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Q80 — Dopamine and GABA Neuron Activity in Stress-Induced Anhedonia

Theoretical 2 Real exam question — full text reproduced under IBO's CC BY-NC-SA 4.0 license
Nervous System Physiology

One of the main symptoms of depression, which often develops after stress, is anhedonia, the loss of the ability to feel pleasure. Dopamine (DA) signals, which are sent to certain parts of the brain, play a key role in the pursuit of rewards, motivation for behavior and learning from positive experiences. One assumption is that stress leads to anhedonia through dopamine neurons, although little evidence is available to support this claim. Experiments were performed on mice to study the neurological mechanisms of anhedonia using the method of intracranial electroencephalography (iEEG), assessing the activity of dopaminergic (DA) and Gamma-Aminobutyric Acid (GABA) neurons. Mice in the experimental group (+stress) were rewarded with water after being exposed to stress. Mice in the control group were not exposed to stress. A sound stimulus (as a CS+) alerted mice to the reward of water.

Two-panel figure of neuronal firing rate versus time from the CS+ sound cue. Panel a (DA neurons): control (black) and stress (red) traces overlap almost exactly, both showing a sharp, brief peak in firing rate (~15 Hz) right after the cue before returning to baseline (~5-7 Hz). Panel b (GABA neurons): the control (black) trace rises gradually to ~30 Hz and then declines gradually; the stress (red) trace rises even higher (~35-37 Hz) and stays persistently elevated (well above control) throughout the recording window without returning to baseline. Figure 1. The activity of (a) DA neurons and (b) GABA neurons in the midbrain (firing rate in hertz) in response to a sound signal. Time 0 marks the onset of the CS+ sound signal. Black curve: control mice; red curve: experimental (stress) mice.

Using the information and the figures above, determine whether the following statements are true or false.

A. The response of DA to the expected reward does not change after stress.
B. DA responds to expected reward with a short-term increase in activity, while GABA-neurons respond long-term.
C. Both DA and GABA neurons participate in the development of post-stress anhedonia.
D. GABA-neuronal activity does not decrease back to baseline even after the stress is over, potentially leading to alterations in further downstream processes.
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Question reproduced from IBO 2022, Theoretical Paper 2, licensed under CC BY-NC-SA 4.0 — attributed to the International Biology Olympiad. Open the full exam PDF · Community solutions (unofficial)