Blocking L-type calcium channels enhances long-term depression induced by low-frequency stimulation at hippocampal CA1 synapses

Specific contributions of voltage-dependent calcium channels (VDCCs) to induction of long-term depression (LTD) have not been thoroughly elucidated. The present study examined roles of T- and L-type VDCCs in N-methyl-D-aspartate (NMDA) receptor-dependent LTD induced at several different levels of synaptic activation (0.5- to 10-Hz presynaptic stimulations) at Schaffer collateral-CA1 synapses in rat hippocampal slices. Blockade of T-type VDCCs with nickel ions failed to change LTD magnitude at all levels of stimulation. However, blockade of L-type VDCCs reduced LTD in response to stimulation at 1 and 2 Hz and, conversely, enhanced LTD at a lower frequency (0.5 Hz). The enhancement of 0.5-Hz LTD under L-type VDCC blockade was shown pharmacologically to depend on NMDA receptors (NMDARs) and intracellular Ca(2+) release. Calcium imaging revealed that contribution of L-type VDCC-mediated calcium influx to the total calcium increase was greater during 0.5-Hz stimulation than during 1.0-Hz stimulation. This finding, combined with the reported suppression of NMDARs mediated by L-type VDCCs, may be relevant to the present enhancement of 0.5-Hz LTD due to L-type VDCC blockade.     

Turning off of GluN2B subunits and turning on of CICR in hippocampal LTD induction after developmental GluN2 subunit switch

NMDA receptors (NMDARs) are essential for the induction of synaptic plasticity that mediates activity‐dependent refinement of neural circuits during development. GluN2B subunits of NMDARs are abundant at synapses in the immature hippocampus and begin to be replaced by GluN2A subunits with the help of casein kinase 2 activity in the second postnatal week, the critical period for the GluN2 subunit switch (Sanz‐Clemente et al. (2000) Neuron 67:984–996). However, the physiological role of GluN2B subunits in the hippocampus during this critical period has not been elucidated. Here, we report that GluN2B subunits mediate the induction of long‐term depression (LTD) in the CA1 region of the hippocampus only until this period. Ifenprodil and Ro25‐6981, selective inhibitors of NMDARs containing GluN2B subunits, blocked LTD in postnatal Day 11–14 (P11–14) rat hippocampal slices but not in P18–22 hippocampus. Just a few days after P14, synaptic NMDAR currents became narrower than those at P11–14, and calcium influx through NMDARs must be reduced. We found that calcium‐induced calcium release (CICR) through ryanodine receptors starts to support the induction of NMDAR‐dependent LTD at P18–22. Intracellular application of thapsigargin and ryanodine, inhibitors of Ca²⁺‐ATP pumps on internal stores and ryanodine receptors, respectively, did not at all affect LTD in the hippocampus at P11–14 but completely blocked LTD in the P18–22 hippocampus. Therefore, calcium influx through NMDAR with GluN2B subunits is sufficient to induce LTD at P11–14, after which CICR compensates for the decrease in calcium influx during LTD induction. © 2015 Wiley Periodicals, Inc.     

Mechanisms regulating spill‐over of synaptic glutamate to extrasynaptic NMDA receptors in mouse substantia nigra dopaminergic neurons

