At weaning, male pups within a litter were equally distributed to experimental groups; male offspring from the same litter were uniformly assigned to single (1 rat/42 × 22 × 20 cm cage) or group (5 rats/52 × 28 × 20 cm cage) polycarbonate cages and remained in these conditions until behavioral evaluations were performed (postnatal day 70 or 73) or brains were harvested for quantification of cannabinoid receptor density and endocannabinoid content (postnatal day 70). Rearing manipulations occurred continuously throughout adolescence, a critical period of development (Einon and Morgan, 1977) and lasted 7 weeks (for review see Lapiz et al., 2003), as robust changes in behavior may require continuous isolation rearing (Bakshi and Geyer, 1999, Varty et al., 1999a). Cages were cleaned and refilled with sawdust bedding weekly for single-housed and biweekly for group-housed rats. Single housing prevented, whereas group housing permitted, cohort-induced tactile stimulation.
