Manganese induces oxidative impairment in cultured rat astrocytes

Excessive free radical formation has been implicated as a causative factor in neurotoxic damage associated with exposures to a variety of metals, including manganese (Mn). It is well established that Mn accumulates in astrocytes, affecting their ability to indirectly induce and/or exacerbate neuronal dysfunction. The present study examined the effects of Mn treatment on the following endpoints in primary astrocyte cultures: (1) oxidative injury, (2) alterations in high-energy phosphate (adenosine 5′-triphosphate, ATP) levels, (3) mitochondrial inner membrane potential, and (4) glutamine uptake and the expression of glutamine transporters. We quantified astrocyte cerebral oxidative damage by measuring F²-isoprostanes (F²-IsoPs) using stable isotope dilution methods followed by gas chromatography-mass spectrometry with selective ion monitoring. Our data showed a significant (p < 0.01) elevation in F²-IsoPs levels at 2 h following exposure to Mn (100μM, 500μM, or 1mM). Consistent with this observation, Mn induced a concentration-dependent reduction in ATP and the inner mitochondrial membrane potential (ΔΨ_m), measured by the high pressure liquid chromatography method and the potentiometric dye, tetramethyl rhodamine ethyl ester, respectively. Moreover, 30 min of pretreatment with Mn (100μM, 500μM, or 1mM) inhibited the net uptake of glutamine (GLN) (³H-glutamine) measured at 1 and 5 min. Expression of the messenger RNA coding the GLN transporters, SNAT3/SN1 and SNAT1, was inhibited after 100 and 500μM Mn treatment for 24 h. Our results demonstrate that induction of oxidative stress, associated mitochondrial dysfunction, and alterations in GLN/glutamate cycling in astrocytes represent key mechanisms by which Mn exerts its neurotoxicity.