Stimulation of the uterine cervix in the rat induces a pattern of PRL secretion composed of two daily surges: a diurnal surge occurring as the lights go off and a nocturnal surge occurring as the lights go on. We recently demonstrated that these PRL surges are a true circadian rhythm in that they are entrained to an alternating photoperiod but free-run in constant dark. To determine if this induced rhythm is generated by the suprachiasmatic nuclei (SCN), as are the other endogenous rhythms, we examined the pattern of PRL secretion in rats which were first cervically stimulated and then lesioned in the SCN. The cervices of rats maintained in a 12-h light, 12-h dark photoperiod were stimulated electrically for 30 sec at 1900 h (day 0). The next day, cannulae were inserted in their carotid arteries, and radiofrequency lesions were made in the SCN. Blood samples for PRL analysis were obtained at 2-h intervals from 0900 h on day 3 to 1100 h on day 4. Sham-operated rats and rats in which the SCN were missed or incompletely lesioned exhibited the diurnal and nocturnal surges of PRL. However, the PRL surges were not present in rats in which the SCN were completely lesioned. These results suggest that the SCN are critical for generation of the PRL surges induced by cervical stimulation. Thus, PRL secretion after cervical stimulation is a circadian rhythm generated by the SCN and, as such, is analogous to other endogenous or spontaneous rhythms. It is unique in that it is not expressed until cervical stimulation is applied, and therefore, we postulate the participation of additional neural elements in this rhythm of hormone secretion.
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