Zn2+ blocks the NMDA- and Ca2+ -triggered postexposure current ipe in hippocampal pyramidal cells

Aberrant NMDA-dependent LTD after perinatal ethanol exposure in young adult rat hippocampus

Irreversible cognitive deficits induced by ethanol exposure during fetal life have been ascribed to a lower NMDA-dependent synaptic long-term potentiation (LTP) in the hippocampus. Whether NMDA-dependent long-term depression (LTD) may also play a critical role in those deficits remains unknown. Here, we show that in vitro LTD induced with paired-pulse low frequency stimulation is enhanced in CA1 hippocampus field of young adult rats exposed to ethanol during brain development. Furthermore, single pulse low frequency stimulation, ineffective at this age (LFS600), induced LTD after ethanol exposure accompanied with a stronger response than controls during LFS600, thus revealing an aberrant form of activity-dependent plasticity at this age. Blocking NMDA receptor or GluN2B containing NMDA receptor prevented both the stronger response during LFS600 and LTD whereas Zinc, an antagonist of GluN2A containing NMDA receptor, was ineffective on both responses. In addition, LFS600-induced LTD was revealed in controls only with a reduced-Mg(2+) medium. In whole dissected hippocampus CA1 field, perinatal ethanol exposure increased GluN2B subunit expression in the synaptic compartment whereas GluN2A was unaltered. Using pharmacological tools, we suggest that LFS600 LTD was of synaptic origin. Altogether, we describe a new mechanism by which ethanol exposure during fetal life induces a long-term alteration of synaptic plasticity involving NMDA receptors, leading to an aberrant LTD. We suggest this effect of ethanol may reflect a delayed maturation of the synapse and that aberrant LTD may also participates to long-lasting cognitive deficits in fetal alcohol spectrum disorder.

Persistent ictal-like activity in rat entorhinal/perirhinal cortex following washout of 4-aminopyridine

Application of 4-aminopyridine (4-AP, 100μM) in a solution containing 0.6mM Mg(2+) and 1.2mM Ca(2+) to hippocampal-entorhinal-perirhinal slices of adult rat brain induced ictal-like epileptiform activity in entorhinal and perirhinal cortices as revealed by electrophysiological field potential recordings. The ictal-like activity persisted after washing out the 4-AP. This persistence indicated that a change had occurred in the slice so that it was now "epileptic" in the absence of the convulsant 4-AP. Induction of persistent ictal-like activity was dependent upon the concentration of divalent cations during 4-AP exposure; that is, although 4-AP caused ictal-like activity in approximately half the slices in solution containing 1.6mM Mg(2+) and 2.0mM Ca(2+), this ictal-like activity did not persist upon washout of the 4-AP. Expression of the persistent ictal-like epileptiform activity required ionotropic glutamate-mediated synaptic transmission: application of the AMPA/kainate receptor antagonist NBQX after 4-AP washout reduced persistent ictal-like activity, and the combined application of NBQX and the NMDA receptor antagonist d-AP5 completely blocked it. In order to investigate the mechanism of induction of persistent ictal-like activity, several agents were applied before the introduction of 4-AP. Application of d-AP5 did not block the onset of ictal-like activity upon introduction of 4-AP but did prevent the persistence of the ictal-like activity upon washout of the 4-AP. In contrast, induction of persistent ictal-like activity was not prevented by simultaneous application of the group I metabotropic glutamate receptor (mGluR) antagonists LY 367385 and MPEP or by application of the protein synthesis inhibitor cycloheximide. In conclusion, we have characterized a new in vitro model of epileptogenesis in which induction of ictal-like activity is dependent upon NMDA receptor activation but not upon group I mGluR activation or protein synthesis.

