Biomedical subjects

P Licht

Publications and source records attributed to P Licht.

At least 55 records · Page 3Linked to original sources

PubMed1993

Interactions of temperature and steroids on larval growth, development, and metamorphosis in a toad (Bufo boreas).

The effects of temperature and steroids [testosterone (T), estradiol (E2), and corticosterone (B)] on premetamorphic growth and development were investigated in the toad (Bufo boreas). The effects of steroids were both temperature and age dependent. In the first experiment, steroids (1.1-1.4 microM) were administered by dissolving them in the water beginning 1 day after hatching at 22 degrees C or 27 degrees C. At 22 degrees C, B inhibited growth (P < 0.001) but had no significant effect on development. Forelegs never emerged in B-treated animals and all died before complete tail resorption. Discontinuation of B treatment allowed normal growth and metamorphosis, but the resulting post-metamorphic animals were significantly shorter (snout-vent length, P < 0.001) than after other treatments. At 22 degrees C, T and E2 had no effect on larval growth and development or size at metamorphosis (P > 0.05), but T induced early foreleg emergence (FLE) (P < 0.005). At 27 degrees C, B was fatal after 2 weeks of treatment, and T and E2 inhibited growth (P < 0.001) and development (P < 0.001), but did not affect time to FLE. In a second experiment at 27 degrees C, treatment with 1.1 microM B starting 15 days after hatching induced early metamorphic events (P < 0.001), such as tail resorption and emergence of the left foreleg (but not the right), but jaw and head restructuring failed to occur. All B-treated animals died before complete tail resorption. In a third experiment, 0.275 and 1.11 microM B, starting at day 43 (stage 43), induced early FLE (P < 0.05) and decreased snout-vent length at tail resorption (P < 0.005) without a dose effect. A higher dose of B (4.44 microM) decreased snout-vent length at tail resorption and time to FLE (P < 0.05) but did not affect body weight at metamorphosis (P > 0.05). Animals in this experiment survived to complete tail resorption and transformed normally. The actions of B in these experiments closely resemble those observed with administration of thyroid hormones, suggesting that steroids may interact with endogenous thyroid hormones.

Animals↗

PubMed1993

A mechanism for virilization of female spotted hyenas in utero.

Female spotted hyenas exhibit male-like genitalia and dominance over males. Hyena ovarian tissues incubated in vitro produced large quantities of the steroid hormone precursor androstenedione. The activity of aromatase, which converts androstenedione to estrogen, was one-twentieth as great in hyena versus human placental homogenates. In comparison, the activity of 17 beta-hydroxysteroid dehydrogenase, which converts androstenedione to testosterone, was equal in the two homogenates. The limited aromatase activity may allow the hyena placenta to convert high circulating concentrations of androstenedione to testosterone, which results in virilization of the fetal external genitalia and possibly destruction of fetal ovarian follicles. Androstenedione production by residual ovarian stromal cells during reproductive life accounts for the epigenetic transmission of virilization in female spotted hyenas.

17-Hydroxysteroid Dehydrogenases↗

PubMed1993

In vitro exposure of frog (Rana pipiens) pituitaries to gonadotropin-releasing hormone results in a persistent stimulation of gonadotropin biosynthesis.

To further investigate the action of gonadotropin-releasing hormone (GnRH) on gonadotropin (Gth) secretion and synthesis, pituitaries from frogs, Rana pipiens, were incubated in the presence or absence of 100 ng/ml GnRH for 12 h. GnRH was then removed from the medium and glands were labeled with 35S-methionine either 12, 24, or 36 h after the conclusion of GnRH exposure. Immunoassayable LH and FSH (iLH and iFSH; both labeled and nonlabeled hormone) were determined by separate radioimmunoassays. Newly synthesized (radiolabeled) Gth was quantified by immunoprecipitation. When GnRH was present in the medium, it stimulated the secretion of both iLH and iFSH (compared to unexposed glands). Following withdrawal of the secretagogue, iLH and iFSH secretion diminished until they returned to control levels (within ca. 12 h). Previous exposure to GnRH enhanced the biosynthesis of Gth; this elevation was detectable up to 36 h after removal of the secretagogue. The magnitude of the elevation in Gth synthesis did not diminish with time, as secretion had, but remained constant. Gth synthesis was comparable in glands that had been exposed to GnRH 12 h and those exposed to the secretagogue 36 h prior. Thus, the GnRH-induced elevation in Gth secretion and synthesis can be dissociated; the elevation in synthesis is more chronic than the elevation in secretion.

