Observations on the epidermis of desert-living iguanids

The epidermis of 146 specimens of Dipsosaurus dorsalis and 182 Uma notata collected throughout the active period of the animals' year has been examined. The morphology of the epidermis is essentially similar to previously described lacertilians but differs in the relatively great degree of development of the mesos layer and the complete keratinization of the lacunar tissue prior to sloughing. Analysis of sloughing frequency throughout the year suggests that species specific patterns may exist, but these do not correlate with any particular known ecologic datum. The patterns do not reflect the reproductive activity of the two species supporting previous experimental conclusions on the lack of effect of gonadial hormones on epidermal activity. There appears to be no evidence of association of femoral gland activity with epidermal activity in D. dorsalis, but the situation is not clearcut in U. notata. These data are discussed in the light of recent studies of the evolutionary origin of epidermal glands in lizards.     

Glucocorticoid modulation of prolactin receptors on mammary cells of lactating mice

Prolactin receptors were monitored by measuring ¹²⁵I-labeled prolactin binding to collagenase-dissociated mammary epithelial cells of lactating BALB/c mice. Specific receptors for iodine-labeled prolactin with an apparent dissociation constant (K_d) of 0.99 · 10^?9 M were present on the dissociated mammary cells. The binding was inhibited by ovine prolactin, human growth hormone and human placental lactogen but not by follicle stimulating hormone, luteinizing hormone, thyroid stimulating hormone, bovine growth hormone or insulin. Adrenal ablation of nursing mothers caused a reduction of the number of prolactin receptors and this effect was preventable by hydrocortisone therapy. Hydocortisone injections to mothers 3 days after adrenalectomy also induced a replenishment of the prolactin receptors on the mammary cells. Injections of progesterone failed to sustain the high level of mammary cell prolactin receptors in adrenalectomized animals. Stimultaneous injections of hydrocortisone and progesterone to animals 3 days after adrenalectomy caused a partial suppression of the stimulatory action of hydrocortisone alone. The results suggest that hydrocortisone can exert a modulatory influence on mammary cell prolactin receptors in non-hypophysectomized post-partum mice without altering the dissociation constant (K_d) of the receptors.     

Ultrastructural contributions to an understanding of the cellular mechanisms involved in lizard skin shedding with comments on the function and evolution of a unique Lepidosaurian phenomenon

Previous reports on the fine structure of lizard epidermis are confirmed and extended by SEM and TEM observations of cell differentiation and the form of shed material from the American anole Anolis carolinensis. Attention is drawn to two issues: 1) the tips of the spinules arising from the mature oberhautchen are markedly curved; this morphology can be seen during differentiation; 2) the median keels of scales from all parts of the body show “naked” oberhautchen cells that lack characteristic spinules, but have a membrane morphology comprising a complex system of serpentine microridges. Maderson's ([1966] J. Morphol. 119:39–50) “zip-fastener” model for the role of the shedding complex formed by the clear layer and oberhautchen is reviewed and extended in the light of recent SEM data. Apparently periodic lepidosaurian sloughing permits somatic growth; understanding how the phenomenon is brought about requires integration of data from the organismic to the molecular level. The diverse forms of integumentary microornamentation (MO) reported in the literature can be understood by considering how the cellular events occurring during the renewal phase prior to shedding relate to the emergence of the form-function complex of the β-layer, which provides physical protection. Issues concerning the evolutionary origin of lepidosaurian skin-shedding are discussed. J. Morphol. 236:1–24, 1998. © 1998 Wiley-Liss, Inc.     

