Photic induction of Fos in the suprachiasmatic nucleus of African mole-rats: responses to increasing irradiance.
African mole-rats (family Bathyergidae) are strictly subterranean rodent species that are rarely exposed to environmental light. Morphological and physiological adaptations to the underground environment include a severely reduced eye size and regressed visual system. Responses of the circadian system to light, however, appear to be intact, since mole-rats are able to entrain their circadian activity rhythms to the light-dark cycle and light induces Fos expression in the suprachiasmatic nucleus (SCN). Social organization varies from solitary species to highly elaborated eusocial structures, characterized by a distinct division of labor and in which one reproductive female regulates the behavior and reproductive physiology of other individuals in the colony. The authors studied light-induced Fos expression in the SCN to increasing light intensities in four mole-rat species, ranging from strictly solitary to highly social. In the solitary Cape mole-rat, light induces significant Fos expression in the SCN, and the number of Fos-immunopositive cells increases with increasing light intensity. In contrast, Fos induction in the SCN of social species was slightly greater than, but not statistically different from, the dark-control animals as is typical of most rodents. One species showed a trend for an increase in expression with increased light, whereas a second species showed no trend in expression. In the naked mole-rat, Fos expression appeared higher in the dark-controls than in the animals exposed to light, although the differences in Fos expression were not significant. These results suggest a gradient in the sensitivity of the circadian system to light in mole-rats, with a higher percentage of individuals that are unresponsive to light in correlation with the degree of sociality. In highly social species, such as the naked mole-rat that live in a relatively stable subterranean milieu in terms of food availability, temperature, constant darkness, and devoid of 24-h cyclic environmental cues, the temporal coordination of rest-wake activities may be dependent on social interactions and social status rather than on photic regulation of the circadian timing system.
Oosthuizen
Maria K
MK
Mammal Research Institute, Department of Zoology and Entomology, University of Pretoria, Pretoria, South Africa. moosthuizen@zoology.up.ac.za
Bennett
Nigel C
NC
Cooper
Howard M
HM
D000818:Animals; D057906:Circadian Clocks; D002940:Circadian Rhythm / Q000502:physiology; D003624:Darkness; D005260:Female; D008027:Light; D019577:Mole Rats / Q000378:metabolism*; D010775:Photic Stimulation; D017440:Photoperiod; D016760:Proto-Oncogene Proteins c-fos / Q000378:metabolism*; D051381:Rats; D013493:Suprachiasmatic Nucleus / Q000166:cytology / Q000378:metabolism*
D016760:Proto-Oncogene Proteins c-fos