, it was observed that weak low-frequency stimulation (WLFS) could induce E-LTD in the proximal region along with LTD-specific tag-setting at the synapses. In conclusion, the current study suggests unique findings that the synaptic and cross-tagging mediate L-LTD expression is maintained in the proximal location of hippocampus apical CA1 dendrites.

Golgi satellites are essential for polysialylation of NCAM and expression of LTP at distal synapses

The complex cytoarchitecture of neurons poses significant challenges for the maturation of synaptic membrane proteins. It is currently unclear whether locally secreted synaptic proteins bypass the Golgi or whether they traffic through Golgi satellites (GSs). Here, we create a transgenic GS reporter mouse line and show that GSs are widely distributed along dendrites and are capable of mature glycosylation, in particular sialylation. We find that polysialylation of locally secreted NCAM takes place at GSs. Accordingly, in mice lacking a component of trans-Golgi network-to-plasma membrane trafficking, we find fewer GSs and significantly reduced PSA-NCAM levels in distal dendrites of CA1 neurons that receive input from the temporoammonic pathway. Induction of long-term potentiation at those, but not more proximal, synapses is severely impaired. We conclude that GSs serve the need for local mature glycosylation of synaptic membrane proteins in distal dendrites and thereby contribute to rapid changes in synaptic strength.

Denervated mouse CA1 pyramidal neurons express homeostatic synaptic plasticity following entorhinal cortex lesion

Structural, functional, and molecular reorganization of denervated neural networks is often observed in neurological conditions. The loss of input is accompanied by homeostatic synaptic adaptations, which can affect the reorganization process. A major challenge of denervation-induced homeostatic plasticity operating in complex neural networks is the specialization of neuronal inputs. It remains unclear whether neurons respond similarly to the loss of distinct inputs. Here, we used in vitro entorhinal cortex lesion (ECL) and Schaffer collateral lesion (SCL) in mouse organotypic entorhino-hippocampal tissue cultures to study denervation-induced plasticity of CA1 pyramidal neurons. We observed microglia accumulation, presynaptic bouton degeneration, and a reduction in dendritic spine numbers in the denervated layers 3 days after SCL and ECL. Transcriptome analysis of the CA1 region revealed complex changes in differential gene expression following SCL and ECL compared to non-lesioned controls with a specific enrichment of differentially expressed synapse-related genes observed after ECL. Consistent with this finding, denervation-induced homeostatic plasticity of excitatory synapses was observed 3 days after ECL but not after SCL. Chemogenetic silencing of the EC but not CA3 confirmed the pathway-specific induction of homeostatic synaptic plasticity in CA1. Additionally, increased RNA oxidation was observed after SCL and ECL. These results reveal important commonalities and differences between distinct pathway lesions and demonstrate a pathway-specific induction of denervation-induced homeostatic synaptic plasticity.

The properties of long-term potentiation at SC-CA1/ TA-CA1 hippocampal synaptic pathways depends upon their input pathway activation patterns

Long-term potentiation (LTP) has been considered as a cellular mechanism of memory. Since the Schaffer collateral (SC) and temporoammonic (TA) inputs to CA1 are distinct synaptic pathways that could mediate different cognitive functions, this study was therefore aimed to separately study and compare the properties of LTP of these two synaptic pathways. In the current study we used slice electrophysiological methods to compare various properties of these two synaptic pathways in response to single, paired pulse stimulation, and to three standard protocols for inducing LTP: the high frequency electrical stimulation (HFS), theta-burst (TBS), and primed burst (PBs) stimulation. We found that the SC-CA1 synapses could produce bigger maximum synaptic responses than TA-CA1 synapses. In addition, we showed that paired-pulse ratios of the SC-CA1 synapses were higher than TA-CA1 synapses at certain inter-pulses intervals. Finally, we showed a higher LTP% was induced by PBs or TBS at the SC-CA1 synapse than the TA-CA1 synapse. Briefly, our findings suggest the differential basal synaptic transmission, paired-pulse evoked synaptic responses, and LTP exhibition of the hippocampal SC-CA1/ TA-CA1 synaptic pathways, which may rely on spontaneous and evoked activity pattern at the local circuit level.

