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.     

Reduced sensitivity of the N-methyl-D-aspartate component of synaptic transmission to magnesium in hippocampal slices from immature rats

This study describes the measurement of N-methyl-D-aspartate (NMDA)-mediated excitatory postsynaptic potentials (EPSPs) of the CA1 subregion of transverse hippocampal slices from immature and adult rats. Our methods permit extracellular measurement of NMDA-mediated depolarizations in the presence of magnesium (Mg2+) ions. In comparison to slices from adult rats (75-90 days old), NMDA EPSPs in hippocampus from immature rats (25-35 days old) were of significantly greater amplitude and were significantly less sensitive to magnesium. It is suggested that developmental plasticity may be related to changes in magnesium regulation of the NMDA channel complex.     

NMDA-receptors on Purkinje cell dendrites in guinea pig cerebellar slices

The Mg2+-dependent depolarizing action of N-methyl-D-aspartate (NMDA) was intrasomatically investigated, in comparison with quisqualate (QA), in Purkinje cells in cerebellar slices from adult guinea pigs. NMDA applied iontophoretically to the proximal dendritic region (about 100 micron from the Purkinje cell soma) induced depolarizations and spike firings in about the half of the Purkinje cells tested in nominal Mg2+-free medium (contaminated with 4-11 microM Mg2+), and 1 mM Mg2+ almost completely blocked this NMDA action. Application of NMDA onto the distal dendritic region (about 200 micron from the soma) caused no depolarization at all even in the Mg2+-free medium. QA applied onto either the proximal or distal dendritic region consistently showed Mg2+-independent depolarizations. The amplitude of NMDA-induced depolarization in the Mg2+-free medium was non-linearly related to the membrane potential, i.e. smaller at a hyperpolarized potential level. 2-Amino-5-phosphonovalerate blocked the NMDA action partially but more selectively than the QA action, while the reverse was the case for glutamic acid diethylester. These results suggest that the Mg2+-dependent, NMDA-sensitive receptor, which is distinct from the QA receptor and probably similar to the well-known NMDA receptor, is present on the proximal dendrite of the cerebellar Purkinje cell of the guinea pig.     

Calcium-dependent subthreshold oscillations determine bursting activity induced by N-methyl-D-aspartate in rat subthalamic neurons in vitro

We used whole-cell patch recordings in current clamp to investigate the ionic dependence of burst firing induced by N-methyl-d-aspartate (NMDA) in neurons of the subthalamic nucleus (STN) in slices of rat brain. NMDA (20 microm) converted single-spike firing to burst firing in 87% of STN neurons tested. NMDA-induced bursting was blocked by AP5 (50 microm), and was not mimicked by the non-NMDA receptor agonist AMPA (0.6 microm). Tetrodotoxin (1 microm) converted bursts to oscillations of membrane potential, which were most robust when oscillations ranged between -50 and -70 mV. The NMDA bursts were blocked by an elevated extracellular concentration of Mg(2+), but superfusate containing no added Mg(2+) either reduced or increased burst firing, depending upon the amount of intracellular current injection. Block of K(+) conductances by apamin and tetraethylammonium prolonged burst duration, but iberiotoxin had no effect. NMDA-induced burst firing and membrane oscillations were completely blocked by superfusate containing no added Ca(2+), and they were significantly reduced when patch pipettes contained BAPTA. Selective antagonists for T-type (mibefradil, 10 microm), L-type (nifedipine, 3 microm), and N-type (omega-conotoxin GVIA, 1 micro m) Ca(2+) channels had no effect on NMDA burst firing. Superfusate containing a low concentration of Na(+) (20 mm) completely abolished NMDA-induced burst firing. Flufenamic acid (10 microm), which blocks current mediated by Ca(2+)-activated nonselective cation channels (I(CAN)), reversibly abolished NMDA-depended bursting. These results are consistent with the hypothesis that NMDA-induced burst firing in STN neurons requires activation of either an I(CAN) or a Na(+)-Ca(2+) exchanger.     

