Remodeling of Hippocampal Synaptic Networks by a Brief Anoxia–Hypoglycemia

Abstract

Cerebral ischemia is a major cause of brain dysfunction. Using a model of delayed death induced by a brief, transient oxygen and glucose deprivation, we studied here how this affected the structural organization of hippocampal synaptic networks. We report that brief anoxic-hypoglycemic episodes rapidly modified the structure of synapses. This was characterized, at the electron microscopic level, by a transient increase in the proportion of perforated synapses, followed after 2 hr by an increase in images of multiple synapse boutons. These changes were considerable because 10-20% of all synapses were affected. This structural remodeling was correlated by three kinds of modifications observed using two-photon confocal microscopy: the growth of filopodia, occurring shortly (5-20 min) after anoxia-hypoglycemia, enlargements of existing spines, and formation of new spines, both seen mainly 20-60 min after the insult. All of these structural changes were calcium and NMDA receptor dependent and thus reproduced, to a larger scale, those associated with synaptic plasticity. Concomitantly and related to the severity of anoxia-hypoglycemia, we could also observe spine loss and images of spine, dendrite, or presynaptic terminal swellings that evolved up to membrane disruption. These changes were also calcium dependent and reduced by NMDA receptor antagonists. Thus, short anoxic-hypoglycemic episodes, through NMDA receptor activation and calcium influx, resulted in a profound structural remodeling of synaptic networks, through growth, formation, and elimination of spines and synapses.

Excitatory synaptic activity is associated with a rapid structural plasticity of inhibitory synapses on hippocampal CA1 pyramidal cells

“…Short episodes of oxygene glucose deprivation were produced by switching to a medium containing 10 mM sucrose instead of glucose and exposing the slice cultures to an atmosphere composed of 95% N 2 , 5% CO 2 for 10 min, followed by recovery under the standard conditions in the incubator for 30 or 60 min before fixation for EM. Ten minutes exposure to OGD in the interface configuration induces robust and long-lasting LTP (Jourdain et al, 2002), while shorter OGD episodes in this configuration were unable to induce sufficient potentiation (unpublished observations).…”

“…Brief oxygeneglucose deprivation episode was applied to induce the anoxic LTP (Hammond et al, 1994;Hsu and Huang, 1997;Jourdain et al, 2002). The organotypic cultures grown on confetti cut from porous Millipore membranes (Millipore) were placed in an interface chamber perfused with an artificial cerebro-spinal medium containing (in mM): 124 NaCl, 1.6 KCl, 2.5 CaCl 2 , 1.5 MgCl 2 , 24 NaHCO 3 , 1.2 KH 2 PO 4 , 2 ascorbic acid, and 10 glucose (pH 7.4; 32 C), bubbled with a gas mixture containing 95% O 2 and 5% CO 2 (Jourdain et al, 2002).…”

“…The organotypic cultures grown on confetti cut from porous Millipore membranes (Millipore) were placed in an interface chamber perfused with an artificial cerebro-spinal medium containing (in mM): 124 NaCl, 1.6 KCl, 2.5 CaCl 2 , 1.5 MgCl 2 , 24 NaHCO 3 , 1.2 KH 2 PO 4 , 2 ascorbic acid, and 10 glucose (pH 7.4; 32 C), bubbled with a gas mixture containing 95% O 2 and 5% CO 2 (Jourdain et al, 2002). Short episodes of oxygene glucose deprivation were produced by switching to a medium containing 10 mM sucrose instead of glucose and exposing the slice cultures to an atmosphere composed of 95% N 2 , 5% CO 2 for 10 min, followed by recovery under the standard conditions in the incubator for 30 or 60 min before fixation for EM.…”

“…Previous work has shown that application of a brief OGD to hippocampal slices results in a lasting increase of excitatory synaptic strength, mimicking theta burst-induced LTP and without effects on immediate cell survival (Crepel et al, 1993;Hammond et al, 1994;Hsu and Huang, 1997;Jourdain et al, 2002). This lasting synaptic potentiation was therefore called anoxic LTP.…”

“…Changes in synaptic strength, but also remodeling of excitatory synapses, including enlargement of dendritic spine heads, modifications of the postsynaptic density or growth of new protrusions, have been largely described as a result of application of high frequency stimulation patterns or other activity paradigms such as short anoxic/hypoglycemic episodes or seizure induction (Buchs and Muller, 1996;Jourdain et al, 2002;Bourne and Harris, 2008). Far less is known about modifications of inhibitory synapses under similar conditions of activity, although inhibitory inputs play a crucial role in shaping the activity of neuronal networks (Cobb et al, 1995;Freund and Buzsaki, 1996;reviewed in Freund, 2003;Klausberger and Somogyi, 2008).…”