BDNF genotype Val66Met interacts with acute plasma BDNF levels to predict fear extinction and recall
Introduction
Brain derived neurotrophic factor (BDNF) is an abundant neurotrophin that plays a critical role in long-term potentiation and synaptic plasticity underlying learning and memory (Minichiello, 2009). BDNF and its high affinity receptor Tropomyosin-related Kinase B (TrkB) are located throughout the central nervous system, including key areas involved in memory such as the hippocampus, basolateral amygdala, and prefrontal cortex (Bredy et al., 2007; Chhatwal, Stanek-Rattiner, Davis, & Ressler, 2006; Lubin, Roth, & Sweatt, 2008; Rattiner, Davis, French, & Ressler, 2004). BDNF is released as a synaptic messenger seconds to minutes following commencement of activity (Brigadski, Hartmann, & Lessmann, 2005; Hartmann, Heumann, & Lessmann, 2001; Sasi, Vignoli, Canossa, & Blum, 2017) and indirectly activates NMDA receptors through several pathways, including TrkB signalling. This results in increased dendritic growth and other aspects of synaptic plasticity which underlie long-term memory (Kuczewski, Porcher, & Gaiarsa, 2010; Park & Poo, 2013).
In some cases, the experience of a traumatic event leads to Posttraumatic Stress Disorder (PTSD) which is characterised by distressing symptoms such as hyperarousal, avoidance behaviours, and flashback memories (American Psychiatric Association, 2013). Fear extinction is a specific form of learning and memory consolidation process where aversive memories are overridden by safety learning and is believed to be impaired in PTSD (Bouton, 2004; Zuj & Norrholm, 2019). Fear extinction paradigms are developed in such a way that they parallel the learning processes involved in exposure therapy, the key psychological therapy for PTSD (Graham, Callaghan, & Richardson, 2014; Milad & Quirk, 2012; Zuj & Norrholm, 2019). Fear extinction paradigms have the ability to succinctly translate preclinical models to human participants and are thus critical to understanding the neurobiology of PTSD in the context of improving exposure outcomes (Gogos, Ney, Seymour, Van Rheenen, & Felmingam, 2019; Li & Graham, 2017; Lonsdorf & Kalisch, 2011; Milad & Quirk, 2012; Milton & Holmes, 2019; Ney, Akhurst, et al., 2021; Ney, Matthews, Bruno, & Felmingham, 2018). Since the processes involved in memory consolidation play such an important role in PTSD aetiology, a role for BDNF in the disorder has also begun to be explored in fear extinction.
The role of BDNF in fear extinction in humans has been researched largely through the measurement of a single nucleotide polymorphism of rs6265 on the human BDNF gene (Val66Met), which mediates BDNF trafficking and release (Egan et al., 2003). Some studies have reported that mutation of this polymorphism by substitution of a Val allele with a Met allele is associated with higher fear acquisition and impaired fear extinction learning in healthy humans (Hajcak et al., 2009; Lonsdorf et al., 2010; Mühlberger et al., 2014; Soliman et al., 2010). Other studies of healthy participants, however, did not find significant associations between physiological measures of fear extinction and presence of the Met allele, but did report reduced prefrontal cortex and increased amygdala activation using fMRI in these participants relative to Val homozygotes (Lonsdorf et al., 2014). Studies of the Val66Met polymorphism in PTSD have revealed that higher PTSD prevalence is associated patients who carry at least one Met allele (Bruenig et al., 2016; Notaras, Hill, & van den Buuse, 2015; Pitts et al., 2019), that impaired extinction learning is evident in patients that are Met carriers (Felmingham et al., 2018), and that faster treatment response is associated with participants who are Val homozygotes (Felmingham, Dobson-Stone, Schofield, Quirk, & Bryant, 2013).
Meta-analysis shows that, on average, blood BDNF levels are higher in PTSD patients compared to healthy controls (Mojtabavi, Saghazadeh, van den Heuvel, Bucker, & Rezaei, 2020). However, this meta-analysis shows that plasma and sandwich ELISA assays are far better at discriminating group differences in PTSD subjects compared to serum and standard ELISA assays. BDNF crosses the blood-brain barrier (Pan, Banks, Fasold, Bluth, & Kastin, 1998), however serum BDNF levels are 20–50 times higher than in plasma due to the ability of platelets to synthesise and store BDNF (Chacon-Fernandez et al., 2016; Fujimura et al., 2002; Gejl et al., 2019). Serum BDNF has therefore been proposed to largely reflect changes occurring in peripheral tissue instead of the central nervous system and detecting subtle group differences may be difficult (Chacon-Fernandez et al., 2016). Conversely, it has been reported that 70–80 % of plasma BDNF is released from the brain (Rasmussen et al., 2009), is more closely associated with psychopathology compared to serum (Fernandes et al., 2015), and is highly correlated with cerebral spinal fluid BDNF (Pillai et al., 2010). Plasma BDNF shows diurnal variability and is correlated with salivary cortisol throughout the day (Begliuomini et al., 2008; Cain et al., 2017). It appears to be correlated with cognitive performance (Ney, Felmingham, Nichols, & Matthews, 2020; Sungkarat, Boripuntakul, Kumfu, Lord, & Chattipakorn, 2018) and shows exercise-responsivity, similarly to serum BDNF (Dinoff, Herrmann, Swardfager, & Lanctot, 2017).
