Reduced Hippocampal Volume and Neurochemical Response to Adult Stress Exposure in a Female Mouse Model of Urogenital Hypersensitivity

Abstract

Early life stress exposure significantly increases the risk of developing chronic pain syndromes and comorbid mood and metabolic disorders later in life. Structural and functional changes within the hippocampus have been shown to contribute to many early life stress-related outcomes. We have previously reported that adult mice that underwent neonatal maternal separation (NMS) exhibit urogenital hypersensitivity, altered anxiety- and depression-like behaviors, increased adiposity, and decreased gene expression and neurogenesis in the hippocampus. Here, we are using magnetic resonance imaging and spectroscopy (MRI and MRS) to further investigate both NMS- and acute stress-induced changes in the hippocampus of female mice. Volumetric analysis of the whole brain revealed that the left hippocampus of NMS mice was 0.038 mm³ smaller compared to naïve mice. MRS was performed only on the right hippocampus and both total choline (tCho) and total N-acetylaspartate (tNAA) levels were significantly decreased due to NMS, particularly after WAS. Phosphoethanolamine (PE) levels were decreased in naïve mice after WAS, but not in NMS mice, and WAS increased ascorbate levels in both groups. The NMS mice showed a trend toward increased body weight and body fat percentage compared to naïve mice. A significant negative correlation was observed between body weight and phosphocreatine levels post-WAS in NMS mice, as well as a positive correlation between body weight and glutamine for NMS mice and a negative correlation for naïve mice. Together, these data suggest that NMS in mice reduces left hippocampal volume and may result in mitochondrial dysfunction and reduced neuronal integrity of the right hippocampus in adulthood. Hippocampal changes also appear to be related to whole body metabolic outcomes.

Keywords: early life stress; magnetic resonance imaging; magnetic resonance spectroscopy; obesity; pain.

Figure 1

Voxel Based Morphometry analysis revealed regions that were significantly different in size between NMS (n = 9) and naïve (n = 5) female mice. Maximum intensity projections in the sagittal (A), coronal (B), and horizontal (C) plane are represented on the Mortimer Space Atlas, where the blue lines represent bregma corresponding to the left hippocampus.

Figure 2

Magnetic Resonance Spectroscopy was performed on the right hippocampus prior to (baseline) and following water avoidance stress (Post-WAS) in naïve (n = 5) and NMS (n = 9) female mice. (A) Examples of the region of interest containing the right hippocampus for the collection of our MRS spectral data. From the resulting spectra (B) we measured absolute chemical concentrations. (C) A significant effect of NMS and an NMS/WAS interaction was observed on the level of total Choline (tCho). The post-WAS levels of tCho in NMS mice were significantly lower than in naïve mice. (D) A significant effect of NMS and an NMS/WAS interaction was observed on the combined level of N-acetylaspartate and N-acetylaspartylglutamate (tNAA). The level of tNAA in post-WAS NMS mice was significantly lower compared to both that in naïve mice and to their baseline measurements. (E) A significant effect of WAS and an NMS/WAS interaction was observed on phosphoethanolamine (PE). The level of PE in naïve mice, post-WAS, was significantly lower than in NMS mice and compared to their baseline measurements. (F) A significant effect of WAS was observed on the level of ascorbate. Brackets indicate a significant impact of NMS (§, §§p < 0.05, 0.01), WAS (ω, ωωp < 0.05, 0.01), or a NMS/WAS interaction effect (&p < 0.05) two-way RM ANOVA; *p < 0.05 vs. naïve, #p < 0.05 vs. baseline, Bonferroni posttest.

Figure 3

Body weight and percent body fat were measured prior to imaging and correlated with spectral contents after water avoidance stress exposure in naïve (n = 5) and NMS (n = 9) female mice. A non-significant increase in body weight (A) and percent body fat (B) was observed in female NMS mice compared to naïve mice. (C) A significant negative correlation was observed between hippocampal phosphocreatine (PCr) levels and body weight in NMS mice. (D) Significant negative and positive correlations were observed in naïve and NMS mice, respectively, between hippocampal glutamine levels and body weight. *, **p < 0.05, 0.01, Pearson correlation.

Figure 4

Visceromotor response (VMR) during urinary bladder distention (UBD) was measured seven months after water avoidance stress exposure and MR imaging in naïve and NMS female mice. An increase in VMR during UBD was observed in NMS mice (p = 0.0094) compared to naïve mice. Two-way ANOVA with Bonferroni posttest. Naïve, n = 5, NMS, n = 7.