Alteration by liquid protein diet of TRH and cyclo(His-Pro) in the young rat brain

Effects of liquid protein diet (LPD), known to be poor in protein quality, on brain thyrotropin-releasing hormone (TRH) and histidylproline diketopiperazine (cyclo(His-Pro] were examined in young male rats. At day 23 of weaning, the animals were fed either a 20% casein diet ad libitum or a 20% LPD, or were pair-fed with a 20% casein diet. They were decapitated at day 48. The body weight of those in the LPD and pair-fed groups decreased significantly, and the loss was sustained in comparison with that in the ad libitum group. Cyclo(His-Pro) concentrations in the whole brain were significantly greater in the LPD group than in the pair-fed and ad libitum groups, whereas its concentrations were similar in the pair-fed and ad libitum groups. No significant differences were observed in concentrations of brain TRH in the three groups. These findings indicate that quality of the protein component in the diet contributes to alterations in the neuropeptide, cyclo(His-Pro), in the young rat brain.