Some small polar molecules do not cross cell membranes easily and carrier proteins are used to get them into cells. This may be the means of removing the molecule from its site of action and so ending its action: 5HT is taken up into neurones, drugs which block this process prolong its action, eg fluoxetine (Prozac), a Specific Serotonin Reuptake Inhibitor used as an antidepressant in people and to modify the behaviour of dogs and cats.
