Histidyl-proline diketopiperazine: its biological role as a regulatory peptide.

Histidyl-proline diketopiperazine [cyclo(His-Pro)] is a metabolite of thyrotropin releasing hormone (TRH). This review summarizes the literature concerning cyclo (His-Pro) and, in addition, some studies dealing with TRH and other peptide that are considered of interest. The enzymes concerned with the metabolism of TRH are discussed. Distribution studies of peptides by immunological methods show that, while TRH is concentrated in synaptosomes, cyclo (His-Pro) is not, suggesting that cyclo (His-Pro) is not a classical neurotransmitter. Rat brain contains approximately three times as much cyclo (His-Pro) as TRH, mainly localized in the pituitary and hypothalamus. While the TRH is found in a free form, the cyclo (His-Pro) is bound to a carrier of molecular weight approximately 70,000. While specific membrane receptors for TRH have been detected in pituitary cells, no such receptors for cyclo (His-Pro) have yet been found in brain or pituitary; however, there is a specific binding of cyclo (His-Pro) to adrenal cortex membranes. Both TRH and cyclo (His-Pro) have effects in the central nervous system or pituitary. These include effects on prolactin release, thermoregulation, CNS depression, stereotypic behavior and cyclic nucleotide levels. Possible mechanisms and interrelations of these effects are discussed.