Omega-conotoxin- and nifedipine-insensitive voltage-operated calcium channels mediate K(+)-induced release of pro-thyrotropin-releasing hormone-connecting peptides Ps4 and Ps5 from perifused rat hypothalamic slices

Modulation of the growth hormone-releasing activity of thyrotropin-releasing hormone in the chicken by its gene-related peptide preproTRH((160-169))(Ps4): enhanced somatostatinergic tone?

Recent research demonstrated that endocrine actions of thyrotropin (TSH)-releasing hormone (TRH) are modulated by gene-related products within proTRH. In the present report we show that the growth hormone (GH) response to TRH is clearly inhibited after the preincubation of chicken pituitary glands with preproTRH((160-169))Ps4, whereas the TSH response is not impaired. Binding sites for(125)I-[Tyr(0)]-Ps4 were, however, not detected on chicken pituitary membranes, although (as a control) they were readily detectable on membranes from rat pituitary glands. An indirect action may therefore take place within the pituitary by modulating the action of somatostatin (SRIH), the inhibitor of GH release in the chicken. This hypothesis is strengthened by the observation that Ps4 increases the binding of(125)I-[Tyr(1)]-SRIH to chicken pituitary membranes in a dose-related way. Since Ps4 is also produced by pituitary tissue, this may reflect a local or paracrine action on the regulation of GH release.

omega-Conotoxin GVIA blocks nicotine-induced catecholamine secretion by blocking the nicotinic receptor-activated inward currents in bovine chromaffin cells

We have studied the contribution of N-type voltage-dependent Ca2+ channels to both norepinephrine and epinephrine secretion from bovine chromaffin cells induced by high K+ or nicotine using omega-conotoxin GVIA, a selective blocker of N-type voltage-dependent Ca2+ channels. We found that high K+ (75 mM) induced catecholamine secretion was not affected by exposure of bovine chromaffin cells to omega-conotoxin GVIA (1 microM). However, nicotine-induced both norepinephrine and epinephrine secretion were similarly blocked (about 25%) by omega-conotoxin GVIA (1 microM). This effect could be explained by a potent (about 80%) and reversible blockade of the inward current induced by nicotine receptor activation in bovine chromaffin cells. The results indicate that besides the blockade of N-type voltage-dependent channels, omega-conotoxin GVIA is a potent and reversible blocker of the nicotinic receptor-induced currents in chromaffin cells.

Analysis of structural requirements for TRH-potentiating peptide receptor binding by analogue design

Previous studies established that the [125I-Tyr0]Ps4 derivative of TRH-potentiating peptide (Ps4), Ser-Phe-Pro-Trp-Met-Glu-Ser-Asp-Val-Thr, displays high affinity and selectivity for an orphan membrane receptor in rat anterior pituitary. To identify the sites in Ps4 that determine receptor binding affinity, we have synthesized and screened a panel of 15 single-point substituted analogues for their ability to displace bound [125I-Tyr0]Ps4. The affinity of [Tyr0]Ps4 for rat anterior pituitary membranes [inhibitory constant (Ki), approximately 5 nM] was drastically reduced by the substitution of either Tyr0 with Gly or Asp8 with Asn (Ki approximately 95 and approximately 51 nM, respectively). Deamination of [Tyr0]Ps4 also sharply reduced affinity (Ki approximately 1100 nM). In contrast, Ser1-->Ala, Pro3-->Ala and Thr10-->Val substitutions led to analogues showing a tenfold increase in binding affinity relative to the parent peptide. The change of Phe2-->Leu, Trp4-->Ala, Glu6-->Gln, Val9-->Thr and carboxamidation of Thr10 had no effect on binding affinity. The data suggest that substitutions of the amino-terminal group, Asp8 and Tyr0, have a marked effect on the ability of [Tyr0]Ps4 to compete with [125I-Tyr0]Ps4 for binding to TRH-potentiating peptide pituitary receptor.

Modulation of the biological activity of thyrotropin-releasing hormone by alternate processing of pro-TRH

Thyrotropin-releasing hormone prohormone contains multiple copies of TRH linked together by connecting sequences. Like other plurifunctional prohormone proteins, pro-TRH undergoes differential proteolytic processing in various tissues to generate, beside authentic TRH, several other novel peptides corresponding to C-terminally extended forms of TRH and connecting fragments. The pro-TRH connecting peptides are, together with TRH, predominant storage forms of TRH-precursor related peptides in the rat hypothalamus. Connecting peptides are co-localized with TRH in the median eminence nerve endings and co-released through a mechanism involving voltage-operated Ca2+ channels. The connecting peptide Ps4 is involved in potentiation of the action of TRH on thyrotropin hormone release by pituitary in vitro and in vivo through interactions with a specific pituitary cell receptor coupled to dihydropyridine and omega-connotoxin sensitive Ca2+ channels of the L-type. It also causes dose-dependent increases in the steady state levels of mRNAs of TSH and prolactin through stimulation of the respective gene promoter activities. These findings indicate that Ps4 and TRH, two peptides which originate from a single multifunctional biosynthetic precursor, can function on the same target tissues in a coordinate manner to promote hormonal secretion. This suggests that differential processing of the TRH prohormone may have the potential to modulate the biological activities of TRH.

Isolation and amino acid sequence of the TRH-potentiating peptide from bovine hypothalamus

A neuropeptide termed TRH-potentiating peptide, which potentiates TRH-evoked thyrotropin secretion by antehypophysis in vitro, was isolated from an acetonic powder of bovine hypothalamus. The peptide was purified to homogeneity by a 3-step protocol involving molecular sieve filtration, ion-exchange chromatography and reverse phase high performance liquid chromatography. The complete amino acid sequence of the decapeptide was determined as Ser-Phe-Pro-Trp-Met-Glu-Ser-Asp-Val-Thr by automated Edman degradation with a solid-phase sequencer. Bovine TRH-potentiating peptide is structurally identical to Ps4, a decapeptide which was deduced from the cDNA encoding the rat TRH precursor. This study provides for the first time a direct chemical evidence for the existence of non-TRH peptides originating from posttranslational processing of the TRH precursor in vivo.