Carbon monoxide, a novel neural messenger, does not modulate extracellular glutamate concentration in forebrain ischemia

We investigated the role of carbon monoxide as a neural modulator of extracellular glutamate concentration in rat hippocampus CA1 in transient forebrain ischemia by using metalloporphyrins, which block the production of carbon monoxide through the inhibition of heme oxygenase (HO) activity. Infusion of 10 and 100 microM zinc protoporphyrin IX, which inhibits nitric oxide synthase activity as well as HO activity, significantly increased glutamate concentration compared with that on the vehicle-treated side. However, infusion of 100 microM tin mesoporphyrin IX, which inhibits only HO activity, did not affect glutamate concentration in ischemia. Our results therefore do not support the hypothesis that carbon monoxide acts as a neural messenger through the modulation of extracellular glutamate concentration in ischemia.