Long-term changes in gastrin, cholecystokinin and insulin in response to oxytocin treatment

The present study was designed to investigate how repeated injections of oxytocin influence plasma levels of vagally controlled hormones such as gastrin, cholecystokinin (CCK), insulin and somatostatin, as well as of endogenous oxytocin and glucose. Since oxytocin may enhance the activity of centrally located alpha2-adrenoreceptors, a second aim of this study was to explore whether these receptors are involved in the effects. For this purpose, oxytocin (1.0 mg/kg) or NaCl was given subcutaneously (s.c.) once a day during 5 days to male rats. Rats were decapitated 1, 3 and 10 days after the last injection, blood was collected and hormone levels were radioimmunoassayed. The oxytocin treatment caused an elevation of plasma levels of oxytocin 1 day (p < 0.05) but not 3 and 10 days after treatment. Gastrin levels were decreased on day 1, 3 and 10 (ANOVA; p < 0.01). In addition, both insulin and CCK levels were decreased in response to the oxytocin treatment when measured 3 and 10 days after the last injection (ANOVA; insulin p < 0.01, CCK p < 0.05). When the alpha2-adrenoreceptor agonist clonidine (2.5 microgram/kg intracerebroventricularly) was administered 3 days after the 5-day treatment period with oxytocin or saline, plasma levels of insulin and CCK increased significantly (p < 0.05) in the oxytocin-treated rats, when compared to saline-treated controls receiving clonidine only. No change in glucose or somatostatin levels was found in response to the oxytocin treatment. In conclusion, these results show that oxytocin induces long-lasting changes in plasma levels of gastrin, CCK and insulin, without affecting somatostatin or glucose levels. These effects may be mediated by changes in vagal nerve activity.