Photic regulation of c-fos gene expression in the suprachiasmatic nucleus and the circadian rhythm of photosensitivity in the mink

The relationship between the photic stimulation of the c- fos gene product in cells of the suprachiasmatic nuclei and the photoperiodic control of testicular activity were examined in mink. Mink were kept in a short photoperiod (control, LD4:20), or in ‘asymmetric skeleton photoperiods’ (groups A and B). The light period for groups A and B was divided into two fractions (3 h 30 min and 30 min); the shorter fraction occurred in the night, 4 h (group A) or 8 h (group B) after the end of the main light period. There was no photic activation of the proto-oncogene c- fos on the control or group A, and 4 weeks on this photoperiodic treatment produced marked testicular development. In contrast, in group B, c- fos mRNA was induced 30 min after the end of the secondary photofraction, was maximal 30 min later and then decreased. Fos-like immunoreactivity was detected 2 h after the end of the secondary photofraction, with activity peaking 1 h later. The animals of this group remained sexually quiescent. These results suggest that photo-induction of the photo-oncogene c- fos is implicated in the gonadal inhibition induced in this species when the light period, extends into the photosensitive phase of the circadian rhythm of photosensitivity.