N‐Methyl‐d ‐aspartate glutamate receptors (NMDARs) contribute to neural development, plasticity and survival, but they are also linked with neurodegeneration. NMDARs at synapses are activated by coincident glutamate release and depolarization. NMDARs distal to synapses can sometimes be recruited by ‘spill‐over’ of glutamate during high‐frequency synaptic stimulation or when glutamate uptake is compromised, and this influences the shape of NMDAR‐mediated postsynaptic responses. In substantia nigra dopamine neurons, activation of NMDARs beyond the synapse during different frequencies of presynaptic stimulation has not been explored, even though excitatory afferents from the subthalamic nucleus show a range of firing frequencies, and these frequencies change in human and experimental Parkinson's disease. This study reports that high‐frequency stimulation (80 Hz/200 ms) evoked NMDAR‐excitatory postsynaptic currents (EPSCs) that were larger and longer lasting than those evoked by single stimuli at low frequency (0.1 Hz). MK‐801, which irreversibly blocked NMDAR‐EPSCs activated during 0.1‐Hz stimulation, left a proportion of NMDAR‐EPSCs that could be activated by 80‐Hz stimulation and that may represent activity of NMDARs distal to synapses. TBOA, which blocks glutamate transporters, significantly increased NMDAR‐EPSCs in response to 80‐Hz stimulation, particularly when metabotropic glutamate receptors (mGluRs) were also blocked, indicating that recruitment of NMDARs distal to synapses is regulated by glutamate transporters and mGluRs. These regulatory mechanisms may be essential in the substantia nigra for restricting glutamate diffusion from synaptic sites and keeping NMDAR‐EPSCs in dopamine neurons relatively small and fast. Failure of glutamate transporters may contribute to the declining health of dopamine neurons during pathological conditions.     

Activation of extrasynaptic NMDA receptors induces LTD in rat hippocampal CA1 neurons

In the adult rat hippocampus, activation of N-methyl-d-aspartate receptors (NMDARs) is required for the induction of certain forms of synaptic plasticity, including long-term potentiation (LTP) and long-term depression (LTD). Several studies have indicated the opposing role of synaptic NMDARS (S-NMDARs) versus extrasynaptic NMDARs (ES-NMDARs) in CREB-dependent gene regulation and neuronal survival/death. The contribution of ES-NMDARs in synaptic plasticity, however, remains unclear. Here we investigated the contribution of ES-NMDARs on LTD induction in CA1 neurons of rat hippocampal slices. ES-NMDARs were selectively activated by theta burst stimulation (TBS) after selective blockade of S-NMDARs with pairing of 5 Hz stimulation and MK-801, an irreversible use-dependent antagonist of NMDARs. Application of TBS in naïve slices evoked a transient potentiation. In contrast, the activation of ES-NMDARs evoked a robust LTD. These results suggest the involvement of ES-NMDARs in LTD induction.     

Synchronized population oscillation of excitatory synaptic potentials dependent of calcium-induced calcium release in rat neocortex layer II/III neurons

We examined the roles played by calcium-induced calcium release from ryanodine-sensitive calcium stores in induction of neocortical membrane potential oscillation by using caffeine, an agonist of ryanodine receptors. Intracellular recordings were made from neurons in layer II/III of rat visual cortex slices in a caffeine-containing medium. White matter stimulation initially evoked monophasic synaptic potentials. As low-frequency stimulation continued for over 10 min, an oscillating synaptic potential gradually became evoked, in which a paroxysmal depolarization shift was followed by a 8-10-Hz train of several depolarizing wavelets. This oscillating potential was not induced in a medium containing no caffeine with 2 or 0.5 mM [Mg2+](o). Under blockade of N-methyl-D-aspartate receptors, induction of this oscillating potential failed even with caffeine application. Experiments with the calcium store depletor, thapsigargin, revealed that this oscillating potential is induced in a manner dependent on intracellular calcium release. Dual intracellular recordings revealed that the oscillation was synchronized in pairs of layer II/III neurons. The oscillating potential was detectable by field potential recordings also, suggesting that the present oscillation seems to reflect a network property.     

