Molecular mechanisms contributing to long-lasting synaptic plasticity at the temporoammonic-CA1 synapse

Labile glutamate synaptic transmission in the adult CA1 stratum-lacunosum-moleculare region

The excitatory monosynaptic activation of hippocampal CA1 pyramidal cells is spatially segregated such that the proximal part of the apical dendritic tree in stratum radiatum (SR) receives input from the hippocampal CA3 region while the distal part in the stratum-lacunosum-moleculare (SLM) receives input mainly from the entorhinal cortex. The AMPA receptor-mediated (AMPA) signalling of SLM synapses in slices from neonatal rats was previously found to considerably differ from that of the SR synapses. In the present study, AMPA signalling of SLM synapses in 1-month-old rats has been examined, that is, when the hippocampus is essentially functionally mature. For the SR synapses, this time is characterized by a facilitatory shift in short-term plasticity, in the disappearance of labile postsynaptic AMPA signalling, a property thought to be important for early activity-dependent organization of neural circuits, and the expression of an adult form of long-term potentiation. We found that the SLM synapses alter their short-term plasticity similarly to that of the SR synapses. However, the labile postsynaptic AMPA signalling was not only maintained but substantially enhanced in the SLM synapses. The long-term potentiation observed was not of the adult form but like that of the neonatal SR synapses based on unsilencing of AMPA labile synapses. We propose that these features of the SLM synapses in the mature hippocampus will help to produce a flexible map of the multimodal sensory input reaching the SLM required for its conjunctive operation with the SR input to generate a proper functional output from the CA1 region.

Repeated Morphine Exposure Alters Temporoamonic-CA1 Synaptic Plasticity in Male Rat Hippocampus

In this study, the electrophysiological and biochemical consequences of repeated exposure to morphine in male rats on glutamatergic synaptic transmission, synaptic plasticity, the expression of GABA receptors and glutamate receptors at the temporoammonic-CA1 synapse along the longitudinal axis of the hippocampus (dorsal, intermediate, ventral, DH, IH, VH, respectively) were investigated. Slice electrophysiological methods, qRT-PCR, and western blotting techniques were used to characterize synaptic plasticity properties. We showed that repeated morphine exposure (RME) reduced excitatory synaptic transmission and ability for long-term potentiation (LTP) in the VH as well as eliminated the dorsoventral difference in paired-pulse responses. A decreased expression of NR2B subunit in the VH and an increased expression GABAA receptor of α1 and α5 subunits in the DH were observed following RME. Furthermore, RME did not affect the expression of NR2A, AMPA receptor subunits, and γ2GABAA and GABAB receptors in either segment of the hippocampus. In sum, the impact of morphine may differ depending on the region of the hippocampus studied. A distinct change in the short- and long-term synaptic plasticity along the hippocampus long axis due to repeated morphine exposure, partially mediated by a change in the expression profile of glutamatergic receptor subunits. These findings can be useful in further understanding the cellular mechanism underlying deficits in information storage and, more generally, cognitive processes resulting from chronic opioid abuse.

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.

Entorhinohippocampal cholecystokinin modulates spatial learning by facilitating neuroplasticity of hippocampal CA3-CA1 synapses

The hippocampus is broadly impacted by neuromodulations. However, how neuropeptides shape the function of the hippocampus and the related spatial learning and memory remains unclear. Here, we discover the crucial role of cholecystokinin (CCK) in heterosynaptic neuromodulation from the medial entorhinal cortex (MEC) to the hippocampus. Systematic knockout of the CCK gene impairs CA3-CA1 LTP and space-related performance. The MEC provides most of the CCK-positive neurons projecting to the hippocampal region, which potentiates CA3-CA1 long-term plasticity heterosynaptically in a frequency- and NMDA receptor (NMDAR)-dependent manner. Selective inhibition of MEC CCKergic neurons or downregulation of their CCK mRNA levels also impairs CA3-CA1 LTP formation and animals' performance in the water maze. This excitatory extrahippocampal projection releases CCK upon high-frequency excitation and is active during animal exploration. Our results reveal the critical role of entorhinal CCKergic projections in bridging intra- and extrahippocampal circuitry at electrophysiological and behavioral levels.

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.

Astrocytic α4-containing nAChR signaling in the hippocampus governs the formation of temporal association memory

Everyday episodic memories involve linking together related events that are temporally separated. However, the mechanisms of forming this temporal association have remained unclear. Here, using astrocyte-specific manipulations, we show that potentiating astrocyte Ca²⁺ signaling in the hippocampal cornu ammonis 1 (CA1) enhances the strength of such temporal association, in parallel with long-term potentiation (LTP) enhancement of temporoammonic pathway to CA1, whereas attenuation of astrocyte Ca²⁺ signaling has the opposite effect. Moreover, we identify that these effects are mediated by astrocytic α4 subunit-containing nicotinic acetylcholine receptors (α4-nAChRs) via mechanisms involving NMDAR co-agonist supply. Finally, astrocytic α4-nAChRs underlie the cognitive enhancer nicotine's physiological effects. Together, these findings highlight the importance of astrocyte Ca²⁺ signaling in cognitive behavior and reveal a mechanism in governing the temporal association of episodic memory formation that operates through α4-nAChRs on hippocampal astrocytes.

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.

G-protein coupled estrogen receptor (GPER1) activation promotes synaptic insertion of AMPA receptors and induction of chemical LTP at hippocampal temporoammonic-CA1 synapses

It is well documented that 17β estradiol (E2) regulates excitatory synaptic transmission at hippocampal Shaffer-collateral (SC)-CA1 synapses, via activation of the classical estrogen receptors (ERα and ERβ). Hippocampal CA1 pyramidal neurons are also innervated by the temporoammonic (TA) pathway, and excitatory TA-CA1 synapses are reported to be regulated by E2. Recent studies suggest a role for the novel G-protein coupled estrogen receptor (GPER1) at SC-CA1 synapses, however, the role of GPER1 in mediating the effects of E2 at juvenile TA-CA1 synapses is unclear. Here we demonstrate that the GPER1 agonist, G1 induces a persistent, concentration-dependent (1-10 nM) increase in excitatory synaptic transmission at TA-CA1 synapses and this effect is blocked by selective GPER1 antagonists. The ability of GPER1 to induce this novel form of chemical long-term potentiation (cLTP) was prevented following blockade of N-methyl-D-aspartate (NMDA) receptors, and it was not accompanied by any change in paired pulse facilitation ratio (PPR). GPER1-induced cLTP involved activation of ERK but was independent of phosphoinositide 3-kinase (PI3K) signalling. Prior treatment with philanthotoxin prevented the effects of G1, indicating that synaptic insertion of GluA2-lacking α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors underlies GPER1-induced cLTP. Furthermore, activity-dependent LTP occluded G1-induced cLTP and vice versa, indicating that these processes have overlapping expression mechanisms. Activity-dependent LTP was blocked by the GPER1 antagonist, G15, suggesting that GPER1 plays a role in NMDA-dependent LTP at juvenile TA-CA1 synapses. These findings add a new dimension to our understanding of GPER1 in modulating neuronal plasticity with relevance to age-related neurodegenerative conditions.

