Postsynaptic IP3 receptor-mediated Ca2+ release modulates synaptic transmission in hippocampal neurons

Ca(2+)-dependent mechanisms are important in regulating synaptic transmission. The results herein indicate that whole-cell perfusion of inositol 1,4,5-trisphosphate receptor (IP(3)R) agonists greatly enhanced excitatory postsynaptic current (EPSC) amplitudes in postsynaptic hippocampal CA1 neurons. IP(3)R agonist-mediated increases in synaptic transmission changed during development and paralleled age-dependent increases in hippocampal type-1 IP(3)Rs. IP(3)R agonist-mediated increases in EPSC amplitudes were inhibited by postsynaptic perfusion of inhibitors of Ca(2+)/calmodulin, PKC and Ca(2+)/calmodulin-dependent protein kinase II. Postsynaptic perfusion of inhibitors of smooth endoplasmic reticulum (SER) Ca(2+)-ATPases, which deplete intracellular Ca(2+) stores, also enhanced EPSC amplitudes. Postsynaptic perfusion of the IP(3)R agonist adenophostin (AdA) during subthreshold stimulation appeared to convert silent to active synapses; synaptic transmission at these active synapses was completely blocked by 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX). Postsynaptic IP(3)R-mediated Ca(2+) release also produced a significant increase in spontaneous EPSC frequency. These results indicate that Ca(2+) release from intracellular stores play a key role in regulating the function of postsynaptic AMPARs.     

Calcium dynamics are altered in cortical neurons lacking the calmodulin-binding protein RC3

RC3 is a neuronal calmodulin-binding protein and protein kinase C substrate that is thought to play an important regulatory role in synaptic transmission and neuronal plasticity. Two molecules known to regulate synaptic transmission and neuronal plasticity are Ca(2+) and calmodulin, and proposed mechanisms of RC3 action involve both molecules. However, physiological evidence for a role of RC3 in neuronal Ca(2+) dynamics is limited. In the current study we utilized cultured cortical neurons obtained from RC3 knockout (RC3-/-) and wildtype mice (RC3+/+) and fura-2-based microscopic Ca(2+) imaging to investigate a role for RC3 in neuronal Ca(2+) dynamics. Immunocytochemical characterization showed that the RC3-/- cultures lack RC3 immunoreactivity, whereas cultures prepared from wildtype mice showed RC3 immunoreactivity at all ages studied. RC3+/+ and RC3-/- cultures were indistinguishable with respect to neuron density, neuronal morphology, the formation of extensive neuritic networks and the presence of glial fibrillary acidic protein (GFAP)-positive astrocytes and gamma-aminobutyric acid (GABA)ergic neurons. However, the absence of RC3 in the RC3-/- neurons was found to alter neuronal Ca(2+) dynamics including baseline Ca(2+) levels measured under normal physiological conditions or after blockade of synaptic transmission, spontaneous intracellular Ca(2+) oscillations generated by network synaptic activity, and Ca(2+) responses elicited by exogenous application of N-methyl-D-aspartate (NMDA) or class I metabotropic glutamate receptor agonists. Thus, significant changes in Ca(2+) dynamics occur in cortical neurons when RC3 is absent and these changes do not involve changes in gross neuronal morphology or neuronal maturation. These data provide direct physiological evidence for a regulatory role of RC3 in neuronal Ca(2+) dynamics.     

Distinct regulation of expressed calcium channels 2.3 in Xenopus oocytes by direct or indirect activation of protein kinase C

Protein kinase C (PKC)-dependent regulation of voltage-gated Ca (Ca(v); with alpha(1)beta1Balpha2/delta subunits) channel 2.3 was investigated using phorbol 12-myristate 13-acetate (PMA), or by M(1) muscarinic receptor activation in Xenopus oocytes. The inward Ca(2+)-current with Ba(2+) (I(Ba)) as the charge carrier was potentiated by PMA or acetyl-beta-methylcholine (MCh). The inactivating [I(inact)] and non-inactivating [I(noninact)] components of I(Ba) and the time constant of inactivation tau(inact) were all increased by MCh or PMA. This may be a PKC-dependent action since the effect of MCh and PMA was blocked by Ro-31-8425 or beta-pseudosubstrate. MCh effect was blocked by atropine, guanosine-5'-O-(2-thiodiphosphate) trilithium (GDPbetaS) or U-73122. The effect of MCh but not PMA was blocked by the inhibition of inositol-1,4,5-trisphosphate (IP3) receptors, intracellular Ca(2+) ([Ca(2+)](i)) or the translocation of conventional PKC (cPKC) with heparin, BAPTA and betaC2.4, respectively. While a lower concentration (25 nM) of Ro-31-8425 blocked MCh, a higher concentration (500 nM) of Ro-31-8425 was required to block PMA action. This differential susceptibility of MCh and PMA to heparin, BAPTA, betaC2.4 or Ro-31-8425 is suggestive of the involvement of Ca(2+)-dependent cPKC in MCh action, whereas cPKC and Ca(2+)-independent novel PKC (nPKC) in PMA action. PMA led to additional increase in I(Ba) that was already potentiated by preadministered MCh (1 or 10 microM), leading to the suggestion that differential phosphorylation sites for cPKC and nPKC may be present in the alpha(1)2.3 subunit of Ca(v) 2.3 channels.     

