Age-related changes in body composition, leptin, and hypothalamic-pituitary expression
of the leptin receptor and associative relationships of these factors to constituents
of the growth hormone (GH) axis were evaluated. Seventy wethers were randomly assigned
at birth to one of four treatment groups: control; treatment 1 implanted with the
estrogenic compound zeranol (12 mg, Ralgro on days 0, 45, and 90; treatment 2 received
zeranol on days 45 and 90; and treatment 3 received zeranol on day 90. Serum and tissues
were collected from wethers (n > or = 5) from each group on days 28, 73, 118, and
135. Percent body fat and leptin increased linearly (P < 0.01) with age, but were
not influenced (P > or = 0.14) by zeranol. The leptin receptor in the pituitary appeared
to be differentially (P = 0.097) expressed across days 73-135, but no differences
(P > or = 0.43) were detected in expression of this receptor in the hypothalamus among
treatments and ages. Leptin and % body fat were negatively correlated (r > or = -0.52,
P < 0.05) to mRNA levels of factors involved in pituitary synthesis and secretion
of GH. Serum leptin increased with age as did percent body fat, but zeranol did not
influence body composition, serum leptin, or expression of the leptin receptor in
the hypothalamus or pituitary; however, the leptin receptor appeared to be differentially
expressed among the hypothalamus and pituitary with level of body fat and leptin being
inversely associated to transcriptional-factors involved in somatotrope synthesis
and secretion of GH.