Age-dependent occurrence of synchronized population oscillation suggestive of a developing functional coupling between NMDA and ryanodine receptors in the neocortex

Synchronized population oscillation of delta to alpha range frequencies was synaptically induced in rat visual cortex neurons, depending on (1) the extent of N-methyl-D-aspartate (NMDA) receptor activation, (2) occurrence of calcium-induced calcium release (CICR), and (3) the age of animals. In adult slices bathed with 0.1 mM Mg(2+), as white matter stimulation continued, solitary synaptic potentials gradually became enlarged and overridden by small wavelets oscillating at alpha range frequencies. The calcium store depeletor thapsigargin prevented appearance of these wavelets. When Mg(2+) block of NMDA receptors was intensified with 0.5 or 2.0 mM Mg(2+), the oscillation failed to be induced, but became inducible by caffeine, which facilitates CICR. NMDA receptor blockade abolished induction of the oscillation even under caffeine application. In immature slices, the induction of oscillation was possible only with 0.1 mM Mg(2+) and bath-applied caffeine, but failed with 0.5 mM Mg(2+) or without caffeine. The oscillation induced in immature tissue was smaller in amplitude and frequency (delta-theta; range) than in adult tissue (alpha range). Immature tissue thus requires more restricted conditions for inducing the oscillation. We propose that NMDA and ryanodine receptors become coupled during maturation to enhance synaptically induced intracellular calcium releases, thereby facilitating induction of the oscillation at later stages.     

Synergistic action of GABA-A and NMDA receptors in the induction of long-term depression in glutamatergic synapses in the newborn rat hippocampus

We show that activation of GABA(A) receptors (GABA(A)Rs) promotes induction of N-methyl-D-aspartate (NMDA) receptor (NMDAR)-dependent long-term depression (LTD) of glutamatergic synapses in the newborn rat hippocampal area CA1 in a developmentally restricted manner. In the newborn rat hippocampus two mechanistically different types of LTD of glutamatergic synapses could be induced under similar experimental conditions. The form of the LTD induced depended on the stimulation protocol and on the age of the animal. Low-frequency stimulation (1 Hz) with 100 stimuli induced a robust homosynaptic, reversible LTD at postnatal days 2-8 (P2-P8) but not at P14. This LTD was blocked by the NMDAR antagonist AP5 or by the GABA(A)R antagonist picrotoxin. Use of a low-chloride solution in the patch pipette resulting in E(GABA-A) < -70 mV blocked the NMDAR-dependent LTD, whereas clamping the cell to -40 mV during induction rescued it. In addition, it was possible to induce LTD at P14 with 100 stimuli if the cells were clamped to -40 mV during induction. Low-frequency stimulation with 900 stimuli induced a robust homosynaptic, reversible LTD both at P2-P8 and at P14. However, neither AP5 nor picrotoxin affected the LTD induced by 900 pulses at P2-P8. Instead, the 900 stimuli-induced LTD was blocked by the metabotropic glutamate receptor antagonists when co-applied with AP5. We suggest that during the first postnatal week postsynaptic depolarization provided by the activation of GABA(A)Rs shifts the threshold for the LTD induction, making the synapses more prone to activity-induced plasticity. From the second postnatal week onwards, when the GABA(A) responses are already hyperpolarizing, different mechanisms for LTD induction prevail.     

A novel form of long‐term depression in the CA1 area of the adult rat hippocampus independent of glutamate receptors activation

In young rats, low frequency (1–2 Hz) stimulation of the Schaffer collaterals for 15 min induces in the CA1 area of the hippocampus a homosynaptic and N-methyl-d -aspartate receptor-dependent form of long-term depression (LTD) of synaptic efficacy. In the adults, while a similar stimulation paradigm is able to depress previously potentiated synapses, it leads to conflicting results when applied to naive synapses. In the present experiments, different stimulation paradigms have been used to induce LTD in the CA1 area of the adult rat hippocampus in vitro. Thus, stimulation of the afferent pathway at frequencies higher than those used to produce LTD in young animals (5–10 Hz, for 15 min) reliably induced a homosynaptic form of LTD. This form of LTD was associated with a significant increase in paired-pulse facilitation ratio and was insensitive to ionotropic (CNQX, 10 μm and CPP, 20 μm ) and metabotropic (S-MCPG, 1 mm ) glutamate receptors antagonists, suggesting a presynaptic mechanism for both LTD induction and expression. In conclusion, our experiments clearly show that LTD is not a purely developmental phenomenon but is present also in mature rats, which possess the whole machinery for LTD induction and this will greatly enhance the flexibility and the storing capacity of neuronal circuits.     

