DO THE SUPRACHIASMATIC NUCLEI OSCILLATE IN OLD RATS AS THEY DO IN YOUNG ONES

The basis of the decline in circadian rhythms with aging was addressed by comparing the patterns of three behavioral rhythms in young and ol d rats with the in vitro rhythm of neuronal activity in the suprachias matic nuclei (SCN), the primary circadian pacemaker. In some old rats, rhythms of body temperature, drinking, and activity retained signific ant 24-h periodicities in entraining light-dark cycles; in others, one or two of the rhythms became aperiodic. When these rats were 23-27.5 mo old they were killed, and single-unit firing rates in SCN brain sli ces were recorded continuously for 30 h. There was significant damping of mean peak neuronal firing rates in old rats compared with young. S CN neuronal activities were analyzed with reference to previous entrai ned behavioral rhythm patterns of individual rats as well. Neuronal ac tivity from rats with prior aperiodic behavioral rhythms was erratic, as expected. Neuronal activity from rats that were still maintaining s ignificant 24-h behavioral rhythmicity at the time they were killed wa s erratic in most cases but normally rhythmic in others. Thus there wa s no more congruence between the behavioral rhythms and the brain slic e rhythms than there was among the behavioral rhythms alone. These res ults, the first to demonstrate aberrant SCN firing patterns and a decr ease in amplitude in old rats, imply that aging could either disrupt c oupling between SCN pacemaker cells or their output, or cause deterior ation of the pacemaking properties of SCN cells.