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.

Altered A-Type Potassium Channel Function Impairs Dendritic Spike Initiation and Temporoammonic Long-Term Potentiation in Fragile X Syndrome

Fragile X syndrome (FXS) is the leading monogenetic cause of cognitive impairment and autism spectrum disorder. Area CA1 of the hippocampus receives current information about the external world from the entorhinal cortex via the temporoammonic (TA) pathway. Given its role in learning and memory, it is surprising that little is known about TA long-term potentiation (TA-LTP) in FXS. We found that TA-LTP was impaired in male fmr1 KO mice. Although there were no significant differences in basal synaptic transmission, synaptically evoked dendritic calcium signals were smaller in KO neurons. Using dendritic recording, we found no difference in complex spikes or pharmacologically isolated Ca2+ spikes; however, the threshold for fast, Na+-dependent dendritic spikes was depolarized in fmr1 KO mice. Cell-attached patch-clamp recordings found no difference in Na+ channels between wild-type and fmr1 KO CA1 dendrites. Dendritic spike threshold and TA-LTP were restored by blocking A-type K+ channels with either 150 µm Ba2+ or the more specific toxin AmmTx3. The impairment of TA-LTP shown here, coupled with previously described enhanced Schaffer collateral LTP, may contribute to spatial memory alterations in FXS. Furthermore, as both of these LTP phenotypes are attributed to changes in A-type K+ channels in FXS, our findings provide a potential therapeutic target to treat cognitive impairments in FXS.SIGNIFICANCE STATEMENT Alterations in synaptic function and plasticity are likely contributors to learning and memory impairments in many neurologic disorders. Fragile X syndrome is marked by dysfunctional learning and memory and changes in synaptic structure and function. This study shows a lack of LTP at temporoammonic synapses in CA1 neurons associated with biophysical differences in A-type K+ channels in fmr1 KO CA1 neurons. Our results, along with previous findings on A-type K+ channel effects on Schaffer collateral LTP, reveal differential effects of a single ion channelopathy on LTP at the two major excitatory pathways of CA1 pyramidal neurons. These findings expand our understanding of memory deficits in FXS and provide a potential therapeutic target for the treatment of memory dysfunction in FXS.

Regulation of hippocampal synaptic function by the metabolic hormone leptin: Implications for health and disease

Significant advances have been made in our understanding of the hormone, leptin and its CNS actions in recent years. It is now evident that leptin has a multitude of brain functions, that extend beyond its established role in the hypothalamic control of energy balance. Additional brain regions including the hippocampus are important targets for leptin, with a high density of leptin receptors (LepRs) expressed in specific hippocampal regions and localised to CA1 synapses. Extensive evidence indicates that leptin has pro-cognitive actions, as it rapidly modifies synaptic efficacy at excitatory Schaffer collateral (SC)-CA1 and temporoammonic (TA)-CA1 synapses and enhances performance in hippocampal-dependent memory tasks. There is a functional decline in hippocampal responsiveness to leptin with age, with significant reductions in the modulatory effects of leptin at SC-CA1 and TA-CA1 synapses in aged, compared to adult hippocampus. As leptin has pro-cognitive effects, this decline in leptin sensitivity is likely to have negative consequences for cognitive function during the aging process. Here we review how evaluation of the hippocampal actions of leptin has improved our knowledge of the regulatory brain functions of leptin in health and provided significant insight into the impact of leptin in age-related neurodegenerative disorders linked to cognitive decline.

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.

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.

Inactivation of ATRX in forebrain excitatory neurons affects hippocampal synaptic plasticity

α-Thalassemia X-linked intellectual disability (ATR-X) syndrome is a neurodevelopmental disorder caused by mutations in the ATRX gene that encodes a SNF2-type chromatin-remodeling protein. The ATRX protein regulates chromatin structure and gene expression in the developing mouse brain and early inactivation leads to DNA replication stress, extensive cell death, and microcephaly. However, the outcome of Atrx loss of function postnatally in neurons is less well understood. We recently reported that conditional inactivation of Atrx in postnatal forebrain excitatory neurons (ATRX-cKO) causes deficits in long-term hippocampus-dependent spatial memory. Thus, we hypothesized that ATRX-cKO mice will display impaired hippocampal synaptic transmission and plasticity. In the present study, evoked field potentials and current source density analysis were recorded from a multichannel electrode in male, urethane-anesthetized mice. Three major excitatory synapses, the Schaffer collaterals to basal dendrites and proximal apical dendrites, and the temporoammonic path to distal apical dendrites on hippocampal CA1 pyramidal cells were assessed by their baseline synaptic transmission, including paired-pulse facilitation (PPF) at 50-ms interpulse interval, and by their long-term potentiation (LTP) induced by theta-frequency burst stimulation. Baseline single-pulse excitatory response at each synapse did not differ between ATRX-cKO and control mice, but baseline PPF was reduced at the CA1 basal dendritic synapse in ATRX-cKO mice. While basal dendritic LTP of the first-pulse excitatory response was not affected in ATRX-cKO mice, proximal and distal apical dendritic LTP were marginally and significantly reduced, respectively. These results suggest that ATRX is required in excitatory neurons of the forebrain to achieve normal hippocampal LTP and PPF at the CA1 apical and basal dendritic synapses, respectively. Such alterations in hippocampal synaptic transmission and plasticity could explain the long-term spatial memory deficits in ATRX-cKO mice and provide insight into the physiological mechanisms underlying intellectual disability in ATR-X syndrome patients.

