Size heterogeneity of plasma prolactin in the rat: TRH and serotonin-induced changes

Sexual dimorphism in the age changes of the pituitary lactotrophs in rats

It is known that aging is associated with alterations in hypothalamic and pituitary functions. In the present study, we have undertaken a quantitative immunohistochemical assessment of the lactotroph cell population as well as prolactin (PRL) secretion, in male and female rats of different ages. Pituitaries from young (3 months), old (20 months) and senescent (29 months) male and female Sprague-Dawley rats were processed for the immunohistochemical detection of lactotrophs. Serum PRL was measured by a homologous RIA. Additionally, the in vitro PRL secretory activity was estimated by perifusion of pituitary cells from senescent animals. Analysis of morphometric parameters revealed age-related changes of PRL cell population in animals of both sexes. The cell density (CD), surface density (SD) and volume density (VD) decreased with age in both male and female rats. However, CD as well as SD appeared to have increased in females when compared to males, either in young or old animals, while VD was higher only in old females. The pituitaries of senescent females displayed chromophobic microadenomas on a background of diffuse PRL cell hyperplasia. Prolactin serum levels showed a marked increase with age in females, but only a modest elevation in males. In senescent females, PRL production per cell was reduced. We conclude that in rats, there exists a clear sexual dimorphism in the age-related changes of pituitary PRL cells.

Age-dependent prolactin-releasing activity of nucleoproteins

In previous studies we demonstrated that histone preparations possess multiple effects in vivo on pituitary hormone secretion and that these effects tend to disappear with age. We have now evaluated the in vitro effects of histone and nucleohistone preparations on the secretion of prolactin (PRL) in perifused pituitary cells from young (4 months) and senescent (29-33 months) female rats. Freshly dispersed pituitary cells were packed into short columns and were continuously perifused with serum-free medium. The substances to be tested were pumped through the perifusion circuit, at the end of which perifusate fractions were collected and hormones measured by specific radioimmunoassay (RIA). Quantitative immunohistochemistry was carried out on the pituitary glands from seven young and six senescent females. In vitro basal PRL release was similar in both age groups. Perfusion of cells with median eminence extract (1/90 to 1/10), histone H2A (100 to 1000 micrograms/ml) or nucleohistone (200 to 1000 micrograms/ml), generated PRL responses which were higher in young than in senescent cells. The pituitaries of the senescent animals were characterized, in most cases, by the presence of chromophobic microprolactinomas against a background of diffuse prolactotroph hyperplasia. Our results confirm previous evidence that circulating nucleohistones and histones may act as hypophysotropic signals. The morphologic alterations in PRL cell populations found in the sencscent rats may play role in the desensitization of the pituitary gland to nucleoproteins, and possibly to other hypophysiotropic molecules, with age.

Effect of naltrexone on stress-induced bioactive prolactin release in aging male rats

Effects of the opiate receptor antagonist naltrexone (NALT, 10 mg/kg) on basal and novel environment stress-induced prolactin (PRL) release were examined in young (3-5 month) and old (22-24 month) male Copenhagen-Fischer 344 rats. Radioimmunoassay and Nb2 lymphoma bioassay were used to determine plasma immunoreactive (ir-) and bioactive (bio-) PRL levels, respectively. Although basal plasma irPRL levels were greater, bioPRL levels were significantly lower in old as compared to young rats. NALT induced significant decreases in basal plasma ir- and bioPRL concentrations in young rats, but had little or no effect on irPRL or bioPRL levels in old rats. Stress-induced elevations in irPRL levels were similar, but increases in bioPRL levels were attenuated in vehicle-treated old as compared to young rats. NALT pretreatment blocked stress-induced ir- and bioPRL release in both age groups. These results indicate that the role of endogenous opioid peptides (EOP) in regulating basal PRL secretion appears to be diminished, whereas EOP continue to be involved in stress-induced PRL release in the aging animal.

Production and secretion of the 21-23.5 kDa prolactin-like molecules

We recently described the presence of a series of prolactin (PRL)-like molecules (PLMs) in the rat pituitary gland and showed that their formation was not due to artifactual proteolysis of 24 kDa PRL during extraction or to degradation of PRL in lysosomes. In this study we have found (1) in vitro translation of pituitary cell RNA to result in the production of only 24 kDa monomer isoform 2 and no PLMs, (2) that secretion of newly synthesized PLMs is differently regulated than at least a proportion of newly synthesized monomers, (3) that secretion of newly synthesized PLMs occurs after at least a 6 h delay, (4) that cysteamine (a) inhibits the release of the PLMs, (b) causes an increase in their amount versus isoform 2, and (c) causes an intracellular accumulation of pleiomorphic, immature secretory granules, and (5) that cells grown under degranulating culture conditions do not contain PLMs. These results, using normal anterior pituitary cells in primary culture, demonstrate the potential for differential release of the PLMs versus monomer PRL in vivo and are consistent with the production of the PLMs from 24 kDa monomer isoform 2 during secretory granule condensation.

