Pharmacological dissection of calcium channel subtype-related components of strontium inflow in large mossy fiber boutons of mouse hippocampus

Delta-opioid receptor-mediated modulation of excitability of individual hippocampal neurons: mechanisms involved

BACKGROUND

Delta-opioid receptor (DOR)-mediated modulation of hippocampal neural networks is involved in emotions, cognition, and in pathophysiology and treatment of mood disorders. In this study, we examined the effects of DOR agonist (SNC80) and antagonist (naltrindole) on the excitability of individual hippocampal neurons.

METHODS

Primary neuronal cultures were prepared from hippocampi of newborn rats and cultivated in vitro for 8-14 days (DIV8-14). The effects of SNC80 naltrindole on evoked and spontaneous action potentials (APs) were measured at DIV8-9 and DIV13-14, respectively.

RESULTS

SNC80 (100 µM) potentiated spontaneous AP firing and stimulated sodium current; naltrindole had opposite effects. The stimulatory effect of 100 µM of SNC80 was revoked by pre-administration of 1 µM of naltrindole. SNC80 and naltrindole induced similar inhibitory effects on the evoked AP firing and on the calcium current. Further, SNC80 inhibited both peak and sustained potassium currents. Naltrindole had no effect on potassium currents.

CONCLUSION

We suggest that the effects of naltrindole and high concentration of SNC80 on the sodium currents are mediated via DORs and underlying the changes in spontaneous activity. The inhibitory effects of SNC80 on calcium and potassium currents might also be DOR-dependent; these currents might mediate SNC80 effect on the evoked AP firing. The inhibitory effects of naltrindole on calcium and of low doses of SNC80 on sodium currents might be however DOR independent. The behavioral effects of SNC80 and naltrindole, observed in previous studies, might be mediated, at least in part, via the modulatory effect of these ligands on the excitability of hippocampal neurons.

Calcium channel subtypes on glutamatergic mossy fiber terminals synapsing onto rat hippocampal CA3 neurons

The current electrophysiological study investigated the functional roles of high- and low-voltage-activated Ca2+ channel subtypes on glutamatergic small mossy fiber nerve terminals (SMFTs) that synapse onto rat hippocampal CA3 neurons. Experiments combining both the “synapse bouton” preparation and single-pulse focal stimulation technique were performed using the conventional whole cell patch configuration under voltage-clamp conditions. Nifedipine, at a high concentration, and BAY K 8644 inhibited and facilitated the glutamatergic excitatory postsynaptic currents (eEPSCs) that were evoked by 0.2-Hz stimulation, respectively. However, these drugs had no effects on spontaneous EPSCs (sEPSCs). Following the use of a high stimulation frequency of 3 Hz, however, nifedipine markedly inhibited eEPSCs at the low concentration of 0.3 µM. Moreover, ω-conotoxin GVIA and ω-agatoxin IVA significantly inhibited both sEPSCs and eEPSCs. Furthermore, SNX-482 slightly inhibited eEPSCs. R(−)-efonidipine had no effects on either sEPSCs or eEPSCs. It was concluded that glutamate release from SMFTs depends largely on Ca2+ entry through N- and P/Q-type Ca2+ channels and, to a lesser extent, on R-type Ca2+ channels. The contribution of L-type Ca2+ channels to eEPSCs was small at low-firing SMFTs but more significant at high-firing SMFTs. T-type Ca2+ channels did not appear to be involved in neurotransmission at SMFTs.

NEW & NOTEWORTHY Action potential-evoked glutamate release from small mossy fiber nerve terminals (SMFTs) that synapse onto rat hippocampal CA3 neurons is regulated by high-threshold but not low-threshold Ca2+ channel subtypes. The functional contribution mainly depends on N- and P/Q-type Ca2+ channels and, to a lesser extent, on R-type Ca2+ channels. However, in SMFTs stimulated at a high 3-Hz frequency, L-type Ca2+ channels contributed significantly to the currents. The present results are consistent with previous findings from fluorometric studies of large mossy fiber boutons.

