Modulation of neuropeptide-stimulated pituitary hormone secretion in hatchling turtles.

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 nM 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 nM) 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 nM) was present only during the 2-hr basal period; however, reduction of TSH and GH responses to TRH required the presence of SRIH (60 nM) during the basal period and the period of stimulation. TSH responses to 25 nM oCRH and rGHRH and GH responses to rGHRH were significantly reduced by preincubation with 60 nM SRIH. The biogenic amines, dopamine (DA), serotonin (5HT), and norepinephrine (NE) (50 or 500 nM) were tested for possible direct actions on basal and neuropeptide-stimulated pituitary TSH and GH secretion.(ABSTRACT TRUNCATED AT 250 WORDS)