Analysis of Variance↗

PubMed1993

Olfactory bulb removal lengthens the period of circannual rhythms and disrupts hibernation in golden-mantled ground squirrels.

Removal of the olfactory bulbs lengthened the period of circannual rhythms (CARs) of body mass and plasma testosterone (T) concentrations in male golden-mantled ground squirrels, but did not otherwise alter their expression. The period of the CAR was approximately 2 months longer in bulbectomized than in sham-operated animals. Peak values of body mass and T were unaffected by bulbectomy. All neurologically intact, but only 50% of bulbectomized, squirrels displayed normal hibernation patterns. We conclude that the olfactory bulbs are not the site of essential circannual oscillators. Circannual reproductive and hibernation cycles of ground squirrels are less subject to modulation by the olfactory bulbs than are the corresponding rhythms of several non-circannual hamster species.

Animals↗

PubMed1993

Comparative studies of thyroxine binding proteins in turtle plasma using column chromatography.

Thyroxine (T4) binding proteins in turtle plasma were compared with those of various mammalian and nonmammalian (fish, birds) species using gel filtration chromatography (Bio-Gel). High-affinity T4 binding was observed in all mammals studied, in the chicken, and in one species of turtle, Trachemys scripta. This high affinity T4 binding protein (TBP) appears to be the major component of T4 transport in the turtle; selective removal of this TBP greatly diminished binding activity in T. scripta plasma. In all other nonmammalian species studied (including other turtles), T4 binding appeared to be associated primarily with relatively low affinity components such as albumin. Albumin concentrations in turtle (T. scripta, and Chelydra serpentina) plasmas were examined by measurement of dry weight, protein assay (Bradford), radioimmunoassay, and electrophoretic densitometry. While C. serpentina may have total protein concentrations approaching that of human plasma, albumin content in both turtles was estimated at about 3-4 mg/ml, less than 10% that of human plasma albumin; larger molecular weight components that do not bind T4 comprise a greater proportion of turtle plasma protein. Additionally, purified T. scripta albumin exhibits lower T4 binding than does human albumin; binding to turtle albumin is also reduced by fatty acids. Taken together, these results indicate a lesser role for albumin in plasma T4 binding in T. scripta than in humans, and, thus, a larger role for the T4 binding protein, TBP. Other turtles lacking a TBP may rely wholly on low affinity albumin binding that also has a lower capacity than that in mammals.

Adult↗

PubMed1993

Inhibition of in vitro pituitary gonadotropin secretion by 17 beta-estradiol in the frog Rana pipiens.

We tested the effects of low concentrations of 17 beta-estradiol (E2) on the in vitro pituitary responsiveness to gonadotropin-releasing hormone (GnRH) in frogs (Rana pipiens). We first showed that the pH indicator phenosulfonpthalein (phenol red, PR) at levels normally used in the culture medium did not significantly affect pituitary basal or GnRH-stimulated gonadotropin secretion in male glands, as may occur in mammals. Incubation of male glands with E2 at 10 pg/ml for 24 hr in medium lacking PR did not alter basal follicle-stimulating hormone (FSH) or luteinizing hormone (LH) secretion, but significantly attenuated responsiveness of both gonadotropins to GnRH (0.2, 1, and 5 ng/ml); E2 did not affect responses at the highest dose of GnRH (25 ng/ml). GnRH responsiveness of female pituitaries was also inhibited by E2 at all doses of GnRH; 1 ng/ml E2 had greater effects than did 10 pg/ml. The present results demonstrate that E2 at concentrations commonly found in the circulation of both sexes have a consistent direct inhibitory effect on FSH and LH secretion at the level of the pituitary.

Animals↗

PubMed1993

GnRH-induced desensitization of in vitro luteinizing hormone secretion in the turtle, Trachemys scripta.

