AJP - Regulatory, Integrative and Comparative Physiology, Vol 244, Issue 1 93-105, Copyright © 1983 by American Physiological Society
Sex differences in the circadian control of hamster wheel-running activity
F. C. Davis, J. M. Darrow and M. Menaker
The circadian pacemaker that underlies the wheel-running activity of
hamsters was studied in males and females. Sex differences were found in
the mechanism by which the pacemaker entrains to light-dark cycles and in
the timing of activity onset. When exposed to a light-dark cycle with a
period of 24.75 h (with 1 h of light/cycle), males show a greater ability
to maintain entrainment than do females. This difference in the upper limit
of entrainment appears due to a sex difference in the magnitude of
light-induced phase shifts. A small difference in free-running period may
also contribute to the sex difference in entrainment. Two weeks after
castration of adults, the sex difference in entrainment is not affected,
indicating that the difference does not depend on circulating gonadal
steroids or on estrous cyclicity of the female. However, castration of
females at an early age increases their ability to entrain, whereas
long-term castration of males seems to reduce entrainment ability. During
entrainment to a 24-h light-dark cycle (LD 14:10), females were found to
begin their daily activity before males and before castrated females. This
difference is consistent with a sex difference in the magnitude of
light-induced phase shifts and in entrainment of the pacemaker. However,
evidence is given that the sex difference in activity onset might also be
caused by a sex difference in the relationship of locomotor activity to the
pacemaker in intact males and females.
