P2 receptor-mediated modulation of noradrenaline release by electrical field stimulation and ischemic conditions in superfused rat hippocampus slices

In this study the regulation of the release of noradrenaline by P2 receptors was investigated in hippocampus slices preincubated with [³H]NA. Electrical field stimulation (EFS; 2 Hz, 240 shock, 1 ms) enhanced the release of noradrenaline in a [Ca]_o-dependent manner. In contrast, the excess release in response to ischemic-like conditions (combined oxygen and glucose deprivation) was [Ca]_o-independent. The Na⁺ channel blocker tetrodotoxin (1–3 μM) abolished both EFS-evoked and in vitro ischemia-evoked release of tritium. The P2 receptor agonists ATP, ADP and 2-MeSADP concentration-dependently decreased the tritium overflow with the potency order of ADP > 2-MeSADP > ATP. The inhibition by ATP (300 μM) was prevented by the P2 receptor antagonist PPADS (30 μM), by the P2Y₁ receptor antagonist MRS2179 (10 μM) and by the P2Y_12/13 receptor antagonist 2-MeSAMP (10 μM). Under ischemic-like conditions the P2X₁ receptor antagonist PPNDS (1 μM) inhibited the outflow of [³H]NA, whereas MRS2179 (10 μM) significantly increased the tritium outflow. PPADS and 2-MeSAMP did not affect ischemia-evoked [³H]NA efflux. RT-PCR analysis revealed that mRNA encoding P2Y₁₂ and P2Y₁₃ receptor subunits were expressed in the brainstem including locus coeruleus. The pharmacological profile of the underlying receptor subtype resembles the P2Y₁ and P2Y₁₃ receptor phenotype, and the endogenous activation of P2X₁ and P2Y₁ receptors contribute to the modulation of noradrenaline efflux upon ischemic-like conditions.