Up-regulation of Oxytocin Receptor Gene and Protein in the Sheep Anterior Pituitary by a Dopamine Derivative (Salsolinol)

Specific oxytocin receptors (OTR) have been identified in the anterior pituitary (AP), and their expression has been shown to change in relation to the animal physiological stage, whereas salsolinol (a derivative of dopamine) has been shown to stimulate the synthesis and release of oxytocin (OT) in lactating sheep. In the present study, the expression of both OTR mRNA and OTR protein in the AP were examined by real-time quantitative PCR and enzyme-linked immunosorbent assay, either in anestrous or lactating sheep 48 h after weaning lambs. Moreover, the effect of salsolinol administered via an intracerebroventricular (i.c.v.) infusion was tested in additional sheep at the same physiological stages. The i.c.v. infusions of Ringer-Locke (control) and salsolinol solutions were carried out from 10:00 to 15:00 h in a serial manner, i.e. five 30-min infusions at 30-min intervals. We observed both OTR gene and OTR protein expression in the AP, in both anestrous and lactating sheep, but it was significantly (P < 0.01 and P < 0.05, respectively) higher in the AP of lactating animals compared to anestrous animals. Salsolinol i.c.v. treatment in anestrous sheep evoked a significant (P < 0.05) increase in both OTR gene and OTR protein expression compared to control animals. In contrast, salsolinol did not affect either OTR gene or OTR protein expression in lactating sheep. In conclusion, the expression of OTR in the sheep AP is upregulated by salsolinol. The effect of salsolinol was more pronounced in non-lactating sheep, with a reduced response due to ongoing OTR expression in lactating animals. Increased expression of OTR in the AP of lactating sheep may be related to the stimulation of pituitary lactotrophs by OT following the release of prolactin during suckling.