Bioactivity of plasma prolactin in ovariectomized, diethylstilbestrol-treated Long-Evans and Holtzman rats after thyrotropin-releasing hormone or bromocriptine administration

The objective of this study was to determine the effects of thyrotropin-releasing hormone (TRH) and bromocriptine on plasma levels of biologically active prolactin in ovariectomized, diethylstilbestrol (DES)-treated rats. Female Long-Evans and Holtzman rats were ovariectomized and each was given a subcutaneous implant of diethylstilbestrol (DES). One week later, groups of DES-treated rats were fitted with indwelling intra-atrial catheters, and 2 days later blood samples were withdrawn before and at 1, 2, 5, 10, and 20 min after intravenous administration of TRH (250, 500, or 1000 ng/rat). Blood samples were obtained from other groups at 4 weeks of DES treatment by orbital sinus puncture under ether anesthesia before and at 30, 60, and 120 min after bromocriptine administration (2.5 mg/rat sc). Plasma was assayed for prolactin by conventional radioimmunoassay (RIA) and by Nb2 lymphoma bioassay (BA). Holtzman rats released significantly more prolactin following TRH than did Long-Evans rats when the RIA was used to measure prolactin. However, when the BA was used to assay prolactin in the same samples, the Long-Evans rats released more prolactin than did the Holtzman rats. In addition, the ratio of the BA to RIA values was significantly increased in both strains following TRH, but the greatest increase was observed in the Long-Evans rats, in which the ratio was 4.5 at the peak of the TRH-induced rise in plasma prolactin. Gel filtration chromatography of plasma obtained at 5 min after TRH treatment in Long-Evans rats revealed large molecular forms of prolactin with BA to RIA ratios of 4-5. In addition, monomeric prolactin had a BA to RIA ratio of 2. Bromocriptine treatment reduced prolactin levels in both strains, but the effect was more rapid in Holtzman than in Long-Evans rats. In addition, bromocriptine treatment of Holtzman, but not Long-Evans, rats significantly reduced the BA to RIA ratio of plasma prolactin. The results indicate that TRH and bromocriptine affect the release of biologically active prolactin to a greater extent than prolactin detected by antibody in the RIA, and that Long-Evans and Holtzman rats respond to these secretagogues differently with regard to BA to RIA comparisons.     

Temporal effects of estradiol and diethylstilbestrol on pituitary and plasma prolactin levels in ovariectomized Fischer 344 and Holtzman rats: a comparison of radioimmunoassay and Nb2 lymphoma cell bioassay.

An Nb2 lymphoma cell bioassay (Nb2BA) and a radioimmunoassay (RIA) were used to compare plasma and pituitary levels of prolactin in ovariectomized Fischer 344 (F344) and Holtzman rats treated with either diethylstilbestrol (DES) or estradiol for up to 8 weeks. The objectives were to determine whether there were temporal differences in prolactin responses in strains with different genetic predispositions to estrogen-induced pituitary tumor formation and to determine whether the results of the two assay methods were equivalent. All rats were ovariectomized for 7 days and all except controls received subcutaneous Silastic implants of DES or 17 beta-estradiol and were sacrificed at intervals from 2 days to 8 weeks later. Pituitary content and plasma levels of prolactin were determined by Nb2BA and RIA and the ratio of these measurements was calculated. DES induced a significant increase in pituitary prolactin in F344 rats by 2 days of treatment, as measured by RIA. Pituitary content increased to a peak by Day 4, after which a gradual decline occurred until the end of the experiment. Nb2BA measurements were similar to those obtained by RIA, except at 8 weeks, when the content determined by Nb2BA was significantly higher than the content determined by RIA. When estradiol was given to F344 rats a pattern of increase and subsequent decrease in pituitary content similar to that seen with DES was observed and levels measured by Nb2BA and RIA were essentially equivalent. Plasma levels of prolactin in DES-treated F344 rats increased exponentially through the 8 weeks, and the Nb2BA measurements were significantly greater than levels determined by RIA throughout the treatment period. Estradiol treatment produced a pattern of change in plasma levels of prolactin similar to that observed with DES, except that RIA and Nb2BA measurements were not different. Comparable results were obtained in Holtzman rats, except plasma levels were not increased to the same degree as seen in F344 rats. From these results, we conclude that DES, but not estradiol, can selectively increase the secretion of prolactin that is more bioactive than immunoreactive and that this effect of DES is observed in F344 and Holtzman rats, although F344 rats released more prolactin in response to estrogens than did Holtzman females.     