NGL-1/LRRC4C Deletion Moderately Suppresses Hippocampal Excitatory Synapse Development and Function in an Input-Independent Manner

Netrin-G ligand-1 (NGL-1), also known as LRRC4C, is a postsynaptic densities (PSDs)-95-interacting postsynaptic adhesion molecule that interacts trans-synaptically with presynaptic netrin-G1. NGL-1 and its family member protein NGL-2 are thought to promote excitatory synapse development through largely non-overlapping neuronal pathways. While NGL-2 is critical for excitatory synapse development in specific dendritic segments of neurons in an input-specific manner, whether NGL-1 has similar functions is unclear. Here, we show that Lrrc4c deletion in male mice moderately suppresses excitatory synapse development and function, but surprisingly, does so in an input-independent manner. While NGL-1 is mainly detected in the stratum lacunosum moleculare (SLM) layer of the hippocampus relative to the stratum radiatum (SR) layer, NGL-1 deletion leads to decreases in the number of PSDs in both SLM and SR layers in the ventral hippocampus. In addition, both SLM and SR excitatory synapses display suppressed short-term synaptic plasticity in the ventral hippocampus. These morphological and functional changes are either absent or modest in the dorsal hippocampus. The input-independent synaptic changes induced by Lrrc4c deletion involve abnormal translocation of NGL-2 from the SR to SLM layer. These results suggest that Lrrc4c deletion moderately suppresses hippocampal excitatory synapse development and function in an input-independent manner.

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.

Hippocampal GABAergic Inhibitory Interneurons

In the hippocampus GABAergic local circuit inhibitory interneurons represent only ~10-15% of the total neuronal population; however, their remarkable anatomical and physiological diversity allows them to regulate virtually all aspects of cellular and circuit function. Here we provide an overview of the current state of the field of interneuron research, focusing largely on the hippocampus. We discuss recent advances related to the various cell types, including their development and maturation, expression of subtype-specific voltage- and ligand-gated channels, and their roles in network oscillations. We also discuss recent technological advances and approaches that have permitted high-resolution, subtype-specific examination of their roles in numerous neural circuit disorders and the emerging therapeutic strategies to ameliorate such pathophysiological conditions. The ultimate goal of this review is not only to provide a touchstone for the current state of the field, but to help pave the way for future research by highlighting where gaps in our knowledge exist and how a complete appreciation of their roles will aid in future therapeutic strategies.

Food for thought: Leptin regulation of hippocampal function and its role in Alzheimer's disease

Accumulating evidence indicates that diet and body weight are important factors associated with Alzheimer's disease (AD), with a significant increase in AD risk linked to mid-life obesity, and weight loss frequently occurring in the early stages of AD. This has fuelled interest in the hormone leptin, as it is an important hypothalamic regulator of food intake and body weight, but leptin also markedly influences the functioning of the hippocampus; a key brain region that degenerates in AD. Increasing evidence indicates that leptin has cognitive enhancing properties as it facilitates the cellular events that underlie hippocampal-dependent learning and memory. However, significant reductions in leptin's capacity to regulate hippocampal synaptic function occurs with age and dysfunctions in the leptin system are associated with an increased risk of AD. Moreover, leptin is a potential novel target in AD as leptin treatment has beneficial effects in various models of AD. Here we summarise recent advances in leptin neurobiology with particular focus on regulation of hippocampal synaptic function by leptin and the implications of this for neurodegenerative disorders like AD. This article is part of the Special Issue entitled 'Metabolic Impairment as Risk Factors for Neurodegenerative Disorders.'

In vitro characterization of cell-level neurophysiological diversity in the rostral nucleus reuniens of adult mice

KEY POINTS

The nucleus reuniens (Re), a nucleus of the midline thalamus, is part of a cognitive network including the hippocampus and the medial prefrontal cortex. To date, very few studies have examined the electrophysiological properties of Re neurons at a cellular level. The majority of Re neurons exhibit spontaneous action potential firing at rest. This is independent of classical amino-acid mediated synaptic transmission. When driven by various forms of depolarizing current stimulus, Re neurons display considerable diversity in their firing patterns. As a result of the presence of a low threshold Ca2+ channel, spike output functions are strongly modulated by the prestimulus membrane potential. Finally, we describe a novel form of activity-dependant intrinsic plasticity that eliminates the high-frequency burst firing present in many Re neurons. These results provide a comprehensive summary of the intrinsic electrophysiological properties of Re neurons allowing us to better consider the role of the Re in cognitive processes.

ABSTRACT

The nucleus reuniens (Re) is the largest of the midline thalamic nuclei. We have performed a detailed neurophysiological characterization of neurons in the rostral Re of brain slices prepared from adult male mice. At resting potential (-63.7 ± 0.6 mV), ∼90% of Re neurons fired action potentials, typically continuously at ∼8 Hz. Although Re neurons experience a significant spontaneous barrage of fast, amino-acid-mediate synaptic transmission, this was not predominantly responsible for spontaneous spiking because firing persisted in the presence of glutamate and GABA receptor antagonists. With resting potential preset to -80 mV, -20 pA current injections revealed a mean input resistance of 615 MΩ and a mean time constant of 38 ms. Following cessation of this stimulus, a significant rebound potential was seen that was sometimes sufficiently large to trigger a short burst of very high frequency (100-300 Hz) firing. In most cells, short (2 ms), strong (2 nA) current injections elicited a single spike followed by a large afterdepolarizing potential which, when suprathreshold, generated high-frequency spiking. Similarly, in the majority of cells preset at -80 mV, 500 ms depolarizing current injections to cells led to a brief initial burst of very high-frequency firing, although this was lost when cells were preset at -72 mV. Biophysical and pharmacological experiments indicate a prominent role for T-type Ca2+ channels in the high-frequency bursting of Re neurons. Finally, we describe a novel form of activity-dependent intrinsic plasticity that persistently eliminates the burst firing potential of Re neurons.

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'.

