Information for Problem 4.

            Serotonin is a major neurotransmitter in the brain; many psychoactive drugs (such as Prozac & Ecstasy) affect release or uptake of serotonin. Scientists are studying serotonin uptake in platelets, which are much easier to obtain than neurons from the brain. Neurons release serotonin by exocytosis of serotonin filled vesicles. After a short time, the neurons take the serotonin back up, and pack it into vesicles again for another cycle. Platelets do the same, presumably by a very similar mechanism. Here is the situation with platelets:

            You can isolate vesicles formed from the plasma membrane of platelets. These vesicles accumulate serotonin in vitro when appropriate ion gradients are present. This process requires a high external concentration of Na⁺ and is inhibited by imipramine (an antidepressant).

            Most of the serotonin that accumulates inside platelets is found in intracellular storage organelles (storage vesicles). When these organelles are isolated, their internal pH is acid, and they catalyze the uptake of serotonin. This uptake (in vitro) normally requires ATP, and is inhibited by reserpine (an antipsychotic).

            Suppose you do experiments 1-4. You lyse some cells in a solution with a low Na⁺ concentration, and you get a lysate – a mixture that contains both reformed plasma membranes vesicles and storage organelles.  You then measure uptake of serotonin in the lysate under different conditions. The serotonin that was taken up could be inside the plasma membrane vesicles or inside the storage organelles. (You have to figure out where it is.)

In experiment 1, you add Na⁺ to the lysate. You get uptake of serotonin. Uptake is inhibited by imipramine.

 

In experiment 2, you add ATP (but no Na⁺). You get uptake of serotonin. Uptake is inhibited by reserpine.

 

In experiment 3, you acidify the organelles. You get uptake of serotonin. Uptake is inhibited by reserpine.

 

In experiment 4, you don’t do anything to the lysate. You get no uptake of serotonin.

Here is a (corrected) table summarizing the results:

* Uptake = total uptake = measurement of amount of serotonin in storage organelles + amount in vesicles from plasma membrane.