Aspects of neural plasticity in the central nervous system—IV. Chemical anatomical studies on the aging brain

Some effects of aging processes on the neurochemical features of central transmitter-identified neuronal populations have been investigated by means of immunocytochemistry and receptor autoradiographic techniques coupled with image analysis. A selective decrease of tyrosine hydroxylase immunoreactivity in the ventrolateral region of the arcuate nucleus in aged rats was observed. The level and turnover (recovery after irreversible blockade of monoamine receptors with the peptide coupling agent N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline) of 2-adrenergic ([³H]paraaminoclonidine binding) and D2 dopamine ([³H]spiperone binding) receptors were reduced in most regions of the rat brain. Peptide receptors showed a more complex pattern of change, since while μ opiate receptors (preferentially labeled with [³H]etorphine binding) were reduced in the old animals, δ opiate ([³H]DSTLenkephalin binding) receptors were affected only in certain areas. The effect of irreversible blockade of monoamine receptors on μ and δ opiate receptors was also studied in young adult and aged rats. A δ but not μ opiate receptor up-regulation was observed after monoamine receptor blockade in the young adult animals. This effect was greatly reduced in the n. caudatus-putamen, n. accumbens and tuberculum olfactorium of the old animals.