Strain-dependent regulation of hippocampal long-term potentiation by dopamine D1/D5 receptors in mice

The magnitude and persistency of long-term potentiation (LTP) in the rodent hippocampus is species-dependent: rats express more robust and more prolonged LTP in response to a broader afferent frequency range than mice. The C57Bl/6 mouse is an extremely popular murine strain used in studies of hippocampal synaptic plasticity and spatial learning. Recently it was reported that it expresses impoverished LTP compared to other murine strains. Given the important role of the dopamine D1/D5 receptor (D1/D5R) in the maintenance of LTP and in memory consolidation, we explored to what extent strain-dependent differences in LTP in mice are determined by differences in D1/D5R-control. In CaOlaHsd mice, robust LTP was induced that lasted for over 24 h and which was significantly greater in magnitude than LTP induced in C57Bl/6 mice. Intracerebral treatment with a D1/D5R-antagonist (SCH23390) prevented both the early and late phase of LTP in CaOlaHsd mice, whereas only late-LTP was impaired in C57Bl/6 mice. Treatment with a D1/D5R-agonist (Chloro-PB) facilitated short-term potentiation (STP) into LTP (> 24 h) in both strains, whereby effects became evident earlier in CaOlaHsd compared to C57Bl/6 mice. Immunohistochemical analysis revealed a significantly higher expression of D1-receptors in the stratum lacunosum moleculare of CaOlaHsd compared to C57Bl/6 mice. These findings highlight differences in D1/D5R- dependent regulation of strain-dependent variations in hippocampal LTP in C57Bl/6 and CaOlaHsd mice, that may be mediated, in part, by differences in the expression of D1R in the hippocampus.

Early life GABAA blockade alters the synaptic plasticity and cognitive functions in male and female rats

Gamma-aminobutyric acid (GABA), the major inhibitory neurotransmitter in adults, has a critical contribution to balanced excitatory-inhibitory networks in the brain. Alteration in depolarizing action of GABA during early life is connected to a wide variety of neurodevelopmental disorders. Additionally, the effects of postnatal GABA blockade on neuronal synaptic plasticity are not known and therefore, we set out to determine whether postnatal exposure to bicuculline, a competitive antagonist of GABA_A receptors, affects electrophysiologic changes in hippocampal CA1 neurons later on. To this end, male and female Wistar rats received vehicle or bicuculline (300 μg/kg) on postnatal days (PNDs) 7, 9 and 11, and then underwent different behavioral and electrophysiological examinations in adulthood. Postnatal exposure to bicuculline did not affect basic synaptic transmission but led to a pronounced decrease in paired-pulse facilitation (PPF) in CA1 pyramidal neurons. Bicuculline treatment also attenuated the long-term potentiation (LTP) and long-term depression (LTD) of CA1 neurons accompanied by decreased theta-burst responses in male and female adult rats. These electrophysiology findings together with the reduced brain-derived neurotrophic factor (BDNF) levels in the hippocampus and prefrontal cortex reliably explain the disturbance in spatial reference and working memories of bicuculline-treated animals. This study suggests that postnatal GABA_A blockade deteriorates short- and long-term synaptic plasticity of hippocampal CA1 neurons and related encoding of spatial memory in adulthood.

Leptin regulation of synaptic function at hippocampal TA-CA1 and SC-CA1 synapses

Increasing evidence indicates that the metabolic hormone, leptin markedly influences the functioning of the hippocampus. In particular, exposure to leptin results in persistent changes in synaptic efficacy at both temporoammonic (TA) and Schaffer Collateral (SC) inputs to hippocampal CA1 neurons. The ability of leptin to regulate TA-CA1 and SC-CA1 synapses has important functional implications, as both synaptic connections play important roles in hippocampal-dependent learning and memory. Here we review the modulatory actions of the hormone leptin at these hippocampal CA1 synapses and explore the impact on learning and memory processes.

