TRH analog administration increases endogenous TRH levels in the central nervous system.

Experiments were carried out to increase the endogenous levels of TRH in brain and spinal cord. After 7 days of continuous oral ingestion of TRH analogs (D-pGlu-Leu-ProNH2, pGlu-Leu-Pro, pGlu-Pro-ProNH2, or pGlu-Leu-pyrrolidide), spinal cord and brain levels of TRH were elevated, whereas the content of TRH metabolite, cyclo(His-Pro), was decreased. In addition, the TRH analogs inhibited in vitro metabolism of [3H-Pro]-TRH by brain and spinal cord extracts. Our data show that certain TRH analogs can elevate tissue levels of TRH by inhibiting its metabolism. Furthermore, because these analogs do not affect interaction between TRH and its receptor, the elevated TRH can effectively enhance TRH-related bioactivity.

Synthesis of thyrotropin-releasing hormone analogues. 2. Tripeptides structurally greatly differing from TRH with high central nervous system activity.

A new series of thyrotropin-releasing hormone (TRH) analogues, obtained by further modifications of our most potent central nervous system (CNS) stimulating neutral tripeptides at both termini, were synthesized by the pentafluorophenyl ester method and tested for CNS and thyrotropin (TSH) releasing activity. Replacement of pyroglutamic acid by pyro-2-aminoadipic acid, 2-oxoimidazolidine-4-carboxylic acid or gamma-butyrolactone-gamma-carboxylic acid and that of proline by pipecolic acid, thiazolidine-4-carboxylic acid, or homoproline in [Leu2]- and [Nva2]TRH led to tripeptides structurally widely different from TRH. In spite of this fact, 7 of the 17 analogues (1, 2, 8-10, 16, and 17) have stronger anticataleptic effect than TRH, with negligible or no hormonal potency. The highest CNS activity was achieved when pyroglutamic acid was replaced by pyro-2-aminoadipic acid at the N-terminus [pAad-Leu-Pro-NH2, 1 (RGH 2202), and pAad-Nva-Pro-NH2,2]. A novel synthesis of L-2-aminoadipic acid suitable for large-scale preparation is also described.

Synthesis of thyrotropin-releasing hormone analogues. 1. Complete dissociation of central nervous system effects from thyrotropin-releasing activity.

Twenty-four thyrotropin-releasing hormone (TRH) analogues containing mainly aliphatic amino acids in position 2 were synthesized and tested for central nervous system (CNS) and hormonal (TSH) activity. Application of the pentafluorophenyl ester method in the syntheses resulted in optimal yields and high purity of the products. The neutral tripeptides pGlu- Nva -Pro-NH2 (9), pGlu-Nle-Pro-NH2 (10), and pGlu-Leu-Pro-NH2 (3) with a three- or four-membered straight or branched alkyl side chain in the position of the central amino acid had 2.5 to 10 times stronger anticataleptic effect than TRH, demonstrating that the presence of histidine is not essential for the CNS activity. Analogue 9 exhibited tenfold anticataleptic activity as compared to TRH, and it was found to be fully inactive in the release of TSH.

Reexamination of sodium-liquid ammonia reduction in the peptide chemistry.

Sodium-liquid ammonia reduction has been used for over 50 years for removal of benzyl-type protecting groups in peptide chemistry. Up until now a definitely blue end-point has generally been accepted for detection of the completion of reaction. Systematic investigation with model compounds has revealed that this is not only unnecessary for the complete removal of the protecting groups but also that the application of sodium in excess results in many undesired transformations which can simply be suppressed or even eliminated by optimizing the sodium consumption. Cleavage of tert.-butyloxycarbonyl group and N-C alpha bond, reduction of carboxamide groups to carbinol derivatives, transpeptidation and formation of a hydantoin derivative have been observed in model experiments by using sodium in excess.

Synthetic tripeptides with anorexogenic effect.

Thirty two synthetic tripeptides structurally related both to thyrotropin releasing hormone (TRH) and anorexogenic tripeptide (Glp-His-Gly-OH) were investigated for anorexogenic effect in rats. While the two endogenous peptides, TRH and Glp-His-Gly-OH, were ineffective in rats deprived of food for 96 hours when they were administered intracerebroventricularly, some of the synthetic analogues showed significant food intake reducing effect under the same conditions. This anorexogenic effect of the tripeptides is similar--though much weaker--to that of satietin, a highly potent anorexogenic glycopeptide in human and mammalian serum. These tripeptides were ineffective when they were administered intravenously.