Strontium chloride hexahydrate as a candidate molecule for long-term treatment of allergic rhinitis

Background & objectives:

Neurogenic inflammation plays a role in the pathophysiology of allergic rhinitis (AR). Strontium salts are highly effective in reducing the sensory irritation. This study was aimed to investigate the efficacy of strontium chloride (SC) on AR symptoms based on the duration of SC use before the symptoms begin.

Methods:

Wistar albino rats (n=18) were randomly divided into three groups: Group 1, received 1μg mometasone furoate (MF); Group 2, three per cent SC; and Group 3 received five per cent SC (2 μl/site). Drugs were administered to the each nasal cavity for three weeks every morning. On the days 7, 14 and 21, histamine dihydrochloride (HD) 5 μmol (2 μl/site) was administered and the frequencies of nasal rubbing and sneezing were counted for 15 min.

Results:

After 7, 14 and 21 day medication period, the groups were compared in terms of the frequency of sneezing and nasal rubbing following HD. There was a significant difference among the groups in terms of the frequency of sneezing on the day 7 (P< 0.05). Intragroup comparisons for the nasal rubbing showed significance (P< 0.05). In Group 3, there was a decrease in the number of nasal rubbings on the day 14 and 21; however, the difference was not significant.

Interpretation & conclusions:

Our results showed that three and five per cent SC were less effective than MF for sneezing during the first week, but the efficiency was equal to that of MF after the first 14 days. Long-term use of SC was as effective as MF on nasal rubbing. SC can be as effective as MF on both sneezing and nasal rubbing on regular use over three weeks.

Effect of tolbutamide on tetraethylammonium-induced postsynaptic zinc signals at hippocampal mossy fiber-CA3 synapses

The application of tetraethylammonium (TEA), a blocker of voltage-dependent potassium channels, can induce long-term potentiation (LTP) in the synaptic systems CA3-CA1 and mossy fiber-CA3 pyramidal cells of the hippocampus. In the mossy fibers, the depolarization evoked by extracellular TEA induces a large amount of glutamate and also of zinc release. It is considered that zinc has a neuromodulatory role at the mossy fiber synapses, which can, at least in part, be due to the activation of presynaptic ATP-dependent potassium (K_ATP) channels. The aim of this work was to study properties of TEA-induced zinc signals, detected at the mossy fiber region, using the permeant form of the zinc indicator Newport Green. The application of TEA caused a depression of those signals that was partially blocked by the K_ATP channel inhibitor tolbutamide. After the removal of TEA, the signals usually increased to a level above baseline. These results are in agreement with the idea that intense zinc release during strong synaptic events triggers a negative feedback action. The zinc depression, caused by the LTP-evoking chemical stimulation, turns into potentiation after TEA washout, suggesting the existence of a correspondence between the observed zinc potentiation and TEA-evoked mossy fiber LTP.

Characterization of L-type voltage-gated Ca(2+) channel expression and function in developing CA3 pyramidal neurons

Voltage-gated calcium channels (VGCCs) play a major role during the development of the central nervous system (CNS). Ca(2+) influx via VGCCs regulates axonal growth and neuronal migration as well as synaptic plasticity. Specifically, L-type VGCCs have been well characterized to be involved in the formation and refinement of the connections within the CA3 region of the hippocampus. The majority of the growth, formation, and refinement in the CNS occurs during the third trimester of human pregnancy. An equivalent developmental time period in rodents occurs during the first 2weeks of post-natal life, and the expression pattern of L-type VGCCs during this time period has not been well characterized. In this study, we show that Cav1.2 channels are more highly expressed during this developmental period compared to adolescence (post-natal day 30) and that L-type VGCCs significantly contribute to the overall Ca(2+) currents. These findings suggest that L-type VGCCs are functionally expressed during the crucial developmental period.

