Questions: Dopamine Receptor Subtypes and Signaling Pathways

Question 1

A patient takes a drug that selectively activates D2 receptors in the striatum. Based on the receptor's location on indirect-pathway neurons and its Gi signaling, what is the most likely effect on motor output?

• A. Movement is suppressed, because D2 activation inhibits striatal neurons
• B. Movement is facilitated, because reducing indirect-pathway activity releases the brake on the thalamus
• C. Movement is unaffected, because D2 is a presynaptic autoreceptor only
• D. Movement is suppressed, because Gi signaling decreases neuronal excitability throughout the basal ganglia

Answer: (B)

D2 receptors are enriched on indirect-pathway neurons that normally inhibit movement. When dopamine activates D2 (via Gi, lowering cAMP), these indirect-pathway neurons become less active, which removes a brake on thalamic activity and facilitates movement. The common misconception is that D2 'suppresses' movement simply because it decreases cAMP — but the effect depends on which neurons carry D2 receptors and what those neurons do in the circuit.

Question 2

Why do both first-generation antipsychotics (which block D2) and Parkinson's disease (which depletes dopamine) produce similar movement deficits?

• A. Both reduce dopamine binding at D1 receptors on direct-pathway neurons
• B. Both eliminate the pro-movement effect of dopamine on both direct (D1) and indirect (D2) pathways simultaneously
• C. Both increase cAMP levels in indirect-pathway neurons, slowing movement
• D. Both cause degeneration of the substantia nigra pars compacta

Answer: (B)

In Parkinson's, dopamine loss means D1 neurons in the direct pathway are underactivated (less cAMP, less facilitation) AND D2 neurons in the indirect pathway lose inhibition (more activity, more braking). First-generation antipsychotics block D2 receptors, interfering specifically with the indirect pathway arm. Both manipulations reduce the net pro-movement drive that dopamine normally exerts — which is why D2 blockade produces drug-induced Parkinsonism as a side effect.

Question 3

D2 receptor activation directly facilitates movement by increasing cAMP in indirect-pathway neurons.

• T. True
• F. False

Answer: False

This is doubly wrong. D2 receptors couple to Gi proteins, which *decrease* cAMP — the opposite of Gs-coupled D1 receptors. And D2 activation on indirect-pathway neurons reduces their activity, which *disinhibits* the thalamus and thereby facilitates movement. D2's pro-movement contribution operates by suppressing a suppressor, not by directly exciting anything.

Question 4

Parkinson's disease produces both underactivation of D1-class direct-pathway neurons and underactivation of D2-class indirect-pathway neurons.

• T. True
• F. False

Answer: True

Dopamine depletion affects both populations simultaneously. D1-bearing direct-pathway neurons lose excitation (less cAMP). D2-bearing indirect-pathway neurons lose the inhibition that normally dampens their activity — so they become overactive, applying more brake. Both effects converge on reduced thalamic drive to motor cortex, producing bradykinesia and rigidity.

Question 5

Why do first-generation antipsychotics, which are effective D2 antagonists, often cause Parkinsonian side effects?

Model answer: D2 receptors on striatal indirect-pathway neurons are normally activated by dopamine, which reduces their activity and releases the brake on movement. When antipsychotics block D2 receptors in the striatum, dopamine can no longer dampen the indirect pathway, so it becomes overactive and suppresses movement — mimicking the circuit state seen in Parkinson's disease.

The side effect is a direct pharmacological consequence of the receptor's circuit function. Antipsychotics block D2 to reduce psychosis (excess mesolimbic D2 signaling), but D2 receptors in the nigrostriatal pathway are blocked simultaneously, disrupting motor control. This forced clinicians to develop second-generation antipsychotics with lower D2 affinity — a design goal only intelligible with knowledge of receptor subtype distribution.