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

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 (A1R)-mediated depression of normoxic NMDAR activity is well documented. However, experiments examining the role of A1Rs in regulating NMDAR activity during anoxia have yielded inconsistent results. The aim of this study was to examine the role of A1Rs 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 A1R or Gi protein modulation during normoxia (95% O2/5% CO2) and anoxia (95% N2/5% CO2). NMDAR currents were unchanged during normoxia and decreased 51 ± 4% following anoxic exposure. Normoxic agonism of A1Rs with adenosine or N6-cyclopentyladenosine (CPA) decreased NMDAR currents 57 ± 11% and 59 ± 6%, respectively. The A1R 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 Gi 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 A1R activity.