Biological embedding of early trauma: the role of higher prefrontal synaptic strength

ABSTRACT

Background: Early trauma predicts poor psychological and physical health. Glutamatergic synaptic processes offer one avenue for understanding this relationship, given glutamate’s abundance and involvement in reward and stress sensitivity, emotion, and learning. Trauma-induced glutamatergic excitotoxicity may alter neuroplasticity and approach/avoidance tendencies, increasing risk for psychiatric disorders. Studies examine upstream or downstream effects instead of glutamatergic synaptic processes in vivo, limiting understanding of how trauma affects the brain.

Objective: In a pilot study using a previously published data set, we examine associations between early trauma and a proposed measure of synaptic strength in vivo in one of the largest human samples to undergo Carbon-13 (¹³C MRS) magnetic resonance spectroscopy. Participants were 18 healthy controls and 16 patients with PTSD (male and female).

Method: Energy per cycle (EPC), which represents the ratio of neuronal oxidative energy production to glutamate neurotransmitter cycling, was generated as a putative measure of glutamatergic synaptic strength.

Results: Results revealed that early trauma was positively correlated with EPC in individuals with PTSD, but not in healthy controls. Increased synaptic strength was associated with reduced behavioural inhibition, and EPC showed stronger associations between reward responsivity and early trauma for those with higher EPC.

Conclusion: In the largest known human sample to undergo ¹³C MRS, we show that early trauma is positively correlated with EPC, a direct measure of synaptic strength. Our study findings have implications for pharmacological treatments thought to impact synaptic plasticity, such as ketamine and psilocybin.

HIGHLIGHTS

• Abnormalities in the strength of synaptic connections have been implicated in trauma and trauma-related disorders but not directly examined.

• We used magnetic resonance spectroscopy to investigate the association between early trauma and an in vivo measure of synaptic strength.

• For people with posttraumatic stress disorder, as early trauma severity increased, synaptic strength increased, highlighting the potential for treatments thought to change synaptic connections in trauma-related disorders.

Antecedentes: El trauma temprano predice una mala salud psicológica y física. Los procesos sinápticos glutamatérgicos ofrecen una vía para entender esta relación, dada la abundancia de glutamato y su participación en la recompensa y sensibilidad al estrés, la emoción y el aprendizaje. La excitotoxicidad glutamatérgica inducida por el trauma puede alterar la neuroplasticidad y las tendencias de aproximación/evitación, aumentando el riesgo de trastornos psiquiátricos. Los estudios examinan los efectos pre y postsinápticos en lugar de los procesos sinápticos glutamatérgicos in vivo, limitando la comprensión de cómo el trauma afecta al cerebro.

Objetivo: En un estudio piloto utilizando un conjunto de datos publicado previamente, examinamos las asociaciones entre el trauma temprano y una medida propuesta de la fuerza sináptica in vivo en una de las muestras humanas más grandes sometidas a espectroscopía de resonancia magnética de Carbon-13 (13C MRS). Los participantes fueron 18 controles sanos y 16 pacientes con TEPT (hombres y mujeres).

Método: Energía por ciclo (EPC), que representa la relación entre la producción de energía oxidativa neuronal y el ciclo del neurotransmisor glutamato, se generó como una medida putativa de la fuerza sináptica glutamatérgica.

Resultados: Los resultados revelaron que el trauma temprano se correlacionó positivamente con EPC en individuos con TEPT, pero no en controles sanos. Una mayor fuerza sináptica se asoció con una reducción de la inhibición conductual, y la EPC mostró asociaciones más fuertes entre la capacidad de respuesta a las recompensas y el trauma temprano en aquellos con una EPC más alta.

Conclusión: En la muestra humana más grande conocida que se sometió a 13C MRS, mostramos que el trauma temprano se correlaciona positivamente con EPC, una medida directa de la fuerza sináptica. Los hallazgos de nuestro estudio tienen implicaciones para los tratamientos farmacológicos que se cree que afectan la plasticidad sináptica, como la ketamina y la psilocibina.

背景：早期创伤预示着心理和身体健康状况不佳。谷氨酸能突触加工为理解这种关系提供了一种途径，因为谷氨酸含量丰富且参与奖赏和压力敏感性、情绪和学习。 创伤引起的谷氨酸兴奋性毒性可能会改变神经可塑性和接近/回避倾向，增加精神障碍的风险。研究考查上游或下游效应，而不是体内谷氨酸能突触加工，限制了对创伤如何影响大脑的理解。

目的：在一项使用先前发表过的数据集的初步研究中，我们在接受碳 13 (13C MRS) 磁共振波谱分析的最大人类样本之一中考查了早期创伤与提出的体内突触强度测量之间的关联。参与者包括 18名健康对照者和 16 名 PTSD 患者（男性和女性）。

方法：周期能（EPC）代表神经元氧化能量产生与谷氨酸神经递质循环的比率，是作为谷氨酸突触强度的推测测量值而产生的。

结果：结果显示，在患有 PTSD 的个体中，早期创伤与 EPC 呈正相关，但在健康对照中则不然。 更强的突触强度与减少的行为抑制相关，对于具有较高 EPC 的人来说，EPC 显示奖赏反应性与早期创伤之间有更强的关联。

