Pulsatile secretion of prolactin in the male rat after pimozide administration is not due to pulsatile inhibition of PIF secretion.

Sequential blood samples were taken every 2 min from intact male rats implanted with a permanent indwelling right atrial cannula. The relationship between pimozide dose and prolactin secreation was established by administering graded doses of pimozide (30–3000 μg/kg) as a single bolus injection through the indwelling cannula. The maximum response of prolactin secretion was achieved with 300 μg/kg pimozide. Higher doses of pimozide did not raise further the circulating prolactin concentration suggesting that the receptors for the presumed prolactin inhibiting factor (PIF) were blocked completely at this dose. Marked pulsatile fluctuations in circulating prolactin concentration were observed after administration of pimozide, at all dosages, or of another ‘specific’ dopaminergic receptor blocking agent, d-butaclamol. Since we assume that PIF receptors are completely blocked by the higher doses of pimozide, we conclude that this pulsatile secretion of prolactin cannot be due to the inhibition of PIF secretion but may be due either to the stimulation of prolactin releasing factor (PRF) secretion, or to an inherent rhythmicity in the prolactin secreting cells.     

Paradoxical effects of oxytocin and vasopressin on basal prolactin secretion and the estrogen-induced prolactin surge

The roles of oxytocin (OT) and vasopressin (AVP) on both basal and estrogen-induced prolactin (PRL) secretion were examined. Adult female Sprague-Dawley rats that were ovariectomized for 3 weeks and received estrogen treatment for 1 week were used. Intravenous administration of hormones and serial blood sampling were accomplished through indwelling intraatrial catheters which were implanted two days before. Plasma PRL levels were measured by radioimmunoassay. Oxytocin at a dose of 20 micrograms/rat stimulated a moderate PRL release in the morning and lower doses (5 and 10 micrograms) were without effect. Vasopressin was most effective at a dose of 5 micrograms/rat in stimulating PRL release, while consecutive injections of higher doses (10 and 20 micrograms) were less effective. In contrast, TRH, ranging from 1 to 8 micrograms/rat, induced a dose-dependent increases in PRL secretion. Using the effective dosages determined from the morning studies, repeated injections of either OT, AVP or their specific antagonists MPOMeOVT [( 1-(beta-mercapto-beta, beta-cyclopentamethylene propanoic acid), 2-(O-methyl)tyrosine, 8-ornithine]-vasotocin) and d (CH2)5Tyr(Me)AVP ([1-(beta-mercapto-beta, beta-cyclo-pentamethylene propionic acid), 2-(O-methyl)tyrosine, 8-arginine]-vasopressin), were given hourly between 1300 to 1800 h and blood samples were obtained hourly from 1100 to 1900 h. It was found that either OT or AVP significantly reduced the afternoon PRL surge, while their antagonists were not as effective. When OT or AVP were administered together with their specific antagonists, the inhibitory effects of either hormone on PRL surge were reversed. Thus it is concluded that both OT and AVP assume a non-specific stress-like effect on PRL release, in which basal secretion is stimulated and surge secretion is inhibited.     

Pulsatile secretion of gonadotrophins, ovarian steroids and ovarian oxytocin during prostaglandin-induced regression of the corpus luteum in the cow

A luteolytic dose (500 micrograms) of cloprostenol was given on Day 12 of the oestrous cycle to 5 heifers. Blood samples were collected simultaneously from the caudal vena cava and jugular vein at 5-20-min intervals from -6 to 0 (control period), 0 to 12 and 24 to 36 h after PG injection. Pulses of LH were secreted concomitantly with pulses of FSH during all sampling periods. However, during the control period separate FSH pulses were detected resulting in a shorter (P less than 0.01) interpulse interval for FSH than LH (93 versus 248 min). LH and FSH pulse frequencies increased (P less than 0.01) beginning 1-3 h after PG to interpulse intervals of 59 and 63 min, respectively, and continued to be maintained 24-36 h after PG. Concomitantly there was a 2-3-fold increase (P less than 0.01) in basal concentrations and pulse amplitude for LH (but not FSH). FSH basal concentrations and pulse amplitudes decreased (P less than 0.05) in 3 heifers 24-36 h after PG. Pulsatile secretion of oestradiol was observed at frequencies similar to LH during the periods 4-12 h (3 heifers) and 24-36 h (2 heifers) after PG, respectively, resulting in higher (P less than 0.05) mean oestradiol concentrations. Progesterone concentrations in the vena cava increased (P less than 0.01) 5-10 min after PG but decreased (P less than 0.01) 67% by 20 min after PG. This decrease was followed by a rise (P less than 0.05) beginning 2-3 h after PG and lasting for an average of 3.3 h.(ABSTRACT TRUNCATED AT 400 WORDS)     

