Intracellular calcium stores modulate miniature GABA‐mediated synaptic currents in neonatal rat hippocampal neurons

Abstract

The whole-cell configuration of the patch clamp technique was used to record miniature gamma-aminobutyric acidA (GABAA) receptor-mediated currents (in tetrodotoxin, 1 microM and kynurenic acid 1 mM) from CA3 pyramidal cells in thin hippocampal slices obtained from postnatal (P) day (P6-9) old rats. Switching from a Ca2+-containing to a nominally Ca2+-free medium (in which Ca2+ was substituted with Mg2+, in the presence or in the absence of 100 microM EGTA) did not change significantly the frequency or amplitude of miniature events. Superfusion of thapsigargin induced a concentration-dependent increase in frequency but not in amplitude of tetrodotoxin-resistant currents that lasted for the entire period of drug application. Mean frequency ratio (thapsigargin 10 microM over control) was 1.8+/-0.5, (n = 9). In nominally Ca2+-free solutions thapsigargin was ineffective. When bath applied, caffeine (10 mM), reversibly reduced the amplitude of miniature postsynaptic currents whereas, if applied by brief pressure pulses, it produced an increase in frequency but not in amplitude of spontaneous GABAergic currents. Superfusion of caffeine (10 mM) reversibly reduced the amplitude of the current induced by GABA (100 microM) indicating a clear postsynaptic effect on GABAA receptor. Superfusion of ryanodine (30 microM), in the majority of the cells (n = 7) did not significantly modify the amplitude or frequency of miniature events. In two of nine cells it induced a transient increase in frequency of miniature postsynaptic currents. These results indicate that in neonatal hippocampal neurons, mobilization of calcium from caffeine-ryanodine-sensitive stores facilitates GABA release.

Maximinis

“…Over the past two years, it has become clear that clathrin-mediated endocytosis is also required for hippocampal long-term depression [3][4][5][6] , with AMPARs being internalized rapidly into clathrin-coated vesicles 5 . Conversely, induction of long-term potentiation is accompanied by insertion of AMPARs from intracellular sites into the postsynaptic density 7,8 , presumably by exocytotic membrane fusion.…”

“…The ligands ryanodine and IP 3 can trigger release of calcium from endoplasmic reticulum. Experimental manipulations of neuronal stores affect action-potential-evoked release, or the frequency of mPSCs, but manipulations of stores have not seemed to directly affect mPSC amplitudes 5,6 .…”

Presynaptic calcium stores underlie large-amplitude miniature IPSCs and spontaneous calcium transients

“…Fourth, caffeine and/or ryanodine modify presynaptic Ca 2+ i signals in autonomic ganglia 12,13 and in photoreceptors 14 . Finally, in hippocampal pyramidal cells, caffeine or thapsigargin can increase the frequency of miniature IPSCs 15 . Hence, spontaneous Ca 2+ release from presynaptic Ca 2+ stores may provide the synchronization mechanism that leads to multivesicular miniatures.…”

Action Potential-Evoked and Ryanodine-Sensitive Spontaneous Ca2+Transients at the Presynaptic Terminal of a Developing CNS Inhibitory Synapse

“…Spontaneous Ca 2ϩ transients have also been observed at presynaptic boutons of hippocampal pyramidal cells (Emptage et al, 2001) and in frog vestibular hair cells (Lelli et al, 2003), in which electrophysiological studies support a role of RyRs on regulating transmitter release, and have recently been shown at nerve terminals from hypothalamic neurons (De Crescenzo et al, 2004). The observation of SCaTs adds to a growing body of evidence favoring the role of intracellular Ca 2ϩ stores on exocytosis at various mammalian CNS synapses [CA3 hippocampal pyramidal cells (Savic and Sciancalepore, 1998); cortical pyramidal cells (Simkus and Stricker, 2002)], as well as in salamander photoreceptors (Krizaj et al, 2003) and at the neuromuscular junctions of frog (Narita et al, 1998) and lizard (Melamed-Book et al, 1993). Although the implication of Ca 2ϩ stores in exocytosis is still controversial at some synapses (Carter et al, 2002;Lim et al, 2003), the available data indicate that the phenomenon may be widespread (for review, see Bouchard et al, 2003).…”