It is well documented that the hormone leptin signals information regarding the status of fat stores to hypothalamic nuclei, which in turn control feeding behaviour and body weight. However, leptin and its receptor are widely expressed in many extra-hypothalamic brain regions, including hippocampus, brain stem and cerebellum. Moreover, evidence is accumulating that leptin has other neuronal functions that are unrelated to its effects on energy homeostasis. Indeed a role for leptin in neuronal development has been suggested as leptin-deficient rodents display abnormal brain development and leptin actively participates in the development of the hypothalamus. In the hippocampus, leptin is a potential cognitive enhancer as genetically obese rodents with dysfunctional leptin receptors display impairments in hippocampal synaptic plasticity. Moreover, direct administration of leptin into the hippocampus can facilitate hippocampal LTP (long-term potentiation) in vivo and improve memory processing in mice. At the cellular level, we have also shown that leptin has the capacity to convert short-term potentiation into LTP. Here, we review the data that leptin influences hippocampal synaptic plasticity via enhancing NMDA (N-methyl-D-aspartate) receptor function. We also provide evidence that rapid trafficking of NMDA receptors to the plasma membrane may underlie the effects of leptin on excitatory synaptic strength.
- cognitive enhancer
- N-methyl-D-aspartate receptor (NMDA receptor)
- synaptic plasticity
- phosphoinositide 3-kinase (PI3K)
It is well documented that the hormone leptin plays a key role in regulating food intake and body weight via its actions on specific hypothalamic nuclei . Hypothalamic leptin receptors also play an important role in controlling thermogenesis, neuroendocrine function and bone formation [2–4]. However, leptin receptors are widely expressed in numerous extra-hypothalamic regions of the brain, including the hippocampus, cerebellum, amygdala and brain stem [5–7], suggesting that leptin has additional functions in these brain regions. In addition, leptin mRNA, protein and immunoreactivity are all widely expressed in a number of brain regions [8,9], indicating that leptin may be released locally in the brain.
The leptin receptor is a class I cytokine receptor , that signals via association with JAKs (Janus tyrosine kinases). Several pathways are activated downstream of JAKs including insulin receptor substrate proteins, which can in turn stimulate the activation of PI3K (phosphoinositide 3-kinase). Indeed PI3K is a key component of leptin receptor signalling in neurons [7,14,15] and peripheral cells [16,17]. In addition to the PI3K signalling cascade, leptin can also rapidly stimulate the Ras-Raf-MAPK (where MAPK stands for mitogen-activated protein kinase) pathway. Leptin receptor-driven activation of this pathway has been observed in numerous peripheral cells [18–20]. There is also evidence that leptin activates this pathway in neurons [21,22].
Recent studies have identified a potential role for leptin in hippocampal synaptic plasticity as genetically obese rodents with dysfunctional leptin receptors display impairments in hippocampal LTP (long-term potentiation) and long-term depression. These obese rodents also show impaired ability to perform spatial memory tasks in the Morris water maze . More recent studies have demonstrated that direct administration of leptin into the dentate gyrus enhances the level of LTP , whereas leptin administration into the hippocampal CA1 region improves memory processing in mice . In the hippocampus, NMDA (N-methyl-D-aspartate) receptor activation is required for the induction of LTP . In this review, we summarize recent results showing that modulation of NMDA receptor function underlies the effects of leptin on hippocampal synaptic plasticity.
Leptin promotes conversion of STP (short-term potentiation) into LTP
It is well documented that the NMDA receptor subtype of ionotropic glutamate receptors, contribute little to basal synaptic transmission but are activated during periods of high frequency transmission. Indeed, in the hippocampal CA1 region, the synaptic activation of NMDA receptors and a concomitant rise in post-synaptic intracellular Ca2+ are required for the induction phase of LTP . STP of excitatory-synaptic transmission can also be evoked at hippocampal CA1 synapses using a primed burst stimulation paradigm (a single shock followed by four consecutive shocks 100 ms apart). Exposure of hippocampal neurons to the hormone leptin (50 nM) results in a marked enhancement of this form of NMDA receptor-dependent synaptic plasticity, as leptin promotes the conversion of STP into robust LTP .