Presynaptic calcium stores contribute to nicotine-elicited potentiation of evoked synaptic transmission at CA3-CA1 connections in the neonatal rat hippocampus

Nicotine acetylcholine (ACh) receptors (nAChRs) are ligand-gated ion channels that are widely expressed throughout the central nervous system. It is well established that presynaptic, alpha7-containing nAChRs modulate glutamate release in several brain areas, and that this modulation requires extracellular calcium. However, the intracellular mechanisms consecutive to nAChR opening are unclear. Recent studies have suggested a role for presynaptic calcium stores in the increase of neurotransmitter release following nAChR activation. Using the minimal stimulation protocol at low-probability Schaffer collateral synapses in acute hippocampal slices from neonatal rats, we show that nicotine acting on presynaptic alpha7 nAChRs persistently upregulates glutamate release. We tested the role of calcium stores in this potentiation. First, we examined the relationship between calcium stores and glutamate release. We found that bath application of SERCA pump inhibitors (cyclopiazonic acid and thapsigargin), as well as an agonist of ryanodine receptors (ryanodine 2 microM) increases the probability of glutamate release at CA3-CA1 synapses, decreases the coefficient of variation and the paired-pulse ratio, indicating that presynaptic activation of calcium-induced calcium release can modulate glutamatergic transmission. Next, we investigated whether blocking calcium release from internal stores could alter the effect of nicotine. Preincubation with thapsigargin (10 microM), cyclopiazonic acid (30 microM), or with a high (blocking) concentration of ryanodine (100 microM) for 30 min to 5 h failed to block the effect of nicotine. However, after preincubation in ryanodine, nicotine-elicited potentiation was significantly shortened. These results indicate that at immature Schaffer collateral-CA1 synapses, activation of presynaptic calcium stores is not necessary for but contributes to nicotine-elicited increase of neurotransmitter release.     

Nitric-oxide-guanylyl-cyclase-dependent and -independent components of multiple forms of long-term synaptic depression

Long-term depression (LTD) of synaptic strength is induced by glutamate-triggered increases in postsynaptic [Ca²⁺], through either influx or release from intracellular stores. Induction of LTD has also been reported to require release of Ca²⁺ from presynaptic stores and activation of presynaptic Ca²⁺/calmodulin-dependent protein kinase II. This finding leads to the hypothesis that the intercellular messenger nitric oxide (NO) may be a means by which postsynaptic Ca²⁺ triggers changes expressing LTD in presynaptic terminals. We report that bath application of the oxadiazoloquinoxalone derivative ODQ (5 μM), a selective inhibitor of NO-sensitive guanylyl cyclase (NOGC), markedly attenuated (90%) the magnitude of LTD induced by low-frequency stimulation (LFS; 1 Hz/15 min) of Schaffer collateral-CA1 synapses in hippocampal slices in vitro. Both the NO donor S-nitroso-N-acetylpenicillamine (100 μM) and the membrane-permeant cyclic guanine 3′,5′-monophosphate (cGMP) analogue 8-(4-chlorophenylthio) guanosine (8-pCPT)-cGMP (50 μM) enhanced the magnitude of LTD, which is consistent with the hypothesis that activation of NOGC plays a role in the induction of LTD. Nicotinamide (20 mM), an inhibitor of NO-activated ADP ribosyltransferase, did not impair the induction of LTD. In contrast to de novo LTD, the reversal of long-term potentiation by LFS (depotentiation) was only partially blocked (55%) by ODQ, and heterosynaptic LTD was not impaired at all, suggesting that there are both NOGC-dependent and -independent forms of LTD. Because postsynaptic intracellular infusion of ODQ (500 μM) failed to block the induction of LTD, we conclude that activation of presynaptic NOGC is a necessary step in the induction of an NOGC-dependent component of LTD. Hippocampus 7:286–295, 1997. © 1997 Wiley-Liss, Inc.     

