EFFECTS OF DEXMEDETOMIDINE ON HYPOXIA-EVOKED GLUTAMATE RELEASE AND GLUTAMATE-RECEPTOR ACTIVITY IN HIPPOCAMPAL SLICES

Background: The selective alpha(2) agonist dexmedetomidine may improve neurologic outcome after incomplete ischemia in animals when it is ad ministered either before or after the start of the ischemic insult. To clarify further the mechanisms by which alpha(2) agonists may provide neuroprotective effects, the authors tested the hypotheses that dexme detomidine decreases synaptic and extrasynaptic glutamate release stim ulated by potassium chloride or hypoxia, and decreases postsynaptic gl utamate receptor activity during aerobic or hypoxic conditions. Method s: Glutamate released from brain slices (300-mu m thick) from rat hipp ocampus was measured in a cuvette during two experimental stresses: (1 ) potassium chloride-evoked depolarization (30 mM) with and without 10 nM, 100 nM, or 1,000 nM dexmedetomidine; and (2) hypoxia (95% N-2 - 5 % CO2) with and without 100 mi dexmedetomidine. Glutamate release was quantified by fluorescence assay using 1 mM nicotinamide adenine dinuc leotide, and 5 international units per ml glutamate dehydrogenase. The formation of nicotinamide dinucleotide reduced from nicotinamide aden ine dinucleotide by glutamate dehydrogenase was measured fluorometrica lly (excitation light 340 nn, emission intensity 460 nm) in the soluti on above the slice. Glutamate receptor activity was determined by the change in cytosolic calcium concentration in CA1 neurons in the presen ce and absence of 100 nM dexmedetomidine during administration of N-me thyl-D-aspartate (100 mu M) and during simulated ischemic penumbra con ditions (P-O2 = 20 mmHg, glutamate 3 nM). Calcium concentration was me asured using a microscope fluorometer in fura 2-loaded rat hippocampal brain slices. Results: Dexmedetomidine attenuated potassium chloride- evoked glutamate release by 37%, 51%, (P = 0.03) and 27%, respectively , for the 10 nM, 100 nM, and 1,000 nM concentrations, and decreased (a t 100 nM) the increase in glutamate release in response to hypoxia by 61% (P < 0.0001). Dexmedetomidine (100 nM) had no effect on N-methyl-D -aspartate or hypoxia plus 3 mar L-glutamate-mediated calcium changes. Conclusions: The selective alpha(2)-adrenergic agonist dexmedetomidin e decreases evoked glutamate release from hippocampal rat brain slices during depolarization or hypoxic stress, but does not alter calcium c hanges mediated by the stimulation of glutamate receptors during aerob ic or hypoxic conditions.