The influence of blinding, olfactory bulbectomy and pinealectomy on plasma and anterior pituitary prolactin levels and on uterine and anterior pituitary weights in normal and neonatally androgenized rats

Lactating Sprague-Dawley rats had their litters adjusted to 8-10 pups on day 3 of lactation favoring females and some litters were injected with 1.25 mg of testosterone propionate to neonatally androgenize (NA) them. At 22-25 days of age both normal and NA animals were ovariectomized (OVX) and subjected to either sham olfactory bulbectomy (ANOS) + sham pinealectomy (PX), blinding (BLD) + ANOS or BLD + ANOS + pinealectomy (PX). At 13 weeks of age all animals were injected with 0.5 mg of polyestradiol phosphate. At 15 weeks of age the animals were fitted with atrial catheters and at 16 weeks of age blood samples (0.3 ml) were obtained every 3 hours over a 24 hour period. Uterine and anterior pituitary (AP) weights were recorded at sacrifice. Plasma and AP were assayed for prolactin (PRL) by RIA and AP were also assayed for PRL using the Nb2 lymphoma cell PRL bioassay. In both normal and NA animals, BLD + ANOS suppressed plasma PRL levels and PX partially prevented this response. Uterine weights were similar among groups while AP weights were significantly lower for sensory deprived animals. Anterior pituitaries extracted at pH 7.6 had a PRL concentration that was higher for the BLD + ANOS groups when estimated by either RIA or BA, a result that was not observed when the AP were extracted at pH 10.6. The amount of PRL extracted at pH 10.6 was twice that obtained at pH 7.6. Sensory deprived animals that were OVX prepubertally and administered estrogen as adults had a small but significant increase in mean plasma prolactin at 1700 hr. Both normal and NA animals responded in a similar manner to experimental manipulation.     

Relationship between bioassay and radioimmunoassay measurement of prolactin in the IPL rat, a hypoprolactinemic rat strain

IPL (Institut Pasteur, Lyon) nude, hypoprolactinemic rats exhibit delayed puberty and a complete lack of lactation. To characterize the secretion of circulating forms of prolactin (PRL) of these rats, PRL concentrations were measured in serum and pituitaries of males and females under various physiological conditions. Two assay methods, a radioimmunoassay (RIA) and a sensitive bioassay (NB2BA) were employed. Normal rats of the Sprague-Dawley strain were tested simultaneously, as controls. The pituitary content of PRL, estimated either by RIA or by NB2BA, in IPL nude males and females was similar to that of normal male and female rats. On the contrary, serum PRL levels of IPL male rats, measured by RIA or NB2BA, were significantly reduced when compared to normal rats. In both groups, there was a close correlation between the results obtained by the two methods, the NB2BA estimates being higher. However, the NB2BA/RIA ratio was significantly decreased in serum from IPL nude rats compared to controls, indicating that the circulating form of PRL was less bioactive in this group. Castrated male rats injected with estradiol showed sharply increased PRL values as estimated by RIA or NB2BA. The increase was greater (35-fold) in IPL nude rats then in normal rats (9-fold), but these increases resulted in serum PRL levels being similar in the two groups. However, the NB2BA/RIA ratio remained significantly reduced in IPL nude rats. In female rats, PRL was measured during different physiological states: estrus, diestrus, proestrus at 1000, 1200, and 1600 h and Days 1 and 21 of gestation and 2 days postpartum.(ABSTRACT TRUNCATED AT 250 WORDS)     

The effect of steroids on the size heterogeneity of pituitary prolactin in castrate male rats