Gonadotropin-releasing hormone (GnRH)-induced desensitization of pituitary luteinizing hormone (LH) secretion in female turtles (Trachemys scripta) was investigated by in vitro superfusion of whole or hemipituitaries with the endogenous forms of the peptide. In sub-adults, LH secretion was rapidly attenuated (desensitized) by both prolonged and pulsatile treatments of chicken II GnRH (cII-GnRH), but pulsatile GnRH administration was more effective than prolonged administration in stimulating LH release. Prolonged administration of chicken I GnRH (cI-GnRH) also desensitized LH secretion; cI-GnRH was slightly less potent than cII-GnRH. GnRH-induced attenuation of LH secretion was not the result of tissue deterioration or depletion of releasable LH: GnRH responsiveness of desensitized pituitaries was fully restored after 4 hr of in vitro recovery, and further LH secretion was induced by tetraethylammonium chloride, a membrane depolarizing agent, in the desensitized glands; TRH-stimulated TSH release did not show desensitization in the same glands. Pituitaries from most adult (vitellogenic) females failed to respond to GnRH challenges; however, when responsive, adult pituitaries also desensitized under prolonged GnRH stimulation and were more resistant to GnRH-induced desnsitization under pulsatile GnRH treatments. Current results show that adult T. scripta typically display lower GnRH sensitivity than the subadults; the latter are like mammals, birds, and goldfish in their susceptibility to desensitization by both forms of endogenous GnRH.

Animals↗

PubMed1993

Binding affinities of thyroxine-binding proteins in turtle plasma.

Binding affinities (Ka) for thyroxine (T4) by blood plasma and purified plasma proteins from two turtles, the slider (Trachemys scripta) and snapper (Chelydra serpentina), were compared with those of a human using equilibrium dialysis. The purified T4 binding protein (TBP) from T. scripta had a high affinity that was lower by about fivefold than that of human TBG (3.2 x 10(9) vs 1.7 x 10(10) M-1). The affinity of T4 binding by TBP was similar to that determined for whole plasma from this species. T. scripta plasma stripped of TBP by affinity chromatography showed a greatly reduced Ka (2.8 x 10(5) M-1) similar to that of albumin purified from the same species (3.6 x 10(5) M-1), as well as a marked increase in free T4 (9.7%) as compared to whole turtle plasma (0.13%). The T4 binding of snapping turtle plasma (Ka = 5 x 10(5) and free T4 = 2.7%) was similar to that of the TBP-stripped plasma and albumin. Binding affinities for the two turtle and human albumins were similar. Because of low concentrations of albumin in the turtle blood (estimated at ca. 10 mg/ml based on binding), TBP probably accounts for a greater proportion of T4 binding in the slider turtle than does TBG in the human; e.g., when plasma T4 and TBP are elevated in T. scripta, > 98% of bound T4 would be associated with TBP. When both TBP and T4 are depressed (e.g., as in hatchlings), free T4 may actually be higher than in conditions when total T4 is elevated.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗

PubMed1993

In vivo GnRH responsiveness of LH secretion in the female turtle, Trachemys scripta, in relation to the reproductive stage.

The effects of several forms of gonadotropin-releasing hormone (GnRH) on the in vivo secretion of luteinizing hormone (LH) were investigated in the female turtle Trachemys scripta. Serial injections of large vitellogenic turtles with 1-2 micrograms (ca. 0.15 micrograms/100 g turtle) of chicken GnRH-I (cGnRH-I) and multiple blood sampling revealed that mature females were unresponsive to GnRH. In contrast, both forms of avian GnRH, cGnRH-I and cGnRH-II, and an avian GnRH analogue (GnRHa) significantly increased circulating levels of LH in previtellogenic turtles. In these turtles, plasma LH increased within 10 min after the initial injection of 0.5 microgram GnRHa or cGnRH-I, but this increase in LH was generally transient and could not be sustained for more than 2 hr even with repeated GnRH injections. Turtles treated with GnRHa on Day 1 showed attenuated pituitary responsiveness to subsequent GnRH challenges on Day 2, whereas turtles treated with both native forms of chicken GnRH remained equally responsive on Day 2. In vitro, GnRHa was more resistant to degradation in plasma than both native forms of GnRH, and the clearance rate of GnRHa in vivo was also slower than that of cGnRH-I. The results show, for the first time, that in vivo GnRH administration stimulates and desensitizes LH secretion in the turtles, but the GnRH responsiveness is related to reproductive status. Furthermore, the results suggest that the desensitizing effects of various forms of GnRH in vivo may be related to their clearance rate.

