Leptin prevents aberrant targeting of tau to hippocampal synapses via PI 3 kinase driven inhibition of GSK3ß.

Amyloid-ß (Aß) and hyper-phosphorylated tau are key hallmarks of Alzheimer's disease (AD), with an accumulation of both proteins linked to hippocampal synaptic dysfunction. Recent evidence indicates that Aß drives mis-localisation of tau from axons to synapses, resulting in AMPA receptor (AMPAR) internalisation and impaired excitatory synaptic function. These tau-driven synaptic impairments are thought to underlie the cognitive deficits in AD. Consequently, limiting the synapto-toxic effects of tau may prevent AD-related cognitive deficits. Increasing evidence links leptin dysfunction with higher AD risk, and numerous studies have identified neuroprotective properties of leptin in AD models of Aß-induced toxicity. However, it is unclear if leptin protects against tau-related synaptic dysfunction. Here we show that Aß1-42 significantly increases dendritic and synaptic levels of tau and p-tau in hippocampal neurons, and these effects were blocked by leptin. In accordance with GSK-3ß being involved in tau phosphorylation, the protective effects of leptin involve PI 3-kinase (PI3K) activation and inhibition of GSK-3ß. Aß1-42 -driven synaptic targeting of tau was associated with the removal of GluA1-containing AMPARs from synapses, which was also inhibited by leptin-driven inhibition of GSK-3ß. Direct application of oligomeric tau to hippocampal neurons caused internalisation of GluA1-containing AMPARs and this effect was blocked by prior application of leptin. Similarly, leptin prevented the ability of tau to block induction of activity-dependent long-term potentiation (LTP) at hippocampal SC-CA1 synapses. These findings increase our understanding of the neuroprotective actions of leptin in the early pre-clinical stages of AD and further validate the leptin system as a therapeutic target in AD.

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 17ß estradiol (E2) regulates excitatory synaptic transmission at hippocampal Shaffer-collateral (SC)-CA1 synapses, via activation of the classical estrogen receptors (ERα and ERß). 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-based hexamers facilitate memory and prevent amyloid-driven AMPA receptor internalisation and neuronal degeneration.

Key pathological features of Alzheimer's disease (AD) include build-up of amyloid ß (Aß), which promotes synaptic abnormalities and ultimately leads to neuronal cell death. Metabolic dysfunction is known to influence the risk of developing AD. Impairments in the leptin system have been detected in AD patients, which has fuelled interest in targeting this system to treat AD. Increasing evidence supports pro-cognitive and neuroprotective actions of leptin and these beneficial effects of leptin are mirrored by a bioactive leptin fragment (leptin116-130 ). Here we extend these studies to examine the potential cognitive enhancing and neuroprotective actions of 8 six-amino acid peptides (hexamers) derived from leptin116-130 . In this study, we show that four of the hexamers (leptin116-121, 117-122, 118-123 and 120-125 ) replicate the ability of leptin to promote α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor trafficking and facilitate hippocampal synaptic plasticity. Moreover, the pro-cognitive effects of the hexamers were verified in behavioural studies, with the administration of leptin117-122 enhancing performance in episodic memory tasks. The bioactive hexamers replicated the neuroprotective actions of leptin by preventing the acute hippocampal synapto-toxic effects of Aß, and the chronic effects of Aß on neuronal cell viability, Aß seeding and tau phosphorylation. These findings provide further evidence to support leptin and leptin-derived peptides as potential therapeutics for AD.

Food for Thought: Leptin and Hippocampal Synaptic Function.

It is well documented that the endocrine hormone, leptin controls energy homeostasis by providing key signals to specific hypothalamic nuclei. However, our knowledge of leptin's central actions has advanced considerably over the last 20 years, with the hippocampus now established as an important brain target for this hormone. Leptin receptors are highly localised to hippocampal synapses, and increasing evidence reveals that activation of synaptically located leptin receptors markedly impacts cognitive processes, and specifically hippocampal-dependent learning and memory. Here, we review the recent actions of leptin at hippocampal synapses and explore the consequences for brain health and disease.

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.

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.

Leptin regulation of hippocampal synaptic function in health and disease.