Neuroprotectant minocycline depresses glutamatergic neurotransmission and Ca(2+) signalling in hippocampal neurons

The mechanism of the neuroprotective action of the tetracycline antibiotic minocycline against various neuron insults is controversial. In an attempt to clarify this mechanism, we have studied here its effects on various electrophysiological parameters, Ca(2+) signalling, and glutamate release, in primary cultures of rat hippocampal neurons, and in synaptosomes. Spontaneous excitatory postsynaptic currents and action potential firing were drastically decreased by minocycline at concentrations known to afford neuroprotection. The drug also blocked whole-cell inward Na(+) currents (I(Na)) by 20%, and the whole-cell Ca(2+) current (I(Ca)) by about 30%. Minocycline inhibited glutamate-evoked elevation of the cytosolic Ca(2+) concentration ([Ca(2+)](c)) by nearly 40%, and K(+)-evoked glutamate release from synaptosomes by 63%. Minocycline also depressed the frequency and amplitude of spontaneous excitatory postsynaptic currents, but did not affect the whole-cell inward current elicited by gamma-aminobutyric acid or glutamate. This pharmacological profile suggests that the neuroprotective effects of minocycline might be associated with the mitigation of neuronal excitability, glutamate release, and Ca(2+) overloading.     

Upregulation of KCC2 activity by zinc-mediated neurotransmission via the mZnR/GPR39 receptor

Vesicular Zn(2+) regulates postsynaptic neuronal excitability upon its corelease with glutamate. We previously demonstrated that synaptic Zn(2+) acts via a distinct metabotropic zinc-sensing receptor (mZnR) in neurons to trigger Ca(2+) responses in the hippocampus. Here, we show that physiological activation of mZnR signaling induces enhanced K(+)/Cl(-) cotransporter 2 (KCC2) activity and surface expression. As KCC2 is the major Cl(-) outward transporter in neurons, Zn(2+) also triggers a pronounced hyperpolarizing shift in the GABA(A) reversal potential. Mossy fiber stimulation-dependent upregulation of KCC2 activity is eliminated in slices from Zn(2+) transporter 3-deficient animals, which lack synaptic Zn(2+). Importantly, activity-dependent ZnR signaling and subsequent enhancement of KCC2 activity are also absent in slices from mice lacking the G-protein-coupled receptor GPR39, identifying this protein as the functional neuronal mZnR. Our work elucidates a fundamentally important role for synaptically released Zn(2+) acting as a neurotransmitter signal via activation of a mZnR to increase Cl(-) transport, thereby enhancing inhibitory tone in postsynaptic cells.     

Calcium-triggered exit of F-actin and IP(3) 3-kinase A from dendritic spines is rapid and reversible

The structure of the actin cytoskeleton in dendritic spines is thought to underlie some forms of synaptic plasticity. We have used fixed and live-cell imaging in rat primary hippocampal cultures to characterize the synaptic dynamics of the F-actin binding protein inositol trisphosphate 3-kinase A (IP3K), which is localized in the spines of pyramidal neurons derived from the CA1 region. IP3K was intensely concentrated as puncta in spine heads when Ca(2+) influx was low, but rapidly and reversibly redistributed to a striated morphology in the main dendrite when Ca(2+) influx was high. Glutamate stimulated the exit of IP3K from spines within 10 s, and re-entry following blockage of Ca(2+) influx commenced within a minute; IP3K appeared to remain associated with F-actin throughout this process. Ca(2+)-triggered F-actin relocalization occurred in about 90% of the cells expressing IP3K endogenously, and was modulated by the synaptic activity of the cultures, suggesting that it is a physiological process. F-actin relocalization was blocked by cytochalasins, jasplakinolide and by the over-expression of actin fused to green fluorescent protein. We also used deconvolution microscopy to visualize the relationship between F-actin and endoplasmic reticulum inside dendritic spines, revealing a delicate microorganization of IP3K near the Ca(2+) stores. We conclude that Ca(2+) influx into the spines of CA1 pyramidal neurons triggers the rapid and reversible retraction of F-actin from the dendritic spine head. This process contributes to changes in spine F-actin shape and content during synaptic activity, and might also regulate spine IP3 signals.     