Age-dependent appearance of an insulo-parietal cortical signal propagation that elicits a synchronized population oscillation in the parietal cortex in rats

We investigated postnatal development of a functional connectivity from the gustatory insular cortex to the parietal cortex, which is known to contain many more neurons responding to oral somesthetic stimulation than the insular cortex, in slices obtained from 9-35-day-old rats. Field potentials were evoked by stimulation to the insular cortex. In the mature cortex, insular stimulation elicited a solitary field potential in both the insular and parietal cortices and, as the simulation continued, the initial solitary potential came to be followed by a population oscillation of field potential in the parietal cortex, but not in the insular cortex. In the immature cortex, by contrast, insular stimulation failed to evoke both the initial solitary potential and the subsequent population oscillation in the parietal cortex. In the mature cortex, application of neither thapsigargin nor AP5 prevented elicitation of the initial solitary potential in the parietal cortex, but either of them abolished the parietal oscillation. In immature cortex bathed with low Mg(2+) medium, insular stimulation elicited both the initial solitary potential and the subsequent parietal oscillation, which were both prevented by thapsigargin or AP5. Theses results suggest that the insular and parietal cortices are anatomically connected but functionally unlinked at an early postnatal stage, and that a functional linkage, dependent both on NMDA and ryanodine receptors, is formed during the first postnatal month.     

Caffeine-dependent stimulus-triggered oscillations in the CA3 region of hippocampal slices from rats chronically exposed to lead

Yoshimura et al. [Yoshimura, H., Sugai, T., Onoda, N., Segami, N., Kato, N., 2002. Age-dependent occurrence of synchronized population oscillation suggestive of a developing functional coupling between NMDA and ryanodine receptors in the neocortex. Dev. Brain Res., 136, 63-68.] have shown that caffeine can elicit synchronized oscillations (10-12 Hz) dependent on calcium-induced calcium release in rat neocortex neurons. In the present work, synchronized oscillations in the CA3 region of rat hippocampus were studied by recording field excitatory postsynaptic potentials (fEPSPs) in vitro. In the presence of 0.1 mM caffeine, in CA3 of 44 of 45 (97.8%) slices from chronic lead-exposed rats, single electrical stimuli triggered a burst of high-frequency oscillations (approximately 230 Hz), whereas in CA3 of caffeine-treated slices from control rats, such oscillations could be elicited in only 2 of 24 (8.3%) slices. The complete (but fully reversible) block of caffeine-dependent oscillations by 6-cyano-7-nitro-quinoxaline-2,3-dione (CNQX; 20 microM) indicates that alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate (AMPA) receptors are necessary for the high-frequency synchronized oscillations. 2-Amino-5-phosphonopentanoate (AP-5; 50 micoM) partially reduced the amplitude of caffeine-dependent oscillations without significantly altering their frequency. Caffeine-dependent oscillations could be abolished by application of AP-5 and 3 mM Mg2+ during the initial period of bursting, indicating that N-methyl-D-aspartate (NMDA) receptors play an important role in the generation of oscillations. The Ca2+ chelator ethylene glycol bis-(beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid (EGTA; 5 mM) added in standard artificial cerebrospinal fluid (ACSF) containing 0.1 mM caffeine fully blocked the oscillations. Caffeine-dependent oscillations are insensitive to an antagonist of gamma-aminobutyric acid (GABAA) receptors (10 microM bicuculline), L-type Ca2+ channels (10 muM nicardipine), L-type and N-type voltage-dependent calcium channels (100 microM Cd2)), and T-type Ca2+ channels (100 microM Ni2+). Previous studies have demonstrated that expression and function of NMDA and AMPA receptors are altered in the hippocampus of chronic lead-exposed rats. We propose that caffeine-dependent stimulus-induced oscillations in CA3 area of hippocampus from chronic lead-exposed rats are mainly mediated by the entry of extracellular Ca2+ through NMDA and non-NMDA receptors, without participation of GABAA receptors. Additionally, the underlying mechanisms are also discussed.     

Epileptiform activity induced by low extracellular magnesium in the human cortex maintained in vitro.