结论：在已知接受 13C MRS 的最大人类样本中，我们展示了早期创伤与作为突触强度直接测量的EPC呈正相关。我们的研究结果被认为对突触可塑性的药物治疗（例如氯胺酮和裸盖菇素）具有影响。

KEYWORDS:

• Synaptic plasticity
• glutamate
• magnetic resonance spectroscopy
• childhood maltreatment
• childhood trauma
• PTSD

PALABRAS CLAVE:

• Plasticidad sináptica
• glutamato
• espectroscopia de resonancia magnética
• maltrato infantil
• trauma infantil
• TEPT

关键词:

• 突触可塑性
• 谷氨酸
• 磁共振波谱分析
• 童年期虐待
• 童年期创伤
• PTSD

1. Introduction

Childhood trauma is prevalent, with a third of the population in the world estimated to have early trauma exposure (Krystal et al., 2017). This makes early trauma a significant public health concern, further magnified by the associated risk for maladaptation in psychiatric, behavioural, and biological areas of development. Particularly concerning is the greater susceptibility to psychopathology, with childhood adversity accounting for an estimated 29.8% of mood, anxiety, behaviour, and substance use disorders (Akiki & Abdallah, 2019; Averill et al., 2020; Krystal et al., 2017; Murrough et al., 2016). Sequelae are not limited to childhood. Early trauma can begin a toxic developmental cascade that continues throughout the lifespan into adulthood. Effects may persist 9–10 years after the trauma even in the ‘best-case-scenario’ according to one study that found brain and behavioural alterations in individuals for whom child abuse took place before 3 years of age, were unlikely to even remember it, and were adopted by high socioeconomic status parents (Lally et al., 2016). In addition to personal suffering, early trauma represents a significant societal burden, with a yearly estimated cost in the United States of USD $124 billion for childhood maltreatment alone (Newbury et al., 2018).

The theory of latent vulnerability suggests that long before psychiatric conditions emerge in adulthood, biological and neurocognitive systems calibrate to the adverse childhood environment in which early trauma is experienced (Abdallah et al., 2019; Wrocklage et al., 2017). Early experience molds brain structure and function through pruning of synaptic connections in the central nervous system, taking place at different rates for different brain areas (Akiki et al., 2017). Frequently activated synapses are selectively preserved, whereas rarely active connections will disappear (Akiki et al., 2017). Because this process occurs at the highest rate in early life (Akiki et al., 2018), children are likely to be particularly susceptible to the effects of early stress. The brain is thought to develop in a use-dependent fashion, so during traumatic experiences, the child’s brain may be in a constant state of fear-related/reward insensitive activation (Abdallah et al., 2019; Catlow et al., 2013). Use-dependence suggests that this state can result in enhanced or inhibited neural maturation for brain regions involved in psychological and emotional processes and endocrine and immune responses to stress (Jonassaint et al., 2007; Letourneau et al., 2018; Weathers et al., 2018). This altered development can lead to maladaptive persistence of these impaired approach/avoidance states, resulting in increased risk of later psychopathology.

Glutamate is among the most ubiquitous neurotransmitters and is involved in many processes in the brain, including reward sensitivity, emotion, stress responsivity, learning, and memory (McLaughlin et al., 2020; Wojtas et al., 2022). Animal studies have found that stress impairs glutamatergic synaptic signalling and its accessories, including reductions in brain-derived neurotrophic factor (BDNF) and downstream intracellular signalling (Hoffman, 1987). Aberrant glutamatergic functioning has been implicated in mood, anxiety, and trauma-related disorders, all of which are more common in those exposed to early trauma (Rosso et al., 2017; Rothman et al., 2019; Sibson et al., 1997; Stone et al., 2009; Wojtas et al., 2022). Trauma and stress are thought to activate glutamatergic circuits and cause glutamatergic ‘spillover’, which can result in adaptive plasticity that may transition into glutamatergic excitotoxicity once it surpasses a threshold (Wojtas et al., 2022) (Figure 1). Excitotoxicity reduces richness of synaptic connections, particularly in corticolimbic circuits e.g. hippocampus and prefrontal circuits (Cross et al., 2017; Hebb, 1949; Schür et al., 2016) that regulate stress responsiveness, memory, and emotions and have been found to be altered following early traumas (Abdallah et al., 2018; Rothman et al., 2011; Weathers et al., 2018). This in turn may undermine neuroplasticity and impair the processes underpinned by these neural regions, such as fear extinction (Foster et al., 1996; Hoffman, 1987; McCrory et al., 2017; Oquendo et al., 2003), with implications for psychiatric disorders.

Figure 1. The vicious cycle of trauma and stress. Adapted with permission from Averill et al. (2017).