Modulation of neuronal [Ca]i by caffeine is altered with aging

Voltage-dependent calcium channels play an important role in controlling many neuronal processes such as neuronal excitability and synaptic transmission. Any slight alteration in intracellular calcium concentration ([Ca2+]i) can have a considerable impact on various neuronal functions. The effects of caffeine on [Ca2+]i were studied in CA1 hippocampal neurons of young (2 months) and old (24 months) C57BL mice. Fura 2-AM fluorescence photometry was used to measure [Ca2+]i in the presence and absence of caffeine (100 microM) in response to KCl (26 mM) application. Caffeine enhanced the peak [Ca2+]i as compared to control solution in young mice (control: 325+/-8 nM, caffeine: 402+/-10 nM), but had no effect on the peak [Ca2+]i in old mice (control: 222+/-6 nM, caffeine: 223+/-7 nM). These results indicate that caffeine can impact neuronal functions through the modification of [Ca2+]i. The lack of caffeine-induced modulation of [Ca2+]i in old mice suggests that this role of caffeine has been compromised with aging.     

Acute treatment with 17beta-estradiol attenuates astrocyte-astrocyte and astrocyte-neuron communication

Astrocytes are now recognized as dynamic signaling elements in the brain. Bidirectional communication between neurons and astrocytes involves integration of neuronal inputs by astrocytes and release of gliotransmitters that modulate neuronal excitability and synaptic transmission. The ovarian steroid hormone, 17beta-estradiol, in addition to its rapid actions on neuronal electrical activity can rapidly alter astrocyte intracellular calcium concentration ([Ca2+]i) through a membrane-associated estrogen receptor. Using calcium imaging and electrophysiological techniques, we investigated the functional consequences of acute treatment with estradiol on astrocyte-astrocyte and astrocyte-neuron communication in mixed hippocampal cultures. Mechanical stimulation of an astrocyte evoked a [Ca2+]i rise in the stimulated astrocyte, which propagated to the surrounding astrocytes as a [Ca2+]i wave. Following acute treatment with estradiol, the amplitude of the [Ca2+]i elevation in astrocytes around the stimulated astrocyte was attenuated. Further, estradiol inhibited the [Ca2+]i rise in individual astrocytes in response to the metabotropic glutamate receptor agonist, trans-(+/-)-1-amino-1,3-cyclopentanedicarboxylic acid. Mechanical stimulation of astrocytes induced [Ca2+]i elevations and electrophysiological responses in adjacent neurons. Estradiol rapidly attenuated the astrocyte-evoked glutamate-mediated [Ca2+]i rise and slow inward current in neurons. Also, the incidence of astrocyte-induced increase in spontaneous postsynaptic current frequency was reduced in the presence of estradiol. The effects of estradiol were stereo-specific and reversible following washout. These findings may indicate that the regulation of neuronal excitability and synaptic transmission by astrocytes is sensitive to rapid estradiol-mediated hormonal control.     

Involvement of CDC25Mm/Ras-GRF1-Dependent Signaling in the Control of Neuronal Excitability

Ras-GRF1 is a neuron-specific guanine nucleotide exchange factor for Ras proteins. Mice lacking Ras-GRF1 (-/-) are severely impaired in amygdala-dependent long-term synaptic plasticity and show higher basal synaptic activity at both amygdala and hippocampal synapses (Brambilla et al., 1997). In the present study we investigated the effects of Ras-GRF1 deletion on hippocampal neuronal excitability. Electrophysiological analysis of both primary cultured neurons and adult hippocampal slices indicated that Ras-GRF1-/- mice displayed neuronal hyperexcitability. Ras-GRF1-/- hippocampal neurons showed increased spontaneous activity and depolarized resting membrane potential, together with a higher firing rate in response to injected current. Changes in the intrinsic excitability of Ras-GRF1-/- neurons can entail these phenomena, suggesting that Ras-GRF1 deficiency might alter the balance between ionic conductances. In addition, we showed that mice lacking Ras-GRF1 displayed a higher seizure susceptibility following acute administration of convulsant drugs. Taken together, these results demonstrated a role for Ras-GRF1 in neuronal excitability.     