It is widely accepted that the metabolic hormone leptin regulates food intake and body weight via activation of hypothalamic leptin receptors. However, as leptin receptors are also highly expressed in other brain regions, such as the hippocampus, alterations in leptin responsiveness also impacts on key functions of the hippocampus, like learning and memory. Within the hippocampus, high levels of leptin receptors are expressed at excitatory synapses, and in accordance with a synaptic localization, leptin potently regulates synaptic transmission at both Schaffer collateral (SC) and temporoammonic (TA) inputs to CA1 pyramidal neurons. Increasing evidence from cellular and behavioral studies examining leptin action at CA1 synapses support the notion that leptin is a potential cognitive enhancer. However, the capacity of leptin to regulate synaptic efficacy at SC-CA1 and TA-CA1 synapses declines in an age-dependent manner. Moreover, clinical evidence that supports a link between circulating leptin levels and the risk of the age-related neurodegenerative disorder, Alzheimer's disease (AD) is accumulating. Consequently, it has been proposed that the leptin system is a potential therapeutic target in AD, and that boosting the hippocampal actions of leptin may be beneficial in the treatment of AD. Here we review recent progress in our understanding of the neuronal and hippocampal synaptic functions that are regulated by leptin and how alterations in the leptin system influence age-related CNS-related disorders like AD.

The Neuronal Actions of Leptin and the Implications for Treating Alzheimer's Disease.

It is widely accepted that the endocrine hormone leptin controls food intake and energy homeostasis via activation of leptin receptors expressed on hypothalamic arcuate neurons. The hippocampal formation also displays raised levels of leptin receptor expression and accumulating evidence indicates that leptin has a significant impact on hippocampal synaptic function. Thus, cellular and behavioural studies support a cognitive enhancing role for leptin as excitatory synaptic transmission, synaptic plasticity and glutamate receptor trafficking at hippocampal Schaffer collateral (SC)-CA1 synapses are regulated by leptin, and treatment with leptin enhances performance in hippocampus-dependent memory tasks. Recent studies indicate that hippocampal temporoammonic (TA)-CA1 synapses are also a key target for leptin. The ability of leptin to regulate TA-CA1 synapses has important functional consequences as TA-CA1 synapses are implicated in spatial and episodic memory processes. Moreover, degeneration is initiated in the TA pathway at very early stages of Alzheimer's disease, and recent clinical evidence has revealed links between plasma leptin levels and the incidence of Alzheimer's disease (AD). Additionally, accumulating evidence indicates that leptin has neuroprotective actions in various AD models, whereas dysfunctions in the leptin system accelerate AD pathogenesis. Here, we review the data implicating the leptin system as a potential novel target for AD, and the evidence that boosting the hippocampal actions of leptin may be beneficial.

Brain energy rescue: an emerging therapeutic concept for neurodegenerative disorders of ageing.

The brain requires a continuous supply of energy in the form of ATP, most of which is produced from glucose by oxidative phosphorylation in mitochondria, complemented by aerobic glycolysis in the cytoplasm. When glucose levels are limited, ketone bodies generated in the liver and lactate derived from exercising skeletal muscle can also become important energy substrates for the brain. In neurodegenerative disorders of ageing, brain glucose metabolism deteriorates in a progressive, region-specific and disease-specific manner - a problem that is best characterized in Alzheimer disease, where it begins presymptomatically. This Review discusses the status and prospects of therapeutic strategies for countering neurodegenerative disorders of ageing by improving, preserving or rescuing brain energetics. The approaches described include restoring oxidative phosphorylation and glycolysis, increasing insulin sensitivity, correcting mitochondrial dysfunction, ketone-based interventions, acting via hormones that modulate cerebral energetics, RNA therapeutics and complementary multimodal lifestyle changes.

Activation of oestrogen receptor α induces a novel form of LTP at hippocampal temporoammonic-CA1 synapses.

BACKGROUND AND PURPOSE: 17ß estradiol (E2) rapidly regulates excitatory synaptic transmission at the classical Schaffer collateral (SC) input to hippocampal CA1 neurons. However, the impact of E2 on excitatory synaptic transmission at the distinct temporoammonic (TA) input to CA1 neurons and the oestrogen receptors involved is less clear. EXPERIMENTAL APPROACH: Extracellular recordings were used to monitor excitatory synaptic transmission in hippocampal slices from juvenile male (P11-24) Sprague Dawley rats. Immunocytochemistry combined with confocal microscopy was used to monitor the surface expression of the AMPA receptor (AMPAR) subunit, GluA1 in hippocampal neurons cultured from neonatal (P0-3) rats. KEY RESULTS: Here, we show that E2 induces a novel form of LTP at TA-CA1 synapses, an effect mirrored by the ERα agonist, PPT, and blocked by an ERα antagonist. ERα-induced LTP is NMDA receptor (NMDAR)-dependent and involves a postsynaptic expression mechanism that requires PI 3-kinase signalling and synaptic insertion of GluA2-lacking AMPARs. ERα-induced LTP has overlapping expression mechanisms with classical Hebbian LTP, as HFS-induced LTP occluded PPT-induced LTP and vice versa. In addition, activity-dependent LTP was blocked by the ERα antagonist, suggesting that ERα activation is involved in NMDA-LTP at TA-CA1 synapses. CONCLUSION AND IMPLICATIONS: ERα induces a novel form of LTP at juvenile male hippocampal TA-CA1 synapses. As TA-CA1 synapses are implicated in episodic memory processes and are an early target for neurodegeneration, these findings have important implications for the role of oestrogens in CNS health and neurodegenerative disease.

