Paradoxical nifedipine facilitation of 45Ca uptake into rat hippocampal synaptosomes

Non-genomic Actions of Methylprednisolone Differentially Influence GABA and Glutamate Release From Isolated Nerve Terminals of the Rat Hippocampus

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 [¹⁴C]Glu release from rat hippocampal nerve terminals, without affecting [³H]GABA outflow. Facilitation of [¹⁴C]Glu release by MP is mediated by a Na⁺-dependent Ca²⁺-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 [¹⁴C]Glu release by DL-TBOA. Depolarization-evoked [³H]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 [¹⁴C]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 Ca²⁺ levels decrease below the threshold required for transmitter release.

Under stressful conditions activation of the ionotropic P2X7 receptor differentially regulates GABA and glutamate release from nerve terminals of the rat cerebral cortex

γ-Aminobutyric acid (GABA) and glutamate (Glu) are the main inhibitory and excitatory neurotransmitters in the central nervous system (CNS), respectively. Fine tuning regulation of extracellular levels of these amino acids is essential for normal brain activity. Recently, we showed that neocortical nerve terminals from patients with epilepsy express higher amounts of the non-desensitizing ionotropic P2X7 receptor. Once activated by ATP released from neuronal cells, the P2X7 receptor unbalances GABAergic vs. glutamatergic neurotransmission by differentially interfering with GABA and Glu uptake. Here, we investigated if activation of the P2X7 receptor also affects [³H]GABA and [¹⁴C]Glu release measured synchronously from isolated nerve terminals (synaptosomes) of the rat cerebral cortex. Data show that activation of the P2X7 receptor consistently increases [¹⁴C]Glu over [³H]GABA release from cortical nerve terminals, but the GABA/Glu ratio depends on extracellular Ca²⁺ concentrations. While the P2X7-induced [³H]GABA release is operated by a Ca²⁺-dependent pathway when external Ca²⁺ is available, this mechanism shifts towards the reversal of the GAT1 transporter in low Ca²⁺ conditions. A different scenario is verified regarding [¹⁴C]Glu outflow triggered by the P2X7 receptor, since the amino acid seems to be consistently released through the recruitment of connexin-containing hemichannels upon P2X7 activation, both in the absence and in the presence of external Ca²⁺. Data from this study add valuable information suggesting that ATP, via P2X7 activation, not only interferes with the high-affinity uptake of GABA and Glu but actually favors the release of these amino acids through distinct molecular mechanisms amenable to differential therapeutic control.