The interaction of benzodiazepines with thyrotropin-releasing hormone receptors on clonal pituitary cells

Thyrotropin-releasing hormone induces Ca2+ increase in a subset of vagal nodose ganglion neurons

Thyrotropin-releasing hormone (TRH) plays a central role in metabolic homeostasis, and single-cell sequencing has recently demonstrated that vagal sensory neurons in the nodose ganglion express thyrotropin-releasing hormone receptor 1 (TRHR1). Here, in situ hybridization validated the presence of TRHR1 in nodose ganglion (NG) neurons and immunohistochemistry showed that the receptor is expressed at the protein level. However, it has yet to be demonstrated whether TRHR1 is functionally active in NG neurons. Using NG explants transduced with a genetically encoded Ca²⁺ indicator (GECI), we show that TRH increases Ca²⁺ in a subset of NG neurons. TRH-induced Ca²⁺ transients were briefer compared to those induced by CCK-8, 2-Me-5-HT and ATP. Blocking Na⁺ channels with TTX or Na⁺ substitution did not affect the TRH-induced Ca²⁺ increase, but blocking Gq signaling with YM-254890 abolished the TRH-induced response. Field potential recordings from the vagus nerve in vitro showed an increase in response to TRH, suggesting that TRH signaling produces action potentials in NG neurons. These observations indicate that TRH activates a small group of NG neurons, involving Gq pathways, and we hypothesize that these neurons may play a role in gut-brain signaling.

Carboxyl Tail Cysteine Mutants of the Thyrotropin-Releasing Hormone Receptor Type 1 Exhibit Constitutive Signaling: Role of Palmitoylation

We studied the role of carboxyl tail cysteine residues and their palmitoylation in constitutive signaling by the thyrotropin-releasing hormone (TRH) receptor type 1 (TRH-R1) in transfected mammalian cells and in Xenopus laevis oocytes. To study palmitoylation, we inserted a factor Xa cleavage site within the third extracellular loop of TRH-R1, added a carboxyl-terminal C9 immunotag and expressed the mutant receptor in Chinese hamster ovary cells. We identified TRH-R1-specific palmitoylation in the transmembrane helix-7/carboxyl-tail receptor fragment mainly at Cys-335 and Cys-337. In contrast to a mutant truncated at Cys-335 that was reported previously to be constitutively active, a receptor truncated at Lys-338 (K338Stop), which preserves Cys-335 and Cys-337, and C337Stop and N336Stop, which preserve Cys-335, did not exhibit increased constitutive signaling. TRH-R1 mutants substituted singly by Gly or Ser at Cys-335 or Cys-337 did not exhibit constitutive signaling. By contrast, substitution of both cysteines (C335G/C337G or C335S/C337S) yielded TRH-R1 mutants that exhibited marked constitutive signaling in mammalian cells. In the oocyte, constitutive signaling by C335G/C337G resulted in homologous (of C335G/C337G) and heterologous (of M1 muscarinic receptor) desensitization. Because both Cys-335 and Cys-337 have to be substituted or deleted for constitutive signaling, we propose that a single palmitoylation site in the proximal carboxyl tail is sufficient to constrain TRH-R1 in an inactive conformation.

Effects of central and peripheral type benzodiazepine ligands on thyrotropin and prolactin secretion

The cold-stimulated thyrotropin (TSH) levels in the rat were decreased by clonazepam (a central type benzodiazepine agonist), diazepam (a mixed agonist), FG 7142 (an inverse central type agonist) and Ro 5-4864 (a peripheral type agonist), clonazepam being the most potent and Ro 5-4864 the least active. Clonazepam and diazepam also decreased while FG 7142 increased prolactin (PRL) levels. Ro 5-4864 did not have any significant action. Clonazepam (1 and 5 mg/kg) and diazepam (15 mg/kg but not 25 mg/kg) decreased even the TRH-induced PRL levels. Only Ro 5-4864 (25 mg/kg) decreased TRH-induced TSH secretion but not significantly. The actions of central type compounds were antagonized by flumazenil but not by PK 11195. The weak effects of Ro 5-4864 were not antagonized by either antagonists. While the peripheral type benzodiazepine agonist only weakly affected the secretion of anterior pituitary hormones, the central type inhibition of TSH appears to be mediated through the hypothalamic TRH and that of PRL rather through the anterior pituitary gland. The sedating (or agitating in case of FG 7142) effect of high doses of benzodiazepine ligands may contribute to the changes in TSH and PRL levels.

Thyrotropin-releasing hormone receptor occupancy determines the fraction of the responsive pool of inositol lipids hydrolysed in rat pituitary tumour cells

We report that there are distinct thyrotropin-releasing hormone (TRH)-responsive and -unresponsive pools of inositol (Ins) lipids in rat pituitary tumour (GH3) cells, and present evidence that the size of the responsive pool is determined by the number of activated TRH-receptor complexes. By use of an experimental protocol in which cycling of [3H]Ins is inhibited and resynthesis occurs with unlabelled Ins only, we were able to measure specifically the effects of TRH on the hydrolysis of the Ins lipids present before stimulation. A maximally effective dose of TRH (1 microM) caused a time-dependent decrease in 3H-labelled Ins lipids that attained a steady-state value of 42 +/- 1% of the initial level between 1.5 and 2 h. After 2 h, even though there was no further decrease in 3H-labelled Ins lipids, and no increase in [3H]Ins or [3H]Ins phosphates, turnover of Ins lipids, as assessed as incorporation of [32P]Pi into PtdIns, continued at a rate similar to that in cells incubated without LiCl or unlabelled Ins. These data indicate that Ins lipid turnover was not desensitized during prolonged TRH stimulation. Depletion of lipid 3H radioactivity by TRH occurred at higher TRH doses on addition of the competitive antagonist chlordiazepoxide. Addition of 1 microM-TRH after 3 h of stimulation by a sub-maximal (0.3 nM) TRH dose caused a further decrease in 3H radioactivity to the minimum level (40% of initial value). We propose that the TRH-responsive pool of Ins lipids in GH3 cells is composed of the complement of Ins lipids that are within functional proximity of activated TRH-receptor complexes.

Non-peptide ligands for peptide receptors

Considerable progress has been made in synthesizing peptide analogs with improved stability for probing function of peptide-receptor systems. However, since peptide ligands are usually unsuitable for development as potent orally active long-duration therapeutic agents, considerable research effort is being directed to the development of non-peptidal ligands. Roger Freidinger compares the lead compounds that have now been described in the opioid, CCK-gastrin and angiotensin II fields, and discusses the progress that has been made in other receptor fields. Most of the successes have been achieved through screening natural sources and synthetic collections. Rational design based on the natural peptide ligand has been more difficult, although ACE inhibitors have been effectively developed from a nonapeptide lead.