Milking- and suckling-induced secretion of oxytocin and prolactin in parturient dairy cows

Serum concentrations of prolactin were unaffected by either suckling or milking on Day 2 or 3 postpartum in cows housed with their calves following parturition. In contrast, among cows housed without their calves milking elicited a four- to sixfold increase in serum prolactin concentrations. Serum oxytocin levels increased in response to both suckling and milking among cows housed with their calves with suckling being a more potent stimulus (257 ± 32 vs 189 ± 23 pg/ml at peak). However, the greatest increase in oxytocin levels accompanied milking in cows housed without their calves (375 ± 36 pg/ml at peak). These results suggest that stimuli associated with the presence or the absence of the calf can alter maternal secretion of oxytocin and prolactin. Greater understanding of factors which regulate secretion of these hormones may result in techniques to modify milk synthesis and milk ejection in dairy cows.     

Do pup ultrasonic cries provoke prolactin secretion in lactating rats?

Marked prolactin (PRL) secretion in response to the ultrasonic distress vocalizations of rat pups in lactating dams deprived of their pups for 6 hr was reported by others. In two experiments, this phenomenon could not be confirmed under our testing conditions at either 1 or 2 weeks postpartum, although behavioral responses to the ultrasounds were noted. In addition, suckling-induced PRL secretion did not differ consistently as a function of the tape recording (pup ultrasounds, 45 kHz artificially produced ultrasounds, or blank tape) heard prior to the return of pups. The functional significance of rat pup ultrasounds is considered.     

Plasma relaxin levels during suckling and oxytocin stimulation in the lactating sow

Plasma relaxin levels were measured in animals at different stages of lactation and related to the amount of nuzzling and suckling behavior exhibited by the piglets. Only in some acute suckling episodes was relaxin secreted rapidly and episodically in spite of normal piglet and sow behavior and interaction. However, when the piglets were removed from the dams 6 h before suckling, the sows were very restless and the relaxin response to suckling was delayed. Oxytocin injection in lactating but nonsuckled sows caused an episodic secretion of relaxin similar to suckling itself. The source of relaxin in the lactating sow may be the old corpus luteum, since progesterone levels increased acutely, somewhat reflecting the profile of relaxin increase over the suckling episode.     

Dopamine antagonism does not potentiate the effects of oxytocin and vasopressin on prolactin secretion

The roles of oxytocin and vasopressin on prolactin secretion were studied. Adult female Sprague-Dawley rats ovariectomized for two weeks and treated with a long-acting estrogen, polyestradiol phosphate for one week were used. Hormone administration and serial blood sampling were accomplished through indwelling intra-atrial catheters which were implanted two days before the experiment. Both oxytocin (20 micrograms/rat) and vasopressin (5 micrograms/rat) stimulated prolactin secretion within 10 min after injection and the effects were diminished by 30 min. In animals pretreated with a small dose of dopamine antagonist, sulpiride (1 microgram/rat), the effect of TRH on prolactin secretion was repeatedly shown to be potentiated. Same pretreatments with two different time intervals (30 and 60 min) between sulpiride and oxytocin/vasopressin administration, however, had no effect on oxytocin- or vasopressin-stimulated prolactin secretion. A vasopressin analog, 1-deamino-[D-Arg8]-vasopressin (dDAVP), with antidiuretic but no vasopressor activity was also used in the study. It was found that unlike vasopressin, dDAVP had no effect on prolactin secretion. In conclusion, both oxytocin and vasopressin can have a stimulatory effect on prolactin secretion when given in vivo. Unlike TRH, however, the action of oxytocin or vasopressin was not augmented by pretreatments of dopamine antagonist. The action of vasopressin on prolactin secretion may be a side effect of its vasopressor activity.     

