The circadian system of African mole-rats : behavioural activity rhythms and early gene expression ( c-fos ) in the suprachiasmatic nucleus

Mole-rats from the family Bathyergidae are endemic to Africa, this family exhibits a continuum of sociality, comprising both solitary and social species. Sociality is related to the degree of aridity and the distribution of the underground food resource. All the members of the bathyergid family are strictly subterranean, and occur in a niche which is devoid of light cues and thermally buffered from ambient and surface extremes. Since vision is redundant in a lightless environment, mole-rats have subsequently undergone ocular regression over evolutionary time. As a consequence of the minute eyes, the visual system of mole rats is severely regressed and, in addition, the proportional retinal innervation to different structures is modified. The classical visual system is reduced while the circadian system is expanded. Retinal projection studies on the giant Zambian mole-rat, Cryptomys mechowi and an albino highveld mole-rat, Cryptomys hottentotus pretoriae, confirmed sparse contralateral retinal projections to structures of the visual system, while the circadian system received relatively dense bilateral innervation. The innervation pattern of an albino Damaraland mole-rat, Cryptomys damarensis differed from the other animals. Investigations of Fos expression in neurons over circadian time suggested that the phase response curve of the solitary mole rat, Georychus capensis, resembles that of aboveground mammals whereas the social Cryptomys hottentotus pretoriae, does not display differential sensitivity to light in the subjective day and night. The influence of increasing light intensities showed that higher light intensities elicit a more pronounced Fos expression in SCN of all the species investigated. In addition, longer light pulses also increases the Fos induction in the SCN. A preliminary investigation into the effect of temperature on the Fos induction in the SCN of three mole-rat species, demonstrated that a higher Fos response could be expected with higher ambient temperatures. However the sample size was very small, and could have influenced the outcome of the experiment. Behavioural locomotor activity rhythms of the solitary species, Georychus capensis, and the social species Cryptomys hottentotus pretoriae and Cryptomys damarensis, confirmed that activity patterns correlate with trends displayed in Fos expression. The solitary species exhibited much more defined rhythmicity than the social species and a higher percentage of the animals displayed distinct endogenous rhythms. African mole rats provide an interesting model to study not only the features of the circadian system in a group of animals with a naturally regressed visual system, but also the influence of sociality on the degree of regression.