Comparison between rat prolactin radioimmunoassay and bioassay values under different experimental and physiological conditions

Serum rat PRL concentrations were compared using values determined by RIA and the Nb2 lymphoma cell bioassay (Nb2BA). Rat serum samples were obtained under different physiological conditions and after the administration of pharmacological agents known to affect PRL secretion. Of the treatments examined, estrogen, morphine bromocriptine and haloperidol significantly altered the relationship between Nb2BA and RIA estimates of PRL. The estrogen-induced increase in PRL levels of ovariectomized females and the proestrus surge of PRL in intact females led to higher bioassay than RIA estimates of PRL. Treatment with haloperidol, bromocriptine and morphine altered the relationship, favoring immunoreactive more than bioactive hormone, and reversing the pretreatment Nb2BA/RIA ratio. No discrepancies between estimates of PRL by the two assays were noted in untreated males, diestrous, estrous and ovariectomized females, or following ether or TRH administration. These results confirm previous observations of discrepancies between rat serum bioassay and RIA estimates, and the data suggest that different forms of prolactin are present in the circulation at different times.

Behavioral effects of TRH, 3-Me-His2-TRH and naloxone in Hemichromis bimaculatus

Twenty minutes after each injection of TRH (2, 5, 10 micrograms/g bw), Hemichromis bimaculatus displayed frequent chafing (rubbing body on substrate) at rates as high as 65 times in 5 minutes. Chafing continued even after 5 hours. Such frequent displacement activities were not observed in untreated fish. These activities could not be suppressed completely with the opiate-antagonist naloxone. Five minutes after application of 3-Me-His2-TRH (1, 2, 5 micrograms/g bw), H. bimaculatus was chafing at higher rates compared to those injected with TRH. Calling movements, which are regulated by a very high prolactin level, also occurred in some fish. When naloxone (2, 4, 8, 12, 16 micrograms/g body weight) was injected, Hemichromis showed excessive spitting and chewing. When 16 micrograms naloxone was administered, H. bimaculatus started to tremble and tried to escape by the presence of a fish net. The fact that chafing was not completely suppressed after naloxone-application implies that naloxone may mediate opiate and non opiate effects [17].

Prolactin levels and molecular heterogeneity in rat strains with high and low incidence of DMBA-induced mammary tumors

We compared the following parameters in Long-Evans (LE) and Sprague-Dawley (SD) female rats: 1) mammary tumor incidence after DMBA, 2) plasma prolactin (PRL) during the estrous cycle before and after DMBA, 3) plasma PRL in immature females from 0900 hr on day 29 to 0900 hr on day 30, 4) plasma PRL from 1200 to 1700 hr and before and 10 min after i.p. TRH administration in ovariectomized (OVX) rats treated with 200 micrograms polyestradiol phosphate (PEP), 5) anterior pituitary (AP) PRL concentration in OVX rats treated with 200 micrograms PEP, and 6) levels and Sephadex G-100 gel filtration patterns of plasma PRL 10 min after i.p. TRH administration in OVX rats treated with 200 micrograms PEP. We observed marked mammary tumor incidence in SD rats from one supplier (S-SD, Spartan) (96%) compared to SD from another supplier (CR-SD, Charles River) (40%) or LE rats (10%). Plasma PRL was significantly decreased on metestrus-diestrus and increased on proestrus-estrus in SD (both suppliers) but not in LE rats 90 days after DMBA compared to rats not given DMBA and sacrificed at same stages of the estrous cycle on day 55 of age. Immature LE and SD-CR females exhibited significant late afternoon and early morning prolactin surges on days 29-30 whereas SD-H rats had either no surges or poorly synchronized surges at the same times.(ABSTRACT TRUNCATED AT 250 WORDS)

Size heterogeneity of prolactin secreted from and stored in the rat anterior pituitary gland in vitro

The size heterogeneity of rat pituitary prolactin was investigated using anterior pituitary glands from female rats incubated in vitro and gel filtration on Sephadex G-100. Monomeric prolactin was preferentially secreted compared with dimeric and 'trimeric' material. When glands were incubated with dopamine, prolactin secretion was inhibited and the relative proportion of dimer in the gland (but not the medium) was decreased. Morphine sulphate reversed the effect of dopamine on prolactin secretion and on the proportion of prolactin in the gland that was in the dimeric form. The results suggest that monomeric prolactin is more readily secreted than dimer, and that dopamine decreases the production or stability of the dimer.