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Reproductive Sciences Program and the Departments of Obstetrics and Gynecology, Biology, and Physiology, University of Michigan, Ann Arbor, Michigan 48109
Address all correspondence and requests for reprints to: Dr. Douglas L. Foster, Reproductive Sciences Program, Room 1101SW, 300 North Ingalls Building, University of Michigan, Ann Arbor, Michigan 48109-0404. E-mail: dlfoster{at}umich.edu
Growth retardation induced by dietary restriction results in hypogonadotropism, and thus, puberty is delayed. The present studies determined 1) whether reduced LH secretion in the growth-retarded condition is due to a reduction in the frequency and/or in the amplitude of GnRH secretion, and 2) whether the mechanism regulating LH secretion is being actively inhibited via central mechanisms. To determine whether GnRH pulse frequency and/or amplitude are reduced during growth restriction, blood samples were simultaneously collected from pituitary portal blood for GnRH and from jugular blood for LH determinations over a 4-h period in ovariectomized lambs (52 wk of age) that were either growth restricted (28 kg; n = 8) or growing normally (60 kg; n = 7). As expected, the growth-restricted females were hypogonadotropic and exhibited a long LH interpulse interval compared with the normally growing females. However, although the GnRH interpulse interval was longer in the growth-restricted lambs compared with that in the normally growing lambs, the pattern of GnRH secretion did not directly correspond with that of LH secretion in the growth-restricted group. In addition, high amplitude GnRH pulses that coincided with LH pulses and small, low amplitude GnRH pulses without a concomitant LH pulse occurred.
The second study tested the hypothesis that diet-induced
hypogonadotropism is the result of actively inhibited central
mechanisms by investigating the effects of the nonspecific central
nervous system inhibitor, sodium pentobarbital, on pulsatile LH
secretion in the growth-restricted lamb. Serial blood samples were
collected from 11 ovariectomized lambs that were maintained at weaning
weight (
20 kg) by reduced diet. After a 4-h pretreatment period, six
of the lambs were anesthetized with sodium pentobarbital for 4 h;
the other five lambs were untreated and served as controls.
Pentobarbital anesthesia reduced the LH interpulse interval (increased
the frequency) and increased mean LH levels.
These findings suggest that during growth restriction hypogonadotropism arises from a central inhibition of GnRH neurons and is manifest as a decrease in both frequency and amplitude of GnRH pulses.
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