Distribution of histidyl-proline diketopiperazine [cyclo (His-Pro)] and thyrotropin-releasing hormone (TRH) in the primate central nervous system

The concentrations of cyclo (His-Pro) and its precursor, thyrotropin-releasing hormone (TRH) were measured in 47 different loci of monkey brain using specific radioimmunoassays. Cyclo (His-Pro) concentrations were higher than those of TRH in all loci excepting the hypothalamus, where TRH concentration was found to be the highest of all the loci and twice those of cyclo (His-Pro). The high levels of cyclo (His-Pro) were seen within the cerebellar system (inferior olivary nucleus>nucleus interpositus>fastigial nucleus>posterior vermis). The great variations in TRH-cyclo (His-Pro) ratios among different loci suggest that other factors in addition to TRH concentration must play roles in determining the unique distribution pattern of cyclo (His-Pro) in the primate brain.