Interaction of nucleus reuniens and entorhinal cortex projections in hippocampal field CA1 of the rat

The nucleus reuniens (RE) and entorhinal cortex (EC) provide monosynaptic excitatory inputs to the apical dendrites of pyramidal cells and to interneurons with dendrites in stratum lacunosum moleculare (LM) of hippocampal field CA1. However, whether the RE and EC inputs interact at the cellular level is unknown. In this electrophysiological in vivo study, low-frequency stimulation was used to selectively activate each projection at its origin; field excitatory postsynaptic potentials (fEPSPs) were recorded in CA1. We applied (1) paired pulses to RE or EC, (2) combined paired pulses to RE and EC, and (3) simultaneously paired pulses to RE/EC. The main findings are that: (a) stimulation of either RE- or EC-evoked subthreshold fEPSPs, displaying paired pulse facilitation (PPF), (b) subthreshold fEPSPs evoked by combined stimulation did not display heterosynaptic PPF, and (c) simultaneous stimulation of RE/EC resulted in enhanced subthreshold fEPSPs in proximal LM displaying a nonlinear interaction. CSD analyses of RE/EC-evoked depth profiles revealed a nonlinear enlargement of the 'LM sink-radiatum source' configuration and the appearance of an additional small sink-source pair close to stratum pyramidale, likely reflecting (peri)somatic inhibition. The nonlinear interaction between both inputs indicates that RE and EC axons form synapses, at least partly, onto the same dendritic compartments of CA1 pyramidal cells. We propose that low-frequency activation of the RE-CA1 input facilitates the entorhinal-hippocampal dialogue, and may synchronize the neocortical-hippocampal slow oscillation which is relevant for hippocampal-dependent memory consolidation.

Cellular and System Biology of Memory: Timing, Molecules, and Beyond

The storage of information in the mammalian nervous systems is dependent on a delicate balance between change and stability of neuronal networks. The induction and maintenance of processes that lead to changes in synaptic strength to a multistep process which can lead to long-lasting changes, which starts and ends with a highly choreographed and perfectly timed dance of molecules in different cell types of the central nervous system. This is accompanied by synchronization of specific networks, resulting in the generation of characteristic "macroscopic" rhythmic electrical fields, whose characteristic frequencies correspond to certain activity and information-processing states of the brain. Molecular events and macroscopic fields influence each other reciprocally. We review here cellular processes of synaptic plasticity, particularly functional and structural changes, and focus on timing events that are important for the initial memory acquisition, as well as mechanisms of short- and long-term memory storage. Then, we cover the importance of epigenetic events on the long-time range. Furthermore, we consider how brain rhythms at the network level participate in processes of information storage and by what means they participating in it. Finally, we examine memory consolidation at the system level during processes of sleep.

Altered Intrinsic Pyramidal Neuron Properties and Pathway-Specific Synaptic Dysfunction Underlie Aberrant Hippocampal Network Function in a Mouse Model of Tauopathy

The formation and deposition of tau protein aggregates is proposed to contribute to cognitive impairments in dementia by disrupting neuronal function in brain regions, including the hippocampus. We used a battery of in vivo and in vitro electrophysiological recordings in the rTg4510 transgenic mouse model, which overexpresses a mutant form of human tau protein, to investigate the effects of tau pathology on hippocampal neuronal function in area CA1 of 7- to 8-month-old mice, an age point at which rTg4510 animals exhibit advanced tau pathology and progressive neurodegeneration. In vitro recordings revealed shifted theta-frequency resonance properties of CA1 pyramidal neurons, deficits in synaptic transmission at Schaffer collateral synapses, and blunted plasticity and imbalanced inhibition at temporoammonic synapses. These changes were associated with aberrant CA1 network oscillations, pyramidal neuron bursting, and spatial information coding in vivo. Our findings relate tauopathy-associated changes in cellular neurophysiology to altered behavior-dependent network function.

SIGNIFICANCE STATEMENT

Dementia is characterized by the loss of learning and memory ability. The deposition of tau protein aggregates in the brain is a pathological hallmark of dementia; and the hippocampus, a brain structure known to be critical in processing learning and memory, is one of the first and most heavily affected regions. Our results show that, in area CA1 of hippocampus, a region involved in spatial learning and memory, tau pathology is associated with specific disturbances in synaptic, cellular, and network-level function, culminating in the aberrant encoding of spatial information and spatial memory impairment. These studies identify several novel ways in which hippocampal information processing may be disrupted in dementia, which may provide targets for future therapeutic intervention.

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.

Role of GABA(B) receptors in learning and memory and neurological disorders

Although it is evident from the literature that altered GABAB receptor function does affect behavior, these results often do not correspond well. These differences could be due to the task protocol, animal strain, ligand concentration, or timing of administration utilized. Because several clinical populations exhibit learning and memory deficits in addition to altered markers of GABA and the GABAB receptor, it is important to determine whether altered GABAB receptor function is capable of contributing to the deficits. The aim of this review is to examine the effect of altered GABAB receptor function on synaptic plasticity as demonstrated by in vitro data, as well as the effects on performance in learning and memory tasks. Finally, data regarding altered GABA and GABAB receptor markers within clinical populations will be reviewed. Together, the data agree that proper functioning of GABAB receptors is crucial for numerous learning and memory tasks and that targeting this system via pharmaceuticals may benefit several clinical populations.

Dendritic sodium spikes are required for long-term potentiation at distal synapses on hippocampal pyramidal neurons

Dendritic integration of synaptic inputs mediates rapid neural computation as well as longer-lasting plasticity. Several channel types can mediate dendritically initiated spikes (dSpikes), which may impact information processing and storage across multiple timescales; however, the roles of different channels in the rapid vs long-term effects of dSpikes are unknown. We show here that dSpikes mediated by Nav channels (blocked by a low concentration of TTX) are required for long-term potentiation (LTP) in the distal apical dendrites of hippocampal pyramidal neurons. Furthermore, imaging, simulations, and buffering experiments all support a model whereby fast Nav channel-mediated dSpikes (Na-dSpikes) contribute to LTP induction by promoting large, transient, localized increases in intracellular calcium concentration near the calcium-conducting pores of NMDAR and L-type Cav channels. Thus, in addition to contributing to rapid neural processing, Na-dSpikes are likely to contribute to memory formation via their role in long-lasting synaptic plasticity.

