Alterations in K+ evoked profiles of neurotransmitter and neuromodulator amino acids after focal ischemia-reperfusion

Abstract

Secondary elevations in extracellular amino acids occur during reperfusion after transient cerebral ischemia. The delayed accumulation of excitatory amino acids may contribute to the progressive development of neuronal injury. In this study, we explored the mechanisms that may be involved in this phenomenon. Microdialysis samples from probes located in rabbit cortex were analysed with a chiral amino acid procedure. Concentrations of neurotransmitters (l-Glu, GABA), N-methyl-d-aspartate receptor modulators (d-Ser, Gly), an inhibitory neuromodulator (Tau), the lipid component phosphoethanolamine, and l-Gln, l-Ser and l-Ala were measured. Depolarization via perfusion with potassium was used to assess the status of release/reuptake systems at 2 and 4h reperfusion after 2h transient focal ischemia. Background experiments classified potassium evoked responses as calcium dependent or calcium-independent by inclusion of 30μM omega-conopeptide MVIIC or by inclusion of 20mM magnesium and ommision of calcium. During ischemia, large elevations of almost all amino acids occured. During reperfusion, secondary elevations in transmitter amino acids (l-Glu, GABA) and N-methyl-d-aspartate receptor modulators (d-Ser, Gly) occured. Tau remained slightly elevated whereas the lipid component phosphoethanolamine remained high and stable during reperfusion. Reperfusion significantly potentiated the potassium response for amino acids with calcium-dependent responses (l-Glu and GABA). In contrast, calcium-independent responses (Tau, phosphoethanolamine, l-Gln) were significantly attenuated. Intermediate behavior was observed with Gly, while no potassium responses were observed for d-Ser, l-Ser or l-Ala.These data demonstrate that perturbations in evoked amino acid profiles after ischemia-reperfusion are selective. Reduction of calcium-independent responses implicate a general decline in efficacy of transporter mechanisms that restore transmembrane gradients of ions and transmitters. Decreased efficacy of transporter systems may reduce transmitter reuptake and account for the amplified release of l-Glu and GABA, thus contributing to progressive neural dysfunction after cerebral ischemia.