Suppressors of cytokine signalling (SOCS) negatively regulate cytokine-induced signalling pathways and may be involved in leptin and prolactin (PRL) interactions. Herein, we examined the effect of PRL on SOCS-3 mRNA expression in pituitary explants and investigated whether leptin could modify the expression of SOCS-3 mRNA in pituitary explants. In the first experiment, we used pituitaries isolated from 16 ewes decapitated in March, May, July and October (four per month). Tissues were cut into

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... mg explants, which were treated with control or medium containing PRL (100 or 300 ng/ml). Incubation was maintained for different time intervals: 0, 60, 120, 180, 240 or 300 min. Real-time PCR was used to measure SOCS-3 mRNA levels. In the second study, we used 24 ewes surgically fitted with third ventricle cannulas (12 were used during the long-day period, and 12 were used during the short-day (SD) period). Each ewe was administered an i.c.v. injection of Ringer-Locke buffer or leptin (0 . 5 or 1 . 0 mg/kg body weight). Explants of anterior pituitaries were collected and snap frozen 1 h after injection. Semiquantitative expression of SOCS-3 mRNA was performed using reverse transcription-PCR. PRL stimulated SOCS-3 expression in the pituitaries collected in March (P!0 . 05) and May (P!0 . 01 and P!0 . 05 for lower and higher doses respectively), inhibited SOCS-3 expression in pituitaries collected in July (P!0 . 01) and had no effect in pituitaries collected in October. Treatment with leptin increased SOCS-3 expression during the SDs in a dose-dependent manner (P!0 . 01). The results demonstrated that photoperiod may be involved in leptin and PRL effects on SOCS-3 expression in sheep.