Endocannabinoid-dependent plasticity at GABAergic and glutamatergic synapses in the striatum is regulated by synaptic activity

Long-term depression (LTD) at striatal synapses is mediated by postsynaptic endocannabinoid (eCB) release and presynaptic cannabinoid 1 receptor (CB₁R) activation. Previous studies have indicated that eCB mobilization at excitatory synapses might be regulated by afferent activation. To further address the role of neuronal activity in synaptic plasticity we examined changes in synaptic strength induced by the L-type calcium channel activator 2,5-dimethyl-4-[2-(phenylmethyl)benzoyl]-1H-pyrrole-3-carboxylic acid methyl ester (FPL 64176, FPL) at glutamatergic and γ-aminobutyric acid (GABA)ergic synapses in the striatum. We found that the basic mechanisms for FPL-mediated eCB signaling are the same at glutamatergic and GABAergic synapses. FPL-induced LTD (FPL-LTD) was blocked in slices treated with the CB₁R antagonist AM251 (2 μ m ), but established depression was not reversed by AM251. FPL-LTD was temperature dependent, blocked by protein translation inhibitors and prevented by intracellular loading of the anandamide transporter inhibitor VDM11 (10 μ m ) at both glutamatergic and GABAergic synapses. FPL-LTD at glutamatergic synapses required paired-pulse afferent stimulation, while FPL-LTD at GABAergic synapses could be induced even in the absence of explicit afferent activation. By evaluating tetrodotoxin-insensitive spontaneous inhibitory postsynaptic currents we found that neuronal firing is vital for eCB release and LTD induction at GABAergic synapses, but not for short-term depression induced by CB₁R agonist. The data presented here suggest that the level of neuronal firing regulates eCB signaling by modulating release from the postsynaptic cell, as well as interacting with presynaptic mechanisms to induce LTD at both glutamatergic and GABAergic synapses in the striatum.     

Heterosynaptic long‐term depression mediated by ATP released from astrocytes

Heterosynaptic long‐term depression (hLTD) at untetanized synapses accompanying the induction of long‐term potentiation (LTP) spatially sharpens the activity‐induced synaptic potentiation; however, the underlying mechanism remains unclear. We found that hLTD in the hippocampal CA1 region is caused by stimulation‐induced ATP release from astrocytes that suppresses transmitter release from untetanized synaptic terminals via activation of P2Y receptors. Selective stimulation of astrocytes expressing channelrhodopsin‐2, a light‐gated cation channel permeable to Ca²⁺, resulted in LTD of synapses on neighboring neurons. This synaptic modification required Ca²⁺ elevation in astrocytes and activation of P2Y receptors, but not N‐methyl‐D ‐aspartate receptors. Furthermore, blocking P2Y receptors or buffering astrocyte intracellular Ca²⁺ at a low level prevented hLTD without affecting LTP induced by SC stimulation. Thus, astrocyte activation is both necessary and sufficient for mediating hLTD accompanying LTP induction, strongly supporting the notion that astrocytes actively participate in activity‐dependent synaptic plasticity of neural circuits. © 2012 Wiley Periodicals, Inc.     

Astrocyte calcium signals at Schaffer collateral to CA1 pyramidal cell synapses correlate with the number of activated synapses but not with synaptic strength

