1. ^*National Institute on Drug Abuse, Baltimore, Maryland, U.S.A.
2. ^†National Institute on Aging, Baltimore, Maryland, U.S.A.
3. ^‡Case Western Reserve University, Cleveland, Ohio, U.S.A.
4. ^§Johns Hopkins University, Baltimore, Maryland, U.S.A.
5. ^¶Massachusetts General Hospital, Boston, Massachusettes, U.S.A.

Correspondence: Evan D Morris, Brain Imaging Section, National Institute on Drug Abuse, 5500 Nathan Shock Drive, Baltimore, MD 21224, U.S.A.

Received 19 January 1998; Revised 5 May 1998; Accepted 6 May 1998.

Abstract

The relation between striatal dopamine D₂ receptor binding and aging was investigated in rhesus monkeys with PET. Monkeys (n = 18, 39 to 360 months of age) were scanned with ¹¹C-raclopride; binding potential in the striatum was estimated graphically. Because our magnetic resonance imaging analysis revealed a concomitant relation between size of striatum and age, the dynamic positron emission tomography (PET) data were corrected for possible partial volume (PV) artifacts before parameter estimation. The age-related decline in binding potential was 1% per year and was smaller than the apparent effect if the age-related change in size was ignored. This is the first in vivo demonstration of a decline in dopamine receptor binding in nonhuman primates. The rate of decline in binding potential is consistent with in vitro findings in monkeys but smaller than what has been measured previously in humans using PET. Previous PET studies in humans, however, have not corrected for PV error, although a decline in striatal size with age has been demonstrated. The results of this study suggest that PV correction must be applied to PET data to accurately detect small changes in receptor binding that may occur in parallel with structural changes in the brain.