Histidyl-proline, a rapidly degraded metabolite of thyrotropin releasing hormone, has behavioural activity

The effects of intracerebroventricular (ICV) administration of histidyl-proline (His-Pro; 0, 1, 10 and 100 micrograms/rat), a rapidly degraded metabolite of thyrotropin releasing hormone (TRH), were assessed using three behavioural paradigms. Locomotor activity was significantly increased over 30, but not 90, min following administration of the highest dose. This stimulatory effect was more apparent when naive rats were tested in an open field without any prior exposure to the apparatus; the effect was attenuated on a second exposure. His-Pro treated subjects also tended to prefer the area near the walls of the apparatus. The effects of the dipeptide on visual perception were studied, using a two-choice task. A low but not high (10, versus 100 micrograms/rat) dose of His-Pro increased response switching, but did not affect response repetition, latencies or percent correct performance. The results indicate that His-Pro can alter behaviour by affecting motor output, as well as emotional processes.