Male Sprague-Dawley rats were castrated and given daily sc. injections of estradiol (E2, 10 micrograms/day), testosterone propionate (TP, 1.0 mg/day), dihydrotestosterone (DHT, 1.0 mg/day) or sesame oil (SO, 0.2 ml/day). A group of sham castrate males received daily sc. injections of SO (0.2 ml/day). On day 8 of steroid treatment animals were decapitated and anterior pituitaries were removed and hemisected. Each half was homogenized in PBS buffer (0.01 M Na2HPO4-NaH2PO4; 0.14 M NaCl; 0.1% bovine serum albumin) at either pH 7.6 or 10.6. Homogenates were chromatographed on Sephadex G-100 columns and eluted fractions were assayed for prolactin (PRL) by RIA. Four immunoreactive forms of PRL, designated as "void volume," "big big," "big" and "little," were eluted from the pituitary homogenates of each experimental group. Homogenates obtained at pH 7.6 contained a greater percentage of PRL in the "void volume" and less activity in the "big" and "little" forms than pH 10.6 homogenates in all experimental groups. Pituitaries from SO- and TP-treated castrate animals contained significantly greater percentages of activity in the "void volume" at pH 7.6 compared to the other groups. At pH 10.6, the pituitary homogenates from the E2-treated group eluted a significantly greater percentage of "big" PRL and a smaller percentage of "little" PRL compared to all other groups. These findings suggest that androgenic and estrogenic steroids may play a role in the pituitary PRL molecular size profile of the male rat.     

Milk prolactin is biologically active

Fat-free milk from cow and goat was chromatographed on Sephadex G-100 and the prolactin (PRL) activity of the fractions determined by radioimmunoassay (RIA). A single prolactin component was observed in 3 cow and 3 goat milk samples with a Vf/Vt ratio of approximately 0.5. Fractions in which PRL was detected by RIA and fractions on either side of the PRL peak were combined, dialyzed and freeze dried. The fractions were assayed for biological activity using the pseudopregnant rabbit mammary gland in organ culture; the degree of secretory response was evaluated histologically. Milk prolactin was biologically active. In the RIA cow milk PRL and one of 2 samples of goat milk PRL gave dose response curves parallel with the bovine PRL standard. In the bioassay the dose response curves for cow milk PRL and ovine PRL were parallel while goat milk PRL was parallel when the results were compared on a weight basis but not on the basis of prolactin content of the preparations assayed by RIA.     

生长激素和催乳素放射免疫测定法的建立与应用

目的：建立测定大鼠垂体和血浆中生长激素（GH）和催乳素（PRL）含量的高特异性、高灵敏度的双抗放射免疫测定（RIA）法;研究急性低氧对垂体激素GH和PRL的作用。方法：用氯胺－T法进行抗原放射性碘标记;采用平衡饱和加样程序的双抗RIA法测定。结果：用该方法测定急性低氧（0．5h）时血浆和垂体GH和PRL含量,7km低氧,垂体GH含量明显升高（P＜0．05）,血浆则相反;7km低氧,明显降低垂体和血浆PRL含量（P＜0．01）;而5km低氧对GH和PRL的作用与对照组比无统计学差异。结论：本双抗RIA法具有高特异性、高灵敏度及简便易行等特性;用该法测定提示急性低氧可抑制大鼠GH和PRL的分泌。     

Lithium-induced changes in the plasma and pituitary levels of luteinizing hormone,follicle stimulating hormone and prolactin in rats

Adult male Sprague-Dawley rats, maintained under a controlled photoperiod of LD 14:10 (white lights on at 06:00 h, CST), were injected with lithium chloride and changes in the levels of plasma and pituitary homogenates of luteinizing hormone (LH), follicle-stimulating hormone (FSH) and prolactin (PRL) were examined to evaluate the effects of this anti-manic drug on reproductive function. Two groups of rats were injected with lithium chloride intraperitoneally, twice daily at 09:00 and 16:00 h, for 2 and 7 days at a dosage of 2.5 meg/Kg body weight. Plasma and pituitary levels of LH, FSH and PRL were measured by radioimmunoassay. Plasma levels of LH were significantly (P<0.05) increased after 2 days of lithium treatment. In contrast, a significant (P<0.005) reduction in plasma levels of LH was evident when lithium injections were continued for 7 days. The plasma levels of FSH remained unaffected by lithium treatment by either time period. Lithium administered for 2 days did not bring about any significant alteration in the plasma levels of PRL, although there was a significant (P<0.002) reduction in plasma PRL levels after 7 days treatment. The concentrations of pituitary LH, FSH and PRL remained unchanged after 2 and 7 days of lithium treatment.     

