Effect of tolbutamide on tetraethylammonium-induced postsynaptic zinc signals at hippocampal mossy fiber-CA3 synapses

Abstract

The application of tetraethylammonium (TEA), a blocker of voltage-dependent potassium channels, can induce long-term potentiation (LTP) in the synaptic systems CA3-CA1 and mossy fiber-CA3 pyramidal cells of the hippocampus. In the mossy fibers, the depolarization evoked by extracellular TEA induces a large amount of glutamate and also of zinc release. It is considered that zinc has a neuromodulatory role at the mossy fiber synapses, which can, at least in part, be due to the activation of presynaptic ATP-dependent potassium (K) channels. The aim of this work was to study properties of TEA-induced zinc signals, detected at the mossy fiber region, using the permeant form of the zinc indicator Newport Green. The application of TEA caused a depression of those signals that was partially blocked by the K channel inhibitor tolbutamide. After the removal of TEA, the signals usually increased to a level above baseline. These results are in agreement with the idea that intense zinc release during strong synaptic events triggers a negative feedback action. The zinc depression, caused by the LTP-evoking chemical stimulation, turns into potentiation after TEA washout, suggesting the existence of a correspondence between the observed zinc potentiation and TEA-evoked mossy fiber LTP.

FAD-Linked Autofluorescence and Chemically-Evoked Zinc Changes at Hippocampal Mossy Fiber-CA3 Synapses

“…Interestingly, the time course of these changes is similar to that of the corresponding fluorescence signals recorded from Newport Green loaded slices. The TEA triggered fluorescence changes are depressed during the perfusion of TEA and recover to or above the baseline level upon washout, as previously reported [24]. In the present study, after subtracting the autofluorescence component, the real zinc signals evoked by TEA have about half the amplitude of that of the total fluorescence traces.…”

“…In the present experimental conditions (excitation wavelength of 480 nm, emission light collected above 500 nm) and taking into account the spectral properties of FAD, the autofluorescence detected is considered to have FAD origin. As previously mentioned, KCl depolarization causes the entry of both calcium and zinc ions to the postsynaptic region [10,22,24,45,46]. When increases in cytosolic zinc concentration are high, zinc ions enter the mitochondria, and if in excess may have neurotoxic effects [4,47,48].…”

“…The experiments were performed in the synaptic system mossy fibers-CA3 pyramidal cells, using the brain of pregnant Wistar rat females (16-18 weeks old), kindly provided by the CNC (Center for Neurosciences) laboratory (as stated in the acknowledgments). The hippocampal slices were prepared as previously described [22,24]. Briefly, after cervical dislocation, the isolated brain was rapidly cooled (5-8°C) in oxygenated (95% O 2 , 5% CO 2 ) artificial cerebrospinal fluid (ACSF), containing (in mM): NaCl 124; KCl 3.5; NaHCO 3 24; NaH 2 PO 4 1.25; MgCl 2 2; CaCl 2 2 and D-glucose 10, pH 7.4.…”

“…However, the application of intense tetanic stimulation caused a depression of zinc and glutamate release, which was reduced by the application of the K ATP channel blocker tolbutamide [14]. Furthermore, in the presence of the potassium channel blocker tetraethylammonium (TEA), which induces a weaker depolarization in the mossy fiber region [23], the zinc signals were reversibly depressed being this depression also reduced by tolbutamide [24]. Since the depolarizing effect of KCl is stronger than that of TEA, it is expected that the increase of the synaptic activity will be more intense in the presence of KCl.…”

Probiotics and Prebiotics as a Therapeutic Strategy to Improve Memory in a Model of Middle-Aged Rats

“…The transient perfusion of the potassium channel blocker tetraethylammonium (TEA) induces a different form of LTP that primarily depends on the activation of voltage-dependent calcium channels rather than NMDA receptors activity (Bastos et al, 2017). Thus, in the next series of experiments, we tested whether the supplementation with E. faecium + Inulin influences the induction of this form of hippocampal plasticity.…”