Glial cells respond to neuronal activity by transient increases in their intracellular calcium concentration. At hippocampal Schaffer collateral to CA1 pyramidal cell synapses, such activity-induced astrocyte calcium transients modulate neuronal excitability, synaptic activity, and LTP induction threshold by calcium-dependent release of gliotransmitters. Despite a significant role of astrocyte calcium signaling in plasticity of these synapses, little is known about activity-dependent changes of astrocyte calcium signaling itself. In this study, we analyzed calcium transients in identified astrocytes and NG2-cells located in the stratum radiatum in response to different intensities and patterns of Schaffer collateral stimulation. To this end, we employed multiphoton calcium imaging with the low-affinity indicator dye Fluo-5F in glial cells, combined with extracellular field potential recordings to monitor postsynaptic responses to the afferent stimulation. Our results confirm that somata and processes of astrocytes, but not of NG2-cells, exhibit intrinsic calcium signaling independent of evoked neuronal activity. Moderate stimulation of Schaffer collaterals (three pulses at 50 Hz) induced calcium transients in astrocytes and NG2-cells. Astrocyte calcium transients upon this three-pulse stimulation could be evoked repetitively, increased in amplitude with increasing stimulation intensity and were dependent on activation of metabotropic glutamate receptors. Activity-induced transients in NG2-cells, in contrast, showed a rapid run-down upon repeated three-pulse stimulation. Theta burst stimulation and stimulation for 5 min at 1 Hz induced synaptic potentiation and depression, respectively, as revealed by a lasting increase or decrease in population spike amplitudes upon three-pulse stimulation. Synaptic plasticity was, however, not accompanied by corresponding alterations in the amplitude of astrocyte calcium signals. Taken together, our results suggest that the amplitude of astrocyte calcium signals reflects the number of activated synapses but does not correlate with the degree of synaptic potentiation or depression at Schaffer collateral to CA1 pyramidal cell synapses.     

Kinetics of neurotransmitter release in neuromuscular synapses of newborn and adult rats

The kinetics of the phasic synchronous and delayed asynchronous release of acetylcholine quanta was studied at the neuromuscular junctions of aging rats from infant to mature animals at various frequencies of rhythmic stimulation of the motor nerve. We found that in infants 6 (P6) and 10 (P10) days after birth a strongly asynchronous phase of quantal release was observed, along with a reduced number of quanta compared to the synapses of adults. The rise time and decay of uni-quantal end-plate currents were significantly longer in infant synapses. The presynaptic immunostaining revealed that the area of the synapses in infants was significantly (up to six times) smaller than in mature junctions. The intensity of delayed asynchronous release in infants increased with the frequency of stimulation more than in adults. A blockade of the ryanodine receptors, which can contribute to the formation of delayed asynchronous release, had no effect on the kinetics of delayed secretion in the infants unlike synapses of adults. Therefore, high degree of asynchrony of quantal release in infants is not associated with the activity of ryanodine receptors and with the liberation of calcium ions from intracellular calcium stores.     

Group I metabotropic glutamate receptors enable two distinct forms of long-term depression in the rat dentate gyrus in vivo

The existence of long-term depression (LTD) in the dentate gyrus of freely moving rats, as well as the contribution of different types of metabotropic glutamate receptors (mGluRs) to this form of plasticity, has been the subject of much debate. Here, we describe two distinct forms of mGluR-dependent hippocampal LTD in the dentate gyrus of freely moving adult rats. LTD, induced by low-frequency stimulation (LFS) of the medial perforant path (LFS-LTD), was prevented by antagonism of the phospholipase C-coupled receptors, mGluR1 but not mGluR5. Chemical LTD, induced by intracerebral application of the group I mGluR agonist (R,S)-3,5-dihydroxyphenylglycine, was blocked by antagonism of both mGluR5 and mGluR1. Selective activation of mGluR5, using (R,S)-2-chloro-5-hydroxyphenylglycine (CHPG), also led to chemical LTD. To test whether LFS-LTD and chemical LTD share common induction mechanisms, we applied LFS following the induction of chemical LTD by CHPG (CHPG-LTD). Surprisingly, LFS impaired CHPG-LTD. Further analysis revealed that induction of CHPG-LTD led to altered calcium dynamics sufficient for its reversal by LFS. We found that LTD induced by (R,S)-3,5-dihydroxyphenylglycine, but not by CHPG, is impaired by N-methyl-d-aspartate receptor antagonism. Both forms of chemical LTD strongly require calcium influx through L-type voltage-gated calcium channels. This contrasts with previous findings that LFS-LTD in the dentate gyrus is both N-methyl-d-aspartate receptor and voltage-gated calcium channel independent. LFS-LTD and LTD induced by group I mGluR agonists thus appear to comprise distinct forms of LTD that require the activation of specific group I mGluRs and recruit calcium from different sources.     