Animals↗

PubMed1993

Metabolism of exogenous steroids by anuran larvae.

Uptake and metabolism of exogenous steroids [testosterone (T), estradiol (E2), and corticosterone (B)] were examined in anuran larvae. Radioimmunoassay (RIA) of steroids dissolved in water revealed that as much as 0.03 mg of T or E2 were cleared from water by a single Xenopus laevis tadpole in 24 hr. Studies using RIA in combination with counting of total [3H]steroids suggested that clearance was mainly due to metabolism and not to uptake and storage by the tadpole. Thin-layer chromatography and HPLC analysis confirmed metabolism and revealed that T was metabolized to at least three ether-extractable compounds as well as to water soluble compounds (nonextractable in ether) by nine anuran species, representing eight families. Most species metabolized E2 to nonextractable compounds, but some produced organic metabolites. In contrast, B was not taken up, cleared, or metabolized by any of the nine species. Studies in X. laevis using [3H]T and radioinert T in the same container confirmed that the decrease in radioimmunoassayable T was due to metabolism as determined by TLC. In a separate study, [3H]T was metabolized more extensively when injected directly into the tadpole than when dissolved in water. Additional studies with X. laevis revealed that steroids were also metabolized in tadpole-conditioned water (water exposed to tadpoles), but the metabolites produced represented < 10% of those produced when tadpoles were present. This metabolism was confined to particulate matter from the gut of tadpoles. Metabolites extracted from tadpoles were similar to metabolites found in the water.

Animals↗

PubMed1993

Further studies on the influence of GnRH on the biosynthesis of gonadotropins in female frogs (Rana pipiens).

To further investigate the action of gonadotropin-releasing hormone (GnRH) on pituitaries of female frogs (Rana pipiens), gonadotropin (Gth) secretion and synthesis was measured following either the in vivo or in vitro exposure to GnRH. Treating frogs with a GnRH-agonist (GnRHa) in vivo for either 2 or 5 days led to an elevation in plasma Gth compared to that of saline-injected controls. In general, the pattern of Gth secretion in vitro paralleled changes observed in their plasma levels. GnRHa-treated frogs also had depressed pituitary content of Gth as compared to controls. Pituitaries from frogs treated with GnRHa for 5 days had higher levels of Gth synthesis than those of controls; however Gth synthesis in females treated with GnRHa for only 2 days was not elevated. After 30 hr of continuous exposure to 100 ng/ml GnRH in vitro, glands from females secreted markedly higher levels of LN and FSH than glands not exposed to the secretagogue. The levels of LH and FSH contained within pituitaries exposed to GnRH were significantly less than those of control glands. Prolonged exposure to GnRH elevated both the secretion of radiolabeled Gth as well as increased the total synthesis of Gth (compared to those of controls). These results indicate that GnRH can stimulate the biosynthesis of Gth in pituitaries of female frogs; however these results differ from those previously obtained with males. It appears that female glands require a much longer duration of GnRH stimulation than males before Gth synthesis is elevated.

Animals↗

PubMed1993

Effects of corticosterone on social behavior of male lizards.

Adrenal corticoid release is a major component of the stress response which can affect many body functions including behavior. The purpose of our studies was to examine the effects of corticosterone (B) on both the agonistic and courtship components of social behavior in male side-blotched lizards (Uta stansburiana). Of particular interest was the effect of B on plasma testosterone (T), a hormone known to influence aggression, and the importance of this action on the behavioral effects of B. Experiments included either castrated males, hormone (either B, T, or B + T)-implanted intact males, or sham males. Behavioral observations were recorded when these males were challenged with "immigrant" males; measurement of plasma steroids confirmed the efficacy of implants in elevating B and T. Castration, B implantation, and combined B and T implantation significantly reduced aggressive behavior to varying degrees, while T implants in intact animals had no effect. B implants significantly decreased plasma T levels (from 1.87 to 0.33 ng/ml), but this decrease was not essential for the inhibitory effect of B on aggression since B + T implantation also reduced aggression, even though plasma T was elevated above normal (67.6 ng/ml). In contrast, B implantation did not affect male courtship and copulatory behavior when males were presented with estrogenized females. These results suggest that the effect of B on social behavior is not through just a single route, that of decreasing plasma T, and that B can affect various intra- and intersexual behaviors differently.