Extracellular field potentials and [K+]o were recorded in slices of human epileptogenic neocortex maintained in vitro during perfusion with Mg(2+)-free artificial cerebrospinal fluid (ACSF). The human neocortex was obtained during neurosurgical procedures for the relief of seizures that were resistant to medical treatment. Spontaneous epileptiform activity and episodes of spreading depression appeared within 1.5 to 2 hours of perfusion with Mg(2+)-free ACSF. The epileptiform discharges consisted of negative field potential shifts (amplitude, 0.8-10 mV) that lasted 2.5 to 80 seconds and recurred at intervals ranging between 4 and 160 seconds. Both duration and frequency of occurrence of epileptiform events were not significantly different when measured in slices obtained from spiking tissue compared with those gathered from nonspiking neocortical areas. Transient increases in [K+]o of up to 10.5 mM were associated with each epileptiform discharge; these changes were maximal and fastest in the middle neocortical layers. Spreading depression episodes were characterized by 20 to 30-mV negative shifts that lasted up to 200 seconds and were accompanied by increases in [K+]o of approximately 100 mM. Epileptiform discharges and spreading depressions did not occur during perfusion with Mg(2+)-free ACSF that contained either competitive or noncompetitive antagonists of the N-methyl-D-aspartate (NMDA) receptor subtype. In contrast, pharmacological blockade of non-NMDA receptors did not influence the epileptiform activity observed in Mg(2+)-free ACSF. These findings demonstrate that decreasing [Mg2+]o leads to the appearance of both spontaneous epileptiform discharges and spreading depression in the human epileptogenic neocortex.(ABSTRACT TRUNCATED AT 250 WORDS)     

Long-term potentiation in the nucleus accumbens requires both NR2A- and NR2B-containing N-methyl-D-aspartate receptors

N-methyl-d-aspartate (NMDA) receptors play crucial roles in several forms of long-term changes in the efficacy of glutamatergic synaptic transmission. The suggestion that the NR2A subunit of the NMDA receptor may be selectively involved in the induction of long-term potentiation (LTP) in the hippocampus and cortex has been challenged. However, the contribution of NR2B in the induction of LTP is not always clearly established. The present study investigates the role of NR2A and NR2B in the induction of LTP in the nucleus accumbens (NAc), a brain region that expresses high levels of NR2B and an NMDA-dependent form of LTP. We recorded extracellular field excitatory postsynaptic potentials/population spikes in slices of mouse NAc. High-frequency stimulation of glutamatergic fibers consistently induced LTP of the field excitatory postsynaptic potential/population spike in the NAc. LTP was abolished in the presence of selective antagonists of either NR2B [R-(R*,S*)-alpha-(4-hydroxyphenyl)-beta-methyl-4-(phenyl-methyl)-1-piperidine propanol and Ifenprodil] or NR2A ([(R)-[(S)-1-(4-bromo-phenyl)-ethylamino]-(2,3-dioxo-1,2,3,4-tetrahydroquinoxalin-5-yl)-methyl]-phosphonic acid) subunits. Recordings performed in a low concentration of Mg(2+) ions in the perfusion solution did not reveal a selective involvement of a particular NMDA receptor subunit because either NR2A or NR2B antagonists were able to block LTP. LTP was also abolished in the presence of a low concentration of the non-subunit-selective NMDA receptor antagonist dl-2-amino-5-phosphonopentanoic acid in normal Mg(2+) and low Mg(2+) in the perfusion solution. These results show that the degree of NMDA receptor activation, and not their subunit composition, determines whether LTP is induced in the NAc.     

High potassium induces taurine release by osmosensitive and osmoresistant mechanisms in the rat hippocampus in vivo

The high potassium-evoked taurine efflux in the nervous tissue has been entirely considered to be the result of the cell swelling produced by KCl influx via passive Donnan forces. However, the extracellular taurine increase evoked in the hippocampus by applying 6-100 mM KCl through microdialysis probes, which saturates at a concentration of 25 mM KCl, is not congruent with the mentioned osmosensitive release of taurine stimulated by high potassium. Therefore, we studied whether the taurine release elicited by different high KCl concentrations (25, 50, 75, or 100 mM) was blocked under hypertonic conditions (+100 mM sucrose). Taurine release stimulated by 25 mM KCl was totally osmosensitive, but that released by higher KCl concentrations became progressively osmoresistant, achieving more than the 60% of the extracellular taurine enhancement during 100 mM KCl perfusion. The osmoresistant taurine release evoked by 100 mM KCl perfusion was partially reduced by a solution without Ca(2+) and with high Mg(2+), or by D,L-2-amino-5-phosphopentanoic acid, an N-methyl-D-aspartic acid (NMDA) receptor antagonist. Moreover, the release of taurine induced by a hypoosmotic solution was reduced by the presence of either high K(+) (75 mM) or NMDA (100 microM). These results indicate that although moderately high [K(+)] evoke the osmosensitive release of taurine, higher [K(+)] inhibit it and trigger the release of taurine by an osmoresistant mechanism. This last component is partially mediated by NMDA receptors activated by the glutamate released during potassium-induced depolarization.     

