Within versus between group designs, and not timing of onset of puberty, influence sex and age differences in intake of palatable food in rats

Developmental trajectory of social reward motivation from early adolescence into adulthood in female and male Long‐Evans rats

“…We predicted that we would see a shift in social motivation from pre‐ to postpuberty at different ages for females and males given the sex difference in the timing of the onset of puberty. We also predicted that male rats may show a peak in social motivation at P50 similar to that observed for the peak in motivation for palatable food reward (Friemel et al., 2010; Marshall et al., 2017; Murray & McCormick, 2022). We also predicted that adolescents would have greater motivation for a social reward than would adults.…”

“…We also predicted that male rats may show a peak in social motivation at P50 similar to that observed for the peak in motivation for palatable food reward (Friemel et al, 2010;Marshall et al, 2017;Murray & McCormick, 2022). We also predicted that adolescents would have greater motivation for a social reward than would adults.…”

“…There is some evidence of dynamic changes in behavior across adolescence. For example, sensitivity to palatable food rewards declined from early adolescence to mid adolescence in male and female rats (Murray & McCormick, 2022) and rose from mid adolescence to late adolescence in male rats (Friemel et al., 2010; Marshall et al., 2017; Murray & McCormick, 2022) but not in female rats (Marshall et al., 2017; Murray & McCormick, 2022). We investigated whether such shifts were evident for social reward using operant conditioning in two studies involving prepubertal early adolescent (P30), mid‐adolescent (P40), late adolescent (P50), and adult (P70) female and male Long Evans rats.…”

“…In most studies of food reward in rats, however, the developmental comparison has typically investigated one adolescent age group to one adult age group. Three exceptions are studies that have investigated the development of sensitivity to a palatable food reward, with evidence of a peak in consumption of palatable foods in postpubertal adolescent male rats (Friemel et al., 2010; Marshall et al., 2017; Murray & McCormick, 2022) that was not found in female rats (Marshall et al., 2017; Murray & McCormick, 2022), and that may depend on the type of design used (an adolescent male peak was evident in within‐group design and not in a between‐group design, Murray & McCormick, 2022).…”

“…As with studies on food reward, most existing research of adolescents has compared one adolescent age to one adult age. Given sex differences in the developmental trajectories (e.g., with female rats’ puberty onset occurring at ∼P33, and male rats at ∼P40; Murray & McCormick, 2022), age‐matching results in comparisons of postpubertal female rats to prepubertal male rats (Bell, 2018; Delevich et al., 2021). The rise in gonadal hormones with the onset of puberty that occurs during adolescence may lead to further sexual differentiation of the brain and thereby influence social behavior (reviewed in Bell, 2018; Brown et al., 2015).…”