Modulation of neuropeptide-stimulated pituitary hormone secretion in hatchling turtles

Abstract

Neuropeptides that have relatively narrow actions on mammalian pituitary secretion may have divergent effects on pituitary hormone secretion in ectothermal vertebrates. In turtles, secretion of both thyrotropin (TSH) and growth hormone (GH) can be stimulated in vitro by thyrotropin-releasing hormone (TRH) and by members of corticotropin-releasing hormone (CRH) and growth hormone-releasing hormone (GHRH) peptide families. To determine if these neuropeptides share common modes of action, and to study other potential regulators of the turtle pituitary, somatostatin-14 (SRIH) and monoamines were tested for direct effects on in vitro basal and neuropeptide-stimulated TSH and GH secretion. Pituitary glands from young turtles ( Pseudemys scripta) were cultured in the presence of 25 n M TRH, ovine CRH, or rat GHRH with or without SRIH. Glands were incubated for several 2-hr periods in medium alone or in medium containing peptides. Preincubation for 4 hr with SRIH (6 or 60 n M) significantly reduced basal and TRH-stimulated TSH and GH output (SRIH present during entire incubation). In another experiment, basal hormone secretion was reduced when SRIH (60 n M) was present only during the 2-hr basal period; however, reduction of TSH and GH responses to TRH required the presence of SRIH (60 n M) during the basal period and the period of stimulation. TSH responses to 25 n M) oCRH and GGHRH and GH responses to GGHRH were significantly reduced by preincubation with 60 n M) SRIH. The biogenic amines, dopamine (DA), serotonin (5HT), and norepinephrine (NE) (50 or 500 n M) were tested for possible direct actions on basal and neuropeptide-stimulated pituitary TSH and GH secretion. Only 5HT at the highest dose tested (500 n M) elevated TSH secretion in the absence of neuropeptides. None of the amines had any effect on neuropeptide-stimulated TSH release. In contrast, GH responses to 25 n M) TRH and GHRH were enhanced by DA (50 and 500 n M) and reduced by 5HT (500 n M) Only NE (500 n M) elevated GH secretion in the absence of neuropeptides, and NE enhanced TRH-stimulated GH secretion (NE was not tested for effects on CRH and GHRH responses). These data show that mammalian SRIH and monoamines can modify basal and neuropeptide-stimulated TSH and GH secretion in turtles. While the mechanism(s) by which the different neuropeptides stimulate the release of TSH and GH from turtle pituitaries is not known (i.e., direct or paracrine), some commonality in their mode of action seems likely, since exposure to SRIH or amines affected the responses to each of the different neuropeptides similarly. Finally, these results implicate several other brain factors in the regulation of pituitary hormone secretion in turtles.