Adenosine A1 receptor activation mediates NMDA receptor activity in a pertussis toxin-sensitive manner during normoxia but not anoxia in turtle cortical neurons

Abstract

Adenosine is a defensive metabolite that is critical to anoxic neuronal survival in the freshwater turtle. Channel arrest of the N-methyl- d-aspartate receptor (NMDAR) is a hallmark of the turtle's remarkable anoxia tolerance and adenosine A1 receptor (A 1R)-mediated depression of normoxic NMDAR activity is well documented. However, experiments examining the role of A 1Rs in regulating NMDAR activity during anoxia have yielded inconsistent results. The aim of this study was to examine the role of A 1Rs in the normoxic and anoxic regulation of turtle brain NMDAR activity. Whole-cell NMDAR currents were recorded for up to 2 h from turtle cortical pyramidal neurons exposed to pharmacological A 1R or G i protein modulation during normoxia (95% O 2/5% CO 2) and anoxia (95% N 2/5% CO 2). NMDAR currents were unchanged during normoxia and decreased 51 ± 4% following anoxic exposure. Normoxic agonism of A 1Rs with adenosine or N6-cyclopentyladenosine (CPA) decreased NMDAR currents 57 ± 11% and 59 ± 6%, respectively. The A 1R antagonist 8-cyclopentyl-1,3-dimethylxanthine (DPCPX) had no effect on normoxic NMDAR currents and prevented the adenosine and CPA-mediated decreases in NMDAR activity. DPCPX partially reduced the anoxic decrease at 20 but not 40 min of treatment. The G i protein inhibitor pertussis toxin (PTX) prevented both the CPA and anoxia-mediated decreases in NMDAR currents and calcium chelation or blockade of mitochondrial ATP-sensitive K + channels also prevented the CPA-mediated decreases. Our results suggest that the long-term anoxic decrease in NMDAR activity is activated by a PTX-sensitive mechanism that is independent of A 1R activity.