Leptin Induces a Novel Form of NMDA Receptor-Dependent LTP at Hippocampal Temporoammonic-CA1 Synapses

Hippocampal CA1 pyramidal neurons receive two anatomically distinct glutamatergic inputs that have distinct roles in learning and memory. The hormone leptin markedly influences excitatory synaptic transmission at the indirect Schaffer-collateral pathway to CA1 neurons.

It is well documented that the hormone leptin regulates many central functions and that hippocampal CA1 pyramidal neurons are a key target for leptin action. Indeed, leptin modulates excitatory synaptic transmission and synaptic plasticity at the Schaffer-collateral input to CA1 neurons. However the impact of leptin on the direct temporoammonic (TA) input to CA1 neurons is not known. Here we show that leptin evokes a long-lasting increase [long-term potentiation (LTP)] in excitatory synaptic transmission at TA-CA1 synapses in rat juvenile hippocampus. Leptin-induced LTP was NMDA receptor-dependent and specifically involved the activation of GluN2B subunits. The signaling pathways underlying leptin-induced LTP involve the activation of phosphoinositide 3-kinase, but were independent of the ERK signaling cascade. Moreover, insertion of GluA2-lacking AMPA receptors was required for leptin-induced LTP as prior application of philanthotoxin prevented the effects of leptin. In addition, synaptic-induced LTP occluded the persistent increase in synaptic efficacy induced by leptin. In conclusion, these data indicate that leptin induces a novel form of NMDA receptor-dependent LTP at juvenile TA-CA1 synapses, which has important implications for the role of leptin in modulating hippocampal synaptic function in health and disease.

Neurophysiological modification of CA1 pyramidal neurons in a transgenic mouse expressing a truncated form of disrupted-in-schizophrenia 1

A t(1;11) balanced chromosomal translocation transects the Disc1 gene in a large Scottish family and produces genome-wide linkage to schizophrenia and recurrent major depressive disorder. This study describes our in vitro investigations into neurophysiological function in hippocampal area CA1 of a transgenic mouse (DISC1_tr) that expresses a truncated version of DISC1 designed to reproduce aspects of the genetic situation in the Scottish t(1;11) pedigree. We employed both patch-clamp and extracellular recording methods in vitro to compare intrinsic properties and synaptic function and plasticity between DISC1_tr animals and wild-type littermates. Patch-clamp analysis of CA1 pyramidal neurons (CA1-PNs) revealed no genotype dependence in multiple subthreshold parameters, including resting potential, input resistance, hyperpolarization-activated ‘sag’ and resonance properties. Suprathreshold stimuli revealed no alteration to action potential (AP) waveform, although the initial rate of AP production was higher in DISC1_tr mice. No difference was observed in afterhyperpolarizing potentials following trains of 5–25 APs at 50 Hz. Patch-clamp analysis of synaptic responses in the Schaffer collateral commissural (SC) pathway indicated no genotype-dependence of paired pulse facilitation, excitatory postsynaptic potential summation or AMPA/NMDA ratio. Extracellular recordings also revealed an absence of changes to SC synaptic responses and indicated input–output and short-term plasticity were also unaltered in the temporoammonic (TA) input. However, in DISC1_tr mice theta burst-induced long-term potentiation was enhanced in the SC pathway but completely lost in the TA pathway. These data demonstrate that expressing a truncated form of DISC1 affects intrinsic properties of CA1-PNs and produces pathway-specific effects on long-term synaptic plasticity.

Visuospatial learning and memory in the Cebus apella and microglial morphology in the molecular layer of the dentate gyrus and CA1 lacunosum molecular layer

We investigated whether the morphology of microglia in the molecular layer of the dentate gyrus (DG-Mol) or in the lacunosum molecular layer of CA1 (CA1-LMol) was correlated with spatial learning and memory in the capuchin monkey (Cebus apella). Learning and memory was tested in 4 monkeys with visuo-spatial, paired associated learning (PAL) tasks from the Cambridge battery of neuropsychological tests. After testing, monkeys were sacrificed, and hippocampi were sectioned. We specifically immunolabeled microglia with an antibody against the adapter binding, ionized calcium protein. Microglia were selected from the middle and outer thirds of the DG-Mol (n=268) and the CA1-LMol (n=185) for three-dimensional reconstructions created with Neurolucida and Neuroexplorer software. Cluster and discriminant analyses, based on microglial morphometric parameters, identified two major morphological microglia phenotypes (types I and II) found in both the CA1-LMol and DG-Mol of all individuals. Compared to type II, type I microglia were significantly smaller, thinner, more tortuous and ramified, and less complex (lower fractal dimensions). PAL performance was both linearly and non-linearly correlated with type I microglial morphological features from the rostral and caudal DG-Mol, but not with microglia from the CA1-LMol. These differences in microglial morphology and correlations with PAL performance were consistent with previous proposals of hippocampal regional contributions for spatial learning and memory. Our results suggested that at least two morphological microglial phenotypes provided distinct physiological roles to learning-associated activity in the rostral and caudal DG-Mol of the monkey brain.

GINIP, a Gαi-interacting protein, functions as a key modulator of peripheral GABAB receptor-mediated analgesia

One feature of neuropathic pain is a reduced GABAergic inhibitory function. Nociceptors have been suggested to play a key role in this process. However, the mechanisms behind nociceptor-mediated modulation of GABA signaling remain to be elucidated. Here we describe the identification of GINIP, a Gαi-interacting protein expressed in two distinct subsets of nonpeptidergic nociceptors. GINIP null mice develop a selective and prolonged mechanical hypersensitivity in models of inflammation and neuropathy. GINIP null mice show impaired responsiveness to GABAB, but not to delta or mu opioid receptor agonist-mediated analgesia specifically in the spared nerve injury (SNI) model. Consistently, GINIP-deficient dorsal root ganglia neurons had lower baclofen-evoked inhibition of high-voltage-activated calcium channels and a defective presynaptic inhibition of lamina IIi interneurons. These results further support the role of unmyelinated C fibers in injury-induced modulation of spinal GABAergic inhibition and identify GINIP as a key modulator of peripherally evoked GABAB-receptors signaling.

