In rodent models, the time frame encompassing adolescence has yet to reach consensus. Adolescence in the rodent has been defined using various factors indicative of developmental transitions in human adolescents. These factors include changes in behavioral patterns, hormonal patterns, and/or primary sexual characteristics. In a comprehensive review, Spear (2000) argues that adolescence cannot be defined based solely on the characteristics associated with puberty and sexual maturation, but rather, that adolescence is a period of ‘soft events’ that should be viewed as a time of transitions that cannot be clearly delineated. Using the appearance of growth spurt, pruning of excitatory synapses as well as unique behavioral transitions in the rat (e.g., increased peer interaction, play and exploratory behavior in the wild), Spear (2000) defined adolescence broadly from postnatal day (PND) 28 to PND 42, with the acknowledgement that in males some traits may appear as late as PND 55. Other reviews have suggested similar broad classifications defined by the appearance of mature hormonal cycling (PND 28–30) and the subsequent appearance of reproductive maturity which can occur as late as PND 60