Studies that examine stress and glutamate have used indirect measures of synaptic transmission, including functional connectivity, brain structure, neurochemical levels, or glutamatergic receptor binding potentials (Gogtay et al., 2004; Holmes et al., 2017; Miskolczi et al., 2019; Wojtas et al., 2022). For instance, we previously reported that occipital glutamine and glutamate/glutamine cycling were positively correlated with early trauma burden (Abdallah et al., 2017). However, most prior studies have not directly examined glutamatergic synaptic processes such as synaptic strength in vivo, instead examining upstream or downstream effects as a proxy measure (Gerin et al., 2019).

In the present pilot study, we use a novel neuroimaging estimate of glutamate neurotransmission of glutamatergic synaptic transmission to understand the relationship between early trauma and synaptic processes more directly (Abdallah et al., 2015; Maren & Holmes, 2016; Schmaal et al., 2017; Widom et al., 2015). In particular, we are interested in synaptic strength: the magnitude of the postsynaptic response to a presynaptic action potential (Averill et al., 2022; Gerin et al., 2019). We examined putative synaptic strength using dynamic ¹³C MRS, in which MRS is acquired during administration of ¹³C-labeled acetate to measure Energy per Cycle (EPC) (Averill et al., 2022), the only method that directly measures glutamate-glutamine cycling in the human brain (Abdallah et al., 2015). In this process, ¹³C-acetate is administered intravenously over 2 h during MRS acquisition. ¹³C is integrated into glutamine and glutamate and produces distinct signals on the ¹³C spectrum. ¹³C-acetate metabolites then enter the mitochondrial tricarboxylic (TCA) Krebs cycle and label glutamate through exchange with α-ketoglutarate. Subsequently, ¹³C glutamate is discharged into the synaptic cleft and taken up by astrocytes, which transforms the ¹³C glutamate to ¹³C-glutamine and transfers it to neurons. Thus, rate of the rate of ¹³C glutamate enrichment reflects oxidative energy production through the TCA cycle, and the rate of glutamine enrichment reflects glutamate-glutamine neurotransmitter cycling. EPC represents the ratio of these factors – rate of neuronal energy production by the rate of glutamate/glutamine cycling (V_TCAn/V_Cycle). This ratio corresponds with neuronal energy consumed per glutamate-glutamine cycle (Gerin et al., 2019).

We previously reported that PTSD was associated with reduced prefrontal EPC in the same sample (Gerin et al., 2019). We aim to extend this work by investigating whether early trauma is associated with reduced EPC. We examine this question in a sample of adults given that neurobiological consequences of trauma can be observed even decades after the original trauma with significant consequences for adult psychopathology (Park et al., 2015). In addition, we aim to extend our prior findings of a relationship between occipital glutamine and early trauma (Abdallah et al., 2017) using a more direct measure of putative synaptic strength in the prefrontal lobe. The prefrontal lobe is an area of significant interest that was not previously accessible to ¹³C MRS (Gerin et al., 2019) and has been related to both early trauma and psychopathology (Rothman et al., 2011). It is hypothesized that early trauma burden will be negatively correlated with EPC, and that this relationship will be observed regardless of psychiatric diagnosis (Abdallah et al., 2018; Murrough et al., 2016). A third exploratory aim is to examine whether putative synaptic strength mediates the relationship between early trauma and behavioural approach/avoidance using measures of behavioural inhibition systems and reward responsivity.

2. Methods

2.1. Sample

Our sample consisted of 16 patients with PTSD and 18 healthy controls between the ages of 18 and 65. Most participants were female (see Table 1) with a mean age of 36.8 (Standard Deviation [SD] = 13.0).

Table 1. Sociodemographic characteristics of the sample (N = 34).

	PTSD (n = 16)	Healthy control (n = 18)	t or Chi-squared	p Value
Female	10 (62%)	11 (61%)	0.01	.93
White	9 (56%)	11 (61%)	0.08	.77
Age (years)	39.5 (13.25)	34.3 (12.61)	−1.16	.25
Height (inches)	65.7 (1.0)	65.3 (0.8)	−0.301	.39
Weight (lbs)	169 (12)	148 (8)	−1.46	.08
PCL-5	45.4 (15.63)	1.8 (2.7)	−7.36	<.001*
QIDS	14.4 (4.9)	1.8 (2.3)	−9.73	<.001*
Unmedicated	7 (44%)	100% (18)	34.00	<.001*
Behavioural inhibition	23.2 (3.6)	17.3 (4.5)	−4.02	<.001*
Reward responsivity	15.1(2.8)	16.8 (2.4)	1.85	.08
Total ETI	14.1 (6.2)	3.2 (3.2)	−6.19	<.001*
EPC	2.2 (0.9)	3.0 (0.7)	−3.00	.003*

Note: ETI = self-report version of the early trauma inventory; PTSD = posttraumatic stress disorder; EPC = energy per cycle; PCL-5 = PTSD Checklist for DSM-5; QIDS = Quick Inventory of Depressive Symptomatology self-report.

Values are presented as weighted mean (SD) or n (%). *denotes statistically significant difference between groups as evaluated by two-sided test.

Given the high comorbidity between PTSD and depression, and our group’s focus on treatments for severe PTSD, all patients met criteria for a secondary diagnosis of major depressive disorder (Chang et al., 2008).