Ultrastructure of the epidermis of the lizard (Lacerta vivipara) at the resting stage of the sloughing cycle

The ultrastructure of the epidermis of the lizard ( Lacerta vivipara ) one day after sloughing is described. The non-keratinized layers of the epidermis are essentially similar in structure to those of amphibians and mammals. The cells of the basal layer are not however separated from each other by the large spaces described in the amphibian (Farquhar & Palade, 1965). The middle layers of the epidermis at this stage of the sloughing cycle produce neither the characteristic mucous granules found in amphibians nor the keratohyalin granules of mammals. A small number of granules corresponding in size and location to the "Odland bodies" of both mammalian and amphibian epidermis are, however, present. The intermediate layer cells also contain a number of bodies similar in appearance to those described by Farquhar & Palade as lysosomes in amphibian skin. These structures are both osmium iodide and acid phosphatase positive. Unlike the condition in amphibians and mammals, the cytoplasm of cells in the layer immediately beneath the keratinized strata is honeycombed with small vesicles, and contains large irregular vacuoles of uncertain content. Certain nonkeratinizing elements within the epidermis are tentatively interpreted as nerve terminations. Two morphologically distinct keratinized strata can be distinguished, the inner stratum consisting of flattened cells similar to those of the stratum corneum of mammalian epidermis; individual cell outlines cannot be distinguished in the outer stratum, which has a structure similar to that of avian feather keratin. A shallow surface zone of the outer keratinized stratum has been identified as the Oberhautchen. This consists of longitudinally disposed leaflets or laminae which are responsible for the sculptured pattern of the epidermal surface. The observations reported here provide a basis for analysis of changes occurring at other stages of the sloughing cycle.     

Fine structure of deep-layer sloughing and epithallial regeneration in Lithophyllum neoatalayense (Corallinales, Rhodophyta)

Transmission electron microscopic study of Lithophyllum neoatalayense Masaki demonstrated that this alga used two mechanisms by which it sloughed cells from its surface. Epithallial filaments, produced by a layer of underlying initial cells, were two to four cells long. Terminal epithallial cells, those closest to the thallus surface, underwent senescence and shedding as previously described for other coralline algae. In addition, deep-layer sloughing occurred. During this process, which is known for only one other coralline alga, cell death occurred uniformly within a surface layer 40-50 μm thick. No pathogens or evidence of mechanical damage was detected. The zone of dead cells included the epithallial layer, the layer of initial cells, and some cortex cells beneath the initials. Pit plugs connecting living and dead cells at the edge of the necrotic zone were sealed by wall material deposited by the surviving cells, but cells within the necrotic zone showed no evidence of sealed pit plugs. The outermost surviving cells became new initial cells, which then divided to form new epithallial cells. Regeneration of the epithallial layer occurred before the thick layer of dead cells peeled away. The extensive deep shedding of both epithallial and initial cells in L. neoatalayense and the effectiveness of regeneration from cortex cells challenge the assumption that protection of initial cells is an important function of epithallial cells.     

Estradiol-induced follicular atresia in rhesus monkeys is not prevented by exogenous gonadotropins

Estradiol-17β (E₂) induces atresia of the dominant preovulatory follicle (DF) when administered on day 6 of the menstrual cycle. The present study was designed to determine whether the atretogenic effect of E₂ could be averted by the administration of exogenous gonadotropins, in an attempt to determine whether E₂-induced atresia in primates is due to a direct action at the ovarian level or is mediated via pituitary secretion. After identification of the DF during laparoscopy, cyclic monkeys received Silastic capsules containing E₂ placed s.c. for 24 hours, plus one of the following treatments: phosphate-buffered saline, or 25 I.U. of either human urinary menopausal gonadotropin (hMG), FSH-rich hMG, human urinary FSH (uFSH), or human pituitary FSH (pFSH) injected i.m. twice daily for 2 days. The control treatment resulted in atresia of the DF and extended follicular phases (26.3 ± 5.9 days, x? ± S.D.), but in normal luteal phases following ovulation of a substitute DF. Similar results occurred in all animals receiving FSH-rich hMG or pFSH, and in 11 of 16 animals receiving hMG or uFSH (P > 0.05). Since all possible routes and regimens of gonadotropin administration were not attempted, a central action of E₂ cannot be ruled out. However, we believe that the experimental observations support our contention that the atretogenic action of E₂ is exerted in part at the ovary.     