Pathway-specific TNF-mediated metaplasticity in hippocampal area CA1

Long-term potentiation (LTP) is regulated in part by metaplasticity, the activity-dependent alterations in neural state that coordinate the direction, amplitude, and persistence of future synaptic plasticity. Previously, we documented a heterodendritic metaplasticity effect whereby high-frequency priming stimulation in stratum oriens (SO) of hippocampal CA1 suppressed subsequent LTP in the stratum radiatum (SR). The cytokine tumor necrosis factor (TNF) mediated this heterodendritic metaplasticity in wild-type rodents and in a mouse model of Alzheimer’s disease. Here, we investigated whether LTP at other afferent synapses to CA1 pyramidal cells were similarly affected by priming stimulation. We found that priming stimulation in SO inhibited LTP only in SR and not in a second independent pathway in SO, nor in stratum lacunosum moleculare (SLM). Synapses in SR were also more sensitive than SO or SLM to the LTP-inhibiting effects of pharmacological TNF priming. Neither form of priming was sex-specific, while the metaplasticity effects were absent in TNFR1 knock-out mice. Our findings demonstrate an unexpected pathway specificity for the heterodendritic metaplasticity in CA1. That Schaffer collateral/commissural synapses in SR are particularly susceptible to such metaplasticity may reflect an important control of information processing in this pathway in addition to its sensitivity to neuroinflammation under disease conditions.

Microdissected Pyramidal Cell Proteomics of Alzheimer Brain Reveals Alterations in Creatine Kinase B-Type, 14-3-3-γ, and Heat Shock Cognate 71

Novel insights on proteins involved in Alzheimer’s disease (AD) are needed. Since multiple cell types and matrix components are altered in AD, bulk analysis of brain tissue maybe difficult to interpret. In the current study, we isolated pyramidal cells from the cornu ammonis 1 (CA1) region of the hippocampus from five AD and five neurologically healthy donors using laser capture microdissection (LCM). The samples were analyzed by proteomics using ¹⁸O-labeled internal standard and nano-high-performance liquid chromatography coupled to tandem mass spectrometry (HPLC-MS/MS) for relative quantification. Fold change between AD and control was calculated for the proteins that were identified in at least two individual proteomes from each group. From the 10 cases analyzed, 62 proteins were identified in at least two AD cases and two control cases. Creatine kinase B-type (CKB), 14-3-3-γ, and heat shock cognate 71 (Hsc71), which have not been extensively studied in the context of the human AD brain previously, were selected for further studies by immunohistochemistry (IHC). In hippocampus, semi-quantitative measures of IHC staining of the three proteins confirmed the findings from our proteomic analysis. Studies of the same proteins in the frontal cortex revealed that the alterations remained for CKB and 14-3-3-γ but not for Hsc71. Protein upregulation in CA1 neurons of final stage AD is either a result of detrimental, pathological effects, or from cell-specific protective response mechanisms in surviving neurons. Based on previous findings from experimental studies, CKB and Hsc71 likely exhibit protective effects, whereas 14-3-3-γ may represent a detrimental pathway. These new players could reflect pathways of importance for the development of new therapeutic strategies.

Neonatal NMDA blockade alters the LTP, LTD and cognitive functions in male and female Wistar rats

There is compelling evidence that neonatal blockade of NMDA receptors by phencyclidine (PCP) is associated with cognitive impairment in adulthood but little is known about the effects of early life PCP treatment on synaptic function later in life. Here, we sought to determine whether early life exposure to PCP alters the electrophysiologic function of hippocampal CA1 neurons in adult rats. To this end, male and female Wistar rats received either saline or PCP (10 mg/kg) on postnatal days (PND) 7, 9, and 11, and then underwent separate behavioral and electrophysiology tests in adulthood. Neonatal PCP treatment did not alter basic synaptic transmission and had only a modest effect on frequency following (FF) capacity but significantly decreased the paired-pulse facilitation (PPF) in the Schaffer collateral (SC)-CA1 pathway. We found that PCP treatment significantly attenuated the long-term potentiation (LTP) and long-term depression (LTD) in CA1 neurons accompanied by pronounced alteration in complex response profile in adult rats. The electrophysiology data were comparable in male and female rats and reliably associated with impaired spatial reference and working memories in these animals. Overall, this study suggests that blockade of NMDA receptors during early life deteriorates the short-term and long-term synaptic plasticity and complex response profile of CA1 neurons in adulthood.

