Prolactin secretion after cervical stimulation of rats maintained in constant dark or constant light

The pattern of PRL secretion induced by stimulation of the uterine cervix is composed of two daily surges; the diurnal surge occurs as the lights go off and the nocturnal surge occurs as the lights come on. To determine if this PRL secretory rhythm was truly circadian, we determined the pattern of PRL secretion in rats maintained in constant dark (D:D) and in constant light (L:L). We stimulated the cervix electrically for 30 sec at 1900 h in rats previously blinded and maintained in D:D for at least 40 days. Blood samples for PRL analysis by RIA were obtained at 2-h intervals for 48 h from aortic cannulae inserted 2 days before beginning the experiment. The PRL surges in D:D animals were of the same size and periodicity as the surges in animals maintained in a 12-h light, 12-h dark lighting schedule. However, the surges were not synchronous among individual D:D animals nor with the surges of 12-h light, 12-dark animals. Thus, the PRL secretory pattern was freerunning in D:D and is a true circadian rhythm. Rats maintained in L:L until persistent vaginal estrus occurred were mated with vasectomized males. Cannulae were inserted in their carotid arteries the next day and, after an additional 2 days, blood samples for PRL analysis were obtained at 2-hr intervals for 24 h. A PRL secretory rhythm, characterized by attenuated and abbreviated surges with a periodicity of 6–8 h, was observed in these animals. These results demonstrate that photoperiod is an important controlling factor in the pattern of PRL secretion engendered by cervical stimulation. The pattern is disrupted by L:L but becomes free-running in the absence of light. The similarity of the PRL rhythm to other endogenous rhythms which are known to be generated by the suprachiasmatic nuclei suggests that the PRL rhythm may be generated by these nuclei also.