PTSD patients had (1) clinician-assessed PTSD diagnosis; (2) no changes in antidepressants for past 4 weeks and were not being treated with drugs that affect amino acid transporters; (3) absence of bipolar disorder, psychotic disorder, and substance use disorders; and (4) were not at high risk for suicide. Healthy controls had no history of psychiatric disorder. Exclusion criteria for the study included contraindication to magnetic resonance imaging (MRI), neurocognitive impairment diagnoses, traumatic brain injury (TBI), pregnancy or breast-feeding, or not using medically acceptable birth control method (i.e. oral, injectable or implant birth control; condom; diaphragm with spermicide; abstinence; partner with vasectomy; intrauterine device). It was also required that participants provide a negative urine toxicology test and negative pregnancy test for women. The study protocol was approved by the Yale Institutional Review Board. All individuals provided informed consent.

2.2. Dynamic Carbon-13 Magnetic Resonance Spectroscopy (¹³C MRS) acquisition and processing

MRS acquisition and processing methods have been described in detail in Gerin et al. (2019). Briefly, 120 min of MRS was obtained with a 4.0 T whole-body magnet interfaced to a Bruker AVANCE spectrometer (Bruker Instruments, Billerica, MA, U.S.A.), using custom coil and specialized shimming to target the prefrontal area based on the coil efficiency as shown in supplementary Figure S1 (Puetz et al., 2014). ¹³C MR spectra were acquired in a 6.5-minute blocks over 120 min, concurrent with [1 ¹³C]-acetate intravenous infusion. MR data processing was conducted while blinded to the behavioural data. Steady-state spectra, acquired after the first 70 min of [1 ¹³C]-acetate infusions, were averaged to measure the peak areas of glutamate C5 and glutamine C5 using a linear combination model approach of spectral basis functions (Gerin et al., 2019). The ¹³C-glutamate/¹³C-glutamine enrichment ratio was computed using peak areas of glutamate C5 and glutamine C5, (i.e. glutamate-C5/glutamine-C5 * f), where f is the ratio of glutamate/glutamine, measured by reference (Grassi-Oliveira et al., 2015). Then, EPC was calculated based on the relative ¹³C enrichment of glutamate over glutamine at steady state, as follows: V_TCAn/V_Cycle= [1 – (¹³C-glutamate/¹³C-glutamine)]/(¹³C-glutamate/¹³C-glutamine), where ¹³C-glutamine and ¹³C-glutamate represent the steady state ¹³C enrichments during the infusion of [1 ¹³C]-acetate (i.e. ∼ 70–120 min after starting infusion) (Widom et al. (2015)).

2.3. Psychiatric measures

The clinician administered PTSD scale for DSM-5 (Dannlowski et al., 2013) and the PTSD Checklist 5 PCL-5, (Averill et al., 2018) were used to assess PTSD diagnosis and severity. Index trauma could be any lifetime traumatic event. These were not the primary outcome measures but were considered as covariates. All participants completed the Early Trauma Inventory (ETI). The ETI comprises 56 items. Responses require selection of ‘yes’ or ‘no’ to each of the 56 items (e.g. were you ever slapped in the face with an open hand?; yes = 1, no = 0). Items are subsumed under four domains of childhood trauma that occur before the age of 18: general trauma (e.g. natural disaster, involvement in serious accident), physical abuse, emotional abuse, and sexual abuse (Sydnor et al., 2021). Items are summed to generate four domain scores. The measure also yields a total trauma score consisting of the total of those four domains.

The Quick Inventory of Depressive Symptomatology (QIDS) self-report was used to measure symptoms of depression. This 16-item measure has previously been shown to be highly correlated with other depression severity scales and has a Cronbach’s alpha of 0.69–0.89 (Shao et al., 2021).

2.4. Behavioural inhibition and approach systems (BIS/BAS)

The BIS/BAS Scales (O’Brien et al., 2021) consist of a 20-item questionnaire that yields four scales: BIS, BAS-Reward Responsiveness, BAS-Fun Seeking, and BAS-Drive. Items are rated on a 4-point Likert scale ranging from 1 (strongly agree) to 4 (strongly disagree). The BAS items are thought to represent sensitivity to appetitive stimuli, whereas the BIS items are thought to represent sensitivity to aversive stimuli.

2.5. Statistical analyses

Of the 34 participants in the study, one patient was missing data for an early trauma measure, and a second was missing data for an early trauma measure and BIS/BAS. Results of a Little’s Missing Completely at Random (MCAR) test suggested that data are likely to be missing at random (χ² = 3.9, df = 3, p = .3).

¹³C MRS was used to obtain a measure of glutamatergic synaptic strength via (1) the rate of neuronal oxidative energy production (V_TCAn) and (2) the rate of glutamate neurotransmission cycling (V_Cycle). The ratio of these two variables provided an Energy Per Cycle (EPC) measure, a putative measure of glutamatergic synaptic strength (Puetz et al., 2014). Supplementary Figure S2 depicts a histogram of EPC.

