Gladys Shaw, Gretchen Neigh
Background: Persistent high levels of stress have been linked to systemic inflammation and neuroplasticity in a sex-dependent manner. Chronic stress beginning in adolescence can disrupt neural mitochondrial function —an organelle that modulates hormone synthesis and neurotransmitter release at the presynaptic terminal. Because mitochondrial function and neurotransmission may co-associate as risk factors for neurocognitive disorders later in life, this study used transmission electron microscopy (TEM) to assess the interaction between sex, chronic stress, mitochondrial function, and changes in synaptic vesicle presence.
Methods: Male and female C57Bl/6 mice were split between chronic repeated predation stress (CRPS) for 15 days during adolescence and adulthood, or daily handling (NS). For 8 weeks following stress, mice were injected with saline (S) or 7.5×105 EU/kg of lipopolysaccharide (LPS) every third day. Four weeks later, tissue was collected, and synaptosomes were isolated and imaged for vesicle presence (male-NS-S n=7; male-NS-LPS n=9; male-CRPS-S n=11; male-CRPS-LPS n=10; female-NS S n=12; female-NS-LPS n=11; female-CRPS-S n=8; female-CRPS-LPS n=10). TEM images were counted manually by a blind counter and analyzed for group differences.
Results: A 3-way ANOVA revealed a stress-sex interaction (p=0.0347) and sex-treatment interaction (p=0.0119). Within males, a posthoc 2-way ANOVA, with the factors of stress and treatment, shows a trend towards a significant decrease in number of vesicles per pre-synaptic body in males with a history of stress (p=0.0572). Within females, a posthoc 2-way ANOVA, with the factors of stress and treatment, shows a main effect of treatment with LPS treated females displaying an increased number of vesicles per synaptic body (p=0.0066).
Conclusion: Data suggest sex and stress-dependent changes in the average number of vesicles per pre-synaptic body in C57Bl/6 mice. In males, there is a trend towards a significant effect of stress, suggesting a potential long-lasting decrease in neurotransmitter release following CRPS. In females, there is a significant treatment difference, suggesting increased neurotransmitter release following a chronic inflammatory challenge. Paired with behavioral and mitochondrial data from these same animals in a past study, these data provide evidence that males and females regulate identical environmental challenges using varying methods of synaptic transmission.