The effects of osmotic experiments and prolactin on the mucous cells in the skin and the ionocytes in the gills of the teleost Cichlasoma biocellatum

Summary Adult Cichlasoma biocellatum were kept in deionized water, 25% sea-water and tap-water. The mucous cells in the epithelium of the gills and the buccal floor were not affected by the osmotic experiments. In animals kept in deionized water the mucous cells in the skin remained unaffected, but the ionocytes (chloride cells) in the gills were strongly stimulated. Keeping the animals in salt water led to a strong regression of the epidermis including the mucous cells, and of the ionocytes. The regression could be counteracted by prolactin injections. The changes in the skin and the ionocytes could be correlated with the activity of the prolactin producing cells in the adenohypophysis. It is suggested that the epidermis and its mucous cells as well as the ionocytes are under prolactin control.The authors gratefully acknowledge the interest and helpful suggestions of Prof. Dr. P. G. W. J. van Oordt. Thanks are also due to Mr. H. van Kooten for making the photographs, to Dr. L. Boomgaart for correcting the english, and to the N. I. H., Bethesda, Maryland (U.S.A.) for presenting the ovine prolactin.     

Circadian rhythms of testosterone-dependent behaviors,crowing and locomotor activity,in male Japanese quail

Summary An apparatus was devised to record crowing (mate calling by males) together with locomotor activity and recorded data was analyzed by several methods for rhythm analysis. Crowing and locomotor activity of Japanese quail held on long days were recorded during sexual development as estimated from circulating gonadotropins and testosterone. Both behaviors were testosterone-dependent but commencement of crowing preceded the increase in locomotor activity. When the two behaviors attained their maximum levels, crowing showed consistent daily rhythms in which two peaks were apparent, a major one at the onset of light and a broader one 8 hours later. Locomotor activity also showed a clear daily rhythm with a peak between the two peaks of crowing rhythm suggesting a fixed phase relationship between the two rhythms.Both rhythms free-ran under constant dim light with periods shorter than 24 h. They persisted in birds which had been castrated and then supplied with exogenous testosterone via implanted Silastic capsules. The durations of both rhythms were quite comparable to each other and they maintained a fixed phase relationship similar to that found under LD cycles.The results indicate that testosterone is essential for the induction of crowing and for the enhancement of locomotor activity but the formation of the rhythms in behavior was strictly dependent on a circadian oscillatory mechanism.Abbreviations LH luteinizing hormone - FHS follicle-stimulating hormone - LD light-dark - LDim light-dim light     

Role of Non-Aromatizable Androgens in the Central Control of Pubertation of Female Rats

In 30-postnatal day-old rat females we created a decreased or increased level of 5-reduced (non-aromatizable) androgens, using injections of an inhibitor of 5-reductase or exogenous dihydrotestosterone, respectively. In these animals, we measured the levels of noradrenaline (NA) and serotonin (5-HT) in the hypothalamus, those of gonadotropins in the hypophysis, and testosterone and estradiol in the blood plasma; the process of pubertation was also monitored. In rats with a deficiency of non-aromatizable androgens, the NA level increased, while that of 5-HT dropped. In animals with an excess of these hormones, the 5-HT level remained stable against the background of a certain increase in the NA level. In the first group of animals, the estradiol level in the blood increased by a factor of 3, while the testosterone level increased insignificantly. The beginning of pubertation in these animals was not shifted, while the first ovulation occurred somewhat earlier, as compared with the control. In the second group of rats, treatment with exogenous dihydrotestosterone accelerated the beginning of pubertation, but the interval between opening of the vagina and first ovulation even somewhat increased. The neurotransmitter hypothalamic mechanisms mediating acceleration of an initial phase of the female pubertation and retardation of its final phase under the influence of non-aromatizable androgens are discussed.     

The nature of the pituitary gonadotropins and their role in ovulation in a urodele amphibian (Ambystoma tigrinum).