To our knowledge, only one study has assessed the relationship between the Val66Met polymorphism and fear extinction recall (Keyan & Bryant, 2019) and none have tested blood BDNF levels during any extinction task. In Keyan and Bryant (2019) (Keyan & Bryant, 2019), participants were allocated to exercise or rest conditions following extinction learning and had extinction recall measured 24 h later. Exercise was associated with higher fear at recall but only in Met carriers, suggesting that exercise may only confer a benefit towards fear extinction recall in Val homozygotes. In the current study, we exposed healthy participants to a 2-day fear conditioning, extinction, and extinction recall paradigm. Genetic samples were genotyped for the Val66Met polymorphism and a smaller group of participants had blood samples assayed for BDNF level on day 1. We hypothesised that lower BDNF plasma levels and presence of the Met allele would be associated with poorer extinction learning. There is limited data concerning the effect of BDNF on extinction recall; however, given the close relationship between extinction learning and subsequent recall, we hypothesised that lower BDNF levels and the Met allele would again be associated with higher fear as indexed by skin conductance responses during recall.
Section snippets
Participants
Participants (N = 246) aged 18–58 (141 female) were recruited to this study from the University of Tasmania, University of Melbourne, and surrounding communities. Participants were screened by a basic, self-report medical questionnaire where they described medical and psychiatric history and were excluded on the basis of current pregnancy or lactation, previous head injury, current or historical neurological, cardiac, physiological, or psychiatric conditions, epilepsy, alcohol dependence, use
Materials and procedure
Experimental sessions were completed between 11am and 6pm (to control for circadian effects) over two days, with 48h between sessions. Sessions were conducted either at the University of Tasmania or University of Melbourne. Participants were instructed to avoid consuming food, caffeine, or nicotine for at least an hour before beginning the experiment, but to drink at least four glasses of water prior to beginning the experiment. They first provided a saliva sample via passive drool into an
Demographics and clinical data
Demographic data are reported and compared between groups in Table 1. There were no significant differences between Val66Met genotypes in terms of gender or age. For those who provided blood samples, there was a trend (t(54) = 1.81, p = .076, Cohen's d = 0.49) towards higher BDNF plasma levels in the Val/Val homozygotes (M = 0.28 ng/mL, SD = 0.13) compared to Met carriers (M = 0.22 ng/mL, SD = 0.12). The sample was largely of Caucasian background (n = 131, 68.6 %), with 59 (30.9 %) participants
Discussion
In the current study, we aimed to determine the role of the BDNF Val66Met polymorphism and acute BDNF plasma levels on fear extinction and extinction recall in a sample of healthy participants. We had hypothesised that Met carriers would have poorer fear extinction learning and extinction recall compared to Val homozygotes. Met carriers showed higher SCRs to the CS + compared to the CS- during extinction learning, which supported our hypothesis and was in the direction reported by previous
Author Credit Statement
AM, LJN, RB, DZ, and KF conceptualised the study. BH, BG, and KF contributed to the design of the work. CMH, EN, DZ, and LJN acquired the data. TS, DN, and LJN interpreted and analysed the data.
LJN drafted the manuscript and all authors revised it critically for important intellectual content.
All authors approved the final version of the manuscript for submission and agreed to be accountable for all aspects of the work in ensuring that questions related to the accuracy or integrity of any part
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This work was supported by and an NHMRC Program grant to KF (APP1073041). We would like to thank Prof Rick Richardson for his generous feedback in preparing our manuscript. We would also like to thank Hannah Robert-Tissot and Claudia Payne who assisted in participant recruitment during the study.
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2022, Behaviour Research and TherapyCitation Excerpt :Multiple other paradigm manipulations allow examining of avoidance behaviours, fear generalisation, and other conditioning phenomena that are reviewed extensively elsewhere (Bouton, 2004; Craske et al., 2014; Dunsmoor, Niv, Daw, & Phelps, 2015; Dymond, Dunsmoor, Vervliet, Roche, & Hermans, 2015; Lipp, Waters, Luck, Ryan, & Craske, 2020; Lonsdorf et al., 2017; Vervliet & Boddez, 2020; Vervliet et al., 2013). Using this paradigm, it is possible to study the association between biological, genetic, affective and cognitive indices and fear extinction performance (Blechert, Michael, Vriends, Margraf, & Wilhelm, 2007; Cameron, Schlund, & Dymond, 2015; Fullana et al., 2016; Lipp et al., 2020; Luck, Patterson, & Lipp, 2020; Ney, Matthews, Hsu, et al., 2021; Ney, Vicario, Nitsche, & Felmingham, 2021; Schenker et al., 2021), as well as the effect of different interventions on fear extinction (Antov, Wolk, & Stockhorst, 2013; Bouton, Vurbic, & Woods, 2008; Graham & Milad, 2013; Kuriyama, Honma, Soshi, Fujii, & Kim, 2011; Merz, Hermann, Stark, & Wolf, 2014; Rabinak et al., 2013; Schiller et al., 2010). Modern conditioning experiments are limited by a number of issues.