Neuropeptides and their receptors in aged-rat brain.

Age-associated changes in methionine-enkephalin (ENK) and thyrotropin releasing hormone (TRH) concentrations, and their receptors were examined in discrete regions of the rat brain. The ENK and TRH concentrations in aged rats were nearly identical to those in young adult rats, except for a slightly lower TRH value in the hypothalamus of the aged rats. On the other hand, the ENK and TRH receptor levels in the cerebral cortex of aged rats was markedly lower than that of young adults rats. The results suggest that determinations of both neuropeptide and receptor levels are indispensable for evaluation of peptide-mediated neural systems in the central nervous system.