Gonadotropic hormones were fractionated from pituitaries of the urodele amphibian $\text{Ambystoma}$$\text{tigrinum}$ (tiger salamander) by methods previously employed to separate these hormones in other species of tetrapod vertebrates. These procedures yielded two distinct fractions that resembled the FSH and LH from other species in regard to their biological profiles in several nonmammalian bioassays and by their chromatographic behavior. The $\text{Ambystoma}$-FSH was free of LH activity (< 0.1%). The major $\text{Ambystoma}$-LH was highly potent in ovulation assays for LH; it also had a high activity in the $\text{Anolis}$ lizard assay, but it is not clear whether this reflects high intrinsic activity or incomplete separation of FSH.$\text{In vitro}$ studies with these and other (frog, turtle, mammalian) gonadotropins indicate that the induction of ovulation in $\text{Ambystoma}$, as in anurans, is highly specific for LH, independent of the source of the gonadotropins. These data support the view that two separate gonadotropins existed early in tetrapod evolution.     

Immunocytochemical and ultrastructural study of the frog (Rana pipiens) pars distalis with special reference to folliculo-stellate cell function during in vitro superfusion

Summary The colloidal gold immunocytochemical technique was used to determine the ultrastructural features of the glandular cells in the pituitaries of male frogs, Rana pipiens, both in vivo and after superfusion in vitro. Specific reactions to antisera against bullfrog gonadotropins, human prolactin, and synthetic 1–39 corticotropin allowed identification of the 3 corresponding types of glandular cells. No immunoreaction was obtained with antisera against human or ovine-growth hormone, human -thyrotropin hormone, and bovine S-100 protein. General morphological features of these immunocytochemically identified glandular cells were similar to those of equivalent cells previously described in other amphibian species. Non-glandular folliculo-stellate cells were distinctive. In freshly removed pituitaries, these folliculo-stellate cells contained lysosome-like structures, but did not show phagocytic vacuoles in the cytoplasm; they contained many mitochondria, and the Golgi complex and endoplasmic reticulum were relatively undeveloped. After 4 or 18 h of superfusion, some immunoreactive gonadotropic, prolactin, and corticotropic cells showed degeneration and destruction. In the same gland, folliculo-stellate cells retained a viable appearance, but showed phagocytic vacuoles containing secretory granule-like structures which were immunoreactive to gonadotropic, prolactin, and corticotropic antibodies. Some folliculo-stellate cells showed phagocytic vacuoles containing complete glandular cells. These results suggest that superfusion causes a destruction of some of the glandular cells, and that folliculo-stellate cells act as phagocytes when cellular debris or moribund cells are present in the intercellular space in the pituitary parenchyma.Supported by grant DCB 8710462 from the National Science Foundation, grant 2148-83 from the CAICYT (Spain) and the Junta de Andalucia (Spain)     

Polymorphic microsatellite markers for the Iberian rock lizard species,Iberolacerta cyreni,and cross‐species priming in other Iberolacerta species

Eight polymorphic microsatellite loci are described for the Iberian rock lizard species, Iberolacerta cyreni. Loci were isolated from a partial genomic library that had been enriched for AAAG repeat sequence. Number of alleles per locus ranged from two to 13 in a sample of 24 individuals from the Sierra de Guadarrama (central Spain). Observed heterozygosity ranged from 0.13 to 0.96. At least three loci were amplified and polymorphic in four other Iberian rock lizard species: Iberolacerta monticola, Iberolacerta bonnali, Iberolacerta aranica and Iberolacerta aurelioi. These markers will be used to study mating strategies and reproductive success in I. cyreni.     