Serum prolactin and growth hormone determined by radioimmunoassay and a two-site immunoradiometric assay: comparison with the Nb2 cell bioassay

Serum levels of the two lactogenic hormones prolactin (PRL) and growth hormone (GH) were compared when determined by radioimmunoassay (RIA) and two-site immunoradiometric (IRMA) assays in 83 normal premenopausal women. The mean values for the PRL and GH results determined by RIA were higher than those obtained by IRMA, despite strong correlations between the two (PRL, r = 0.92; GH, r = 0.79). The lactogenic hormones were also determined together by the Nb2 cell bioassay (BA) in 38 of these same women, and the results compared with the sum of the PRL and GH immunoassays. There was a strong correlation between the BA and RIA (r = 0.75), and the BA/PRL+GH RIA ratio averaged 1.6 +/- 0.5. Corresponding values for IRMA were r = 0.66, and BA/PRL + GH IRMA 3.3 +/- 1.1. Thus, the polyclonal RIA antisera appeared to recognize bioactive hormone components not determined by the double monoclonal antibody IRMA. Another 23 women at risk for familial breast cancer, and 14 cystic breast disease patients were also studied. High BA, but normal RIA results, giving mean ratios of 2.4 +/- 1.1 and 3.6 +/- 3.0 respectively, suggest the presence of a further variant with high bioactivity not detected by RIA in these two clinical situations.     

Further evidence that peptide histidine isoleucine (PHI) may function as a prolactin releasing factor in rats

Intraventricular administration of peptide histidine isoleucine (PHI) (200 ng, 1, 5 and 10 micrograms/rat) resulted in a significant and dose-related increase in plasma prolactin (PRL) levels in urethane-anesthetized rats and in conscious rats with intraatrial and intraventricular catheters. Intravenous injection of PHI (10 micrograms/rat) also raised plasma PRL levels in these animals. In in vitro studies, PRL release from superfused rat anterior pituitary cells was stimulated by PHI (10(-9), 10(-8) and 10(-7) M) in a dose-related manner. The stimulating effect of PHI (10(-7)M) on PRL release in vitro was as potent as that of vasoactive intestinal polypeptide (VIP) (10(-7) M) and was observed even in the presence of dopamine (10(-7) M). These results suggest that PHI plays a stimulating role in regulating PRL secretion by acting, at least in part, directly on the pituitary in the rat.     

Size homogeneity of plasma prolactin in the female rat following various natural, pharmacological and stressful stimuli

Gel filtration on Sephadex G-150 was performed on freshly drawn plasma from ovariectomized, estrogen-treated rats following 10 minutes of ether inhalation, intraperitoneal administration of TRH (1 μg/rat) or pimozide (500 μg/kg body weight) and at the peak of the estrogen-induced afternoon surge of prolactin (1700 h). Plasma from intact lactating rats 30 minutes after suckling was also subjected to gel filtration. For comparison, homogenates of fresh and frozen pituitaries from ovariectomized, estrogen-treated rats were chromatographed. Prolactin activity was determined by RIA in each fraction eluted between the void and total volumes of the column. Immunoreactive prolactin in plasma following all experimental procedures eluted as a single component with a K_av of approximately 0.6. Chromatography of the fresh pituitary homogenate showed prolactin immunoactivity at the void volume and at a K_av of 0.4 and 0.6. A homogenate of frozen pituitary contained a component with a K_av of 0.3 in addition to those seen in fresh pituitary. These studies demonstrate that although the rat pituitary contains multiple molecular forms of immuno-reactive prolactin, only one small component is found in plasma.     

Elevated growth hormone levels in sera from breast cancer patients

The concentrations of growth hormone (GH) and prolactin (PRL) in the serum of 42 breast cancer patients were determined by radioimmunoassay (RIA). Forty percent of the patients had elevated GH levels while only 17% had elevated PRL levels. These findings suggest a relationship between GH and breast cancer; a weaker correlation exists between PRL and this malignancy. In addition, total lactogens in the serum were measured by a bioassay (BA). The BA/RIA (GH + PRL) ratio was greater in the breast cancer patients than the controls, indicating that variant forms of the hormones with higher than normal biological activity might be present.     

