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Ionic basis of the postsynaptic depolarizing GABA response in hippocampal pyramidal cells
1Department of Pharmacology, State University of New York Health Science Center, Brooklyn 11203, USA.
Journal of Neurophysiology
|December 1, 1996
Summary
This study reveals that the late inward current of the giant postsynaptic current (GPSC) in guinea pig hippocampal neurons is primarily carried by bicarbonate ions (HCO3-). This finding refines our understanding of gamma-aminobutyric acid (GABA) signaling mechanisms.
Area of Science:
- Neuroscience
- Cellular Electrophysiology
- Ion Channel Physiology
Background:
- Gamma-aminobutyric acid (GABA) is a major inhibitory neurotransmitter in the brain.
- GABAergic signaling can exhibit complex kinetics, including biphasic responses.
- The ionic basis of these biphasic GABA responses, particularly the late component, requires detailed investigation.
Purpose of the Study:
- To elucidate the ionic mechanisms underlying the depolarizing (late inward) component of giant GABA-mediated postsynaptic currents (GPSCs) in CA3 pyramidal neurons.
- To determine the relative contributions of different ions, specifically chloride (Cl-) and bicarbonate (HCO3-), to the biphasic GPSC.
- To evaluate existing models of GABAergic signaling in light of experimental findings.
Main Methods:
- Whole-cell voltage-clamp recordings were performed on hippocampal slices from adult guinea pigs.
- Giant GABA-mediated postsynaptic currents (GPSCs) were isolated by blocking GABAB receptors, 4-aminopyridine, and excitatory amino acid receptors.
- Recording pipette solutions with varying concentrations of intracellular ions (K+, Cl-, HCO3-) were used to manipulate ionic gradients and assess their impact on GPSC components.
Main Results:
- The late inward current (GABAD component) of the GPSC was found to be insensitive to changes in intracellular potassium concentration ([K+]i).
- The GABAD component's reversal potential shifted with changes in bicarbonate equilibrium potential, indicating significant HCO3- permeability.
- The GABAA component showed greater sensitivity to intracellular chloride ([Cl-]i), while the GABAD component was more sensitive to intracellular bicarbonate ([HCO3-]i).
Conclusions:
- The late inward current (GABAD component) of the GPSC is predominantly mediated by bicarbonate (HCO3-) ions.
- These findings support, but also necessitate refinement of, the chloride accumulation model for GABAergic signaling.
- The biphasic nature of the GPSC suggests either a more complex chloride accumulation/depletion model or the involvement of distinct GABA channels with differential anion permeabilities.