The development of afternoon minisurges of luteinizing hormone secretion in prepubertal female rats is ovary dependent

We have recently disclosed sustained episodes of LH release (minisurges) during the afternoons of the juvenileperipubertal transition period in the female rat. To determine if these LH minisurges are gonad independent, i.e. develop in the absence of the ovaries, animals were ovariectomized when neonates, and the mode of LH release was examined using a 5-min blood-sampling regimen at one of three ages corresponding to the juvenile, peripubertal, or adult phase of development. In no instance was a minisurge of LH secretion detected. We were Concerned, however, that LH minisurges may have been obscured by the exceedingly high level of LH secretion in these long term ovariectomized rats and, therefore, decided to pursue the study employing a short term (48-h) ovariectomy paradigm. Late juvenile rats which had been ovariectomized for 48 h exhibited conspicuous LH pulses, but, again, LH minisurges were not detected, further suggesting that these sustained secretory episodes do not occur in the absence of the ovaries. Next, plasma estradiol (E₂) levels were differentially raised in 48-h ovariectomized juvenile rats via sc implantation of Silastic capsules containing the steroid (dissolved in corn oil at various concentrations), and plasma LH was measured at 1-h intervals. The highest E₂ concentration elicited a midafternoon LH increase of preovulatory magnitude in all cases, whereas the lowest E₂ concentrations consistently suppressed plasma LH. However, intermediate E₂ concentrations (producing plasma E₂ levels 20–30% greater than those found in intact controls) elicited, in several instances, an increase in LH release of intermediate magnitude. To clarify the nature of these LH responses, we examined, using a 5-min blood-sampling regimen, the afternoon pulsatile pattern of LH release after treatment with appropriate doses of E₂. As expected from the results of the infrequent bloodsampling paradigm, the highest E₂ dose induced proestrus-like surges of LH secretion, while the lowest dose suppressed pulsatile LH release. Moreover, an intermediate E₂ dose which raised plasma E₂ levels just above those of intact animals, was again able to cause suppression of LH pulses, a proestrus-like increase in LH output, or, more importantly, a minisurge of LH secretion. These data indicate that the appearance of minisurges of LH release during sexual development of the female rat is the consequence of subtle increases in ovarian E₂ secretion. They also suggest that E₂-dependent minisurges of LH secretion represent a gradual development of the large preovulatory LH surge that occurs at first proestrus.