N-Palmitoylglycine and other N-acylamides activate the lipid receptor G2A/GPR132.

The G-protein-coupled receptor GPR132, also known as G2A, is activated by 9-hydroxyoctadecadienoic acid (9-HODE) and other oxidized fatty acids. Other suggested GPR132 agonists including lysophosphatidylcholine (LPC) have not been readily reproduced. Here, we identify N-acylamides in particular N-acylglycines, as lipid activators of GPR132 with comparable activity to 9-HODE. The order-of-potency is N-palmitoylglycine > 9-HODE ≈ N-linoleoylglycine > linoleamide > N-oleoylglycine ≈ N-stereoylglycine > N-arachidonoylglycine > N-docosehexanoylglycine. Physiological concentrations of N-acylglycines in tissue are sufficient to activate GPR132. N-linoleoylglycine and 9-HODE also activate rat and mouse GPR132, despite limited sequence conservation to human. We describe pharmacological tools for GPR132, identified through drug screening. SKF-95667 is a novel GPR132 agonist. SB-583831 and SB-583355 are peptidomimetic molecules containing core amino acids (glycine and phenylalanine, respectively), and structurally related to previously described ligands. A telmisartan analog, GSK1820795A, antagonizes the actions of N-acylamides at GPR132. The synthetic cannabinoid CP-55 940 also activates GPR132. Molecular docking to a homology model suggested a site for lipid binding, predicting the acyl side-chain to extend into the membrane bilayer between TM4 and TM5 of GPR132. Small-molecule ligands are envisaged to occupy a "classical" site encapsulated in the 7TM bundle. Structure-directed mutagenesis indicates a critical role for arginine at position 203 in transmembrane domain 5 to mediate GPR132 activation by N-acylamides. Our data suggest distinct modes of binding for small-molecule and lipid agonists to the GPR132 receptor. Antagonists, such as those described here, will be vital to understand the physiological role of this long-studied target.

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.

Regulation of Hippocampal Synaptic Function by the Metabolic Hormone, Leptin: Implications for Health and Neurodegenerative Disease.

The role of the endocrine hormone leptin in controlling energy homeostasis in the hypothalamus are well documented. However the CNS targets for leptin are not restricted to the hypothalamus as a high density of leptin receptors are also expressed in several parts of the brain involved in higher cognitive functions including the hippocampus. Numerous studies have identified that in the hippocampus, leptin has cognitive enhancing actions as exogenous application of this hormone facilitates hippocampal-dependent learning and memory, whereas lack or insensitivity to leptin results in significant memory deficits. Leptin also markedly influences some of the main cellular changes that are involved in learning and memory including NMDA-receptor dependent synaptic plasticity and glutamate receptor trafficking. Like other metabolic hormones, there is a significant decline in neuronal sensitivity to leptin during the ageing process. Indeed, the capacity of leptin to modulate the functioning of hippocampal synapses is substantially reduced in aged compared to adult tissue. Clinical studies have also identified an association between circulating leptin levels and the risk of certain neurodegenerative disorders such as Alzheimer's disease (AD). In view of this, targeting leptin and/or its receptor/signaling mechanisms may be an innovative approach for developing therapies to treat AD. In support of this, accumulating evidence indicates that leptin has cognitive enhancing and neuroprotective actions in various models of AD. Here we assess recent evidence that supports an important regulatory role for leptin at hippocampal CA1 synapses, and we discuss how age-related alterations in this hormonal system influences neurodegenerative disease.

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.

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.'

Cannabinoid Receptor-Related Orphan G Protein-Coupled Receptors.