Increased Excitatory Synaptic Transmission Associated with Adult Seizure Vulnerability Induced by Early-Life Inflammation in Mice

Early-life inflammatory stress increases seizure susceptibility later in life. However, possible sex- and age-specific differences and the associated mechanisms are largely unknown. C57BL/6 mice were bred in house, and female and male pups were injected with lipopolysaccharide (LPS; 100 μg/kg, i.p.) or vehicle control (saline solution) at postnatal day 14 (P14). Seizure threshold was assessed in response to pentylenetetrazol (1% solution, i.v.) in adolescence (∼P40) and adulthood (∼P60). We found that adult, but not adolescent, mice treated with LPS displayed ∼34% lower seizure threshold compared with controls. Females and males showed similar increased seizure susceptibility, suggesting that altered brain excitability was age dependent, but not sex dependent. Whole-cell recordings revealed no differences in excitatory synaptic activity onto CA1 pyramidal neurons from control or neonatally inflamed adolescent mice of either sex. However, adult mice of both sexes previously exposed to LPS displayed spontaneous EPSC frequency approximately twice that of controls, but amplitude was unchanged. Although these changes were not associated with alterations in dendritic spines or in the NMDA/AMPA receptor ratio, they were linked to an increased glutamate release probability from Schaffer collateral, but not temporoammonic pathway. This glutamate increase was associated with reduced activity of presynaptic GABA_B receptors and was independent of the endocannabinoid-mediated suppression of excitation. Our new findings demonstrate that early-life inflammation leads to long-term increased hippocampal excitability in adult female and male mice associated with changes in glutamatergic synaptic transmission. These alterations may contribute to enhanced vulnerability of the brain to subsequent pathologic challenges such as epileptic seizures.SIGNIFICANCE STATEMENT Adult physiology has been shown to be affected by early-life inflammation. Our data reveal that early-life inflammation increases excitatory synaptic transmission onto hippocampal CA1 pyramidal neurons in an age-dependent manner through disrupted presynaptic GABA_B receptor activity on Schaffer collaterals. This hyperexcitability was seen only in adult, and not in adolescent, animals of either sex. The data suggest a maturation process, independent of sex, in the priming action of early-life inflammation and highlight the importance of studying mature brains to reveal cellular changes associated with early-life interventions.

Frequency-dependent layer-specific differences in short-term synaptic plasticity in the dorsal and ventral CA1 hippocampal field

Information from the entorhinal cortex arrives to the hippocampal CA1 microcircuit directly through the temporoammonic path (TA) that terminates in the stratum lacunosum-moleculare (SLM), and indirectly through Schaffer collateral pathway (SC) that terminates in the stratum radiatum (SR). By virtue of this input convergence, CA1 circuitry may act to compare and integrate incoming cortical information. Although a remarkable dorsal-ventral difference in short-term plasticity (STP) has been recently described at SC-CA1 synapses, the corresponding properties at TA-CA1 synapses have not been examined. Here, we report that stimulation of TA in the dorsal hippocampus produces significant facilitation of all conditioned responses evoked by 1-30 Hz, peaking at 20-30 Hz, and significant depression of steady-state responses to 50-100 Hz. Dorsal SC-CA1 synapses display a similar pattern of responses, yet, facilitation peaked at 10 Hz and depression (at 75-100 Hz) is weaker. Strikingly, stimulation of TA in the ventral hippocampus produces facilitation of steady-state responses to 1-30 Hz and highly contrasts with the depression of SC-CA1 synapses. Steady-state responses to 40-100 Hz in the ventral hippocampus depress in both layers similarly. High-frequency TA input (40-100 Hz) to the dorsal hippocampus depresses more in proximal than in distal SLM, while low-frequency (1-3 Hz) TA input to the ventral hippocampus facilitates more in distal than in proximal SLM. The present evidence suggests that direct and indirect entorhinal cortical inputs across the septotemporal extent of hippocampal CA1 field display frequency selectivity both in the radial and transverse axes, and that a rapid information processing may take place through direct ventral hippocampal CA1-EC circuit interactions independently of trisynaptic circuit.

Alternating sequences of future and past behavior encoded within hippocampal theta oscillations

Neural networks display the ability to transform forward-ordered activity patterns into reverse-ordered, retrospective sequences. The mechanisms underlying this transformation remain unknown. We discovered that, during active navigation, rat hippocampal CA1 place cell ensembles are inherently organized to produce independent forward- and reverse-ordered sequences within individual theta oscillations. This finding may provide a circuit-level basis for retrospective evaluation and storage during ongoing behavior. Theta phase procession arose in a minority of place cells, many of which displayed two preferred firing phases in theta oscillations and preferentially participated in reverse replay during subsequent rest. These findings reveal an unexpected aspect of theta-based hippocampal encoding and provide a biological mechanism to support the expression of reverse-ordered sequences.

