Effect of Keeping Rats in Conditions of Increased Crowding on Anxiety and Conditioned Reflex Fear

The effects of long-term keeping (postnatal days 30–150) in conditions of elevated crowding (15–17 rats in a standard cage measuring 31 × 52 × 20 cm, 106–120 cm² per rat) on anxiety behavior and the acquisition and extinction of a classical conditioned defensive reflex were studied in adult rats. Activation of the immune system was induced in half the rats in early ontogenesis by administration of bacterial lipopolysaccharide on postnatal days 3 and 5 at a dose of 50 μg/kg (the LPS group), while the other half of the animals were given physiological saline (the PS group). Starting from day 90, rats were tested in the open field and the elevated plus maze and were trained to conditioned reflex freezing to sound. Both males and females were used in the experiments. Testing revealed no signs of increased anxiety levels in response to crowding, but did demonstrate a decrease in movement activity and an increase in displacement activity (grooming) in both males and females. During development of conditioned reflex fear to a sound, all males kept in crowded conditions had shorter freezing times than controls. However, only males in the LPS group showed disruption of the memory of the signal in response to crowding, with impairment to differentiation of context and signal and faster attenuation of the reflex. Thus, males were more susceptible than females to the adverse effects of crowding during development of conditioned reflex fear. Early proinflammatory stress experienced by rats aggravated the effects of crowding.