Changes in angiotensin II receptor bindings in the hen neurohypophysis before and after oviposition.

The present study was performed to elucidate whether the angiotensin II (ANG II) receptor exists in the plasma membrane fraction of the neurohypophysis in hens, to estimate the time of action of ANG II on the neurohypophysis before and after oviposition, and to examine relationships between the action of ANG II on the neurohypophysis and those of estrogen and prostaglandin F(2α) (PGF(2α)) in relation to arginine vasotocin (AVT) release. The specific binding had a binding specificity to chicken ANG II (cANG II), reversibility, and saturation in the [(125)I]cANG II binding assay. Scatchard analysis revealed that the binding sites are of a single class. The equilibrium dissociation constant (K(d)) obtained by kinetic analysis and Scatchard analysis suggested a high affinity, and the maximum binding capacity (B(max)) obtained by Scatchard analysis suggested a limited capacity. These results suggest that an ANG II receptor exists in the neurohypophysis of hens. The K(d) and the B(max) value was significantly smaller in laying hens than in nonlaying hens, which suggests that bindings of the cANG II receptor change, depending on the difference in laying condition. Values of the K(d) and the B(max) decreased approximately 15 min before oviposition in laying hens, and decreased 1 h after an intramuscular injection of estradiol-17ß and 5 min after an intravenous injection of cANG II in nonlaying hens. The amount of specific binding of PGF(2α) receptor in the neurohypophysis also decreased and AVT concentration in blood increased after the cANG II injection. It seems likely that the action of cANG II in the neurohypophysis increases due to the effect of estrogen approximately 15 min before oviposition, and the cANG II action stimulates AVT release through the increase in the PGF(2α) action in this tissue.

Changes in prostaglandin F2α receptor bindings in the hen oviduct uterus before and after oviposition.

The specific binding component for prostaglandin F(2α) (PGF(2α)) that exists in the plasma membrane fraction of the oviduct uterus myometrium of laying hens was shown to possess receptor properties for PGF(2α), such as binding specificity to PGF(2α), binding saturation, high affinity, and limited capacity. The value of the equilibrium dissociation constant (K(d)) for the receptor was not different between laying hens and nonlaying hens, but the value of the maximum binding capacity (B(max)) was smaller in laying hens than in nonlaying hens. During an oviposition cycle, the K(d) value did not show a significant change, but the B(max) value decreased at 3 and 0.5 h before oviposition and 2 h after oviposition. Neither the K(d) nor B(max) value changed in nonlaying hens during a 24-h period. An intravenous injection of PGF(2α) (5 µg/hen) decreased the B(max) value, but not the K(d) value, of the PGF(2α) receptor. It is thought from the results that PGF(2α) may act directly on the oviduct uterus myometrium at a fixed time before and after oviposition in laying hens.

Calcitonin receptor bindings in the hen hypothalamus before and after oviposition.

To demonstrate the presence of a receptor for calcitonin (CT) in the hen hypothalamus and to determine when CT acts on this tissue during the oviposition cycle, bindings of (125)I labeled CT in the plasma membrane fraction of the hen hypothalamus were measured by radioligand binding assay. The specific CT binding component in the plasma membrane fraction of the hypothalamus containing the preoptic area (HPOA) possessed properties of a receptor: binding specificity to CT, saturable binding, high affinity, and limited capacity. As for the median eminence area, no specific binding component was found in the present study. Therefore, the binding component for CT in the plasma membrane fraction of HPOA is likely to be a receptor for CT. In laying hens, the binding affinity of CT receptor increased at 30 min before oviposition and the binding capacity was decreased at 30 min before oviposition but not changed in nonlaying hens during a 24-h period. These results suggest that the action of CT on the hen HPOA may increase 30 min before oviposition.

Parathyroid hormone-related peptide directly increases adrenocorticotropic hormone secretion from the anterior pituitary in hens.

The presence of the receptor for parathyroid hormone-related peptide (PTHrP) and the effect of PTHrP on adrenocorticotropic hormone (ACTH) secretion in the hen anterior pituitary were examined. The plasma membrane fraction of the anterior pituitary was found to contain a specific chicken PTHrP (cPTHrP) binding component. The binding component had properties of a receptor, such as binding specificity to cPTHrP, reversibility, saturable binding, high affinity, and limited capacity; therefore, it was elucidated that the PTHrP receptor exists in the plasma membrane of the hen anterior pituitary. A third ventricular injection of cPTHrP in nonlaying hens caused a decrease in the chicken ACTH level of the anterior pituitary and an increase in the chicken ACTH level of blood plasma, with an increase in the binding affinity and a decrease in the binding capacity of PTHrP receptor in the anterior pituitary. The present study suggests that PTHrP may act directly on the anterior pituitary via its receptor binding and may enhance ACTH secretion from this tissue in hens.

Effect of estradiol-17ß on calcitonin receptor bindings in the hen neurohypophysis.

