Comparison of effects induced by toxic applications of kainate and glutamate by glucose deprivation on area CA1 of rat hippocampal slices

Blockers of adenosine A1, but not muscarinic acetylcholine, receptors improve excessive extracellular glutamate-induced synaptic depression

We investigated adenosinergic and cholinergic effects on excessive glutamate-induced depressions of central excitatory synaptic transmissions in vitro. From the CA1 region in rat hippocampal slices, orthodromically elicited population spikes (PSs) and field excitatory postsynaptic potentials (fEPSPs) at 0.1Hz were simultaneously recorded. ANOVA was used for statistics, and p<0.05 was accepted as significant. Glutamate (10mM for 10min) completely depressed PSs and fEPSPs, which were partially recovered by the following washout for 40min (67.5±15.7% and 65.4±13.9% of the control, respectively, p<0.01, n=12). The recoveries in PSs and fEPSPs were exacerbated by edrophonium and carbamoylcholine but improved by non- and A1-selective adenosine receptor antagonists (p<0.01, n=6). The recovery in PSs, not that in fEPSPs, was exacerbated by adenosine, adenosine A1-receptor agonist and A2a-receptor antagonist (p<0.01, n=6). The effects of edrophonium were blocked by non-, M2- and M4-selective muscarinic acetylcholine receptor antagonists (p<0.01, n=6). Excessive glutamate depresses glutamatergic excitatory synaptic transmissions, which are exacerbated by muscarinic acetylcholine receptor stimulation but improved by adenosine A1 receptor block. Somatic excitability is impaired by excessive glutamate with adenosine A1 receptor stimulation.

Glutamate-induced depression of EPSP-spike coupling in rat hippocampal CA1 neurons and modulation by adenosine receptors

The presence of high concentrations of glutamate in the extracellular fluid following brain trauma or ischaemia may contribute substantially to subsequent impairments of neuronal function. In this study, glutamate was applied to hippocampal slices for several minutes, producing over-depolarization, which was reflected in an initial loss of evoked population potential size in the CA1 region. Orthodromic population spikes recovered only partially over the following 60 min, whereas antidromic spikes and excitatory postsynaptic potentials (EPSPs) showed greater recovery, implying a change in EPSP-spike coupling (E-S coupling), which was confirmed by intracellular recording from CA1 pyramidal cells. The recovery of EPSPs was enhanced further by dizocilpine, suggesting that the long-lasting glutamate-induced change in E-S coupling involves NMDA receptors. This was supported by experiments showing that when isolated NMDA-receptor-mediated EPSPs were studied in isolation, there was only partial recovery following glutamate, unlike the composite EPSPs. The recovery of orthodromic population spikes and NMDA-receptor-mediated EPSPs following glutamate was enhanced by the adenosine A1 receptor blocker DPCPX, the A2A receptor antagonist SCH58261 or adenosine deaminase, associated with a loss of restoration to normal of the glutamate-induced E-S depression. The results indicate that the long-lasting depression of neuronal excitability following recovery from glutamate is associated with a depression of E-S coupling. This effect is partly dependent on activation of NMDA receptors, which modify adenosine release or the sensitivity of adenosine receptors. The results may have implications for the use of A1 and A2A receptor ligands as cognitive enhancers or neuroprotectants.

Adenosine preconditions against ouabain but not against glutamate on CA1-evoked potentials in rat hippocampal slices

Hypoxic and ischaemic brain damage are believed to involve excessive release of glutamate, and recent work shows that glutamate-induced damage in brain slices can be reduced by preconditioning with hypoxia or glutamate itself. Because adenosine is a powerful preconditioning agent, we have investigated whether adenosine could precondition against glutamate in vitro. In rat hippocampal slices, glutamate depolarization reduced the amplitudes of antidromic- and orthodromic-evoked potentials, with only partial recovery. Applying adenosine before these insults failed to increase that recovery. Ouabain also produced depolarization with partial reversibility, but adenosine pretreatment increased the extent of recovery. The preconditioning effect of adenosine on ouabain responses was prevented by blocking receptors for N-methyl-D-aspartate (NMDA), but not receptors for kainate or alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA), and was blocked by inhibiting nitric oxide synthase. Preconditioning was also abolished by the ATP-dependent potassium channel blockers, glibenclamide (cytoplasmic) or 5-hydroxydecanoate (mitochondrial). We conclude that adenosine does not precondition against glutamate in hippocampal slices, but that it does precondition against ouabain with a pharmacology similar to studies in vivo. Ischaemic neuronal damage is a complex of many factors, and because adenosine can precondition against ischaemic neuronal damage, its failure to protect against glutamate highlights limitations of using glutamate alone as a model for ischaemia. Because damage following ischaemia, trauma or excitotoxicity also involves reduced Na(+),K(+)-ATPase activity, and adenosine can precondition against ouabain, we propose that ouabain-induced damage represents an additional or alternative model for the contribution to cell damage of Na(+),K(+)-ATPase loss, this being more relevant to the mechanisms of preconditioning.

