After release into the synaptic cleft, the neurotransmitter dopamine is actively

Question

After release into the synaptic cleft, the neurotransmitter dopamine is actively taken up by the cells via specific dopamine transporters. The drug cocaine interferes with this process and is therefore called a reuptake inhibitor. The inhibition of the transporter at a certain cocaine concentration is qualitatively represented in the following graph. Based on this graph, which of the following describes the effect of cocaine on the kinetics of dopamine reuptake by the transporter?

a) In the presence of cocaine, the maximal rate of transport (Vmax) is unaffected, but the apparent affinity of the transporter for dopamine is reduced

b) In the presence of cocaine, the maximal rate of transport (Vmax) is reduced, but the apparent affinity of the transporter for dopamine is unaffected.

c) In the presence of cocaine, the maximal rate of transport (Vmax) is enhanced, but the apparent affinity of the transporter for dopamine is unaffected.

d) In the presence of cocaine, both the maximal rate of transport (Vmax) and the apparent affinity of the transporter for dopamine are enhanced.

e) In the presence of cocaine, both the maximal rate of transport (Vmax) and the apparent affinity of the transporter for dopamine are unaffected, but the transporter is nevertheless inhibited.

Explanation / Answer

cocaine can always bind with dopamine transporters no matter whether dopamine is bound in dopamine transporters or not. Dopamine cannot reach its binding site if cocaine binds with dopamine transporters.

The affinity of cocaine binding with dopamine transporters in the presence of dopamine is much lower than that of cocaine binding with dopamine transporters in the absence of dopamine. When both dopamine and cocaine bind with dopamine transporters, the transporter protein is not expected to undergo the conformational change necessary for the transport of dopamine.

So, cocaine inhibit the transport of dopamine through both blocking the initial binding of dopamine with dopamine transporters and reducing the transporter after dopamine binds with dopamine transporters.

ans is (a) In the presence of cocaine, the maximal rate of transport (Vmax) is unaffected, but the apparent affinity of the transporter for dopamine is reduced

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