Social interaction with rat exposed to constant light during lactation prevents depressive-like behavior induced by constant light in adulthood.

Circadian rhythm disruptions are often observed in depressed patients, and changes in the light/dark cycle promote depressive-like behavior in animal models. Prolonged exposure to constant light (LL) is known to lead to arrhythmicity of circadian locomotor activity and depressive-like behavior in rats. Interestingly, neonatal exposure to LL prevents both arrhythmicity and depressive behavior in adulthood. Arrhythmic rats under LL conditions that cohabitate with a rhythmic rat exhibit improvement in circadian rhythms. We tested whether such cohabitation also protects against LL-induced depressive-like behavior. Wistar rats were assigned to conditions of either neonatal constant light (neonatal-LL) on postnatal days 10-22 or a regular light/dark cycle (neonatal-LD). On day 45, the animals were assigned to three possible pair combinations. After a baseline sucrose preference test, half of the pairs were placed under LL conditions. Weekly sucrose preference tests were conducted to evaluate depressive-like behavior. The animals were isolated by an aluminum wall on the test day. At week 2 of LL, sucrose preference was reduced in neonatal-LD/neonatal-LD pairs of animals. At week 5, neonatal-LD/neonatal-LD pairs exhibited anhedonic-like behavior, but the pairs with at least one neonatal-LL rat did not. The LL cycle was returned to an LD cycle, and the neonatal-LD/neonatal-LD pairs exhibited a restoration of sucrose preference 2 weeks later. We conclude that social interaction can prevent depressive-like behavior induced by circadian rhythm disruption as long as one of the animals is more prone to present a strong rhythm.