Change in bi-directional plasticity at CA1 synapses in hippocampal slices taken from 6-hydroxydopamine-treated rats: the role of endogenous norepinephrine

The object of the present study is to investigate the role of endogenous adrenergic innervation in regulating bi-directional synaptic plasticity in rat hippocampal CA1 synapses. The endogenous adrenergic system was eliminated by giving subcutaneous injection of 6-hydroxydopamine (6-OHDA) to rats immediately after birth, and the animals were killed for experiments at postnatal ages of 25-35 days. In hippocampal slices taken from 6-OHDA-treated animals, theta-burst stimulation at 100 Hz failed to induce long-term potentiation (LTP) at CA1 synapses. However, the induction of long-term depression (LTD) by prolonged low frequency stimulation at 1 Hz was unaffected in slices from 6-OHDA-treated animals. Bath application of norepinephrine (NE) restored LTP to control levels and blocked LTD. This effect was mimicked by beta- but not alpha-adrenergic receptor agonists, i.e. by isoproterenol but not phenylephrine. The activators of adenylyl cyclase and protein kinase A (PKA), i.e. forskolin and 8-bromoadenosine-3', 5'-cyclic monophosphate, respectively, restored LTP in slices from 6-OHDA-treated animals. In addition, application of the D1/D5 receptor agonist, dihydrexidine, also restored LTP in slices from 6-OHDA-treated animals. These results suggest that physiologically the recruitment of catecholamine innervation may be important for induction of LTP at hippocampal CA1 synapses during tetanic stimulation, while it may not be essential for LTD induction by prolonged 1 Hz stimulation. The released NE and dopamine exert their role in modulating synaptic plasticity via activation of beta-adrenergic and D1/D5 receptors, respectively, which in turn increase the levels of cytoplasm adenosine-3',5'-cyclic monophosphate and PKA.     

Modulatory metaplasticity induced by pregnenolone sulfate in the rat hippocampus: A leftward shift in LTP/LTD‐frequency curve

We recently have found that an acute application of the neurosteroid pregnenolone sulfate (PREGS) at 50 μM to rat hippocampal slices induces a long‐lasting potentiation (LLP_PREGS) via a sustained ERK2/CREB activation at perforant‐path/granule‐cell synapses in the dentate gyrus. This study is a follow up to investigate whether the expression of LLP_PREGS influences subsequent frequency‐dependent synaptic plasticity. Conditioning electric stimuli (CS) at 0.1–200 Hz were given to the perforant‐path of rat hippocampal slices expressing LLP_PREGS to induce long‐term potentiation (LTP) and long‐term depression (LTD). The largest LTP was induced at about 20 Hz‐CS, which is normally a subthreshold frequency, and the largest LTD at 0.5 Hz‐CS, resulting in a leftward‐shift of the LTP/LTD‐frequency curve. Furthermore, the level of LTP at 100 Hz‐CS was significantly attenuated to give band‐pass filter characteristics of LTP induction with a center frequency of about 20 Hz. The LTP induced by 20 Hz‐CS (termed 20 Hz‐LTP) was found to be postsynaptic origin and dependent on L‐type voltage‐gated calcium channel (L‐VGCC) but not on N‐methyl‐D ‐aspartate receptor (NMDAr). Moreover, the induction of 20 Hz‐LTP required a sustained activation of ERK2 that had been triggered by PREGS. In conclusion, the transient elevation of PREGS is suggested to induce a modulatory metaplasticity through a sustained activation of ERK2 in an L‐VGCC dependent manner. © 2009 Wiley‐Liss, Inc.     

