The effect of overcrowding on anxiety and conditioned fear in rats

The effect of prolonged housing (from the 30th to the 150th postnatal day) in increased crowding (15–17 rats in a standard cage measuring 31 × 52 × 20 sm, 106–120 sm2 per rat) on anxiety behavior, as well as the acquisition and extinction of the conditioned fear in adult animals was studied. In half of the rats in early ontogenesis, activation of the immune system was induced by the administration of bacterial lipopolysaccharide on the 3rd and 5th postnatal days at a dose of 50 mcg/kg (LPS group). The other half of the rats were injected with saline solution (SAL group). Starting from the 90th day, the rats were tested in an open field, an elevated plus maze, and conditioned fear to sound was developed. Both males and females participated in the experiments. Testing revealed that the level of anxiety didn’t increase, but the locomotor activity decreased and displacement activity (grooming) increased in both males and females under the influence of crowding. During fear conditioning, all males kept in crowded conditions had a shorter freezing time compared to the control. However, only in males of the LPS group, the memory of the signal was disturbed under the influence of crowding, the differentiation of the context and the signal suffered, the extinction of the freezing reaction passed faster. Thus, males compared to females were more susceptible to the negative impact of crowding. Experienced early proinflammatory stress in the LPS group aggravated the effect of crowding.