Early structural and functional defects in synapses and myelinated axons in stratum lacunosum moleculare in two preclinical models for tauopathy

The stratum lacunosum moleculare (SLM) is the connection hub between entorhinal cortex and hippocampus, two brain regions that are most vulnerable in Alzheimer's disease. We recently identified a specific synaptic deficit of Nectin-3 in transgenic models for tauopathy. Here we defined cognitive impairment and electrophysiological problems in the SLM of Tau.P301L mice, which corroborated the structural defects in synapses and dendritic spines. Reduced diffusion of DiI from the ERC to the hippocampus indicated defective myelinated axonal pathways. Ultrastructurally, myelinated axons in the temporoammonic pathway (TA) that connects ERC to CA1 were damaged in Tau.P301L mice at young age. Unexpectedly, the myelin defects were even more severe in bigenic biGT mice that co-express GSK3β with Tau.P301L in neurons. Combined, our data demonstrate that neuronal expression of protein Tau profoundly affected the functional and structural organization of the entorhinal-hippocampal complex, in particular synapses and myelinated axons in the SLM. White matter pathology deserves further attention in patients suffering from tauopathy and Alzheimer's disease.

Alternative polyadenylation and differential expression of Shank mRNAs in the synaptic neuropil

The stability and dynamics of synapses rely on tight regulation of the synaptic proteome. Shank proteins, encoded by the three genes Shank1, Shank2 and Shank3 are scaffold molecules in the postsynaptic density of excitatory neurons that contribute to activity-dependent neuronal signalling. Mutations in the Shank genes are associated with neurological diseases. Using state-of-the-art technologies, we investigated the levels of expression of the Shank family messenger RNAs (mRNAs) within the synaptic neuropil of the rat hippocampus. We detected all three Shank transcripts in the neuropil of CA1 pyramidal neurons. We found Shank1 to be the most abundantly expressed among the three Shank mRNA homologues. We also examined the turnover of Shank mRNAs and predict the half-lives of Shank1, Shank2 and Shank3 mRNAs to be 18-28 h. Using 3'-end sequencing, we identified novel 3' ends for the Shank1 and Shank2 3' untranslated regions (3' UTRs) that may contribute to the diversity of alternative polyadenylation (APA) for the Shank transcripts. Our findings consolidate the view that the Shank molecules play a central role at the postsynaptic density. This study may shed light on synaptopathologies associated with disruption of local protein synthesis, perhaps linked to mutations in mRNA 3' UTRs or inappropriate 3' end processing.

Short- and long-term plasticity in CA1 neurons from mice lacking h-channel auxiliary subunit TRIP8b

Hyperpolarization-activated cyclic nucleotide-gated nonselective cation channels (HCN or h-channels) are important regulators of neuronal physiology contributing to passive membrane properties, such as resting membrane potential and input resistance (R(N)), and to intrinsic oscillatory activity and synaptic integration. The correct membrane targeting of h-channels is regulated in part by the auxiliary h-channel protein TRIP8b. The genetic deletion of TRIP8b results in a loss of functional h-channels, which affects the postsynaptic integrative properties of neurons. We investigated the impact of TRIP8b deletion on long-term potentiation (LTP) at the two major excitatory inputs to CA1 pyramidal neurons: Schaffer collateral (SC) and perforant path (PP). We found that SC LTP was not significantly different between neurons from wild-type and TRIP8b-knockout mice. There was, however, significantly more short-term potentiation in knockout neurons. We also found that the persistent increase in h-current (I(h)) that normally occurs after LTP induction was absent in knockout neurons. The lack of I(h) plasticity was not restricted to activity-dependent induction, because the depletion of intracellular calcium stores also failed to produce the expected increase in I(h). Interestingly, pairing of SC and PP inputs resulted in a form of LTP in knockout neurons that did not occur in wild-type neurons. These results suggest that the physiological impact of TRIP8b deletion is not restricted to the integrative properties of neurons but also includes both synaptic and intrinsic plasticity.

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

In terms of its sub-regional differentiation, the hippocampal CA1 region receives cortical information directly via the perforant (temporoammonic) path (pp-CA1 synapse) and indirectly via the tri-synaptic pathway where the last relay station is the Schaffer collateral-CA1 synapse (Sc-CA1 synapse). Research to date on pp-CA1 synapses has been conducted predominantly in vitro and never in awake animals, but these studies hint that information processing at this synapse might be distinct to processing at the Sc-CA1 synapse. Here, we characterized synaptic properties and synaptic plasticity at the pp-CA1 synapse of freely behaving adult rats. We observed that field excitatory postsynaptic potentials at the pp-CA1 synapse have longer onset latencies and a shorter time-to-peak compared to the Sc-CA1 synapse. LTP (>24 h) was successfully evoked by tetanic afferent stimulation of pp-CA1 synapses. Low frequency stimulation evoked synaptic depression at Sc-CA1 synapses, but did not elicit LTD at pp-CA1 synapses unless the Schaffer collateral afferents to the CA1 region had been severed. Paired-pulse responses also showed significant differences. Our data suggest that synaptic plasticity at the pp-CA1 synapse is distinct from the Sc-CA1 synapse and that this may reflect its specific role in hippocampal information processing.

Synaptic integration by different dendritic compartments of hippocampal CA1 and CA2 pyramidal neurons

Pyramidal neurons have a complex dendritic arbor containing tens of thousands of synapses. In order for the somatic/axonal membrane potential to reach action potential threshold, concurrent activation of multiple excitatory synapses is required. Frequently, instead of a simple algebraic summation of synaptic potentials in the soma, different dendritic compartments contribute to the integration of multiple inputs, thus endowing the neuron with a powerful computational ability. Most pyramidal neurons share common functional properties. However, different and sometimes contrasting dendritic integration rules are also observed. In this review, we focus on the dendritic integration of two neighboring pyramidal neurons in the hippocampus: the well-characterized CA1 and the much less understood CA2. The available data reveal that the dendritic integration of these neurons is markedly different even though they are targeted by common inputs at similar locations along their dendrites. This contrasting dendritic integration results in different routing of information flow and generates different corticohippocampal loops.