Immunoneutralization of oxytocin attenuates stress-induced corticotropin secretion in the rat

Recent studies have demonstrated that oxytocin (OT) is released during certain stresses and that OT can potentiate the activity of CRF in vitro. To better define the role of OT during stress, the effect of injections of anti-OT antiserum on stress-induced corticotropin (ACTH) secretion was studied in vivo. A dose of antiserum which completely neutralized the increase in plasma OT levels during tail-hang stress caused a 59% decrease in plasma ACTH concentrations (P less than 0.005). The data support a physiologic role for OT in the regulation of ACTH secretion.     

Pulsatile growth hormone secretion decreases S-adenosylmethionine synthetase in rat liver

S-adenosylmethionine synthetase (AdoMet synthetase) is responsible for the synthesis of the major methyl donor S-adenosylmethionine. The AdoMet synthetase gene was identified by subtractive suppressive hybridization as being expressed at higher levels in the liver of rats continuously exposed to growth hormone (GH) than in rats intermittently exposed to the hormone. Further studies on the regulation of AdoMet synthetase showed that the activity and mRNA levels were higher in female than in male rats. Hypophysectomy increased AdoMet synthetase mRNA in both male and female rats. Combined thyroxine and cortisol treatment of hypophysectomized rats had no effect on AdoMet synthetase mRNA levels. Two daily injections of GH for 7 days, mimicking the male secretory pattern of GH, decreased AdoMet synthetase activity and mRNA levels. A continuous infusion of GH, mimicking the female secretory pattern of GH, had small or no effects on AdoMet synthetase activity and decreased the mRNA levels to a lesser degree than two daily injections. It is concluded that the lower AdoMet synthetase activity in male rats is due to an inhibitory effect of the male characteristic pulsatile secretory pattern of GH on AdoMet synthetase mRNA expression.     

Proteolytic modification of rat prolactin by subcellular fractions of the lactating rat mammary gland

The current study explored prolactin proteolysis by rat lactating mammary gland. 125I-labelled rat prolactin was incubated with tissue fractions of lactating mammary gland and the extent of prolactin degradation and fragment formation was visualized and densitometrically quantitated from autoradiographs derived from SDS-polyacrylamide gel electrophoresis under reducing conditions. At pH 4.5, the 25 000 X g pellet of mammary gland converted intact prolactin (23 kDa band) to proteolytic fragments (8-16 kDa bands) in a time- and tissue concentration-dependent fashion similar to that reported previously for rat ventral prostate. The prolactin-degrading and -fragmenting activity in lactating mammary gland was 5-10-times that observed for ventral prostate, the most active male tissue. This activity at acid pH was also demonstrable in other fractions of mammary gland but appeared to predominate in the cytosol. The above activities in mammary gland virtually disappeared at pH 7.4, appeared sensitive to aspartate and sulfhydryl proteinase inhibitors, and insensitive to serine and metalloenzyme proteinase inhibitors. The distribution of this activity could not be correlated with a particular enzyme marker. These characteristics of mammary gland activity differed significantly from those reported previously for prostate. When electrophoresis was conducted under non-reducing conditions, prolactin proteolysis in prostate and mammary gland was primarily associated with the formation of a more slowly migrating product (24 kDa band) with little spontaneous 8-16 kDa fragment formation. Re-electrophoresis of the 24 kDa band under reducing conditions resulted in the appearance of the 8 and 16 kDa fragments. In conclusion, prolactin is proteolytically modified by prostate and lactating mammary gland to a variant of intact hormone (24 kDa band) with a cleavage site in its large loop, by two or more widely distributed, acid-dependent proteinases. Lactating mammary gland, the principal target for prolactin, has the capacity to cleave the hormone in its loop at rates higher than any other tissue examined to date.     

