EFFECTS OF A DAYLIGHT CYCLE REVERSAL ON LOCOMOTOR-ACTIVITY IN SEVERALINBRED STRAINS OF MICE

There is some evidence of melatonin implication in the nycthemeral reg ulation of running activity rhythm in rodents. Because some inbred str ains of mice such as C57BL/6 and BALB/c have been generally found to p resent no nocturnal melatonin peak, in contrast to others such as C3H/ He and CBA mice, the aim of this study was to examine the adaptation o f daily locomotor activity to a light/dark cycle phase shift in these four strains. An apparatus consisting of two boxes connected by a tunn el was used to record spontaneous locomotor activity, defined as the n umber of transitions between the two boxes. Locomotor activity was mon itored continuously during 3 days before and 14 days after a 12-h phas e delay of the light/dark cycle. Results essentially showed that the a daptation of the locomotor activity rhythm to the phase shift was fast er in C57BL/6 and BALB/c mice than in C3H/He and CBA mice. This could be related, at least in part, to the differences in melatonin synthesi s between the former strains and the latter ones. Although melatonin n octurnal peak is not necessary to a daylight regulation of circadian f unctions in rodents, it could be considered as an endocrine message th at takes part in the anticipation of the following light/dark cycle. ( C) 1998 Elsevier Science Inc.