Growth, endocrine profiles and reproductive responses of ewe lambs after medium and long-term treatment with ovine growth hormone

Two experiments were conducted to examine effects of exogenous ovine growth hormone (oGH) on growth and reproductive traits of ewe lambs. In the first trial, 30 Debouillet ewe lambs (4 months old) received either 0 or 2.5 mg, s.c. of oGH (Day 0 = first day of 98-day treatment). Ovarian cyclicity was determined by monitoring serum progesterone. Efficiency of gain (first 50 days of treatment) was more (P < 0.10) desirable in oGH-treated animals, but did not differ (P > 0.20) between groups during the last 48 days of treatment. Serum GH in alternate-day samples was elevated five-fold (P < 0.01). First estrus occurred 10 days earlier (P = 0.14) in oGH-treated ewe lambs. In a second trial, 45 ewe lambs were evenly divided into three groups receiving 0 mg of oGH (CON; 50 injections), 2.5 mg of oGH (GH98, 50 injections) or 25 injections containing 2.5 mg of oGH followed by 25 injections of 0 mg of oGH (GH48) on alternate days for 98 days before a breeding season. Ewe lambs receiving GH48 had higher (P < 0.05) gains the first 24 days than those receiving CON or GH98. Ewes receiving GH48 demonstrated first estrus (P = 0.06) 22 days before control ewes and 28 days before GH98 ewes. Other reproductive traits did not differ (P > 0.25). Serum GH was greatly elevated by injections of exogenous oGH, but neither serum insulin nor prolactin was affected. Short-term elevation of serum GH resulting from exogenous oGH injections marginally enhances reproductive and growth responses, but does not induce major changes in these traits in ewe lambs after 4 months of treatment.     

Effect of thyroidectomy on rhythmic gonadotropin release.

Nonstress blood samples were obtained from intact and thyroidectomized (TE) male rats at 3-hr intervals over a 24-hr period via rapid decapitation. The animals were thyroidectomized when 40 days old and used 6 weeks later. Intact animals showed periodicity in serum LH (P less than 0.01) and prolactin (P less than 0.01). Both gonadotropins began increasing after 8 PM and peak levels occurred at 11 PM. In contrast, 24-hr periodicity was not observed in serum FSH. Corticosterone levels in these same serum samples showed the expected circadian periodicity. After TE, the 24-hr pattern in all gonadotropins was altered significantly. Serum LH increased (P less than 0.01) and circadian periodicity appeared to be absent. FSH and prolactin levels were increased and decreased, respectively (P less than 0.01), with serum prolactin showing a 9-hr phase shift. Prolactin began increasing at 2 AM and reached a peak at 8 AM. Corticosterone in TE animals showed a 24-hr rhythm similar to that of intact rats. These findings confirm our previous observations that nonstress serum LH and prolactin levels fluctuate with a 24-hr periodicity and suggest that the level of, and the phase angle betweeen, these rhythms is markedly influenced by pituitary-thyroid activity.     

A diurnal rhythm of pituitary prolactin activity with diurnal effects of mammalian and teleostean prolactin on total body lipid deposition and liver lipid metabolism in teleost fishes*

Previous investigations have shown that mammalian prolactin has a pronounced effect on lipid deposition in teleost fishes (e.g. Meier et al., 1971; de Vlaming & Sage, 1972). Prolactin can be either lipogenic or lipolytic depending on when during the day it is injected. Studies were initiated to determine whether the diurnal effects of prolactin on lipid metabolism in Fundulus similis vary with season or photoperiod acclimation. Data presented here indicate that the diurnal rhythm of response to prolactin persists in the pre-spawning and post-spawning seasons if F. similis is maintained on a long photoperiod. When animals are acclimated to a short photoperiod, treatment with mammalian prolactin either early or late in the photoperiod causes lipid depletion. Homogenates of the portion of the pituitary containing only prolactin cells cause lipid depletion in F. similis when injected early during a long photoperiod, but stimulate lipogenesis if injected late in the photoperiod; these results imply that fish prolactin may be involved in lipid metabolism. Furthermore, prolactin activity in the pituitary of F. similis varies diurnally. In vitro studies of lipid metabolism in Notemigonous crysoleucas and Ictalurus natalis suggest that the liver may be one of the target organs for prolactin.     

