[The relationship between free-running period and monoamines in the SCN]

Free-running period of blinded rats that had kept in a cage with a running wheel varied markedly between 23.4-24.7 h. The free-running period correlated negatively with motor activity (r = -0.61, P less than 0.005). In the SCN, 5-HT concentration correlated positively with motor activity (r = 0.70, P less than 0.005), consequently correlated negatively to the free-running period (r = -0.79, P less than 0.005). Similarly, 5-HIAA content showed weakly negative correlation to the free-running period (r = -0.45, P less than 0.05) but such correlation was never observed in other monoamines and their metabolites examined (NE, DA, DOPAC and HVA). Further, no correlation was seen between free-running period and contents of amines and their metabolites, including 5-HT and 5-HIAA, in the areas tested other than SCN. These facts suggest that motor activity affects the activity of serotonergic system in SCN, resulting in the change in the free-running period.