It was a big day in psychedelic neuroscience on Monday, with the publication of an article titled "Psychedelics promote plasticity by directly binding to BDNF-receptor TrkB" from Moliner & colleagues.
This article comes in the height of the psychedelic research era, where the question of 5-HT2A-dependent therapeutic mechanisms is continuously being investigated. The work presented in this article is a continuation of previous findings, in which the same group discovered that different antidepressants - both typical and fast-acting - promote synaptic localization of TrkB and activation by agonist BDNF by binding directly to TrkB receptors.
What does this mean to the non-scientist? Well, BDNF is a neurotrophic growth factor that when in its active form and bound to its receptor, TrkB, it promotes synaptic plasticity and structural plasticity. This means that more BDNF activating the receptor, more adaptive brain changes. Interestingly, BDNF is suggested to be decreased in people with depression in regions such as the frontal cortex and hippocampus, which are two important regions for learning, memory and emotional regulation. So essentially, if antidepressants and psychedelics are promoting BDNF to bind more to its receptor, they are promoting neuroplasticity. For more background on neuroplasticity, you can check out our episode from season 1.
A little more background about the TrkB receptor is necessary before diving into the paper. This receptor needs to dimerize (meaning combine two receptors) for BDNF to bind. This dimerization can happen with either an agonist associated with the receptor, or when allosteric modulators associate with the receptors. An allosteric modulator is something that can bind the receptor and induce a conformational change, but doesn't activate the receptor or bind in the same place as the agonist. Keep this in mind as we talk about these incredible findings and what they mean for the psychedelic field moving forward...
Hypothesis: Psychedelics will promote neuroplasticity, which is considered a critical component of their potential therapeutic effects. Therefore, psychedelics must be mediating either directly - or indirectly - the effects of BDNF and its receptor to promote neuroplasticity.
Binding of Psychedelics to TrkB
Using HEK cells (human embryonic kidney cells) that express the receptor of interest, TrkB, they use a radioligand binding assay to determine if psychedelics LSD and psilocybin bind to the TrkB receptor.
LSD and psilocybin both bind to the TrkB receptor with high affinity, while lisuride (non-psychedelic 2A agonist) bound at higher concentrations or with low affinity. Other LSD-related compounds did not bind. 5-HT2A antagonists, volinanserin (M100) and ketanserin did not bind to TrkB.
Something important to note within the binding data: the concentration of radiolabeled LSD is
very large compared to other groups (10 nM vs. 0.1 nM) and the nonspecific unlabeled LSD
is also quite large. Binding results should be replicated and analyzed with other
concentrations of LSD.
Using mutations to alter the transmembrane domain (TMD; where the receptor attaches to the membrane) and point-mutations to alter single amino acids that are necessary for compounds to interact with the receptor, they find that the TMD and a specific tyrosine (Y433) and valine (V437) is important and necessary for the binding of LSD and psilocybin.
The association of psychedelics to the TrkB is spontaneous and different from the binding of antidepressants like the SSRI fluoxetine. Both LSD and psilocybin bind specifically in the TMD, stronger than fluoxetine, and in a way that stabilizes the dimer to attract more BDNF molecules. Lisuride has a similar binding conformation to LSD and psilocybin but the binding is not as strong.
BDNF Signaling
Staying with the mutation in Y433 residue, they use wildtype TrkB receptors (so normal configuration) and the Y433 heterodimer (which is mutated at that residue) to see if the amino acid is also necessary for BDNF signaling via psychedelics.
LSD was found to increase phosphorylation of residues important for dimerization in hippocampal and cortical brain samples, which was not inhibited by pretreatment with antagonists. The stablization of the dimer via LSD is thought to potentiate the effects of low concentrations of BDNF.
LSD increased TrkB interaction with phospholipase C (PLC), which is necessary in regulating intracellular calcium release and antidepressant action. This interaction is increased in both the hippocampus and cortex of wildtype mice, but not the Y433 mutant.
LSD was also found to increase phosphorylation of ERK, a protein implicated in neuronal growth and plasticity. It was also found to have lasting changes on BDNF mRNA, including upregulation up to 24 h post-administration.
These finding are interesting because psychedelics also promote calcium signaling and upregulation of ERK through direct agonist activation of 5-HT2A receptors. These receptors are Gq coupled and produce signaling through PLC.