Due to a statistically significant difference in mean ETI for the PTSD (M = 14.1, SD = 6.2) and healthy control groups (M = 3.2, SD = 3.3) (t(31) = −6.53, p < .001), we limited the primary analysis to PTSD and studied the groups separately. The primary analysis was a non-parametric correlation analysis examining the relationship between EPC and early trauma severity in individuals with PTSD. Follow-up correlation analyses were performed to examine the role of trauma domains and to determine whether healthy controls show a relationship between EPC and early trauma. Correlations between EPC and trauma were compared between groups using Fisher r-to-z transformation. Post-hoc analyses were conducted to examine the moderating role of EPC in the relationship between early trauma, inhibition, and reward responsivity. Because BIS/BAS were non-disease specific measures and have been used in healthy samples (e.g. Pechtel et al., 2014) we combined healthy and PTSD groups. We used a Bonferroni corrected alpha = 0.01 (0.05/4 tests) for these post-hoc analyses.

3. Results

3.1. Descriptive statistics

78% (n = 14) of the healthy control group had a score of at least 1 on the ETI, with a mean score of 3.2 (SD = 3.3). In PTSD group, all participants had a score of at least 3 on the ETI.

Chi-squared and independent t-test revealed no statistically significant differences in sex, race, age, height, or weight between the two groups. As expected, ETI, QIDS, PCL, and EPC differed significantly between groups (see Table 1).

3.2. Relationship between synaptic strength and early trauma severity in individuals with PTSD

A correlation analysis performed in individuals who met criteria for PTSD showed that total ETI score was positively correlated with EPC, such that greater severity of early trauma was associated with greater EPC (Rho = 0.71, n = 15, p = .003; see Figure 2).

Figure 2. Graph illustrating energy per cycle as a function of total early trauma severity in PTSD patients. Line represents regression line, with 95% confidence interval.

Subsequent analyses were conducted to examine whether any subdomains of ETI were driving the relationship between ETI and EPC. General ETI and physical ETI were both associated with EPC in correlation analyses (Rho  = 0.68, n = 15, p = .005; and Rho = 0.52, n = 15, and p = .045, respectively), such that greater general and physical abuse were associated with greater EPC (see Figures 3(a,b) and 4). When general and physical abuse variables were controlled for in the analysis using a partial correlation, total trauma score was no longer significantly associated with EPC (r = 0.07, n = 11, df = 11, p = .83), suggesting that general and physical childhood abuse drive most of the relationship between EPC and ETI.

Figure 3. (a,b) Graph illustrating energy per cycle as a function of general childhood abuse severity (3a) and physical abuse severity (3b) in PTSD patients. Lines represent regression lines, with 95% confidence intervals.

Figure 4. Graph illustrating energy per cycle as a function of total early trauma severity healthy controls. Line represents regression line, with 95% confidence interval.

Age, medication status, PCL score, and sex were all considered as potential confounding variables to be included in the analyses. However, none were included in the final analyses, as they did not account for the relationship between ETI and EPC upon examination of the correlation matrix.

3.3. Relationship between synaptic strength and early trauma severity in healthy controls

Total ETI did not significantly correlate with EPC in healthy controls (Spearman’s Rho (Rho) = −0.07, n = 18, p = .79; see Figure 1). A Fisher r-to-z transformation was used to examine whether the relationship between ETI and EPC in healthy controls was significantly different to the relationship between these variables in individuals with PTSD. Results of the transformation showed that the correlation between total ETI and EPC was significantly stronger in participants with PTSD than participants without PTSD (z = 2.47, p = .01), demonstrating an interaction between early trauma severity and PTSD diagnosis.

3.4. Effects of trauma on inhibition/reward responsivity

Post-hoc nonparametric correlation analyses were conducted to examine whether baseline early trauma severity was associated with BIS and BAS (BAS-Reward Responsivity, BAS-Fun Seeking, and BAS-Drive). We found that there were statistically significant correlations between early trauma severity and BIS score (Rho = 0.49, n = 32, p = 0.004) and BAS-Reward Responsivity (Rho = −0.48, n = 32, p = .006). Relations of total ETI with BAS-Fun Seeking and Drive were negative but not statistically significant.

3.5. Correlation between EPC and inhibition/reward responsivity

Next, correlation analyses were conducted to examine the association between EPC and behavioural inhibition/reward responsivity. EPC was significantly correlated with behavioural inhibition (Rho = −0.57, n = 32, p < .001). This finding was not driven by PTSD diagnosis. When correlations between EPC and inhibition were compared for both groups, the correlation was negative and non-significant for both the PTSD group (Rho = −0.32, n = 14, p > .05) and the healthy group (Rho = −0.21, n = 18, p = 0 > .05), suggesting that PTSD symptoms do not drive the relationship between BIS and EPC. The relationship was such that higher BIS was associated with lower synaptic strength (Figure 5).

Figure 5. Graph illustrating the relationship between behavioural inhibition and energy per cycle for PTSD patients (blue) and healthy controls (red). Line represents regression line, with 95% confidence interval.