The Influence of Magnesium Ions on the NMDA Mediated Responses of Ventral Rhythmic Neurons in the Spinal Cord of Xenopus Embryos

Xenopus embryos immobilized in tubocurarine respond to natural skin stimulation with fictive swimming. This can also occur in saline without Mg2+ and is blocked by NMDA antagonists. Ventral spinal cord neurons which are rhythmically active during swimming are depolarized by bath applied N-methyl-D-aspartate (NMDA) (in 1 microM tetrodotoxin (TTX) to block indirect effects). By using current clamp techniques this depolarization is shown to be partially blocked by 0.5 and 1 mM Mg2+ in a voltage-dependent manner similar to that described in cultured neurons. Mg2+ partially and reversibly reduces the slow NMDA-mediated component of excitatory post-synaptic potentials (EPSPs) in ventral neurons. However, in 1 mM Mg2+ fictive swimming can still be evoked by natural stimulation. The frequency of swimming is slightly lower than in nominally 0 mM Mg2+, but the pattern of ventral root activity and synaptic drive to ventral neurons seems little affected. Fictive swimming can also be induced by applying NMDA to spinal preparations. In 0 mM Mg2+, such rhythmic activity is unstable and transient over a narrow NMDA concentration range. In 0.5 mM Mg2+, continuous rhythmic activity is induced over a wide range of NMDA concentrations. Lower spinal preparations need higher NMDA concentrations to induce activity. We conclude that the neurons rhythmically active in swimming have NMDA receptor channels which show a voltage dependent block in the presence of Mg2+. However, while Mg2+ exerts a powerful stabilizing influence on rhythmic activity induced in spinal embryos by exogenous NMDA, its influence on 'naturally' evoked fictive swimming is less clear. The fictive swimming machinery in the brain and spinal cord can produce stable swimming with or without Mg2+ induced voltage dependency of the NMDA channels.     

Cyclic AMP-dependent attenuation of oscillatory-activity-induced intercortical strengthening of horizontal pathways between insular and parietal cortices

Cyclic AMP (cAMP) is a key intracellular second messenger, and the intracellular cAMP signaling pathway acts to modulate various brain functions. We have previously reported that low-frequency insular cortex stimulation in rat brain slices switches on a voltage oscillator in the parietal cortex that delivers signals horizontally back and forth under caffeine application. The oscillatory activities are N-methyl-D-aspartate (NMDA) receptor-dependent, and the role of oscillation is to strengthen functional intercortical connections. The present study investigated actions of the cAMP signaling pathway on caffeine-induced strengthening of intercortical connections and tried to confirm the role of oscillation on intercortical strengthening by focusing on the cAMP pathway. After induction of parietal oscillation by insular cortex stimulation in caffeine-containing medium, application of membrane-permeable cAMP analog, bromo-cAMP, diminished oscillatory signal delivery from the parietal cortex, but initial insulo-parietal signal propagation remained strong. When oscillatory activities were reduced with co-application of caffeine and bromo-cAMP from the beginning, initial insulo-parietal propagation was established, but amplitudes of propagating wavelets and propagating velocity were reduced. Thus, cAMP-dependent diminution of caffeine-induced NMDA-receptor-dependent oscillatory signal delivery causes attenuation of intercortical strengthening of horizontal pathways between insular and parietal cortices. This finding suggests that the intracellular cAMP signaling pathway has the ability to regulate extracellular communications at the network level, and also that full expression of strengthened intercortical signal communication requires sufficient NMDA-receptor-dependent oscillatory neural activities.     

