Examining mGlu5 Receptor Availability as a Predictor of Vulnerability to PTSD: An [18F]FPEB and PET Study in Male and Female Rats

Abstract

Background: Females are twice as likely to experience post-traumatic stress disorder (PTSD) than males, yet specific factors contributing to this greater risk are not fully understood. Our clinical and recent preclinical findings suggest a role for the metabotropic glutamate receptor 5 (mGlu5) in PTSD and differential involvement between males and females.

Methods: Here, we further investigate whether mGlu5 receptor availability may contribute to individual and sex differences in PTSD susceptibility by quantifying receptor availability using the mGlu5 receptor-specific radiotracer, [¹⁸F]FPEB, and positron emission tomography in male (n = 16) and female (n = 16) rats before and after traumatic footshock exposure (FE) and assessment of stress-enhanced fear learning (SEFL) susceptibility, as compared with no-shock controls (CON; n = 7 male; n = 8 female).

Results: Overall, FE rats displayed greater fear generalization as compared with CON (p < .001). Further, greater mGlu5 receptor availability at baseline (p = .003) and post-test (p = .005) was significantly associated with expression of the SEFL phenotype. Notably, FE female rats displayed a shift to more passive coping (ie, freezing), and displayed greater SEFL susceptibility (p = .01), and had lower baseline mGlu5 availability (p = .03) relative to their FE male rat counterparts.

Conclusion: Results are consistent with clinical findings of higher mGlu5 receptor availability in PTSD, and add to growing evidence implicating these receptors in the pathophysiology of PTSD and sex-differences in susceptibility for this disorder.

Keywords: metabotropic glutamate receptor 5; positron emission tomography; post-traumatic stress disorder; sex-differences; stress-enhanced fear learning.

Figure 1.

PET evaluation of mGlu5 receptor availability during SEFL. (A) Schematic of the experimental timeline, including PET with [¹⁸F]FPEB for in vivo quantification of mGlu5 receptor availability before (‘Baseline’) and after (‘Post-test’) completion the SEFL procedure. (B) Details of the 4-day SEFL procedure. Abbreviations: mGlu5, metabotropic glutamate receptor 5; PET, positron emission tomography; SEFL, stress-enhanced fear learning.

Figure 2.

Footshock exposure in females is associated with a shift to a more passive stress response. (A) The repeated measure analysis of freezing during the first 3 min of the day 2 test of context A fear memory relative to freezing in context B during the 3 min prior to the tone presentation during the day 4 SEFL test in n = 16 male (left panel) and n = 16 female (right panel) FE rats. Data for individual rats across sessions are linked. Post hoc (Bonferroni) p-values for the effect of context/time within each sex are provided. (B) The difference in freezing between the 2 contexts (B-A) was used to identify rats as either expressing the SEFL phenotype (SEFL(+): B-A $>$ 0), or those that did not (SEFL(−): B-A $\le$ 0). This resulted in the classification of n = 8 SEFL(+) and n = 8 SEFL(−) males and n = 12 SEFL(+) and n = 4 SEFL(−) females. Data are presented as the mean ± SD with the result of the independent sample t-test (p-value). Abbreviation: FE, footshock exposed; SD, standard deviation; SEFL, stress-enhanced fear learning.

Figure 3.

Prior footshock exposure is associated with sensitization and generalization of fear learning in male and female rats. Freezing behavior of FE (n = 16 male, n = 16 female) and no-shock control (CON, n = 7 male, n = 8 female) rats during the (A) 90 min footshock-exposure session in context A on day 1 and (B) the context A fear memory test on day 2. (C-E) Freezing during the day 3 cued fear condition session in context B, including freezing (C) before, (D) during, and (E) after the single tone/shock pairing. (F-G) Freezing during the day 4 SEFL test in context B including freezing (F) before, (G) during, and (H) after the single presentation of the tone in the absence of any footshocks. Data (mean ± SD) are presented as the time spent freezing as the percentage of the total observation time (%freezing). The main effect of treatment group (ie, CON vs FE) for each analysis is indicated by the p-value displayed on each graph. Abbreviation: CON, control; FE, footshock exposed; SEFL, stress-enhanced fear learning.

Figure 4.

MANOVA, assessing regional difference in mGlu5 receptor availability as estimated by binding potential (BP_ND) using [¹⁸F]FPEB and PET. (A) Baseline and (B) post-test analysis of mGlu5 receptor availability as a function of behavioral phenotype (SEFL(−) versus SEFL(+)) and sex (male vs female) in 4 brain ROI: AMY, HIP, PFC, and STR. Data are shown for n = 7 SEFL(−) and n = 8 SEFL(+) males and n = 3-4 SEFL(−) and n = 11–12 SEFL(+) females and presented as the mean ± SD. Differences (%) between SEFL(−) versus SEFL(+) are provided for each ROI. Results of regional univariate tests, main effect of behavioral phenotype: *p < .05, **p < .01. Abbreviations: AMY, amygdala; HIP, hippocampus; MANOVA, multivariate analysis of variance; mGlu5, metabotropic glutamate receptor 5; PET, positron emission tomography; PFC, prefrontal cortex; ROI, regions of interest; SEFL, stress-enhanced fear learning; STR, striatum.