Passive spatial perception facilitates the expression of persistent hippocampal long-term depression

Learning-facilitated plasticity describes the ability of hippocampal synapses to respond with synaptic plasticity when weak afferent stimulation is coupled with a spatial learning event. Qualitative differences appear to influence whether long-term potentiation or long-term depression (LTD) are facilitated by spatial learning. At many hippocampal synapses, LTD is facilitated when rats actively explore a novel spatial context. We investigated whether learning-facilitated plasticity is expressed when an unconstrained but stationary rat observes a computer-generated spatial environment. Visual fields were separated. Novel object configurations were presented to one field; familiar constellations were presented to the other field. LTD was facilitated in the CA1 region of the hemisphere to which novel object constellations were presented. Familiar constellations had no effect. LTD facilitation was prevented by treatment with the protein translation inhibitor, anisomycin. LTD in the dentate gyrus was not facilitated by novel object constellations, suggesting that effects are not common to all hippocampal subfields. These data support a unique association of LTD in the CA1 region with learning about spatial context and indicate that rats can passively perceive space.

Grid cells generate an analog error-correcting code for singularly precise neural computation

Entorhinal grid cells in mammals fire as a function of animal location, with spatially periodic response patterns. This nonlocal periodic representation of location, a local variable, is unlike other neural codes. There is no theoretical explanation for why such a code should exist. We examined how accurately the grid code with noisy neurons allows an ideal observer to estimate location and found this code to be a previously unknown type of population code with unprecedented robustness to noise. In particular, the representational accuracy attained by grid cells over the coding range was in a qualitatively different class from what is possible with observed sensory and motor population codes. We found that a simple neural network can effectively correct the grid code. To the best of our knowledge, these results are the first demonstration that the brain contains, and may exploit, powerful error-correcting codes for analog variables.

Endocannabinoids differentially modulate synaptic plasticity in rat hippocampal CA1 pyramidal neurons

BACKGROUND

Hippocampal CA1 pyramidal neurons receive two excitatory glutamatergic synaptic inputs: their most distal dendritic regions in the stratum lacunosum-moleculare (SLM) are innervated by the perforant path (PP), originating from layer III of the entorhinal cortex, while their more proximal regions of the apical dendrites in the stratum radiatum (SR) are innervated by the Schaffer-collaterals (SC), originating from hippocampal CA3 neurons. Endocannabinoids (eCBs) are naturally occurring mediators capable of modulating both GABAergic and glutamatergic synaptic transmission and plasticity via the CB1 receptor. Previous work on eCB modulation of excitatory synapses in the CA1 region largely focuses on the SC pathway. However, little information is available on whether and how eCBs modulate glutamatergic synaptic transmission and plasticity at PP synapses.

METHODOLOGY/PRINCIPAL FINDINGS

By employing somatic and dendritic patch-clamp recordings, Ca(2+) uncaging, and immunostaining, we demonstrate that there are significant differences in low-frequency stimulation (LFS)- or DHPG-, an agonist of group I metabotropic glutamate receptors (mGluRs), induced long-term depression (LTD) of excitatory synaptic transmission between SC and PP synapses in the same pyramidal neurons. These differences are eliminated by pharmacological inhibition with selective CB1 receptor antagonists or genetic deletion of the CB1 receptor, indicating that these differences likely result from differential modulation via a CB1 receptor-dependent mechanism. We also revealed that depolarization-induced suppression of excitation (DSE), a form of short-term synaptic plasticity, and photolysis of caged Ca(2+)-induced suppression of Excitatory postsynaptic currents (EPSCs) were less at the PP than that at the SC. In addition, application of WIN55212 (WIN) induced a more pronounced inhibition of EPSCs at the SC when compared to that at the PP.

CONCLUSIONS/SIGNIFICANCE

Our results suggest that CB1 dependent LTD and DSE are differentially expressed at the PP versus SC synapses in the same neurons, which may have an impact on synaptic scaling, integration and plasticity of hippocampal CA1 pyramidal neurons.

Conditional gene deletion reveals functional redundancy of GABAB receptors in peripheral nociceptors in vivo

BACKGROUND

gamma-aminobutyric acid (GABA) is an important inhibitory neurotransmitter which mainly mediates its effects on neurons via ionotropic (GABA(A)) and metabotropic (GABA(B)) receptors. GABA(B) receptors are widely expressed in the central and the peripheral nervous system. Although there is evidence for a key function of GABA(B) receptors in the modulation of pain, the relative contribution of peripherally- versus centrally-expressed GABA(B) receptors is unclear.

RESULTS

In order to elucidate the functional relevance of GABA(B) receptors expressed in peripheral nociceptive neurons in pain modulation we generated and analyzed conditional mouse mutants lacking functional GABA(B1) subunit specifically in nociceptors, preserving expression in the spinal cord and brain (SNS-GABA(B1)-/- mice). Lack of the GABA(B1) subunit precludes the assembly of functional GABA(B) receptor. We analyzed SNS-GABA(B1)-/- mice and their control littermates in several models of acute and neuropathic pain. Electrophysiological studies on peripheral afferents revealed higher firing frequencies in SNS-GABA(B1)-/- mice compared to corresponding control littermates. However no differences were seen in basal nociceptive sensitivity between these groups. The development of neuropathic and chronic inflammatory pain was similar across the two genotypes. The duration of nocifensive responses evoked by intraplantar formalin injection was prolonged in the SNS-GABAB(1)-/- animals as compared to their control littermates. Pharmacological experiments revealed that systemic baclofen-induced inhibition of formalin-induced nociceptive behaviors was not dependent upon GABA(B1) expression in nociceptors.

CONCLUSION

This study addressed contribution of GABA(B) receptors expressed on primary afferent nociceptive fibers to the modulation of pain. We observed that neither the development of acute and chronic pain nor the analgesic effects of a systematically-delivered GABA(B) agonist was significantly changed upon a specific deletion of GABA(B) receptors from peripheral nociceptive neurons in vivo. This lets us conclude that GABA(B) receptors in the peripheral nervous system play a less important role than those in the central nervous system in the regulation of pain.

Recruitment of N-Type Ca(2+) channels during LTP enhances low release efficacy of hippocampal CA1 perforant path synapses

The entorhinal cortex provides both direct and indirect inputs to hippocampal CA1 neurons through the perforant path and Schaffer collateral synapses, respectively. Using both two-photon imaging of synaptic vesicle cycling and electrophysiological recordings, we found that the efficacy of transmitter release at perforant path synapses is lower than at Schaffer collateral inputs. This difference is due to the greater contribution to release by presynaptic N-type voltage-gated Ca(2+) channels at the Schaffer collateral than perforant path synapses. Induction of long-term potentiation that depends on activation of NMDA receptors and L-type voltage-gated Ca(2+) channels enhances the low efficacy of release at perforant path synapses by increasing the contribution of N-type channels to exocytosis. This represents a previously uncharacterized presynaptic mechanism for fine-tuning release properties of distinct classes of synapses onto a common postsynaptic neuron and for regulating synaptic function during long-term synaptic plasticity.

