Membrane current noise in toad retinal rods exposed to low external calcium

Area of Science:

• Neuroscience
• Phototransduction
• Cellular Physiology

Background:

• Rod photoreceptors are essential for vision in low light.
• Membrane currents and noise in photoreceptors are critical for signal transduction.
• Extracellular calcium influences photoreceptor function and membrane properties.

Purpose of the Study:

• To investigate the origin of noise in toad rod outer segment membrane currents.
• To determine the relationship between extracellular calcium, dark current, and noise.
• To explore the role of intracellular voltage fluctuations in generating this noise.

Main Methods:

• Recording outer segment membrane current of single toad rod photoreceptors using suction electrodes.
• Manipulating extracellular calcium concentration by altering saline composition.
• Analyzing noise power spectra using Lorentzian equations.
• Comparing noise in electrically coupled vs. isolated rod outer segments.

Main Results:

• Low extracellular calcium significantly increased dark current and broadband noise (1-800 Hz).
• Noise power spectra fitted a single Lorentzian equation, with corner frequencies around 40 Hz.
• A rebound increase in dark current after light stimulation was accompanied by similar noise.
• Noise was reduced or absent in electrically coupled rods but present in isolated rods, implicating intracellular voltage fluctuations.

Conclusions:

• The observed noise in toad rod outer segments is likely generated by intracellular voltage fluctuations, not directly by light-sensitive channels.
• The corner frequency of the noise power spectrum may represent the membrane time constant of isolated rods.
• Extracellular calcium levels play a crucial role in modulating dark current and associated noise in rod photoreceptors.