Different changes in spontaneous field potential oscillations precede epileptiform bursting in hippocampal slices perfused with penicillin or reduced magnesium

Power spectra were used to analyse spontaneous field potentials (SFPs) recorded in the CA3 distal apical dendritic region of guinea pig hippocampal slices perfused with either penicillin or reduced Mg2+. High concentrations of penicillin (2000 IU/ml) progressively converted the low amplitude, irregular oscillations observed in control medium to higher amplitude, low frequency, rhythmic oscillations at approximately 2-3 Hz just prior to the onset of spontaneous, synchronized bursting. Low concentrations (50-300 IU/ml) increased the power of frequencies below 10 Hz and suppressed higher frequencies in a dose-dependent fashion. Although Mg2(+)-free medium also increased the magnitude of the SFPs prior to the onset of synchronous bursting, the changes were smaller than with penicillin and the frequency distribution was completely different. Low concentrations of Mg2+ (0.0-0.5 mM) increased the power across all frequencies, however, the maximal effect was on frequencies between 5 and 25 Hz. The transition from normal to epileptiform activity may proceed through at least 2 distinct intermediate states. When recurrent inhibition is blocked (penicillin), synchronous synaptic activity precedes the onset of bursting, whereas non-specific increases in excitability and activation of NMDA receptors (reduced Mg2+) produce an asynchronous transition state.     

Hypoosmolar conditions reduce extracellular volume fraction and enhance epileptiform activity in the CA3 region of the immature rat hippocampus

The osmolarity of the extracellular space (ECS) compartment is an important factor determining the excitability of neuronal tissue. In the adult hippocampus an important role of osmolarity and ECS diffusion parameters on the susceptibility to epileptic events is well established, but the influence of hypo- and hyperosmolar conditions on the immature hippocampus remains elusive. To investigate the influence of osmolarity on epileptiform activity, extracellular field potentials were recorded in the CA3 region of hippocampal slices of immature (postnatal days 4-7) Wistar rats. The ECS diffusion parameters were determined by the real-time tetramethylammonium (TMA+) iontophoretic method with ion-selective microelectrodes in immature hippocampal slices and showed a lack of diffusion anisotropy; a tortuosity of about 1.39; and a volume fraction, alpha, of 0.41 +/- 0.01 (n = 10 slices). A reduction in osmolarity of -90 mOsm induced a decrease in alpha to 0.17 +/- 0.02 (n = 4 slices). The frequency of epileptiform activity elicited in 10-50 microM 4-AP-containing low-Mg2+ solution was increased under -90 mOsm and -40 mOsm hypoosmolar conditions by 39.9% +/- 8.1% (n = 16) and 24.1% +/- 4.8% (n = 10), respectively, whereas hyperosmolar solutions decreased the frequency. A -90-mOsm reduction in the osmolarity of low-Mg2+ solution induced epileptiform activity in nine of 19 slices. In summary, these results demonstrate that hypoosmolar conditions increased excitability and susceptibility to epileptiform activity in immature hippocampal slices, suggesting a functional role of the larger alpha in suppression of seizures.     

Calcium-induced calcium release contributes to action potential-evoked calcium transients in hippocampal CA1 pyramidal neurons

Calcium-induced calcium release (CICR) is a mechanism by which local elevations of intracellular calcium (Ca2+) are amplified by Ca2+ release from ryanodine-sensitive Ca2+ stores. CICR is known to be coupled to Ca2+ entry in skeletal muscle, cardiac muscle, and peripheral neurons, but no evidence suggests that such coupling occurs in central neurons during the firing of action potentials. Using fast Ca2+ imaging in CA1 neurons from hippocampal slices, we found evidence for CICR during action potential-evoked Ca2+ transients. A low concentration of caffeine enhanced Ca2+ transient amplitude, whereas a higher concentration reduced it. Simultaneous Ca2+ imaging and whole-cell recordings showed that membrane potential, action potential amplitude, and waveform were unchanged during caffeine application. The enhancement of Ca2+ transients by caffeine was not affected by the L-type channel blocker nifedipine, the phosphodiesterase inhibitor IBMX, the adenylyl cyclase activator forskolin, or the PKA antagonist H-89. However, thapsigargin or ryanodine, which both empty intracellular Ca2+ stores, occluded this effect. In addition, thapsigargin, ryanodine, and cyclopiazonic acid reduced action potential-evoked Ca2+ transients in the absence of caffeine. These results suggest that Ca2+ release from ryanodine-sensitive stores contributes to Ca2+ signals triggered by action potentials in CA1 neurons.     

