Drug Actions and Drug Effects
2. How Drugs Alter Neurotransmission
How SSRI Antidepressants Work
It is not unusual to hear that Prozac and other antidepressant drugs classified as selective serotonin reuptake inhibitors (SSRIs) have "revolutionized" the treatment of mood disorders. Although evidence from carefully controlled studies casts doubt on claims about the Prozac "revolution," it does appear that SSRIs are just as effective as older antidepressants such as MAO inhibitors and tricyclics in relieving symptoms of depression. What is different about SSRIs is that they are safer than other antidepressants because of their reduced risk of toxic and lethal effects, including intentional overdose. As their name suggests, SSRIs selectively target the reuptake mechanism for serotonin unlike older antidepressants that affected the action of several neurotransmitters. By inhibiting the process of serotonin reuptake, more of this neurotransmitter remains in the synapse. Pharmacologists believe that this increase in the synaptic supply of serotonin relieves the symptoms of depression. The following abstract and video on SSRIs from the Mayo Clinic provide a clear account of this particular drug action and antidepressant effect.
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Cocaine and Dopamine Reuptake
The mechanism through which the stimulant, cocaine, acts upon neurotransmission is similar in some ways to the reuptake inhibiting mechanism of SSRIs. However, in contrast to the more selective action of SSRIs, cocaine acts upon several neurotransmitters, including dopamine, norepinephrine, and glutamate as well as serotonin. Cocaine appears to increase the release of these neurotransmitters from the terminal vesicles, and it also interferes with their reuptake. The diagram shown below depicts the action of cocaine in blocking the normal reuptake of dopamine, a neurotransmitter that is at least partly responsible for the euphoric effects of this stimulant drug.
It is important to note that all forms of cocaine—powder, "crack" or "freebase"—have identical actions on the process of neurotransmission at the synaptic level. What makes the effects of these alternative forms of cocaine different is their respective routes of administration. For instance, when power cocaine is snorted and absorbed through nasal membranes, it reaches the brain more slowly than does cocaine that is smoked and inhaled through the lungs. The rapid and concentrated dose of cocaine that hits the brain after crack is inhaled produces an intense "rush" that is highly reinforcing and potentially habituating.
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