Contextual fear conditioning leads to hypo-innervation of the left ventricular myocardium in female but not male C57BL/6J mice

Objective: Contextual fear conditioning is an effective way of inducing and modelling anxiety-like states in rodents like mice. We examined the sex-dependent effects of contextual fear conditioning on the autonomic innervation of the left ventricular myocardium in female and male mice. Methods: Contextual fear conditioning was induced in male and female C57BL/6J mice and compared to male and female controls (n = 5 each). The experimental procedure consisted of two days of habituation, one day of training and one day of context fear memory retrieval. 24 h after context fear memory retrieval, hearts were harvested. PGP9.5-staining was performed to visualize autonomic innervation of left ventricular myocardium. Design-based stereology was used to quantify nerve fibre length density and total length at light microscopic and cardiomyocyte ultrastructure at electron microscopic level. Results: Contextual fear conditioning led to a hypo-innervation of the left ventricular myocardium in female but not male mice, implicating a sex-dependent autonomic response to and integration of stressful stimuli. Cardiomyocyte ultrastructure was not significantly affected by fear conditioning in both sexes. Conclusion: Contextual fear conditioning had a profound morphological effect on the innervation of the left ventricular myocardium of female but not male mice. This may have implications for cardiac function in female patients with anxiety disorder and requires further investigation.