Effect of Growth Retardation on Pituitary Luteinizing Hormone and Hypothalamic Neuropeptide Y in Ovariectomized Sheep

Abstract

The possible role of neuropeptide Y (NPY) in mediating the relationship between pituitary LH secretion and growth retardation due to restricted feeding was examined in ovariectomized (OVX) prepubertal ewe lambs. One specific objective examined whether there was an inverse relationship between concentrations of NPY in four diencephalic brain regions and pituitary LH secretion in ewe lambs 29-30 weeks old which had been growth retarded since 8 weeks and OVX at 24 weeks. Through dietary restriction, body weight was held constant at 20.7 ± 0.5 kg in 13 growth-retarded ewes as compared with 48.0 ± 0.6 kg for 5 age-matched control ewes at 29-30 weeks of age. Episodic LH was quantified at 10-min intervals for 190 min/day on 3 of the 8 days immediately before euthanasia. Serum LH averaged 6.5 ± 0.6 ng/ml in control ewes with a mean pulse frequency of 1.0 ± 0.1 pulses/h. Serum LH in growth-retarded ewes was much less episodic (0.2 ± 0.05 pulses/h) and averaged only 1.2 ± 0.2 ng/ml. All ewes were euthanized during week 30, and the following brain regions were dissected: basal forebrain, preoptic area, median eminence and remainder of hypothalamus. Following extraction, NPY concentrations (pg/mg of original tissue) were quantified by radioimmunoassay. In each brain region, NPY concentrations were greater (p < 0.05) in 6 growth-retarded ewes than in 5 control ewes (median eminence: 5.2 vs. 0.6; remainder of hypothalamus: 3.3 vs. 0.8; preoptic area: 3.1 vs. 0.8, and basal forebrain: 2.2 vs. 1.2). A secondary objective examined whether the LH and NPY parameters were rapidly altered by transient changes in feed consumption. Therefore, a subgroup of 7 growth-retarded ewes was given access to increased feed during the last week of the experiment (denoted as refed growth-retarded), but feed consumption and weight gain were highly variable as only 1 ewe maintained her initially increased feed consumption for the entire 7 days. Secretion of LH as reflected in mean concentrations (2.1 ± 0.5 ng/ml) and pulse frequency (0.4 ± 0.1 pulses/h) was slightly increased (p < 0.05) in refed ewes. However, NPY concentrations (median eminence: 6.6; remainder of hypothalamus: 3.6; preoptic area: 3.6, and basal forebrain: 2.5) in the 7 refed growth-retarded ewes were similar to those of the 6 growth-retarded ewes which were not refed. Among individual refed growth-retarded ewes, there was no clear relationship among weight gain/feed consumption and hypothalamic NPY. In summary, steroid-independent suppression of LH secretion in chronically growth-retarded ewes was inversely related to elevated levels of NPY in median eminence and other diencephalic regions. Increased food availability forpnly 7 days reversed only slightly the suppression of LH and did not affect the elevated levels of hypothalamic NPY. Except for these divergent changes in refed ewes, the main results of this study are consistent with the hypothesis that increased neural production of NPY contributed to suppression of LH release in growth-retarded OVX ewes.