Research reportRole of TRH receptors as possible mediators of analeptic actions of TRH-like peptides
Introduction
Thyrotropin-releasing hormone (pGlu-His-Pro-NH2), EEP (pGlu-Glu-Pro-NH2), and Leu2-TRH (pGlu-Leu-Pro-NH2) have been reported to have antidepressant, analeptic, neuroprotective, euphoric, and antiamnesic effects [7], [8], [9], [11], [12], [13], [15]. Peptides with the structure pGlu-X-ProNH2 are referred to here as TRH-like peptides. These, and other TRH-like peptides including Val2-TRH, Phe2-TRH, and Tyr2-TRH, occur in high concentration within rat brain regions associated with the regulation of mood [11], [12], [13]. A recently described TRH receptor 2 (TRHR2) is highly expressed in rat brain regions involved in the regulation of attention, learning, arousal, sleep, central motor control and processing of sensory information [5]. In limbic regions such as the perirhinal cortex, entorhinal cortex and shell of the accumbens, areas associated with mood regulation, TRHR1 predominates [5]. We have inquired whether TRHR2 has physiologically significant affinity for TRH-like peptides with known CNS effects and whether these interactions contribute to the analeptic effect of TRH-like peptides. We began our survey of possible mediators of TRH-like peptide action using transformed cells expressing the known TRH receptors. We measured the affinity and signal transduction capacity of TRH-like peptides in cell lines stably transfected with either TRHR1 or TRHR2, allowing us to identify effects mediated by the two receptors selectively. The binding specificity of TRHR2 has not yet been characterized as extensively as it has for TRHR1 [2]. We also tested the ability of TRH-like peptides to stimulate a calcium response, allowing us to determine whether the peptides behave as agonists. We show for the first time that EEP, Val2-TRH and Leu2-TRH are analeptics following intracisternal (i.c.) injection while Phe2-TRH and Tyr2-TRH are not. The present results suggest that these novel TRH-like peptides with important action in the CNS and peripheral tissues do not function via TRHR1 or TRHR2.
Section snippets
Materials
pGlu-Glu-Pro-NH2 was purchased from Sigma (St Louis, MO). pGlu-Phe-Pro-NH2 and pGlu-Gln-Pro-NH2 were prepared by Peninsula Laboratories. All other TRH-like peptides were custom synthesized by Phoenix Pharmaceuticals (Belmont, CA). Cell culture media, sera, and HBSS were from Gibco-BRL (Grand Island, NY). Fura2AM was from Molecular Probes (Eugene, OR), and TRH from Calbiochem (La Jolla, CA). [3H]TRH was from Dupont/New England Nuclear Corporation (Boston, MA). A plasmid encoding TRHR2 in pcDNA3
Results
Peptides were tested for their analeptic effect, defined as reduction in pentobarbital-induced sleep time. Injection (i.c.) of 1 μg of TRH or TRH-like peptides did not result in a significant analeptic effect. The results in Table 1 show that i.c. injections of 10 μg TRH, EEP, Leu2-TRH or Val2-TRH had an analeptic effect. Interestingly, the injection of TRH, Leu2-TRH and Val2-TRH was frequently accompanied by ‘wet dog’ shaking while the animal was still anesthetized. On the other hand, EEP
Discussion
We have shown that EEP, Val2-TRH and Leu2-TRH are analeptics, like TRH, but Phe2-TRH and Tyr2-TRH are not. These results indicate that the imidazole ring is not necessary for the analeptic activity of this novel family of peptides, and a hydrophobic residue in the 2-position is not sufficient. Although the analeptic effect of TRH is well established, His2-substituted peptides had not previously been known to exert this activity. The receptor mediating the analeptic effects of TRH and related
Acknowledgements
This work was supported in part by NIH grant DK19974 (PMH) and the Research Service of the US Department of Veterans Affairs (AEP and AS). The authors are grateful to John A. Puskas for excellent technical assistance.
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Global view of neuropeptides and their receptors in the brain and pituitary of grass carp (Ctenopharyngodon idellus)
2019, AquacultureCitation Excerpt :As reflected by the considerable expression levels of the ligands and receptors throughout the brain, those peptides might also play a variety of roles in grass carp neuroendocrine system. In mammals, thyrotropin-releasing hormone (TRH) is a well-known hypothalamic neuropeptide for inducing pituitary TSH secretion and synthesis, and then indirectly regulates the secretion of thyroid hormones (Hinkle et al., 2002; Vella and Hollenberg, 2009). In contrast to mammals, teleost TRH has no or only a minor effect on the production of TSH, but could significantly induce pituitary growth hormone (GH), prolactin (PRL), and α-melanocyte–stimulating hormone (α-MSH) secretion (Kagabu et al., 1998; Tran et al., 1989).
TRH
2013, Handbook of Biologically Active Peptides[Glu<sup>2</sup>]TRH dose-dependently attenuates TRH-evoked analeptic effect in mice
2010, Brain Research BulletinTRH and TRH-like peptide expression in rat following episodic or continuous corticosterone
2008, PsychoneuroendocrinologyCitation Excerpt :However, to date no TRH receptor antagonists have been found. Of the TRH-like peptides, only the endogenous pGlu-Glu-Pro-NH2 has a documented receptor, and only on the sperm (Adeoya-Osiguwa et al., 1998, and see Hinkle et al., 2002 for negative results). The molecular precursors of the TRH-like peptides have not yet been determined (their sequence is not found in prepro-TRH; work in progress).