Polyunsaturated fatty acids modify mouse hippocampal neuronal excitability during excitotoxic or convulsant stimulation

The n-3 polyunsaturated fatty acids (PUFAs) reduce cardiac membrane excitability and prevent cardiac arrhythmias in animals and probably in humans. In this study, we assessed the effects of n-3 PUFAs on membrane excitability in mouse hippocampal neurons with both whole-cell current and voltage-clamp methods. Extracellular application of 20 microM eicosapentaenoic acid (EPA, C20:5n-3) significantly reduced the frequency of electrical-evoked action potentials in CA1 neurons of hippocampal slices from 3.8+/-0.7 Hz of control to 2.1+/-0.5 Hz. In addition, EPA significantly hyperpolarized the resting membrane potential and raised the stimulatory threshold of action potentials in CA1 neurons. Another n-3 PUFA, docosahexaenoic acid (DHA, C22:6n-3), had effects on membrane excitability similar to those of EPA. In contrast, EPA ethyl ester, oleic acid (OA, C18:n-9), and stearic acid (SA, C18:0) did not alter the membrane excitability in CA1 neurons. Bath application of pentylenetetrazole (PTZ) or glutamate reduced the stimulatory threshold and increased the frequency of action potentials of hippocampal neurons. EPA restored PTZ- or glutamate-enhanced neuronal excitability to the control level. EPA also suppressed glutamate-activated inward currents. Furthermore, EPA and DHA significantly inhibited the frequency of action potentials without effecting the stimulatory threshold of CA3 neurons. These data demonstrate that n-3 PUFAs modify neuronal membrane excitability under control and drug-stimulated conditions. The sensitivity to these effects of PUFAs varies from neurons of different hippocampal regions.     

Caffeine modulates potassium currents in Drosophila neurons

We investigated the effects of caffeine on the delayed-rectifier potassium current (IK(DR)) which is important in repolarizing the membrane potential, and the transient A-type potassium current (IK(A)) which regulates neuronal firing threshold and the rate of repetitive action potentials. The whole-cell patch-clamp technique was used to measure the currents from cultured Drosophila neurons derived from embryonic neuroblasts. The currents were measured from neurons before and after the application of 1mM caffeine to the external saline of the same neuron. IK(DR) measured in the caffeine-containing solution (470+/-36 pA, n=18), was smaller than that measured in the control 6K/0Ca Tris solution (745+/-51 pA, n=18). IK(A) measured in the caffeine-containing solution (17+/-2 pA, n=16) was smaller than that measured in the control 6K/0Ca Tris solution (35+/-4 pA, n=16). These results indicate that caffeine reduces IK(DR) and IK(A) amplitudes and possibly leads to increased action potential frequency and enhanced neuronal excitability.     

Facilitation of spontaneous glycine release by anoxia potentiates NMDA receptor current in the hypoglossal motor neurons of the rat

Deficiency in energy supply, such as occurs during hypoxia, anoxia, metabolic stress and mitochondrial failure, strongly affects the excitability of central neurons. Such lowered energy supply evokes various changes in spontaneous synaptic input to the hippocampal and cortical neurons. However, how this energy deprivation affects synaptic input to motor neurons, which are also vulnerable to energy deprivation, has never been addressed. Here we report for the first time the effect of metabolic stress on synaptic input to motor neurons by recording postsynaptic currents in the hypoglossal nucleus. Chemical anoxia with NaCN (1 mm) and anoxia with 95% N(2) induced a persistent inward current and a marked and robust increase in action potential-independent synaptic input. This increase was abolished by strychnine, but not by picrotoxin, CNQX or MK-801, indicating glycine release facilitation. Blockade of voltage-dependent Ca(2+) channels and extracellular Ca(2+) deprivation strongly attenuated this facilitation. The amplitude of inward currents evoked by local application of NMDA to the motor neurons in the presence of strychnine was significantly increased during NaCN application. A saturating concentration of d-serine occluded this potentiation, suggesting that released glycine activated the glycine-binding sites of NMDA receptors. By contrast, neurons in the dorsal motor nucleus of the vagus showed no detectable change in synaptic input in response to NaCN. These data suggest that increase in synaptically released glycine in response to metabolic stress may play an exacerbating role in NMDA receptor-mediated excitotoxicity in motor neurons.     

