The effects of excess salt consumption on contextual fear generalization in mice.

Excess salt (NaCl) consumption is prevalent worldwide. Most Americans consume more than twice the daily recommended amount of sodium (Na; 2300mg). Excessive salt intake is a risk factor for cardiovascular pathology such as hypertension. Anxiety is often comorbid with cardiovascular diseases. Research has been done to examine the relationship between salt consumption and cardiovascular and other related peripheral diseases. However, far less research has been done to examine the relationship between salt consumption and brain health or anxiety. To examine this, mice were used because their salt intake and stress exposure can be rigorously controlled, unlike in humans. Specifically, we focused on fear generalization, a key characteristic of anxiety symptoms involving contextually inappropriate fear of situations not currently present. We hypothesized that mice consuming a diet with excess salt would exhibit higher levels of fear generalization relative to mice consuming a low salt diet. Adult (9 wk old) male and female C57BL/6J mice were given either a low (0.4% NaCl, w/w) or high (4.0% NaCl) salt diet ad libitum. Mice underwent contextual fear training 2 or 6 wks after commencing diet manipulation. Testing for contextually appropriate (fear expression) and contextually inappropriate (fear generalization) fear behavior occurred either 2 days (2 and 6 wk groups) or 4 wks (6 wk group) after training. Regardless of diet condition, mice of both sexes acquired contextual fear similarly when trained at the 2 wk timepoint, and exhibited equivalent fear expression and contextual discrimination when tested 2 days later. Similarly, and as expected, male mice consuming a low salt diet expressed normal levels of fear generalization at 4 wks. Contrary to our hypothesis, however, male mice consuming a high salt diet did not exhibit fear generalization at 4 wks. Female tests of 4 wk fear generalization, and both sexes in the 6 wk group being tested after 2 days, are ongoing. Thus far, our findings indicate that male mice consuming a high salt diet do not generalize their contextual fear after a standard time delay (4 wks), or alternatively display enhanced contextual discrimination at the 4 wk timepoint post-training. Our findings suggest intake of salt, a non-caloric dietary component, can have a significant effect on a measure directly related to anxiety symptoms. Future experiments will evaluate the contribution of neuroinflammation to these behavioral changes, as well as investigate cued fear responses in both sexes.