Effect of peptide histidine isoleucine on in vitro and in vivo prolactin secretion in the turkey.

Studies were conducted both in vitro, using monolayer cultures of turkey pituitary cells, and in vivo, using ovariectomized turkey hens, steroid-primed ovariectomized hens, and immature toms to compared the effectiveness of peptide histidine isoleucine (PHI) with that of vasoactive intestinal peptide (VIP) in stimulating prolactin (PRL) secretion. Vasoactive intestinal peptide, a putative PRL-releasing factor (PRF) in the turkey, was approximately 1000-fold more effective than PHI in stimulating PRL secretion in vitro. Prolactin secretion was significantly enhanced by the exposure of pituitary cells to 10(-7) M PHI and a similar stimulation was observed with 10(-10) M VIP. Injection of cannulated, unrestrained turkeys with PHI at doses up to 100 micrograms per kg of BW caused no significant change in circulating PRL concentrations, but injection of 10 micrograms of VIP per kg resulted in a 7 to 22-fold increase in plasma PRL concentration within 10 to 30 min following injection. These results demonstrate a marked difference between the turkey and the rat in their response to a PRL-stimulating neuropeptide. In contrast to what was observed in the turkey, PHI is a strong PRF in the rat, with a potency equal to or greater than that of VIP. Earlier studies have shown that thyrotropin-releasing hormone, another strong PRF in mammals, has little consistent PRF activity in the turkey. Thus, the present studies add additional evidence of major differences between mammals and birds in the control of PRL secretion by hypothalamic neuropeptides.