S.C. Fehrer

Effects of estrogen and progesterone on serum prolactin and luteinizing hormone levels in ovariectomized turkeys (Meleagris gallopavo)

Serum prolactin (PRL) and luteinizing hormone (LH) levels from mature female turkeys were determined following ovariectomy and daily subcutaneous injections for 4 or 9 days of 0.002-2.0 mg/kg estradiol benzoate (EB), 0.05-2.0 mg/kg progesterone (P), or their combinations. Serum PRL levels were increased (P less than 0.05) at the onset of sexual maturity in intact controls, but not in ovariectomized turkeys. Injection of 0.02 mg/kg EB into ovariectomized turkeys resulted in elevated (P less than 0.05) serum PRL levels after 8 treatment days. Higher doses failed to elicit a response of greater magnitude. EB at doses of 0.02-2.0 mg/kg reduced LH levels while 0.002 mg/kg EB increased LH levels above (P less than 0.05) those of ovariectomized controls. The 1.0-mg/kg P dose increased (P less than 0.05) serum PRL within 24-48 hr of administration while the 0.05-, 0.5-, or 2.0-mg/kg P doses failed to elicit a response. When P was injected in doses of 0.5-2.0 mg/kg, LH levels were decreased in a dose-response manner. The injection of EB combined with P had a variable effect on serum PRL levels. The 0.5- or 1.0-mg/kg dose of P facilitated (advanced in time) the rise in serum PRL level induced by 0.02 mg/kg of EB. In contrast, the positive feedback effect of 0.2 mg/kg of EB was blocked when administered with 0.5 mg/kg of P. Serum LH levels were dramatically decreased by all steroid combinations used. These results indicate that daily injections of EB and/or P in ovariectomized turkeys have a variable effect on serum PRL and LH levels depending upon the dose, the duration of treatment, or the ratio between EB and P.

Gonadal steroid modulation of basal and vasoactive intestinal polypeptide-stimulated prolactin release by Turkey anterior pituitary cells☆

Porcine vasoactive intestinal polypeptide (VIP; 10(-9) to 10(-7) M) was a potent stimulator of prolactin (PRL) release by anterior pituitary cells from immature and laying turkey hens. Basal and VIP-induced PRL release of cells from laying hens were diminished (P less than 0.05) when the cells were cultured for 48 hr in the presence of charcoal-stripped laying hen serum, but not when the cells were cultured in the presence of whole laying hen serum. This change in VIP-induced PRL release was not evident when cells were derived from immature hens. Basal PRL release by cells from laying hens was not altered by the presence of estradiol (E2; 10(-12) to 10(-5) M), although such release was generally enhanced in cultures of cells from immature hens containing E2. The presence of E2 enhanced (P less than 0.05) the magnitude of the VIP-induced PRL release by cultures of cells from laying hens and diminished (P less than 0.05) the magnitude of this release in cultures of cells from immature hens. Cells from immature and laying hens exposed to progesterone (P4; 10(-5) M) for 96 hr exhibited enhanced basal PRL release, though lower P4 concentrations had no effect. Utilizing cells from laying hens, P4 exposure for 24 hr resulted in diminished (P less than 0.05) VIP-induced PRL release, while P4 exposure for 96 hr resulted in markedly enhanced (P less than 0.05) VIP-induced PRL release. Basal PRL release was generally not altered by the presence of testosterone (T). The VIP-induced PRL release by cells derived from immature and laying hens was diminished (P less than 0.05) by the presence of T. Prolactin release in the turkey is likely modulated by gonadal steroids acting directly on the cells of the anterior pituitary.

The influence of acute or repeated immobilization on plasma prolactin levels in the turkey (Meleagris gallopavo)

Three experiments were conducted to evaluate the effect of immobilization on plasma prolactin (PRL) levels in the immature turkey. Acute immobilization for 120 min resulted in elevated PRL levels (P less than 0.05) starting 15 min after the onset of immobilization. Release from immobilization caused PRL levels to return to those of the nonimmobilized controls by 120-180 min following replacement of the turkeys in cages. Repeated immobilization for 3 or 5 consecutive days diminished PRL response (P less than 0.05) to subsequent immobilization. It is suggested that the PRL controlling mechanism(s) of the young turkey is both susceptible and able to habituate to changes induced by immobilization stress.