Alpha2 nicotine receptors function as a molecular switch to continuously excite a subset of interneurons in rat hippocampal circuits

Rapid activation of nicotinic acetylcholine receptors (nAChRs) at various anatomical and cellular locations in the hippocampus differentially modulates the operation of hippocampal circuits. However, it is largely unknown how the continued presence of nicotine affects the normal operation of hippocampal circuits. Here, we used single and dual whole-cell recordings to address this question. We found that horizontally oriented interneurons in the stratum oriens/alveus continuously discharged action potentials in the presence of nicotine. In these interneurons, bath application of nicotine produced slow inward currents that were well maintained and inhibited by the non-alpha 7 antagonist dihydro-beta-erythroidine. Single-cell reverse transcription-polymerase chain reaction analysis showed that nicotine-responding interneurons were consistently positive for the alpha2 subunit mRNA. These observations suggest that in the presence of nicotine, a subset of interneurons in the stratum oriens/alveus are continuously excited due to the sustained activation of alpha2* nAChRs. These interneurons were synaptically connected to pyramidal cells, and nicotine increased inhibitory baseline currents at the synapses and suppressed phasic inhibition at the same synapses. Nicotine-induced inhibitory activity increased background noise and masked small phasic inhibition in pyramidal cells, originating from other interneurons in the stratum radiatum. Thus, the continued presence of nicotine alters the normal operation of hippocampal circuits by gating inhibitory circuits through activating a non-desensitizing alpha2 nAChR subtype on a distinct population of interneurons.

Membrane potential response profiles of CA1 pyramidal cells probed with voltage-sensitive dye optical imaging in rat hippocampal slices reveal the impact of GABA(A)-mediated feed-forward inhibition in signal propagation

The spatial and temporal distribution of excitatory and inhibitory membrane potential responses on a cell plays an important role in neuronal calculations in local neuronal circuits in the brain. The electrical dynamics of excitatory and inhibitory inputs along the somatodendritic extent of CA1 pyramidal cells during circuit activation were examined by stimulating strata radiatum (SR), oriens (SO), and lacunosum-moleculare (SLM) and measuring laminar responses with voltage-sensitive dye (VSD) optical recording methods. We first confirmed the linearity of the optical signal by comparing fluorescence changes in CA1 to global membrane potential changes when slices were bathed in high-potassium ([K+](O)=25 mM) solution. Except for a TTX-sensitive component in stratum pyramidale, fluorescence changes were equal in all strata, indicating that VSD sensitivity had reasonable linearity across layers. We then compared membrane potential profiles in slices exposed to picrotoxin, a GABA(A) receptor antagonist. We attributed the picrotoxin-induced changes in the first peak of the excitatory membrane potential to feed-forward inhibition and the later response (appearing 30 ms after stimulation) to feedback inhibition. A difference in feed-forward components was observed in perisomatic and distal apical dendritic regions after SR stimulation. SLM stimulation produced large differences in perisomatic and apical dendritic regions. SO stimulation, however, produced no feed-forward inhibition at the perisomatic region, but produces feed-forward inhibition in distal dendritic regions. These results suggest that actual inhibition of membrane potential response by feed-forward inhibition is greater at perisomatic regions after SR or SLM stimulation but is smaller at distal dendritic regions after SR, SO, and SLM stimulation.

Developmental changes in short-term facilitation are opposite at temporoammonic synapses compared to Schaffer collateral synapses onto CA1 pyramidal cells

CA1 pyramidal neurons receive two distinct excitatory inputs that are each capable of influencing hippocampal output and learning and memory. The Schaffer collateral (SC) input from CA3 axons onto the more proximal dendrites of CA1 is part of the trisynaptic circuit, which originates in Layer II of the entorhinal cortex (EC). The temporoammonic (TA) pathway to CA1 provides input directly from Layer III of the EC onto the most distal dendrites of CA1 pyramidal cells, and is involved in spatial memory and memory consolidation. We have previously described a developmental decrease in short-term facilitation from juvenile (P13-18) to young adult (P28-42) rats at SC synapses that is due to feedback inhibition via synaptically activated mGluR1 on CA1 interneurons. It is not known how short-term changes in synaptic strength are regulated at TA synapses, nor is it known how short-term plasticity is balanced at SC and TA inputs during development. Here we describe a novel developmental increase in short-term facilitation at TA synapses, which is the opposite of the decrease in facilitation occurring at SC synapses. Although short-term facilitation is much lower at TA synapses when compared with SC synapses in juveniles, short-term plasticity at SC and TA synapses converges at similar levels of paired-pulse facilitation in the young adult rat. However, in young adults CA3-CA1 synapses still exhibit more facilitation than TA-CA1 synapses during physiologically-relevant activity, suggesting that the two pathways are each poised to uniquely modulate CA1 output in an activity-dependent manner. Finally, we show that there is a developmental decrease in the initial release probability at TA synapses that underlies their developmental decrease in facilitation, but no developmental change in release probability at SC synapses. This represents a fundamental difference in the presynaptic function of the two major inputs to CA1, which could alter the flow of information in hippocampus during development.

Frequency-dependent signal transmission and modulation by neuromodulators

The brain uses a strategy of labor division, which may allow it to accomplish more elaborate and complicated tasks, but in turn, imposes a requirement for central control to integrate information among different brain areas. Anatomically, the divergence of long-range neuromodulator projections appears well-suited to coordinate communication between brain areas. Oscillatory brain activity is a prominent feature of neural transmission. Thus, the ability of neuromodulators to modulate signal transmission in a frequency-dependent manner adds an additional level of regulation. Here, we review the significance of frequency-dependent signal modulation in brain function and how a neuronal network can possess such properties. We also describe how a neuromodulator, dopamine, changes frequency-dependent signal transmission, controlling information flow from the entorhinal cortex to the hippocampus.