The present study was performed to elucidate whether estradiol-17ß (E2) would affect calcitonin (CT) receptor binding in the hen neurohypophysis. The equilibrium dissociation constant (K(d)) and the maximum binding capacity (B(max)) of the CT receptor in the plasma membrane fraction of the hen neurohypophysis were examined by Scatchard analysis of specific binding of (125)I-labeled chicken CT. A single i.m. injection of E2 into nonlaying hens caused a decrease in K(d) and B(max) values of the CT receptor. The K(d) and B(max) values of the CT receptor were smaller in laying hens than in nonlaying hens. The present study suggests that E2 may increase the action of CT on the neurohypophysis in hens.

Calcitonin receptor binding in the hen neurohypophysis before and after oviposition.

To demonstrate the presence of a receptor for calcitonin (CT) in the hen neurohypophysis and to estimate the time of action of CT on the neurohypophysis during the oviposition cycle in relation to arginine vasotocin (AVT) release, binding of (125)I-labeled chicken CT in plasma membrane fractions of the hen neurohypophysis was measured by the use of a radioligand binding assay. The binding specificity, reversibility, high affinity, and limited capacity are characteristics of a CT receptor. Therefore, it was elucidated that the CT receptor might exist in the plasma membrane of the neurohypophysis of hens. The binding affinity of CT receptor increased at 30 min before oviposition and the binding capacity was decreased at 15 min before oviposition. However, no change was found in non-laying hens during a 24-h period. Such changes in the CT receptor binding were found at 10 min after an i.v. injection of chicken CT into non-laying hens with an increase in the blood level of AVT. The changes in the binding affinity and capacity of CT receptor of the neurohypophysis may be related to AVT release partly at oviposition time in the hen.

Calcitonin directly increases adrenocorticotropic hormone-stimulated corticosterone production in the hen adrenal gland.

The present study was performed to elucidate whether the receptor for calcitonin (CT) exists in the adrenocortical cells of hens and to determine the effect of CT on adrenocorticotropic hormone (ACTH)-stimulated corticosterone production in its cell. The binding site of CT in the membrane fraction of the adrenal gland in hens was determined using a [125I]CT binding assay system. The binding properties in the adrenal gland satisfied the criteria of a receptor-ligand interaction in terms of specificity, reversibility, and saturation. When the cortical cells were incubated in vitro with chicken ACTH in the presence of CT, greater corticosterone production was observed. The result suggested that CT acts directly on the adrenocortical cells via its receptor binding and increases responsiveness of ACTH on corticosterone production in the laying hen.

2,3,7,8-tetrachlorodibenzo-p-dioxin decreases responsiveness of the hypothalamus to estradiol as a feedback inducer of preovulatory gonadotropin secretion in the immature gonadotropin-primed rat.

Sprague-Dawley rats (23-day-old) were dosed with TCDD (32 microg/kg) in corn oil or vehicle alone. Equine chorionic gonadotropin (eCG) was injected (5 IU, sc) 24 h later to induce follicular development. Another 24 h later, half of TCDD- or corn oil-treated rats were injected (sc) with 17 beta-estradiol-cypionate (ECP, at 0.004 to 0.5 mg/kg). Blood and ovaries were collected on expected proestrous (preovulatory period) at 51, 54, and 58 h after eCG injection as well as in the morning after ovulation (72 h after eCG). Serum concentrations of 17 beta-estradiol (E), progesterone (P), luteinizing hormone (LH), and follicle-stimulating hormone (FSH) were determined by radioimmunoassay. The number of ova shed was measured at 72 h after injection of eCG by irrigating ova from oviducts. During the preovulatory period (approximately 58 h after eCG injection), a circulating level of 70-100 pg E/ml coincided with LH and FSH surges and later normal ovulation of 10 to 12 ova/rat was observed in controls. However, the same concentration of E was not associated with LH and FSH surges in rats treated with TCDD (32 microg/kg), resulting in reduced ovarian weight gain and reduction of ovulation by 70 to 80% (2-3 ova/rat). Blockage of the gonadotropin surge, reduced ovarian weight gain, and ovulation were all reversed completely by the lowest effective dose of ECP (0.1 mg/kg). At 72 h after eCG, serum P secretion was reduced and serum E levels were significantly increased compared to those of corn oil-treated controls. ECP alone had no effect on serum P levels at any time point, but in rats treated with TCDD and ECP, both the reduction of P (at 58 and 72 h) and the increase in E secretion (72 h) were completely reversed. Further studies confirmed that restoration by ECP of gonadotropin surges and associated ovulation could not be attained until circulating levels of E rose sufficiently high to trigger the LH and FSH surges. The new action threshold of E for inducing gonadotropin surges in rats treated with TCDD (32 microg/kg) was determined to be eight- to 10-fold higher than that in controls. Thus, it is apparent that TCDD decreased the responsiveness of the hypothalamus to E as a feedback inducer of preovulatory gonadotropin secretion.