Effect of cysteamine on suckling-induced prolactin secretion in the rat

We have examined the effects of the thiol agent cysteamine on physiological prolactin secretion in the female rat. Administration of cysteamine completely abolishes suckling-induced prolactin secretion in a dose-dependent manner. Cysteamine treatment does not alter nursing behavior of the mothers. Further, we have found that the prolactin-depleting ability of cysteamine is not altered by a prior suckling stimulus. These results indicate that cysteamine administration inhibits physiologically-induced prolactin secretion with similar potency and efficacy as previously reported for cysteamine effects on basal and pharmacologically-induced prolactin secretion. Furthermore, the effect of cysteamine is not compromised by a previous suckling stimulus, suggesting that "depletion-transformation" of pituitary prolactin stores does not protect against the effect of cysteamine.     

Freeze-fracture observations of the lactating rat mammary gland. Membrane events during milk fat secretion

Membrane events during milk fat secretion were analyzed by freeze-fracture of the rat mammary gland. Two modes of milk fat secretion were observed: extrusion of fat droplets surrounded by a portion of the apical plasma membrane of the alveolar epithelial cells and, less frequently, release into the alveolar lumen of fat droplets contained in intracytoplasmic vacuoles. The extrusion process consists of two asynchronous events: clearing of membrane particles (probably including integral membrane proteins) and bulging of the apical plasma membrane. Most fat droplets are extruded with a bilayer membrane envelope (milk fat globule membrane) partially devoid of particles. The segregation of membrane particles may represent the onset of a process of structural degradation of the milk fat globule membrane.     

Activation of subfornical organ efferents stimulates oxytocin secretion in the rat

The effects of activation of subfornical organ (SFO) efferents on plasma oxytocin concentrations were examined in conscious freely moving male Sprague-Dawley rats. Blood samples were obtained through chronically implanted atrial catheters and SFO efferents were activated electrically using chronically implanted bipolar stimulating electrodes. Hormone concentrations were measured by radioimmunoassay, and experimental animals were assigned to one of 3 experimental groups according to histologically verified anatomical locations of stimulating electrodes in either the SFO, the hippocampal commissure (HC), or the medial septum (MS). Electrical stimulation in the SFO resulted in increased plasma concentrations of oxytocin from control values of 2.54 +/- 0.9 pg/ml, to a post-stimulation level of 65.6 +/- 27.0 pg/ml. In contrast, stimulation in immediately adjacent structures including HC and MS was found to be without effect on plasma concentrations of oxytocin. These studies provide the first definitive evidence that SFO efferents may play a significant role in controlling the secretion of oxytocin from the posterior pituitary.     

Effects of naloxone or transient weaning on secretion of LH and prolactin in lactating sows

Sows (N = 16) were infused intravenously for 8 h with saline or naloxone (200 mg/h) or their litters were transiently weaned for 8 h. Before infusion, 200 mg naloxone were administered to elevate quickly concentrations of naloxone. Blood samples were collected from sows at 15 min intervals for 24 h, beginning 8 h before and continuing until 8 h after imposition of treatments during the middle 8-h segment. Frequency of episodic release of LH and concentrations of prolactin were similar before, during and after infusion of saline. Average concentration of LH was greater during the last than during the middle 8-h segment when sows were given saline. Frequency of episodic release of LH increased and concentrations of prolactin decreased during infusion of naloxone or transient weaning; however, average concentration of LH increased during transient weaning, but not during infusion of naloxone. After transient weaning or infusion of naloxone, frequency of release of LH decreased, returning to pretreatment values in sows infused with naloxone but remaining above pretreatment values in sows subjected to transient weaning. At the resumption of suckling by litters in sows subjected to transient weaning, prolactin increased to levels not different from those observed during the 8-h pretreatment segment. Prolactin did not increase until 4-5 h after cessation of naloxone infusion. We conclude that continuous infusion of naloxone altered secretory patterns of LH and prolactin. Collectively these results provide evidence that the immediate effects of weaning on LH and prolactin in sows are mediated in part through a mechanism involving endogenous opioid peptides.