Modulation of Serum Brain-Derived Neurotrophic Factor by a Single Dose of Ayahuasca: Observation From a Randomized Controlled Trial

CONCLUSION

In this study, we found an interaction between serum BDNF and serum cortisol levels at baseline. The serum cortisol was predictor of baseline serum BDNF of volunteers. Individuals with hypocortisolemia, both patients and controls, presented lower serum BDNF levels when compared to volunteers with eucortisolemia (patients and controls). Moreover, we found a negative correlation between serum cortisol and serum BDNF in eucortisolemic volunteers. However, neither BDNF nor cortisol levels were predictors of major depression and none of the clinical characteristics of patients predicted their BDNF levels. Forty-eight hours after treatment (D2), we observed higher BDNF levels in both groups (patients and controls) who were treated with ayahuasca, compared to those treated with placebo. In addition, at D2 patients treated with ayahuasca, and not with placebo, presented a negative significant correlation between BDNF levels and MADRS scores. However, the number of previous unsuccessful antidepressant treatments was the main predictor of remission rates found at D2. These results corroborate in part our initial hypotheses. On one hand, we found a relationship between serum cortisol and serum BDNF at baseline. Among the various responses induced by the glucocorticoid, the direct modulation of cortisol on BDNF appears to be extremely significant in understanding the etiology and treatment of major depression. The expression of BDNF in the central nervous system is modulated by various brain molecules, including cortisol. It is known that one interaction between these two molecules occurs at the level of messenger RNA. It is observed that high concentrations of cortisol induce reduction of BDNF gene expression, and hippocampal and neocortex neurogenesis. Analogously, low concentrations of the cortisol interfere negatively in the expression of BDNF. An inverted U-curve relationship between cortisol and BDNF has been proposed, where intermediate levels of cortisol are the most suitable for the expression of BDNF. In addition, it is suggested that there is an interaction between TrkB and GR receptor activity. In order TrkB receptor to be activated by BDNF, the GR receptor must be coupled to the TrkB, forming a complex. When cortisol levels are high, cortisol binds to its GR receptor and removes it from the complex, reducing the potential response of BDNF. Therefore, the literature suggests that allostatic levels of cortisol are important to adequate BDNF expression and function. Herein, this hypothesis is corroborated by the model where baseline serum cortisol levels predict serum BDNF levels and by the negative correlation between BDNF and cortisol found only for eucortisolemic volunteers that consequently induced a baseline difference in serum BDNF levels of eucortisolemic and hypocortisolemic volunteers in our study. Brain-derived neurotrophic factor is a broad-acting neurotrophin that plays important roles in protection, differentiation, viability and growth of new neurons, and its effects go beyond the central nervous system. Outside of nervous system, the BDNF is mainly synthesized in platelets, which also is the main storage site of peripheral tissues. Rodent studies point out to the existence of bidirectional transport of BDNF in the blood-brain barrier, and peripheral administration of BDNF enhances hippocampal neurogenesis in mice, indicating that blood levels of BDNF are significant for brain function. Recent evidence suggests that several pathways of neuronal plasticity are activated by psychedelic compounds. The induction of neuroplasticity and neurogenesis by psychedelics are a relevant aspect of their action as potential antidepressants since several studies have shown that the efficacy of clinical antidepressants is related to an increase in hippocampal plasticity induced after monoaminergic enhancement. Even though most studies suggest that the serum BDNF levels of patients with depression are significantly lower than healthy controls, this finding is not unanimous. In our study, baseline serum BDNF levels, baseline serum cortisol levels, sex and income did not predict major depression and we did not find any clinical characteristics that predict baseline levels of BDNF in the group of patients. Both results are opposite to our hypothesis. Here, the variable age could predict the presence of depressive symptoms. However, as our patients were older than controls, this find might be biased by this baseline difference. It has been speculated that wash-out periods may mask the expected reduced BDNF levels found in depression, as it is been proposed that antidepressants stimulate BDNF synthesis. In fact, studies indicate that the reduction of BDNF is most consistently observed after a 4week wash-out periodor in severe untreated patients. Herein, all patients were in a 2-week wash-out period, which might not have been enough to allow the detection of lower BDNF levels at baseline. Furthermore, there are still controversies regarding the correlation between blood and central BDNF levels. There is no consensus in the literature about how far human blood BDNF reflects accurately the concentration of BDNF in the central nervous system. Therefore, this could also explain why we did not find difference in BDNF levels between healthy controls and depressed patients and the baseline serum BDNF levels did not predict major depression in our study. In addition, it is important to note that more recently three isoforms of BDNF have been identified. The intracellular synthesis of BDNF is initiated as the pre-pro-BDNF precursor form, and then the pre-region is removed resulting in the pro-BDNF form, which after a new cleavage forms mature BDNF (m-BDNF) and BDNF pro-peptide. In vitro assays suggest that pro-BDNF is the major secreted form, not m-BDNF, but in the extracellular medium, the