Remodeling of hippocampal network in pilocarpine-treated mice expressing synaptopHluorin in the mossy fiber terminals

Pilocarpine-induced seizures induce an ectopic projection of hippocampal mossy fibers (MFs). Here, the sprouting was directly examined using TV-42 mice that express synaptopHluorin (SpH) selectively in the MF boutons. The SpH was ectopically expressed in the inner molecular layer (IML) of the dentate gyrus in typical mice after seizures, but were not always accompanied by the zinc fluorescence. The expression of SpH also has a tendency to be enhanced in layers of the CA3a region. It is suggested that the abnormal connection of neurons is more widespread than expected based on the previous zinc-detecting methods.

VOLTAGE-GATED CALCIUM CHANNELS ARE NOT AFFECTED BY THE NOVEL ANTI-EPILEPTIC DRUG LACOSAMIDE

The novel anti-epileptic drug lacosamide targets two proteins - voltage-gated sodium channels and collapsin response mediator protein 2 (CRMP-2) - suggesting dual modes of action for lacosamide. We recently identified the neurite outgrowth and axonal guidance protein CRMP-2 as a novel partner and regulator of the presynaptic N-type voltage-gated Ca(2+) channel (CaV2.2) [Brittain et al., J. Biol. Chem. 284: 31375-31390 (2009)]. Here we examined the effects of lacosamide on voltage-gated Ba(2+) channels. Lacosamide did not affect Ba(2+) currents via N- and P/Q- channels in rat hippocampal neurons or L-type Ca(2+) channels in a mouse CNS neuronal cell line, respectively. N-type Ba(2+) currents, augmented by CRMP-2 expression, were also unaffected by acute or chronic lacosamide exposure. These results establish that the anti-epileptic mode of action of lacosamide does not involve these voltage-gated Ca(2+) channels.

Contributions of SERCA pump and ryanodine-sensitive stores to presynaptic residual Ca2+

The presynaptic Ca2+ signal, which triggers vesicle release, disperses to a broadly distributed residual [Ca2+] ([Ca2+](res)) that plays an important role in synaptic plasticity. We have previously reported a slowing in the decay timecourse of [Ca2+](res) during the second of paired pulses. In this study, we investigated the contributions of organelle and plasma membrane Ca2+ flux pathways to the reduction of effectiveness of [Ca2+](res) clearance during short-term plasticity in Schaffer collateral terminals in the CA1 field of the hippocampus. We show that the slowed decay timecourse is mainly the result of a transport-dependent Ca2+ clearance process; that presynaptic caffeine-sensitive Ca2+ stores are not functionally loaded in the unstimulated terminal, but that these stores can effectively take up Ca2+ even during high frequency trains of stimuli; and that a rate limiting step of sarco-endoplasmic reticulum Ca2+-ATPase (SERCA) kinetics following the first pulse is responsible for a large portion of the observed slowing of [Ca2+](res) clearance during the second pulse. We were able to accurately fit our [Ca2+](res) data with a kinetic model based on these observations and this model predicted a reduction in availability of unbound SERCA during paired pulses, but no saturation of Ca2+ buffer in the endoplasmic reticulum.

Amyloid precursor protein regulates Cav1.2 L-type calcium channel levels and function to influence GABAergic short-term plasticity

