Nonphysical Contact Between Cagemates Alleviates the Social Isolation Syndrome in C57BL/6 Male Mice

In rodents, social isolation during the early postweaning phase induces several behavioral abnormalities, commonly referred to as the isolation syndrome. We attempted to identify the contribution of the selective deprivation of physical contact to the emergence of the isolation syndrome. To this end, we devised a pseudoisolated housing condition in which male 3-week-old C57BL/6 mice were caged in pairs, but were separated by a transparent perforated partition allowing only nonphysical contact, and compared it with the classical isolation procedure and standard laboratory group housing. Locomotor activity, acoustic startle reactivity, and amphetamine-induced hyperactivity were enhanced by isolation, but neither anxiety nor prepulse inhibition of the acoustic startle response was affected. Pseudoisolated mice were comparable to grouped controls in their acoustic startle response and locomotor reactivity to amphetamine, but they were as active as isolated animals in the predrug sessions of the open field. Furthermore, pseudoisolation also exerted its own unique effects, namely, anxiolysis. Our results demonstrated for the first time the relevance of nonphysical contact including its ability to undermine the emergence of the isolation syndrome in mice.