Age-related changes in biogenic amines, opiate, and steroid receptors in the prepubertal bull calf

Events leading to the increase in pulsatile LH secretion during prepubertal development in the bull calf may include removal of inhibitory or the development of stimulatory mechanisms affecting the hypothalamic release of GnRH. To examine possible contributing systems, serial blood samples were collected from Holstein bull calves at 2, 5, and 10 wk of age one day prior to receiving either no treatment (controls) or two injections of alpha-methyl-p-tyrosine (alpha-MPT), an inhibitor of catecholamine synthesis. Blood was sampled every 10 min for 5 h and serum was analyzed for LH by RIA. Following treatment, animals were killed and hypothalamic and pituitary tissues were removed for analysis of total opiate receptors, micro-opiate receptors, estrogen and androgen receptors and concentrations of monoamines: dopamine, the dopamine metabolite 3,4-dihydroxyphenylacetic acid (DOPAC), norepinephrine, serotonin, and its metabolite 5-hydroxyindole-3-acetic acid (5-HIAA). Pulses of LH increased from nondetectable at 2 wk to nearly 1.5 pulses per sampling period at 10 wk. Pulse height rose to 0.95 +/- 0.16 ng/ml at 10 wk. Total opiate receptor number as determined by binding to naloxone was unchanged in all tissues between 2 and 10 wk. In contrast, micro-opiate receptors (DAGO binding) increased 2-fold in the preoptic-anterior hypothalamic area between 5 and 10 wk. No age-related changes in estrogen receptor concentrations were observed in any tissue except the anterior pituitary in which binding increased 3.2-fold between 2 and 10 wk. A similar increase was not noted for androgen receptors in the pituitary; however, testosterone binding in the preoptic-anterior hypothalamic area was 4.6-fold higher at 5 wk compared to levels at 2 and 10 wk. Concentrations of norepinephrine, dopamine, and DOPAC increased 2- to 3-fold in the anterior hypothalamic-preoptic area in control calves between 5 and 10 wk of age while alpha-MPT treatment reduced dopamine concentrations in this same tissue by 55% 10 wk of age. These findings suggest that development of pulsatile LH secretion in the postnatal calf is not associated with a decline in steroid or opiate inhibitory tone through alterations in receptor numbers, but is associated with increased catecholaminergic neuronal activity within the anterior hypothalamic-preoptic area.