Q46 — Channelrhodopsin-2 (ChR2) in Hippocampal Neurons

Theoretical A Real exam question — full text reproduced under IBO's CC BY-NC-SA 4.0 license

The discovery of light-gated cation channels quickly led to the development of new tools and applications, notably the optical control of neural activity using algal channelrhodopsin-2 (ChR2).

Viral vectors were used to express ChR2 in rat hippocampal neurons, and ChR2 membrane localization was observed. A series of experiments was conducted to investigate the properties of these transduced cells, and the resulting data are presented below.

Voltage traces from a ChR2-expressing hippocampal neuron showing action potential spikes evoked by 1-second light pulses, subthreshold depolarizations from weaker pulses, and voltage responses to a brief series of light pulses where the fourth pulse fails to trigger a spike. Figure 1. A Voltage traces and spikes in a hippocampal neuron evoked by 1-second periods of light (4 repeated experiments). B Voltage traces showing subthreshold depolarizations in response to light pulses. C Voltage traces in response to brief light pulse series. Bars below the records indicate light pulses.

On your answer sheet, indicate “T” for true statements and “F” for false ones.

A. During steady illumination, the consistency of spikes is limited by the neuron's action potential firing frequency limit.

B. There is no action potential initiated during the fourth stimulation in C1 due to the absolute refractory period of the neuron.

C. Light-induced depolarization would happen regardless of whether ChR2 is selective for sodium alone or both sodium and potassium.

D. Consistent induction of subthreshold depolarizations, compared to action potentials, requires more precise tuning of the light pulse length.

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Question reproduced from IBO 2024, Theoretical Exam Part A, licensed under CC BY-NC-SA 4.0 — attributed to the International Biology Olympiad. Open the full exam PDF · Community solutions (unofficial)