Neuroprotective Use-Dependent Blockers of Na+and Ca2+Channels Controlling Presynaptic Release of Glutamatea

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Abstract
We have originated a family of N,N'-disubstituted guanidines that block the voltage-activated Ca2+ and Na+ channels governing glutamate release. These compounds, CNS 1237 (N-acenaphthyl-N'-methoxynaphthyl guanidine) and its analogues, are "use dependent" in their ability to attenaute neurotransmitter release: they block glutamate release with greater efficacy under conditions of persistent or repetitive depolarization, as would be encountered under pathophysiological circumstances, relative to their ability to block glutamate release elicited by brief, transient depolarizations more characteristic of normal physiological release events in nonischemic brain. Using electrophysiological and rapid kinetic methods, we have differentiated the use-dependent block of the relevant Na+ and Ca2+ channels governing neurotransmitter release from the mechanism of channel antagonism exhibited by, respectively, the substituted guanidine Na+ channel blocker tetrodotoxin (TTX) and venom peptide Ca2+ antagonists. To characterize use-dependent Na+ channel block by CNS 1237, we have employed whole-cell voltage-clamp recordings from a Chinese hamster ovary (CHO) cell line expressing cloned mammalian type II Na+ channels. These experiments demonstrated that, in contrast to the actions of TTX under the same conditions, the potency of Na+ channel block by CNS 1237 is greatly enhanced by depolarizing stimuli in a frequency-dependent manner. Ca2+ channel-activated glutamate release from brain nerve terminal preparations was measured with approximately 300 msec time resolution over a 5-second period of high K(+)-depolarization, using a rapid superfusion technique. CNS 1237 and analogues, at 1-3 microM, accelerated the decay of glutamate release by 40-70%, reflecting depolarization-induced enhancement of block. In contrast, blockade of glutamate release by the Ca2+ channel antagonist peptide toxins omega-aga IV-A (from spider venom) and omega-conotoxin M-VII-C (from cone snail venom) exhibited "reverse-use-dependence:" at concentrations of 0.3 microM, which blocked the initial amplitude of glutamate release by 40-60%, the decay time constant for glutamate release was significantly increased, indicating depolarization-induced relief of block. These findings establish that CNS 1237 and other members of this compound series are use-dependent blockers of the voltage-activated ion channels governing glutamate release. Studies of CNS 1237 in the rat middle cerebral artery occlusion (MCAO) focal stroke model have indicated infarct size reduction comparable to that observed by the same investigators for the glutamate release blocker (BW 619C89 (Burroughs-Wellcome, now in clinical development). Maximal infarct size reduction is achieved with a 3-mg/kg bolus followed by a 4-hour infusion of 0.75 mg/kg/hr.(ABSTRACT TRUNCATED AT 400 WORDS)