ABSTRACT
An in vitro model of ischemia was developed and characterized using the acute rat hippocampal slice preparation. Neuroprotective concentrations of several competitive and noncompetitive glutamate subtype-selective antagonists (CGS-19755, MK-801, YM90K and GYKI-52466) were initially determined in anoxia-enhanced agonist-induced excitotoxicity experiments. Concentrations which proved to be effective in these studies were subsequently tested for their effectiveness against an ischemic episode. Ischemia was defined as a 30-min exposure to aglycemic media ending in 5 min of concurrent anoxia, a protocol which was arrived at by empirically determining the effect of various hypoglycemic and anoxic insults on the ability of hippocampal slices to retain their electrophysiological viability. Exposure to such an ischemic episode resulted in a loss of viability by most slices, an effect which was strongly dependent on extracellular calcium. AMPA antagonists applied alone produced no neuroprotective effect in the present model of in vitro ischemia, while NMDA antagonists applied alone had a modest neuroprotective effect. In contrast, the coapplication of 10 microM MK-801 and 300 microM GYKI-52466, noncompetitive NMDA and AMPA receptor antagonists, respectively, resulted in almost complete neuroprotection. This protection was comparable to that obtained by withholding extracellular calcium, indicating that the toxic effects of glutamate receptor overstimulation can be accounted for solely by calcium influx. The effect of this combination treatment on the survival rate of hippocampal slices was synergistic, that is greater than the sum of the effects of the individual compounds. The results indicate that neuroprotection against acute ischemic insults may require a combination therapy approach.
Subject(s)
Benzodiazepines , Brain Ischemia/drug therapy , Excitatory Amino Acid Antagonists/therapeutic use , Neuroprotective Agents/therapeutic use , Receptors, AMPA/antagonists & inhibitors , Receptors, N-Methyl-D-Aspartate/antagonists & inhibitors , Animals , Anti-Anxiety Agents/therapeutic use , Dizocilpine Maleate/therapeutic use , Hypoxia/drug therapy , Male , Pipecolic Acids/therapeutic use , Quinoxalines/therapeutic use , Rats , Rats, WistarSubject(s)
Colchicine/pharmacology , Motor Neurons/drug effects , Muscles/drug effects , Neuromuscular Junction/drug effects , Vinblastine/pharmacology , Acetylcholine/pharmacology , Action Potentials/drug effects , Animals , In Vitro Techniques , Membrane Potentials/drug effects , Membranes/drug effects , Microscopy, Electron , Neuromuscular Junction/anatomy & histology , Rats , Sciatic Nerve/drug effects , Tetrodotoxin/pharmacologyABSTRACT
Tetrodotoxin can exist as a zwitterion or two different cations in aqueous solutions. The cation forms are more potent in nerve block. Thus the lactone-hemilactal configuration of tetrodotoxin influences potency. Electrostatic attraction, hydrogen bonding, and intermolecular hemilactal formation are possible mechanisms of interaction with membrane receptors.
