Potassium-evoked Glutamate Release Liberates Arachidonic Acid from Cortical Neurons

Abstract

Ischemic cell death in the brain accompanies any condition characterized by a significant interruption in cerebral blood flow, including stroke and head trauma. Although many of the cellular and molecular mechanisms that lead to ischemic cell death have been described, efficacious methods of intervention remain elusive. Brain levels of free fatty acids, specifically arachidonic acid (AA), 1 can be released rapidly during cerebral ischemia and can reach up to 10-fold higher concentrations than levels estimated during normal physiological conditions (1, 2). Thus, it is perhaps not surprising that pharmacological inhibition of AA metabolism has proven to be an effective strategy for ameliorating cerebral ischemic damage in animal models (3-7), perhaps through inhibition of deleterious metabolite (8 -11) and/or oxygen-derived free radical formation (8,(12)(13)(14)(15). However, AA itself can have effects independent from its metabolites, some of which can have deleterious consequences for a cell. For instance, AA may directly decrease membrane integrity by acting as a detergent, thereby altering membrane fluidity (16). Further, AA directly enhances NMDA receptor currents (17) and may enhance the concentration of glutamate in the synaptic cleft by increasing its release (18,19) and/or blocking its reuptake (20 -22). These effects could potentiate excitotoxicity, a major contributor to ischemic cell death (23). In addition, unesterified AA has been reported to inhibit mitochondrial respiration (24, 25) and may directly signal apoptosis through the concomitant activation of caspase 3 (26) and down-regulation of bcl-2 (27). Given this, inhibition of AA release from cellular membranes may be a more effective neuroprotective strategy than inhibition of its metabolism alone. Thus, it is of great import to understand the mechanism by which AA is released following cerebral ischemia.Since a large rise in extracellular [K ϩ ] (up to 60 mM) has been measured during cerebral ischemia (16, 28), we questioned whether this might be a trigger for AA release. Studies addressing this question are discrepant. Rat cortical synaptosomes (i.e. isolated nerve terminals) or isolated cerebellar glomeruli release AA when bathed in a depolarizing K ϩ buffer (29 -31), but primary cultures of striatal or hippocampal neurons do not (32-34). The ability of elevated [K ϩ ] o to stimulate AA release from intact cortical cells, which are highly susceptible to cerebral ischemic damage, has not been explored. Thus, the aim of the present study was to determine whether a rise in extracellular [K ϩ ] was a sufficient stimulus for AA release in a murine mixed cortical cell culture system, and if so, by what mechanism. Part of this work has appeared in abstract form (35). EXPERIMENTAL PROCEDURESMaterials-MK-801 (dizocilpine maleate) and CNQX (6-cyano-7-nitroquinoxaline-2,3-dione) were purchased from Research Biochemicals Inc. (Natick, MA). DL-threo-␤-benzyloxyaspartate was purchased from Tocris (Ellisville, MO). CdCl 2 , iodoacetamide, and triethanolami...

Spontaneous Glutamatergic Synaptic Activity Regulates Constitutive COX-2 Expression in Neurons OPPOSING ROLES FOR THE TRANSCRIPTION FACTORS CREB (cAMP RESPONSE ELEMENT BINDING) PROTEIN AND Sp1 (STIMULATORY PROTEIN-1)

“…4 and 5). The failure of tetanus toxin, a Clostridium toxin that prevents evoked, but not spontaneous, synaptic vesicle exocytosis in our and other systems (23)(24)(25)(26), to reduce both constitutive neuronal activity and basal COX-2 mRNA expression in our cultures (Fig. 6) led us to conclude that spontaneous synaptic activity regulates basal neuronal COX-2 expression.…”

Synaptic lipid signaling

“…Depolarizing concentrations of K ϩ promote active release of AA in cortical neurons (15), retina (16), and synaptosomes (17). One interpretation of these results is that high K ϩ promotes a depolarization-dependent Ca 2 ϩ influx through voltage-gated Ca 2 ϩ channels.…”

“…One interpretation of these results is that high K ϩ promotes a depolarization-dependent Ca 2 ϩ influx through voltage-gated Ca 2 ϩ channels. This in turn activates presynaptic glutamate release that modulates excitatory glutamate receptors (15). Thus Ca 2 ϩ influx through the N -methyl-d -aspartate (NMDA) receptor, and direct Ca 2 ϩ entrance through voltage-dependent calcium channels, activate phospholipase A 2 (PLA 2 ) to result in AA release via phospholipid hydrolysis.…”

Relationships between superoxide levels and delayed calcium deregulation in cultured cerebellar granule cells exposed continuously to glutamate

“…Phospholipase A2 inhibitors prevent the increase in superoxide following DCD Ionomycin, metabolic inhibition or glutamate have each been reported to activate phospholipase A2 in CGNs (Lazarewicz et al 1990;Gunasekar et al 1995;Chen et al 1999) and cortical neurons (Tapia-Arancibia et al 1992;Stella et al 1995;Taylor and Hewett 2002) while PLA2 inhibitors decrease cell death measured by lactic dehydrogenase release (Ciani et al 1996). In addition Lafon-Cazal et al (1993) reported that superoxide production from glutamate-exposed CGNs was due in part to arachidonic acid release.…”