In vivo assessment of mechanisms underlying the neurovascular basis of postictal amnesia

Long-lasting confusion and memory difficulties during the postictal state remain a major unmet problem in epilepsy that lacks pathophysiological explanation and treatment. We previously identified that long-lasting periods of severe postictal hypoperfusion/hypoxia, not seizures per se, are associated with memory impairment after temporal lobe seizures. While this observation suggests a key pathophysiological role for insufficient energy delivery, it is unclear how the networks that underlie episodic memory respond to vascular constraints that ultimately give rise to amnesia. Here, we focused on cellular/network level analyses in the CA1 of hippocampus in vivo to determine if neural activity, network oscillations, synaptic transmission, and/or synaptic plasticity are impaired following kindled seizures. Importantly, the induction of severe postictal hypoperfusion/hypoxia was prevented in animals treated by a COX-2 inhibitor, which experimentally separated seizures from their vascular consequences. We observed complete activation of CA1 pyramidal neurons during brief seizures, followed by a short period of reduced activity and flattening of the local field potential that resolved within minutes. During the postictal state, constituting tens of minutes to hours, we observed no changes in neural activity, network oscillations, and synaptic transmission. However, long-term potentiation of the temporoammonic pathway to CA1 was impaired in the postictal period, but only when severe local hypoxia occurred. Lastly, we tested the ability of rats to perform object-context discrimination, which has been proposed to require temporoammonic input to differentiate between sensory experience and the stored representation of the expected object-context pairing. Deficits in this task following seizures were reversed by COX-2 inhibition, which prevented severe postictal hypoxia. These results support a key role for hypoperfusion/hypoxia in postictal memory impairments and identify that many aspects of hippocampal network function are resilient during severe hypoxia except for long-term synaptic plasticity.

Distance-dependent regulation of NMDAR nanoscale organization along hippocampal neuron dendrites

Hippocampal pyramidal neurons are characterized by a unique arborization subdivided in segregated dendritic domains receiving distinct excitatory synaptic inputs with specific properties and plasticity rules that shape their respective contributions to synaptic integration and action potential firing. Although the basal regulation and plastic range of proximal and distal synapses are known to be different, the composition and nanoscale organization of key synaptic proteins at these inputs remains largely elusive. Here we used superresolution imaging and single nanoparticle tracking in rat hippocampal neurons to unveil the nanoscale topography of native GluN2A- and GluN2B-NMDA receptors (NMDARs)-which play key roles in the use-dependent adaptation of glutamatergic synapses-along the dendritic arbor. We report significant changes in the nanoscale organization of GluN2B-NMDARs between proximal and distal dendritic segments, whereas the topography of GluN2A-NMDARs remains similar along the dendritic tree. Remarkably, the nanoscale organization of GluN2B-NMDARs at proximal segments depends on their interaction with calcium/calmodulin-dependent protein kinase II (CaMKII), which is not the case at distal segments. Collectively, our data reveal that the nanoscale organization of NMDARs changes along dendritic segments in a subtype-specific manner and is shaped by the interplay with CaMKII at proximal dendritic segments, shedding light on our understanding of the functional diversity of hippocampal glutamatergic synapses.

Proximodistal Heterogeneity in Learning-promoted Pathway-specific Plasticity at Dorsal CA1 Synapses

Contextual learning requires the delivery of AMPA receptors to CA1 synapses in the dorsal hippocampus. However, proximodistal heterogeneity of pathway-specific plasticity remains unclear. Here, we examined the proximodistal heterogeneity in learning-induced plasticity at the CA1 synapses with inputs from the entorhinal cortex layer III (ECIII) or from CA3. We subjected male rats to an inhibitory avoidance task and prepared acute hippocampal slices for whole-cell patch clamp experiments, where we stimulated ECIII-CA1 or CA3-CA1 input fibers to analyze evoked excitatory postsynaptic currents (EPSCs). Compared to untrained controls, trained rats exhibited higher AMPA/NMDA current ratios at CA3-CA1 synapses of proximal and intermediate, but not distal CA1 neurons, which suggested that region-specific plasticity occurred after learning. Moreover, trained rats exhibited higher AMPA/NMDA current ratios at ECIII-CA1 synapses of intermediate and distal, but not proximal CA1 neurons. These findings suggested the presence of proximodistal heterogeneity in pathway-specific postsynaptic plasticity. Regarding presynaptic plasticity, training slightly, but significantly increased the paired-pulse ratios of CA3-CA1 synapses of proximal and intermediate, but not distal CA1 neurons. Moreover, trained rats exhibited higher paired-pulse ratios at ECIII-CA1 synapses of intermediate and distal, but not proximal CA1 neurons, which suggested region-specific presynaptic plasticity. Finally, learning was clearly prevented by the bilateral microinjection of a plasticity blocker in the proximal or intermediate, but not distal CA1 subfields, which suggested functional heterogeneity along the proximodistal axis. Understanding region- and pathway-specific plasticity at dorsal CA1 synapses could aid in controlling encoded memory.