Hypoglycemic seizures during transient hypoglycemia exacerbate hippocampal dysfunction

Severe hypoglycemia constitutes a medical emergency, involving seizures, coma and death. We hypothesized that seizures, during limited substrate availability, aggravate hypoglycemia-induced brain damage. Using immature isolated, intact hippocampi and frontal neocortical blocks subjected to low glucose perfusion, we characterized hypoglycemic (neuroglycopenic) seizures in vitro during transient hypoglycemia and their effects on synaptic transmission and glycogen content. Hippocampal hypoglycemic seizures were always followed by an irreversible reduction (>60% loss) in synaptic transmission and were occasionally accompanied by spreading depression-like events. Hypoglycemic seizures occurred more frequently with decreasing "hypoglycemic" extracellular glucose concentrations. In contrast, no hypoglycemic seizures were generated in the neocortex during transient hypoglycemia, and the reduction of synaptic transmission was reversible (<60% loss). Hypoglycemic seizures in the hippocampus were abolished by NMDA and non-NMDA antagonists. The anticonvulsant, midazolam, but neither phenytoin nor valproate, also abolished hypoglycemic seizures. Non-glycolytic, oxidative substrates attenuated, but did not abolish, hypoglycemic seizure activity and were unable to support synaptic transmission, even in the presence of the adenosine (A1) antagonist, DPCPX. Complete prevention of hypoglycemic seizures always led to the maintenance of synaptic transmission. A quantitative glycogen assay demonstrated that hypoglycemic seizures, in vitro, during hypoglycemia deplete hippocampal glycogen. These data suggest that suppressing seizures during hypoglycemia may decrease subsequent neuronal damage and dysfunction.

NMDA-dependent, but not group I metabotropic glutamate receptor-dependent, long-term depression at Schaffer collateral-CA1 synapses is associated with long-term reduction of release from the rapidly recycling presynaptic vesicle pool

Postsynaptic alterations have been suggested to account for NMDA receptor (NMDAR)-dependent long-term depression (LTD) and long-term potentiation of synaptic strength, although there is substantial evidence supporting changes in presynaptic release. Direct chemical activation of either NMDA or group I metabotropic glutamate receptor (mGluR1) elicits LTD of similar magnitudes, but it is unknown whether they share common expression mechanisms. Using dual-photon laser-scanning microscopy of FM1-43 [N-(3-triethylammoniumpropyl)-4-(4-(dibutylamino)styryl)pyridinium dibromide] to directly visualize presynaptic vesicular release from the rapidly recycling vesicle pool (RRP) at Schaffer collateral terminals in field CA1 of rat hippocampal slices, we found that a persistent reduction in vesicular release from the RRP is induced by NMDA-LTD but not by mGluR1-LTD. Variance-mean analyses of Schaffer collateral release probability (P(r)) at varying extracellular calcium concentrations confirmed that NMDA-LTD was associated with reduced P(r), whereas mGluR1-LTD was not. Pharmacological isolation of NMDAR-dependent and mGluR-dependent forms of stimulus-evoked LTD revealed that both are composed of a combination of presynaptic and postsynaptic alterations. However, when group I mGluR-dependent LTD was isolated by combining an NMDAR blocker with a group II mGluR antagonist, this form of LTD was purely postsynaptic. The nitric oxide synthase inhibitor N omega-nitro-L-arginine blocked the induction of NMDA-LTD but did not alter mGluR-LTD, consistent with a selective role for nitric oxide as a retrograde messenger mediating NMDA-LTD. These data demonstrate that single synapses can express multiple forms of LTD with different sites of expression, that NMDA-LTD is a combination of presynaptic and postsynaptic alterations, but that group I mGluR-LTD appears to be expressed entirely postsynaptically.

