Spontaneous wheel running in laboratory rats genetically selected for activity level in a novel environment

Wistar rats were genetically selected for either high (+A) or low (-A) locomotor activity level in a new environment. The response to the novelty was tested in all filial generations at the age between 75-80 days of life. Groups of 15 +A male rats selected from the 11th and 12th generations were placed in the individual cages connected with rotation wheels at the age 149 +/- 9 days. For a period of 32 days, the spontaneous wheel running activity was recorded for the time intervals 0.00-6.00 a.m., 6.00-12.00, 12.00-18.00, and 18.00-24.00 h. The -A group exhibited a generally low level of running activity over the entire experimental period, whereas the time of running in the +A tended to increase. Inside the +A group, homogeneous as to the short- term activity level, there gradually differentiated two sub-groups, one with relatively constant medium values and the other which showed an enormous prolongation of the time spent with running. The +A and -A groups differed also in their daily activity rhythm. Whereas the +A rats reached maximum of their activity between 18-24 h., followed by a rapid decrease in the morning hours, the -A animals showed a tendency to a later onset of night-type activity, and a higher level continued to morning hours.