Agonistic Behavior↗

PubMed1993

Sustained hormonal responses of Siberian hamsters (Phodopus sungorus) to a single longer day at weaning.

Siberian hamsters undergo gonadal development for several weeks after exposure to a single longer day at weaning. To characterize changes in gonadotropin secretion after a single acute light stimulus, hamsters housed in a long photoperiod (16L:8D) were given a single longer day (20L:4D) or maintained in the 16L:8D photoperiod at 19 days of age and transferred to a short photoperiod (8L:16D) on Day 20. Elevated plasma FSH concentrations were detected in male hamsters at 5, 7, and 12 but not at 17 days after the single longer day. Melatonin treatment during light exposure and on two succeeding nights blocked the stimulatory effect of light on the reproductive axis; melatonin injections limited to one night were marginally effective. Pinealectomy during the dark phase of the photocycle and the resultant truncation of the melatonin signal for one night did not stimulate a greater degree of gonadal development than pinealectomy during the light phase. We conclude that the single extra 4-h light pulse at weaning alters hypothalamic-pituitary function for approximately 2 wk. Trophic effects of the light pulse appear to be mediated by suppression of melatonin secretion for several days; one truncate melatonin signal is not sufficient to simulate the effects of a single long day on the reproductive axis.

Animals↗

PubMed1993

Prolactin and testosterone inhibit torpor in Siberian hamsters.

Female Siberian hamsters maintained in a winter photoperiod (8 h light/day) ceased to undergo daily torpor during infusion of prolactin (PRL) from osmotic minipumps; winter torpor was reinstated within 3 days of discontinuation of treatment. By contrast, PRL infusion was ineffective in suppressing daily torpor elicited by restricting food intake in female hamsters housed in a summer photoperiod (16 h light/day). Summer daily torpor was, however, completely inhibited in long-day gonadectomized male hamsters treated with testosterone (T). We suggest that the hyperprolactinemia, which in previous studies is characteristic of hamsters that sustain ablation of the suprachiasmatic nucleus, is incompatible with winter torpor. Summer torpor may be controlled by a mechanism less responsive to variations in plasma PRL concentration. Both winter and summer torpor are inhibited by exogenous T; it remains uncertain, however, whether sustained decreases in endogenous T secretion are as essential for the expression of summer as they are for winter torpor.

Animals↗

PubMed1993

Novel expression of human chorionic gonadotropin/luteinizing hormone receptor gene in brain.

LH from anterior pituitary and hCG from placenta bind to a common receptor in gonadal and nongonadal reproductive tissues. There have been numerous examples suggesting that the brain may also contain hCG/LH receptors, yet there has been no evidence for their existence so far. We now demonstrate by reverse transcription-nested polymerase chain reaction and northern blotting that the rat brain contains hCG/LH receptor mRNA. A major receptor transcript of 2.6 kilobases and minor transcripts of 1.8 and 4.4 kilobases were found. Western immunoblotting, ligand blotting, and covalent receptor cross-linking studies have shown that rat brain also contains an 80-kilodalton receptor protein that can bind hCG and hLH, but not hFSH. Rat testis used as a positive control showed a higher abundance of multiple transcripts and an 80-kilodalton receptor protein that can bind [125I]hCG. Rat liver used as a negative control did not contain any receptor transcripts or protein. In situ hybridization, dot blotting, immunocytochemistry, and topical autoradiography have revealed that hCG/LH receptors are present in rat hippocampus; dentate gyrus; hypothalamus; cerebellum; choroid plexus; ependymal cells of third, fourth, and lateral ventricles; cortex; brainstem; bovine hypothalamus; and human area postrema. These novel findings could potentially explain numerous previous observations and suggest new possibilities concerning the regulation of brain functions by hCG and LH.

Animals↗

PubMed1993

Novel self-regulation of human chorionic gonadotropin biosynthesis in term pregnancy human placenta.

