PHOTIC INDUCTION OF Fos IN THE SUPRACHIASMATIC NUCLEUS OF AFRICAN MOLE-RATS: RESPONSES TO INCREASING IRRADIANCE

Abstract

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.

Perspectives of Homo sapiens lifespan extension: focus on external or internal resources?

“…Moreover, the epiphysis has not been found in H. glaber ([113], single-animal study, confirmed by V. Manskikh in our group). H. glaber does not respond significantly to the increased Fos gene expression level in the suprachiasmatic nucleus of the hypothalamus in response to increased illumination, which is different from the response of other visually impaired fossorial species, such as the common mole rat Cryptomys hottentotus [114,115].…”

Locomotor Activity and Body Temperature Patterns over a Temperature Gradient in the Highveld Mole-Rat (Cryptomys hottentotus pretoriae)

“…However, the mole-rat circadian system remains intact, and compares anatomically in size to the circadian structures in the brains of other rodents [10–12]. Light also reaches the circadian clock in the SCN, as illustrated by expression of Fos protein in response to light stimulation [13,14]. Furthermore, many mole-rat species can entrain their locomotor activity patterns to light cycles and also display endogenous rhythms of locomotor activity in the absence of light cues [15–18].…”

Physiological rhythms are influenced by photophase wavelength in a nocturnal and a diurnal rodent species from South Africa

“…In a previous study, we showed that changes in light intensity of the photophase did not affect locomotor activity, urine production or urinary 6-SMT in the same manner in M. namaquensis and R. pumilio (Van der Merwe et al, 2017). These variations could stem from differences in the SCN response to light of different intensities and wavelengths (Bonmati-Carrion et al, 2017;Dkhissi-Benyahya et al, 2000;Oosthuizen et al, 2010), and most likely reflected adaptations to the vastly distinct photo-environments of the temporal niches to which these two species are confined in their natural habitats.…”