Suppression of excitatory synaptic transmission can facilitate low-calcium epileptiform activity in the hippocampus in vivo

It has been reported that the inhibitory postsynaptic potential (IPSP) is abolished before the excitatory postsynaptic potential (EPSP) when the extracellular concentration of Ca(2+) ([Ca(2+)](o)) is removed gradually in hippocampal slices. However, the low-Ca(2+) nonsynaptic epileptiform activity does not appear until the [Ca(2+)](o) is decreased to a level sufficient to depress the excitatory synaptic transmission. This suggests the hypothesis that the suppression of excitatory synaptic transmission itself could facilitate the generation of epileptiform activity. In the present study, we tested this hypothesis and developed a new model of nonsynaptic epileptiform activity by gradually raising the neuronal excitability and blocking the synaptic transmission with high K(+), zero Ca(2+) and calcium chelator ethylene glycol-bis (beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid (EGTA) in the CA1 region of hippocampus in vivo. The changes of synaptic transmission and recurrent inhibitory activity during this process were evaluated by measuring the amplitude of the population spikes (PS) in response to paired-pulse orthodromic stimulation. The results show that the epileptiform activity appeared only when the excitatory synaptic transmission was depressed by further lowering [Ca(2+)](o) with EGTA. Similar epileptiform activity could be induced when EGTA was replaced by the excitatory postsynaptic amino acid antagonists D-(-)-2-amino-5-phosphonopentanoic acid (APV) plus 6,7-dinitroquinoxaline-2,3-dione (DNQX) or APV alone but not DNQX alone. The combination application of APV and cadmium enhanced the epileptiform activity. These results suggest that the suppression of excitatory synaptic transmission can facilitate the appearance of epileptiform activity in solution with high K(+) and low Ca(2+) in vivo. These data provide new information to be considered in the development of antiepileptic drugs. They also suggest a possible mechanism to explain the fact that low-frequency electrical stimulation can suppress epileptiform activity.     

Nicotine modulates the spontaneous synaptic activity in cultured embryonic rat spinal cord interneurons

The nicotine-induced modulation of the synaptic activity was studied in cultured spinal cord neurons from embryonic rats, using the patch-clamp technique, alone or in combination with Ca(2+) imaging. Morphologically, neurons could be divided into two populations: multipolar nerve cells and bipolar, spindle-shaped neurons. Neurons were predominantly GABAergic, with approximately 70% of bipolar cells and 60% of multipolar cells positive for GABA immunostaining. Nicotine (Nic) did not affect the activity of the spontaneous postsynaptic current (sPSC) in multipolar neurons, whereas bipolar cells responded to Nic applications with an enhancement of both inhibitory and excitatory synaptic activity (threefold for 100 microM Nic). No change in the mean event amplitude was observed. The increase of sPSC frequency was detectable at 1-10 microM Nic, and was prevented by dihydro-beta-erythroidine (DHbetaE) but not by alpha-bungarotoxin. Choline, a selective alpha7-nAChR agonist, did not mimic the Nic action. Simultaneous treatment with inhibitors of ionotropic glutamate receptors, CNQX (20 microM) and AP5 (20 microM), completely blocked the excitatory sPSC activity but did not prevent the Nic-induced enhancement of inhibitory sPSC activity. Tetrodotoxin (1 microM) reduced the basal spontaneous activity but did not block the Nic-induced effects on bipolar neurons. In a subset of bipolar neurons (12%) exposed to AP5 and CNQX, Nic activated DHbetaE-sensitive inward currents, associated with an elevation of cytosolic Ca(2+) ([Ca(2+)](i)). Our results provide the first evidence of modulation of both excitatory and inhibitory neurotransmitter release in embryonic spinal cord interneurons by non-alpha7-containing nicotinic receptors.     