Term pregnancy human placenta contains hCG/LH receptor mRNA transcripts and immunoreactive receptor protein. Both the receptor transcripts and receptor proteins are present only in trophoblasts. These findings led us to investigate whether hCG can regulate its own synthesis in term pregnancy human placenta. Treatment of placental tissue in static cultures or in a dynamic superfusion system with increasing concentrations of highly purified hCG provoked a biphasic effect on the steady state hCG subunit mRNA levels. Although low concentrations of hCG (< 200 mIU/ml) were not effective, moderate concentrations (200-1000 mIU/ml) increased, and high concentrations (> or = 5000 mIU/ml) either had no effect or actually decreased mRNA levels relative to the control values. This response was specific, because none of the hCG concentrations tested had any effect on glyceraldehyde-3-phosphate dehydrogenase or beta-actin mRNA levels. The effects of hCG on steady state hCG subunit mRNA levels were paralleled by corresponding changes in tissue hCG protein levels. Endogenous hCG appears to down-regulate alpha-subunit mRNA levels and hCG secretion. The hCG effect is probably receptor mediated, because a receptor antagonist, deglycosylated hCG, partially antagonized the hCG action. Treatment with exogenous hCG also down-regulated its own receptor mRNA and receptor protein levels. hCG regulation of its alpha-subunit and receptor levels involved both transcriptional as well as posttranscriptional mechanisms. In summary, this is the first demonstration of hCG regulating its own synthesis in term pregnancy human placenta. The findings of this study could offer a potential molecular explanation for the profile of hCG levels in normal pregnant women.

Antibodies↗

PubMed1992

Identification and purification of a high-affinity thyroxine binding protein that is distinct from albumin and prealbumin in the blood of a turtle, Trachemys scripta.

Fractionation of plasma proteins in the turtle, Trachemys scripta, confirmed the presence of a high-affinity thyroxine (T4) binding protein (TBP) that was distinct from albumin (ALB) and prealbumin (PA). The TBP was isolated by adsorption on a T4-affinity column and a high degree of purification was achieved by gel filtration and preparative electrophoresis. Analysis by reversed-phase HPLC showed a single peak of protein with T4 binding activity. The electrophoretic mobility of the TBP, based on staining and binding to [125I]T4 on nondenaturing polyacrylamide gels (PAGE), corresponded to that of the major T4 binding activity previously identified in plasma (ca. 60 kDa). PA was fractionated as a complex with retinol binding protein (PA-RBP) based on retinol associated fluorescence using ion exchange chromatography on DEAE-Sephacel and gel filtration. This complex behaved as a larger and more highly charged molecule than TBP; it was partially dissociated in low ionic strength basic solution. SDS-PAGE of the PA-RBP-enriched fraction revealed a major component of about 48 kDa (possibly free PA), with smaller components corresponding to those expected for free RBP (ca. 22 kDa) and subunits of PA (e.g., 14 and 28 kDa). ALB was purified by ion exchange chromatography on DEAE and gel filtration; it behaved as less basic than PA with MW approximately 67 kDa. TBP accounted for virtually all the T4 binding activity of whole plasma: TBP was about 100 times as active and PA and ALB were less than 1% as active as plasma. The binding affinity of purified TBP was similar to that of whole plasma from turtle and human (e.g., approx. 10(9) M-1 on Sephadex G-25).

Animals↗

PubMed1992

Changes of LH level in the pituitary gland and plasma in hibernating frogs, Rana temporaria.

Pituitary and plasma luteinizing hormone (LH) levels were measured in female frogs, Rana temporaria, during and immediately after hibernation (0-4 degrees in darkness; 22-25 weeks) to study regulation of gonadotropin leading to posthibernation ovulation. Pituitary LH content began to rise progressively during the last third of hibernation (e.g., from starting levels of 5.3 +/- 0.09 micrograms/gland, 1.5 micrograms/mg between 8 and 11 weeks, to 18.6 +/- 9.7 micrograms/gland, 5.0 +/- 2.7 micrograms/mg at 19-22 weeks). Plasma LH increased in parallel, but with some delay (from 10 +/- 8 to 25 +/- 24 ng/ml). Frogs kept in light at low temperatures showed similar responses. Release of LH (rise in plasma levels with a drop in pituitary content) was observed during 32 hr immediately after termination of hibernation in association with the onset of ovulation. These data indicate the existence of regulatory mechanisms operating during hibernation under conditions of constant cold. Altogether, these and our previous results obtained after surgical deafferentiation of median eminence support the hypothesis of a progressive reduction of a CNS inhibition that results in the release of GnRH during hibernation in this frog.

Analysis of Variance↗