The identification of the thyrotropic cells in the adenohypophysis of the cichlid fish Cichlasoma biocellatum and the role of these cells and of the thyroid in osmoregulation

Summary In order to identify the thyrotropic cells in the pituitary gland of Cichlasoma biocellatum, adult animals were treated with thyroxine. Periodic acid Schiff (PAS)- and aldehyde fuchsin (AF)-positive, and Alcian blue (AB)-negative basophils in the meso-adenohypophysis as well as the thyroid were strongly inactivated by such treatment. Since other cell types in the adenohypophysis were not affected, it is concluded that the PAS-positive and AB-negative basophils in the meso-adenohypophysis are the thyrotrops. In normal animals the thyrotrops and the thyroid are very active. Propylthiouracil did not cause a further activation of TSH cells. Keeping the animals in 25% sea-water caused a strong inactivation of the prolactin cells, the thyrotrops and the thyroid. In deionized water the prolactin cells were stimulated, but the thyroid was less active than in controls. Injections of ovine prolactin did not enhance the activity of the thyroid in salt water animals. It is suggested that in Cichlasoma biocellatum prolactin does not have a direct stimulatory effect on the thyroid.Thanks are due to Prof. Dr. P. G. W. J. van Oordt for his continual interest and valuable suggestions, and to Dr. L. Boomgaart for correcting the english. We are indebted also to Messrs. H. van Kooten and E. van der Vlist for making the photographs. The National Institutes of Health, Bethesda, Maryland (U.S.A.) kindly presented the ovine prolactin.     

Circadian organization inAplysia: internal desynchronization and amplitude of locomotor rhythm

Summary We have tested the hypothesis that the circadian oscillators in the eyes ofAplysia are coequal driver oscillators for the circadian locomotor rhythm. Three predictions based on this hypothesis were tested. Prediction 1: at a time when the phase difference between the eye rhythms is small, the amplitude of the locomotor rhythm in two eyed animals will be as great or greater than the amplitude in one eyed animals. Prediction 2: the amplitude of the locomotor rhythm of two eyed animals will decline under conditions in which the two eye rhythms become out of phase with each other. Prediction 3: the form of the locomotor rhythm will broaden or become biphasic in two eyed animals when the two eye rhythms become out of phase with each other.None of the predictions was confirmed. One eyedAplysia had higher amplitude locomotor rhythms than two eyedAplysia, even under conditions in which the two eye rhythms were probably not far out of phase with each other. There was no tendency for the amplitude of the locomotor rhythm of two eyed animals to decline under circumstances in which the phase difference between the two eye rhythms changes from less than 4 h to as much as 11.5 h. There was no tendency in two eyed animals for the locomotor rhythm to broaden or become biphasic as the eye rhythms became more out of phase with each other.The results led us to reject the hypothesis that the eyes are co-equal drivers for the locomotor rhythm. The ocular influence on locomotion is more likely to be mediated via mechanisms in the central nervous system that do not faithfully conserve the phase of the eye rhythms. One possibility is that the driver is a third circadian oscillator that interacts with the two eye oscillators.Abbreviations CAP compound action potentials - CC constant conditions - CT circadian time - DO driver oscillator - EO eye oscillator - RSD relative standard deviations (see Methods)     

Indomethacin causes a simultaneous decrease of both prolactin binding and fluidity of mouse liver membranes

Indomethacin suppressed the numbers of prolactin receptors detectable in the liver membranes of both male and female C₃H mice. This occurred in a dose-dependent fashion with 7.5 μg/gm body weight injected every 4 hours exerting a maximal effect within 20 hours. While injection of 50 Vg prolactin every 4 hr increased the number of prolactin receptors in control animals it could not in the indomethacin-treated animals. Membrane fluidity was estimated by fluorescence polarization techniques using the lipid probe 1,6-diphenylhexatriene. Indomethacin caused a decrease in membrane fluidity, whereas, exogenous prolactin increased the fluidity of the recipients' liver membranes but again could not overcome these suppressive effects of indomethacin. The data suggest that prolactin induces its own membrane-associated receptor by means of the prostaglandin cascade, perhaps by altering the fluidity of the supporting lipid bilayer.     