Basal and hypothalamic extract-induced luteinizing hormone and prolactin secretion by cultured anterior pituitary cells from female turkeys in various stages of the reproductive cycle.

The wet weight of the anterior pituitary gland of the domestic turkey increases as the hen progresses through the reproductive cycle. The greatest wet weight was observed with anterior pituitary glands from incubating hens, whose weight was twice that of anterior pituitary glands from nonphotostimulated hens. Anterior pituitary glands from hens in each of the various stages of the reproductive cycle were dissociated and cultured, and subsequently basal and hypothalamic extract (HE)-induced releases of prolactin (PRL) and luteinizing hormone (LH) were compared between cultures containing a defined number of anterior pituitary cells. Basal and HE-induced releases of PRL were greatest (P less than 0.05) in the cultures of anterior pituitary cells derived from incubating hens, with those of laying, photorefractory, and nonphotostimulated hens being successively less (P less than 0.05), respectively. HE-induced release of LH was greatest (P less than 0.05) in cell cultures derived from nonphotostimulated hens, with cultures of cells derived from laying, photorefractory, and incubating hens exhibiting successively smaller releases of LH (P less than 0.05), respectively. The concentration of HE that induced the first significant increase in the release of PRL or LH above that of basal levels also varied with the reproductive stage of the donor hens. The changes in circulating PRL levels during the various stages of the reproductive cycle reflect changes in anterior pituitary sensitivity to hypothalamic releasing activity and/or changes in the PRL releasing capacity of the anterior pituitary. In contrast, there does not appear to be a strong correlation between anterior pituitary LH releasing capacity in vitro and circulating LH levels in the domestic turkey hen.

Hypothalamic and serum factors influence on prolactin and luteinizing hormone release by the pituitary gland of the young turkey (Meleagris gallopavo).

Intravenous administration of 1.0 or 3.0 eq hypothalamic extract (HE) to 8-week-old male and 7-week-old female turkeys, respectively, induced an increase in circulating prolactin (PRL) levels but had no effect on circulating luteinizing hormone (LH) levels. The incubation of dissociated anterior pituitary cells from 13-week-old female turkeys with HE induced a dose-related increase in PRL release; however, only the highest dose of HE induced an increase in LH release. Coincubation of a hypothalamic fragment with anterior pituitary cells from 9-week-old females induced a release of both PRL and LH. Dissociated pituitary cells from 11-week-old females initially incubated for 3 hr in medium containing charcoal-treated (stripped) turkey serum yielded a larger release of PRL and LH in the presence of HE than did cells initially incubated with turkey serum or no serum. Luteinizing hormone-releasing hormone (LHRH) induction of LH release was greatest from cells initially incubated with stripped serum for 3 or 24 hr. The LHRH-induced LH release was completely blocked in cells initially incubated for 24 hr with turkey serum. The initial incubation of cells for 24 hr with stripped serum yielded a larger release of PRL and LH in response to HE than did cells initially incubated with serum. The hypothalamus of the young turkey contains substantial PRL-releasing activity as well as LH-releasing activity. The ability of the releasing factors to stimulate pituitary hormone release is influenced by factors present in the blood of the young turkey. This is especially evident in the LHRH-induced LH release where serum factors inhibited the release.

The influence of thyrotropin releasing hormone on in vivo prolactin release and in vitro prolactin, luteinizing hormone, and growth hormone release from dispersed pituitary cells of the young turkey (Meleagris gallopavo) ☆

Intravenous administration of 0.025, 0.25, or 2.5 micrograms/kg thyrotropin releasing hormone (TRH) to 4-week-old female turkeys induced a dose-dependent increase (P = 0.004) in serum prolactin (PRL) 15 min post-treatment. Dispersed anterior pituitary cell cultures were utilized to determine the effect of TRH on cellular release of PRL, luteinizing hormone (LH), and growth hormone (GH). In the first experiment, cells from 13-week-old male turkeys were initially incubated for 24 hr in Medium 199 (M-199) plus 10% turkey serum and then placed in M-199 plus 10(-10) to 10(-4) M TRH for 5 hr. Incubation with TRH produced no change in PRL release from that of spontaneous release (P = 0.854). However, 10(-5) and 10(-4) M TRH induced LH release (P less than 0.0001). The TRH-induced GH response was parabolic (P less than 0.0001), with the maximal release at 10(-8) M. The second experiment, utilizing pituitary cells from 7-week-old females, studied these responses on 3, 5, and 7 days of monolayer incubation. TRH failed to induce a PRL release in all tests (P greater than 0.162), although hypothalamic extract induced a large release (P less than 0.0001) of PRL each time. Both 10(-6) and 10(-4) M TRH induced a LH release on Day 3 while only 10(-4) M did so on Day 5, and none of the doses elicited a release on Day 7. The parabolic GH response generally persisted in all tests.(ABSTRACT TRUNCATED AT 250 WORDS)

