Anxiety from mild qualms to disabling panic is rated as more common than any other class of psychiatric disorder. (And this estimate predates 9/11.) But because anxiety is seldom recognized as an illness, it usually goes untreated.
Also, there’s a reversible gray area between anxiety and clinical depression. One may progress to the other, or vice versa. The best-known antidepressant drug is Prozac fluoxetine hydrochloride fielded by Eli Lilly & Co. in the mid-1990s. In 1995 its sales of the oral antidepressant reached $2 billion. The fine print in Prozac’s pharmacological bill of particulars also lists its psychiatric indications as including obsessive-compulsive disorder and bulimia nervosa binge eating.
Today’s Proceedings of the National Academy of Sciences (PNAS), dated March 5, 2002, carries a paper titled, “Involvement of striatal and extrastriatal DARPP-32 in biochemical and behavioral effects of fluoxetine (Prozac).” Its senior author is Nobelist and neuroscientist Paul Greengard, who directs the Laboratory of Molecular and Cellular Neuroscience at Rockefeller University in New York. Its first author is neuropharmacologist Per Svenningsson, a research associate in Greengard’s lab.
“In this study,” Svenningsson told BioWorld Today, “we have found several different proteins inside the cells that are mediating the effects of Prozac. They are both protein kinases and phosphatases, which haven’t been studied before in relation to depression. Now we find that all these different phosphatases and kinases may be normal targets for developing the next generation of antidepressant drugs.
“In this lab we do very basic research, so we know that several of these kinases and phosphatases that are intracellular proteins are located in the right place in the brain. But we haven’t been able to capture them to a very important drug or medication before. However, now it turns out that these proteins are important for the action of Prozac.”
More Serotonin In Brain, Less Depression
“What Prozac does is release a neurotransmitter called serotonin,” he went on. “Serotonin then activates a lot of signaling cascades within the cell, which later on cause the action of Prozac. And an important component of this signaling cascade is protein phosphorylation. That means you put the phosphate group on the protein and thereby activate it. For example, in our case, a protein called DARPP-32 takes away that phosphatase, and thus inactivates the protein. We have now linked them to the action of Prozac.
“DARPP-32 was discovered by Paul Greengard and his colleagues in the beginning of the 1980s,” Svenningsson recalled, “as a major substrate of dopamine, which together with serotonin are perhaps the most important neurotransmitters in terms of drug targets. DARPP-32 is enriched in the dopamine-innervated areas of the brain, such as the caudate putamen. That’s in a region,” he pointed out, “where we have a lot of neuronal defects, for example, in Parkinson’s disease. But DARPP-32 is also expressed outside the caudate putamen in the hippocampus, the prefrontal cortex and the amygdala. These brain regions are very important anatomical targets for treating defects in depression. And now it turns out that it’s also very important for innervation of serotonin.
“We also know that fluoxetine is very good as an anxiolytic. In this particular study, we have focused on depression, but one can already see that these phosphatases and kinases may also be targets for anxiolytic compounds. One of the compounds that work on both of these diseases is Prozac.”
Svenningsson described the in vivo experiments he conducted on transgenic mice lacking DARPP-32: “We have methods by which to measure the activities of these phosphatase and kinase proteins in the intact animal. We injected Prozac either acutely or over a prolonged period,” he recounted. “Then we dissected out different brain regions we knew were important in the etiology of depression, such as the prefrontal cortex and hippocampus.
“I also conducted behavioral studies,” he related, ”together with a friend of mine, George Nomikos,” working at Eli Lilly’s Corporate Center, Neuroscience Discovery Research, in Indianapolis. “He did a classical test for antidepressant efficacy, which is called the tail suspension test. It’s very simple in a way. You attach the tail of a mouse so that it’s hanging freely. Normally the animal is struggling from being in this position. Then after a while it goes immobile. And we measured the interval that it’s motionless. This time is decreased by all antidepressant agents in use today. It has a high validity for finding novel antidepressant drugs. This was done in KO mice that lack DARPP-32. We expect to repeat the same thing in rats.
“Another experiment was the locomotor test,” Svenningsson continued. “We put the mice in what’s called an open field. It’s actually a box where we have a light beam that measures the animals’ movements. We knew from before that the serotonergic system increases this locomotion parameter. Therefore we were interested in seeing whether DARPP-32 is involved in this component of serotonergic neurotransmission. We found that DARPP-32 is also mediated in locomotion.”
But There’s More Taking On Anxiety
“Now we want to see whether DARPP-32 is involved also in the anxiolytic properties of Prozac. We may extend this ongoing research to other compounds that we know are anxiolytic,” Svenningsson concluded, “and see whether DARPP-32 is involved in that disorder as well.”
Back-to-back with Svenningsson and Greengard’s PNAS paper is a complementary article by the same authors. Its title: “DARPP-32 mediates serotonergic neurotransmission in the forebrain.” The first paper is an in-depth analysis on how these kinases and phosphatases that mediate the phosphorylation event are involved in the action of Prozac. The second article is a broader description of which receptors may cause the action of Prozac on this regulation of the kinases and phosphatases.