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...