Inhibitory influence of catecholamines on prolactin release in the turkey (Meleagris gallopavo)

The involvement of catecholamines (CAs) in the control of prolactin (PRL) secretion was investigated in female turkeys. The birds were treated with DL-alpha-methyltyrosine methyl ester HCl (alpha-MT; 10, 100, or 200 mg/kg), diethyldithiocarbamate (DDC; 200 or 400 mg/kg), or phentolamine (5, 10, and 20 mg/kg), all antiadrenergic drugs. Reduction in central CAs by alpha-MT or DDC, or blockade of adrenergic receptors by phentolamine resulted in elevated serum PRL levels in a dose-dependent manner. Prior administration of the adrenergic stimulant clonidine (15, 150, or 1500 micrograms/kg) attenuated the elevated PRL response to phentolamine (20 mg/kg). These results are consistent with the view that the release of PRL in the turkey may be under the inhibitory influence of an adrenergic system.

Serotonergic stimulation of prolactin release in the young turkey (Meleagris gallopavo)

Serum prolactin (PRL) levels were determined by homologous radioimmunoassay in 6- to 10-week-old domestic white turkeys treated by intraperitoneal injection of agents that alter serotonergic activity. Quipazine (0.1-10.0 mg/kg), a serotonin (5-hydroxytryptamine; 5-HT) agonist, induced a dose-dependent rise in serum PRL level 1 hr after injection. The 5-HT precursor, 5-hydroxytryptophan (5-HTP), at doses of 80 and 150 mg/kg produced over a twofold elevation in PRL level 1 hr after administration, though the 50 mg/kg dose failed to produce any change. Administration of fluoxetine (10 mg/kg), a 5-HT reuptake blocker, induced an elevation in PRL level persisting 3 hr. When fluoxetine injection preceded administration of a weakly stimulatory dose of 5-HTP, a prolonged elevation in PRL level was observed. methysergide (MES), a 5-HT antagonist, administered in a 10 mg/kg dose produced no change in PRL level, while the 25 mg/kg dose initially produced a spike in PRL level which subsequently dropped slightly below the control level. Prior injection of 20 mg/kg MES completely blocked the serum PRL rises induced by quipazine and 5-HTP. These results suggest that serotonergic mechanisms are involved in the regulation of pituitary PRL release beyond basal levels in young domestic turkeys.

Gonadal steroid-mediated alteration of luteinizing hormone secretion by anterior pituitary cells of young turkeys☆

The magnitude of luteinizing hormone (LH) release during a 3-hr test incubation was diminished (P less than 0.05) when anterior pituitary cells from young turkeys were cultured for 24 to 120 hr. This trend was evident with basal LH release and with LH release induced by luteinizing hormone-releasing hormone (LH-RH) or hypothalamic extract. Anterior pituitary cells were cultured with various concentrations (10(-14) to 10(-6) M) of estradiol (E2), progesterone (P4), or testosterone (T) for 24 hr and then exposed to LH-RH or control medium for 3 hr, still in the presence of steroids. Basal LH release was potentiated (P less than 0.05) when cells were cultured with 10(-8) or 10(-6) M T, but not with E2 or P4. When cells were cultured with E2, LH release in the presence of 10(-8) M LH-RH was enhanced (P less than 0.05) in a dose-dependent fashion. LH-RH mediated LH release was also enhanced (P less than 0.05) when cells were cultured with 10(-8) M P4 or 10(-6) M T. Gonadal steroids can act directly on the anterior pituitary of the young domestic turkey to modulate LH release, with T enhancing basal LH release and E2 potentiating LH-RH-mediated LH release.