Actions of TRH and cyclo-(His-Pro) on spontaneous and evoked activity of cortical neurones.

Thyrotrophin releasing hormone (TRH) and its metabolite histidyl-proline diketopiperazine (cyclo-(His-Pro) have been applied by microiontophoresis to neurons in the rat cerebral cortex. Both TRH and cyclo-(His-Pro) had little effect on the spontaneous firing of neurons but both were able to enhance excitatory responses to acetylcholine. Both peptides also caused an enhancement of excitatory responses to quisqualic acid while having little consistent effect on responses to N-methyl-DL-aspartic acid although excitation of a small proportion of cells was enhanced. On 3 cells the increase in size of responses to quisqualate was sufficient to cause overdepolarisation. No antagonism of responses to quisqualate or N-methyl-DL-aspartate was seen.