Alkylxanthines as research tools

(1) The methylxanthine caffeine has many pharmacological effects, most of which can be linked to blockade of adenosine receptors, inhibition of phosphodiesterases, and augmentation of calcium-dependent release of calcium from intracellular stores. (2) A variety of xanthines have been developed as potent and/or selective antagonists for adenosine receptors. (3) Several xanthines have been developed that are more potent and more selective inhibitors of cyclic nucleotide phosphodiesterase than caffeine or theophylline. (4) Caffeine remains the xanthine of choice for activation of intracellular calcium-sensitive calcium release channels although millimolar concentrations are required, which can have effects on other aspects of calcium regulation.     

Activation of NMDA receptors in hippocampal area CA1 by low and high frequency orthodromic stimulation and their contribution to induction of long-term potentiation

N-methyl-D-aspartate (NMDA) receptors are important in many instances of synaptic plasticity. In hippocampal area CA1, long-term potentiation (LTP) can be induced by both NMDA receptor-dependent and -independent mechanisms. Using intracellular recordings and single-electrode voltage clamp, we isolated and characterized NMDA receptor-mediated synaptic responses. NMDA receptor-mediated responses evoked by low frequency orthodromic stimulation were inhibited in a dose-dependent manner by the competitive antagonist D,L-2-amino-5-phosphonovaleric acid (APV). High frequency (tetanic) stimulation, which facilitates synaptic release of glutamate, failed to overcome the blockade of NMDA receptors by APV. Using extracellular recordings of field potentials, we studied the contribution of NMDA receptors to LTP induced by different patterns of tetanic stimulation. LTP was inhibited in a dose-dependent manner by APV, but was more sensitive to APV than were NMDA receptor-mediated synaptic responses. This most likely reflects a threshold for NMDA receptor activation in LTP induction. A component of LTP that resisted blockade by APV was induced by high (200 Hz), but not low (25 Hz), frequency tetanization. This NMDA receptor-independent component of LTP persisted for > 4 hours and accounted for approximately half the potentiation induced by 200 Hz tetanization. Procedures necessary to induce LTP at the Schaffer collateral/ commissural synapses in area CA1 by both NMDA receptor-dependent and -independent mechanisms are now well characterized. Using the same neuronal population, it will be possible to ask if processes involved in the maintenance of LTP are shared even when LTP is induced through two different mechanisms. © 1994 Wiley-Liss, Inc.     

Glutamatergic synaptic responses and long-term potentiation are impaired in the CA1 hippocampal area of calbindin D(28k)-deficient mice.

The contribution of the cytosolic calcium binding protein calbindin D(28K) (CaBP) to glutamatergic neurotransmission and synaptic plasticity was investigated in hippocampal CA1 area of wild-type and antisense transgenic CaBP-deficient mice, with the use of extracellular recordings in the ex vivo slice preparation. The amplitude of non-N-methyl-D-aspartate receptor (non-NMDAr)-mediated extracellular field excitatory postsynaptic potentials (fEPSPs) recorded in control medium was significantly greater in CaBP-deficient mice, whereas the afferent fiber volley was not affected. In contrast, the amplitude of NMDAr-mediated fEPSPs isolated in a magnesium-free medium after blockade of non-NMDAr and GABAergic receptors was significantly depressed in these animals. No alteration in the magnitude of paired-pulse facilitation was found, indicating that the presynaptic calcium mechanisms controlling glutamate release were not altered in CaBP-deficient mice. The magnitude and time course of the short-term potentiation (STP) of fEPSPs induced by a 30 Hz conditioning stimulation, which was blocked by the NMDAr antagonist 2-amino-5-phosphonovalerate acid (2-APV), was not impaired in the transgenic mice, whereas long-term potentiation (LTP) induced by a 100 Hz tetanus was not maintained. The long-term depression (LTD) induced by low-frequency stimulation (1 Hz, 15 min) in the presence of the GABA antagonist bicuculline was not altered. These results argue for a contribution of CaBP to the mechanisms responsible for the maintenance of long-term synaptic potentiation, at least in part by modulating the activation of NMDA receptors.