Correlations between BAS variables and EPC did not reach statistical significance, but there were trend level negative associations between EPC and BAS drive (Rho = −0.45, n = 14, p = .011). There were negative associations with BAS reward responsivity which were not statistically significant (Rho = −0.15, n = 14, p = .60), and positive associations with BAS fun in the PTSD group (Rho = 0.25, n = 14, p = .39).

3.6. EPC: moderator of the relation between early trauma severity and inhibition/reward responsivity?

We then examined whether the relationship between early trauma severity and inhibition/reward was moderated by EPC using a stepwise regression model. These analyses were performed with and without controlling for PCL score given that EPC was significantly correlated with both ETI baseline severity and PCL score (p < .01). There was a statistically significant interaction between EPC and ETI in predicting reward responsivity, regardless of whether PTSD symptoms were controlled for with the PCL (t(1,28) = −3.31, p = .003)) or not (t(1,30) = −2.61, p = .01). The relationship was such that increased ETI was associated with reduced reward responsivity, with the relationship stronger for those with higher synaptic strength. There were no other statistically significant interactions for each of the other BIS and BAS variables (p > .01), suggesting independent main negative effects of BIS on EPC. To examine possibly confounding effects of PTSD diagnosis, a Fisher’s r-to-z transformation showed no statistically significant difference in the BIS–EPC relationship between the PTSD and no PTSD group (p = .76).

4. Discussion

Contrary to our hypothesis, we found that increased early trauma severity was related to stronger synaptic connections. This unexpected finding may indicate a relationship between early trauma and prefrontal synaptic connectivity in individuals with stress and trauma-related disorders (vs. those without). Consistent with this interpretation, we found no associations between early trauma and synaptic strength in the healthy participants. Furthermore, we previously reported positive association between early trauma and both cortical glutamine levels and glutamate cycling in patients with major depressive disorder (Abdallah et al., 2017). This data suggests a negative association between early trauma and synaptic strength in depression, considering that glutamate cycling is the denominator in the EPC. Together, these findings raise questions of whether the early trauma associations with synaptic connectivity are specific to individuals with current trauma or stress-related psychopathology vs. those without, or extend to other psychiatric groups. These questions remain to be investigated in future studies.

Accumulating evidence underscore the broad overlap between PTSD and depression pathophysiology, including (a) preclinical data implicating synaptic loss as common pathology (Hoffman, 1987), (b) both are stress-related and have high comorbidity in humans (Reilly et al., 2015), (c) neuroimaging data demonstrating gray matter deficits in both disorders (Bovin et al., 2016), and (d) traditional antidepressants have shown efficacy in both PTSD and depression (Abdallah & Mason, 2021; Krystal et al., 2010). However, functional connectivity data has been increasingly distinguishing between PTSD and depression network alterations. For example, while gray matter deficit is reported in both disorders (e.g. cortical thinning (Faye et al., 2018; Sheridan & McLaughlin, 2014) and smaller hippocampus (Kessler et al., 2010; Seckl & Meaney, 2004)), prefrontal connectivity was negatively associated with depression (Heim & Nemeroff, 2001) but positively correlated with PTSD symptoms of reexperiencing and arousal (Hilton et al., 2006; Nichter et al., 2019). Similarly, depression is believed to be related to high default mode network connectivity (Daftary et al., 2009). In contrast, PTSD was repeatedly associated with reduced default mode network connectivity (Bermudo-Soriano et al., 2012). Future studies could investigate the role of synaptic connectivity as measured by EPC in PTSD-specific network disturbances and whether these alterations are affected by early trauma.

Our data cannot determine the mechanisms underlying the association between early trauma and synaptic connectivity. However, to guide future research, we here speculate on possible mechanisms involved. One such mechanism may be increased synaptic strength in the PFC to compensate for deficits elsewhere. It may be that early trauma severity results in early over-strengthening of synapses to increase learning in the early adverse environment (Lebon et al., 2002). Specifically, increased glutamate neurotransmission may result in increased long-term potentiation and stronger, more intractable connections (Aleksandrova & Phillips, 2021). This theory is supported by findings that stress can enhance synaptic strength in the nucleus accumbens, ventral tegmental area (VTA), and ventral hippocampus in animal studies (Averill et al., 2017; Averill et al., 2017; Rothbaum et al., 2014). In addition, post-hoc analyses revealed that general trauma and childhood physical abuse were individually associated with EPC and ETI. This suggestion is supported by work suggesting that glutamate concentration shortly after trauma (and thus less likely to reflect the long-term changes of chronic PTSD on the brain), correlated positively with PTSD symptoms (Li et al., 2014). This increase may then be followed by reductions resulting from the toxic effects of psychopathology or subsequent trauma that then reduces synaptic strength over time (Letourneau et al., 2018) (Figure 6). Individuals with higher early trauma severity may have the initial buffer of increased synaptic strength that compensates for this reduction, resulting in higher net strength among those with higher ETI compared to those with lower ETI.

