Comparison of glutamine-enhanced glutamate release from slices and primary cultures of rat brain

Increased extracellular glutamate has been associated with a wide range of effects including production of neurotoxicity. Glutamine has previously been shown to cause increased release of glutamate from a variety of preparations. Extracellular central nervous system (CNS) glutamine levels are known to increase with neurotoxin exposures, hepatic failure, renal failure, head trauma or stroke. However, the action of glutamine to enhance the release of glutamate under nondepolarizing conditions has not been well studied. Since glutamine-mediated increases in extracellular glutamate are potentially of significance in cellular damage as a result of CNS insult, further examination of this phenomenon is important. Striatal and hippocampal slices or virtually neuron-free primary striatal glial cultures were employed in studies to further elucidate the mechanism(s) of glutamine-enhanced glutamate release. Elevated extracellular glutamine caused increased glutamate release in all three preparations. In hippocampal and striatal slices elevated glutamine caused an enhancement of N-methyl-D-aspartate (NMDA) receptor-mediated [3H]catecholamine release equivalent to that produced by high concentrations (up to 100 microM) of exogenous glutamate. In both striatal slices and primary cultures kynurenate increased glutamate release in the presence of 500 microM glutamine, while kainate either had no effect or decreased glutamate levels in the presence of glutamine. Since several presynaptic modulators of release did not affect the glutamate release produced by glutamine in slices, vesicular release of glutamate from nerve terminals was probably not involved in the effects of the exogenous glutamine. The similarities between striatal slices and primary striatal cultures indicate that enzymatic conversion of glutamine to glutamate within glia may be an important factor in the glutamine-mediated elevation of extracellular glutamate levels.