Could psychedelics be promoting plasticity both through increased interactions with PLC in two different pathways, perhaps synergistically?
Psychedelic-induced plasticity
Using a model called FRAP, which is where you basically bleach neurons and then see how long the fluorescent labeled proteins (or in this case, the TrkB receptor) reappears.
This assay revealed that LSD and PSI robustly increased fluorescent recovery in hippocampal neurons when compared with vehicle treatment. This effect relied on the Y433 residue.
They then looked at neuronal cultures from mice to determine if there was indeed structural plasticity as evidenced by dendritic spine growth. They found LSD and psilocybin increased spine density of mature neurons 24 h after treatment in only the wildtype mice.
They cite that 5-HT2A receptor antagonists did not prevent the spinogenic effect of LSD, but they do not specify whether these neuronal cultures are hippocampal or cortical. If they are
hippocampal, it would be unsurprising due to the very low density of 5-HT2A receptors in this
brain region.
As an example of functional plasticity, the authors demonstrate that LSD promotes visual plasticity and facilitates a shift in ocular dominance in favor of an open eye in a assay that deprives mice of using one of their eyes. This suggests that psychedelics may prove to be adaptive in behavior as well as in structural plasticity.
Psychedelics effects on behavior
The authors aim to demonstrate that the effects of psychedelics and TrkB signaling are independent of the 5-HT2A receptor signaling.
Using their wildtype and Y433 het mice they show that head twitch response (a 5-HT2A-mediated behavioral output following psychedelics) occurs in both. Normal response in the het mice suggests no involvement of TrkB on head twitch. Consistent with other papers, the effects of LSD on head twitch are blocked by 5-HT2A receptor antagonists.
In a model of chronic stress, where mice are exposed multiple times to a swim test in which they must swim or float for a period of time, they want to know if LSD will reduce the antidepressant-like behavior. This behavior is typically an increased immobility time in the water suggested to be like learned helplessness in mice.
LSD was found to produce a reduction in immobility time, suggesting the mice are not in despair. Interestingly, the Y433 mice did not have any response to the LSD suggesting this effect may be mediated by BDNF signaling. They also report that LSD's effects on the behavior were not reversed by antagonists.
Finally, using fear extinction assays they find LSD decreased conditioned fear responses in mice at 3 days and persisting 4 weeks following a single administration. These effects were not seen in the Y433 mice. An interesting caveat of this experiment is that LSD alone did not produce a decreased fear response, training in extinguishing the response was required.
Taken together, these results may suggest that psychedelics can produce lasting changes in the behaviors associated with depression and anxiety, but may require BDNF-dependent mechanisms involved in learning and memory.
Major Conclusions & Thoughts
This paper is truly groundbreaking. This gives us the first major publication that evidences a direct chemical and molecular interaction between the psychedelics and the BDNF receptor mechanism to induce changes in the brain. It's important to note that while psychedelics directly bind to the TrkB receptor, they ARE NOT direct agonists, but instead they act as allosteric modulators by facilitating the effects of BDNF. Extracellular BDNF is still required for the effects on TrkB dimerization and the induced plasticity.
While many are touting these findings as a 5-HT2A receptor independent mechanism of psychedelic-induced plasticity, it's important to think about the brain as a system where circuits and neurotransmission overlap. For example, extracellular BDNF has to come from somewhere right? The release of BDNF into the system could still be facilitated through some mechanisms downstream of 5-HT2A receptor activation. Many of the findings in this paper (single molecular localization, FRAP and long-term neuronal survival) were done in hippocampal neurons where this is little 5-HT2A receptor expression. It would be interesting to see these findings in frontal cortex neurons, since there is a higher receptor density there. Like mentioned above, the activation of 5-HT2A receptors in the cortex can also increase ERK signaling and influence upregulation of mTOR and BDNF mRNA, potentially allowing for synergy within the system.
BDNF/ERK/5-HT2A receptor relationships have also been evidenced in studies related to stress and gonadal hormones, suggesting potential influences of sex and gender on the system. While this study mentions the use of male and female animals for their chronic stress swim tests, they don't mention any analysis of sex as an independent variable. This could largely impact findings. Since psychedelic research and the interactions with psychedelics and BDNF pathways are so new, it is important to understand the impact of sex on these results.
Overall, we can't wait to see what this group does next!