Amyloid precursor protein (APP) has been strongly implicated in the pathogenesis of Alzheimer's disease (AD). Although impaired synaptic function is believed to be an early and causative event in AD, how APP physiologically regulates synaptic properties remains poorly understood. Here, we report a critical role for APP in the regulation of L-type calcium channels (LTCC) in GABAergic inhibitory neurons in striatum and hippocampus. APP deletion in mice leads to an increase in the levels of Ca(v)1.2, the pore-forming subunit of LTCCs, and subsequent increases in GABAergic calcium currents (I(Ca(2+))) that can be reversed by reintroduction of APP. Upregulated levels of Ca(v)1.2 result in reduced GABAergic paired-pulse inhibition and increased GABAergic post-tetanic potentiation in both striatal and hippocampal neurons, indicating that APP modulates synaptic properties of GABAergic neurons by regulating Ca(v)1.2. Furthermore, APP physically interacts with Ca(v)1.2, suggesting a mechanism in which loss of APP leads to an inappropriate accumulation and aberrant activity of Ca(v)1.2. These results provide a direct link between APP and calcium signaling and might help explain how altered APP regulation leads to changes in synaptic function that occur with AD.

ALTERED CALCIUM CURRENTS AND AXONAL GROWTH IN Nf1 HAPLOINSUFFICIENT MICE

Mutations of the neurofibromin gene (NF1) cause neurofibromatosis type 1 (NF1), a disease in which learning disabilities are common. Learning deficits also are observed in mice with a heterozygous mutation of Nf1 (Nf1(+/-)). Dysregulation of regulated neurotransmitter release has been observed in Nf1(+/-) mice. However, the role of presynaptic voltage-gated Ca(2+) channels mediating this release has not been investigated. We investigated whether Ca(2+) currents and transmitter release were affected by reduced neurofibromin in Nf1(+/-) mice. Hippocampal Ca(2+) current density was greater in neurons from Nf1(+/-) mice and a greater fraction of Ca(2+) currents was activated at less depolarized potentials. In addition, release of the excitatory neurotransmitter, glutamate, was increased in neuronal cortical cultures from Nf1(+/-) mice. Dendritic complexity and axonal length were also increased in neurons Nf1(+/-) mice compared to wild-type neurons, linking loss of neurofibromin to developmental changes in hippocampal axonal/cytoskeletal dynamics. Collectively, these results show that altered Ca(2+) channel density and transmitter release, along with increased axonal growth may account for the abnormal nervous system functioning in NF1.

Nerve Terminal GABAA Receptors Activate Ca2+/Calmodulin-dependent Signaling to Inhibit Voltage-gated Ca2+ Influx and Glutamate Release

gamma-Aminobutyric acid type A (GABA(A)) receptors, a family of Cl(-)-permeable ion channels, mediate fast synaptic inhibition as postsynaptically enriched receptors for gamma-aminobutyric acid at GABAergic synapses. Here we describe an alternative type of inhibition mediated by GABA(A) receptors present on neocortical glutamatergic nerve terminals and examine the underlying signaling mechanism(s). By monitoring the activity of the presynaptic CaM kinase II/synapsin I signaling pathway in isolated nerve terminals, we demonstrate that GABA(A) receptor activation correlated with an increase in basal intraterminal [Ca(2+)](i). Interestingly, this activation of GABA(A) receptors resulted in a reduction of subsequent depolarization-evoked Ca(2+) influx, which thereby led to an inhibition of glutamate release. To investigate how the observed GABA(A) receptor-mediated modulation operates, we determined the sensitivity of this process to the Na-K-2Cl cotransporter 1 antagonist bumetanide, as well as substitution of Ca(2+) with Ba(2+), or Ca(2+)/calmodulin inhibition by W7. All of these treatments abolished the modulation by GABA(A) receptors. Application of selective antagonists of voltage-gated Ca(2+) channels (VGCCs) revealed that the GABA(A) receptor-mediated modulation of glutamate release required the specific activity of L- and R-type VGCCs. Crucially, the inhibition of release by these receptors was abolished in terminals isolated from R-type VGCC knock-out mice. Together, our results indicate that a functional coupling between nerve terminal GABA(A) receptors and L- or R-type VGCCs is mediated by Ca(2+)/calmodulin-dependent signaling. This mechanism provides a GABA-mediated control of glutamatergic synaptic activity by a direct inhibition of glutamate release.