The importance of thyroid hormone in experimental ovarian cyst formation in gilts

The role of the thyroid gland in ovarian cyst formation in farm animals and in women has rarely been considered. Experimental data on the induction of polycystic ovarian disease (PCOS) in rats indicates the importance of thyroid function to the mechanism of this disorder. The objective of this work was to prove the role of thyroid hormones in gonadotropin-induced cystic ovarian disease (COD) in gilts.In hypothyroid gilts (oral administration of 1 g of methylthioracyl (MTU) daily for 24 days), ovarian cysts were induced by injections of pregnant mares' serum gonadotropin (PMSG) (equine chorionic gonadotropin (eCG)) and human chorionic gonadotropin (hCG) (400 IU and 200 IU daily for 10 days, respectively). Gonadotropins were also injected into hyperthyroid gilts (400 μg of L-thyroxine daily for 24 days). Suitable control groups (no treatment, injected with gonadotropins, hypothyroid by application of MTU and hyperthyroid by administration of L-thyroxine) were set up. Thyroid function was monitored by estimating the total thyroxine in blood plasma using the radio-immunoassay (RIA) method. After treatment, all animals were laparatomized on Days 5–6 of the cycle and the blood samples from peripheral and utero-ovarian veins were collected by cannulation for 2–3 days following surgery. All gilts were then slaughtered and ovaries and other hormonal glands were excised, inspected and preserved for further analysis.The experimental results showed that thyroid hormones in gilts demonstrate an antagonistic influence on the cyst-formative action of gonadotropins. Hypothyroid status increased ovarian sensitivity to gonadotropin action. This was visualised by marked hypertrophy of the ovaries and multiple follicular cysts were also found in both ovaries. In contrast, the hyperthyroid animals showed a reduced sensitivity to the cyst-formative action of gonadotropins (decrease of ovarian dimensions, small numbers of cysts). The mechanism of antagonistic thyroid-gonadotropin relations may be based on negative interactions between thyroid hormones and gonadotropin receptors in the ovaries, and/or on central or peripheral interrelations between thyroid hormones and oestrogens.     

Repeated exposure to prolactin is required to induce luteal regression in the hypophysectomized rat

We investigated whether prolactin (PRL) treatments resembling the intermittent PRL surges of estrous cycles could induce luteal regression in hypophysectomized rats. Immature female rats were stimulated to ovulate and form corpora lutea with exogenous gonadotropins, and were hypophysectomized following ovulation. A single s.c. injection of either vehicle (VEH) or PRL was administered to each rat on post-hypophysectomy Day 8 and again on Day 11. The four resulting treatment groups consisted of rats that received two injections of VEH, VEH followed by PRL, PRL followed by VEH, or two injections of PRL. Rats were killed 24 or 72 h following the second injection. Plasma 20alpha-dihydroprogesterone, luteal weight, and total luteal protein were determined. One ovary was sectioned for immunohistochemistry for monocytes/macrophages, apoptotic nuclei, and major histocompatibility class II (MHC II) molecules. No effect of time (following injection) was observed on any endpoint, indicating that PRL does not have an ongoing regressive action. Time groups from within each treatment group were therefore pooled for analysis. Significant declines (P: < 0.05) in plasma concentrations of 20alpha-dihydroprogesterone, luteal weight, and protein per corpus luteum occurred only after two injections of PRL. Numbers of luteal monocytes/macrophages, apoptotic nuclei, and MHC II-positive cells were low in all groups; numbers of luteal monocytes/macrophages increased following two injections of PRL (P: < 0.05). We conclude that PRL has a cumulative regressive effect on the corpus luteum of the hypophysectomized rat. Drawing a parallel with the estrous cycle, we suggest that continued exposure to PRL, over several cycles, is necessary to induce full luteal regression.