Developmental changes in metabotropic glutamate receptor-mediated calcium homeostasis

Neurons of the chick cochlear nucleus, nucleus magnocellularis (NM), require eighth nerve activation of metabotropic glutamate receptors (mGluRs) for maintenance of intracellular calcium homeostasis. Interrupting this activation results in an increase in intracellular calcium concentration ([Ca(2+)](i)) followed by cell atrophy, degeneration, and death of many neurons. Although these phenomena are well characterized in late embryonic and posthatch chicks, little is known about the role of mGluRs and calcium homeostasis during the development of synaptic activity in NM. Using Fura-2 imaging, fluorescent immunohistochemistry, and Western immunoblotting, we investigated (1) the expression and function of group I mGluRs and their role in calcium regulation during development of NM, and (2) the expression of two other key molecules involved in regulating neuronal [Ca(2+)](i) : inositol trisphosphate receptors (IP(3)Rs) and sarcoplasmic/endoplasmic reticulum calcium ATPases (SERCAs). Confocal imaging of Fluo-3-labeled NM was used to investigate the kinetics of global NM neuron calcium signals. Measurements were made at four ages that extend from before synaptic function begins in NM, through functional onset, to mature patterns of spontaneous activity, namely, embryonic days (E) 10, 13, 15, and 18. mGluR5, mGluR1, and SERCA expression peaked at E13 and then decreased with age. IP(3)R expression increased to peak at E18. [Ca(2+)](i) response to mGluR activation increased with age. The rise time of [Ca(2+)](i) signals in NM neurons did not change with development, but E13 neurons were slower to reestablish baseline [Ca(2+)](i). These results suggest that the mGluR-mediated calcium homeostasis of NM neurons develops in parallel with synaptic activity and appears to be refined with increasing synaptic activity.     

Spontaneous release from mossy fiber terminals inhibits Ni2+-sensitive T-type Ca2+ channels of CA3 pyramidal neurons in the rat organotypic hippocampal slice

Mossy fibers (axons arising from dentate granule cells) form large synaptic contacts exclusively onto the proximal apical dendrites of CA3 pyramidal neurons. They can generate large synaptic currents that occur in close proximity to the soma. These properties mean that active conductance in the proximal apical dendrite could have a disproportionate influence on CA3 pyramidal neuron excitability. Ni(2+)-sensitive T-type Ca(2+) channels are important modulators of dendritic excitability. Here, we use an optical approach to determine the contribution of Ni(2+) (100 microM)-sensitive Ca(2+) channels to action potential (AP) elicited Ca(2+) flux in the soma, proximal apical and distal apical dendrites. At resting membrane potentials Ni(2+)-sensitive Ca(2+) channels do not contribute to the Ca(2+) signal in the proximal apical dendrite, but do contribute in the other cell regions. Spontaneous release from mossy fiber terminals acting on 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX)-sensitive postsynaptic channels underlies a tonic inhibition of Ni(2+)-sensitive channels. Chelating Zn(2+) with CaEDTA blocks CNQX-sensitive changes in Ca(2+) flux implicating a mechanistic role of this ion in T-type Ca(2+) channel block. To test if this inhibition influenced excitability, progressively larger depolarizing pulses were delivered to CA3 pyramidal neurons. CNQX significantly reduced the size of the depolarizing step required to generate APs and increased the absolute number of APs per depolarizing step. This change in AP firing was completely reversed by the addition of Ni(2+). This mechanism may reduce the impact of T-type Ca(2+) channels in a region where large synaptic events are common.     

Effects of insulin-like growth factor 1 on synaptic excitability in cultured rat hippocampal neurons

Insulin-like growth factor 1 (IGF-1) has important functions in the brain, including metabolic, neurotrophic, neuromodulatory and neuroendocrine actions, and it also prevents beta amyloid-induced death of hippocampal neurons. However, its functions in the synaptic excitability remain uncertain. Here we investigated the effects of IGF-1 on synaptic excitability in cultured rat hippocampal neurons using whole-cell patch clamp recordings. Incubation the hippocampal neurons with different concentrations of IGF-1 for 24 h or 30 min significantly increased the frequency of spontaneous excitatory postsynaptic currents (sEPSCs), but had no effect on the frequency of miniature EPSCs (mEPSCs) and spontaneous inhibitory postsynaptic currents (sIPSCs). The mean amplitudes, rise, and decay kinetics of sEPSCs, mEPSCs, and sIPSCs were not significantly affected by IGF-1, indicating that IGF-1 increased the probability of neurotransmitter release but did not modulate postsynaptic receptors. The effects of IGF-1 were mediated by mitogen-activated protein kinase (MAPK). IGF-1 activated the ERK1/2 signaling pathway in cultured hippocampal neurons, and the inhibitor PD98059 blocked the enhancement of sEPSCs induced by IGF-1. These results demonstrated the regulatory function of IGF-1 on synaptic excitability in hippocampal neurons and its underlying signaling mechanism.