A new microbioassay for the measurement of lactogenic hormones in human serum

The standard Nb2 assay for biologically active prolactin has been modified to allow a rapid convenient microbioassay without loss of specificity or accuracy. Lactogenic hormones specifically stimulate the replication of Nb2 node rat lymphoma cells in suspension culture and form the basis of a currently available bioassay to measure prolactin and growth hormone in human serum. A new microbioassay was developed using microtest plates enabling a large number of samples to be assayed simultaneously whilst maintaining the overall sensitivity of the bioassay for lactogenic hormones. Growth of the Nb2 node lymphoma cells, measured by a light scattering technique using optical density on a spectrophotometer, was shown to be closely correlated with the cell number determined on a Coulter counter. Addition of excess anti-human prolactin and anti-human growth hormone completely inhibited the growth stimulatory effects of both human prolactin and human growth hormone. This new microbioassay (BA) and conventional radioimmunoassay (RIA) were used to measure lactogenic hormones in 48 normal subjects. There was a close correlation between the results of both assays for each hormone studied in the control sera. The mean basal BA/RIA ratio was 1.5 (range 0.8-2.0) for prolactin, 0.7 (range 0-4.5) for growth hormone and 1.3 (range 0.5-1.9) for total lactogenic activity.     

Effect of experimentally induced hyperprolactinemia on growth hormone secretion

In order to study the existence of possible interrelation-ships between prolactin (PRL) and growth hormone (GH) secretions, adult male rats bearing an anterior pituitary graft under the kidney capsule since day 90 of life and their sham-operated controls were submitted to a single i.p. administration of L-dopa (50 mg/kg weight) or saline 30 days after the operation. Plasma PRL and GH levels were measured by using specific RIA methods. Dopamine (DA) and norepinephrine (NE) contents in the hypothalamus and in the in situ anterior pituitary gland were measured by using a specific radioenzymatic assay. An increase in plasma PRL levels and a decrease in plasma GH levels were shown in grafted rats. Hypothalamic contents of DA and NE were increased in these animals, while the anterior pituitary content of DA was not modified as compared to controls. The administration of a single injection of L-dopa led to decreases of plasma PRL and GH levels in both grafted and control rats, but while marked increases in hypothalamic and anterior pituitary contents of DA were shown in both groups, the hypothalamic content of NE was only increased in control animals. These data suggest that PRL and GH secretions were closely related. Dopamine could be mediating the action of PRL on GH, while NE would be less involved.     

Simultaneous radioimmunoassay for luteinizing hormone and prolactin

A combined radioimmunoassay (RIA) for the measurement of the anterior pituitary proteins luteinizing hormone (LH) and prolactin (PRL) is described and compared with individual RIAs for these hormones. The standard curves and the sample values for LH and PRL were identical when determined in a combined or in an individual RIA. This technique may prove useful to a number of laboratories where it is desirable to determine levels of more than one hormone in limited sample volumes.     

Possible involvement of endogenous opioid peptides in prolactin secretion induced by alpha 2-adrenergic stimulation in rats

Noradrenergic mechanisms have a stimulatory role in regulating prolactin (PRL) secretion in the rat. We investigated the mechanism by which the alpha 2-adrenergic system stimulates PRL release in urethane-anesthetized male rats. Intracerebroventricular injection of norepinephrine (2 micrograms/rat) or epinephrine (100 ng and 1 microgram/rat) caused an increase in plasma PRL levels. The PRL increase induced by epinephrine was much greater than that by norepinephrine. Intracerebroventricular injection of phentolamine (1 microgram/rat), an alpha-antagonist, blunted the plasma PRL increase induced by epinephrine (100 ng intracerebroventricularly). Plasma PRL levels were increased by intravenous injection of alpha 2-agonists, clonidine (15 micrograms/100 g of body wt), and xylazine (200 micrograms/100 g of body wt). Plasma PRL increase induced by clonidine or xylazine was suppressed by intravenous injection of naloxone (125 micrograms/100 g of body wt), an opiate antagonist. These findings suggest that alpha 2-adrenergic mechanisms stimulate pituitary PRL secretion, at least partly, by activating endogenous opioid peptides in the rat.     

