How do antidepressants actually work?
The Abraham in the genealogy of antidepressants is a synthetic compound called imipramine. As shown below, imipramine belongs to an ancient line of compounds that dates back to the dawn of psychopharmacology, when neurotransmitters ruled the Earth:
A conserved ethylamine side chain (in red) pervades the psychopharmacopoeia
Discovered in the early 1950s, imipramine is the original tricyclic antidepressant, and it begat second- and third-generation antidepressants with fewer side effects but not necessarily greater efficacy, including the selective-serotonin reuptake inhibitors (SSRIs): Zoloft, Prozac, and Paxil, among others.
So how do antidepressants work at a molecular level? Which cellular bits do they bind to? You’d think we’d know the answer to that question after more than 50 years of antidepressant use. As suggested by the name SSRI, antidepressants inhibit with different degrees of selectivity the reuptake of monoamine neurotransmitter transporter proteins at synapses in the brain. Neurotransmitter transporter proteins include SERT, which is specific for serotonin, or 5-hydroxytryptamine (5-HT). Evolution has conserved the SERT gene in mammals, flies and even nematode worms. The human version of SERT is thought to be the primary drug target of SSRI antidepressants. In other words, SSRIs bind to SERT expressed on the surface of serotonin-producing neurons and then inhibit serotonin reuptake into cells.
But there’s a catch. The serotonin-centric model has an exposed, soft underbelly. In the mid-90s, psychopharmacologists Eric Nestler and Ronald Duman and others showed that antidepressants induce neurogenesis in laboratory rodents after chronic but not acute treatment. The chronic vs acute part is key. Inhibition of SERT by antidepressants occurs within minutes to hours, yet people who respond to antidepressants don’t feel better until they’ve been taking them for weeks or months. So the rodent observation meshes with long-standing clinical observations in people taking antidepressants who experience a “therapeutic lag.”
For the record, acute is defined as 30 minutes after injection of antidepressant. One of the behavioral assays employed in the antidepressant R&D pipeline is the tail suspension test. Briefly, mice are suspended upside down by their tails. As you or I would if subjected to a comparable stress position, inverted mice struggle to right themselves but eventually submit to gravity. Turns out mice treated with antidepressants fidget longer than their placebo-treated compatriots. This difference between treated and untreated mice has traditionally been the green light to move a candidate antidepressant onto human clinical trials.
In 2011, Randy Blakely’s lab at Vanderbilt published an insightful paper in PNAS that informs any serious discussion of antidepressant pharmacology, which is clearly complex. The Blakely Lab has a unique place in the history of antidepressant research. Blakely led the team that first cloned SERT from rats in 1991, validating the crude extract-based neurotransmitter reuptake assays that were originally employed by the pharmaceutical industry in the 1960s and 1970s in R&D and that ultimately yielded the SSRIs. With the 2011 PNAS paper, Blakely’s efforts came full circle. He and his group found that acute behavioral effects of antidepressants did not occur in a special SERT mutant mouse they genetically engineered.
This mutant mouse’s SERT gene was mutated at a single amino acid position in the SERT protein. As a result, this one mutation rendered the mouse SERT protein unable to be bound by many antidepressants but still able to bind and transport serotonin, its natural substrate. As such, this mSERT mutant mouse is a clean genetic test of the role of serotonin reuptake in the complex pharmacology of antidepressants.
As shown in Figure 1A of their paper, mutant SERT (M172) interacts with and transports serotonin, or 5-HT, just as well as wildtype (I172). However, several SSRI antidepressants (but interestingly not paroxetine/Paxil), as well as cocaine, must be used at higher concentrations to block serotonin transport by mutant SERT. In one case (E), up to 1000-times more citalopram/Celexa is required:
The single amino acid change, which swapped an isoleucine for a methionine at position 172, was not made by serendipity but on the basis of data in a 2007 Nature paper by Eric Gouaux, whose lab is at Oregon Health & Science University. The Gouaux Lab studies neurotransmitter transporter proteins. However, the Gouaux Lab couldn’t crystallize a SERT protein of mammalian origin, but they were able to crystallize an old relative of mammalian SERT – from bacteria! Bacteria don’t produce serotonin, but they express a distantly related yet structurally similar transporter that recognizes the amino acid leucine (see featured image).
Blakely’s group has demonstrated that SERT is required for the acute behavioral effects of antidepressants in laboratory rodents. So here’s the $64,000 question: is SERT required for neurogenesis in laboratory rodents after chronic antidepressant treatment?
Two years later this experiment still hasn’t been performed to my knowledge — what are we waiting for?