Axonal transport of zinc transporter 3 and zinc containing organelles in the rodent adrenergic system

Zinc is the second most abundant trace metal (after iron) in mammalian tissues, and it is an essential element for growth, development, DNA synthesis, immunity, and other important cellular processes. A considerable amount of zinc in the brain exists as a pool of free or loosely bound zinc ions in synaptic vesicles with zinc transporter 3 (ZnT3) in their membranes. Here we demonstrate that also in the peripheral sympathetic nervous system zinc handling neurons exist. In autonomic ganglia of rats and mice a subset of neuronal cell bodies contain zinc, visualized by the autometallographic (AMG) and TSQ histochemical methods. The Zn-transporter 3 is, as shown by immunofluorescence, also present in tyrosine hydroxylase (TH)-positive neurons, but rarely in cell bodies with neuropeptide Y (NPY)-immunoreactivity (IR). In axons of crush-operated sciatic nerves a rapid bidirectional accumulation of AMG granules occurred. Also ZnT3-IR was found to accumulate rapidly in anterograde as well as retrograde direction, colocalized with TH-IR. So far nerve terminals with ZnT3-IR have not been observed. The functional significance of zinc ions in the sympathetic system is not known.

Electrophysiological mechanisms of delayed excitotoxicity: positive feedback loop between NMDA receptor current and depolarization-mediated glutamate release

Delayed excitotoxic neuronal death after insult from exposure to high glutamate concentrations appears important in several CNS disorders. Although delayed excitotoxicity is known to depend on NMDA receptor (NMDAR) activity and Ca(2+) elevation, the electrophysiological mechanisms underlying postinsult persistence of NMDAR activation are not well understood. Membrane depolarization and nonspecific cationic current in the postinsult period were reported previously, but were not sensitive to NMDAR antagonists. Here, we analyzed mechanisms of the postinsult period using parallel current- and voltage-clamp recording and Ca(2+) imaging in primary hippocampal cultured neurons. We also compared more vulnerable older neurons [about 22 days in vitro (DIV)] to more resistant younger (about 15 DIV) neurons, to identify processes selectively associated with cell death in older neurons. During exposure to a modest glutamate insult (20 microM, 5 min), similar degrees of Ca(2+) elevation, membrane depolarization, action potential block, and increased inward current occurred in younger and older neurons. However, after glutamate withdrawal, these processes recovered rapidly in younger but not in older neurons. The latter also exhibited a concurrent postinsult increase in spontaneous miniature excitatory postsynaptic currents, reflecting glutamate release. Importantly, postinsult NMDAR antagonist administration reversed all of these persisting responses in older cells. Conversely, repolarization of the membrane by voltage clamp immediately after glutamate exposure reversed the NMDAR-dependent Ca(2+) elevation. Together, these data suggest that, in vulnerable neurons, excitotoxic insult induces a sustained positive feedback loop between NMDAR-dependent current and depolarization-mediated glutamate release, which persists after withdrawal of exogenous glutamate and drives Ca(2+) elevation and delayed excitotoxicity.

Zinc chelation during non-lesioning overexcitation results in neuronal death in the mouse hippocampus

