Static culture techniques were used to examine the effects of neuropeptides on thyrotropin (TSH), growth hormone (GH), and luteinizing hormone (LH) secretion by turtle pituitaries (Pseudemys scripta). Pituitaries were exposed to 2.5-2,500 nM thyrotropin-releasing hormone (TRH), ovine corticotropin-releasing hormone (oCRH), sauvagine (SV), carp urotensin I (UI), rat growth hormone-releasing hormone (rGHRH), human GHRH (hGHRH), carp GHRH-like immunoreactive peptide (cGHRH-LIP), vasoactive intestinal peptide (VIP), arginine vasotocin (AVT), salmon gonadotropin-releasing hormone (sGnRH) or chicken I1 GnRH (cGnRH). Medium concentrations of TSH, GH, and LH were estimated by RIA. Significant dose responses in TSH secretion were observed with TRH, oCRH, rGHRH, and hGHRH; VIP was also a potent stimulator of TSH, but a dose response was not observed. The two CRH-like molecules, SV and UI, also stimulated TSH at the single dose tested (25 nM). All these neuropeptides had similar potencies. In one test in which PRL was measured, it was stimulated by 25 nM TRH but not by an equivalent dose of the CRH-like neuropeptides. GH secretion was elevated by TRH, rGHRH, hGHRH, and VIP. oCRH at the highest dose (250 nM) stimulated GH, but SV and UI had no effect on GH at the single dose tested (25 nM). Neither AVT, cGHRH-LIP, nor the two GnRH molecules had any effect on TSH or GH secretion. Data obtained by bioassay of TSH in representative samples were consistent with results of RIA.
In contrast to the diversity of neuropeptides affecting pituitary TSH and GH, only GnRH stimulated significant and marked increases in LH secretion. AVT produced a small increase in LH output at the higher doses tested (250 and 2,500 nM), and all other neuropeptides were inactive at all doses. Taken together with other comparative data, these results suggest that in ectothermal vertebrates, TSH and GH responses are generally not as specific for neuropeptides as they are in normal mammals but may resemble mammals in certain abnormal states. However, there is not a clear phylogenetic pattern in which neuropeptides may be stirnulatory to TSH or GH. The gonadotropes in both ectotherms and mammals appear to be relatively specific for molecules of the GnRH family.