Effects of strontium on the permeation and gating phenotype of calcium channels in hair cells

To minimize the effects of Ca2+ buffering and signaling, this study sought to examine single Ca2+ channel properties using Sr2+ ions, which substitute well for Ca2+ but bind weakly to intracellular Ca2+ buffers. Two single-channel fluctuations were distinguished by their sensitivity to dihydropyridine agonist (L-type) and insensitivity toward dihydropyridine antagonist (non-L-type). The L- and non-L-type single channels were observed with single-channel conductances of 16 and 19 pS at 70 mM Sr2+ and 11 and 13 pS at 5 mM Sr2+, respectively. We obtained KD estimates of 5.2 and 1.9 mM for Sr2+ for L- and non-L-type channels, respectively. At Ca2+ concentration of approximately 2 mM, the single-channel conductances of Sr2+ for the L-type channel was approximately 1.5 and 4.0 pS for the non-L-type channels. Thus the limits of single-channel microdomain at the membrane potential of a hair cell (e.g., -65 mV) for Sr2+ ranges from 800 to 2,000 ion/ms, assuming an ECa of 100 mV. The channels are >or=4-fold more sensitive at the physiological concentration ranges than at concentrations>10 mM. Additionally, the channels have the propensity to dwell in the closed state at high concentrations of Sr2+, which is reflected in the time constant of the first latency distributions. It is concluded that the concentration of the permeant ion modulates the gating of hair cell Ca2+ channels. Finally, the closed state/s that is/are altered by high concentrations of Sr2+ may represent divalent ion-dependent inactivation of the L-type channel.

Ultrastructural evidence for pre- and postsynaptic localization of Cav1.2 L-type Ca2+ channels in the rat hippocampus

In the hippocampal formation, Ca(v)1.2 (L-type) voltage-gated Ca(2+) channels mediate Ca(2+) signals that can trigger long-term alterations in synaptic efficacy underlying learning and memory. Immunocytochemical studies indicate that Ca(v)1.2 channels are localized mainly in the soma and proximal dendrites of hippocampal pyramidal neurons, but electrophysiological data suggest a broader distribution of these channels. To define the subcellular substrates underlying Ca(v)1.2 Ca(2+) signals, we analyzed the localization of Ca(v)1.2 in the hippocampal formation by using antibodies against the pore-forming alpha(1)-subunit of Ca(v)1.2 (alpha(1)1.2). By light microscopy, alpha(1)1.2-like immunoreactivity (alpha(1)1.2-IR) was detected in pyramidal cell soma and dendritic fields of areas CA1-CA3 and in granule cell soma and fibers in the dentate gyrus. At the electron microscopic level, alpha(1)1.2-IR was localized in dendrites, but also in axons, axon terminals, and glial processes in all hippocampal subfields. Plasmalemmal immunogold particles representing alpha(1)1.2-IR were more significant for small- than large-caliber dendrites and were largely associated with extrasynaptic regions in dendritic spines and axon terminals. These findings provide the first detailed ultrastructural analysis of Ca(v)1.2 localization in the brain and support functionally diverse roles of these channels in the hippocampal formation.

Differential gating and recruitment of P/Q-, N-, and R-type Ca2+ channels in hippocampal mossy fiber boutons

