In the present study we investigated the extent to which expression of the diurnal pattern of LH secretion in prepubertal female rats is driven by an ovarian-independent neuroendocrine mechanism. To remove gonadal influences, rats were ovariectomized (OVX) during the early juvenile, late juvenile, or peripubertal phases of development (20-22, 26-27, and 32-33 days of age, respectively), and blood samples were collected throughout the day (every 30-60 min) 2 or 4 days after surgery. Morning plasma LH levels were relatively low 2 days after ovariectomy (30-120 ng/ml), but rose during the afternoon to reach levels ranging from 180-300 ng/ml. This afternoon elevation was sustained both in late juvenile and peripubertal rats, but not in early juvenile rats. The predominant change observed 4 days after ovariectomy was an overall 3- to 6-fold increase in plasma LH levels, which masked the afternoon elevation. Analysis of plasma LH profiles from individual rats, however, revealed that at each of the three ages studied the peak plasma LH levels occurred in the afternoon, and these were most pronounced in the peripubertal animals. To further clarify the existence of such a diurnal pattern of LH release in the 4- day OVX animals pulsatile LH release profiles were obtained from individual animals, using a 5-min bleeding paradigm, at either 24 days (in the morning) or 36 days of age (in either the morning or the afternoon). LH release was episodic in all of the animals studied, with pulses occurring on average of about once every 30 min. Analysis of the plasma LH profiles using the PULSAR algorithm revealed that the overall mean plasma LH levels of the peripubertal animals was enhanced during the afternoon, compared to that in the morning or to the afternoon LH levels in the early juvenile rats. Mean nadir and mean peak LH levels were also greater. No differences in LH pulse amplitude and only marginal differences in LH pulse frequency were detected among the three groups studied. The enhancement of afternoon LH secretion in the OVX rats could not be attributed to age-related or diurnal changes in adenohypophyseal responsiveness to LHRH. The results suggest that the initiation of enhanced afternoon LH secretion, previously shown to occur in intact female rats during the juvenile-peripubertal transition period, results primarily from the activation of a central neuroendocrine mechanism. Although the ovaries might play a role in the development of this diurnal pattern, it is clear that its activation is ovarian independent.
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