Leptin Regulation of Synaptic Function at Hippocampal TA-CA1 and SC-CA1 Synapses: Implications for Health and Disease

Growing evidence indicates that the endocrine hormone leptin regulates hippocampal synaptic function in addition to its established role as a hypothalamic satiety signal. Indeed, numerous studies show that leptin facilitates the cellular events that underlie hippocampal learning and memory including activity-dependent synaptic plasticity and glutamate receptor trafficking, indicating that leptin may be a potential cognitive enhancer. Although there has been extensive investigation into the modulatory role of leptin at hippocampal Schaffer collateral (SC)-CA1 synapses, recent evidence indicates that leptin also potently regulates excitatory synaptic transmission at the anatomically distinct temporoammonic (TA) input to hippocampal CA1 neurons. The cellular mechanisms underlying activity-dependent synaptic plasticity at TA-CA1 synapses differ from those at SC-CA1 synapses and the TA input is implicated in spatial and episodic memory formation. Furthermore, the TA input is an early target for neurodegeneration in Alzheimer's disease (AD) and aberrant leptin function is linked to AD. Here, we review the evidence that leptin regulates hippocampal synaptic function at both SC- and TA-CA1 synapses and discuss the consequences for neurodegenerative disorders like AD.

Age-dependent regulation of excitatory synaptic transmission at hippocampal temporoammonic-CA1 synapses by leptin

The hippocampus is a key target for the hormone leptin and leptin regulation of excitatory synaptic transmission at Schaffer-collateral-CA1 synapses during aging are well documented. However, little is known about the age-dependent actions of leptin at the temporoammonic (TA) input to CA1 neurons. Here we show that leptin induces a novel form of N-methyl-D-aspartate receptor-dependent long-term depression (LTD) at adult (12-24 weeks old) TA-CA1 synapses. Leptin-induced LTD requires activation of canonical Janus tyrosine kinase 2- signal transducer and activator of transcription signaling and removal of GluA1-containing α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors from synapses. Moreover, leptin-induced LTD is occluded by activity-dependent LTD at TA-CA1 synapses. By contrast, leptin has no effect on excitatory synaptic transmission at aged (12-14 months old) TA-CA1 synapses, and low-frequency stimulation also fails to induce LTD at this age. These findings demonstrate clear age-related alterations in the leptin sensitivity of TA-CA1 synapses and provide valuable information on how the leptin system alters with age. As leptin has been linked to Alzheimer's disease, these findings have important implications for understanding of age-related disorders such as Alzheimer's disease.

Long term potentiation, but not depression, in interlamellar hippocampus CA1

Synaptic plasticity in the lamellar CA3 to CA1 circuitry has been extensively studied while interlamellar CA1 to CA1 connections have not yet received much attention. One of our earlier studies demonstrated that axons of CA1 pyramidal neurons project to neighboring CA1 neurons, implicating information transfer along a longitudinal interlamellar network. Still, it remains unclear whether long-term synaptic plasticity is present within this longitudinal CA1 network. Here, we investigate long-term synaptic plasticity between CA1 pyramidal cells, using in vitro and in vivo extracellular recordings and 3D holography glutamate uncaging. We found that the CA1-CA1 network exhibits NMDA receptor-dependent long-term potentiation (LTP) without direction or layer selectivity. By contrast, we find no significant long-term depression (LTD) under various LTD induction protocols. These results implicate unique synaptic properties in the longitudinal projection suggesting that the interlamellar CA1 network could be a promising structure for hippocampus-related information processing and brain diseases.