Figure 6. Proposed mechanisms of relationship between synaptic strength and early trauma (6a), late trauma only (6b), and healthy development with no trauma exposure (6c). It may be that early trauma results in early over-strengthening of synapses to increase learning in the early adverse environment (Lebon et al., 2002). This may then be followed by reductions resulting from the toxic effects of psychopathology or subsequent trauma that then reduces synaptic strength over time (Letourneau et al., 2018). Individuals with early trauma may have the initial buffer of increased synaptic strength that compensates for this reduction, resulting in higher net strength among those with higher ETI compared to those with lower ETI. Note: ^ = increased synaptic strength, with these synapses most likely to survive.

Our finding of increased synaptic strength with high early trauma severity was limited to the patient population. Overall, the healthy group had higher synaptic strength than the PTSD group. Healthy controls had a negative association between increased early trauma and synaptic strength, such that higher early trauma severity predicted reduced synaptic strength, though this relationship was not statistically significant. One explanation for a null finding in the healthy group is that variance in trauma severity in the healthy population was limited due to our sample size, necessitating future larger studies with healthy controls. Further, in future studies, non-PTSD participants need to be significantly trauma-exposed to better test the questions asked herein. Though most participants had indicated exposure to one or more of the traumatic events, there were floor effects in healthy controls, with far greater trauma exposure in PTSD patients. Alternatively, our findings may be specific to stress- and trauma-related disorders. Future work should investigate whether our findings of increased EPC with increasing trauma exposure extend to other psychiatric stress-related conditions that have been differentially associated with synaptic density (Pitman et al., 2012).

Glutamatergic synaptic disturbances, regardless of their direction, have important implications given the neurotransmitter’s critical roles in overall brain function, metabolism (Letourneau et al., 2018), as well as emotional and cognitive processes (Abdallah et al., 2018; Harvey & Shahid, 2012; Holmes et al., 2019; Littlefield et al., 2010; Logue et al., 2021; Teicher & Samson, 2013). Although our findings do not allow us to infer a network effect, glutamatergic synaptic abnormalities may translate to brain-wide network disturbances based on the neurotransmitter’s cardinal role across the brain (Teicher & Samson, 2013). Depending on sensitive period when the trauma occurred, different patterns of network disturbance may be observed with differing implications for psychopathology (Abdallah et al., 2017; Lebon et al., 2002). Future work should examine effects of trauma onset and chronicity at trauma on the relationship between EPC and trauma (Lebon et al., 2002).

Our study was novel in examining glutamatergic synaptic strength in the PFC in vivo. The PFC has an important role in fear and reward learning. Impaired plasticity in the PFC mediated by increased synaptic strength may result impair emotion regulation, result in failures of the PFC to regulate impulsive and risky decision making (Fortress et al., 2018), and failures to approach prosocial rewards in favour of more stimulating risk taking behaviours (Scott et al., 2010). Contributing to our understanding of these behaviours, our study also provided novel findings of the association between PFC synaptic strength and self-reported tendencies of approach/avoidance of appetitive and aversive stimuli (BIS/BAS). The relationship between early trauma severity and reward responsiveness was moderated by synaptic strength such that the relationship between more severe early trauma and reduced reward responsivity was magnified in the context of high synaptic strength. Higher synaptic strength was associated with reduced behavioural inhibition. One possible explanation of this finding is that lower EPC coupled with the higher BIS in the PTSD versus the control group may drive the relationship between EPC and inhibition in the pooled PTSD and non-PTSD sample. However, results persisted even controlling for PTSD severity. In addition, the correlations between BIS and EPC were largely comparable in the PTSD and healthy groups, with no statistically significant difference between correlations for the two groups. This supports a relationship between EPC and behavioural inhibition independently of PTSD symptoms, though this finding needs to be replicated in research using larger samples.

The present findings have clinical implications. Synaptic strength may represent one neurobiological variable that differs in individuals with PTSD and early trauma, providing an avenue for future research to examine whether targeting this variable is associated with symptom improvement. For instance, both ketamine and psilocybin are associated with normalization of synaptic plasticity in animal studies and to alter fear extinction. For example, findings suggest (Pignatelli et al., 2021) that ketamine ameliorated the negative impacts of social isolation stress on synaptic plasticity in male rats. In addition, others (Sanacora et al., 2012) found that ketamine facilitated extinction of avoidance behaviour in rats, and this was mediated by normalized hippocampal synaptic plasticity. Similarly, psilocybin has been found to modulate glutamatergic neurotransmission (Hesselgrave et al., 2021) and synaptic remodelling in brain regions such as the PFC implicated in stress-related disorders (Bremner et al., 2000; McCrory & Viding, 2015; Mulders et al., 2015) and enhance fear extinction in mice (Carver & White, 1994). Researchers (Harnett et al., 2017) have found an increase in synaptic event frequency in vivo in rodents was associated with reduced synaptic α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor (AMPAR)-mediated excitatory synaptic current amplitude following ketamine injection. They inferred that this may be the result of a compensatory downregulation of synaptic strength due to an increase in synaptic number, or the addition of new, immature synapses. Thus, ketamine and psilocybin may promote new learning via additional synaptic connections and facilitate the extinction of old learning with additional connections, which may be most beneficial for individuals exposed to early trauma with increased synaptic strength (Harnett et al., 2017). In support of this position, previous work found that a history of childhood physical abuse predicted better treatment outcomes for depression in response to ketamine compared to individuals without history of childhood physical abuse (Abdallah et al., 2017). Future work should attempt to replicate these findings.