Selective enhancement of non-NMDA receptor-mediated responses following induction of long-term potentiation in entorhinal cortex

The contribution of NMDA receptors to the expression of long-term potentiation (LTP) is controversial. In entorhinal cortex (EC) previous studies reported either that LTP was exclusively expressed through NMDA receptors or that both NMDA and non-NMDA receptors were involved in LTP expression. To reexamine this issue, horizontal entorhinal cortical slices were prepared from adult rats and electrical stimulation was delivered in layer II/III, while field potential recordings were made in layer III. In the standard condition (2.5 mM Mg(++)), LTP was reliably induced by theta burst stimulation, but was blocked by 100 microM D-AP5, an NMDA receptor antagonist. This corroborates previous reports that NMDA receptor activation is required for induction of EC LTP. The field potential response was not affected by D-AP5, but completely blocked by 10 microM CNQX, a non-NMDA receptor antagonist. This indicates that the expression of LTP is mediated by non-NMDA receptors in the standard condition. LTP of NMDA receptor-mediated responses was tested by comparing NMDA responses before and after applying theta burst stimulation in medium containing low magnesium (0.4-1 mM). Theta burst stimulation induced 43.2+/-9.7% increase of non-NMDA responses (i.e., AP5-insensitive fast component) but 5.6+/-9.0% decrease of the NMDA receptor component (AP5-sensitive slow component). These results indicate that activation of NMDA receptors is critical for induction of LTP, but LTP expression is mediated by non-NMDA receptors in EC under these experimental conditions.     

NMDA receptor-mediated synaptic excitation selectively inhibited by ethanol in hippocampal slice from adult rat

The effect of ethanol (EtOH) on synaptic transmission mediated by N-methyl-D-aspartate (NMDA) and non-NMDA glutamate receptors was investigated in slices from adult rat hippocampus. Synaptic responses were elicited by stimulation of stratum radiatum and were recorded in CA1 stratum radiatum or stratum pyramidale. Population EPSPs (pEPSPs) mediated by NMDA receptor activation were isolated by application of a solution containing the kainate/quisqualate receptor antagonist 6,7-dinitroquinoxaline-2,3-dione and either low (0.1 mM) Mg2+ or 100 microM bicuculline. Increasing concentrations of EtOH produced increasing inhibition of NMDA receptor-mediated pEPSPs with EtOH concentrations between 1 and 50 mM. At a concentration of 50 mM, EtOH inhibited NMDA receptor-mediated pEPSPS by 43%; the inhibition by 100 mM EtOH was not significantly different from that produced by 50 mM. Methanol and 1-butanol also inhibited the NMDA receptor-mediated pEPSPs; the potency of the alcohols for inhibition of NMDA receptor-mediated pEPSPs was 1-butanol greater than ethanol greater than methanol. pEPSPs mediated by non-NMDA glutamate receptors were isolated by the application of the NMDA receptor antagonist d,1-2-amino-5-phosphonovaleric acid in the presence of 1.5 mM Mg2+. These pEPSPs were not significantly affected by 50 mM EtOH, whereas 100 mM EtOH reduced the amplitude of these pEPSPs by 9%. The observations indicate that synaptic excitation mediated by NMDA receptors in tissue from adult rat is inhibited by intoxicating concentrations of EtOH. The data are consistent with the hypothesis that EtOH-induced inhibition of EPSPs mediated NMDA receptors may contribute to the intoxicating effects of EtOH.     

Zn2+ ions: modulators of excitatory and inhibitory synaptic activity.

The role of Zn(2+) in the CNS has remained enigmatic for several decades. This divalent cation is accumulated by specific neurons into synaptic vesicles and can be released by stimulation in a Ca(2+)-dependent manner. Using Zn(2+) fluorophores, radiolabeled Zn(2+), and selective chelators, the location of this ion and its release pattern have been established across the brain. Given the distribution and possible release under physiological conditions, Zn(2+) has the potential to act as a modulator of both excitatory and inhibitory neurotransmission. Excitatory N-methyl-D-aspartate (NMDA) receptors are directly inhibited by Zn(2+), whereas non-NMDA receptors appear relatively unaffected. In contrast, inhibitory transmission mediated via GABA(A)receptors can be potentiated via a presynaptic mechanism, influencing transmitter release; however, although some tonic GABAergic inhibition may be suppressed by Zn(2+), most synaptic GABA receptors are unlikely to be modulated directly by this cation. In the spinal cord, glycinergic transmission may also be affected by Zn(2+) causing potentiation. Recently, the penetration of synaptically released Zn(2+) into neurons suggests that this ion has the potential to act as a direct transmitter, by affecting postsynaptic signaling pathways. Taken overall, present studies are broadly supportive of a neuromodulatory role for Zn(2+) at specific excitatory and inhibitory synapses.     

