The ability of uterine cervical stimulation (CS) to initiate prolactin surges was studied in immature rats. The cervix was stimulated with a glass rod for 60 sec at 1700 h (lights on 0600-1800 h). CS of 20- to 23-day-oid rats failed to release prolactin the next morning at times corresponding to a nocturnal surge (05001100 h). In contrast, CS on day 24 resulted in typical nocturnal prolactin surges on days 25–27. However, no diurnal surges were observed. After CS on day 23, even though no prolactin surges occurred on day 24, nocturnal surges were observed on days 25 and 26. Therefore, the ability to retain the information from CS develops by day 23 whereas the ability to express the information as prolactin surges does not develop until day 25. Ovarian steroids did not alter the timing of this development, as only the magnitude of the surges was affected by the presence or absence of ovarian steroids. The failure to release prolactin after CS of androgen-sterilized females suggests that development of the ability to release prolactin is primarily a hypothalamic event. The relationship of a time of day signal to prolactin surges was studied in immature rats raised in constant light. Prolactin secretion increased immediately after CS but not at times corresponding to the nocturnal surge. Therefore, in the absence of an appropriate time of day signal, CS becomes a typical neuroendocrine reflex with hormone secretion increasing immediately after the stimulus.
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