Voltage-gated Ca2+ channels in presynaptic terminals initiate the Ca2+ inflow necessary for transmitter release. At a variety of synapses, multiple Ca2+ channel subtypes are involved in synaptic transmission and plasticity. However, it is unknown whether presynaptic Ca2+ channels differ in gating properties and whether they are differentially activated by action potentials or subthreshold voltage signals. We examined Ca2+ channels in hippocampal mossy fiber boutons (MFBs) by presynaptic recording, using the selective blockers omega-agatoxin IVa, omega-conotoxin GVIa, and SNX-482 to separate P/Q-, N-, and R-type components. Nonstationary fluctuation analysis combined with blocker application revealed a single MFB contained on average approximately 2000 channels, with 66% P/Q-, 26% N-, and 8% R-type channels. Whereas both P/Q-type and N-type Ca2+ channels showed high activation threshold and rapid activation and deactivation, R-type Ca2+ channels had a lower activation threshold and slower gating kinetics. To determine the efficacy of activation of different Ca2+ channel subtypes by physiologically relevant voltage waveforms, a six-state gating model reproducing the experimental observations was developed. Action potentials activated P/Q-type Ca2+ channels with high efficacy, whereas N- and R-type channels were activated less efficiently. Action potential broadening selectively recruited N- and R-type channels, leading to an equalization of the efficacy of channel activation. In contrast, subthreshold presynaptic events activated R-type channels more efficiently than P/Q- or N-type channels. In conclusion, single MFBs coexpress multiple types of Ca2+ channels, which are activated differentially by subthreshold and suprathreshold presynaptic voltage signals.

Inflammation reduces the contribution of N-type calcium channels to primary afferent synaptic transmission onto NK1 receptor-positive lamina I neurons in the rat dorsal horn

N-type calcium channels contribute to the release of glutamate from primary afferent terminals synapsing onto nocisponsive neurons in the dorsal horn of the spinal cord, but little is known of functional adaptations to these channels in persistent pain states. Subtype-selective conotoxins and other drugs were used to determine the role of different calcium channel types in a rat model of inflammatory pain. Electrically evoked primary afferent synapses onto lumber dorsal horn neurons were examined three days after induction of inflammation with intraplantar complete Freund's adjuvant. The maximal inhibitory effect of the N-type calcium channel blockers, omega-conotoxins CVID and MVIIA, were attenuated in NK1 receptor-positive lamina I neurons after inflammation, but the potency of CVID was unchanged. This was associated with reduced inhibition of the frequency of asynchronous-evoked synaptic events by CVID studied in the presence of extracellular strontium, suggesting reduced N-type channel contribution to primary afferent synapses after inflammation. After application of CVID, the relative contributions of P/Q and L channels to primary afferent transmission and the residual current were unchanged by inflammation, suggesting the adaptation was specific to N-type channels. Blocking T-type channels did not affect synaptic amplitude under control or inflamed conditions. Reduction of N-type channel contribution to primary afferent transmission was selective for NK1 receptor-positive neurons identified by post hoc immunohistochemistry and did not occur at synapses in laminae II(o) or II(i), or inhibitory synapses. These results suggest that inflammation selectively downregulates N-type channels in the terminals of primary afferents synapsing onto (presumed) nociceptive lamina I NK1 receptor-positive neurons.

Compartmentalized Ca(2+) channel regulation at divergent mossy-fiber release sites underlies target cell-dependent plasticity

Hippocampal mossy fibers (MFs) innervate CA3 targets via anatomically distinct presynaptic elements: MF boutons (MFBs) innervate pyramidal cells (PYRs), whereas filopodial extensions (Fils) of MFBs innervate st. lucidum interneurons (SLINs). Surprisingly, the same high-frequency stimulation (HFS) protocol induces presynaptically expressed LTP and LTD at PYR and SLIN inputs, respectively. This differential distribution of plasticity indicates that neighboring, functionally divergent presynaptic elements along the same axon serve as autonomous computational elements capable of modifying release independently. Indeed we report that HFS persistently depresses voltage-gated calcium channel (VGCC) function in Fil terminals, leaving MFB VGCCs unchanged despite similar contributions of N- and P/Q-type VGCCs to transmission at each terminal. Selective Fil VGCC depression results from HFS-induced mGluR7 activation leading to persistent P/Q-type VGCC inhibition. Thus, mGluR7 localization to MF-SLIN terminals and not MFBs allows for MF-SLIN LTD expression via depressed presynaptic VGCC function, whereas MF-PYR plasticity proceeds independently of VGCC alterations.

