Different actions of intracellular free calcium on resting and GABA-gated chloride conductances

Area of Science:

• Neuroscience
• Cellular Physiology
• Ion Channel Function

Background:

• Voltage-dependent calcium channels play crucial roles in neuronal excitability and signaling.
• Gamma-aminobutyric acid (GABA) is the primary inhibitory neurotransmitter in the central nervous system, mediating chloride (Cl-) currents.
• The interplay between calcium influx and GABAergic inhibition is not fully understood.

Purpose of the Study:

• To investigate the modulatory effects of voltage-dependent calcium current (ICa) on GABA-gated chloride conductances (ICl) in isolated frog sensory neurons.
• To elucidate the mechanism by which calcium influences GABA receptor activity.

Main Methods:

• Patch-clamp electrophysiology was used to record ionic currents in isolated frog sensory neurons.
• Voltage-clamp techniques were employed to isolate and measure both voltage-dependent calcium currents and GABA-gated chloride currents.
• GABA was applied at various concentrations to construct dose-response curves before and during calcium influx.

Main Results:

• A preceding voltage-dependent calcium current (ICa) significantly suppressed the amplitude of the GABA-gated chloride current (ICl).
• Increasing intracellular calcium concentration ([Ca2+]i) shifted the GABA dose-response curve to the right, without altering the maximum response, suggesting competitive or allosteric modulation.
• An ICa-activated chloride current (ICl) was observed as an inward tail current upon cessation of ICa, which saturated with increased calcium influx.

Conclusions:

• Voltage-dependent calcium influx directly modulates GABA-gated chloride conductances in frog sensory neurons.
• Calcium ions likely interact with the GABA receptor complex, altering its sensitivity to GABA or its ion channel gating properties.
• The observed ICa-activated ICl suggests a novel pathway for calcium signaling influencing neuronal inhibition.