Intense proteolysis of cytoskeletal proteins occurs in brain within minutes of transient ischemia, possibly because of the activation of calcium-sensitive proteases (calpains). This proteolytic event precedes overt signs of neuronal degeneration, is most pronounced in regions of selective neuronal vulnerability, and could have significant consequences for the integrity of cellular function. The present studies demonstrate that (i) the early phase of enhanced proteolysis is a direct response to hypoxia rather than other actions of ischemia, (ii) it is possible to pharmacologically inhibit the in vivo proteolytic response to ischemia, (iii) inhibition of proteolysis is associated with a marked reduction in the extent of neuronal death, and (iv) protected neurons exhibit normal-appearing electrophysiological responses and retain their capacity for expressing long-term potentiation, a form of physiological plasticity thought to be involved in memory function. These observations indicate that calcium-activated proteolysis is an important component of the post-ischemic neurodegenerative response and that targeting this response may be a viable therapeutic strategy for preserving both the structure and function of vulnerable neurons.Elevated levels of intracellular calcium during and/or following transient ischemia are widely believed to trigger cellular events that lead to neuronal death (e.g., ref. 1). During ischemia, calcium enters vulnerable neurons through voltage-sensitive and receptor-operated channels and is released from intracellular stores. Consequently, treatments that reduce the entry ofcalcium into vulnerable neurons have achieved some success in protecting neurons (1, 2). However, conflicting results have been reported regarding the effectiveness of calcium antagonists (2), perhaps due to the many routes through which calcium can reach cytoplasmic pools. An alternative strategy for neuroprotection is to identify and target cellular events that are triggered by calcium and likely to be involved in neurodegeneration. An important biochemical mechanism satisfying both ofthese criteria is the activation of calcium-sensitive proteases (calpains). Several prominent cytoskeletal proteins are preferred substrates for calpain [e.g., spectrin, microtubule-associated protein MAP2, and neurofilament proteins (3-5)], and increased proteolysis of spectrin is associated with toxin-induced (6, 7) and lesion-induced neuropathologies (8). Moreover, a marked accumulation of spectrin breakdown products (BDPs) caused by calpain is one of the earliest biochemical changes occurring in vulnerable neurons after transient ischemia (9). Substantial proteolysis of any or all of the substrate proteins (9, 10) for calpain would presumably have severe consequences for the integrity of neuronal structure and function. Calpain is, therefore, in a position to provide a link between transient ischemia and cell death inasmuch as (i) it is associated with a variety of neurodegenerative responses, (ii) it is activated by an appro...

The effects of kainate (KA)-induced epileptic seizures on the binding properties of hippocampal glutamate receptors, on the modulation of DL-a-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)/quisqualate receptor by phospholipase A2 (PLA2), and on the formation of long-term potentiation (LTP) were studied in hippocampal membranes and hippocampal slices. Systemic administration of KA (10 mg/kg; 15 hr survival) produced specific changes in the binding properties of the Although substantial progress has been made in describing the initial events required for the induction of long-term potentiation (LTP), the steps involved in the translation ofthe initial trigger into long-lasting changes in synaptic efficacy continue to be a subject of intense controversy (1). In field CA1 of hippocampus, activation of the N-methyl-D-aspartate (NMDA) receptor (2, 3) and an increase in intracellular calcium in postsynaptic structures (4,5) are critical in producing the selective increase in the synaptic currents mediated by the DL-a-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)/quisqualate subtype of glutamate receptors that maintain LTP (6-8). Thus, it is reasonable to propose that expression of LTP is due to the activation ofone or several calcium-dependent processes that modify the properties of the AMPA/quisqualate receptors. Previous studies have shown that epileptiform activity in the limbic system disrupts the mechanisms that induce LTP (17-19). We reasoned that such a situation might provide a unique opportunity to establish the roles of different calciumdependent processes in LTP induction and especially that of calcium-dependent phospholipases. We report here that epileptic seizures produced by systemic administration of kainic acid (KA) prevent the induction of LTP in field CA1 of hippocampal slices. Moreover, the increase in [3H]AMPA binding elicited by PLA2 treatment of hippocampal membranes is lost after KA administration. These results suggest that PLA2-induced modification of AMPA/quisqualate receptors is a necessary step in the development of LTP. MATERIALS AND METHODS 1893The publication costs of this article were defrayed in part by page charge payment. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. §1734 solely to indicate this fact.