There is a fundamental disconnect between how rapidly antidepressants act molecularly, and how slowly they act clinically.

"Typical antidepressants take 2-4 weeks to act, which is a substantial clinical problem, not only because of the delayed therapeutic effect, but also due to reduced patient compliance," Eero Castren told BioWorld Science.

Now, Castren, who is an academy professor in the Neuroscience Center at the University of Helsinki, and his team have discovered a possible reason for the discrepancy.

In their experiments, which were published in the February 18, 2021, online issue of Cell, the international team showed that both typical and fast-acting atypical antidepressants bind to the brain-derived neurotrophic factor (BDNF) receptor, tyrosine kinase receptor 2 (TRKB).

The mechanism directly links antidepressant activity to neuronal plasticity and may account for the slow onset of action of typical antidepressants.

"This is the first study to recognize a binding site for many, if not all, antidepressants specifically in the transmembrane domain [TMD] of TRKB," said Castren.

Several targets have been identified for antidepressant drug action, but it remains unclear how binding to these translates into clinical antidepressive effects.

TRKB, cholesterol and antidepressants

Typical antidepressants, including serotonin selective reuptake inhibitors (SSRIs) and monoamine oxidase inhibitors, increase synaptic monoamine levels of by inhibiting their reuptake or metabolism, but it is unclear why their clinical effects are delayed, while effects on monoamines are rapid.

All antidepressants increase expression and signaling of BDNF through TRKB, while the effects of SSRIs on BDNF signaling are thought to be indirect, via the respective inhibition of serotonin transporter and N-methyl-D-aspartate- (NMDA)-type glutamate receptors.

BDNF mimics antidepressant effects in rodents, and TRKB signaling inhibition prevents their behavioral effects.

TRKB activation is a key mediator of activity-dependent synaptic plasticity. Antidepressant-induced TRKB signaling reactivates juvenile-like plasticity in the adult brain, which may underlie the effects of antidepressants on mood.

TRKB signaling is also linked to brain cholesterol metabolism, with BDNF promoting cholesterol neuronal production, while cholesterol regulates TRKB signaling.

Cholesterol is essential for neuronal maturation and synaptic transmission, but cannot cross the blood-brain barrier, so neurons rely on cholesterol synthesized by astrocytes.

Synaptic cholesterol levels are low during early life but increase during the third postnatal week in mice, coinciding with increased BDNF expression and appearance of the antidepressant effects on TRKB.

Many antidepressants are now known to interact with phospholipids and to accumulate in cholesterol-rich membrane domains.

This prompted the new study reported in which the international research team led by Castren investigated the potential interactions between TRKB, cholesterol and antidepressants.

The study showed that the TRKB TMD senses changes in cell membrane cholesterol levels that mediate cholesterol's synaptic effects, with the researchers then elucidating its underlying mechanism.

Specifically, they showed that both typical and fast-acting antidepressants directly bind to TRKB, thereby facilitating synaptic localization of TRKB and its activation by BDNF.

"Molecular modeling studies identified a binding site formed by a dimer of two TRKB TMDs, which biochemical binding and mutagenesis studies then verified to be the binding site," said Castren.

Extensive computational approaches, including atomistic molecular dynamics simulations, revealed a binding site at the TMR of TRKB dimers.

"Molecular dynamics simulations are computational methods that create a molecular model and computationally test drug binding, which has much greater resolution than that currently achievable by biochemical studies," explained Castren. "The fact that two TRKB TMRs are needed for the binding site has probably prevented the earlier recognition of this binding site, as previous studies have mostly studied single TRKB molecules."

Mutation of the TRKB antidepressant-binding motif impaired cellular, behavioral and plasticity-promoting responses to antidepressants both in vitro and in vivo.

"We changed several amino acids, which simulations had predicted to be part of the binding site, to other amino acids that should not mediate binding," explained Castren.

The team also introduced one mutation into a mouse model, and found those animals "did not show any responses to antidepressants in tests in which these drugs normally produce behavioral responses," he said.

"This particular mutation has not been found in humans, but if it were, we would predict that these mutation carriers would show reduced response to antidepressant treatment."

The findings suggest that direct binding to TRKB and promotion of BDNF-mediated plasticity is the mechanism of action for antidepressant drugs, which may explain why typical antidepressants act slowly and how the molecular effects of antidepressants translate into clinical mood recovery.

They also have important implications for the development of new antidepressants medications, as the "recognition of a new binding site enables a search for molecules that optimally bind to that site, which might become new antidepressants," said Castren.

However, such drugs would probably not be significantly more effective, as the currently available antidepressants already bind to this site.

"Our previous work has shown that antidepressants promote neuronal plasticity through BDNF-TRKB signaling, but plastic networks need to be guided by rehabilitative psychotherapy to be fully effective and hopefully our current findings will further emphasize this combination approach to treating mood disorders." Looking ahead, said Castren, "we are now searching for molecules that bind to TRKB with a higher affinity and are curious about why typical antidepressants accumulate in neuronal membrane compartments so slowly, which we suggest underlies the slow onset of action of these drugs." (Casarotto, P.C. et al. Cell 2021, Advanced publication).