Changes in Extracellular Action Potential Detect Kainic Acid and Trimethyltin Toxicity in Hippocampal Slice Preparations Earlier than do MAP2 Density Measurements

In vitro electrophysiological techniques for the assessment of neurotoxicity could have several advantages over other methods in current use, including the ability to detect damage at a very early stage, and could further assist in replacing animal experimentation in vivo. We investigated how an electrophysiological parameter, the extracellularly-recorded compound action potential (“population spike”, PS) could be used as a marker of in vitro neurotoxicity in the case of two well-known toxic compounds, kainic acid (KA) and trimethyltin (TMT). We compared the use of this electrophysiological endpoint with changes in immunoreactivity for microtubule-associated protein 2 (MAP2), a standard histological test for neurotoxicity. We found that both toxic compounds reliably caused disappearance of the PS, and that such disappearance occurred after only 1 hour of exposure to the drug. By contrast, densitometric measurements of MAP2 immunoreactivity were unaffected by both KA and TMT after such a short exposure time. We conclude that, in the case of KA and TMT, the extracellular PS was abolished at a very early time-point, when MAP2 immunoreactivity levels were still comparable to those of the untreated controls. Electrophysiology could be a reliable and early indicator of neurotoxicity, which could improve our ability to test for neurotoxicity in vitro, thus further replacing the need for in vivo experimentation.

Persistent block of CA1 synaptic function by prolonged hypoxia

The effects of prolonged hypoxia were studied by field and intracellular recordings from hippocampal slices of the rat, kept submerged at 34 degrees C. When artificial cerebrospinal fluid contained 10 mM glucose, even very long exposures to hypoxia or 300 microM cyanide (21-25 min) did not block field excitatory postsynaptic potentials and population spikes irreversibly. By contrast, in the presence of 4 mM glucose, hypoxia lasting only 9-13 min-ending 2-3 min after the characteristic transient recovery ("hypoxic injury potential")-resulted in irreversible block of synaptic responses. Voltage-dependent sodium channels and N-methyl-D-aspartate receptors are involved, because irreversible block was prevented by tetrodotoxin (0.5 microM), kynurenate (2 mM) or DL-aminophosphonovalerate (50 microM), whereas 6,7-dinitroquinoxaline-2,3-dione (25 microM) suppressed only the transient recovery. The hypoxic suppression of afferent volleys in slices kept in 4 mM glucose was also prevented by kynurenate or aminophosphonovalerate. Intracellular recordings revealed opposite effects of hypoxia according to glucose concentration: in 10 mM glucose, mainly hyperpolarization; in 4 mM glucose, after a brief hyperpolarization, a major and usually irreversible depolarization. In the presence of kynurenate or tetrodotoxin, major depolarizations also occurred, but they were reversible. Thus, large depolarizations of hippocampal neurons do not necessarily lead to irreversible block of synaptic transmission: there is lasting damage only when hypoxia is combined with low glucose, presumably because a reduced supply of glycolytically generated ATP limits the Na+/K+ pump's ability to maintain or restore membrane potentials and thus prevent excessive activation of N-methyl-D-aspartate receptors.

Effects of glucose deprivation in area CA1 of hippocampal slices from adult and juvenile rats

The effects of glucose deprivation were studied in area CA1 of rat hippocampal slices obtained from adult and juvenile rats (postnatal days (PN) 6-8; 13-15; 20-22). Ion-sensitive microelectrodes were employed to monitor baseline and stimulus-induced changes in [Ca2+]0, [K+]0 and field potentials. In slices from juvenile animals, the decline of baseline [Ca2+]0 during glucose deprivation was delayed in comparison to adult slices. The minimum in [Ca2+]0 was reached in slices from adult rats after 50 +/- 8.5 min, in slices from PN 20-22 after 69 +/- 9 min, and in slices from PN 13-15 after 111 +/- 11 min. In slices from PN 6-8, [Ca2+]0 did not decrease significantly even during prolonged exposure of up to 4 h. Alvear stimulation failed to evoke any stimulus-induced responses in field potentials, rises in [K+]0 and decreases in [Ca2+]0 after the minimum in [Ca2+]0 was reached in slices from all age groups except for slices from PN 6-8. In the older age groups, afferent fibre stimulation still induced afferent volleys and small decreases in [Ca2+]0, which were about 20-30% of those under control conditions, suggesting that presynaptic fibres and endings maintained some of their functional properties even after prolonged glucose deprivation. In contrast, stimulation of the stratum radiatum failed to evoke synaptic responses in slices from PN 6-8, presumably due to a failure in synaptic transmission. These findings confirm that similar to hypoxia during the early postnatal stage, hippocampal neurons are much more resistant to glucose deprivation. The findings also show that during early postnatal development, glucose deprivation may result in a block of synaptic transmission independent of postsynaptic excitability.