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.

The GABA content was determined simultaneously in two peripheral organs, i.e., ovary and Fallopian tube. Moreover, the effects of inhibitors of glutamate decarboxylase or gamma-aminobutyrate transaminase (GABA-T) on the GABA concentrations of the two organs were examined, to point out similarities and differences between central and peripheral pathways of GABA biosynthesis and degradation. In ovary, GABA concentration was found to be about 30% of that in total brain tissue. Furthermore, isoniazid and thiosemicarbazide caused significant reduction of GABA levels in peripheral organs. In contrast to the CNS, aminooxyacetic acid failed to increase, but even produced a significant diminution in peripheral GABA content. Gabaculine did not change GABA levels. In conclusion, it has been demonstrated for the first time that a peripheral organ, i.e. fallopian tube, contained higher GABA concentrations than the CNS. On the other hand, in the organs examined GABA seemed to be synthesized similarly, but metabolized by a pathway different from that in the brain.

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