中药降乳散对大鼠催乳素瘤表达Pit-1的作用

目的研究中药降乳散对大鼠催乳素（PRL）瘤组织表达Pit-1的作用。方法用皮下植入雌激素的方法制备大鼠PRL瘤模型,将成功诱发出PRL瘤的大鼠随机分2组,分别给予安慰剂和中药降乳散灌胃,用药4周后处死动物,垂体称重,用放免法测定血清PRL水平。用反转录-PCR方法分析Pit-1在大鼠PRL瘤组织中的表达量。结果降乳散组大鼠血清PRL水平和垂体重量均明显低于安慰剂组（P〈0.001）,Pit-1 mRNA水平在降乳散组较安慰剂组明显降低（P〈0.001）。结论降低Pit-1的表达水平可能是中药降乳散抗PRL瘤的重要机制之一。     

Prolactin in the marsupial Macropus eugenii, during the estrous cycle, pregnancy and lactation

An heterologous double antibody radioimmunoassay (RIA) using a guinea-pig antiserum (33-9) raised against human prolactin and 125I-ovine prolactin has been developed to measure prolactin (Prl) in plasma and pituitary preparations of marsupials. In this system, purified tammar and kangaroo Prl preparations showed parallel dose-response curves as did serial dilutions of crude pituitary homogenates of tammar, possum and eastern grey kangaroo. Serial dilutions of plasma from ovariectomized and lactating female and castrate male tammars showed immunoreactivity, and plasma Prl levels increased after injection of TRH. The assay has been used to monitor changes in plasma Prl in female tammars in various reproductive states. Plasma Prl remained at basal concentrations of 20 to 30 ng/ml throughout the estrous cycle, at estrus and during pregnancy. However, just prior to parturition, there was a 2- to 3-fold increase in Prl concentrations which declined to basal levels after birth. During early lactation, Prl levels were low but increased to maximum concentration in the second half of lactation.     

CRH receptor type 1 mediates continual hypoxia-induced changes of immunoreactive prolactin and prolactin mRNA expression in rat pituitary

We have reported that, in rats, hypoxia (10.8% O2) stimulates prolactin (PRL) release from the pituitary. This study is designed to compare the response of pituitary PRL to acute hypoxia (AH), continual hypoxia (CH), intermittent hypoxia (IH), cold, and restraint, individually and combined with hypoxia. This study also investigates the involvement of the corticotropin-releasing hormone receptor 1 (CRH R1) in the hypoxia-induced PRL response. Hypoxia was induced by exposing the rats to high altitudes of 2 km (16.0% O2) or 5 km (10.8% O2). The PRL levels in the pituitary (iPRL) and in plasma (pPRL) were measured by immunocytochemistry and RIA assay, respectively. The acute hypoxia of 5 km for 2-24 h caused a biphasic change (early decrease and late increase) of PRL. Both CH and IH at 2 or 5 km for 1-5 days markedly increased pPRL but decreased iPRL. Continual severe hypoxia (10.8% O2) for periods of 10, 15, and 25 days significantly enhanced pPRL but this effect was less marked at the lower altitude (16.0% O2) and did not occur during intermittent hypoxia (at both altitudes). The increased pPRL was significantly enhanced by restraint, restraint + hypoxia, hypoxia, and cold + hypoxia exposure. Treatment with a CRH R1 antagonist (CP-154,526) reversed hypoxia-decreased immunoreactive PRL and upregulated PRLmRNA in the pituitary. The data suggest that both CH and IH can stimulate rat PRL release in a time-course- and intensity-dependent manner. However, compared to the relatively low CH-induced response, restraint induced a more powerful response than either cold or hypoxia alone. CRH R1 mediates PRL secretion and PRL mRNA expression in the pituitary under hypoxic exposure. Hypoxia-enhanced PRL response over the lifespan may play a significant role in adaptation to an extreme environment.     

In vitro stimulation of prolactin release by vasoactive intestinal peptide

The effect of vasoactive intestinal peptide (VIP) on anterior pituitary hormone release was examined in a variety of in vitro preparations. Synthetic VIP was capable of stimulating increased prolactin (PRL) release from male rat hemipituitaries in doses as low as 10⁻⁹ M only when the enzyme inhibitor bacitracin was present in the incubation medium. Natural porcine VIP was similarly capable of stimulating PRL release, but only at higher doses (10⁻⁶ M). Additionally, synthetic VIP was capable of stimulating PRL release from dispersed anterior pituitary cells harvested from adult male and lactating female rats and from an enriched population of lactotrophs obtained by unit gravity sedimentation of similar dispersed cells from infantile female rats. No effect of VIP on luteinizing hormone, growth hormone or thyroid stimulating hormone release was seen. These findings taken in concert with the presence of VIP in the hypothalamus, pituitary and hypophyseal portal plasma of the rat suggest a physiological role for VIP in the control of PRL secretion.