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 prod uct in cells of the suprachiasmatic nuclei and the photoperiodic contr ol of testicular activity were examined in mink. Mink were kept in a s hort photoperiod (control, LD4:20), or in 'asymmetric skeleton photope riods' (groups A and B). The light period for groups A and B was divid ed into two fractions (3 h 30 min and 30 min); the shorter fraction oc curred in the night, 4 h (group A) or 8 h (group B) after the end of t he main light period. There was no photic activation of the proto-onco gene c-fos on the control or group A, and 4 weeks on this photoperiodi c treatment produced marked testicular development. In contrast, in gr oup B, c-fos mRNA was induced 30 min after the end of the secondary ph otofraction, was maximal 30 min later and then decreased. Fos-like imm unoreactivity was detected 2 h after the end of the secondary photofra ction, with activity peaking 1 h later. The animals of this group rema ined sexually quiescent. These results suggest that photo-induction of the proto-oncogene c-fos is implicated in the gonadal inhibition indu ced in this species when the light period, extends into the photosensi tive phase of the circadian rhythm of photosensitivity.