Mossy fiber synaptic transmission: communication from the dentate gyrus to area CA3

Communication between the dentate gyrus (DG) and area CA3 of the hippocampus proper is transmitted via axons of granule cells--the mossy fiber (MF) pathway. In this review we discuss and compare the properties of transmitter release from the MFs onto pyramidal neurons and interneurons. An examination of the anatomical connectivity from DG to CA3 reveals a surprising interplay between excitation and inhibition for this circuit. In this respect it is particularly relevant that the major targets of the MFs are interneurons and that the consequence of MF input into CA3 may be inhibitory or excitatory, conditionally dependent on the frequency of input and modulatory regulation. This is further complicated by the properties of transmitter release from the MFs where a large number of co-localized transmitters, including GABAergic inhibitory transmitter release, and the effects of presynaptic modulation finely tune transmitter release. A picture emerges that extends beyond the hypothesis that the MFs are simply "detonators" of CA3 pyramidal neurons; the properties of synaptic information flow from the DG have more subtle and complex influences on the CA3 network.

Silencing of ZnT-1 expression enhances heavy metal influx and toxicity

ZnT-1 reduces intracellular zinc accumulation and confers resistance against cadmium toxicity by a mechanism which is still unresolved. A functional link between the L-type calcium channels (LTCC) and ZnT-1 has been suggested, indicating that ZnT-1 may regulate ion permeation through this pathway. In the present study, immunohistochemical analysis revealed a striking overlap of the expression pattern of LTCC and ZnT-1 in cardiac tissue and brain. Using siRNA to silence ZnT-1 expression, we then assessed the role of ZnT-1 in regulating cation permeation through the L-type Ca(2+) channels in cells that are vulnerable to heavy metal permeation. Transfection of cortical neurons with ZnT-1 siRNA resulted in about 70% reduction of ZnT-1 expression and increased Ca(2+) influx via LTCC by approximately fourfold. Moreover, ZnT-1 siRNA transfected neurons showed approximately 30% increase in synaptic release, monitored using the FM1-43 dye. An increased cation influx rate, through the LTCC, was also recorded for Zn(2+) and Cd(2+) in cells treated with the ZnT-1 siRNA. Furthermore, Cd(2+)-induced neuronal death increased by approximately twofold after transfection with ZnT-1 siRNA. In addition, ZnT-1 siRNA transfection of the ovarian granulosa cell line, POGRS1, resulted in a twofold increase in Cd(2+) influx rate via the LTCC. Finally, a robust nimodipine-sensitive Cd(2+) influx was observed using a low extracellular Cd(2+) concentration (5 muM) in neurons and testicular slice cultures, attesting to the relevance of the LTCC pathway to heavy metal toxicity. Taken together, our results indicate that endogenously-expressed ZnT-1, by modulating LTCC, has a dual role: regulating calcium influx, and attenuating Cd(2+) and Zn(2+) permeation and toxicity in neurons and other cell types.

Developmentally regulated actions of alcohol on hippocampal glutamatergic transmission

Ethanol exposure during fetal development is a leading cause of learning disabilities. Studies suggest that it alters learning and memory by permanently damaging the hippocampus. It is generally assumed that this is mediated, in part, via alterations in glutamatergic transmission. Although NMDA receptors are presumed to be the most sensitive targets of ethanol in immature neurons, this issue has not been explored in the developing hippocampus. We performed whole-cell patch-clamp recordings in hippocampal slices from neonatal rats. Unexpectedly, we found that acute ethanol (10-50 mM) exposure depresses inward currents elicited by local application of exogenous AMPA, but not NMDA, in CA3 pyramidal neurons. These findings revealed a direct effect of ethanol on postsynaptic AMPA receptors. Ethanol significantly decreased the amplitude of both AMPA and NMDA receptor-mediated EPSCs evoked by electrical stimulation. This effect was associated with an increase in the paired-pulse ratio and a decrease in the frequency of miniature EPSCs driven by depolarization of axonal terminals. These findings demonstrate that ethanol also acts at the presynaptic level. Omega-conotoxin-GVIA occluded the effect of ethanol on NMDA EPSCs, indicating that ethanol decreases glutamate release via inhibition of N-type voltage-gated Ca2+ channels. In more mature rats, ethanol did not affect the probability of glutamate release or postsynaptic AMPA receptor-mediated currents, but it did inhibit NMDA-mediated currents. We conclude that the mechanism by which ethanol inhibits glutamatergic transmission is age dependent and challenge the view that postsynaptic NMDA receptors are the primary targets of ethanol early in development.

