Orexinergic neuron numbers in three species of African mole rats with rhythmic and arrhythmic chronotypes

Abstract In the present study, orexinergic cell bodies within the brains of rhythmic and arrhythmic circadian chronotypes from three species of African mole rat (Highveld mole rat— Cryptomys hottentotus pretoriae , Ansell's mole rat— Fukomys anselli and the Damaraland mole rat— Fukomys damarensis ) were identified using immunohistochemistry for orexin-A. Immunopositive orexinergic (Orx+) cell bodies were stereologically assessed and absolute numbers of orexinergic cell bodies were determined for the distinct circadian chronotypes of each species of mole rat examined. The aim of the study was to investigate whether the absolute numbers of identified orexinergic neurons differs between distinct circadian chronotypes with the hypothesis of elevated hypothalamic orexinergic neurons in the arrhythmic chronotypes compared with the rhythmic chronotypes. We found statistically significant differences between the circadian chronotypes of F. anselli , where the arrhythmic group had higher mean numbers of hypothalamic orexin neurons compared with the rhythmic group. These differences were observed when the raw data was compared and when the raw data was corrected for body mass ( M b ) and brain mass ( M br ). For the two other species investigated, no significant differences were noted between the chronotypes, although a statistically significant difference was noted between all rhythmic and arrhythmic individuals of the current study when the counts of orexin neurons were corrected for Mb —the arrhythmic individuals had larger numbers of orexin cells.