No abstract

Corticosteroids exert a dual role in eukaryotic cells through their action via (1) intracellular receptors (slow genomic responses), or (2) membrane-bound receptors (fast non-genomic responses). Highly vulnerable regions of the brain, like the hippocampus, express high amounts of corticosteroid receptors, yet their actions on ionic currents and neurotransmitters release are still undefined. Here, we investigated the effect of methylprednisolone (MP) on GABA and glutamate (Glu) release from isolated nerve terminals of the rat hippocampus. MP favored both spontaneous and depolarization-evoked [ 14 C]Glu release from rat hippocampal nerve terminals, without affecting [ 3 H]GABA outflow. Facilitation of [ 14 C]Glu release by MP is mediated by a Na + -dependent Ca 2+ -independent non-genomic mechanism relying on the activation of membrane-bound glucocorticoid (GR) and mineralocorticoid (MR) receptors sensitive to their antagonists mifepristone and spironolactone, respectively. The involvement of Na + -dependent high-affinity EAAT transport reversal was inferred by blockage of MP-induced [ 14 C]Glu release by DL-TBOA. Depolarization-evoked [ 3 H]GABA release in the presence of MP was partially attenuated by the selective P2X7 receptor antagonist A-438079, but this compound did not affect the release of [ 14 C]Glu. Data indicate that MP differentially affects GABA and glutamate release from rat hippocampal nerve terminals via fast non-genomic mechanisms putatively involving the activation of membrane-bound corticosteroid receptors. Facilitation of Glu release strengthen previous assumptions that MP may act as a cognitive enhancer in rats, while crosstalk with ATP-sensitive P2X7 receptors may promote a therapeutically desirable GABAergic inhibitory control during paroxysmal epileptic crisis that might be particularly relevant when extracellular Ca 2+ levels decrease below the threshold required for transmitter release.

Objective:Mounting evidence suggest that the ATP-sensitive ionotropic P2X4 receptor (P2X4R) improves cardiac performance and optimizes myocardial energy demand by reducing chronotropy. Here, we set to investigate whether the digitalis-like effect of the P2X4R in the sinus node was preserved in rats with right heart failure due to pulmonary arterial hypertension (PAH).Design and method:Male Wistar rats received a single subcutaneous injection of either monocrotaline (MCT; PAH group) or vehicle (CTRL group). Right heart failure associated to PAH normally occurs in Wistar rats 21 to 25 days after MCT administration. Myographic recordings were performed in spontaneously beating atria (chronotropic effect) isolated from both animal groups.Results:Baseline spontaneous atrial rate was 250 ± 5 (n = 26) bpm in CTRL and 237 ± 7 bpm (n = 23) in PAH group (p > 0.05). ATP (0.01–1 mM) concentration-dependently reduced the spontaneous atrial rate of both animal groups. The negative chronotropic effect of ATP (100 M) was smaller in PAH rats (-9 ± 2%; n = 14 vs. -22 ± 4%; n = 25, p < 0.05), but this divergence was attenuated upon increasing the concentration of ATP to 1 mM (CTRL: -54 ± 5%; n = 15 vs. MCT: -50 ± 7%; n = 8; p > 0.05). The P2X4R antagonist, 5-BDBD (10 M), attenuated the negative chronotropic action of ATP (100 M) in CTRL (from -19 ± 5% to -13 ± 3%; n = 5, p > 0.05) and PAH (from -8 ± 3% to 1 ± 8%; n = 4, p > 0.05) rats. The positive allosteric modulator of P2X4R, ivermectin (30 M), potentiated the negative chronotropic effect of ATP (100 M) in CTRL (from -31 ± 5% to -56 ± 14%; n = 5, p > 0.05) and PAH (from -12 ± 6% to -24 ± 9%; n = 4, p < 0.05). The non-selective P2XR agonist, CTP (1 mM), which contrary to ATP does not breakdown into adenosine, also decreased atrial chronotropy in CTRL (-24.31 ± 6.73%; n = 6) and PAH (-11.82 ± 1.98%; n = 6) rats.Conclusions:Data suggest that the negative chronotropic effect of P2X4R activation is preserved in atria of rats with heart failure associated to PAH, thus strengthening the hypothesis that heart rate reductions using this ATP-sensitive receptor may increase cardiac performance in patients with PAH-induced heart failure.