Long-term Potentiation in the Striatum is Unmasked by Removing the Voltage-dependent Magnesium Block of NMDA Receptor Channels

We have studied the effects of tetanic stimulation of the corticostriatal pathway on the amplitude of striatal excitatory synaptic potentials. Recordings were obtained from a corticostriatal slice preparation by utilizing both extracellular and intracellular techniques. Under the control condition (1.2 mM external Mg2+), excitatory postsynaptic potentials (EPSPs) evoked by cortical stimulation were reversibly blocked by 10 microM 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX), an antagonist of dl-alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) ionotropic glutamate receptors, while they were not affected by 30 - 50 microM 2-amino-5-phosphonovalerate (APV), an antagonist of N-methyl-d-aspartate (NMDA) glutamate receptors. In the presence of 1.2 mM external Mg2+, tetanic activation of cortical inputs produced long-term depression (LTD) of both extracellularly and intracellularly recorded synaptic potentials. When Mg2+ was removed from the external medium, EPSP amplitude and duration increased. In Mg2+-free medium, cortically evoked EPSPs revealed an APV-sensitive component; in this condition tetanic stimulation produced long-term potentiation (LTP) of synaptic transmission. Incubation of the slices in 30 - 50 microM APV blocked striatal LTP, while it did not affect LTD. In Mg2+-free medium, incubation of the slices in 10 microM CNQX did not block the expression of striatal LTP. Intrinsic membrane properties (membrane potential, input resistance and firing pattern) of striatal neurons were altered neither by tetanic stimuli inducing LTD and LTP, nor by removal of Mg2+ from the external medium. These findings show that repetitive activation of cortical inputs can induce long-term changes of synaptic transmission in the striatum. Under control conditions NMDA receptor channels are inactivated by the voltage-dependent Mg2+ block and repetitive cortical stimulation induces LTD which does not require activation of NMDA channels. Removal of external Mg2+ deinactivates these channels and reveals a component of the EPSP which is potentiated by repetitive activation. Since the striatum has been involved in memory and in the storage of motor skills, LTD and LTP of synaptic transmission in this structure may provide the cellular substrate for motor learning and underlie the physiopathology of some movement disorders.     

Glutamate-dependent mechanisms in the induction of a calcium long-term potentiation-like phenomenon

The electric synaptic efficacy, in terms of extracellular electrical potentials, and the intracellular postsynaptic efficacy, in terms of inositol phosphate (IP) accumulation, were evaluated in rat hippocampal slices exposed for a brief period (10 min) to a high concentration of calcium (+2.7 mM). In addition, the effects of N-methyl- -asparate (NMDA) ionotropic and metabotropic glutamate receptor (mGluR) antagonists on the induction and the establishment or maintenance of enhanced synaptic efficacy of CA1 pyramidal neurons due to high-calcium exposure were also tested. Elevation of the calcium concentration from 1.3–4 mM in the medium bathing hippocampal slices produced a long-lasting (60 over 90 min) increase in the slope of the CA1 somatic excitatory postsynaptic potential and the amplitude of the population spike (PS). Slice perfusion with NMDA antagonists cyclazocine and cis-4-phosphonomethyl-2-piperidine-carboxylic acid (CGS 19755) or with mGluR antagonists -2-amino-3-phosphonopropionic acid (AP3) or alpha-methyl-4-carboxyphenyl-glycine (all 0.1 mM), during the 10-min period of exposure to high-calcium prevented the induction of such changes. By contrast, slice perfusion with the same concentration of CGS 19755 or -AP3 did not affect the already established long-lasting increase in amplitude of CA1 PS induced by high-calcium. Moreover, high-calcium failed to produce any significant modification of the basal IP accumulation or of the IP accumulation elicited by mGluR agonist 1S,3R-trans-amino cyclo-pentane-1,3-dicarboxylic acid (ACPD). In conclusion, the results confirm that high-calcium induces a long-lasting increase in synaptic efficacy in rat hippocampal slices. Both NMDA ionotropic and mGluR receptors are involved in the induction, but not in the maintenance, of this phenomenon. In line with these data no modifications of basal or ACPD-induced phosphoinositide hydrolysis have been found during the maintenance stage.