In the hippocampus, chelatable zinc is accumulated in vesicles of glutamatergic presynaptic terminals, abounding specially in the mossy fibers, from where it is released with activity and can exert a powerful inhibitory action upon N-methyl-D-aspartate receptors. Zinc is therefore in a strategic situation to control overexcitation at the zinc-rich excitatory synapses, and consequently zinc removal during high activity might result in excitotoxic neuronal damage. We analyzed the effect of zinc chelation with sodium dietyldithiocarbamate under overexcitation conditions induced by non-lesioning doses of kainic acid in the mouse hippocampus, to get insight into the role of zinc under overexcitation. Swiss male mice were injected with kainic acid (15 mg/kg, i.p.) 15 min prior to sodium dietyldithiocarbamate (150 mg/kg, i.p.), and left to survive for 6 h, 1 day, 4 days, or 7 days after the treatment. Cell damage was analyzed with the hematoxylin-eosin and acid fuchsin stainings. Neither control animals treated only with kainic acid nor those treated only with sodium dietyldithiocarbamate suffered seizures or neuronal damage. By contrast, the kainic acid+sodium dietyldithiocarbamate-treated animals showed convulsive behavior and cell death involving the hilus, CA3, and CA1 regions. Pretreatment with the N-methyl-D-aspartate receptor antagonist MK801 (1 mg/kg, i.p.) completely prevented neuronal damage. Experiments combining different doses of sodium dietyldithiocarbamate and kainic acid with different administration schedules demonstrated that the overlap of zinc chelation and overexcitation is necessary to trigger the observed effects. Moreover, the treatment with a high dose of sodium dietyldithiocarbamate (1000 mg/kg), which produced a complete bleaching of the Timm staining for approximately 12 h, highly increased the sensitivity of animals to kainic acid. Altogether, our results indicate that the actions of sodium dietyldithiocarbamate are based on a reduction of zinc levels, which under overexcitation conditions induce seizures and neuronal damage. These findings fully support a protective role for synaptically released zinc during high neuronal activity, most probably mediated by its inhibitory actions on N-methyl-D-aspartate receptors, and argue against a direct action of synaptic zinc on the observed neuronal damage.

Multiple pathways of neuroprotection against oxidative stress and excitotoxic injury in immature primary hippocampal neurons

In the immature brain hydrogen peroxide accumulates after excitotoxic hypoxia-ischemia and is neurotoxic. Immature hippocampal neurons were exposed to N-methyl-D-aspartate (NMDA), a glutamate agonist, and hydrogen peroxide (H(2)O(2)) and the effects of free radical scavenging and transition metal chelation on neurotoxicity were studied. alpha-Phenyl-N-tert.-butylnitrone (PBN), a known superoxide scavenger, attenuated both H(2)O(2) and NMDA mediated toxicity. Treatment with desferrioxamine (DFX), an iron chelator, at the time of exposure to H(2)O(2) was ineffective, but pretreatment was protective. DFX also protected against NMDA toxicity. TPEN, a metal chelator with higher affinities for a broad spectrum of transition metal ions, also protected against H(2)O(2) toxicity but was ineffective against NMDA induced toxicity. These data suggest that during exposure to free radical and glutamate agonists, the presence of iron and other free metal ions contribute to neuronal cell death. In the immature nervous system this neuronal injury can be attenuated by free radical scavengers and metal chelators.

Cumulative effects of glutamate microstimulation on Ca(2+) responses of CA1 hippocampal pyramidal neurons in slice

Glutamate stimulation of hippocampal CA1 neurons in slice was delivered via iontophoresis from a microelectrode. Five pulses (approximately 5 muA, 10 s duration, repeated at 1 min intervals) were applied with the electrode tip positioned in the stratum radiatum near the dendrites of a neuron filled with the Ca(2+) indicator fura-2. A single stimulus set produced Ca(2+) elevations that ranged from several hundred nM to several microM and that, in all but a few neurons, recovered within 1 min of stimulus termination. Subsequent identical stimulation produced Ca(2+) elevations that outlasted the local glutamate elevations by several minutes as judged by response recoveries in neighboring cells or in other parts of the same neuron. These long responses ultimately recovered but persisted for up to 10 min and were most prominent in the mid and distal dendrites. Recovery was not observed for responses that spread to the soma. The elevated Ca(2+) levels were accompanied by membrane depolarization but did not appear to depend on the depolarization. High-resolution images demonstrated responsive areas that involved only a few mu(m) of dendrite. Our results confirm the previous general findings from isolated and cell culture neurons that glutamate stimulation, if carried beyond a certain range, results in long-lasting Ca(2+) elevation. The response characterized here in mature in situ neurons was significantly different in terms of time course and reversibility. We suggest that the extended Ca(2+) elevations might serve not only as a trigger for delayed neuron death but, where more spatially restricted, as a signal for local remodeling in dendrites.