Of the druggable group of G protein-coupled receptors in the human genome, a number remain which have yet to be paired with an endogenous ligand-orphan GPCRs. Among these 100 or so entities, 3 have been linked to the cannabinoid system. GPR18, GPR55, and GPR119 exhibit limited sequence homology with the established CB1 and CB2 cannabinoid receptors. However, the pharmacology of these orphan receptors displays overlap with CB1 and CB2 receptors, particularly for GPR18 and GPR55. The linking of GPR119 to the cannabinoid receptors is less convincing and emanates from structural similarities of endogenous ligands active at these GPCRs, but which do not cross-react. This review describes the evidence for describing these orphan GPCRs as cannabinoid receptor-like receptors.

Canonical JAK-STAT signaling is pivotal for long-term depression at adult hippocampal temporoammonic-CA1 synapses.

The Janus kinase (JAK)-signal transducer and activator of transcription (STAT) signaling pathway is involved in numerous cellular processes and it is implicated in neurodegenerative disorders, like Alzheimer disease. Recent studies identified a crucial role for this pathway in activity-dependent long-term depression (LTD) at hippocampal Schaffer collateral (SC)-CA1 synapses. However, it is unclear whether JAK-STAT signaling also regulates excitatory synaptic function at the anatomically distinct temporoammonic (TA) input to CA1 neurons. Here we demonstrate that LTD at adult TA-CA1 synapses involves JAK-STAT signaling, but unlike SC-CA1 synapses, requires rapid gene transcription. TA-CA1 LTD requires NMDA receptor activation and is independent of PI3K or ERK signaling. JAK-STAT signaling was critical for TA-CA1 LTD as inhibition of JAK or STAT blocked LTD induction and prevented NMDA-induced AMPA (α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid) receptor internalization in hippocampal neurons. Moreover, an increase in phosphorylated JAK2 and STAT3 accompanied chemical induction of LTD and AMPA receptor internalization. STAT3-driven gene transcription was required for LTD as inhibition of STAT3-DNA binding, nuclear export, and gene transcription all prevented LTD induction. These data indicate an essential role for canonical JAK-STAT signaling in activity-dependent LTD at TA-CA1 synapses and provide valuable insight into the role of the TA input in hippocampal synaptic plasticity.-McGregor, G., Irving, A. J., Harvey, J. Canonical JAK-STAT signaling is pivotal for long-term depression at adult hippocampal temporoammonic-CA1 synapses.

Emerging roles for the novel estrogen-sensing receptor GPER1 in the CNS.

Estrogens play a key role in regulating reproductive and neuroendocrine function by activating classical nuclear steroid receptors that act as ligand gated transcription factors. However evidence is growing that estrogens also promote rapid non-genomic responses via activation of membrane-associated estrogen receptors. The G protein-coupled estrogen receptor (GPER1; also known as GPR30) has been identified as one of the main estrogen-sensitive receptors responsible for the rapid non-genomic actions of estrogen. In recent years, our understanding of the CNS actions of GPER1s has significantly increased following the development of selective pharmacological tools and via the use of transgenic technologies to knockout GPER1 in mice. Here we review recent advances that have been made to uncover the role of GPER1s in the CNS. This article is part of the Special Issue entitled 'Lipid Sensing G Protein-Coupled Receptors in the CNS'.

A Leptin Fragment Mirrors the Cognitive Enhancing and Neuroprotective Actions of Leptin.

A key pathology of Alzheimer's disease (AD) is amyloid ß (Aß) accumulation that triggers synaptic impairments and neuronal death. Metabolic disruption is common in AD and recent evidence implicates impaired leptin function in AD. Thus the leptin system may be a novel therapeutic target in AD. Indeed, leptin has cognitive enhancing properties and it prevents the aberrant effects of Aß on hippocampal synaptic function and neuronal viability. However, as leptin is a large peptide, development of smaller leptin-mimetics may be the best therapeutic approach. Thus, we have examined the cognitive enhancing and neuroprotective properties of known bioactive leptin fragments. Here we show that the leptin (116-130) fragment, but not leptin (22-56), mirrored the ability of leptin to promote AMPA receptor trafficking to synapses and facilitate activity-dependent hippocampal synaptic plasticity. Administration of leptin (116-130) also mirrored the cognitive enhancing effects of leptin as it enhanced performance in episodic-like memory tests. Moreover, leptin (116-130) prevented hippocampal synaptic disruption and neuronal cell death in models of amyloid toxicity. These findings establish further the importance of the leptin system as a therapeutic target in AD.

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

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.