Involvement of intracellular Zn2+ signaling in LTP at perforant pathway-CA1 pyramidal cell synapse

Physiological significance of synaptic Zn²⁺ signaling was examined at perforant pathway-CA1 pyramidal cell synapses. In vivo long-term potentiation (LTP) at perforant pathway-CA1 pyramidal cell synapses was induced using a recording electrode attached to a microdialysis probe and the recording region was locally perfused with artificial cerebrospinal fluid (ACSF) via the microdialysis probe. Perforant pathway LTP was not attenuated under perfusion with CaEDTA (10 mM), an extracellular Zn²⁺ chelator, but attenuated under perfusion with ZnAF-2DA (50 μM), an intracellular Zn²⁺ chelator, suggesting that intracellular Zn²⁺ signaling is required for perforant pathway LTP. Even in rat brain slices bathed in CaEDTA in ACSF, intracellular Zn²⁺ level, which was measured with intracellular ZnAF-2, was increased in the stratum lacunosum-moleculare where perforant pathway-CA1 pyramidal cell synapses were contained after tetanic stimulation. These results suggest that intracellular Zn²⁺ signaling, which originates in internal stores/proteins, is involved in LTP at perforant pathway-CA1 pyramidal cell synapses. Because the influx of extracellular Zn²⁺ , which originates in presynaptic Zn²⁺ release, is involved in LTP at Schaffer collateral-CA1 pyramidal cell synapses, synapse-dependent Zn²⁺ dynamics may be involved in plasticity of postsynaptic CA1 pyramidal cells.

Changes in Neuronal Oscillations Accompany the Loss of Hippocampal LTP that Occurs in an Animal Model of Psychosis

The first-episode of psychosis is followed by a transient time-window of ca. 60 days during which therapeutic interventions have a higher likelihood of being effective than interventions that are started with a greater latency. This suggests that, in the immediate time-period after first-episode psychosis, functional changes occur in the brain that render it increasingly resistant to intervention. The precise mechanistic nature of these changes is unclear, but at the cognitive level, sensory and hippocampus-based dysfunctions become increasingly manifest. In an animal model of first-episode psychosis that comprises acute treatment of rats with the irreversible N-methyl-D-aspartate receptor (NMDAR)-antagonist, MK801, acute but also chronic deficits in long-term potentiation (LTP) and spatial memory occur. Neuronal oscillations, especially in the form of information transfer through θ and γ frequency oscillations are an intrinsic component of normal information processing in the hippocampus. Changes in θ-γ coupling and power are known to accompany deficits in hippocampal plasticity. Here, we examined whether changes in δ, θ, α, β and γ oscillations, or θ-γ coupling accompany the chronic loss of LTP that is observed in the MK801-animal model of psychosis. One and 4 weeks after acute systemic treatment of adult rats with MK801, a potent loss of hippocampal in vivo LTP was evident compared to vehicle-treated controls. Overall, the typical pattern of θ-γ oscillations that are characteristic for the successful induction of LTP was altered. In particular, θ-power was lower and an uncoupling of θ-γ oscillations was evident in MK801-treated rats. The alterations in network oscillations that accompany LTP deficits in this animal model may comprise a mechanism through which disturbances in sensory information processing and hippocampal function occur in psychosis. These data suggest that the hippocampus is likely to comprise a very early locus of functional change after instigation of a first-episode psychosis-like state in rodents.

Complementary learning systems within the hippocampus: a neural network modelling approach to reconciling episodic memory with statistical learning

A growing literature suggests that the hippocampus is critical for the rapid extraction of regularities from the environment. Although this fits with the known role of the hippocampus in rapid learning, it seems at odds with the idea that the hippocampus specializes in memorizing individual episodes. In particular, the Complementary Learning Systems theory argues that there is a computational trade-off between learning the specifics of individual experiences and regularities that hold across those experiences. We asked whether it is possible for the hippocampus to handle both statistical learning and memorization of individual episodes. We exposed a neural network model that instantiates known properties of hippocampal projections and subfields to sequences of items with temporal regularities. We found that the monosynaptic pathway-the pathway connecting entorhinal cortex directly to region CA1-was able to support statistical learning, while the trisynaptic pathway-connecting entorhinal cortex to CA1 through dentate gyrus and CA3-learned individual episodes, with apparent representations of regularities resulting from associative reactivation through recurrence. Thus, in paradigms involving rapid learning, the computational trade-off between learning episodes and regularities may be handled by separate anatomical pathways within the hippocampus itself.This article is part of the themed issue 'New frontiers for statistical learning in the cognitive sciences'.