Inhibition of presynaptic activity by zinc released from mossy fiber terminals during tetanic stimulation

Zinc exists in high densities in the giant boutons of hippocampal mossy fibers. On the basis of the evidence that zinc decreases extracellular glutamate concentration in the hippocampus, the presynaptic action of zinc released from mossy fibers during high-frequency (tetanic) stimulation was examined using hippocampal slices. The increase in zinc-specific fluorescent signals was observed in both extracellular and intracellular compartments in the mossy fiber terminals during the delivery of tetanic stimuli (100 Hz, 1 sec) to the dentate granule cell layer, suggesting that zinc released from mossy fibers is immediately retaken up by mossy fibers. When mossy fiber terminals were preferentially double-stained with zinc and calcium indicators and tetanic stimuli (100 Hz, 1 sec) were delivered to the dentate granule cell layer, the increase in calcium orange signal during the stimulation was enhanced in mossy fiber terminals by addition of CaEDTA, a membrane-impermeable zinc chelator, and was suppressed by addition of zinc. The decrease in FM4-64 signal (vesicular exocytosis) during tetanic stimulation (10 Hz, 180 sec), which induced mossy fiber long-term potentiation, was also enhanced in mossy fiber terminals by addition of CaEDTA and was suppressed by addition of zinc. The present study demonstrates that zinc released from mossy fibers may be a negative-feedback factor against presynaptic activity during tetanic stimulation.

Synapse-to-synapse variation of calcium channel subtype contributions in large mossy fiber terminals of mouse hippocampus

Both N- and P/Q-type voltage-dependent calcium channels are involved in fast transmitter release in the hippocampus, but are differentially regulated. Although variable contributions of voltage-dependent calcium channel subtypes to presynaptic Ca2+ influx have been suggested to give a neural network of great diversity, their presence has only been demonstrated in a culture system and has remained unclear in the brain. Here, the individual large mossy fiber presynaptic terminal was labeled with Ca2+/Sr2+-sensitive fluorescent dextrans in the hippocampal slice of the mouse. The fractional contribution of voltage-dependent calcium channel subtypes to presynaptic Ca2+/Sr2+ influx was directly measured by the sensitivity of Ca2+/Sr2+-dependent fluorescent increment to subtype-selective neurotoxins, omega-conotoxin GVIA (an N-type selective blocker), omega-agatoxin IVA (a P/Q-type selective blocker) and SNX-482 (an R-type selective blocker). Synapse-to-synapse comparison of large mossy fiber terminals revealed that the contributions of N- and R-type voltage-dependent calcium channels varied more widely than that of P/Q-type. Even two large mossy fiber presynaptic terminals neighboring on the same axon differed in the fractional contributions of N- and R-type voltage-dependent calcium channels. On the other hand, these terminals were similar in the fractional contributions of P/Q-type voltage-dependent calcium channels. These results provide direct evidence that individual large mossy fiber synapses are differential in the contribution of N- and R-type voltage-dependent calcium channel subtypes to presynaptic Ca2+/Sr2+ influx. We suggest that the synapse-to-synapse variation of presynaptic voltage-dependent calcium channel subtype contributions may be one of the mechanisms amplifying diversity of the hippocampal network.