Leptin facilitates NMDA receptor-mediated responses
Numerous studies have demonstrated that LTP can be modulated by a number of hormones, and one of the main targets for modulation is the NMDA receptor. For example, insulin, which signals via similar signalling cascades to the hormone leptin, is capable of facilitating native and recombinant NMDA receptor-mediated responses [24,25]. Similarly, application of leptin rapidly potentiated pharmacologically isolated NMDA receptor-mediated EPSCs (excitatory post-synaptic currents) evoked in hippocampal slices. In contrast, application of leptin evoked a small depression of AMPA (α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid) receptor-mediated synaptic transmission that was readily reversed on washout. Leptin also facilitated NMDA-induced Ca2+ influx in hippocampal cultures, but was without effect on basal levels of intracellular Ca2+. This effect of leptin was selective for NMDA receptors as leptin did not effect the Ca2+ rise induced by application of either AMPA or high external K+ (10.6–16.8 mM).
To determine whether enhancement of NMDA responses required leptin receptor activation, the effects of leptin were also assessed on recombinant NMDA receptors heterologously expressed in Xenopus oocytes. Oocytes expressing either NR1a/NR2A NMDA receptor subunits or a combination of NR1a/NR2A and the leptin receptor (Ob-Rb) were voltage clamped at −60 mV and NMDA was bath applied to evoke fast inward currents. The threshold and maximally active concentrations of NMDA were similar in oocytes expressing NR1a/NR2A or NR1a/NR2A plus Ob-Rb. Leptin receptor activation was required for enhancement of NMDA responses as application of leptin to oocytes expressing NR1a/NR2A alone had no effect on NMDA currents, whereas leptin facilitated NMDA currents in oocytes expressing NR1a/NR2A plus Ob-Rb. In addition, leptin enhanced currents evoked by maximal, as well as submaximal, concentrations of NMDA (Figures 1A and 1B), which is likely to reflect an increase in the number of functional NMDA receptor channels expressed ats the cell surface. The ability of leptin to promote rapid delivery of NMDA receptors to the cell surface parallels the actions of the hormone insulin, which promotes the insertion of new receptors via SNARE (soluble N-ethylmaleimide-sensitive fusion protein attachment protein receptor)-dependent exocytosis . However, the cellular mechanisms underlying the effects of leptin on NMDA receptor cell surface expression remain to be established.
Leptin facilitation of NMDA responses involves activation of PI3K, MAPK and Src tyrosine kinase
In peripheral cells [16,17] and neurons , PI3K is a key element of leptin receptor signal transduction. Thus the role of PI3K in leptin-induced facilitation of NMDA responses was examined. Application of either LY294002 or wortmannin, two structurally unrelated inhibitors of PI3K, had no effect on NMDA responses. However, both agents significantly attenuated the ability of leptin to enhance NMDA-induced Ca2+ rise, indicating that a PI3K-dependent process contributes to this action of leptin. In addition to PI3K, the Ras/Raf/MAPK signalling cascade is a signalling intermediate for leptin [18–20]. Indeed in a manner similar to the PI3K inhibitors, inhibition of MAPK activation with either PD98059 or U0126 markedly reduced the ability of leptin to facilitate NMDA receptor-mediated Ca2+ influx in hippocampal cultures. In contrast, the inactive analogue of U0126, U0124 failed to inhibit leptin-induced facilitation of NMDA responses. Thus activation of the Ras/Raf/MAPK signalling pathway is also required for the effects of leptin on NMDA receptor function.
