Dissociation of oxytocin effects on body weight in two variants of female Sprague-Dawley rats

As a counterpoint to studies that make a case for the use of oxytocin in short-term inhibition of food intake, the aim of the present study was to determine whether the peptide facilitates weight gain in female rats in a more long-term perspective. Two different variants of female Sprague-Dawley rats were used. The two variants differed in daily weight gain (0.38 g/day vs. 1.41 g/day during 28 days), and were designated slowly and rapidly growing rats (SGR and RGR) respectively. Oxytocin 1 mg/kg given s.c. to SGR for a five-day period increased weight gain significantly in comparison to a previous five-day period with NaCl-treatment (18.0 g/5 days versus 5.0 g/5 days; p < 0.01). In a separate study, oxytocin 1 mg/kg given for four days increased weight gain significantly in SGR versus saline-treated controls (7.5 g/4 days versus 1.6 g/4 days; p < 0.05). The weight-difference persisted six days later (p < 0.001), The weight increase of oxytocin occurred only during estrus (p < 0.05) and was not accompanied by any measurable increase in food intake. In RGR, oxytocin-treatment decreased food intake significantly (p < 0.001) and tended to decrease weight gain, although not significantly. The SGR and RGR also had different endocrine profiles with, for example, twice as high oxytocin (p < 0.01) and insulin levels (p < 0.01) in RGR compared to the SGR. These data suggest that oxytocin influences weight gain and food intake differently in the two variants of Sprague-Dawley rats, perhaps depending on factors such as endocrine profile and oxytocin sensitivity.