These implications should be judged in context of methodological limitations of the study. Due to the pilot nature of this work, there is a need for replication in future studies before we can draw firm conclusions. Though this is the largest human sample using this imaging method, the sample size was too small to adequately address the question of whether increased synaptic strength would also be associated with early trauma in healthy controls For instance, it is possible that higher variance of early trauma in healthy individuals may demonstrate a significant correlation between ETI and EPC comparable to that found in PTSD. Unfortunately, our current sample cannot rule out this possibility. This limitation should be addressed in future studies, particularly those comparing three groups (i.e. a healthy control group with early trauma, a healthy control group without early trauma and a PTSD group) to rule out possible effects of diagnosis. Our study also did not permit us to examine other psychiatric samples. Consequently, our findings may be specific to stress- and trauma-related psychiatric disorders, and do not necessarily translate to other psychiatric disorders Future studies should examine this question in larger samples using ¹³C MRS imaging. Furthermore, as participants with PTSD had secondary diagnoses of comorbid MDD, limiting our conclusions as to whether our findings are indicative of individuals with PTSD or rather about individuals with PTSD and a comorbid MDD. Moreover, the study is correlational, so we cannot determine a causal relationship between early trauma and synaptic plasticity, nor between synaptic plasticity and PTSD. In addition, the present study did not include a measure of childhood neglect. Dimensional theories of adversity exposure suggests that threatening early trauma experiences such as physical or sexual abuse may exert different neurocognitive effects than deprivation experiences such as neglect (Campioni et al., 2009). Specifically, deprivation is thought to cause brain structures to adapt to low complexity environments, for instance conferring reduced association cortex volume, which may be mediated by reduced synaptic strength (Campioni et al., 2009). Therefore, future studies should compare effects of deprivation and threat types of early trauma. Finally, the study used a retrospective measure of early trauma, though notably retrospective trauma is a stronger predictor of psychopathology compared to prospectively measures trauma (Popoli et al., 2012).

These limitations notwithstanding, the present pilot study suggests that early trauma is positively correlated with EPC, a direct measure of synaptic strength. Our study represents the largest known human sample to undergo ¹³C MRS. This method is innovative, novel, and obtains the closest measure of synaptic strength in vivo. In addition, our sample was a relatively severe group with close to chronic PTSD and comorbidities, a population with a high need for treatment that is often treatment-resistant, has worse functional impairment, greater likelihood of suicidal behaviour, and more complex presentation (Miller et al., 2014). Further research is needed to elucidate how the relationship between early trauma and synaptic strength impacts psychiatric symptoms; investigate the impact of chronicity and onset of early trauma and neglect; examine the impact of pharmacological treatments that target synaptic plasticity; and replicate our findings in a larger sample.

Acknowledgements

The authors would like to thank the individuals who participated in this study for their invaluable contribution. GFM acknowledges support from NIAAA grants R01 AA021984, R21 AA028628, and R01 DK108283.

Disclosure statement

CGA has served as a consultant and/or on advisory boards for Aptinyx, Genentech, Janssen, Psilocybin Labs, Lundbeck, Guidepoint, and FSV7, and as editor of Chronic Stress for Sage Publications, Inc. He also filed a patent for using mTORC1 inhibitors to augment the effects of antidepressants (Aug 20, 2018). LAA has served as a consultant, speaker and/or advisory board member for Guidepoint, Transcend Therapeutics, Source Research Foundation, Reason for Hope, Beond and Ampelis. GFM lists patent application U.S. patent number 10,770,276 for techniques of mass spectrometry for isotopomer analysis and related systems and methods. GFM has received consulting payments above $10,000 from Merck & Co. and less than $10,000 from Sumitomo Dainippon Pharma Co., and UCB Pharma.

Data availability statement

The data that support these findings will be made available at https://nda.nih.gov/.

Additional information

Funding

Funding and research support were provided by NIMH (R01MH112668), NIAAA (R01AA021984), the VA National Centre for PTSD, Brain & Behaviour Foundation (BBRF; formerly NARSAD), Clinical Neuroscience Research Unit (CNRU) at Connecticut Mental Health Centre, Yale Centre for Clinical Investigation (YCCI UL1 RR024139), an NIH Clinical and Translational Science Award (CTSA) and the Beth K and Stuart Yudofsky Chair in the Neuropsychiatry of Military Post Traumatic Stress Syndrome. LAA receives salary support from the VA Clinical Sciences Research & Development (IK2-CX0001873) and the American Foundation for Suicide Prevention (YIG-0-004-16). The content is solely the responsibility of the authors and does not necessarily represent the official views of the sponsors, the Department of Veterans Affairs, NIH, or the U.S. Government. GFM was supported by NIH grants AA021984 and R01 DK108283.