It is well documented that the activity of NMDA receptors is enhanced by Src tyrosine kinases [30–32], whereas tyrosine kinase inhibitors block the induction phase of NMDA receptor-dependent LTP . Thus we examined the possible involvement of Src tyrosine kinases in the actions of leptin on native and recombinant NMDA receptors. In hippocampal cultures, the tyrosine kinase inhibitor, lavendustin A, but not its inactive analogue (lavendustin B), significantly reduced leptin-induced enhancement of NMDA-mediated Ca2+ responses. Similarly in acute hippocampal slices, the ability of leptin to facilitate NMDA receptor-mediated EPSCs was completely abolished following whole cell dialysis with Src tyrosine kinase inhibitors PP1 (type-1 protein phosphatase) or PP2. In contrast, dialysis with PP3, an inactive analogue of PP2, failed to affect the actions of leptin. Thus leptin facilitation of native NMDA receptors involves an Src tyrosine kinase-dependent process. The effects of leptin were also assessed in Xenopus oocytes expressing a combination of NR1a/NR2A NMDA receptor subunits and Ob-Rb. Application of the broad spectrum tyrosine kinase inhibitors, genistein or lavendustin A, had no effect on NR1a/NR2A-mediated currents. However, incubation with either agent completely blocked the ability of leptin to facilitate NR1a/NR2A-dependent currents (Figures 2A and 2C). In contrast, the inactive analogues, daidzein or lavendustin B failed to inhibit the effects of leptin (Figure 2B). Thus together these data indicate that leptin facilitates native and recombinant NMDA receptors via a tyrosine kinase-dependent process.
Our studies show that leptin facilitates hippocampal synaptic plasticity, via selective enhancement of NMDA receptors. This process occurs rapidly as enhanced responses to NMDA were observed approx. 5 min after exposure to leptin. Activation of leptin receptors was required for modulation of NMDA responses as leptin only facilitated NMDA receptor-mediated currents in Xenopus oocytes expressing Ob-Rb. It is likely that delivery of new NMDA receptor channels to the cell surface contributes to these effects, as leptin facilitated NR1a/NR2A-mediated currents evoked by saturating concentrations of NMDA. Currently, studies are under way to establish if leptin promotes the recruitment of new functional NMDA receptors in hippocampal neurons and if this process contributes to the effects of leptin on hippocampal synaptic plasticity.
The enhancement of NMDA responses by leptin not only requires the activation of PI3K, but also MAPK and Src tyrosine kinases. It is well documented that activation of all these signalling cascades play a role in NMDA receptor-dependent hippocampal synaptic plasticity [27–29,34–36]. Thus it is feasible that leptin may be released locally in the CA1 region of the hippocampus during high frequency stimulation (and the induction phase of LTP) and it modulates hippocampal synaptic plasticity by enhancing NMDA receptor function via activation of one or more of these signalling pathways. Alternatively, hormonal release of leptin from adipocytes may be transported from the periphery to the brain via the blood–brain barrier, where it may act to alter the threshold for the induction of hippocampal LTP by selective facilitation of NMDA responses.
It is known that obesity is a high risk factor for the development of Type II diabetes and cognitive deficits, ranging from mild amnesia to dementia, have been identified in diabetic patients . As obese individuals have high circulating levels of leptin, but are resistant to leptin, insensitivity to leptin may be a key contributor to the development of these cognitive deficits in diabetics. Although there is no direct evidence that obesity results in cognitive impairments, there is evidence that profound cognitive deficits exist in a number of obesity-related diseases. For example, individuals with Prader–Willi syndrome are morbidly obese and have profound cognitive impairments that are greater than expected for their IQ . It is also well established that lower cognitive functioning is associated with patients with obesity and hypertension , which suggests that obesity may have an adverse effect on cognitive performance. In support of this possibility, functional imaging of cerebral blood flow has demonstrated regional differences in the brain responses of lean and obese subjects, suggesting that the circulating levels of leptin in the human brain are altered in the obese state .
This work was supported by The Wellcome Trust (grants 055291 and 075821) and The Anonymous Trust.
Proteins in Disease: A Focus Topic at BioScience2005, held at SECC Glasgow, U.K., 17–21 July 2005. Edited by B. Austen (St George's Hospital Medical School, London, U.K.), C. Connolly (Dundee, U.K.), B. Irvine (Belfast, U.K.), M. Sugden (Queen Mary, London, U.K.) and V. Zammit (Hannah Research Institute, Ayr, U.K.).
Abbreviations: AMPA, α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid; EPSC, excitatory post-synaptic current; JAK, Janus tyrosine kinase; LTP, long-term potentiation; MAPK, mitogen-activated protein kinase; NMDA, N-methyl-D-aspartate; NR, NMDA receptor; PI3K, phosphoinositide 3-kinase; STP, short-term potentiation
- © 2005 The Biochemical Society