A California scientist has discovered what may be the key to one of the major problems of aging, decline in one's ability to move.

The explanation lies not, as is generally thought, in a decrease in muscle strength or an accumulation of fat, but in changes in the central nervous system.

Based on observations of yound and old rats, Dr. John F. Marshall of the University of California at Irvine theorizes that the portions of the brain that program and coordinate movement remain intact in elderly animals, but the mechanism that transmits the brain's commands is disrupted.

Specifically, Marshall believes that neurons in the brain containing a chemical called dopamine provide a necessary "juice" for movement to occur in the normal way. In the old, these neurons do not function normally and so certain movements are impaired.

Marshall reports that his elderly rats achieved a dramatic though temporary improvement when given either of two drugs: L-dopa (a biosynthetic precursor of dopamine) and apomorphine (a dopamine receptor stimulant). He believes that both of these drugs act to normalize the activity of the dopamine synapses of the brain.

The National Institute on Aging, which supported Marshall's work, feels that the dramatic rejuvenation of the elderly rats holds great potential for work with humans.