Rapid erasure of hippocampal memory following inhibition of dentate gyrus granule cells

The hippocampus is critical for the acquisition and retrieval of episodic and contextual memories. Lesions of the dentate gyrus, a principal input of the hippocampus, block memory acquisition, but it remains unclear whether this region also plays a role in memory retrieval. Here we combine cell-type specific neural inhibition with electrophysiological measurements of learning-associated plasticity in behaving mice to demonstrate that dentate gyrus granule cells are not required for memory retrieval, but instead have an unexpected role in memory maintenance. Furthermore, we demonstrate the translational potential of our findings by showing that pharmacological activation of an endogenous inhibitory receptor expressed selectively in dentate gyrus granule cells can induce a rapid loss of hippocampal memory. These findings open a new avenue for the targeted erasure of episodic and contextual memories.

Dentate gyrus (DG) is critical for memory formation in the hippocampus but its role in memory retrieval is unclear. Here, Gross and colleagues, show that granule cells in DG are not required for memory retrieval but for maintenance, and inhibiting them with a drug leads to rapid loss of memory.

GABA and Endocannabinoids Mediate Depotentiation of Schaffer Collateral Synapses Induced by Stimulation of Temperoammonic Inputs

Long-term potentiation (LTP) of Schaffer collateral (SC) synapses in the hippocampus is thought to play a key role in episodic memory formation. Because the hippocampus is a shorter-term, limited capacity storage system, repeated bouts of learning and synaptic plasticity require that SC synapses reset to baseline at some point following LTP. We previously showed that repeated low frequency activation of temperoammonic (TA) inputs to the CA1 region depotentiates SC LTP without persistently altering basal transmission. This heterosynaptic depotentiation involves adenosine A1 receptors but not N-methyl-D-aspartate receptors, metabotropic glutamate receptors or L-type calcium channels. In the present study, we used rat hippocampal slices to explore other messengers contributing to TA-induced SC depotentiation, and provide evidence for the involvement of cannabinoid-1 and γ-aminobutyric acid (GABA) type-A receptors as more proximal signaling events leading to synaptic resetting, with A1 receptor activation serving as a downstream event. Surprisingly, we found that TA-induced SC depotentiation is independent of α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)/kainate glutamate receptors. We also examined the involvement of mitogen-activated protein kinases (MAPKs), and found a role for extracellular-signal related kinase 1/2 and p38 MAPK, but not c-Jun-N-terminal kinase. These results indicate that low frequency stimulation of TA inputs to CA1 activates a complex signaling network that instructs SC synaptic resetting. The involvement of GABA and endocannabinoids suggest mechanisms that could contribute to cognitive dysfunction associated with substance abuse and neuropsychiatric disorders.

Long-term Potentiation at Temporoammonic Path-CA1 Synapses in Freely Moving Rats

Hippocampal area CA1 receives direct entorhinal layer III input via the temporoammonic path (TAP) and recent studies implicate TAP-CA1 synapses are important for some aspects of hippocampal memory function. Nonetheless, as few studies have examined TAP-CA1 synaptic plasticity in vivo, the induction and longevity of TAP-CA1 long-term potentiation (LTP) has not been fully characterized. We analyzed CA1 responses following stimulation of the medial aspect of the angular bundle and investigated LTP at medial temporoammonic path (mTAP)-CA1 synapses in freely moving rats. We demonstrate monosynaptic mTAP-CA1 responses can be isolated in vivo as evidenced by observations of independent current sinks in the stratum lacunosum moleculare of both areas CA1 and CA3 following angular bundle stimulation. Contrasting prior indications that TAP input rarely elicits CA1 discharge, we observed mTAP-CA1 responses that appeared to contain putative population spikes in 40% of our behaving animals. Theta burst high frequency stimulation of mTAP afferents resulted in an input specific and N-methyl-D-aspartate (NMDA) receptor-dependent LTP of mTAP-CA1 responses in behaving animals. LTP of mTAP-CA1 responses decayed as a function of two exponential decay curves with time constants (τ) of 2.7 and 148 days to decay 63.2% of maximal LTP. In contrast, mTAP-CA1 population spike potentiation longevity demonstrated a τ of 9.6 days. To our knowledge, these studies provide the first description of mTAP-CA1 LTP longevity in vivo. These data indicate TAP input to area CA1 is a physiologically relevant afferent system that displays robust synaptic plasticity.