pro-BDNF may undergo cleavage by the action of plasmin. The pro-BDNF/m-BDNF ratio seems to be critical for the determination of physiological or pathological conditions. Increased pro-BDNF levels in the prefrontal cortex and ventral tegmental area, and reduced levels of m-BDNF in the prefrontal cortex and hippocampus are observed in different studies using animal models of depression. Despite this, studies with humans that analyze pro-BDNF/m-BDNF ratio in blood are infrequent, and few studies showed changed ratio in patients with major depression. Both m-BDNF, BDNF pro-peptide and pro-BDNF can interact with Trk and p75 NTR receptor, although each one exhibits different biological activities. The m-BDNF has high affinity for TrkB receptorsand its activation has been implicated in neural excitability and increased synaptic strength. On the other hand, pro-BDNF and BDNF pro-peptide performs their biological actions mainly through the binding to p75NTR neurotrophin receptors and have been described as inhibitor of excitability and facilitators of synaptic depression. The commercial kit used in our study is not sensitive to different BDNF isoforms, and the measurement is related to the total BDNF amount of both isoforms. We initially supposed that ayahuasca would modulate BDNF levels, and this was observed. After dosing (D2) we found higher levels of BDNF in individuals treated with ayahuasca compared to placebo, regardless of the group (MD or CG). The between-treatment effect size was medium. Additionally, patients that were treated with ayahuasca, and not with placebo, presented a negative significant correlation between BDNF levels and MADRS scores at D2: higher serum BDNF levels were correlated with lower depression symptomatology after the session with ayahuasca. N, N-dimethyltryptamine (N,N-DMT), the main active compound of ayahuasca, activates 5-HT 2A and σ1R pathways involved in neuroplasticity. Ayahuasca also induces an acute increase of cortisol levels. Specifically, these same volunteers showed an increase of approximately 100% of salivary cortisol 1 h 40 min after ayahuasca intake, which probably was important to reach physiological levels of this hormone in hypocortisolemic volunteers. Thus, the cortisol pathway could be critically implied in the mechanism related to how ayahuasca modulation of BDNF molecule. Although this study suggests higher levels of BDNF in volunteers treated with ayahuasca than those treated with placebo, a detailed explanation of specific pathways possibly involved in ayahuasca modulation of serum BDNF expression needs further pre-clinical and clinical investigation. Nevertheless, the early association between ayahuasca treatment and BDNF found herein 48 h after the dosing session gives support to a potential mechanism behind the observed antidepressant effects of ayahuasca. In this sense, it is also important highlight that previous studies also described significant increases in serum BDNF levels just after a longer period of antidepressant treatment, i.e., after 8-12 weeks, only psychedelics with antidepressant potential that demonstrate rapid action via BDNF synthesis and secretion. However, at D2 neither serum BDNF levels nor the type of treatment (AYA or PLA) predicted remission rates at D2. It is important to highlight, however, that we did not observe a significant difference on remission rates at D2 between treatments (AYA and PLA) in our original clinical trial. In that study, the strongest effect size and a significant difference between treatments on response and remission rates were observed only at D7, and we did not measure BDNF at D7. On the other hand, the number of previous antidepressant treatments predicted remission rates at D2, where patients that were in remission at D2 had a smaller number of previous unsuccessful antidepressant treatments than patients that did not achieve remission. Despite previous findings indicating a positive correlation between antidepressant effects and the expression of BDNF, not all studies support this finding. Few studies using double-blind placebo-controlled design have investigated BDNF in major depression, and studies with psychedelic substances evaluating depression patients are scarce. Ketamine, an anesthetic with psychedelic proprieties, have been tested in MD patients and animal models, and patients with treatment resistant depression exhibited increased plasma BDNF after a ketamine session only in responders between 4 h after intake until 1 week later. In addition to some limitations discussed above, other restrictions in the present study should be considered. First, the genetic factor related to BDNF was not considered in this work. Some studies suggest that major depressive disorder is directly linked to single nucleotide functional polymorphism of BDNF, leading to valine (Val) substitution by methionine (Met) at codon 66 (Val66Met), and heterozygous patients presenting the Val66Met polymorphism have more promising responses to antidepressants when compared to Val/Val homozygotes. Second, serum cortisol and BDNF were measured only once, and considering that these molecules have circadian rhythm, studies that cover sequential measurements throughout the day and for different days could point to more precise results. This was the first double-blind randomized placebocontrolled trial for depression that investigated changes in BDNF levels after an intervention with a psychedelic substance. The results point to important observations, such as the report of different baseline serum BDNF levels in respect to serum cortisol, higher levels of BDNF in volunteers treated with ayahuasca than placebo 48 h after the dosing session, as well a clear association between higher serum BDNF and lower symptoms of depression at D2. Our results suggest a potential link between the observed antidepressant effects of ayahuasca and changes in serum BDNF, which contributes to the emerging view of using psychedelics as an antidepressant.