The influence of thyrotropin releasing hormone on in vivo prolactin release and in vitro prolactin, luteinizing hormone, and growth hormone release from dispersed pituitary cells of the young turkey ( Meleagris gallopavo)

Abstract

Intravenous administration of 0.025, 0.25, or 2.5 μg/kg thyrotropin releasing hormone (TRH) to 4-week-old female turkeys induced a dose-dependent increase ( P = 0.004) in serum prolactin (PRL) 15 min post-treatment. Dispersed anterior pituitary cell cultures were utilized to determine the effect of TRH on cellular release of PRL, luteinizing hormone (LH), and growth hormone (GH). In the first experiment, cells from 13-week-old male turkeys were initially incubated for 24 hr in Medium 199 (M-199) plus 10% turkey serum and then placed in M-199 plus 10 −10 to 10 −4 M TRH for 5 hr. Incubation with TRH produced no change in PRL release from that of spontaneous release ( P = 0.854). However, 10 −5 and 10 −4 M TRH induced LH release ( P < 0.0001). The TRH-induced GH response was parabolic ( P < 0.0001), with the maximal release at 10 −8 M. The second experiment, utilizing pituitary cells from 7-week-old females, studied these responses on 3, 5, and 7 days of monolayer incubation. TRH failed to induce a PRL release in all tests ( P > 0.162), although hypothalamic extract induced a large release ( P < 0.0001) of PRL each time. Both 10 −6 and 10 −4 M TRH induced a LH release on Day 3 while only 10 −4 M did so on Day 5, and none of the doses elicited a release on Day 7. The parabolic GH response generally persisted in all tests. In vivo, administration of the serotonin antagonist methysergide to 6-week-old female turkeys failed to diminish the TRH-induced PRL release. TRH induces PRL release in the young turkey, although it fails to act directly on the PRL-secreting cells of the anterior pituitary. In contrast, TRH acts directly on the pituitary GH-secreting cells.