Abstract

This study was designed to test the hypothesis of whether activation of presynaptic P2X receptor-gated ion channels elicits noradrenaline release from central catecholaminergic terminals. ATP, α,β-methylene-adenosine 5′-triphosphate (α,β-methyleneATP), and ADP elicited concentration-dependent [³H]noradrenaline outflow from superfused rat hippocampal slices with the following rank order of agonist potency: α,β-methyleneATP > ATP > ADP. Among P2 receptor antagonists, pyridoxal-phosphate-6-azophenyl-2′,4′-disulphonic acid (30 μM), 4,4′,4′′,4′′′-[carbonylbis(imino-5,1,3-benzenetriyl-bis(carbonylimino))]tetrakis-1,3-benzenedisulfonic acid (100 nM), and 8,8′-[carbonybis(imino-3,1-phenylenecarbonylimino)]bis1,3,5-naphthalenetrisulphonic acid (10 μM) significantly inhibited the outflow of [³H]noradrenaline, evoked by ATP, whereas Brilliant Blue G (100 nM), 2′-deoxy-N⁶-methyladenosine 3′,5′-bisphosphate tetraammonium (10 μM), the A₁ receptor antagonist 8-cyclopentyl-1,3-dipropylxanthine (250 nM), and the A_2A receptor antagonist 3,7-dimethyl-1-propargylxanthine (250 nM) were ineffective. Pretreatment with the G_i protein inhibitor pertussis toxin (2.5 μg/ml) did not change the effect of ATP on [³H]noradrenaline outflow. In contrast, a decrease in extracellular pH from 7.4 to 6.6 significantly attenuated the response by ATP. When extracellular Na⁺ was replaced by choline chloride and in the presence of the noradrenaline uptake inhibitor desipramine (10 μM), the ATP-evoked [³H]noradrenaline outflow was almost completely abolished, indicating that its underlying mechanism is the sodium-dependent reversal of the noradrenaline transporter. Reverse transcription-polymerase chain reaction analysis revealed that mRNA encoding P2X₁, P2X₂, P2X₃, P2X₄, P2X₆, P2X₇, and P2Y₁ receptor subunits were expressed in the brainstem containing catecholaminergic nuclei projecting to the hippocampus, whereas mRNA encoding P2X₅, P2Y₂, P2Y₄, and P2Y₆ receptors were absent. Taken together, these results indicate that noradrenergic terminals of the rat hippocampus are equipped with presynaptic facilitatory P2X receptors, displaying a pharmacological profile similar to homomeric P2X₁ and P2X₃ receptors.